Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
CA 02522616 2005-10-17
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SOLID STATE LIGHT ENGINE OPTICAL SYSTEM
Bv
Mitchell C. Ruda
Tilman W. Stuhlinger
Kevin ,f. Garcia
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] The contents of this application are related to U.S. Provisional
Application No. 60/465,732 (filed April 24, 2003), entitled " Personal Theater
Optical System" the contents of which are incorporated herein by reference in
their entirety.
BACKGROUND
[0002] 1. Field of the Invention ,
[0003] The present invention relates to, light and image projectors and
particularly to illumination devices for projection displays.
[0004]2. Description of the Background Art
[0005] The prior art discloses various light sources and image projectors for
viewing videos and images. The simplest light projector comprises a flashlight
and a more complex device comprises an image projector with an incandescent
tight source such as in US Patent No. 6,227,669.
[0006] However, prior art projectors have disadvantages in that they can be
heavy, complex, or dim; can comprise a great number of expensive parts; be too
large; generate too much heat; require cooling fans; make too much noise; or
have short bulb life. It is therefore desired to have a projection display
that
overcomes one or more of these disadvantages.
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SUMMARY OF THE INVENTION
[0007~The present invention includes a solid-state light engine consisting of
an
illumination subsystem and a projection subsystem.
[0008 In an exemplary implementation, the illumination subsystem, illuminates
a
single liquid crystal on silicon (LCOS) micro-display with light from red,
green,
and blue light emitting diodes (LEDs). Examples of micro-displays that could
be
used with, the present invention also include Ft.COS, HTPS, Texas Instruments
DLP, MEMS, and others. The projection subsystem images the LCOS micro-
display on to either a reflective or transmissive viewing screen for front or
rear-
view projection televisions (RPTU). Gray scale and color are created by
temporally dithering the LCOS micro-display in conjunction with the LEDs.
Other
micro-displays can be used to create gray scale in analog fashion as found in
a
typical LCD. Thus, SXGA resolutions of at least 24-bit color, at least
1280x1024
resolution, and at least 60Hz frame rafie are easily achievable with this
system.
Although, a projection based TV has been exemplified in the present
specification, the present invention may be also be used for computer monitors
and other display devices.
[0009 One exemplary implementation, includes a doubly telecentric
illumination system that diffuses light at the maximum beam spread of the cone
of light from the source object points, thereby providing optimum local
homogenization of the individual sources at the image. The telecentric
illumination system may include a planar array of telecentric light emitting
diode
(LED) sources telecentrically imaged to the micro-display by at least one
Fresnel
lens. Light from the individually colored LED channels may be combined through
a dichroic cube structure while being imaged to the display. The diffusion
mechanism is placed just before the last Fresnel lens where the beam spread is
the largest and where the lens serves as the final imaging component of the
illumination system.
[0010 Additionally, light concentrators such as quad reflective, total
internal
reflection (TIR) differential, or edge ray c6ncentrators, collectively known
as non-
imaging concentrators, used in reverse and hence called emitters may be used
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to collect and subsequently emit light from solid state sources with large
areas in
the illumination subsystem. The quad differential or edge ray concentrators
may
include four individually blended concentrators that are integrated side by
side,
centered on, and covering a quadrant of the solid state source. The shape of
the
perimeter of the end nearest the solid state source may also be described as a
"clover" configuration. This configuration enables an efficient collection and
emission optic with a substantially smaller size than a single concentrator
covering the entire solid state source.
[0011 In one exemplary implementation, a quad TIR compound hyperbolic
emitter (CHE) is used to collect and emit light from large LED die sizes and
is
composed of four individual TIR compound hyperbolic emitters blended side by
side to cover the large LED die. Each CHE is centered on and covers a quadrant
of the LED die enabling en efficient collection and emission optic much
shorter
and smaller than a single concentrator.
[0012] The output from a non-circular die , or a group of dies, may be more
evenly captured and transmitted by orienting each dies corner at'the overlap
between each of the four CHEs forming the quad TIR CHE such as one having
the clover configuration.
[0013] Other features and advantages of the present invention will be set
forth,
in part, in the descriptions which follow and the accompanying drawings,
wherein
the preferred embodiments of the present invention are described and shown,
and in part, will become apparent to those skilled in the art upon examination
of
the following detailed description taken in conjunction with the accompanying
drawings or may be learned by practice of the present invention. The
advantages
of the present invention may be realized and attained by means of the
instrumentalities and combinations particularly pointed out in the appended
claims.
BRIEF DESCRIPTION OF THE DRAWINGS
[0014 FIG. 1 illustrates the system layout of the optical system according to
an
exemplary embodiment of the present invention;
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[0015] FIG. 2 illustrates the principles of Optical System Telecentricity used
in
an exemplary embodiment of the present invention;
[0016] FIG. 3 illustrates the light path propagation of the optical system of
FIG.
1 using the principles of double telecentricity;
[0017] FIGs. 4A and 4B illustrates the Red Compound Hyperbolic Emitter
(CHE) used in an exemplary implementation of the present invention;
[0018] FIG. 4C illustrates the placement of the end surface of a compound
hyperbolic emitter to fit over the LED die for maximizing the collection and
transmission efficiency;
[0019]FIG. 4D illustrates the placement of the end surface of a single quad
hyperbolic emitter to fit over an LED die for maximizing the collection and
transmissioh efficiency;
[0020] FIGs. 5A and 5B illustrates the Green/Blue Quad Compound Hyperbolic
Emitter (CHE) used in an exemplary implementation of the present invention;
[0021] FIG. 5C illustrates the placement of the end surface , of a quad
compound hyperbolic emitter to fit over multiple LED dies for maximizing the
collection and transmission efficiency;
[0022] FIG. 5D illustrates the orientation of multiple LED dies at the end
surface of a quad compound hyperbolic emitter shown in FIG. 5C.
[0023] FIG. 6 illustrates a Dichroic structure comprising thin glass plates
with .
dichroic material used in an exemplary implementation of the present
invention;
[0024] FIG. 7 illustrates the system layout of the optical system according to
another exemplary implementation of the present invention;
[0025] FIG. 8 is an exemplary implementation of the electronics for
controlling
the LED light output;
[0026] FIG. 9 is an exemplary structure of a rear projection television.
[0027] It should be appreciated that for simplicity and clarity of
illustration,
elements shown in the Figures have not necessarily been drawn to scale. For
example, the dimensions of some of the elements are exaggerated relative to
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each other for clarity. Further, where considered appropriate, reference
numerals have been repeated among the Figures to indicate corresponding
elements.
DESCRIPTION OF THE EMBODIMENTS
(0028) Detailed descriptions are disclosed herein; however, it is to be
understood that the disclosed embodiments are merely exemplary of the
invention, which may be embodied in various forms. Therefore, specific
structural
and functional details disclosed herein are not to be interpreted as limiting,
but
merely as a basis for claims and as a representative basis for teaching one
skilled in the art to variously employ the present invention in virtually any
appropriately detailed structure.
(0029) Reference will now be made in detail to that disclosure, which is
illustrated
in the accompanying drawing (Figs. 1-8).
(0030) As shown in FIG. 1 the optical system 10 of is a solid-state projector
consisting of an illumination subsystem 32 and a projection subsystem 30. The
illumination subsystem 32 illuminates at least one micro-display 36 with light
from a plurality of rid 12, green 14, and blue 16 light emitting diodes (LEDs)
arranged in separate color groups. The projection subsystem 30 images the
output from the LCOS micro-display 36 on to a reflective or possibly
transmissive
viewing screen for front or rear-view projection television (RPT1~. Gray scale
and
color are created by temporally dithering the LCOS micro-display 36 in
conjunction with the LEDs. Thus, SXGA resolutions of at least 24-bit color ,
at
least 1280x1024 resolution, and at least 60Hz frame rate are easily achievable
with this system. The illustration of a television is not a limitation, a
display
without the capacity to receive a broadcast, cable satellite or other
television
signal is within the scope of this disclosure. Additionally, those skilled in
the art
will recognize that organic light emitting diodes (OLEDs), solid state lasers,
lasers, and other source of a light within a narrow bandwidth may be used in
place of one or more of the LEDs. In addition, the LEDs may be of any color or
produce light at any wavelength and/or any band of wavelengths.
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[0031] The illumination subsystem 32 is a critical or Abbe illumination system
that images red, green, and blue (RGB) LED sources, 12, 14, and 16
respectively, to the LCOS micro-display 36 or potentially any other general
type
of spatial light valve, modulator, or digital light processor. The critical
illumination
system, attributed to Emst Carl Abbe's use in microscopy, is an illumination
system where the source is imaged directly onto the object. The designed
illumination system 32, in addition to being a critical illumination system,
may be
doubly telecentric. The double telecentricity is an important characteristic
that
optimizes, in particular, light processing by the LCOS micro-display 36 or
other
angular dependant micro-displays, while accommodating the telecentric
configuration of the LEDs.
[003] As will now be explained with reference to FIG. 2, a telecentric pptical
system is one where the aperture stop 60 is located at a focal point of the
optical
system causing either the entrance pupil or the exit pupil to be located at
infinity.
The aperture stop 60 is the physical stop that limits the amount of light or
cone of
light that propagates through the optical system. The entrance pupil is the
image
of the aperture stop formed by all active op~cal elements preceding the
aperture
stop 60. The exit pupil is the image of the aperture stop formed by all active
optical elements following the aperture stop.
[0033] An optical system exhibiting object space telecentricity has its
aperture
stop s0 located at the rear focal point of the optical system or lens 62. The
entrance pupil is therefore at infinity in the object space 64 (a space so
called
because it is where the object is normally located). An incident chief ray 66
propagating from an object point 68 parallel to the optical axis will travel
through
the center of the aperture stop 60 to the image plane 65. The chief ray 66, by
definition, is the ray from an object point 68 that propagates through the
center of
the aperture stop 60 and hence the entrance and exit pupils since they are
images of the aperture stop. A chief ray 66 from another object point 70 will
propagate in a substantially similar manner. In the case of object space
telecentricity, if the object points are shifted, the resulting image point
magnifications do not change and the points are only blurred in the image
plane.
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[0034~Similarly, an optical system exhibiting image-space telecentricity has
its
aperture stop 80 located at the front focal point of the optical system or
lens 82.
The exit pupil is therefore at infinity in a space _ where the image is
normally
located. An incident chief ray 86 propagating from an object point 88 will
travel
through the center of the aperture stop 80 exiting parallel to the optical
axis at the
image plane 85. A chief ray from another object point will propagate in a
substantially similar manner. In the case of image space telecentricity, if
the
image plane is now shifted, the resulting image points magnification do not
change and the points are only blurred in the image plane. Finally, a doubly
telecentric system 90 combines the advantages of the object space
telecentricity
and image space telecentricity, and as illustrated in FIG. 2.
(0035~The micro-display 36, in one aspect, uses ferro-electric liquid crystal
technology to switch the state of polarization of the incident light 37 in the
plane
of the cell. However, the effectiveness of the micro-display's polariza~on
retardation and associated state switching is a function of the path length of
the
light in the ferro-electric material. In other words, the micro-display'36
operates
best when light is normally incident on its active plane and all the rays of
light
travel nearly the same optical path length in the ferro-electric material.
This is
generally true of all liquid crystal based micro-displays. Fof example,
although
the micro-display 36 (such as an LCOS micro-display) can accept up to a 25
degree off axis beam (f/1.2), it performs best and produces best contrast at
f/3 or
approximately a 10-degree maximum incident angle. The f/number is an
indication of the light gathering capabilities of an optical system. Optical
systems
with smaller f/numbers collect more light than large f/number systems. The
image space flnumber is defined as the ratio of the effective focal length of
the
optical system divided by the entrance pupil diameter. However, in a typical
illumination system it is not uncommon to have f numbers very close to f/1 in
order to maximize the amount of light illuminating the object.. The chief ray
at the
edge of the source in a non-telecentric system enters the micro-display 36 at
a
large angle and will be switched significantly differently than an on-axis ray
from
the center of the source. Thus, it is generally of interest to send light into
the
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LCOS micro-display 36, or any similarly behaving micro-display 36, at near
normal incidence from all points of the source. An illumination subsystem 32
that
provides image-space telecentricity at the LCOS micro-display 36 or any
similar
type of micro-display 36 forces the chief ray from each LED (12, 14, 16) to
illuminate the micro-display 36 perpendicular to its plane. The chief ray in
this
case is the ray of light from object points on each end of the sources that
propagates through the center of the aperture stop.
[0036) Thus, light is incident across the micro-display 36 aperture more
uniformly resulting in a more uniform distribution of light that is output
from the
micro-display to the projeckion screen. However, although the bundle of light
around the chief ray may experience slightly longer path lengths, through the
micro-display material, the majority of light from a particular source point
will be
much closer to opfimurn conditions than that from a non-tetecentric system
operating at a similar f number.
[0037) LEDs (12, 14, 16), typically, are arranged on an electrical board with
their emission axis perpendicular to the board. Such a light emitting source
is
considered a telecentric source, thereby suggesting the use of 'a telecentric
optical system for imaging purposes. In this situation, the optical system
will have
an object-space telecentricity. Thus, an illumination subsystem that provides
object-space telecentricity, at the LEDs, forces the chief ray from each LED
to
emit parallel to the optical axis of the LED. In other words, a ray emitting
from
the center of the LED parallel to the optical axis is forced to be the chief
ray.
[0038) The present invention, in an exemplary aspect, utilizes both object and
image space telecentricities to accommodate the requirements of the micro-
display and inherent LED layout on the electrical board. FIG. 3 illustrates
the
doubly telecentric operation of the projector and the path propagation of the
light
rays emitted from the LEDs (12=, 14, 16~. The illumination system may include
glass, plastic, aspheric, or Fresnel condenser lenses (20, 26, 28) to image
the
sources to the micro-display 36. The current system uses, in an exemplary
nature, three Fresnel lenses (20, 26, 28), whose Fresnel side is adjusted to
minimize illumination system aberrations. Standard glass lenses, in addition
or in
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lieu of Fresnel lenses, could also be used. However, it is easier to
manufacture
larger aperture, lower f/number Fresnel lenses than equivalent glass ones.
Furthermore, the Fresnel lenses are easily aspherized to correct for spherical
aberration, and are thin, lightweight, and less expensive than glass condenser
lenses.
[0039] The illumination subsystem 32 includes a solid state source of red,
green, and blue LEDs (12,14,16). In one implementation , an array of each
color
of red and green and blue LEDs are used as sources. The LEDs are
arranged in an array on the electrical board. Another implementation,
described
in detail below, uses LEDs of each color arranged, for example, in an
hexagonal
packed array. The arrays are composed of red and green and blue LEDs.
Alternatively, any LED having a selected waveband and output power may be
used. The red LED die may be enclosed with an appropriate encapsulent
material within the hemispherical dome of the collection optic. The green and
blue LED die also could be enclosed with an appropriate encapsulent material
within a similar hemispherical dome. The green and blue LEDs could be four die
arranged in a 2 x 2 die matrix.. These LED may also have their die enclosed in
an appropriate encapsulent material within a similar domical cavity. The
encapsulent is necessary to provide an index matching material between the die
emission surface and the collection optic with used in total internal
reflection.
[0040] All of the LEDs in the array emit with a hemispherical Lambertian
emission pattern. A Lambertian emission pattern emits with equal brightness in
all directions around the hemisphere vuhile exhibiting a cosine fall off in
intensity
as a function of angle from the normal of the emission surface.
[0041] However, a fundamental problem of using such LEDs is capturing the
available light from the LEDs and concentrating it into an area and emission
angle that can efficiently and physically be imaged by the critical
illumination
system to the micro-display 36. The hemispherical dome lenses are
substantially
large and limit the collecfion and ultimately the concentration of the light
from the
LEDs (12, 14, 16) on the micro-display 36. The theoretical thermo-dynamic
limit
of light concentration, called the conservation of brightness or throughput or
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etendue, is the product of the emission area of the source and its emission
solid
angle and is conserved as light propagates through the optical system. Small
area sources with large emission solid angles cannot be forced, for example,
into
narrow emission solid angles with the same emission area.
[0042] Thus, compound hyperbolic concentrators (CHCs) can be used to
optimize the collection efficiency of light off of the LEDs, which may be in
the
form of a planar surtace. Compound hyperbolic concentrators and their more
common relative, compound parabolic concentrators (CPCs) were originally
developed as solar concentrator technologies concentrating solar energy to a
detector. When used in reverse (i.e., LED replacing the detector), they become
highly efficient illuminators or emitters. As such they will be referred to
hereafter
as compound hyperbolic emitters (CHEs). The CHEs, in one implementation, are
designed to achieve optimal~total internal reflection (TIR), thereby
maximizing the
light collection efficiency from the LED die and subsequently maximizing the
emission efficiency of the combined die and CHE system. The CHEs fit over the
actual LED die, but with the hemispherical lens removed from the briginal LED
package. The surface of the CHE is inherently designed to reflect fight by
total
internal reflection. The cavity at the bottom of the CHE was filled with an
index
matched encapsulent, which coupled light from the die directly to the CHE.
[0043] In one implementation , the red, green, and blue CHEs were different
from the green and blue CHEs. This was due to the difference in size of the
die.
Furthermore, some CHEs are truncated in length to limit their output apertures
to
accommodate magnifying their output to the micro-display. The non-truncated
output aperture size is directly related to the input aperture size through
the
following equation.
[0044] CHE Output Radius - C~ Refractive Index x CHE Input Radius
Sin~CHE At~le
[0045] The truncation only limits the theoretical emission efficiency to about
90%. This number has been verified empirically. As can be seen from FIG. 4,
these CHEs are symmetric in shape.
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(0046 A quad CHE has bilateral symmetry as shown in F1G. 5A-5D. Such
CHEs are generally a "quad" CHE composed of four separate CHEs.
Additionally, some green and blue die are twice as large as the red die and in
fact are made of four individual die.
(0047 Positioning a single CHE over the larger arrays of four individual die
produces an excessively large CHE output size that is ultimately inconsistent
with
that required to image light from the CHEs to the micro-display 36. The quad
CHE can reduce the CHE output size while still maintaining reasonable emission
efficiency. Furthermore, the quad CHE consists of four individual CHEs, each
CHE is centered on the corner "CC" of each of the four dies 119a -119d LEDs
to completely cover all the LEDs. Finally, the surfaces of adjacent CHEs trim
each other along planes centered on the quad CHE. These devices must also be
truncated and the truncation and trimming limits the theoret<cal emission
efficiency to 65%, which also has been verified empirically.
(0048 As can be seen from FIG. 5A- 5D, in an exemplary implementation one
end '110 of the quad CHE includes circular overlapping surface ends 116 from
each of the individual CHE (CHEF, CHE2, CHE3 and CHE4)ends 116a-116d. This
approach ensures complete coverage of the dies 1 ~ 9a-119d with the four by
four
arrays. Of course, this design may be adapted to cover arbitrary number of
solid
state light sources of arbitrary shape, output power, and wavelengths. In this
aspect of the invention, the overlapping surfaces (or apertures) at the end
110
may be of arbitrary shape (e.g., square, triangular, etc.) and arbitrary size
to
ensure complete coverage of an arbitrary shaped dies having the LEDs.
(0049 Thus, a single CHE, possibly with a clover configuration as shown in
FIG.
5C and 5D, (or a triangular, square, circular, or any arbitrary polygonal
shape)
could be oriented with the die in a manner to maximize collection efficiency
of the
CHE and thus maximize emission efficiency of the combined die and CHE
system. Specifically, as can be seen in FIG. 4C, the end 520 of the CHE may be
oriented over the die to completely encompass the LED die 12 . The end 522 of
a single GHE with a quad or clover configuration is shown in FIG. 4D. The
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corners "CC" of the die 12 are substantially oriented in the center of each
lobe
522a-522d.
[0050] In another aspect of the invention, multiple CHE's may be combined to
yield a configuration of end surfaces having an arbitrary configuration,
instead of
the clover configuration (e.g., pentagonal, hexagonal, etc.), and the
periphery of
the back end surfaces of the CHE's may be oriented about the comers/edges of
the die to maximize collection efficiency of the CHE's and thus maximize
emission efficiency of the combined die and CHE system. An example of the
clover configuration formed from the quad CHE is shown in FIG. 5C
encompassing the dies comprising, for example, blue and green LEDs.
[0051] Light from the red, green, and blue (RGB) LEDs (12, 14, 16) has to be
recombined before it is processed by the micro-display 36 in the present
system
10. Color cubes and dichroic filters may be used for color recombination and
splitting and in tri-color liquid crystal display (LCD) projection systems.
[0052] The color cube, sometimes called an "X-cube", is essentially a dichroic
beamsplitter ,composed of four glass prisms coated with special coatings along
the prism sides but not necessarily along the prism's hypotenuse. When the
prisms are glued together their sides form the coated diagonals of the cube
and
hence the name X cube. An X-cube is typically located very close to the LCD
and
hence is required to be of a high optical quality. However, major drawbacks of
a
color cubes include cost, size, and weight.
[0053] Dichroic filters are generally used to split light from a polychromatic
(white) source, typically a discharge lamp, into component red, green, and
blue
(RGB) colors, and are typically located at the source side of the illumination
system. The separate colors, after processing, are then recombined by the
color
cube. However, the major draw back to dichroic filters is their poorer optical
quality as compared to color cubes and the fact that they split two colors and
not
three at a time.
(0054] A dichroic X (DX) structure, as shown in FIGs. 1, 3, or as illustrated
in
detail in FIG. 6 is designed, in an exemplary aspect of the present invention,
to
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recombine the RGB LED light outputs before it is processed by the micro-
display
36. The DX structure 21 is constructed by cutting either a red dichroic filter
19 or
a blue dichroic filter 18, or gradient dichroic filters, in half. Gradient
dichroic filters
may be used instead of uniform dichroic filters to compensate for undesired
color
shifts caused by reflection and transmission variations as a function input
beam
angle of incidence on the dichroic filter. The end of each half is then
secured to
the middle of the other filter, forming another "X" but with thin, glass plate
dichroic
filters. The dichroic filters are not required to be of high optical quality
since they
are located in the illumination end of the present system 10, and not in the
image
path. The obscuration created by the joint between the halves is substantially
small and, furthermore, is not in a conjugate plane to the micro-display 36.
This
and a homogenization component within the optical system mitigate any non-
uniformity created by the seam 23.
(0055 A fold mirrpr 22 is included in one embodiment of the illumination
system
to fold the optical path to make a substantially compact system. The front
surface
of the fold mirror 22 may be enhanced with an aluminum coating'to minimize
reflective loss.
[0056 A diffuser 34 is placed just before the third Fresnel condenser lens.
The
diffuser homogenizes or makes uniform any non-uniformity from the RGB LED
sources. The particular diffuser used, for example, in this design is a Light
Shaping Diffuser (LSD). Plastic or ground glass diffusers could be used as
well,
the choice of diffuser is dependant on the use and other parameters of a
system.
[0057] The diffuser's position in the optical system optimizes homogenization
of
the light reaching the micro-display 36. Placing the diffuser at the source
ends
near the CHEs, for example, which is a conjugate position of the critical
illumination system, does not provide for the best light uniformity output. A
position closest to the final optical element where the beam size (or beam
spread) is at a maximum, on the object space side of the element, provides
optimum homogenization irr this particular design. Diffusing light from the
individual LEDs at their maximum spread in the optical system provides optimum
local homogenization of the light output from the LEDs at the micro-display
36.
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[0058] In another implementation, the fold mirrors may be eliminated, as shown
in FIG. 7, to minimize optical losses by appropriately positioning the CHE-LED
combination, the DX structure 21, the Fresnel lenses 26, 28, and the diffuser
34,
in relation to the polarization beam splitter 24 and micro-display 36.
[0059] Light from the RGB LED sources is non-polarized or natural polarized.
However, light incident on the LCOS micro-display must be linearly polarized.
Furthermore, the micro-display may be a reflective and not a transmissive
device. Hence, a polarization component 24 is (i) used to linearly polarize
the
light entering the micro-display and (ii) reflect the orthogonally polarized
component from the micro-display onto the projection lens. Alternatively, the
system may be designed such that the LEDs emit polarized light. In this case,
the
polarizing beam splitter may be replaced with a regular beam splitter.
[0060] In one implementation the polarizing beam splitter 24 may be a wire
grid
(for e.g" the polarizing beam splitter 24 could be the beam splitter
manufactured
by Moxtek). The polarizing beam splitter 24 serves simultaneously as polarizer
as well as a beam splitter. The wire grid type polarizer is thin,'
lightweight,
relatively inexpensive, and does not introduce a significant, additional glass
thickness into the illumination or the projection optical path. Furthermore,
wire
grid polarizers generally accept smaller f/number beams (larger or wider
acceptance angles), have high extinction ratios, and higher transmission and
reflection of linearly polarized light than the typical polarization beam
$plitters.
[0061] In an another implementation, other types of polarizers could be used,
such as a cube beam splitters with a polarization coating optimized for faster
optical systems. Alternatively, there could also be a wire grid or other type
of
polarizer to pre-polarize the light incident on a polarization beamsplitter.
[0062] The micro-display 36 in one aspect may be an SXGA color reflection
mode Liquid Crystal Display (LCD) capable of displaying full color computer or
video graphics with a substantially high spatial resolution . The liquid
crystal on
silicon (LCOS) device uses a ferroelectric, as opposed to twisted neumatic,
structure to switch the state of incident polarization very rapidly. Gray
scale and
color are achieved by temporally dithering the LCOS micro-display in
conjunction
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with the LEDs. Other types of micro-displays could be used in the current
embodiment. For example, any single-chip technology such as Texas
Instruments DLP may be used for at least one micro-display. Alternatively, the
microdisplay(s) could be either LCOS, FLCOS, HTPS, or MEMS based,
[0063 The light trap 32 is a light trapping box designed to suppress the
orthogonally polarized non-signal light reflected from the polarizing beam
splitter
24. This light, if not suppressed, will contribute to significant contrast
reduction if
it is reflected or scattered back into the signal beam path 39.
[0064 The light trap 32 is composed of an anti-reflection (AR) coated black
glass and a highly absorptive black wall. The AR coated black glass is
oriented at
45 degrees with respect to the incident light. Most of the light is
transmitted
visible light that enters the AR coated black glass and is highly absorbed as
it
propagates through the absorptive material. The small portion of remaining
reflected visible light propagates to the highly absorptive black painted
wall. Any
back scattered light is scattered to the AR coated black-glass where the
majority
of this small, amount of light further absorbed by the absorptive glass. The
trap
cavity and associated aperture is designed to block a direct stray light path
back
to the microdisplay and the imaging path. Many orders of magnitude in stray
light
reduction are achievable with this arrangement. Additional folds can be added
to
further suppress any back-scattered component.
[0065 The projection lens 30 images the micro-display output on to the viewing
screen. The projection lens 30, in one aspect of an implementation , could be
a
nine element in six group, f/1.75 lens, and is designed to project
approximately
40-inch diagonal image at a distance of eround 8 feet. The projection lens may
also contain a wire-grid linear polarizer at the aperture stop position. The
polarizer in the projection lens would be needed to improve the contrast ratio
of
the signal after refection from the wire-grid beamsplitter. The contrast ratio
off of
this component is typically only about 20:1-50:1. The linear polarizer in this
particular design is placed at the stop because this space occupies a minimum
area and the angles of incidence of the light are a minimum. The linear
polarizer
CA 02522616 2005-10-17
WO 2004/097516 PCT/US2004/013103
may also be rotated independently of the lens barrel to accommodate contrast
ratio changes as the lens is rotated to change focus.
[0066 Furthermore, the system employs a dynamic DC power amplifying circuit
200 that provides specific current to individual LED (solid state light
sources)
color channels at specific time intervals as shown in FIG. 8. Inputs to the
circuit
are DC power 210 and a control signal 212 per LED color channel. The circuit
board 200 amplifies the DC power to preset current limits per LED color
channel,
and can be controlled either digitally or manually, and as active control
signals
are received the amplified DC power is supplied to the corresponding LED color
channel resulting in LED light output for that specific channel. The result is
actual
LED light output that can be controlled digitally, and timed LED light output
that
can be color sequential to a corresponding frame on a micro-display. This is
done very differently from traditional lamp ~ designs where AC power is being
supplied to the lamp resulting in light that is outputed at a constant
frequency.
[0067 The general layout of a rear projection television, or other visual
display,
which includes one of the application areas, for the present invention is
shown in
FIG. 9.
[0068 It is to be understood that other embodiments may be utilized and
structural end functional changes may be made without departing from the
respective scope of the present invention. Possible modifications to the
system
include, but are not limited to, smaller or larger die size for the LED's
thereby
permitting a~ non-quad CHE to be placed in conjunction with said LEDs, a
signal
processing means for filtering the signal to the micro-display for improving
the
uniformity of the light signal and for improving the signal-noise-ratio.
16
