Note: Descriptions are shown in the official language in which they were submitted.
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A HI H EFFICIENCY LIGHT VALVE PROJECTION SYSTEM
FIELD OF THE INVENTION
The present invention relates generally to video and
data display devices and more particularly to an improved video
display system employing light valves such as an active matrix
LCD in conjunction with novel projection optics.
BACKGROUND OF THE INVENTION
The mainstay of electronic imaging, since its
beginnings, has been the cathode ray tube (CRT) or kinescope.
Although CRT technology has progressed over the years, several
major drawbacks remain. Picture size is still limited, making
group viewing difficult. CRT picture tubes larger than about 301'
(measured diagonally) become impractical because of size, weight,
expens~ and danger of implosion because of the high vacuum used.
To achieve high brightness they use dangerously high voltages and
may produce health hazards from x-rays and electromagnetic
fields.
Image quality of CRT-based video displays may be
degraded by color distortion, image shape distortions, color
impurity from the influence of the earth's magnetic field, and
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color misconvergence. In addition, CRT disp:Lays are subject,
particularly when viewed at close range, to visual artifacts such
as scanning lines and discre~e phosphor dots or stripes, which
are inherent in such TV displays. These visual artifacts provide
a poorer image quality than images in movie theaters.
Research has been continuing on for many years to
develop other types of li~ht emissive displays which would
overcome some of these drawbacks. Plasma, electroluminescent (~L)
and cold cathode phosphor displays are among the most promising
candidates, although they have not proved themselves to be
practical. Furthermore, it is highly questionable whether these
other emissive displays, if and when successful, would provide
any advances over current CRT brightness or size in practical
applications.
I'Pocket TVsl' with a 2" 3" picture are constructed today
using liguid crystal displays which are addressed via electronic
multiplexing or active matrix addressing. Creating a large
picture for direct viewing however poses many problems which have
heretofore not been overcome. Simple multiplexing cannot produce
a satisfactory image because of cross-talk. An active matrix
relieves the cross-talk problems, bu~ has so many more production
steps and so many switching and storage elements that must be
deposited over a large surface area that production of large,
defect-free active matrix displays ~or direct viewing has not
been possible and may never be economically feasible Eor very
large displays.
Demand for large video imaginy systems and for thin-
profile or "flat screen" imaging systems, both large and small,
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has increased significan~ly in recent years and is expected to
increase dramatically with the advent of hiqh definition
television broadcasts. 'IProjection televisions" have been
developed and commercialized in recent years. Unfortunately, such
projection display devices have exacerhated many of the problems
associated with earlier video display sys~ems and have created
new problems. Projection televisions are more expensive than
standard direct-view televisions and are more cumbersome,
heavier, and larger so that portability is impractical. Two types
of projection television systems have become popular: one using
three C~Ts with projection lenses and the other using an oil film
scanned by an electron beam.
The CRT-based projection system remains relatively dim,
requiring a dimly-lit viewing environment and a costly special
screen which provides a very limited viewing angle. The three
CRTs produce images in the primary colors, blue, green, and red
and are driven with higher anode voltage than conventional
systems to obtain as much ~rightness out of them as possible.
The higher anode voltage lowers ~ube life and increases the
radiation hazards and oth~r problems associated with high
voltage. The three tubes also increase the danger of tube
implosion The standard oil-based system, referred to as an
Eidophor, has three "scanned oil elements" which have a
relatively short life and uses external light sources. In either
system, all three color images utilizing three sets of optics
must be precisely converged onto the viewing screen, in addition
to requiring adjustments o~ hue, saturation, vertical and
horizontal size and linearity, and minimization of pincushion and
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barrel distortion. Proper alignment in either system is therefore
beyond the abilities of the average person. Proper convergence i5
not easily achieved and o~ten requires up to a half hour of
additional set-up time because o the curvature of the lenses and
variations in the performance of the circuits in either system.
If the pro~ector or screen is moved, the convergence procedure
must be repeated.
Experimentation has also been performed on laser
systems which scan out an image on a viewing screen in the same
way an electron beam scans the image onto the face of a CRT. The
laser systems developed thus far are much too large to be
portable, very complex to use and main~ain, extremely expensive,
potentially dangerous and have proven too dim for large images.
Many attempts have been made t~ solve the
above-mentioned problems, resulting in experimentation on several
novel "light valve" based systems. This type of system uses an
external light source which can ~heoretically be as bright as
desired, with a "light valve" to modulate the light carrying the
picture information. The research and experimentation to develop
a workable light valve system has been primarily directed to
using different optical, electronic, physical and other effects
and finding or producing various materials to accomplish the
desired results. The various light-valve system attempts have
mainly utilized crystals (such as ~uartz, Potassium Di-Hydrogen
Phosphate, Lithium Nioba~e, Barium Strontium Niobate, Yttrlum
Aluminum Garnet and Chromium Oxide), liquids (such as Nitro
~enzene) or liquid crystals (of the smectic or nematic type or a
suspension of particles such as iodoquinine sulphate in a liquid
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carrier) or other similar materials ~o take advantage of one or
more optical effects including electro-optical effects, such as
creating a rotated plane of polarization or altering the index of
refraction of the material due to an applied electric field,
magneto-optical effects using an applied magnetic field,
electro-striction effec~s, piezo-optical effects, electrostatic
particle orientation, photoconductivity, acousto-optical effects,
photochromic effects and laser-scan-induced secondary electron
emission. Except for liquid crystal light valves, such light
valves proved impossible to manufacture economically and with a
sufficiently large aperture and have often been toxic, dangerous~
and inconsistent in production quality.
In all light valves, different areas must be supplied
different information or "addressed," so that a different amount
of light emerges through each area, adding up to a complete
picture across the total beam of light. Techniques for addressing
different picture elements (or "pixels") of a light valve have
included methods for deflecting a laser or electron beam to that
area or the use of a tiny criss-cross of electrically conductive
paths, i.e., a matrix, deposited on or adjacent to the material
to be addrassed in order to activate that area of the matrix. In
scanning beam systems, problems have included outgassing and
erosion of material. The eleotrical matrix system has proved
difficult to engineer, requiring deposition with extremPly high
precision of a transparent material having good Gonductivity
characteristics. Further, such matrices must be driven by
extremely fast switching circuits, which are impractical at the
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high voltages required to activate a given area of most
materials.
The most frequently used system for addressing small
areas is often referred to as electronic multiplexing.
Electronic multiplexing works well with only low
voltage-requiring materials such as liquid crystals. With this
method, all pixel addresses are x and y coordinates on the
conductive grid. ~o activate a given pixel area a specific
amount, different voltages must be applied to the x and y
conductors so that, where they meet, they together exceed a
threshold voltage and modulate the area. A major drawback to such
multiplexing is crosstalk, where surrounding areas are affected
by the electric field, causing false data to influence
surrounding pixels, reducing contrast and resolution, as well as
color saturation and accuracy. The crosstaIk problem increases
when resolution increases because liquid crystal materials
respond fairly linearly to applied vol~age Since all pixels are
interconnected within the same system, all pixels are given
partial voltage and are, thus, partially activated when any one
pixel is addressed. Non-linear materials can be added to the
liguid crystal mix, but this still doesn't allow for more than
about 160 lines of resolution before crosstalk significantly
degrades the image.
An "active matrix" light valve in which all pixels from
the matrix are selectively disconnected except for those pixels
which are addressed at any given time eliminates the crosstalk
problem, regardless of the number of pixels or lines in a
display. Recently, active matrix displays have been made
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utilizing transistors, diodes, or an ionizing gas as the
switching element to disconnect the pixels.
Since liquid crystal ligh~ valves have very little
persistence and one pixel or line of pixels is activated at a
time, substantially less light is projected to the screen to be
ultimately viewed since all pixels are "off" most of the time.
This characteristic wastes light, produces a dimmer image with
poorer contrast and generates more heat because of the brighter
source necessary to compensate for the dim image. High refresh
rates are impractical because they would require faster switching
times and faster responding material.
Active matrix displays, however, also utilize a storage
element, such as a capacitor, connected to each pixel, which
allows each pixel to retain the proper charge, and thus, the
proper transmissivity after the pixel has been addressed and
disconnected from the system. Thus, each pixel remains "on" the
correct amount all the time. This increases light throughput and
eliminates flicker.
If high-wattage light sources are used in order to
achieve very bright displays, heat sensitivity can cause a
decrease in contrast and color fidelity. Absorption of high
intensity light by color filters and polarizers (if used), even
if little or no infrared light is present, results in heating of
these elements which can also degrade image quality and may even
damage the light valve. Use of fan cooling causes objectionable
noise, especially in quiet environments when source volume is
kept low.
Another inherent problem of light valve projection
systems relates to the fact that each pixel of the frame is
surrounded by an opaque border that contain~s addressing circuitry
or physical structure. This results in visi~ly discrete pixels
and contributes an objectionable "graininess" to the image that
become progressively more annoying when viewed at close distance
or on large screens. The problem is amplified if a single full-
color light valve is used in which the individual red, green, and
blue color elements of each pixel are not converged or blended
and are visible to the viewer~
Consequently, projection by means of a small light
valve provides the most practical and economical way to produce a
large, bright image. Un~ortunately, such light valve projectors
have, up to the present, exhibited several shortcomings which
fall generaily into at least four hroad categories, namely:
1) light valve restrictions;
2) light source limitations;
3) optical system inefficiencies; and
4) screen performance weaknesses.
These problems must be a~dressed to allow for the successful
production of acceptable quality, practical display systems,
capable of large projection imagery and display of small or large
images rom a device with a "thin profile."
To address these and other problems associated with
prior art video display svstems, it is an object of the present
invention to provide an adjustable size video image which can be
very large, yet possess high ~uality and sufficient brightness to
be visible from wide viewing angles without distvrtions, in a
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normally lit room as well as in environments with high ambient
light.
Furthermore, an objec~ o~ khe inventivn is to create a
~-ideo display system which utilizes a light valve such as a
specially constructed LCD light valve, an independent light
source with a long life, high brightness, average luminance, and
color temperature, and nov 1 optics, providing or high light
efficiency for front or rear projection and which operates
without excess heat or fan noise.
Another object of the invention is to produce such a
system with high resolution an~ con~rast (eliminating the
appearance of stripes, pixels, or lines), with highly accurate
color rendition (equal to or better than that of a CRT).
-An additional object of the in~ention is to produce a
display that reduces eye-strain by the elimination of flicker and
glare and by the broadenins of color peaksO
A further object of the invention is to produce a
small, lightweight, portable system, having a long
maintenance-free operating life, which is operable in conjunction
with or without a special screen and can be mass-produced
relatively inexpensively.
Yet another object of the invention is to produce a
system which requires no convergence or other dificult
adjustments prior to viewing.
Still another object of the present invention is to
produce a system with greatly reduced radiation and hazard of
tube implosion and operates with relatively low voltage.
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An additional object of the invent:ion is to produce a
system which does not require a special screen, can be easily
projected onto a wall or ceiling, and can be~ viewed comfortably
at relatively wide angles.
A further objective of the invent:ion is to produce such
a system capable of three-dimensional projection.
Additional objects of the invention include the
creation of a system which will overcome drawbacks associated
with CRTs in terms of weight, bulk, high voltage, radiation,
implosion hazard and convergence difficulty in 3-CRT projection
systems.
Further objects will include increasing image contrast,
color reproducibility, resolution and yield while reducing color
pixel visibilityt flicker, hea~ sensitivity, image artifacts,
system cooling noise and bleedthrough of non-image bearing light,
while decreasing the cost and complexity of light valve systems.
Additional objects of the invention involve creating a
system to overcome and improve upon light sourc~ limitations by
increasing brightness efficiency, average luminance and color
temperature, while lengthening bulb life and reducing the weight
and bulk of the power supply.
Yet additional objects of the invention involve
creating a system with improved light collection, decreased light
losses due to color selection and polarization, decreased light
valve aperture ratio losses and o~her non-image light waste.
Further objects of the invention involve creating a
system which involves improving performance by use of particular
screen materials with reduced light absorption, while reducing
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lenticular-lens-pattern image degradation, off-axis projection
distortion and off axis brigh~ness fall-offl while reducing the
effect of glare an~ ambient light ~o image visibility.
Moreover, it is an object o~ the invention to create a
system which minimizes and virtually eliminates the wasted space
of projection distance and enables three-dimensional pro~ection.
Other objects will become evident from the disclosure.
SUMMARY OF THE INVENTION
These and other objects of the invention which will
become apparent hereafter are achieved by "A ~IGH EFFICIENCY
LIGHT VALVE PROJECTION SYSTEM" employing a light valve, such as a
liquid crystal display (LCD) device, for the formation of an
image utilizing an "active matrix" for electronically addressing
and activating each of the liquid crystal elements in the matrix.
The matrix is "active" in that a separate transistor or other
suitable material is adjacent to each picture element or l'pixel"
to control each pixel and a storage element, such as a capacitor,
is employed to store the respective pixel video signal. The
system further comprises a direct projection optics arrangement
which includes a light source for illuminating the light valve,
optics which collimate light from the source and improve light
throughput efficiency and quality of the projected image and a
lens system for projecting and focusing an image from the light
valve onto a viewing surface.
An important aspect of one embodiment of the invention
is the use of a dichroic mirror system to superpose color pixel
triads ~rom a single, multicolored LCD to form ~ull-colored
pixels with spaces between them.
Another aspect o~ one embodiment of the invention
relates to the filling o~ spaces between pixels. These spaces may
be filled using a 4-mirror system, in which a first striped
mirror pair duplicates each pixel and the image is shifted
horizontally into the spaces which previously existed between
pixels. A second mirror pair duplicates the newly created rows of
pixels and shifts the original and the duplicated pixel images
vertically to fill the remaining spaces between pixels.
Other methods are described for the filling of spaces
between adjacent pixels through the use of an expanding lens
array and a collimating lens or a second collimating lens array
to expand and collimate individual images o~ the pixels.
The invention will be better understood by the Detai`led
Description of the Preferred Embodiment in conjunction with the
appended drawings, of which:
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 is a schematic view of the invention depicting
three LCDs projecting their image onto one common screen;
.
Figure 2 is a schematic view of a modified embodiment
of the present invention in which the images of three LCDs are
internally superposed and projected onto a common screen
employing one set of projection optics;
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Figure 3 is a schematic view of various pixels with
reduced spaces between them,
Figure 4 is a schematic view of a projected image of
superposed "full color pixels";
Figure 5 is a schematic view of a four-mirror system
depicting a method of filling in spaces between ad3acent pixels;
Figure 6 is a schematic view depicting the filling of
spaces between pixels by the first two mirros (a "striped-mirror
pair") of the four-mirror system of Figure 5;
Figure 7 i~ an enlarged schematic view of a
"striped-mirror pair'l of the four-mirror system of Figure 5;
Figures 8a and ~b are schematic views of lens~system
embodiments of the present invention;
Figure 9a is a schematic view of a dichroic mirror
system of one embodiment of the present invention;
Figure 9b is a schematic view o~ the embodiment of the
dichroic mirror system of Figure 9a, modified to include an
additional light path;
Figure 10 is a graphical plot of transmitted light
intensity over the visible spectrum through two full color LCDS,
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one with a constant LCDs cavity thickness contrasted with a
"stepped thickne~s" LCD cavity;
Figures lla and llb are graphical plots of transmitted
light intensity vs. applied voltage for three wavelengths used in
two full-color LCDs, one for a constant thickness LCD cavity and
one for a "stepped thickness" LCD cavity;
Figure 12 is a maynified schematic view of a "stepped
thickness" LCD cavity showing the different thicknesses of LCD
through which the red, green, and blue light traverse;
Figure 13 is a CIE chromaticity diagram comparing color
ranges of a CRT display, a conventional color LCD display with a
fixed cavity thickness and a 'Istepped thickness" LCD cavity in
accordance with the present invention;
Figure 14 is a schematic view of a rear-screen
projection system utilizing the present invention with a
venetian-blind type of rear-projection screen;
Figure 15a is a schematic view of color filters on
corresponding color-pixel areas in a full-color LCD;
Figure 15b is a schmatic view of an alternate
arrangement of pixels in which three pixels of a color triad are
indicated by a triangle;
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Figure 16 is an open perspective view of a sound
suppression system which may be adapted to the present invention;
Figure 17 is a schematic diagram of the preferred
embodiment of the invention;
Figure 13 is a schematic view of an active matrix
liquid crystal display which utilizes a gas as a switching
element to disconnect pixels from the circuit;
Figure 19 is a schematic view of an embodiment of the
electronic image projection system in which two light valves are
placed together where one light valve would compensate for
defective pixels in the other light valve;
Figure 20 is a schematic view of a projection
arrangement utilizing a reflective light valve;
Figure 21 is a schematic view of a single l.ight valve
divided into three sections for use in full color projection;
Figure 22 is a schematic view of a method of matching
the path lengths of beams ~ravelling from a light valve to a
projection lens utilizing a path length compensation lens;
Figure 23 is a schematic view of a technique utilizing
mirrors to compensate for path length differences of beams
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travelling from the liyht valve to the projlection lens in an
embodiment of the present inYentioni
Figure 24 is an alternate embodiment of the electronic
image projection system utilizing a reflective light valve to
produce a full color image and a ~cNeille prism for polarizing
and analyzing beams,
Figure 25 is a s~hematic view of a section of the
electronic image projetion system in which dichroic mirrors
separate a collimated beam of a white light into colored beams of
light which pass through a double lens array creating demayni~ied
collimated beams of colored light arranged side by side by a
second set of dichroic mirrors for use as a multicolored beam to
illuminate a full color light valve;
Figure 26 a schematic view of an alternate method of
producing a multicolored light beam in the electronic image
projection system for use in illuminating a multicolored light
valve;
Figure 27 is a schematic view of an alternate method of
producing a multicolored light beam utilizing a hologram to
separate a white light beam into red, green and blue beams and a
second hologram to make parallel the resulting beams;
Figure 28 is a schematic viPw of wedges used in the
optical path of a projector to create three overlapping images of
the full color light valv~ so as to merge red, green and blue
pixel color components into Eull color pixel.s in the image;
Figure 29 is a schQmatic view of 2l four mirror system
in the electronic image projection system to overlap red, green
and blue pixel color components creating full color pixels;
Figure 30 is a schematic view of a two mirror system in
an alternate embodiment of the electronic image projection system
used to superimpose red, green and blue pixel color components
creating full color pixels;
Figure 31 is a three mirror system in an alternate
embodiment of the elec~ronic image projection system to
superimpose red, green and blue pixel color components creating
full color pixels;-
Figure 32 is a schema~ic view of the classic method ofspatial filtering using a lens to perform Fourier transformation;
Figure 33 is a schemati~ view of an electronically
controlled prism for image displacement ~o be used with the ~.
present invention;
Figure 34 is a schematic view of pixel holes in a light
valve with a lenslet before and after the pixel hole for use in
analysis of illumination uniformity i.n one aspect of the
electronic image projection system;
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Figure 3~ is a schematic side view of a light valve and
lens arrays ~or further analysis of an aspect of the electronic
image projection system;
Figure 36 is a schematic side view of an embodiment of
a section of the electronic image projection system utilizing
field lens arrays with a light valve;
Figure 37 depicts a ~chematic view of a Section of the
electronic image projection system in which two light sources are
used whose beams are collimated and made continuous by the use of
a prism;
Figure 38 is a schematic view of a section of the
electronic image projection s~stem in which light from two
collimated beams is redistributed by the use of mirrors to
; produce a single beam with a Gaussian-like distribution that
would be found in a single beam;
: Figure 39 is a schematic view of a section of the
electronic image projection system in which a~parabolic reflector
is used in conjunction with a conventional spherical reflector
and condenser lens to capture more light for use in projection;
Figure 40 is a schematic view of a Gallilaen telesccpe
which may be used to reduce a collimated beams diameter to a
smaller collimated beam;
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Figure 41 is a schematic view of an alternate
embodiment of a section of the elec~ronic image projection system
in which an elliptical mirror is used in conjunction with two
collimating lenses to capture and use otherwise lost light;
Figure 42 is a schematic view of a segment of the
electronic image projection system in which multiple condenser
paths are used to capture more light from a light source for use
in projection;
Figure 43 is a schematic view of a section of the
electronic image projection system in which separate light beams
are caused to become a single light beam by bringing the beams to
separate foci and using a mirror to redirect one of the beams so
that the two beams become colinear;
Figure 44 is a schematic view of an embodiment of a
section of the electronic image projection system in which
mirrors are used to rotate the polarization plane of a beam
coming from a McNeille prism to make the resulting beam parallel
with another beam from the McNeille prism;
Figure 45 is a schem~ic view of a section of the
electronic image projection system in which two collimated beams
are made contiguous by the use of a mirror;
:
Figure 46 is a schematic view of an alternate
embodiment of a section of the electronic image projection system
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in which a parabolic surface is used to cap~ure and collimate
light that misses an elliptical reflector in a light collection
system;
Figure 47 is a schematic view of the opera~ion of a
'`Fresnel mirror" used in an analysis of the operation of an
element of the electronic image projection system;
Figure k8 is a schematic view of the one embodiment of
a thin screen section of the electronic image projection system
utilizing a Fresnel mirror and a rear screen;
Figure 49 is a schematic view of a section of the
electronic image projection system utilizing two Fresnel mirrors
and a rear screen;
Figure 50 is a schematic view o~ an alternate
embodiment of a section of the electronic image projection system
in which a section of an elliptical reflector is used to capture
light that is not captured by a spherical reflector and a
condenser lens to bring the light to a focus at the same point at
which the aspheric condenser lens brings light to focus for use
in projection;
Figure 51 is a schematic view of an element of the
electronic image projection system referred to as a Fresnel
parabolic reflector;
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Figure 52 is a schematic view of an embodiment o~ the
electronic image projection system in whicl~ a full color light
valve is followed by a lens array to creat~l demagnified real
images on the light valve pixels in front of the lens array to
allow for the projection of a Eull color image in which the
individual red, green and blue pixels are merged; and
Figure 53 is a schematic view of four lenses in a lens
array placed in front of a full color light valve in an
embodiment of the electronic image projection system creating a
real image of 24 pixel color components after the lens array.
DETAILED DESCRIPTION OF THE PREFERRED
AND ALTERNATE EMBODIMENTS
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~ The present invention is directed to A HIGH EFFICIENCY
; LIGHT VALVE PROJECTION SYSTEM. This overall system was devised to
overcome the problems of video display systems and to meet the
objectives dilineated in the "Background of the Invention"
~ection.
The most promising technology available to circumvent
CRT problems is light valve te.chno~ogy. This technology uses an
external light source and a ~light valve/" which modulates the
light source, imposing image or data information on the light
beaml so that the beam can be projected onto a viewing surface.
Utili~ing the same strategy as in a CRT projection systeml a
light valve projection system can be constructed to produce a
brighter imaye than a CRT projection system. Such a system could
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also be produced to display black and white, monochromatic, or
full-color images.
Of all known light valve video display systems, the one
which presents the greatest potential for solving the problems
associated with CRTs is the LCD with a conductive matrix for
addressing, utilized in transmissive or reflectiv~ moder taking
advantage of the polarization/rotation, birefringence, or
scattering capabilities of the liquid crystals. Various changes
must however be made ~o current video display designs which use
electronic multiplexing to eliminate the current problems.
Although LCD technology is preferred at this time, most of
present the invention is applicable to light valve technology in
~eneral and is to be interpreted with that broader view in mind.
Figure 1 shows three light valves, one displaying red
110, one green 111 and one blue 1l~ picture data, each light
valve illumina~ed with light of the appropriate color (100, 101,
102). The red light from source 100 is collected by condenser
120, colllmated by collimating optics 13U and projected by
projection optics 140 which focuses a red image on screen 150.
Similarly, the green and blue images are projected and made to
converge on the screen, forming a full color image. The
disadvantage of this full color system, however, is that
adjustments must be made to the optics to converge the images
whenever the projector or screen is moved. The need for
convergence is eliminated in the present invention by the use of
dichroic mirrors and a single projection lens as schematically
shown in Figure 2. Red image information from light valve 200
reflects off front~surface mirror 201 to dichroic mirror 204
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which reflects red light but passes blue and green light. Blue
image information from LCD 220 reflects ofe front surface mirror
202 and then off dichroic mirror 203, which reflects blue light
but permits green light to pass and then passes through dichroic
mirror 204. A totally registered full-color image is thus
projected by projection optics 205 onto screen 206. Convergence
is always perfect, regardless of repositioning of the projector
or screen. The same invention can be applied to making a CRT
projector alleviate convergence problems.
If a picture is to be a mosaic of red, blue and green
pixels, each pixel must acquire a precise amount of current to
reproduce the brightness of each picture element's originally
broadcast brightness, as well as its color rendition. Although
present LCD TV displays using electronic multiplexing produce a
satisfactory small image, when such images are projected to a
large picture, the transmitted light never reaches zero, causing
low contrast. Additionally, with electronic multiplexing,
crosstalk and electronic "bleed through" to neighboring pixels
reduces resolution and color fidelity. Furthermore, light is
wasted and the picture appears dim with each pixel being turned
on for only part of a scanning field. The image cannot be
refreshed sufficiently and so flicker, as well as brightness
efficiency, is dependent on the persistence of the LCD, which is
not adjustable.
To solve the above problems, applicant's system can
include a light valve in which the data used to address each
pixel is stored, causing that pixel of the light ~alve to remain
activated the desired amount of time until new data is received,
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dictating a different value for that pixel. The data may be
stored by various means, but pre~erably in a capacitor which is
disGonnected from t~e charging circuit immecliately after it is
charged so as to remove the path for capacitive discharge.
Network analysis shows that when a given pixel is
addressed through its X and ~ conductors, one-third of its
addressing voltage will also appear across other pixels. Since
liquid crystal materials are fairly linear, this results in
partial activation of incorrect pixels with false data. This can
be alleviated by adding means to restrict the liquid crystal from
being activated by increasing the threshold voltage of the liquid
crystal, making its response to voltage non-linear, or by adding
a switching mechanism to disconnect the pixel from the circuit
until it is to be addressed. The pre~erred way to accomplish this
is by adding a "switch" ~o each pixel, creating what is Xnown as
an "active matrix" addressing system.
For instance, as shown in Figure 1~, an X-Y matrix of
pixels made of transparent conductive material, such as indium
tin oxide, is coated on the inner ~aces of a glass container
which is filled with liquid crystal material 1800. Each pixel in
a given horizontal row on one face is put in contact with a gas
such as helium in a reservoir 1810 which requires a threshold
voltage to ionize it and create a pat~ for current flow to the
pixel electrodes in the row. The corresponding pixel electrodes
1820 on the opposite glass plate are connected, for instance, to
video signal inpu~s along vertical lines. When a threshold
voltage is reached at which the gas for a given row on the first
glass plate becomes io'nized, the video signals applied in
r~j
vertical columns to the corresponding pixel electrodes on the
opposite glass plate charge those pixel electrocles, the liquid
crystal material between the plates acting as a dielectric to
form a capacitor. Immediately thereafter, removal of the
threshold voltage necessary to ionize the gas leaves the pixel
electrode capacitors a~ong the horizontal row charged the
required amount to maintain the polarization rotation through the
liquid crystal material along that row until new data is
available to replace the data already stored.
Alternately, an "active matrix" can be created by the
deposition of a thin film transistor next to each pixel and by
using a storage element at each pixel. Each transistor receives
a gate signal, turning it on and allowing the conduction of a
video signal voltage to the pixel associated ~ith the transistor
that is turned on. When the transistor is switched off ~by
removing the gate signal), the pixel electrodes with liquid
crystal material between them act as a capacitor storing the
charge and maintaining the state of activation of the liquid
crystal material until changed by a néw signal. An additional
capacitor can be added to maintain the charge if the liquid
crystal material has too much charge leakage.
This way, each pixel can be addressed, turned on (to
transmit or reflect light) and will remain on until data for the
next frame is presented. With this system, flicker can be
eliminated as in a progressively scanned picture. Each pixel
will be on for the entire length of a frame, immediately changing
to the appropriate level of transmissivity or reflectivity for
the pixel in the next frame. Each pixel will be on (the desired
-- 26 --
amount) all the time, allowing the highest throughput of light
from the external light source. State of the art methods of
deposition of semiconductor material can be utilized to
mass-produce such an active matrix system. Similarly, in
addition to active matrix addressing of light valves such as
LCDs, other methods, including scanned electron and scanned laser
beam addressing can be utilized in a light valve within a
projector.
The light valve can be used in conjunction with direct
projection optics. A general overview o~ the present invention
is depicted schematically in Figure 17 as comprising a light
source 1700 from which emerges a beam of light, collimating
optics 1710 which collimates ~he beam, including a spherical or
parabolic reflector 1720 which re~lects the beam, a condensing
lens 1730 which focuses the beam forward and collimating lenses
1740 which again collimate the beam. The light valve (or light
valves) 1750 is illumina~ed by ~he collimated beam, creating a
full-color optical image thereupon. Projection optics 1780 then
focuses this image onto a viewing surface 1790. To improve the
quality of the projected image as explain~d further herein,
subsystem 1760 is use~ to superpose pixels of color triads
forming full-color pixels with spaces between them and subsystem
1770, also explained herein, may be used to fill in the spaces
between pixels.
An actiYe ~natrix light valve made by the deposition of
thin film transistors also has signi~icant drawbacks. The chances
for defects such as shor~s and opens abound because oE the small
feature dimensions, the many layers of deposition and the high
r~ 1~ r~
~ 27 ~
density of conductive paths, transistors, and other features in
such light valves. A simple defect can cause an entire row of
pixels to be permanently on or permanently off and can render an
entire display useless since defects projected onto a screen
become very noticeable and unacceptable. The display yield
accordingly goes down dramatically as the resolution and~or size
o~ the display increases and the cost of an acceptable display
dramatically increases. Techniques such as redundant transistors
at each pixel, redundant conductive pathways and the use of a
laser to eliminate shorted transistors or pathways have been
devised to compensate for such defects. However, even with these
techni~ues, many defects are not correctable, keeping yield low
and costs high.
Applicant's technique of placing two otherwise
unacceptable displays back-to-back with appropriate display
drivers greatly increases the yield and reduces the cost of
producing active matrix displays. (See Figure 19.) Although
each display 1910 and 1920 separately is unacceptable because of
its relatively few uncorrectable clefects, l911 and 1921,
respectively, two rejected displays can be combined where the
defects in one do not correspond to the defects of the other.
The output faces or the input ~aces of the two displays must be
facing one another in a conventional LCD which has a twist angle
of 90 degrees (unless a hal~ wave pla~e is placed between them).
This way, vertically polarized light, for instance, entering the
input face of the first display is rotated 90 degrees by the
liquid crystal material when no current is applied, exiting as
horizonkally polarized light. It can now enter the output face
:~, ;
i;J ~ rt ~ r~
~ 2E~ ~
of the second display and be rotated by the liquid crystal
material to become vertically polarized and lexit the input face
of the second display. Consequently, no polarizer need be placed
between the displays.
Although transmission light valves are preferred in
applicant's system, reflection light valves could be used as
well. When utilizing liquid crystals as the active medium, use of
the twisted nematic effect is currently the most common method of
modulating the light to produce a satisfactory image. However,
use of the twisted nematic effect does not work well in a
reflection light valve. This is because polarized light which
enters the light valve (polarized, for example, in the vertical
direction) will rotate 90 degrees, hit the rear reflector and
rotate back 90 degrees upon passing a second time through the
twisted nematic cell. Thus the light will exit predominantly as
it went in with the initial polarization. When there is a signal
causing a voltage to be imposed on the liquid crystal material,
the nematic liquid crystals will become perpendicular to the cell
faces to some degree (depending on voltage), losing their twisted
orientation with respect to the light. Thus, light entering the
cell will pass through the cell and reflect back out unaltered.
Thus, whether or not a voltage is applied, light comes out of a
reflective cell unaffected by the twisted nematic effect.
A reflective liquid crystal cell can work utilizing
scattering or the birefringence of the liquid crystals~ A
reflective active matrix light valve can be constructed in many
ways. For instanceJ a single silicon chip can bP made into an
active matrix utilizing state of the ar~ silicon chip fabrication
- 2~ -
technology such as proposed by Hughes in th~ 1970s with
re~lective pixel electrodes on the silicon chip made of a
material such as aluminum. The opposite faces of the cell can be
made of glass with transparent indium tin oxide pixel electrodes.
Utilizing the scattering effect (see Figure 20), liyht
which enters the cell 2000 can hi~ a specularly reflecting back
surface and reflect out of the cell for focusing, for instance,
through an aperture 2010, as in a Schlieren type optical system.
When a voltage is applied in a gi~en area, light is scattered in
proportion to the voltage, preventing it from being focused
through the aperture on to the screen 2020. To make use of the
birefringence of liquid crystal molecules, a cell can be
constructed wherein the liquid crystal dipoles are oriented
either parallel or perpendicular to the faces of the cell or
somewhere in between, dependiny upon the applied voltage. In this
case, polarized light entering the call when the molecules are
oriented perpendicular to the faces of the cell, will emerge from
the cell after reflection from a rear reflective surface with its
polarization unchanged. However, with ~he proper cell thickness
when the dipole molecules are completely or partially parallel to
the cell's faces, the birefringence of the liquid crystal
molecules will cause the ~iquid crystal material to act like a
quarter wave plate of varying efficiency. Thus, after passage in
and out of such a reflective cell, polarized light will have its
plane of polarization rotated, to some degree (up to 90 degrees)
depending upon the voltage applied (double passage through the
cell making the cell operate as a half wave plate~.
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- 30 -
Heat and IR rad;ation genera~ed by the required
pro~ection bulb are sources of lowered resolution and contrast,
as well as color and gray-level distortion, and could damage the
light valve. Heat and IR, like the light, irradiates the light
valve in a Gaussian-like pattern, causing a "hot spot" in the
center of the light valve~ Even if the damage threshold is not
reached, image degradation could still occur because the light
valve expands, increasing the distance light must travel through
it. When the polarization rotation effect is usedl the rotation
of the plane of polarization of the light passing through khe
light valve could change, throwing off contrast, resolution and
color and gray-level rendition in a Gaussian-like pattern.
Several steps may be taken to deal with the detrimental
effects of heating of~the light valve~ First, all optics
including the light valve, should-be mounted with good contact to
large heat sinXs, as is done, for instance, with power
transistors. Optics in the system, including the light valve
windows, can be made of or coated with substances such as diamond
and sapphire, which have excellent optical qualities and
unusually high heat conductive capabilities. Additionally, all
optics can be coated with material of proper thickness, such as
is done -Eor dichroic reflectors to reflect the infrared (IR)
spectrum. IR reflecting mirrors and heat absorbing glass can also
be used in the optical path. Additionally, a fluid means such as
a liquid or gas in a container, consisting of a large body of
index~matched high-boiling-point fluid (liquid or gas), can be
used for further cooling. This fluid may be static or circulating
within a contained area and placed in contact with the components
- 31 -
to be cooledO Alternatively, instead of transmissive optics,
reflective optics such as optics made of met:al can be utilized
for further heat sinking and to suppress reflection at IR
wavelengths twith anti-reflection coat.ing for the IR).
Anti-re~lection (AR) coatings can, of course, be used
on all optical surfaces to reduce light leases due to reflection
at those surfaces. Such surfaces include surfaces o~ lenses, hot
mirrors, heat absorber¢, polarizers, prisms and light valves such
as LCDs, including the internal surfaces of the glass faces of
the light valves to reduce reflections at glass - ITQ boundaries
glass - liquid crystal boundaries, IT0 liquid crystal boundaries,
etc. ...
Cooling fans may be used to cool the light valve as
well as the other components of the system. Ducts and narrow
tubes can be used ~o provide cooling to specific spots. However,
a fan can pose a noise problem, particularly noticeable when the
audio volume of the system is at a low level, particularly in a
small room. To suppress the noise, an "air baffle" may be used
between the fan and the ou-tlet of, for example, a housiny for
various components of the invention. Figure 16 shows a sound
suppression system, comprising ~an 1600 resting on platform 1620.
Airflow blockers 1630 forces the air tQ traverse a c~rved path
with deflection prior to exiting the housing through outlet 1640.
The surfaces ~rom which the air and sound reflects are covered
with sound absorbing materials, greatly reducing the noise
entering the listening environment. Since some noise will still
be present at outlet 1640, a further measure may be taken for
noise reduction. This measure could comprise microphone 1650
32 -
which picks up the remaining noise and ~end~; it to an amplifier
which inverts the phase o~ the noise by 1~0 degrees. The inverted
noise is played bac~ through speaker 1~60. ~y properly adjusting
the volume and phasing of the amplifier, the remaining perceived
fan noise could be substantially reduced and made practically
inaudible.
Dependin~ upon the brightness of the light source
utilized and the physical and economic constraints of a given
system, some significant Gaussian-like heat pattern could remain
at the light valve and could change with tim~ as overall heat
builds up during operation. An electronic approach can therefore
be used in conjunction with the other recited remedies to
eliminate the problem. Modifying the electronic field in
opposition to temperature e~fects will substantially cancel the
distortion resultant from such effects, since the degree of
rotation of the plane of polarization of the light is not only
dependent on the thickness of the light valve that it passes
through, but also upon the amount of applied electric field. The
result will be uniform perforrnance across the light valveO Such a
system would use a bias voltage applied differently to different
pixels, distribu ed in a Gaussian-like pattern across the light
valve. A thermistor or other temperature-sensing device, placed
at the light valve, can monitor overall average light valve
temperature, ad~usting the Gaussian-like bias voltage
distribution as the temperature fluctuates, using an electronic
feedback circuit. For even more accurate temperature control, a
therrnistor-type device can be deposited next to each pixel in the
~ Q ~ 3 ~. ~
- 33 -
space between the pixels to independently control the
heat-compensating bias of each pixel.
An "active matrix" will allow for more brightness in
the projected image than a multiplexed array and less heat will
be generated for a given level o~ brightness. Addressing each
pixel separately in this way eliminates crosstalk. However, all
the conductive pathways, transistors, and capacitors create
substantial "dead space" between pixels. These dead spaces are
generally in the area of "overlap" where electric fields from
neighboring pixels could co-mingle and produce false data,
reducing contrast and distorting the color mix. Placing an
opaque, black, reflective or other covering over these areas
serves at least three purposes: it stops passage of improperly
modulated and unmodulated light from passage to the screen,
protects the semiconductors from damage due to irradiation from
the intense light and heat and reduces the chance of discharge of
pixels. The covered area may be a fraction of the size of a
pixel.
As an alternative to using three light valves in a
projection system to produce full-color, there are several ways
to construct a full-color projection system using a single light
valve. A simple, compac~ and inexpensive full-color video
projection system may be constructed using a single "full-color"
light valve. Previously full-color, direct-view video image
displays not using projection ~ad been constructed with a single
"full~color" LCD. When such images were enlarged by projection,
however, several problems explained herein become apparent.
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-- 34 --
In a standard CRT-based TV system, red, blue and green
pixel data are sent to adjacent red, blue and green phosphor
spots on the C~T face. Analogously, in a direct-view full-color
LCD TV system, red, blue and green pixel data are sent to
adjacent areas of the LCD. These areas are then covered by red,
blue and green filters to appropriately color the light passing
through those LCD pixel elements. Figure 15a depicts a simple
arrangement of color pixels in which pixels of a given color are
located above one another creating vertical color stripes. Three
horizontally adjacent pixel areas make up a pixel triad which
represents a single, full-color pixel from the actual image.
Figure 15b depicts an alternate arrangement of pixels in which
the three pixels of a color triad are arranged to form a
triangle. In the preferred single light valve embodiment, such a
full-color light valve can be placed at position 1750 in Figure
17 to produce a full-color image.
In one embodiment, a single light valve 2100 may be
divided into three sections. The red image for instance, can be
made to electronically address the left 1/3 of the light valve
panel 2110, while the electronic data corresponding to the green
component of the image addresses the center 1!3 of the light
valve 2120, and the electronic data representing the blue
component of the image can address the right 1/3 of the light
valve 2130. (See Figure 21.) Light from th~se three images can
then be overlapped and projected through projection optics to the
screen. Since the projec~ion lens 2220 has a yiven focal length,
it must be placed approximately its focal length away from each
color component image. (It must be optically equidistant from
- 35 -
each image.) This can be accomplished in a number of ways. One
or more lensesdi can ~e posi~ioned just a~ter the light valve
2100 to adjust the focus of one or more of t~le three images
through the same projection lens even though the three images may
traverse different light paths. (See Figure 22.) For instance,
correction lens 2201 can correct for the distance difference in
the straight-through path as compared to the reflected paths.
Alternatively, path lengths can be matched by the appropriate use
of mirrors, as for example, depicted in Figure 23. As mentioned
earlier, reflection optics, including a reflection light valve,
can be used to produce the full-color video image. An example of
this type of setup with a single light valve is shown in Figure
24.
In this setup, light from light source 2400 is
collected and collimated ~y condenser optics 2410. After passage
through a quarter wave plate 2420, the light enters a MacNeill
beamsplitter cube 2000. S-polarized light reflects from the
internal face within the cube to front-surace mirror 2430. This
reflects the S-polarized light back through the cube, through the
quarter wave plate, back through the condenser optics and light
bulb, and back through the quarter wave plate. At this point, the
S-polarized light, having passed twice through the quarter wave
plate is rotated 90 degrees ~o become P-polarized light. It can
now pass through the cube, resulting in utilization of a majority
o the source light, even though plane polarization is performed.
Dichroic mirror setup 2440 separates the light into
red, green and blue beams which re~lect from path equaliæation
mirrors 2450 and illuminate three sections of light valve 2100,
: ~ .
- 36 -
which is addressed w~th three c~lor-component images. The light
reflects from the light valve and retraces its path to the
MacNeill prism. Light which should appear in the projected image
is converted by the light valve from P-polarized light to
S-polarized light. It therefore reflects from the inner surface
of the cube and exits through the projection lens 2220 to the
screen. Non-image light remains P-polarized and passes through
the cube and is reinjected into the system, making the projected
image somewhat brighter. A birefringence transmission light valv~
with a mirror behind it could also be used in this arrangement.
In convantionally-made LCDS, color filters are
deposited within the cavity of the ~CD. This must be done because
any difference in physical location o~ the actual LCD pixels and
the color filters coloring them will produce a parallax
difference which will be perceived as misregistered or incorrect
colors when viewing a direct-view LCD from any angle aside from
head-on.
Since the space between the glass plates forming an LC~
is typically less than 10 microns, the deposition of color
filters requires a high degree of thickness control as well as
color transmissivity and overall transmissivity uniformity in
such thin coating thicknesses. In addition, high efficiency
filtering must be used to eliminate the possibility of
contaminating particulate mat~er in ~he coating chemicals which
may be on the order of or larger than the space in between the
glass plates.
Projec~ion, however, presents the unique situation in
which a light valve can be illuminated with substantially
- 37 -
collimated light and viewed on a screen from all angles even
though light passes through the light valve substantially in a
parallel direction eliminating any possible parallax error. ~his
means that the making of ~ull-color light valves specifically for
their use in projection will allow the use of external color
filters whose thicknesses do not have to be as precisely
controlled. Also, being placed outside of the light valve cavity
reduces the risk of contamination as well as the complexity and
thus the cost of production of light valves for that purpose.
Using a "full-color" light valve can create another
problem which, although not very noticeable on small displays,
creates major problems in a large image. This problem results in
a poor contrast ratio and poor color fidelity. To understand and
correct this problem the workings of a full-color LCD display
must be analyzed.
The following discussion explains the nature of the
problem. The transmitted light intensity (TI) from a twisted
nematic liquid crystal dpvice, under no applied voltage, with a
crystal thickness (d) for any given wavelength (A), is dependent
on the refraction anisotropy (~n) and the liquid crystal twist
angle (e). TI can equal zero for only a few unique simultaneous
combinations of values for these parameters. This means that
except for very specific combinations of wavelength (A) and
thickness (d) for any given crystal, zero transmitted intensity
or true "black" will not occur. Thus, if the anisotropy, twist
angle, and crystal ~hickness are fixed, as they are in a
conventional light valve such as an LCD (consisting of liquid
crystal between two flat plates), only one color can go to black
'. : .
f~ `.J~',`', i '
- 38 -
at a time. If a voltage is applied, changing the light rotation,
then a different color can go to blacXO This non-linearity
eliminates the possibility of true black in all colors
simultaneously (and thus limits possi~le contrast) and since
perceived color is produced by addition, this eliminates true
color fidelity.
To further illustrate this problem, the dashed curve of
Figure 10 shows the transmitted intensity over the visible
spectrum of a standard full-color LCD with a given thickness.
Figure 11, plot A shows the non-linear ~ransmittance variations
for the three wavelengths used in a full-color LCD of uniform
thickness plotted against the voltage. When red transmission, for
instance, is at a minimum, blue transmission is over 10 percent
and green transmission is abou~ 5 percent. Ha~ing no true black
results in a low contrast ratio which is one of the main problems
with today's LCDS. To solve this problem, one of the Yariables
given above must be modified ~o produce the desired
transmissivity for a given signal voltage. This can be done by
electronically biasing the pixels, which are addressed with data
corresponding to two of the color components (such as red and
green). This would cause the net transmissivity through the red
and green pixels to equal the transmissivity of the blu~ pixels,
when no signal voltages are present for any pixels. With proper
selection of d, all colors will be at a minimum.
Alternatively, the crystal thickness (the spa~e between
the plates encasing the liquid crystal~ can he selected under
each color filter such that at exactly zero (signal) vol-ts, the
proper rotation is imposed on the polarized light for the
~ 39 ~
specific waveleng~h transmitted by that color filter. By doing
this for each of the three sets of color filtlers, the minimum
amount of light for each color will be tran~mitted with no
voltage applied. This, again, will provide a blacker black and
thus a high contrast. This result i9 accomplished, for instance,
if stepped deposition or etching of one plate is done to produce
steps as illustrated in Figure 12.
By using a light valve with such a "stepped thickness"
cavity, the crystal thickness-wavelen~th combination will allow
true black for all three colors simultaneously and a linear
relationship between applied voltage and transmitted intensity
for all colors simultaneously. This is demonstrated by Figure 10
(solid line) where transmission is nearly zero for all colors
simultaneously with no voltage applied and in Figure 11, plot B,
where the transmission for all colors varies with voltage
simultaneously.
In applicant's demonstration model, using a "stepped
thickness" cavity results in a contrast ratio as high as 100:1
and color fid~lity approaching that of a CRT. This high color
fidelity can be seen in the CIE diagram of Figure 13 in which the
dashed line represents the chromaticity of conventional
multi-color LC displays, the dotted line represents the ~ ~
chromaticity of an LC color display with varying crystal
thicknesses and the solid line represen~s the chromaticity of a
conventional CRT.
The small, closely packed red, blue and green spots of
light that make up a direct-view image create the illusion of
color in a scene as they are supposed to appearO However, when
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this image is magnified by projec~ion, each adjacent red, blue,
and green pixel no longer merges to produce properly colored
areas. Instead, they appear as disjoin~ed red, blue, and green
areas, detracting from the appearance of a naturally colored
image. Furthermore, dead spaces between adjacent pixel areas in
the light valve are magnified as well, further creating a
disjointed, disruptive, unnatural looking image. The appearance
of disjointed red, blue and green spots instead of actual colors
in a full-color light valve can be eliminated by various methods.
The preferred method of eliminating them in the
projected image, utilizing a single, full color light valve,
entails the use of lens arrays. Figure 52 shows a full-color
light valve 5200 with red, green and ~lue pixels arranged in
horizontal rows 5210. The rows are preferably arranged so that
each succeeding row is o~fse~ by 1-1~2 pixels from the previous
row, although many other arrangements are possible~ A lens array
5230 is placed in front of the ligh~ valve and behind the
projection lens 5240. The lens array could comprise spherical
lenses, although cylindrical or other types of lenses could be
used, each of which is 1/2 the width of a pixel on the light
valve. The curvature o~ each lenslet and the distance between
the lens array and the ligh~ valve can be chosen so that each
lenslet 5250 creates a demagnified real image of a portion of the
light valve, floating in space, slightly in front of the lens
array, between the lens array and the projection lens. Other
arrangements are, of course, possible.
As shown in Figure 52 (inset) 5250, the real image
produced by a single lensle~ contains data from 6 pixels. These
- 41 -
6 pixel images come from two horizontal rows with 3 pixels on top
and 3 pixels below. Other lens sizes and curvatures could be
used and each real image could contain a different number of
pixel images while still producing essentially the same result.
The addition of the lens array separates the planes of best focus
of the red, green, and blue pixel data and the image information
displayed on the light valve. The projection lens focuses
through the lens array onto the plane of the best image focus,
near the plane of the light valve. Since 4 lenslets 5300 tsee
Figure 53) occupy the same amount of space as a single pixel 5310
and each lenslet produces an image of 6 pixels in this case, the
image focused on the screen of a single pixel will be the
superposition of 24 red, green and blue dots. These dots,
however, are not 24 different pixels, but contain the data from
only 6 pixels on the light val~e (which may correspond to only
two pixels in the actual scene). The 24 dots that superimpose to
create the image of the next pixel contain some of the same
information as the previous 24 dots or some portion of the same
dots and some new ones. Consequently, each adjacent pixel image
is a weighted average of approxima~ely 2 triads, causing onl~ a
slight reduction in resolution. However~ since each newly
created pixel image is an out-of-focus superposition of 24 dots,
its colors combine to produce a net uniform color~ Thus, a
full-color image is still displayed with correct colors in the
correct locations to a sufficient degree of accuracy so that the
image appears essentially unchanged from that projected without
the lens array, excepk that individual red, green and blue dots
are no longer visible. This ~lending process also eliminates the
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- 42 -
appearance of any spaces between pixels. This combined function
eliminates the appearance of pixels altogether. Use of an
anamorphic lenslet profile, or the optical equivalent formed by
crossed lantinicular lenses is preferred so that the "blur" is
only a mix o~ one red, one green and one blue pixel.
When constructing a rear-screen display unit, an
additional flexibility is provided since the scre n is built into
the unit. This allows ~or the addition of optics just before the
screen. If the image projected onto a rear screen has individual
red, green and blue pixels, a lens array as described, which has
for instance twice as many lenses as there are pixels in each
orthogonal direction, can be placed near the focused image that
is to hit the screen. As explained above, each lens element can
create a demagnified image o~ one or more triads in space. A
second lens array with the same number of lenslets as there are
pixels can then focus a blendPd image of the nkk pixel onto a
nearby screen surface (being ~ocused on a plane near the original
image plane, not on the plane o~ real images of the pixels). As
before, the individual color pixels will be blended into full--
color pixels.
Alternatively, a single lens array can be used if it is
made in a special way. The single array should have the same
number of lenslets as there are individual colored pixels. The
array is placed after the image that is to be focused on the
screen. Two of every three lenses in the array also have a
built-in wedge so that the images of a triad will all be focused
onto a nearby screen overlapped, creating full-color pixels. The
wedges can, of course, be separate from the lenslets. These last
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two techniques can also be applied to a CRT or any imaging device
which normally displays individual red, green and blue pixels.
Another method of creating full color pixels entails
the use of narrow angled prisms or wedges. As shown in Figure
28, these two wedges can be placed wi~h a cl~ear-space between
them at any place in the system as long as they are not placed
too close to the light va~ve. Since ~he light distribution is
usually Gaussian, more light is concentrated in the center. To
make all three images equal in brightness, the clear center
section should thereEore be smaller than each wedge section.
Alternatively, to produce a more uniform image, the wedges can be
divided into thin sections and interdispersed with clear spaces.
If the wedges are placed somewhere between the light source and
the light valve, they will create the equivalent of three very
close light sources, illuminating the light valve from slightly
different angles. This will create three slightly displaced
images on the screen.
The wedges can also be positioned somewhere after the
light valve, such as after the pro~ection lens. Such positioning
will create three images on the screen, each slightly offset from
the other.
If the wedge angles are properly chosen based on simple
geometrical considerations, the images will be offset by the
width of one pixel. The red pixels of one image will then be
superimposed on the neighboring green pixels of the second image,
which will be superimposed on the neighboring blue pixels of the
third image, creating full-color pixels in which individual red,
green and blue pixels will not be visible. This technique will
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work well in most ar~as since most groups of three pixels in animage will most likely have the same color value. The only place
this technique will create a slight problem is at the boundary
between two very different areas. At the boundary, when there is
an abrupt change in color andf~r brightness, two of the pixels
that are overlapped on neighboring pixels will be overlapped on
neighbors that should have different values and there~ore
noticeable distortion will become apparent, creating a more
jagged looking edge at the boundaries of the viewed image. The
larger the areas of constant color within a scene, the less
noticeable this will be.
Another method to eliminate the appearance of the
individual colored pixels is by the use of a dichroic mirror
system as depicted in Figure ga. Assuming the pixel arrangement
of Figure 15a, individual red, blue and green pixels can be made
to overlap by the following arrangement: collimated light 901
passes through the full-color light valve 902 and hits dichroic
mirror 903 which reflects only the blue image. The remaining red
and green images pass through dichroic mirror 903, hitting
dichroic mirror surface 904 which reflects only the red image,
allowing the green image to pass through. The blue image
reflects off front surface mirrors 910 and 911 and then off
dichroic mirror surface 905 which reflects only blue light. Here
the blue image rejoins the green image~ By adjusting front
surface mirrors 910 and 911 the blue pixels can be made to
overlap the green pixels. The red image reflects off front
surface mirrors 920 and 921 and then off dichroic mirror 906
which only reflects red light. At 920 and 921, the red pixels
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can be made to overlap the already joined blue and green pixels.
The path lengths could be matche~ using a compensating lens as
described herein or additional mirrors as also described herein.
At this juncture, we have a full-color image with large spaces
between pixels as illustrated in Figure 4.
If individual colored pixels are arranged on the light
valve as shown by Fig~re 15b, in which a color triad orms a
triangle, bringing the red and blue pixels together, as
described, will not allow them to superimpose on top of the
proper green pixels since the proper green pixels are vertically
displaced from their corresponding red and blue pixels.
Consequently, this type of pixel arrangement could use an
additional dichroic mirror path similar to the paths used by the
red and blue light. This is depicted mors clearly in Figure gb,
which is a si~e view o~ the system in Figure 9a modified to
include an additional light path. Collimated light 901 passes
through full-color light valve 902 as before. ~owever, the
distance between light valve 902 and dichroic mirror 903 is
increased to allow for the insertion of dichroic mirror 950 which
reflects green light and transmits red and blue light. As
before, 903 reflects blue light and transmits red light. Mirror
surfaces 904 and 905 are ~ront surface mirrors. Mirror 906
reflects red liyht and transmits blue ligh~. As before, mirrors
910, 911, 920 and 921 are front surf~ce mirrors. In addition,
mirrors 960 and 970 are also front surface mirrors. Mirror 980
is a dichroic mirror which reflects green light and transmits red
and blue light. By this modi~ied arrangement, proper separation
of mirror 910 from mirror 911 and separation of mirror 920 from
-- 46 --
mirror 921 will still cause the overlap o~ the red and blue
pixels. ~dditionally, proper separation o~ mirrors 960 and 970
will cause the proper green pixels to overlap the already joined
red-blue pixel pair. This overhead mirror a;rrangement may also
be used with the color light valve whose pixel arrangement is
depicted in Figure 15a with the spacing between mirrors 960 and
970 adjusted to prevent vertical displacement of the green pixels
since they are already in line with the red and blue pixels. The
separate mirror path for the green light makes the distance
traversed by each color equal, which is important because the
light, although collimated, still expands with distance traveled
and the projection lens must ~ocus all three images
simultaneously. Now the image can pass through subsystem 930
which can be used to fill the spaces between pixels (as described
elsewhere herein~ for final projection by projection optics 940.
Alternatively, in Figure 9a, mirrors 910, 911 and 920,
921 could be tilted up or down to cause the red and blue pixels
to superimpose on the proper gxeen pixel.
In another embodiment for the elimination of the
appearance of red, green and blue pixels, depicted in Figure 2a,
four special mirrors are used. Each mirrox has clear spaces and
mirrored areas. Two of the mirrors 2910 and 2920 have ordinary
mirrored areas coated, for instance, with silver or aluminum,
which totally reflects light of any color. One of the special
mirror's 2930 reflective coatings is dichroic and reflects blue
light and transmits red and green light. The other special
mirror's 29~0 reflective dichroic coa~ing reflects red light. As
seen in Figure 29, the mirrored areas o~ the four mirrors are
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positioned out of phase with each other. On each mirror, the
clear space between every two mirrored spaces is equal to twice
the width of the mirrored space.
Light from red pixel #1 2g50 passes through the clear
area of the first mirror and reflects off the mirrored area of
the second mirror downward towards the red reflective area of the
first mirror. The red light is then reflected upward, passing
through the clear area of the second mirror and then passes
through the clear areas of the third and fourth mirrors.
The green light coming from green pixel ~2 2960 passes
through the dichroic mirrored area of mirror #1, passes through
the clear area of mirror #2, passes ~hrough the dichroic mirrored
area of mirror #3 and passes through the clear area of mirror #4
and is thus superimposed on the light that came from the red
pixel~
Light from the blue pixel #3 ~970 passes through the
clear spaces in mirrors #1, #2 and #3 and reflects off the
mirrored area in mirror #4 down to the dichroic mirrored area of
mirror #3. This dichrvic mirrored area reflects the blue light
upwards, superimposing it on the light from the red and green
pixels. Thus, we have created ~ull-color pixels with spaces
hetween them.
In an alternate embodiment (see Figure 30), two special
mirrors are used. Each mirror has properly mounted ~5 degree
dichroic mirror sections. The first mirror 3010 reflects red
light and transmits blue and green, while the second mirror 3020
reflects blue light and transmits red and green. In the
arrangement, red light from red pixel #1 reflects off two red
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dichroic surfaces upwardly through ~he second blue dichroic
mirror 3020. Green light from green pixel ~ goes straight
upwards, passing through both the red and b~ue clichroic mirrors.
Blue light from blue pixel #3 passes through the clear space in
the first mirror and reflects of~ two blue clichroic mirror
surfaces in th second mirror, sending it in an upward direckion.
As before, this arrangement superimposes the light from the red,
green and blue pixels into a single beam, creating full-color
pixels separated by spaces.
Three special "mirrors" (see Figure 31) are used in
another method of creating full-color pixels. Each "mirror"
consists of properly placed 45 degree dichroic mirror sections.
The first mirror 3110 is a regular mirror, reflecting red light
but transmitting green and blue light. The second mirror 3120 is
a green dichroic mirror, reflecting green light but transmitting
red light and the third dichroic mirror 3130 is a blue dichroic
mirror reflecting blue light but transmitting red and green
light. In this arrangement, red light from red pixel #1 reflects
off the two regular mirrors 3110 into the upward direction
passing through the green and blue dichroic mirrors. Green light
from green pixel #2 similarly makes two reflections from green
dichroic mirrors 3120 reflecting it in an upwards direction and
superimposing on the light from the red pixel. Light from the
blue pixel #3 also reflects off ~wo blue dichroic mirrors 3130,
upwardly superimposing it on the light from the red and green
pixels. Again, full-color pixels are creaked separated ~y spaces.
Various other arrangements can be devised, also
utilizing dichroic mirrors, to superimpose red, green and blue
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pixels. As another example, the image, emerging from the
projection lens can reflect from two "sandwich" surfaces are
separated by a preciser spacing. ~s an example, the ~irst mirror
sandwich can superimpose the red pixels onto the green pixels by
the action of a red dichroic mirror (see Figure 60). The second
mirror sandwich can ~hen superimpose the blue pixels on the
resulting red and green pixels to ~orm full color pixels. Large
spaces (2 pixels wide~ will ~e formed between resulting full
color pixels which can be eliminated as explained elsewhere
herein.
Visibility of red, green and blue pixels could also be
eliminated by using a single, relatively low resolution light
valve with a "time-share scanning" technique. By dividing time
into small segments, each with different data presented to the
screen, the eye will integrate the data over time, seeing the sum
of the data, as if each different data presen~ation were being
projected simultaneously onto the screen. However, time-sharing
of visually-presented data must be done properly or else
artifacts, such as flic~er and reduced image brightness, will
become apparent to the viewer.
As an example, if the light valve is addressed with red
information only, and only red light is projected through the
light valve during that time, followed by the green and blue
images similarly projected, the viewer will perceive a full-coior
image. However, since a standard video image provides 30 frames
per second and since flicker is almost visible to many viewers at
this frsquency, dividing time into se~ments as described, would
produce 10 images per second for each color, creating a
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noticeable color flicker. In addition, if a large area is only
one color (as o~ten happens in real life), then the entire area
will be black for two out of every three time segments,
decreasing perceived brightness to one-third and creating a
strongly pronounced ~licker of the entire area. This problem was
studied in great detail in the early days of color television,
when CBS attempted to develop their sequential color system,
using a spinning color wheel in front of a monochrome CRT.
Another problem encountered when using this method is a marked
decrease in image brightness, due to another factor. Since,
during any given frame, only one color of light is projected on
the scr~en, two-thirds of the light emitted by the source is
therefore eliminated from every frame, and thus from the viewed
image.
To eliminate these problems, a system can be set up in
the following way. Firstly, the light valve is addressed as a
full-color liyht valve, with pixels arranged in an alternating
fashion in which every even row contains the pixels in the order
o~ one red, one green and one ~lue, repeating throughout the
line. Every odd line may contain pixels in the same arrangement,
but may be displaced some amount such as one and one-half pixels,
with respect to every even line. This creates a more random
appearing pixel pattern . For a single segment of time ( such as
1/30 of a second~ the light valve is addressed in this fashion,
and light of the proper colors is sent to each pixel through a
mosaic of color filters (as previously described) or by the
creation of a matching mosaic of colored light beams, created for
instance by multiple dichroic mirrors as described elsewhere,
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herein. For the next segment of time, the light valv~ is
addressed with all color data addressing shifted by one pixel in
a given direction. Simultaneously, ~he distribution of colored
light beams addressing the light valve is shifted to correspond
to the new positions of the colored data on t:he light valve by
either moving the color filters or by appropriately vibrating
mirrors in the dichroic-colored-rayproduction system.
In this embodiment of time-share scanning, pixel #l of
the light valve is addressed with red data corresponding to pixel
# 1 of the image, ~or the first sec~ment of time. This produces a
red data image in pixel #l on the screen during that segment of
time. In the next segment of time, the color data locations, as
well as the arrangement of the colored beams, are shifted so that
pixel #1 on the light valve is now displaying the green data from
pixel #1 in the original image. This green data from pixel ~1 in
the original image is now projec~ed onto the same location on the
screen t~at displayed ~he red data for pixel #l in the previous
time segment. Similarly, the blue data is projected to pixel #1
on the screen in the next time segment, creating the illusion of
full-color image at every pixel location within 1/10 of a
second. Any large area, which is one color only, now has
one-third of its pixels on with that single color during every
time per`iod (such as 1/30 of a second). Thus, the area appears
that color all the time instead of being black two-thirds o~ the
time, as explained above.
With this arrangement, at least one of every three
pixels sends light to the screen all the time, assuming there is
any light in that area in the image. Utilizing the dichroic
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mirror method (described elsewhere herein) of dividing the light
into multiple-colored beams in the proper arrangements eliminates
the problem of wasting two-thirds of the bul~'s light during any
given time segment since all of the light is used in every time
segment.
As a preferred embodiment of "time share scanning," the
light valve can be addressed so that pixel #l is always addressed
with red data, pixel #2 is always addressed with green data,
pixel #3 is always addressed with blue data and so on. The
illumination is fixed so that pixel #l is always illuminated by a
red beam, pixel #2 i5 always illuminated by a green beam, pixel
#3 is always illuminated by a blue beam, and so on. However, in
this embodiment, pixel #1 of the light valve is addressed with
red data from pixel #l o~ the image in the first time segment and
is then addressed with red data from pixel #2 of the image in the
second time segment and is then addressed with red data from
pixel #3 of the image in the third time segment and then back to
red data from pixel #1 of the image, and so on, for all other
pixels. The light exiting from the light valve before going to
the screen reflects off a mirror. This mirror is oscillated in
synchronization with the time segments by an electronically-
controlled electromagnetic coil or piezo electric crystal stack
on one edge of the mirror. The other edge of the mirror is
hinged. Alternatively, reflection from coun~er-rotating mirrors
is used to stabilize the projected image during a given time
segment but to shift it for the next time segment. The mirror
may also be oscillated with a fluid- or gelfilled piezo-electric
prism (see Figure 33~ with two faces which are flat and rigid and
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hinged along one edge. The other thrse sides of the prism are
collapsible. A stack o~ piezo electric crystals 3300 inside the
prism causes it to change its angle in an oscillating fashion in
synchronization with an oscillating current.
The ne' result in either event will be to shift the
image on the screen by one pixel for the second time segment and
by another pixel for the third time segment. Each screen pixel
will therefore contain red, green and blue information over time,
giving the viewer a full-color image with no discernible color
pixels anywhere, using a single, low resolution light valve. It
should be obvious that other arrangements can be used to
accomplish the same ends. This technique creates the perception
of three times the resolution of the light valve, or the
equivalent of three light valves.
Dead spaces between pixels will be visible whether a
"full-color" light valve or multiple "mono-color" light valves
are used, especially with the use of an "active matrix." Although
such an image may be acceptable in some cases, a better solution
is to have all pixels superimposed exactly in triads (red, green
and blue together forming "full-color pixelsl') with spacing
between such pixel triads eliminated, creating a "continuous
image." In Figure 4, each pixel 401 is â superposition of a
corresponding red, blue and green pixel. 402 represents spaces
which need to be filled. The following are methods to eliminate
these dead spaces between pixels in the projected image.
~ he preferred method of elimination of spaces between
full-color pixels (such as are created by the superimposition of
the images of three light valves) uses lenses. A lens array 801
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(as shown in Figures 8a and 8b) constructed with the same number
of lenses as there are "full-color" pixels (e.q., the number of
color "triads" on the ligh~ values arranged with the center of
each lens over each pixel 802) could be used to maynify each
pixel as depicted in Figures 8a and 8b. Then optionally either a
collimating lens array 803 as depicted in Figure 8a or a large
collimating optic 80~ as depicted in Figure 8b could be used to
recollimate the now enlarged and contiguous pixels for projection
by suitable projection optics.
I~ the spacing between pixels along the vertical is
different than along the horizontal dimension, the pixels can be
intentionally underfilled with light, forming a symmetrical dot
(as explained below) or anamorphic lenses or equivalent could be
used to fill the spaces properly. Although fabrication of small
lens arrays is within the state of the artj it is simpler and
less expensive to use more readily available lenticular lenses.
These cylindrical lens arrays can be overlapped with their axes
perpendicular to one another to accomplish the same goal. The
separation of lens function for each orthogonal dimension
eliminates the need for anamorphic lenses which are difficult to
produce accurately and consistently in such small sizes.
It is important to note that eliminating the space
between pixels utilizing lenses after the pixels (and before the
projection lens) can be done with several different approaches.
The lenslet curvature and spacing from the light valve can be
selected to produce a real or virtual magnified image of the
pixel. These real or virtual images can be magnified just the
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right amount so that they become contiguous at a plane in space.
This plane is then imaged onto the screen by the projection lens.
In actual practice, many virtual and real images of the
pixels exist at various locations in spac~s of differeht sizes.
The projection lens can be accordingly adjusted slightly back or
forth to select the pixel image size which just eliminates the
inter-pixel spaces without overlap.
I~ an arrangement is chosen (as described below) in
which the source is imaged into each pixel hole, then the
distribution of light within a pixel may no~ be uniform. If it
isn't, a repetitive structure will be apparent on the screen,
making pixels visible, even if there actually are no spaces
between pixels. In that even~l ~he projection lens should not
focus an image of the pixel plane or a magnified real or virtual
image of its pixels onto the screen. Instead the projection lens
can focus an image of the lens array onto the screen. Each
lenslet will be uniformly illuminated even if the light
distribution within a pixel isn't uni~orm.
If the lens arrays aren't constructed well enough so
that spacing between lenslets approaches zero, a pixel structure
would again be apparent. To eliminate that problem, a second lens
array could be used to generate a magnified real or virtual image
of the lenslets of the first array. Thus the "pixels't would
appear uniform and be contiguous.
With a rear projection system built into a cabinet in
which the relationship between the projector and the screen will
never be altered, it is possible to build in a system to
eliminate the space between the pixels right before the screen. A
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lens array with the same axrangement as the pixels projected from
the projector, placed just behind the screen, will expand the
image of each pixel just enough to fill the space between the
pixels. This lens array can be built into the screen making it a
rigid component of the screen.
The following is a me~hod for inexpensively making the
lens arrays necessary for the elimination of the spaces between
pixels as well as for other aims which involves creating a master
for making lens arrays. The master can be made by taking a
semi~soft material such as copper or wax and scoring it with
parallel lines with a tool which has a circular curvature at its
end. A spherical lens array master can be made by forming a tool
with a surface matching the lens surface desired and repeatedly
pressing it into such a soft material in a l'step-and-repeat"
fashion. This master can then be made into a hard metal master.
If the master is made in copper, the copper can be immersed in an
~electroplating bath, such as nickel sulfamate. If the master is
made in a non-conductive material such as waxl it can first be
coated with a thin metallic layer o~ electroless nickel or by
spraying with a stannous-chloride silver solution. Once
metalliæed in this ~ashion, it can then be placed into the
electroplati~g bath. The nickel master can then be placed on an
embossing machine and used to emboss replicas into thermoplastic
materials, such as mylar and plexiglass. Such a master can also
be used as a mold for injuction or compression molding.
Another method of producing the master is to use a
computer to make a plot in which the height of the lens is
represented as a density. This plot, turned into a transparency,
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can be photo-reduced and replicated by step-and--repeat procedures
to produce a mask with a density pattern which matches the lens
array layout. The mas~ can then ~e imaged with ultraviolet light
onto a photoresist plate~ The differing densities on the mask
will alter the amount that the photoresis~ is exposed and after
development, will alter the amount of photoresist that will be
washed away at each location. This will create a photoresist
master in the shape of the lens array. This photoresist master
can then by metallized and used for replication.
An alternative method to produce such lens arrays for a
projection system is to use lens arrays produced holographically.
Such holographic lenses are easier to produce than conventional
lens machining at such small dimensions, especially if extremely
small F numbers are required. State-of-the-art methods can be
used to create the necessary interference patterns.
As was done earlier to eliminate the appearance of red,
green and blue pixels, a wedge or wedges may be used to create
offset images on the screen, both vertically and horizontally to
eliminate the spaces between pixels. The wedge or wedge segments
may be conveniently placed at the projec~ion lens to fill each
space in the image with a duplicate of the adjacent image data~
creating a focused, de-pixelated image. This method is an
alternate preferred method of eliminating spaces between pixels
in the image.
Since the spaces between pixels are all horizontal and
vertical lin~s o~ a fixed wid~h, spatial filtering may be used to
eliminate the spaces. The classic method of spatial filtering is
demonstrated in Figure 3~. In the input, image A is acted upon by
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lens 3310, creating a Fourier transform in plane B. Another lens
3320, placed a focal length after plane B, creates a Fourier
trans~orm of that trans~orm which is the original image in plane
C. I~ a particular optical filter is placed in plane B, various
components of the final image will be eliminated due to the
blockage in plane B of the Fourier components. The Fourier
components are arranged in a polar coordinate fashion in plane B
with the highest spatial frequencies which correspond to the
smallest features in the original image located throughout and
towards the outside of the Fourier plane. The low spatial
frequencies in the image are represented in the central area of
the Fourier transform in plane B. Periodic input patterns are
represented as localized concentrations of intensities at that
frequency in the Fourier plane. Since the thin lines
representing the spaces between pixels are high in spatial
frequency, they will form large features, located mostly away
from the center of the Fourier transform. Therefore, if an
appropriate filter is placed in plane B, letting through the
lower spatial frequencies, the retransformed image in plane C
will have greatly diminished, or, if the filter is selected
properly, eliminated higher spatial frequencies (corresponding to
the lines between pixals).
Since all pixels have the same spatial fre~ency in a
given direction, which is different from the higher spatial
frequency of the lines be~ween them, those lines can be separated
out and suppressed. The image plane A is analogous to the light
valve plane in the projector and ~he lens performing the Fourier
transform is analogous to the projection lens. Somewhere in
- 59 -
front of the projection lens will kherefore be an approximation
of a Fourier transform of the image on the light valve. Even
though no second lens is used to re-transform the image after a
certain distance~ a re-trans~orm will occur anyway (at the
focused image on the screen), making a final lens unnecessary.
All that is necessary in actual operation is therefore the
placement of an appropriate ~ilter somewhere a~ter the projection
lens. Since the spatial freguency of the line pattern is known,
state-of-the-art methods can be used to form a Fourier filtex to
block out the desired spatial frequency components. The larger
the difference between the width of the pixels and the width of
the spaces between pixels, the more efficient this spatial
filtering process will be. As the widths approach each other, the
process will become less effective.
Alternatively, if a lens is placed between the light
valve and the projection lens, the light can be made to come to a
small focus within the projector. ~ pinhole can be placed at the
focus, allvwing most of the light to pass through. Passage of
light through a re-transforming lens also placed before the
projection lens will create a focused image in space minus the
high spatial frequencies of the image from the light valve plane.
If the projection lens is then made to ~ocus on that image, most
of the light can be projected onto the 'screen without lines
between the pixels.
Another method of obtaining a brighter image is to use
a holographic phase filter beyond the projection lens,
constructed in ways that are known in the state-of-the art either
with varying thickness material or a hologram properly laid out.
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This will still accomplish spatial ~iltering but will allow more
of the light to pass through to the screen.
An alternate method of filling the ~paces between
pixels is by the use of mirrors, To make a m:irror system that
duplicates the pixels in the proper places with minimum waste o
light, a special "striped-mirror system" can be used. one such
configuration is shown in Figure 5. Light containing full color
image information 501 (laid out as indicated in Figure 4) hits a
"striped-mirror pair" labeled as 50~ and 503. This causes the
entire image to be duplicated and shifted horizontally the width
of one pixel with approximately one~half the brightness of the
original image (which is also reduced to one half of its original
brightness~, filling the spaces between pixels in the horizontal
rows as shown by Figure 6. Vertical rows 601A, 602A, and ~03A are
duplications of ver~ical rows 601, 602, and 603, respectively.
The combined (original and duplicated) image existing in space
504 of Figure 5 then passes through a second "striped-mirror
pair" 50S and 506, which duplicates the image but shifts it
vertically the height o~ one pixel. This produces two images of
equal brightn ss, one above the other, filling in the horizontal
rows indicated in Figure 6 as 610, 611, and 612. Thus, a "solid"
image is cxeated with no blank spaces. Eliminating blank or dead
spaces, separately colored pixels; and thus the distinction
between pixels subjectively improves image resolution even above
today's CRT images at close range since CRTs have discernible
lines, pixels and spaces.
A "striped-mirror pair" is better understood by
reviewing Figure 7. Light from a single pixel 701 impinges upon a
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"clear" space 720 on the first mirror 702 of the mirror pair.
~his first mirror is made of glass, plastic or other suitable
material which is AR coated over the visible spectrum and coated
on its opposite side in stripes of a suitable reflective material
such as aluminum or silver. The striped coating may be
accomplished by, for instance, vacuum deposition with a "striped
mask over the glass." Alternatively, the glass can be coated with
photo-resist and exposed to a projected image of stripes of the
desired size~ After developmen~, the glass will be exposecl for
metal vacuum deposition only in the desired stripes. After
deposition, the remaining resist could be peeled off or dissolved
away, leaving the required clear stripes.
The second mirror 703 of the pair also has alternating
clear and reflective stripes. On this mirror however, the
reflective coating is thinner, creating partial mirrors instead
of full mirrors. The percentage of reflectivity is adjusted so
that the two pixel images which emerge are of equal brightness.
Light from pixel 701, after passing through space 720,
impinges on partial mirror 730, creating a transmitted beam 710
and a reflected beam which hits mirrored surface 740 on first
mirror 702. This reflects light through cl~ar space 750 on mirror
703 creating a second beam 710a which is an exact dupli~ate~-of
beam 710, except that it is contiguously displaced from beam 710.
If the spacing between pixels is not equal to the dimensions of a
pixel, the mirrored areas 740 on mirror 702, as well as clear
spaces 750 on mirror 703, may be adjusted to the dimensions of
the space between pixels.
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The overhead view of Figure 5 shows that "striped-
mirror pair" 502, 503, which has ver~ical stripes, is tilte~ wi~h
re~erence to beam 501 around a "ver~ical til~ axis" to create a
horizontally displace~ duplicate image and a "striped-mirror
pair" 505, 506, which has horizontal stripes/ tilted around a
"horizontal tilt axis" ~which is perpendicular to the tilt axis
of the first "striped-mirror pair" and to the beam 501) to create
a vertically displaced duplicate image.
Arrangements which break up a white collimated beam
into colored collimated beams, as well as configurations which
combine multi-colored collimated beams into a single collimated
white beam are reversible and can be used on either side of a
light valve to make full use o all light in the beam, illuminate
a monochromatic light valve with the properly colored beams,
recombine the colored beams to form full-color images without
individually discerni~le color pixels and produce an image which
is continuou.s, having no spaces between the pixels.
The use of time multiplexing, as previously explained,
can be used to fill dsad spaces between pixels with duplicate
pixels to create a "continuous" image. The three color images can
be slightly offset to somewhat fill the spaces between pixels.
Figure 3, for example, shows blue pixel 301 slightly higher than
red pixel 302 and green pixel 303 slightly to the left of each
red pixel 302. Many other arrangements of offsets of the
different colored pixels are possible to decrease black spaces in
the image; however, ~he individual colors remain visible at close
range.
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To produce a good quality color imaqe, it is important
to have as high a resolution as possible, a6 well as to
superimpose red, green and blue pixels on one another to
eliminate the appearance oE individual color pixels and to
eliminate spaces between pixels. Whether accomplishing this with
three optical paths and three light valves or by dividing up a
single light valve with a large number o pixels into 3 ~ections
to produce the 3 color images, the cost is higher and the system
consumes more space and weight than a simple single light valve
system. However, a single ligh~ valve doesn't have the resolution
of three light valves. It i5 therefore desirable to devise
methods which produce a high quality, high resolution image
without the added cost, complexity, weight and size increas~ as
stated above.
Obviously, increasing the number of pixels in a light
valve will increa~e the resolution of the image. Two or more
projectors used to project contiguous images can produce an image
with higher resolution than can be produced by a single projector
using available light valves. Alternatively, a single projector
can be made which essentially contain5 the components of several
projectors but with the contiguous images produced side-by-side
within the projector so that the composite image can be projected
with a single projection lens. This will eliminate the need for
alignment of extQrnally placed projectors and will produce a
higher resolution than is capable of being produced by a single
light valve system.
Regardless of the relationship between the lines of
pixels with respect to the placement o color dots within them,
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64 -
if any grouping of three colored pixels of the light valve is
used to form a color triad representing the color of a particular
point of the scene displayed, then the resolution capability of
the LCD is reduced by a factor of 3. This resolution limitation
can be reduced, however, if each pixel of the light valve, being
either red, green, or blue, is driven by a signal that
corresponds to that light valve pixel's color at that point in
the original scene, and the data about the remaining two color
values at that point in the original scene is simply discarded.
The eye will tend to blend the color contributions of neighboring
pixels to produce the correct color for that area of the scene,
but retain the capability to distinguish detail as fine as the
actual pixel spacing.
"Time-share scanning" (described herein) can be applied
to create a high resolution image with a lower resolution light
valve. For instance, an image can be projected having a space
between every two pixels, along each horizontal line equal to the
width of a pixel. This can be accomplished, for instance, by
fabricating the light valve that way or by using lenslet arrays
to appropriately change the size of each pixel. l'hus, if a light
valve is capable of, for instance, 500 pixels on a horizontal
line, the resolution can be doubled to 1000 by time-share
scanning. One-half of the time can be used to project an image
from the light valve as it exists onto the screen, while the
other half of the time can be used to project an image of
intermediate pixels onto the screen, giving the image twice the
resolution of the light valve in that direction. Unlike other
time multiplexing schemes, no decrease in brightness i5 created
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since each segment of time projects all of the light from the
light source onto the screen and thus all of the light from the
light source is visible to the viewer at all times. This
technique could also be used to double resolution in the vertical
direction creating, for instance, a high definition image from a
standard resolution light valve.
The systems disclosed in this application can use
discrete and individually addressed and maintained pixels. This
approach provides the basis for true digital television.
Presently both audio and video signals are digitized and stored
as digital bits on laser disks and "C~s." This digitization
preserves the exact values of the signal from micro-second to
micro-second. Distortions in the systems, such as ampli~ier
noise and non-linearity, scratches, dropout and other defects on
the recording material and so on can be completely ignored by a
system looking only at each bit to see if it is on or off, i.e.,
a "o" or a "1," and not caring if it varies in strength or
clarity. This will result in more precise, higher quality
television and video display. The upcoming thrust toward High
Definition Television should move the field toward this type of a
digital display device as the system of choiceO In summary, the
present invention makes possible a viable basis for
implementation of digital and High Vefinition TV, regardless of
the format convention selected.
Use of digital processing makes ik easy to eliminate
the problems inherent in today~s video systems such as ghosts,
chroma crawl, moir~ pakterns, snow and crosstalk between
chrominance and luminance signals. It also makes the creation of
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additional pixels in the receiver by interpolation between any
two pixels possible, thus creating the appearance of even higher
resolution in the receiver than is actually transmitted. It also
makes special fea~ures very easy to implement such as picture in
picture, zoomingt frame freezing, image enhancement, special
effects and so on.
All electronic image production systPms, whoss images
are made of a ~inite number of pixels, have an artifact which
becomes more noticeable as the number of pixels in the image
decreases. This artifact is often referred to as jaggies or
aliasing. When a diagonal line, such as a boundary between two
di~ferent features, is presented in the image, the line becomes
jagged, as if it were a staircase, since the pixels are usually
square with their edges parallel and perpendicular to the
horizon. To reduce the noticeability of these jagged boundaries,
known anti-aliasing techniques can be implemented, especially if
used in a digital system, since it is already computerized. When
a boundary is detected between two areas of different bri~htness
values and/or different color valuesl a calculation can be
performed to find the average brightness and color between the
two value~. Then, making all pixels along the boundary that new
value will c~eate a transition between the boundaries that is
much harder to see, thereby reducing the appearance of a jagged
edge.
The image brightness which can be produced by a
projection system is in part dependent ~n the bulb brightness.
This generally means that for more brightness, a higher wattage
bulb should be ~sed. The bulb wattage that can be used in many
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environments i~, however, limited. ~ home projector shouldn't
draw more than about 5 amps, which corresponds to about 600
watts. A higher wattage projector becomes very expensive to
operate and discharges a great deal of heat. It is therefore
desirable to use a bulb which has as high an efficiency, measured
in lumens produced per watt consumed, as possible. The best light
sourc~ uses a microwave stimulated plasma. This type o~ bulb,
currently in prototype form, can produce up to 130 lumens per
watt. Other sources which can be used include Xe, Hg and metal
halide bulbs which can produce from 75 to 95 lumens per watt.
Tungsten halogen bulbs can produce as much as 40 lumens per watt
and regular tungsten can produce up to 25 lumens per watt.
Instead of using a high powered bulb with a large
filament or arc, two or more bulbs of lower wattage and smaller
filaments or arcs can be used. Using multiple lamps presents
several advantages. If a lamp should ~urn out, the system would
only diminish in brightness, operating with the remaining lamp(s)
until the lamp is replaced. Each bulb, being of lower wattage,
can have a much longer lifetime, and a smaller filament or arc
can maka focusing an image o~ the source into the pixel hole
easier. Various methods can be used to combine the beams for use.
Figure 37 illustrates one example in which two light sources are
collimated and made contiguous by the use of a prism. Figure 45
shows how a mirror can be used to make 2 collimated beams
contiguous. Another method of eliminating space between separate
b~ams is the use of mirrors to take light from one part of a beam
and use it to fill in spaces between beams. An example of this is
illustrated in Figure 38.
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Alternatively, the beams can be made to come to a focus
at an area in space so that the filament or arc images abut one
another, forming a new composite light source. By the use of
mirrors, these point sources can be made to propagate in the same
direction, making it easy to collect with a single condenser lens
to form a single collimated light beam containing most of the
light originally captured. An example o~ this is shown in Figure
43.
The accuracy of reproduced color depends on several
factors. With the use of properly selected color filters or
dichroic mirrors, correction for wavelength versus light valve
cavity thickness versus voltage, as described above, and normal
Gamma correction and other normal TV color circuitry, the
~idelity of color reproduction is still limited by the color
makeup (i.e., color temperature) of the light passing through the
projection system. Incandescen~ lightingl although simple and
inexpensive, produces a low color temperature, resulting in a
"reddened" image, while discharge lamps, such as metal halide,
xenon, mercury and especially microwave driven plasma (which
provides constant brightness and color temperature even with tens
of thousands of hours of operation) produce higher color
temperature with more realistic whites and colors. However these
lamps have the drawbacks o~ being more expensive, have bigger and
heavier power supplies an~ are often more difficult and dangerous
to use and replace. Realistic colors can be produced with the use
of incandescent sources if a color-temperature-compensating
filter is used. At the expense of some brightness, the entire
color spectrum can be shif~ed towards the blue, producing more
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realistic whites and colors. The advantages of using an
incandescent source are ~hat they are rugged, inexpensive, saEe
and easy to replace and need a small power supply or no power
supply at all.
A number of approaches might be taken to extend the
life of the light source. The microwave stimulated plasma bulb
for example has virtually an unlimited lifetime, and is thus best
for eliminating bulb replacement.
To extend the life of a filament bulb, circuitry could
be used to run the filament on smoothed DC. Furthermore, the
circuit could ramp up the voltage slowly whenever the lamp is
turned on to reduce shock due to rapid heating and filament
motion.
For an incandescent bulb to have the highest efficiency
as well a~ high color temperature, it is necessary for it to have
a tightly wound filament which runs on relatively low voltage and
high amperage. This would normally necessitat~ the use of a
large and heavy step-down transformer. To eliminate this burden,
a triac circuit can be used to chop up the duty cycle, utilizing
only part of each cycle. Selecting the proper duty cycle will
provide the filament with the reduced voltage that it requires. A
feedback circuit can also be included to monitor line voltage and
to adjust the duty cycle to compensate for line voltage changes
so that a constant reduced voltage is fed to the filament.
The projection systems described herein have brightness
limitations due to low efficiency at various points in the
system. Various methods can be used to increase the efficiency at
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these points and thereby the overall ef~iciency and brightness o~
the projector can be dramatically increased.
One pro~lem common to all projection systems is the
efficiency of the light collection optics. Usually, only a small
percentage of the light produced by a bulb is actually collected
and utilized in the projection system. To further improve the
efficiency of the system, various methods can be used to increase
the amount of light that is captured from the bulb for use in
projection. In the prior art, a light sourcel such as a filament
or arc, is positioned with a condenser lens, such as an aspheric
condenser, in front of the source with a spherical mirror behind
it. This arrangement is used in most projectors and captures some
of the rearward and ~orward propagating light. The majority of
the light, however, propagates ~o the sides, upwards and
downwards and is wasted.
A preferred method of utilizing this normally wasted
light is the use Q~ multiple condenser paths as shown in Figure
42. Two condenser lenses 4210 and ~220 and two spherical mirrors
4230 and 4240 will capture twice as much light emanating from a
bulb 4200 as in the conventional system. In all bulbs today,
light traveling in one direction can never be utilized since one
side of the bulb is used to connect power into the bulb to the
arc or filament. Light from the remaining (upward~) direction can
be captured by an additional condenser lens 4250 and reflected by
a mirror 4260 into th~ system. The beams can be joined into a
single beam using the methods described elsewhere herein.
Another method to utilize this otherwise wasted light
is to place a section of a parabolic reflector 3910 around the
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lamp 3900 in a conventional condenser setup 3920 as shown in
Figure 39. Light that would otherwise be unused is now collimated
and sent forward to join light emerging from the condenser lens.
To reduce the size of the resulting collimated beam, which will
probably be necessary in most applications, various optical
methods may be used, such as the Galilean telescope made of two
lensesl as depicted in Figure 40.
Another method used to capture more light from a bulb
is depicted in Figure 41. In this arrangement, a source 4100 is
placed at one focus of an elliptical mirror 4110~ Any light which
hits this reflector will be focused to the second focus of the
ellipse where it can be captured for collimation, for instance,
by a condenser lens 4120 with a low F number. However, ligh~
which misses the reflector ~4101 and 4102j, except for light on
axis, is lost. This light can be utilized by placing a
collimating lens 4130 at the second focus. This lens will
collimate light that would miss the second focus, but will have
almost no effect on the light going to the second focus.
Optionally, an additional small lens can be placed on the axis
between the two focus o the ellipse close kot he second focus,
to focus light emanating from the source nearly on axis to come
to a focus at the second focus of the ellipse.
Alternatively, a section of a parabola 4610 can be used
to capture and collimate tha~ otherwise lost light. This can be
seen in Figure 46.
An alternate method of using an elliptical surface
efficiently is depicted in Figure 50~ In this setup a spherical
mirror 5010 makes rearward going light into forward going light.
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A lens 5020 captures forward yoing light and brings it to a
focus. A surrounding elliptical surface ~030 captures light which
misses both the spherical reflector and the iocusing lens and
brings it to the focus of the focusing lens. At this point light
can be gathered from the focal point and collimated by a single
lens 5040.
Collection systems which capture light from wide
angles, such as those disclosed herein, generally have large
apertures. This leads to a large collimated beamO As pointed out
herein, such a beam can be reduced in diameter, for instance, by
a telescope arrangement where the output lens has a shorter focal
length than the input lens. ~his reduction of beam diameter is
accomplished with an increase of angles of non-collimated rays
within the beam. This results in a restxiction of how long the
internal optical path of the projection system can be before
light spreads so much tha~ it doesn't get into the projection
lens.
Several measures can be taken to condition the light to
allow for an increased internal path length if one is desired for
a particular system design.
A preferred method of dealing with this limitation is
depicted in Figure 51. ~his method is accomplished by generating
a reflector surface which will be referred to herein as a Fresnel
Parabolic Reflec~or. (The same logic can be used to produce other
surfaces such as a Fresnel Elliptical Reflector and so on.~
By assembling segments of a parabola (dashed curve), an
equivalent parabola 5110 can be constructed with a narrow opening
tsolid curve). Thus, the collimated beam need not be reduced
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much, if at all~ Thereby, angles are not increased and
collimation length is left longer.
An alternate approach to ~his limitation is to use the
idea used in fiber optic cables~ In such a cable, light can
travel a long distance but, because of continued low/loss
internal reflections, the beam diameter does not increase until
the end of the "tunnel," which in our sys~em can be where the
light valve is placed. Multiple tunnels can be used if multiple
light valves are used. Such a tunnel can ~e made of mirrored
surfaces instead o~ fibers and can ~ake various shapes such as
square, rectangular or circular.
The use of non-imaging concentrator optics can be used
to further reduce the beam diameter, essentially allowing for the
optical reduction of the size of the light source. This will
allow for the use of a briyhter bulb, with a larger arc or
filament. The concentrator optics, normally used to concentrate
light for solar collectors, can concentrate the light to a
smaller area than the original arc or filament. This will allow
for greater collimation and, thus, permit more light into a
longer path system. One name commonly used to describe such a
concentrator is a "compound parabolic concentrator" although the
reflective surface actually has hyperbolic walls. The two
currently known designs for non-imaging concentrators,
originating in the 1~60s, are referred to as "edge-ray"
concentrators and "geometric vector-flux" concentrators~
To further increase the amount of light that gets into
the projection lens and thus, reaches the screen, the distanc~
from the light valve(s) to the projection lens must be kept to a
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minimum (so non-collimated light gets into the projection lens).
To accomplish this the focal length and F number of the
projection lens should be kept to a minimum.
If three light paths are used because three light
valves are used to modulate the red, green and blue images
separately, the colored images must be recombined to form a full-
color image. This can be done with various arrangements, such as
the one depicted in Figure 2. However, to minimize the distance
between the light valves and the projection lens, a dichroic
combiner cube will keep the distances to a minimum. Such a cube,
known in the art, consists of four equila~eral triangular prisms
placed together to form a cube. The faces that touch one another
include dichroic coatings to allow the three colored image-
bearing beams to combine into a full-color image.
Conventional direct-view light valve~ utilize color
filters to create a full-color image. Color filters work by
absorption, which unfortunately wastes approximately two-thirds
of the light, converting it to hea~, which exacerbates the
heating problem.
An alternative method to making such a color mosaic
without the use of absorptive color filters is illustrated in the
following embodiment. Figure 25 shows a collimated beam 3f white
light 2500 which is separated into three collimated beams, one
red 2510, one green 2520 and one blue 2530, by a dichroic mirror
arrangement 2540. These beams then pass through a double lens
array 2550, each array containing the same number of lenses as
the nu~ber of pixels in the ligh~ valve 2560. Each lens pair
formed by one lens from each lens array produces a Gallilean
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telescope, producing a collimated beam of reduced diameter. The
lens curvatures are chosen so as to provide a 3:1 reduction in
diameter of each collimate~ beam. A second dichroic mirror
arrangement 2570 brings the color beams together, but, due to
displacement o~ two of the mirrors, the ~eams do not actually
overlap, forming a mosaic of colors to illuminate the
monochromatic light valve in whatever color arrangement is chosen
(such as the two arrangements described above and depicted in
Figures 15A and 15B).
An alternative method of producing a mosaic of colored
beams is illustrated in Figure 26. Collimated light 2600 passes
through a double lens array 2610, which again contains the same
numbér of lenslets per array as there are pixels in the light
valve 2620. The focal lengths of the two arrays are different,
such that a series of collimated beams is formed 263~. The width
of each beam is the size of a pixel and the spacing between
collimated beams is equal to ~wice the pixel pitch. Each
collimated beam intercepts a stack of 3 special mirrors.
These "mirrors" consist of mirrored areas, separated by
clear spaces which are twice the size of the mirrored areas. The
width of the mirrored areas is chosen so that each collimated
beam will exactly fill each mirrored area when hitting the mirror
at 45 degrees to the normal of the mirrors. Tracing the path of a
single collimated beam emerging from one of the lenslets, the
beam passes through clear areas in the first two mirrors 2640 and
2650 in the stack, hitting a dichroic mirrored sur~ace on the
third mirror 2660. This dichroic mirror transmits the red light
and reflects the blue and green light downward. This blue-green
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beam hits a dichroic mirrored area on the 2nd mirror, which
reflects a collimated green beam in a direction parallel to the
red beam, while transmitting the blue beam. I'he blue beam hits
the first mirror, which is a standard firs~ surface mirror, so
that the beam is parallel to the red and green beams. These red,
green and blue beams illuminate three pixels on the light valve,
which is monochromatic, but is addressed with red, green and blue
data, respectively. ~lternately, the dichroic mirrors could be
replaced with volume holograms to accomplish the same result.
In another embodiment, shown in Figure 27, one of the
collimated mini-beams 2700 (as described above) hits a hologram
2710 which re~racts/diffracts the light, breaking it up into
essentially red, green and blue beams. ~ second hologram 2730 or
series of prisms bends the off-axis beams back on axis, so that
parallel red, green and blue beams are formed, which can then
illuminate a full color light valve 2720, as previously
explained.
Use of a dichronic or holographic system to produce a
mosaic o~ colored beams can be done in conjunction with a color
filter mosaic as well. Since ~he light is properly colored
before hitting the filters, less will be absorbed and selected
saturated coloxs will result.
Light valve systems that utilize rotation of the plane
of polarized light have a major loss of efficiency because, to
rotate polarized light, the light valve must be illuminated with
polarized light. Systems in use today make polarized light by
using sheet polarizers which produce polarized light
(inefficiently) by absorbing all light except that which is
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polarized in the desired direction. This wastes more than
two-thirds of the light and causes ~he polarizer to heat up. In
the light valve systems in use today, the polarizers are mounted
on the light valve. Thus, when tha polarizer heats up, the light
valve heats up, limiting the amount of light that can be sent
through the system.
One solution to this light valve heating problem i5 to
mount the polarizers a sufficient distance away from the light
valve an~ to cool the polarizers directly.
A better solution which also alleviates the
inefficiency of sheet polarizers is to use a MacNeill prism for
polarization. The MacMeill prism makes use of the fact that light
which hits a dielectric surface a~ an angle, such as Brewster's
angle, splits into reflected and transmitted beams which are
somewhat orthogonally polarized. This effect can be maximized by
applying several layers of dielectric coatings, with alternating
indices of refraction, such as by vacuum d~position, onto the
surface between two glass prisms, cemented together to form a
cube.
When the cube is properly constructed, approximately
50% of the ligh~ entering the cube is transmitted as P-polarized
light and approximately 50% of ~he light is reflected by the
dia~onal surface as S-polarized light. Since most sheet
polarizers absorb between 65% and 75% of the light that hits
them, just utilizing one of the beams from this cube will
increase the amount of light available ~or the light valve and
will greatly diminish the light valve heating problem caused by
sheet polarizer heating due to absorption. Both beams can
-- 78 ~
actually be used so that very little light is wasted in the
process of providing polarized light for use by the light valve.
Both beams could be used by employing mirrors which
reflect one of the beams emerging from the cube such that its
plane of polarization is rotated when the two beams are joined as
side-by-side parallel beams of light. As shown in Figure 44/
S-polarized light reflected by the cube 4400 is reflected
downwards by a mirror 4410, rotating the plane of polarization of
the light with respect to the horizon. A second mirror shown in
the diagram as mirror 4420 reflects this light in the direction
of the P-polarized light emerging ~rom the cube while maintaining
its polarization orientation. By positioning this mirror at the
right angle, this beam will be reflected up to the height of the
P-polarized beam emerging from the cube. This beam is then
reflected forward by a mirror or as shown in the diagram,
refracted forward by a prism 4430 forming a second beam of light
parallel to the other beam emerging from the cubel both in its
direction of propagation as well as in its plane of polarization.
Each baam can be brought to a focus with the use of lenses and
mirror right next to each other, forming a single expanding
polarized light beam. Other methods described herein could also
be used to combine the beams so that both would illuminate the
light valve.
A preferred method of utilizing both beams produced by
a polarization beam split~er cube 5400 is depicted in Figure 54.
With this method, a mirror 5410 which is parallel to the
dielectrically coated diagonal of the cube is placed adjacent to
the cube, producing two side-by-side collimated beams with
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orthogonal polarizations~ Placing a half wave plate 5420 in one
of the beams produces two side-by~side paralllel beams which have
the same polarization. The size and aspect ratio of the resulting
beam can be altered by the use of spherical 543Q and cylindrical
5440 lensesr if reguired.
If a large beam must be polarized, using a MacNeill
prism will unfortunately require a heavy, large, solid beam
splitter cube which is expensive to produce and consumes much
space. A small beam of light could therefore be used, although
this may require using additional lenses and additional space to
accommodate the changes to the size of the beam. Unfortunately,
reducing beam size increases the angles of non-collimated rays,
which then polarize inefficiently in such a cube. A MacNeill
plate polarizer which weighs less and consumes less space can be
used but will function only over a very narrow bandwidth. In a
video projection system, as contemplated by the present
invention, a beam of white light could be separated into three
color component beams by, for instance, a dichroic mirror system~
These three separate color components could then be sent to three
MacNeill plate polarizers. Although this does save space and
weight, the optics required to separate and recombine the colored
beams may occupy the same or a greater amount of space and weight -
than was saved. Moreover, the three MacNeill beam splïtter plates
would greatly increase the cost o~ the system. Applicant has
devised a "Fresnel ~acNeill prism," which functions as a MacNeill
prism beam splitter but has, at the outer surfaces of the plates,
a multiplicity O~ tiny saw-tooth surfaces, each behaving as a
normal prism. This device weighs much less than a prisml consumes
~ J ~ 3
- 80
less space, operates over the entire visible spectrum, and costs
less to produce.
Linearly polarized light that passes through an
ordinary lens is no longer strictly linearly polarized. This is
because a lens consists of curved surfaces which can alter the
polarization of light passing through it due to the dielectric
polarization effect mentioned above. As a lens surface is
continually curving and changing its angle with respect to
different portions of the beam of light, di~ferent portions of
the beam's polarization are altered differently. This will reduce
contrast and color fidelity of the image produced by a light
valve using polarized light. To reduce this problem, if a
polarizer is used, it should be positioned after any lenses,
whenever possible. The preferred solution is to use lenses which
are as thin as possible, even if several are used in sequence
coated with highly efficient AR coatings on the curved lens
surfaces to minimize the polarization effects encountered when
light hits a surface at an angle.
Although a MacNeill polarization beam splitter allows
approximately 50% of the input light to be transmitted as
P-polarized light, each beam, specially the reflected S-polarized
beam, is somewhat impure. In other words, the transmitted beam,
although primarily P~polarized, contains some non-P~polarized
lightl while the reflected beam, although primarily S-polarized,
contains some non-S-polarized light. A small amount of such
"contamination" is very noticeable to the eye, making the
projection of completely black areas impossible, reducing 'he
contrast and color saturation. To solve this problem, a polarizer
3 ~ .J
-- 81 --
could be positioned between the MacNeill beam splitter and the
light valve with their axis parallel, causing a relatively small
loss of light, but eliminating light of the unwanted
polaxization, improving the contrast ratio potential from
approximately 20:1 to approximately 1000:1 and only increasing
the light loss from 13~ to 35%, which leaves twice as much light
as with the use of just a polarizer
The use of a dichroic beam combiner cube to produce a
full-color image from three separately colored image-bearing
beams within a small spa~e has been explained above. The same
cube can also be coated to operate as a MacNeill polarization
beam combiner cube. This cube will act as a beam analyzer for
light valves using polarized light. With this arrangement, one
beam will be transmitted through the cube, while the other two
beams will be reflected by the internal surfaces. Co~sequently,
the transmitted beam must be P-polarized while the reflected
beams must be S-polarized. The light exiting the light valve
which is to be transmitted by the cube must be P-polarized while
the other two light valves mus~ be manufactured to provide images
in S-polarized light. Light polarized by the MacNeill methods
disclosed herein, being all of one polarization, can be rotated
by a half wave plate before entering the light valve which
requires orthogonal polarization. However, a simpler and less
expensive alternative is ~he use o~ identical light valves (as to
r~quired polarization) and a halfwave plate after the light valve
which produces a differ nt polarization output from the other
light valves.
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Another loss o~ efficiency which is especially
noticeable in an active matrix liqht valve occurs because there
are spaces between pixels which do not transmit light. Light that
hits these areas does not reach the screen, decreasing the
brightness of the projected image and contributing to heating of
the light valve. Typically between 35~ and 70% of the light
illuminating such a light valve actually passes through it. To
get around this problem, light must be crammed into the pixel
holes, being made to miss ~he opaque areas between pixels.
The preferred technique to do this is the use of a lens
array to focus light coming from the condenser system down into
the pixel holes. For a given light valve, the pixel hole size is
fixed. Selecting a bulb fixes the filament or arc size. To get as
much light as possible from the gelected light source into the
pixel requires taking into account a few factors. The thickness
of the glass used in the light valve limits how closely the lens
array can be to the pixe~ hole and thus how short the focal
length of the lens array can be. The ratio of the focal length of
the condenser lens system to the lens array focal length
determines the demagnification of the filament or arc image.
Although we would like a large condenser focal length so that the
demagnification factor is sufficient to ~QCUS the entire image of
the fllament/arc into the pixel, increasing the condenser focal
length decreases the amount of light it can gather from the
filament. Consequently, we must have the condenser focal length
as short as possible while still demagnifying the image of the
filament/arc sufficien~ly to fit within the pixel (taking into
account diffraction blur). We must therefore select a bulb with
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- 83 -
the smallest filament or arc size that will provide the minimum
acceptable brightness. With a given pixel size, a minimum lens
array focal length, a given filament size, a maximum filament
efficiency per unit area and a minimum condenser lens focal
length, the maximum amount of light that can be put through the
pixel holes is determined. Using ~hese paramaters, a light source
and lenses can be chosen to get as much light through the light
valve as possible for any given ligh~ valve. As disclosed
earlier, techniques such as the use of a collimating hologram or
the use of non-imaging concentrator optics can reduce the
filament/arc size, allowing more ligh~ to be focused into the
pixel holes.
Using a single lens array before the light valve
creates a problem. The illumination at any point on the array
after the light valve tused for depixelization) is proportional
to the brightness of the-source and the solid angle through which
that point is illuminated. As seen in Figure 34, the illumination
angle 3410 from the center of the output lens array 3420,
positioned after the light valve 3430 to maynify the images of
the pixels and eliminate the spaces between the pixels in the
image, is that which is subtended by the array element 3440
placed before the light valve, assuming the pixel hole allows the
entire cone of light to get throuyh to the array element after
the light valve~ When looking at the light which hits a point on
the lower edge 3450 o~ an array element after the light valve, as
also shown in Figure 34, we can see that the lower edge of the
pixel hole limits the cone angle of light 34~0 available to
illuminate the array element after the light valve. Thus,
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illumination along the edge of the array element after the light
valve will peak at about 50% of the illumination at the center of
the element and fall off to about 25% at the corner of the
element.
Another problem can be seen in Figure 35. If the
illuminating light source were a true point source, depicted as
the center of the lamp filament 3~00, light would focus as a
result of passing through the array element 3510 before the light
valve into the center of the pixel 3520 and then fully illuminate
the array element 3530 after the light valve. This would cause a
complete uniform illumination of each pixel on the screen.
However, since the filament is extended and not a true point
source, light will be entering the array element before the light
valve from other positiolls and at other angles. ~s seen in the
figure, the light rays coming from the bottom of the filament
3540 would come to a focus at the top of the pixel hole 3550.
After spreading out from this point, some of the light would miss
the corresponding array element after the light valve. This would
also cause a non-unifo~m illumination of the array element after
the light valve and thus the pixel on the screen, in addition to
sending some light to an adjacent or nearby pixal. If this light
wound up on the screen, it could cause a decrease in contrast and
color fidelity in neighboring pixels.
With each pixel being brightest in its center and dim
around its edges, a pixel structure would still appear visible on
the screen even though there was actually no space between
pixels. To circumvent this problem, ideally a ~ield lens array
3690 at the pixel plane would cause the light that would miss the
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array 3610 after the light valve to be redirected, causing
uniform illumination of tha~ last array and prevent light from
hitting adjacent pixels (see Figure 36)~ In xeality however, the
field lens array cannot be placed exactly in the pixel plane.
Consequently, we can split the field lens array into two lens
arrays, one on either side of the light valve, placed as close to
the light valve as possible. Wi~h this arrangement, the first
lens array focuses an image of the lamp with the first field lens
into the center of the pixel. The second field lens (being the
first lens array after the light valve) helps steer the light
towards the final lens array. This final array magnifies the
image of the pixel forming an image to ~e projected on the screen
by pro~ection lens. This magnified image of the pixel, as
explained earlier, a~uts the magnified image of its neighboring
pixel, causing a continuous image made of contiguous pixels, with
no spaces between them on the screen.
As an alternative method of focusing light into the
pixels, two lens arrays can be used as an array of Galilean
telescopes. By this method, light entering each pixel will still
be collimated, but most or all o~ it will go through the pixel
holes. ~ ~
With these methods, the higher the spatial coherence of
the light source (the more of a '1point-source" it is) the more
efficiently these mthods will operate. HOwever, to produce more
light or to make a bulb with a longer life, requ9ires the use of
a larger lighted area. To take advantage of such sources, with
the techniques described herein, the source size must be reduced
by "funneling" the light down to a small point.
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86 -
Another method of cramming light into the pixel holes
is by using a fiber optic bundle in which the input end is
tightly packed and the ou~pu~ end is arrangecl so that each fiber
is the same size as its adjacent pixel hole.
There is one other source of wasted light in a video
projection system which is never thought of as wasted light. This
is the light that is removed from certain areas in the image
because those areas are supposed to appear as darker areas. This
is light that should not reach the screen so that brightness
variations can be produced on the screen to create an image.
However, this light need not be totally lost.
With the use of a light valve that utilizes polarized
light, a polarizer is used after the light valve to act as an
analyzer. Light that should not appear on the screen exits the
light valve polarized perpendicular to the axis of this
polarizer/analyzer and is thereby absorbed by the polarizer. This
generates some heat as well, which can heat up the light valve,
if the polarizer is near it, and is also inefficient in that only
25% to 35% of the light that should ~e going to the screen makes
it through the polarizer/analyzer. 8y using a MacNeill
polarization beam splitter (as described herein) instead of the
final polarizer/analyzerl several advantages are realized. S~nce
there is no absorption, no heating occurs. Because nearly 50% of
the light appears in each beam, nearly 100% of the light that
should go to the screen passes through the MacNeill analyzer to
the screen. A plane mirror in the path of the beam exiting the
MacNeill analyzer that normally would have been absorbed by a
sheet polarizer can re~lect that normally wasted beam back to the
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light source for reprojection through the system to the extent
the beam is collimated~ The beam will retrace its path through
the system ending up being focussed into the center of the light
source to be gathered by the collecting mirrors for reprojection
through the system. ~lthough a large portion of this light will
not make it to the screen due to non-parallelism, and consequent
inability to retrace i~s path through ~he entire system, and due
to loss of improperly polarized light exiting the first MacNeille
polarization beam splitter on its way back to the bulb, some
brightness will be added to the image that would not have been
available if this technique were not used.
These light saving techniques will greatly increase the
light output of a projection system. To summarize, use of a
double condenser system to collect light from the light source
doubles the light output oYer a conventional system. Use of a
polarization beam splitter, instead oE an absorption polarizer,
again doubles the light output. Use of dichroic mirrors instead
of color filters to produce a colored image more than doubles the
light output onca again. Use of lens arrays before the light
valve approximately doubles the light output again, depending
upon the ratio of clear areas to opaque areas on the light valve.
Using these techniques in tandem means an overall potential
increase in image brightness o~er a conventlonal system of 16
times. Use of a polarization beam splitter with a plane mirror in
place of the final polarizer/analyzer as well as other techniques
outlined herein will potentially further increase the image
brightness and system eEficiently.
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Many projection formats can be used in conjunction with
the disclosed video display systems. In addition to curved,
direction-sensitive, high reflectance screens, less expensive,
more widely dispersive screens can ~e used with this system. A
regular movie screen or even a wall proves adequate with a system
of such high brightness. By vertical mounting of the unit or the
attachment to the projection lens of a ~ront-surface mirror, the
image can be displayed on a bedroom ceiling. This technique
allows for convenient viewing of video imagery while lying in
bed, without causing neck or back strain.
Rear-screen projection can be achieved as wellO
Conventional rear-screen television utilizes a lenticular lens
and a Fresnel lens for adequate brightness. This adds a
discernible pattern to the image and produces a limited angle of
viewing both horizontally and vertically. This type of screen,
like a conventional CRT, reflects ambient light to the viewer,
creating glare which adds to the viewerls eye strain. With the
present system, brightness is much higher, allowing for a broader
viewing angle as well as more streamlined, lightweight and
aesthetically pleasing display units.
The high brightness allows for the use of a gray matte
(i.e., textured) screen ma~erial with wide dispersion angles.
This creates an image that is viewable from practically ~ny angle
with uniform brightness and no glare. This type of glareless
screen, coupled with the ability to vary the brightness and color
temperature of the display by selection of bulb type and
operating voltage, may also provide a significantly less
- 89 -
fatiguing display for individuals who must spend long hours
staring at a video display terminal.
One of the most efficient types of screen (front or
rear) can be made using holography. With a hologram, a diffuser
can be produced with a predetermined dispersion pattern, creating
as much diffusion as desired, with precisely tailored brightness
distribution characteristics. Efficiency can approach 100%. The
interference pattern can be made optically for simple
specifications or by computer generation for more complex
characteristics. Bleached or gelatin phase holograms or metalized
embossed holograms can be used to produce the actual screen with
high efficienc~.
With rear-screen projection, rather than locating the
projector several feet behind the screen to allow the image to
expand sufficiently to fill the screen, one or more mirrors can
be used to reflect the beam one or more times to allow image
expansion within a smaller cabinet size. For instance, a cabinet
approximately 18l' deep could be used to fill a rear projection
screen with a diagonal measurement of 50".
When an image projected on a screen is viewed in an
environment where there is much ambient light, the areas of the
screen that should be dark become filled with the ambient light,
reducing contrast in the image. A type of screen can be
constructed which will provide a bright image with high contrast
in high ambient light situations in both front and rear
projectionO The front projection version of this screen is
depicted in Figure 46 and comprises a regular front projection
screen such as a beaded, flat white or metallic coated screen.
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-- 90 --
On top of the screen is a black mask with relatively thin
horizontal slits. ~ lenticular lens whose cylindrical lenslets
are oriented horiz~ntally is placed on top of the slit mask.
There is one slit for every cylindrical lenslet. For maximum
versatility, the slit mask is adjustable in the vertical
direction. Light from the projector focuses an image on the
lenticular lens sheet of this screen, breiaking the image into
many horizontal siub-images corresponding to the number of
horizontal cylindrical lenslets. Each lenslet focuses its image
component to a thin line which passes throuyh the corresponding
slit in the mask to be re~lected ~rom the screen behind it. This
reflected light is re-expanded by the cylindrical lenslet for
viewing with high visibility from all angles. ~mbient light
arriving at the screen from any height other than that of the
projector ~which makes up most ambient light), will be ~ocussed
by the lenslets onto the black light absorbing layer and will not
be visible to the viewers.
The rear projection version of this screen is
constructed by placiny two horizontally oriented lenticular lens
sheets back-to-back with their flat $ides towards each other. The
slit mask described above is placed b~ween the lenticular~lens
sheets. Optionally a highly transmissive rear screen miaterial can
be placed next to the slit mask (also ~etween the lens sheets)~
The screen operates in the same manner as the front projection
version to eliminate ambien~ light reaching -the viewer. In both
front and rear configurations, the silit mask can be adjusted up
or down to allow the light ~rom the projector to pass exactly
.:
-- 91 --
through the slits, depending on ~he pxojector's height in
relation to the screen.
Another method could be used to reduce ambient light
reflection. ~he video projector~s image can be focussed onto the
input end of a coherant ~iber op~ic bundleO This is shown in
Figure 17 as 1795 which places the input end of the fiber bundle
into the projected beam instead of screen 1790. The other end of
the fibers 1797 can be flat or polished into lenses or can be
coupled to lenses. Thus each fiber, separated from neighboring
fibers, can magnify (due to fiber separation and due to the lens)
and deliver to a rear-screen a portion of the image (preferably
one pixel or part of a pixel per fiber), magnified a
predetermined amount. The composite image will appear continuous,
creating a very large image, with only a few inches of cabinet
thickness since the fibers can ~end. This technique also
eliminates the need for any other subsystem to fill the spaces
between pixels. Using the fiber optic screen with the fibers
spread apart at the output end, no lenses, and no screen, in
conjunction with black, light absor~ing material to fill the
spaces between the fibers will produce a bright i~age in an area
with high ambient light such as in an outdoor stadium. This is
because a majority of the surface area of the output of the fiber
bundle will be absorptive to ambient light, while all of the
image bearing light will still be sent to the viewer. However
this is done at the cost of creating a pixel-like structure due
to the spaces introduced between the fibers. When viewing a large
projected image in this situation however, the viewers are
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- 92 -
generally positioned at some distance from the screen which will
make the pixel structure virtually invisible to the viewers~
An example of an artistic and futuristic projection
system is illustrated by Figure 1~. The video projector 1401 can
be ~ounted to an upright 140~ projecting an image onto a mirror
1403. Mirror 1403 can reflect the image to focus onto a special
rear screen 1404 mounted in a frame which apears to be "hanging
in space." The screen itself can be made of extremely thin slats
1405 o~ almost any rear projection matarial. By mounting an axle
onto the ends of each slat with a gear on each, a motor drive can
be used to open tslats lying flat and parallel to the floor) and
close the slats (lying perpendicular to the floor, creating a
solid rear screen for projection). In the open position, the
screen will appear as a transparent window in spac~. When the
projection unit is turned on, by remote control for instance, the
slats can simultaneously and quickly be closed, creating a "video
image in space."
Whatever projection method is used, two other important
problems can occur. Unless the surface being projected upon is
perpendicular to the optical axis of the projection beam, the
image will suffer from keystoning and blurring of the parts of
the picture not precisely focussed on the screen surface. This
problem is inherent if the projector is mounted on the floor, on
a low table, or on the ceiling while the screen is centered on a
wall. CRT systems handle keystoning by varying the
electromagnetic scan line deflection. Some light-valve based
systems, however, have predefined pixel locations and thus cannot
utilize this technique.
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Consequently a type of anamorphic lens system can be
constructed. A zoom lens normally changes the size of a projected
image by changing the relative positions betwleen the elements of
the projection optics. However this could also be accomplished if
lens elements of differen~ curvatures were used. ~pplicant~s
system could employ a lens which is shaped as if it has added to
it two varying focal length lenses, one above and one below the
standard lens molded into one lens. The cen~ral area of the lens,
large enough to encompass the entire light beam from tht~ valve,
creates a rectangular projected image. Bu~ i~ this lens is raised
or lowered with respect to the light valve, the magnification
varies across the image, causing a trapezoidal image
predistortion with either the top or bottom of the image of the
light valve being the largest side o~ the trapezoid. Thus, the
lens is adjusted up or down, depending on the angle the video
projector is making with the screen and thereby the keystone
effect is cancelled.
The variable ~ocus problem can be corrected by a
little-known photographic technique known as "Scheimpflug
correction." If a scene to be photographed has a large depth and
a fairly large aperture is used, the only way to simultaneously
focus all elements of the scene is to tilt the lens and film
plane such that a line drawn through all objects in the scene
intersects the line drawn through the ~ilm plane at the same
point that it intersec~s a line drawn through the lens plane. In
a camera, this is accomplished by bellows. Using the same logic,
a mechanical adjustment that tilts the light valve plane and the
plane of the projection optics, creating an intersection with a
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- 94 -
line passing ~hrough the screen plane, will cause the entire
image to be in focus, even though the projector's beam is not
perpendicularly aimed at the screen.
Science fiction has always portrayed the video display
of the future as a thin large screen that hangs on the wall and
modern day technologists have been working towards that end for
decades. With an image projected onto a wall, the idea is almost
realized. However, projection onto a wall mandates that the
proj ection distance be included as part of the system because
nothing may be placed between the projection lens and the wall.
Applicant has devised a new type of screen which would eliminate
this intervening space or projection distance. With this screen,
the projector can be placed underneath it or even be built into
the screen itself, an~ yet the entire device thickness need not
exceed a few inches. This screen takes advantags of the
phenomenon that a beam of light of small diameter shone on a
surface at a very oblique angle can be spread over a huge
distance. When the propagation direction of the light beam is
nearly parallel to a surface, the beam can illuminate the entire
surface, even if the surface is hundreds of times larger than the
diameter of the beam, w~th no projection distance necessary
before the light hits the surface. Spreading of a light beam by
shining on an obligue surface "expands" the light beam's
dimensions in one direction. If the surface could then re-direct
the very wide beam, onto another surface, again at an oblique
angle, but orthogonal to the first surface, the beam could again
be spread in the orthogonal direction with no projection distance
required.
- 95 -
This re-direction is realized by a surface with
saw-tooth shape elements with the sloping side of each saw-tooth
mirrored 4700, forming a ~Fresnel mirror." As shown in Figure 47,
this will spread the ligh~ over a large area, but will create
horizontal stripes o~ light with dark horizontal stripes between
them 4710. The smaller these re~lectors, the more of them there
are, and the less noticeable the black bars in the imaye. To
make the light coverage continuous and eliminate the dark
stripes, the sloping surface of each saw-tooth need only be
curved slightly to expand ~he segment of light that hits a given
saw-tooth sufficiently ~o cover half of the dark band on either
side of the light ~and reflected by the saw-tooth.
Alternatively, a lenticular lens can be placed between the saw-
toothed surface and the imaging area.
An alternate method o~ producing a surface that will
~ehave as re~uired is to use known techniques to produce a
holographic surface that will re-direct the light into the right
directions.
If the light beam aimed at such a "Fresnel mirror,"
contains an image, the image will be spread in one direction onto
the surface of the Fresnel mirror. If the Fresnel mirror 4800 is
placed at an oblique angle to a rear screen 4810, as is shown in
Figure 48, the image will now be expanded in the orthogonal
direction, filling the entire screen. However, since the image
viewed from a rear screen appears brightest when looking at the
screen towards the source illuminating the screen, the screen
would be its brightest only when viewed at an oblique angle.
Adding a second Fresnel mirror ~900 to re-direct the light in a
-- 9~
direction normal to the screen 4910 makes the image visible on
the screen brightest when viewing in a normal fashion. (See
Figure 49.)
Alternatively, instead of utilizing curved saw-tooth
surfaces or lenticular lenges after re~lection from each Fresnel
surface, a spherical lens array can be placed just before the
final viewing screen to eliminate spaces between sections of the
image.
Two distortions are created by projecting onto a screen
by way of Fresnel mirrors. Since the image spreads out in all
directions as it propagates, the image will be wider the further
it has to go, with the furthest end ~eing wider than the nearest
end. This trapezoidal distortion will be repeated in the
orthogonal direction when reflecting from the second Fresnel
mirror. These two trapezoidal distoxtions can be corrected by
pre-distorting the image trapezoidally in both axes with
appropriate lenses in the opposite directions of the trapezoidal
distortions that will be encoun~ered due to spreading. The second
distortion is focus distortion due to the widely varying distance
from the projection lens to the near part of the image versus the
distance from the projection lens to the far part of the image.
This focus distortion can be corrected by tilting the projection
lens with respect to the light valve plane in the direction
opposite to the screen tilt. This tilt uses Scheimpflug
correction ~described above) so that the entire image is in focus
on the screen, even though it is being projected at oblique
angles. Such a screen system could be used for the projection of
any type of image, including slides and movies as well.
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Although projection systems generally project their
images on some sort o~ screen, in some instances it would be
advantageous to project directly onto the retina of one's aye.
Since a light valve, such as an LCD, can be made very small and
lightweight, and using some of the techniques listed herein, an
efficient projector can be made which is very compact and
lightweight. It then becomes feasible to mount such a system on
a headband or pair of glasses so as to give the viewer his own
private viewing screen. Because the entire retina can be
projected upon, the viewer can see his entire field of view
covered with the image. If the image is projected into one eye
only, the viewer will be able to see the projected image all
around him, but, it will appear superimposed on the real world.
This technique could be especially useful for private viewing of
a movie or confidential data, without others seeing it, or for
providing a computer screen to be connected to a computer in
place of a monitor. This application would free the viewer's kody
and head from being constrained to one position for long periods
of time.
In place of a conventional projection lens or condenser
system, compact optics such as lens arrays can be used to image
each pixel onto the retina with a corresponding lenslet for each
pixel. Alternatively, compound holographic optical element could
be used or multiple curved reflectors facing each other's
reflective surfaces, with on and off-axis elements to reflect and
image a light valve onto the retina could be used.
The present invention lends itself to three-dimensional
video projection. One method of accomplishing 3-D projection is
d ~ 6 ~
- 9B -
to use two projection systems with the polarizers of one light
valve system perpendicular to the polarizers o~ the other light
valve system. Sending stereoscopic video signals, derived from
two displaced cameras for instance, and pro~ecting onto a
non-depolarizing screen will allow viewers wearing polarized
glasses to see full color 3-D video. A single lens 3-D video
projection system can be constructed by placing both light valve
systems in one enclosure. Internally, the two orthogonally
polarized stereoscopic images can be joined by a McNeill prism.
Alternatively, instead of using the second mirror 503 of the
first "striped mirror pair" 502 and 503 of Figure 5, the
horizontally displaced spaces between the pixels of one light
valve can be filled by the pixels of the other light valve
through a simple beam splitter setup, creating a horizontally
interlaced, orthogonally polarized 3-D image pair for projPction
through the single projection lens. Striped mirror 502 can be
tilted at a 45 degree angle with respect to the axis of the light
from the first light valve. The light from the pixels of this
light valve will pass through the clear areas o~ the striped
~irror. The second light valve, whoss axis is perpendicular to
the axis of the first light valve, reflects its light from the
mirrored areas of the striped mirror, causing an interlaced
composite image made from both images, with orthogonal
polarization.
Another method of 3 D projection which can be used is
auto-stereoscopic 3-D projection. This method does not require
any special glasses for 3-D viewing. Two identical lenticular
lens screens/ wi~h their cylinders oriented vertically, placed
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back-to-back, optionallyr with a t~in translucent screen between
them are projected upon at different angles by two or more video
projectors, bearing stereo or multiple angles-of-view
information. The images can be viewed from the opposite side of
the screen at various locations in ~pace. As one moves to various
locations, around the screen, the images are viewable; one at a
time, without image overlap. This creates several orthoscopic as
well as pseudoscopic viewing zones in space~ ~f one positlons his
eyes in an orthoscopic viewing zone such that one image goes to
each eye, a 3-D view will be visible. Many viewers will be able
to view an orthoscopic 3-D video image from several angles and
positions at once. This type of screen can also be used in front
projectlon with a regular screen behind a lenticular lens.
Another method of preparing stereo visual data for 3-D
viewing uses half waveplate strips to rotate the plane of
polarization 90 for alternating columns of pixels. The columns
would be addressed so that every other column would produce a
right-eye image and the intervening columns w~uld produce a left-
eye image. Alternatively, instead of alternating columns,
alternating rows could be used for the presentation of left and
right eye images. Other presentation patterns could be used to
present a more uniform integration of left and right eye images
such as having each row consist of alternating left and right eye
image pixels followed by a row offset by one pixel such that a
checkerboard pattern of lef~ and right eye pixel images is
produced. All pixels corresponding to one eye's image can be
covered with a half wavepla~e so that one eye's image is
polarized orthogonal to the other eye's image. With this
~ . . ,
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- 100
arrangement a single projector with three or even one light valve
can be used to project onto a non-depolarizing screen for viewing
with polariæed glasses and the stero images will always be in
registration without requiring alignment.
If using any of the methods descrihed herein for
filling in spaces between pixels, the data for each eye's view
can be made to overlap the data for the other eye's view on the
screen. This will cause each eye~s image to appear continuous
without holes, lines, pixels or other spaces.
~ alf waveplates may be made pixel-sized and placed over
the correct pixels by photo-lithography technology. A
photographic mask, corresponding to the pattern of pixels to be
viewed by one eye, is imaged with U.V. onto photoresist which is
coated onto birefringent plastic of the proper thickness. Once
the photoresist is developed away in the exposed area (or
unexposed areas, depending on the resist used), a chemical can be
used to dissolve away the plastic that is exposed. Subseguently,
the remaining resist is washed away, leaving a mask to be placed
on the light valve. Alternatively, a master dye can be similarly
made of metal which can then be used to punch out holes in the
appropriate places in a sheet o~ birefringent plastic to produce
the mask for the light valve.
A light valve that is addressed in alternating vertical
columns of right and left eye views can be projected onto a
lenticular lens ssreen (in front or rear projection~ to produce
an auto-stereoscopic display which can be viewed without glasses
to produce a 3-D image.
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With the use of digital circuits and computer
capability built into the system, the system can be used to
process images so as to turn a two-dimensional image into a
three-dimensional image. One method of doing this requires pre-
processing o~ the movie to conver~ it to 3-D. The conversion need
be done only once, with the converted version being stored for
projection at a later time. With this technique, objects in a
scene which should appear to the viewer to be located somewhere
other than in the plane of the screen can be selected during pre-
processing and marked. Software can direct a computer to follow
the marked object from frame to frame. Tnis allows the operator
to select an object only once until it disappears from view,
eliminating the need to mark the object in every frame. Once an
object in a scene is selected and marked and the depth at which
it is to appear is determined and input, the computer can
generate a duplicate image of that object at a spacing to the
primary image that will cause ~he eyes to see the merged image at
the desired depth. Using, for instance, the stereo system,
described above, in which two projection systems have their
images perpendicularly polarized, to be viewed by someone wearing
polarized glasses, the computer can generate this duplicated
image ~or projection with polarization perpendicular to the first
image. The projector will project this duplicate image on th~
screen next to i~s counterpar~ image, separated by a distance,
which determines the depth at which a viewer will see the
composite image. When an object is selected to change its depth
inputting this ~act and indicating its new depth will cause the
computer to change the distance between the two component images
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to be projected on ~he screen. This will cause the ~iewer to see
the composite image formed in his ~rain by binocular ~usion at
the new depth.
Another technique can bP used to create depth in an
image, utilizing the above-described projection systems. With
this technique however, conversion to 3 D occurs as the image is
projected with no ~uman intervention or preprocessing necessary.
The imagery however should be shot with this system in mind if
the depth created is to be realistic. By having the projector
store, for example, three ~rames at a time and projec-t, as the
stereo frames to be viewed, frames 1 and 4 at any given time t4
being the current frame being shown, for instance, and 1 being
the frame which was shown four frames ago)~ a 3-D view is created
using glasses or an autostereo screen as described herein. The
faster an object moves, ~he larger the distance will be between
the left and right eye images and thus the further behind or in
front of the screen the image will appear to the viewer.
Consequently, motion of objects should be coordinated with their
depth to provide the most realistic three-dimensional imagery.
Various recently developed technological innovations
such as wireless transmission of sound from the projector to
speakers, wireless transmission o~ cable and VCR signals to the
projector~ a built-in VCR and/or a built-in computer when built
into a projection system as described herein will produce a
projection system with much broader use than any other system
available today.
While the preferred and alternate embodiments of the
invention have been illustrated in detail, modifications and
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- 103 -
adaptations of such embodiments will be apparent to those skilled
in the art. However, it is to be expressly understood that such
modifications and adaptations are within the spiri~ and scope of
the present invention as set forth in the following claims.
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