Note: Descriptions are shown in the official language in which they were submitted.
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PROJECTOR SYSTEM WITH LIGHT PIPE OPTICS
Field Of The Invention
The present invention relates to optics and more
particularly to the light engine of an optical projector
system.
Background Of The Invention
At the present time optical projector systems are
widely used in business, educational and consumer
applications. For example, slide and motion picture
projectors are used to show images from film on screens;
projection TV may use one or three LCD panels (Liquid
Crystal Display) (LCLV-Liquid Crystal Light Valve) and
other projectors may use a computer-controlled LCD. The
light for projector systems is provided by a "light engine"
which generally consists of a light source, for example, a
light bulb, a reflector and one or more lenses to direct
the light on the "image gate" such as an LCD panel or film
gate. In general it is desirable that the light engine
have the following characteristics: (1) the light it
provides should be bright; (2) the light engine should not
produce heat in excess of its ability to be cooled, for
example, by a fan, in order to conserve the life of its
bulb and other components; (3) it should produce white
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light without color spots, which is especially a problem in
metal halide arc lamps; (3) the light should be spread
evenly over the image gate. Often, in commercial light
engines, the light in the center may be at least 100% but
the light in the four corners may be less than 60%; (4) the
light engine should be physically as small and as low in
cost as feasible.
An improved light engine would spread the light evenly
so that the light in the corners of a rectangular image
gate ("corner illuminance") is at least 70% of the light at
the gate's center; the colors from the lamp are homogenized
to produce white light without color spots; and there is
reduced wasted light due to "spillage" (light beams which
fall outside of the image gate).
In the article "A Uniform Rectangular Illuminating
Optical System For Liquid Crystal Light Valve Projectors",
Chang et al, Eurodisplay '96, pgs. 256-260, light from a
short arc lamp is gathered by an elliptical reflector and
transmitted through a RPGR (Rectangular Pillar-Like Glass
Rod) to a LCLV (Liquid Crystal Light Valve).
In addition, a number of other articles and patents
have described light pipes for projection systems,
including U.S. Patents 5,146,248 (column 6, lines 35-51);
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4,813,765 (column 1, lines 40-42); 5,696,865 (column 4,
lines 61-65); 5,634,704 (column 8, lines 55-58); 5,625,738;
5,059,013; 4,045,133; 3,913,872 and 3,170,980.
A solid light pipe, when used in a projection system
to transmit light from a light source to an image gate,
presents a number of problems. If the light source
produces a large amount of heat, such as an arc lamp, the
light pipe may have to be made of Pyrex (TM of Corning) or
other high temperature resistant (low thermal expansion)
glass. However, the light transmission of such high
temperature glass is poor compared to regular optical
glass, and consequently light would be lost and the
efficiency of transmission of light would be
unsatisfactory. In addition, although optical glass or
optical plastic appears clear and unblemished to the naked
eye, it generally has microscopic sized bubbles and lines.
Those microscopic sized imperfections cause an
unpredictable and undesirable color shift in the
transmitted light.
It may be difficult, or impossible, to meet a typical
specification for color uniformity of + 200°K due to color
shifts caused by the non-homogeneous solid transparent
material of the light pipe. Furthermore, a solid light
pipe, especially if made from glass, adds to the weight and
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expense of the light engine. It requires special mounting,
for example, by screws having pointed ends, and the
mounting may be delicate and expensive.
An active matrix consists of tiny picture elements
("pixels") which are switched on and off. An organic
fluid, called a "liquid crystal", is held between
transparent plates. Generally the crystals are transparent
but can alter the orientation of polarized light passing
through them when the alignment of their molecules is
changed by applying an electrical field across the
crystals.
In a color LCD having a single plate the two outside
faces of the transparent plate are coated with a polarizing
filter (sheet polarizer) so that only P (parallel to plane
of incidence) or S (perpendicular to plane of incidence)
directed light waves may pass. Each full color pixel
comprises a red, green and blue subpixel which has color
filters so that only red, green or blue light is
transmitted. In a normally open panel, when no power is
applied, light incident on the first polarizer is plane
polarized along a chosen plane. The liquid crystals, with
no power on, are aligned to twist the polarized light
through 90°. The second polarizer/analyzer is set at 90°
to the first one. In this manner, light is transmitted
along a single polarization plane through the panel when
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there is no power on. Most LCD projection panels are of
this type. When a pixel (or sub-pixel) is activated
through the application of an electric field (power on),
the polarized light will not be twisted by 90° by the
Liquid Crystal and will therefore be blocked by the second
polarizer/analyzer.
The active matrix consists of one transistor for each
subpixel, formed directly thereon, and the connecting
printed wires. The wires are generally formed in column
addressing lines and in row addressing lines. The
polarized light is derived from a non-polarized light
source, such as a bulb. Due to the filtering, only one-
half of the light output of the light source is utilized.
It is desirable, in many LCD projector systems, that
all of the light be utilized. This would result in a
brighter picture, using the same size of bulb.
Alternatively, the bulb size may be reduced, which reduces
the heat generated by the light engine. A smaller bulb may
be cooler and may have a longer life. It has been a major
goal in the LCD projection industry to develop ways to
recover part of the light in a projection system that is
not polarized in the required polarization plane. Such
light is lost from the projected beam. If successful, such
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a device, called a "polarizes doubles", will double the
overall light efficiency of a projection system to greatly
improve a projector's performance-to-cost ratio.
A number of prior patents and articles have suggested
that the unpolarized light may be separated into two
polarized beams, the polarization of one of the beams
reversed and the two beams combined. That type of system
is called a "polarizes doubles" as it doubles the amount of
light available in one polarization. Such a polarizes
doubles generally uses a Polarizing Beam Splitter (PBS)
which separates light into its two polarizations.
Polarizes-doubles systems for LCD panels are disclosed
in U.S. Patent 4,913,529; U.S. Patent 5,601,351; European
P.A. 0467-447-A1; U.S. Patents 4,798,448; 5,566,367;
5,653,520; and in the following articles: Nicholas et al,
"Efficient Optical Configuration for Polarized White Light
Illumination of 16/9 LCDs in Projection Display", Japan
Display '92, pgs. 121-124; Shikama et al, "A Polarization
Transforming Optics For High Luminance LCD Projector",
Japan Display '93, pgs. 26-29; Imai et al, "A novel
polarization converter for high-brightness liquid crystal
light valve projector", Euro Display '93, pgs. 257-260 and
Japan Display '93, pgs. 235-237; and DeVaan, Brandt,
"Polarization conversion system LCD projection, Euro-
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Display 1995, pgs. 253-256. U.S. Patent 5,777,789
discloses a light tunnel with straight sides and a
polarizing beam splitter (PBS) which is not a doubler.
The above-cited patents and articles are incorporated
by reference.
Summary Of The Invention
In accordance with the present invention there is
provided a novel and improved light engine for projector
systems. The projector system may include the following
conventional portions: a light source, such as an arc
lamp, a reflector, relay optics such as one or more lenses,
an image gate such as a film gate or LCD panel, and a
projector lens alone or with field optics. The light
engine includes the lamp, reflector and relay optics. In
addition, the present invention provides a novel
collector-reflector assembly which forms the light from the
reflector into a cone, instead of another beam shape, and a
tapered light pipe LPI (Light Pipe Integrator). The
collector optic is formed as a continuous mirror curved
surface. That surface comprises two, or more, ellipses
having different eccentricities (e) and focal points (F).
The ellipses are in profile (cross-sections in planes
through the reflector's optical axis); but cross-sections
of the reflector perpendicular to its optical axis are
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circles. The truncated end of the light cone is
positioned on the entrance pupil of the light pipe. The
relay optics are positioned between the exit pupil (exit
face) of the light pipe and the image gate. In some
embodiments the relay optics may be omitted and the exit
pupil may be positioned directly proximate the image gate.
The light pipe integrator (LPI) functions on the
principle of internal reflection. The entrance pupil
(entrance face) of the light pipe receives the conical beam
from the collection optic (reflector). The entrance pupil
is flat and round, or octagonal, in a cross-section
perpendicular to the optical axis, to best match the
truncated cone shape of the light beam. The exit pupil
(exit face) of the light pipe is flat in a cross-section
and has the shape and aspect ratio of the rectangular image
gate, for example, the LCD panel. This aspect ratio is
typically 4:3, although other aspect ratios may be
utilized, such as HDTV~s (High Definition Television) image
gate aspect ratio of 9:16.
The light pipe integrator integrates the colors from
the arc lamp and produces a homogenized, uniform color
temperature for white light at its exit pupil. The light
pipe also provides a rectangular exit face that may exactly
fit the image gate aperture or Polarizing Beam Splitter
(PBS) (aspect ratio and shape), thus substantially reducing
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the amount of light lost in spillage. The light pipe gives
an excellent distribution of light over the image gate or
input face of the PBS. The light engine consequently
produces a beam at the image gate, easily resulting in
corner illuminance (ANSI) of 70% of center, which is a
major image quality enhancement over presently commercially
available light engines. The light pipe, and the
multiple-ellipse reflector (collector optic), called
"VAREX" (TM of Torch Technologies LLC), improves color
uniformity, light uniformity and collection efficiency.
The cone angle distribution ("angle population") of the
incoming light cone from the light source sets the cone
angle distribution of the outgoing cone. It is desirable
that the outgoing beam cone angle be reduced. The goal is
to match the required angle population of the image gate,
i.e., the LCD. The preferred shape of the light pipe (LPI)
is composed of an entrance tapered section, a narrow center
section and an exit tapered section. The wide part of the
taper of the exit tapered section is toward the image gate.
The entry tapered section has the wide part of the taper
facing the light source. Such a double tapered light pipe
produces a reduction in the angle population of the
outgoing beam cone and reduces the geometric extent, which
results in the reduction of the physical length of the
light engine.
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A preferred embodiment of a light pipe LPI is a hollow
light pipe with a two tapered section shape, an entry
pupil, a center section, which is square, or an octagon in
cross-section, and a flat rectangular exit pupil of the
chosen aspect ratio.
The preferred light pipe integrator (LPI) functions~on
the principle of internal reflection. It is hollow with
mirror interior walls (internal reflection) and is not a
solid piece of glass or other transparent material (total
internal reflection). The interior walls of the LPI are
coated with a "cold mirror coating." That coating is
reflective (mirror) to visible light but passes IR (Infra
Red) light; which is undesirable heat. The LPI is formed
of flat sheets of metal or heat resistive low thermal
expansion glass. The flat sheets permit an even high-
quality cold mirror coating, which is expensive or
impossible on curved glass. The light pipe provides color
uniformity which has been measured at + 50°K.
The present invention provides a polarizer doubler for
rotating a rejected linearly polarized beam by 90°. Its
polarization axis is oriented in the same direction as the
polarization axis of the selected beam. In one embodiment,
the two beams are then joined side-by-side (not
superposition of beams). It uses a Polarizing Beam
Splitter (PBS) to collect the entire initial beam (less
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reflective and transmissive losses) and directs it through
the LCD image plane (LCD image gate) and further onto the
screen. In another embodiment the polarizes doubles PD is
within the LPI.
There are unique major advantages in using the
combination of a PD and an LPI, namely: (1) The beam
cross-sections are shaped appropriately for the particular
aperture at the image gate, which is rectangular A
rectangular beam is propagated, making the beam addition
(recombination) more efficient and optically easier to
accomplish; (2) The collector/LPI combination can be
designed to minimize the cone angle of the beam of light
incident on the LCD, thus enhancing the LCD's performance;
(3) This optical approach will reduce cost and size of the
light engine. These three factors combine to make the
light throughout for the projection system more efficient.
The gain due to the polarizing doubles is close to a factor
of two.
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Brief Description Of The Drawings
In the accompanying drawings of the present invention,
the figures are described as follows:
Figure 1 is a perspective view of the projector system
of the present invention;
Figure 2 is a top view of the projector system of
Figure 1;
Figure 3 is a perspective view of one embodiment of a
light pipe of the present invention;
Figure 4 is a perspective view of a second embodiment
of a light pipe of the present invention;
Figure 5 is a perspective view of a third embodiment of
a light pipe of the present invention;
Figure 6 is a side cross-sectional view of a lamp and
reflector;
Figure 7 is a side view of another embodiment of a
projector system of the present invention;
Figure 8 is an enlarged side view of the entrance
portion of a solid embodiment of the light pipe;
Figure 9 is a polar coordinate system showing two
collector optic (reflector) for elliptical conic sections
having the same focus F but different eccentricities (solid
and dash-dash line). The second focal points are F' and
F '
1
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Figure 10 is a chart plotting eccentricity (e) of
elliptical curves of a reflector (x axis) against
magnification (y axis) for five various collection angles
(LI) from 0° to 110°;
Figure 11 is a chart plotting eccentricity (e) (y axis)
from 0 to 1.00, against convergence angle (iE) (x axis)
from 0° to 50° for three collection angles (iI at 40°,
90°
and 120°), for a half-cone;
Figure 12 is a chart plotting convergence angle (iE) (y
axis) against collection angle (iI) at seven eccentricities
of elliptical curves from e=0.60 to 0.90;
Figure 13 is a chart plotting converging zone f#
(at iE = 120°) (y axis) against magnification (x axis) at
two collection angles (iI = 60° and iI = 40°);
Figure 14 is a cross-sectional view of a light pipe
(cross-sections perpendicular to optical axis and the same
size). It shows a simplified diagram of a light cone
entering the entrance pupil of the light pipe integrator
(hollow, mirrored interior walls);
Figure 15 is a chart plotting the length of the light
pipe required for the first reflection in mm (y axis)
against the angle of convergence (alpha) (x axis) from 0°
to 50° for the three diameters (d) of Figure 14;
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Figure 16 is a simplified cross-sectional diagram of
the tapered light pipe of the present invention (hollow)
having circular cross-sections;
Figure 17 is a chart plotting the ratio of the areas of
the entry pupil : exit pupil (RD = Dp) (y axis) against
( Dc)
ratio of cone angle f#s (Rf = fp) (x axis) where p is the
( fc)
exit pupil having a rectangular shape in the ratio 4:3 and
c is the entry pupil having a round shape;
Figure 18 is a chart plotting the f number at the exit
pupil (1.0-6.0) (left scale) at various f#s (1.0 to 2.0) of
an LPI (Light Pipe Integrator) (y axis) against the exit
pupil diagonal size in mm (x axis) for an entry pupil round
in shape and 16 mm diameter (Dc);
Figure 19 is a side cross-sectional view of an
embodiment of the LPI;
Figure 20 is a perspective view of a tapered and hollow
LPI having an octagonal entry pupil and a rectangular exit
pupil;
Figure 21 is a cross-sectional profile view of a
multiple eccentricity a set of elliptical curves forming a
continuous surface to provide a constant magnification;
Figure 22 is a cross-sectional profile view of a double
eccentricity elliptical curve;
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Figure 23 is a side view of a light engine using a U-
shaped curved LPI;
Figure 24 is a perspective view of a double-lamp light
engine for use with a motion picture film gate;
Figure 25 is Table 1;
Figure 26 is a side cross-sectional view of a double-
tapered LPI;
Figure 27 is a perspective view of a double-tapered
hollow LPI having a square entry pupil and a rectangular
exit pupil;
Figure 28 is a side view and Figure 29 is a cross-
sectional view, along A-A of Figure 28, of a metal casing
for the flat cold mirror sheets forming the light pipe of
Figure 27;
Figure 30 is a cross-sectional view of the system of
the present invention exemplifying the use of its novel
optics to provide a polarization doubler;
Figure 31 is a side view of a PBS, 1/2 wave retarder
assembly;
Figure 32 is a cross-sectional view of the doubled beam
on the LCD plate having an aspect ratio of W:H;
Figure 33 is a cross-sectional view of the beam as it
exits the exit pupil of the LPI and having an aspect ratio
of W/2:H;
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Figure 34 is a perspective view of an LPI having an
octagonal entry pupil and a rectangular exit pupil with an
aspect ratio of W/2:H;
Figure 35 is a cross-sectional view of an embodiment
using a solid LPI;
Figure 36 is a top view showing a PD (Polarization
Doubler) having equal path lengths for the P and S
components and using mirrors;
Figure 37 is a side view of a PD using different focus
points to obtain the effect of equal path lengths for the P
and S components;
Figure 38 is a side view of a PD using a convex lens to
obtain the effect of equal path lengths for the P and S
components;
Figure 39 is a side view of a PD using a concave-
concave mirror to obtain the effect of equal path lengths
for the P and S components;
Figure 40 is a side view of a PD using a glass prism in
one component path to obtain equal path lengths for the P
and S components;
Figure 41 is a side cross-sectional view of a PD;
Figure 42A is a side view of a polarizer doubler;
Figure 42B is a front view of a polarizer doubler of
Figure 42A;
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Figures 43A-43C are side cross-sectional views of
polarizes doublers within a light pipe, the Figures 43A-43C
showing alternative embodiments;
Figure 44 is a side cross-sectional view of the
polarizes doubles of Figure 42A positioned at the exit
pupil of a light pipe; and
Figure 45 is a side cross-sectional view of the
polarizes doubles of Figure 42B positioned within a light
pipe.
Detailed Description
As shown in Figure 1, the first embodiment of the light
engine of the present invention is used in an LCD (Liquid
Crystal Display Projector System). That projector system
includes an arc lamp 10 and reflector 11. The reflector 11
(collector assembly or collector optic) forms the light
into a conical beam 12 (cone) having a peak (tip) 13. The
beam is directed so that the cone peak falls within the
entrance pupil 14 (entrance face) of a light pipe 15 - LPI
(Light Pipe Integrator). The light exits from the exit
pupil 16 (exit face) of the LPI 15 and is transmitted
through the relay optics 17 consisting of first plano-
convex lens 18 and second plano-concave lens 19. In some
cases a field lens is used between the image gate and the
projection lens. Those components form the light engine.
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The relay optics 17 directs the beam onto the image
gate (entry face) of an LCD panel 21. The LCD panel 21 is
preceded in the optical path by a Fresnel lens 20 (Fresnel
lens sheet) and is followed by a second Fresnel lens 22. A
projection lens 23 focuses the image from LCD panel 21 onto
a screen (not shown). A "Fresnel lens" is a plate having
concentric grooves (about 40-300 grooves per inch) which is
molded as a thin plastic sheet and replaces the curved
surface of a conventional lens.
A light pipe (LPI) is an elongated optical element
having an entrance pupil, reflecting internal walls and an
exit pupil. Light entering the entrance pupil is
internally reflected to become homogenized (mixed). The
LPI is preferably hollow with internal mirror walls.
Alternatively, the light pipe may be a solid transparent
member of optical glass or plastic whose outside walls
should not be mirrored because it would lose its total
internal reflective property. The solid light pipe is held
in place by a knife edge or plastic screw supports and
covered (not touching) by a sheath, for example, a sheet
metal sheath.
The entrance pupil 13 is preferably square, cone-shaped
or hemispherical (if solid LPI) or flat and round or
octagonal (if hollow LPI) in a cross-section perpendicular
to the optical axis to accord with the shape of the light
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beam cone. If the LPI is solid the beam cone is truncated
perpendicular to its axis of rotation in the case of a flat
entrance pupil. The light pipe exit pupil 16 is flat and
rectangular and of the same aspect ratio as the aspect
ratio of the image gate, typically 4:3 for LCD panels.
The light pipe section closest to the LCD panel is
tapered so that the exit pupil is at least 50% larger (in
area) than the center section of the light pipe (except if
using a PBS) and the light pipe becomes larger (in cross-
sections perpendicular to its optical axis) towards its
exit pupil, i.e., toward the image gate. This tapered
shape permits an efficient transmittal of the light without
wasting light, due to spillage, at the image gate.
Preferably the ratio of the entrance pupil area to exit
pupil area is in the range of 1:1.5 to 1:5 and most
preferably in the range of 1:2 to 1:4. Preferably the
cross-sectional area of the exit pupil is at least 50%
greater than the cross-sectional area of the center
section.
In the embodiment of Figure 3 the light pipe 25 has a
round and flat entry pupil 26 and a rectangular and flat
exit pupil 27, both of which are flat perpendicular to its
optical axis 28 (dash-dot line). The aspect ratio (Width .
Height) of exit pupil 27 is typically 4:3. The light pipe
25 at the entrance pupil 26 is round and its walls
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gradually become flat. The light pipe 30 of Figure 4 is
similar, except its walls are parabolic in shape, i.e., the
walls are parabolic in cross-section profiles taken in
planes through the optical axis 28.
One embodiment of a light pipe 31 is shown in Figure 5.
In that embodiment the entrance pupil profile 32 is curved
and the exit pupil 33 profile is flat and rectangular in
cross-section.
In the solid LPI embodiment of Figure 19, for example,
the diameter "q" is 14 mm and the height "p" is 24 mm and
the exit pupil width is 18 mm. An entry portion 35 is
round (in cross-sections perpendicular to the optical
axis). The round entry section 35 extends for at least
one-third, and preferably about one-half, of the length of
the light pipe. The hemispherical profile entry pupil 32
receives a conical beam. In Figure 19 the entry pupil
profile 32a is cone-shaped.
Hollow LPIs are preferred to solid transparent LPIs for
a number of reasons. Costs are reduced because the hollow
LPI may use flat, reflective coated material that is
mechanically easy to assemble and integrate into the "light
engine" system. There are no problems with entrance and
exit pupil losses (no AR coatings needed), or heating of
the glass substrate. Sheet metal or low thermal expansion
(high temperature resistant) borosilicate glass can be used
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as a reflector substrate (the base for the internal
reflective walls). The light travels through empty air
space without scattering or other interference.
The internal reflective coating is preferably a cold
mirror coating to remove IR (Infra Red) heat from the light
beam. The removal of IR heat radiation from the beam of
light, without the requirement of using transmissive heat
filters that reduce substantially the visible light in the
beam, is a major advantage. The LPI is formed from flat
sheets of metal or low expansion glass having a cold mirror
coating on its internal surface. Such coatings are more
difficult and expensive to apply to a curved substrate
(curved glass base).
An LPI has been fabricated and tested using a cold
mirror coating (HR98C) manufactured by Optical Coating
Laboratories Inc. (OCLI) of Santa Rosa, California. It has
a reflectivity of 98.5% average over the visible spectrum.
Such coatings are typically used in flat mirrors and are
designed for a specific angle of incidence (such as normal
or 45 degrees). One gets a color shift in the reflected
light if the incident light is more than +/-15 degrees off
the design angle for the coating. This cold mirror coating
(a multilayer or dielectric coating) in an LPI makes the
angle of incidence dependence of the coating not critical.
One of the major properties of the LPI is the
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"homogenizing" of the colors within the LPI, so by the time
the beam exits from the LPI the various colors are well
mixed and color uniformity is excellent.
A suitable cold mirror coating will transmit 90% of the
IR light and reflect at least 98% of the visible light. A
ray having both IR and visible light and which is reflected
twice in the light pipe will lose at least 90% of its IR
heat and be reflected with a loss of less than 4% of
visible light.
Preferably, as shown in Figure 27, the flat glass cold
mirror sheets 130 are sheet metal and form a sheet metal
case 131. The case is the V-BLOCK light pipe and acts as a
heat sink. It may be cooled, if required, by a fan.
It is believed that as much as 95% of the heat of the
beam may be removed by passing through the cold mirror
glass and into the metal casing. Another advantage of a
cold mirror coating on the LPI is that if one takes heat
out via a "cold-mirror" coating on the reflector, the huge
variation in angle of incidence results in losses of
visible light. Of course, that is also possible to some
degree in the V-block LPI, but the angles are not so bad.
The metal casing may be sheet metal with fins and may
be die cast, also with fins. If the lamp is small the heat
may be removed by convection air, for example, through
holes in the bottom and top of the projector casing. The
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metal housing of the LPI acts as a heat sink and permits
novel forms of projector cooling to avoid noise, weight and
unreliability of a conventional cooling fan.
The metal LPI casing may be cooled by thermoelectric
cooling, e.g., Peltier effect, using a solid state
semiconductor N and P junction, see U.S. Patent 5,724,818,
incorporated by reference. Such solid-state thermoelectric
cooling is not suitable for a battery operated projector,
due to its electrical inefficiency. However, it may be
used with a plug-in (household current) projector. The
heat from the opposite end of the Peltier effect
electrocouples may be removed by a heat sink, i.e., a metal
plate with fins, or by convection air or by a fan.
A preferred hollow LPI shape has an octagonal or square
and flat (cross-section) entrance pupil, with a tapered
first section ending in an octagon or square (cross-
section). That octagon or square is attached to a second
section (center section) with a rectangular cross-section
(14 mm x 14 mm) and a larger exit pupil crass-section (exit
section) to match the shape of the PBS or image gate, as
shown in Figure 26. The overall length is 106 mm and the
entrance pupil is 28 mm in diameter and the exit pupil is
18 mm high and 24 mm wide.
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A preferred arc lamp bulb and its reflector is shown in
Figure 6. The reflector 70 (collector optic) is metal with
a mirror interior finish and its profile is elliptical in
shape (17.27 mm F (center), a = 0.746, and may be "tilted
ellipse", e.g., two elliptical sections that do not form a
single ellipse. The lamp bulb 71 is a plasma arc bulb, for
example, type HTI 404W/SE by OSRAM. The light source may
be a xenon arc lamp or an incandescent source such as a
halogen lamp. The reflector diameter "A" is 3.250 inches
(82.55 cm).
The embodiment of Figure 7 shows a configuration of the
light engine 50, but the relay lens component is
eliminated. The LCD panel 51 is placed at the exit pupil
of the light pipe 52 with a field lens 53 between the light
pipe and the panel. A field lens could also be positioned
in position 53a, following the image gate, or two field
lenses could be used one in position 53 and one in position
53a. The design of the projection lens 56 will have to
match the choice of the field optics. In this
configuration, the light pipe exit pupil is of the same
general size as the image gate, i.e., a film gate or an LCD
panel. The plasma arc lamp 54 and reflector/collector 55
may be of the type of Figure 6. The system also includes a
conventional projector lens 56 and screen 57. This
configuration is effective for smaller image apertures,
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under three inches in diagonal. The elimination of the
relay optics results in tighter packaging (shorter length)
for the projector. There is a substantial reduction in the
length of the light path.
Although the description set forth above is in
connection with an LCD panel, the invention is also
applicable to film projection systems. The following light
engine may be used in a film projection system: a light
engine comprised of (a) a plasma arc lamp (or halogen
lamp), (b) a collector optic, (c) a light pipe integrator,
(d) relay optics, (e) field optics, (f) a slide projector
film gate or a motion picture projection film gate or an
overhead projector stage, and (g) a projection lens.
The shape of the reflector (concave mirror) which
gathers the light from the lamp and directs it at the entry
pupil of the light pipe is preferably curved with two
elliptical sections. It is a concave reflector whose back
end (closed end) is formed as an ellipse having a first
eccentricity and secondary focal point Fl' and whose front
end (open end) is an ellipse having a second (and
different) eccentricity and secondary focal point F2'.
This variable ellipse reflector does not form a sharp image
(point or line focus) at a focal point, since it is not
used to form an image. Instead, it forms a fuzzy ball of
light located at, or within, the entrance pupil of the
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tapered light pipe. The internal mirrored reflecting
surface, in profile, is a nonspherical continuously curved
surface having two, or more, difference generators of
curvature (preferably ellipses) and in which the cross-
sections are circular (perpendicular to the optical axis of
the cone of light).
A typical (prior art) collector optic is an elliptical
conic section that collects the light from the lamp and
directs it to the entrance of the light pipe. Figure 9
shows such a collector and defines the various angles of
importance. The angle i(I) is the collection angle (the
angle over which light is collected and directed with the
lamp at F, the primary focus, taken as the center); the
angle i(E) is the convergence angle (the angle of
reflection relative to the optical axis of a light beam on
the reflector); F' and F'1 are the secondary foci of the
two elliptical sections of the collection optic, using the
same primary focus F but of different eccentricities.
A parametric study has been made of the elliptical
collector and a number of insights established related to
its functioning in conjunction with the LPI. For an
efficient collection of light from an arc light source it
is required that the collection angle i(I) sweep through
between 35 and 135 degrees. This angle sweep may not be
mechanically or otherwise attainable for each application,
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so the analysis considers a more modest collection angle
sweep between 40 and 120 degrees although, when conditions
allow, a collection angle between 35 and 135 degrees should
be implemented. Eccentricities of the ellipse between
e=0.60 and e=0.90 are investigated. Most practical
applications would fall in the range of e=0.60 to e=0.75.
This does not mean that special circumstances may not
indicate the use of eccentricities outside this range.
With the collection angle limited to 120 degrees,
parametric plots are made for the magnification vs.
eccentricity at various collection angles (Figure 10) and
the converging angle vs. eccentricity at various collection
angles (Figure 11). A plot of convergence angle vs.
collection angle at various eccentricities is shown in
Figure 12. The following conclusions are drawn:
1. The collector maximum convergence angle at the
collection angle of interest (120 degrees) is solely
dependent on the eccentricity of the ellipse. This is the
maximum convergence angle that a LPI will see.
2. The magnification of the focal spot depends also
primarily on the eccentricity of the ellipse, at various
collection angles.
3. Since the convergence angle gets bigger as the
focal length of an ellipse becomes smaller, an optimization
decision is made when matching a collector optic with an
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LPI. A small spot at a small convergence angle is desired.
Figure 13 shows a plot of the converging cone f# (focal
number) as a function of the magnification.
4. The light coming out from the back end (closed end)
of the elliptical reflector has the smallest convergence
angle and the highest magnification. The light coming out
from the front end (open edge) of the reflector has the
highest convergence angle and the smallest magnification.
5. The light coming out of the arc lamp is fairly
evenly distributed within the collection angle sweep
between 35 and 135 degrees.
A constant cross-section hollow LPI is shown in Figure
14A only for the purpose of explanation. A beam of light
includes angles of incidence of a (alpha) and b (beta)
represents the input from an elliptical collector optic.
The smaller angles b will obtain their first reflection,
within the light pipe wall, further down than the larger
angles a. The larger angles a will have more reflections
as they travel down the LPI. The distance down the LPI for
the first reflection of an incident ray at various angles
of convergence a (alpha) for a number of sites LPI openings
d are depicted parametrically in the plot shown in Figure
15. Subsequent reflections after the first reflection will
occur down the LPI every 2x (twice the distance from entry
pupil to the first reflection). In Figure 16 O (theta) is
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the angle of convergence. The angle a (alpha) represents a
small angle of convergence and the angle b (beta) is the
largest angle of convergence from the reflector. x is the
initial principal ray's first reflection distance in the
LPI; 2x is the distance for its second reflection; and
tan a = 2 /x and x = d/2 tan 8.
A tapered LPI is shown in Figure 16 with the wider
portion toward the film gate. Principal rays coming in at
the entry pupil (smaller end) exit at the exit pupil
(larger end) at a smaller angle. The angle of the incident
ray (e. g. a or b) is changed each time it is reflected from
the wall of the LPI by 0 (theta). A tapered light pipe is
used to change the f# of the incident beam to a larger f#,
thus reducing the angle population maximum value at the
exit of the light pipe. This is an important advantage of
the tapered LPI. Figure 17 is a plot relating the ratio of
the diagonals of a circular entry pupil and rectangular
exit pupil of a tapered light pipe of the ratio of the f#s
at its entry and exit pupils. Figure 18 is a parametric
plot of the light pipe diagonal vs. the exit pupil cone
angle for an entry pupil diagonal of 16 mm, based on a
circular entry pupil and a rectangular exit pupil.
The following conclusions apply from this study:
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1. The incoming light cone from the elliptical
collector optic spans a range of convergence angles, from
about 6 to 45 degrees, and has a variety of spot sizes.
Preferably the LPI optic uses double-taper sections shown
in Figure 26. The larger entry pupil, due to the entry
section taper, enables the LPI to gather the larger spots
(at the lower angles) while not adversely affecting the
smaller spots (at the higher angles).
2. The LPI integrates the incoming light beam in terms
of color and light distribution at the exit pupil of the
light pipe (mixing) by a number of reflections inside the
light pipe, both for the smallest and largest incident
angles.
3. The LPI shapes the beam so that the output LPI
cross-section (exit pupil) is the same shape and aspect
ratio as the aperture (image gate) to be illuminated. The
entry pupil of the LPI is fitted to a round cross-section
incoming beam, i.e., a conical beam.
The LPI changes the f# of the cone angle of the angle
population between the incoming beam and of the outgoing
beam. The "angle population" is the percent of total beam
per angle increment, i.e., if the light is 80% at angles
less than 6 degrees it has a low angle population. This is
important because the LCD panel angle population acceptance
angle is rather limited (under 10 degrees). The tapered
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shape of the LPI reduces the maximum angle of the incoming
angle population and produces an outgoing light beam having
a low angle population (at least 80% is under 10 degrees).
The goals of the LPI based light engine are to provide
the maximum possible amount of light that can be collected
from the arc lamp through the image gate (open aperture)
where the LCD panel is located at the proper angle
population. The angle population should be limited to + 15
degrees, and is preferably within + 10 degrees, and is most
preferably + 6 degrees.
The choices involving the collector optical element, an
ellipse, are between a small spot and a small convergence
angle, which are incompatible. An ellipse in the
eccentricity range between e=0.65 and e=.75 is chosen in
conjunction with a light source 3 mm long (the arc gap is
3 mm). This range of eccentricities provides small spots
and relatively large convergence cone angles. The higher
eccentricity end (e=.75) is dictated by the magnification
that can be tolerated (arc gap 3 mm). For example, at a
collection angle i(I) of 40 degrees the magnification at
e=0.75 is approximately X6. A 3 mm arc gap will be imaged
into an 18 mm spot. At e=0.60, it will be imaged into an
11 mm spot. The maximum angle of convergence for e=0.75 is
28 degrees and for an e=0.65 it is 41 degrees. The maximum
of the converging angle is dictated by the number of
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reflections in the light pipe one can tolerate (the sharper
the angle the more the number of reflections), and the
limit imposed by the critical angle of the medium of the
light pipe medium.
One of the problems with the tapered LPI, and other
LPIs, is that some of the small angles coming into the LPI
will not reflect even once and go through unmixed. This
adversely affects the color uniformity and the light
distribution on the screen. For this reason, and to
optimize the LPI performance, one LPI configuration that
optimizes the LPI functions listed above is shown
schematically in Figure 5. It is a hollow LPI. The center
section is narrow and has a constant cross-section round in
shape. The first section is the mixing section for the
incoming cone. The entrance pupil of the LPI has a cone,
or quasi-spherical, open entrance. This approach avoids
any refractive effects that would tend to reduce the angles
to the normal upon entry which would make distances between
reflections further apart and therefore require a longer
LPI. At the interface between the circular and rectangular
light pipe section (the exit section) a number of
transition shapes are possible. A preferred shape, easy to
fabricate, would require the end of the circular light pipe
of the mixing section to become a rectangle of the proper
aspect ratio with the same diameter as the light pipe.
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This, then, would be mated to the small end of the tapered
section of the light pipe whose purpose is to reduce the
maximum cone of the exit angle population. The light pipe
mixing section should be long enough and with a cross-
section small enough where the lowest converging angle of
the entrance beam can get at least one reflection inside.
Another preferred transition shape for the mixing section
of the LPI or, for that matter, for an LPI consisting
solely of a tapered section, starts with an octagonal LPI
entrance cross-section instead of circular. The octagonal
cross-section can make a smooth transition with a
rectangular LPI exit cross-section, as shown in Figure 20.
This transition geometry is most suitable in developing
good corner coverage for the light distribution on the
screen.
The fundamental trade-offs involved in the
LPI/Collector optic combination are depicted in Figures 13
and 17. Figure 13 details the relationship between the
convergence angle (listed in terms of the f#), the
magnification and the eccentricity a of the elliptical
collector optic. At low eccentricities one obtains low
magnification and large convergence angles. At high
eccentricities one obtains high magnification and smaller
convergence angles. The plot depicts the maximum
convergence angle (originating at the edge of the ellipse)
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and the maximum magnification (originating at the smallest
collection angle of 40 degrees). Figure 17 shows how the
ratio of the diagonals for the entrance circular aperture
of the LPI and the exit diagonal relate to the change in
f#s of the incoming and outgoing one angle populations.
Since the LPI exit area is limited to the size of the panel
diagonal for small LCD panels (or other open apertures) up
to about 3", the smaller the entry diagonal is the better
the cone angle populations will be when incident on the
panel (i.e., smaller cone angles).
Table 1 (Figure 25) describes the first order approach
to optimization. Three typical LCD panels have been
selected, 1.3", 2.0" and 3.0" (4:3 aspect ratio). The arc
gap size is then chosen. A popular metal halide lamp
currently in production is the Osram VIP R 270 which has
the smallest arc gap (1.6 mm) available in that power
range. It puts out 15000 lumens. Another lamp used in
this optimization is an Osram MH lamp 404W/DE with 30000
lumens at 400 watts and an arc gap of 3.0 mm.
With the small gap lamp an eccentricity of 0.75 is
chosen as a first iteration. The results listed in Table 1
show that the spot size at the entrance of the LPI has a
diagonal of 10 mm. The end result indicates that this
choice of arc lamp is a good match for the 2.0" panel; an
easy fit with the 3.0" panel and somewhat of a mismatch for
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the 1.3" panel. The cone angle population has a maximum of
28 degrees from the ellipse while the acceptable cone to
yield an f# of 3.5 at the panel is 20.3 degrees. For the
3.0 mm gap a smaller eccentricity is used to reduce the
magnification of the entrance spot into the LPI. The 3.0"
panel is a good fit with this eccentricity and arc gap.
The other two panels would reject a substantial amount of
light that resides in the higher cone angles.
For the situations where there is not a good fit in the
cone angle population, a good trade-off would be to lose
some incoming light by making the entrance diagonal to the
LPI smaller, i.e., allow only a 10 mm entry pupil
(aperture) for the 3.0 mm arc lamp. That approach would
reject some of the initial spot. However, about 75% of all
the light is concentrated within 50% of the diagonal, so
cutting the outer edge is not as bad as cutting off some of
the converging cone angles. Within the converging cone,
roughly 10% of the total light is within each i0 degree
collection zone. Preferably a hollow "V-BLOCK" (V-8
INTEGRATOR - TM of Torch Technologies LLC) LPI is used
having an entrance tapered section and an exit tapered
section, see Figure 26. That device can accommodate both
the generated spot size and its cone angle population. The
entry tapered section (the entry pupil) collects all the
light from the incoming beam. The embodiment of Figure 26
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is preferably used with the "VAREX" (TM) reflector having a
variable elliptical eccentricity and two, or more, focal
points.
One of the limits imposed on the optimization between
light source and image aperture is due to the elliptical
collector that is defined in terms of a single
eccentricity. This is a severe limitation because once one
selects the eccentricity, it determines a specific value of
the maximum convergent cone angle and the maximum
magnification for the spot. The present invention provides
a design of elliptical collectors with variable
eccentricity. It is desirable to keep the eccentricity of
the back rays collected around the 40-degree collection
angle at a small eccentricity to reduce magnification. It
is also desirable to keep the edge of the ellipse at a
collection angle of 120 or more degrees at low eccentricity
to reduce the converging cone angle population. An
elliptical curve is preferred where the back end starts at
a lower eccentricity than the front end. Figure 21 shows a
curve with a variable eccentricity derived from equal
magnification requirement for all collection angles. The
various zones focus at different points along the optical
axis, e.g., the interfocal distance for each eccentricity
is different. Clearly, such a major change in eccentricity
along the various collection angles is not practical.
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Figure 22 shows a practical example of a variable
eccentricity curve. In this case the spread of
eccentricities is limited to 0.05. The different
elliptical curves have different focal lengths,within + 2mm
of a central focal point. Even though the two beams focus
at different points on the optical axis, the LPI can still
collect all the light projected by the collector. The
light collected by the elliptical collector need not be
focused in a single focal point.
In the variable eccentricity designs an LPI may have a
flared entrance to accommodate extra collection capability
from the incoming cone of light (Figure 22). For small
panels one does not need relay optics and the optical
engine configuration can be as in Figure 7. For panels
larger than about 3.0" in diagonal, the light engine design
will follow the design of Figures 1 and 2.
Figure 26 shows the most preferred embodiment of the
LPI. The entry section 100 starts with the entry pupil
101. The entry pupil is flat (cross-section perpendicular
to optical axis 102) and may be round or octagonal. This
LPI is hollow and constructed from sheet metal mirrors.
The entry section is tapered in shape with the larger
portion toward the light source (left in Figure 26). In
the prototype the entry pupil cross-section is square, but
an octagonal cross-sectional entry pupil is preferred. The
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entry pupil area (cross-section) is in the range of 1:1.5
to 1:5 and most preferably in the range of 1:2 to 1:4, to
the smallest area cross-section (perpendicular to axis 102)
of the center section 103. The exit section 104 is also
tapered and the exit pupil 105 is flat and rectangular
(cross-section). The center section 103 is integral with
the exit section 104. In the embodiment of Figure 26, the
entry pupil profile is flat and square (28 mm x 28 mm) or
preferably octagonal, and if octagonal the entry section
transitions to a square flat which is connected to the
center section. The entry section is 21 mm long. This
embodiment is made of two pieces. At their connection
they are both square (14 mm x 14 mm). The second taper
(center section and exit section) is 85 mm long and goes
from square cross-section (14 mm x 14 mm) to rectangular
(24 mm x 18 mm - 30 mm diagonal) to match the aspect ratio
of the image gate. Its total length is 106 mm.
The reflector-collector 106 is of the double ellipse
type ("VAREX" reflector), described previously, in which
the lamp's primary focal points F1 and F2 are directed to
the secondary focal points F1' and F2'; E1 and E2 are
elliptical curves with the eccentricity of E2 greater than
the eccentricity of E1, i.e., E1 = 0.710 and E2 = 0.730 and
the radius of the reflector goes from 9.0 to 52.3 mm using
an OSRAM 404 lamp. When the VAREX reflector is used with
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the Figure 26 embodiment ("V-BLOCK") all rays over 6o will
have at least one reflection. Typically, the lowest angles
are 6-8°. An extra straight square mixing section (center
section) between the two tapers may be used for additional
mixing, if needed.
Collector optics are positioned with the lamp located
on the optical axis. Such optical elements preferably are
compound elliptical surfaces (that is, made of variable
eccentricities and focal points). Alternatively, one may
use a faceted reflector where each facet is directing the
local rays in the proper direction towards the LPI element,
or a combination of compound and faceted designs. Such
designs can be generated by computer programs and
fabricated by glass molding techniques, similar to the way
automobile headlights are made.
In Figure 23 the LPI (light pipe) is hollow, has an
entry section 71 and makes two 90-degree turns with a right
angle prism 76,77 at each turn. The entry section 71 has a
flat octagonal entry pupil (cross-section) and section 71
gradually changes from octagonal to rectangular in its
cross-sections vertical to the optical axis. The entire
light pipe 71a makes a 180° turn. The reflector 72 is
pointed in the opposite direction from the rectangular face
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73 of the LPI 74, which face 73 is proximate an LCD panel
75. In some cases using a turned light pipe may permit a
shorter and more compact projector.
In the embodiment of Figure 24 a double bulb system 80
is shown in which, if one bulb ceases to work, the
alternative bulb is turned on. The circuit 81, when
switched on, will first apply power to the first bulb 82.
If it does not light, its dark condition is sensed in
milliseconds by circuit 81, as it does not draw power. The
circuit then applies power to the second bulb 83. When
the first bulb is replaced, the circuit will again apply
power to the first bulb. In this embodiment the LPI (light
pipe) is hollow and has two branches 84,85, each of which
preferably is round or octagonal at its entry pupil cross-
section (perpendicular to its turned center axis). The
central section 91 of the LPI is rectangular (in cross-
sections vertical to the optical axis). A right angle
prism 92 is used to reflect the light from branches 84 or
85 into section 91. In addition, a light valve (not shown)
may be provided in each branch 84,85 to prevent loss of
light when the bulb at that branch is not illuminated. The
rectangular face 88 of the light pipe 89 is proximate the
film gate 90. This embodiment may be useful in motion
picture projectors as the bulbs need be replaced less
often.
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In Figure 30 of the present invention a "light engine"
is shown incorporating a Light Pipe Integrator 3 (LPI) and
a Polarizing Beam Splitter 5A (PBS). It is a "polarizer
doubler" PD as it doubles the polarized light intensity at
the image gate. Light from the lamp lA (light source) is
collected by the collector optic 2A (reflector) and
concentrated onto the entrance pupil 23A of the LPI 3A.
The LPI 3A transmits the beam of light from its entrance
pupil 23A to its exit pupil 26A via multiple reflections.
The light exiting the LPI exit pupil and PD is collected by
the relay optics 4 (convex-convex lenses) that forms the
image of the exit pupil of the LPI onto the image gate,
e.g., the LCD plate 7A. The LPI exit pupil is rectangular
and, in aspect cross-section, is half the aspect width W of
the rectangular LCD plate. In Figure 30 W indicates that
width, e.g., this view shows LCD plate 7A on its side.
Preferably, when used with a PD, the light pipe section
of the LPI closest to the LCD panel is tapered so that the
exit pupil is at least 20% larger (in area) than the center
section of the light pipe and the light pipe becomes larger
(in cross-sections perpendicular to its optical axis)
towards its exit pupil, i.e., toward the LCD plate. This
tapered shape permits an efficient transmittal of the light
without wasting light, due to spillage, at the LCD plate.
Preferably the ratio of the entrance pupil area to exit
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pupil area is in the range of 1:1.5 to 1:5 and most
preferably in the range of 1:2 to 1:4. Preferably the
cross-sectional area of the exit pupil is at least 20%
greater than the cross-sectional area of the center
section.
Figure 33 shows the cross-section of the beam as it
exits the exit pupil of the LPI 3A of Figure 2. Figure 32
shows the cross-section beam as it enters the field lens
6A. The field lens 6A directs the beam onto the
rectangular LCD plate 7A and is passed through plate 7A to
the projection lens 8A. The width of the beam is doubled
as it passes through the PD 5A. PD 5A, in Figure 30, is a
half-wave retarder and mirror assembly comprising solid PBS
cube 5A and mirror 9A. Both halves of the beam are
converted to S-polarization (perpendicular to plane of
incidence).
As shown in Figure 30, there is a small gap in the
center of the light beam directed and imaged on the image
gate. This gap can be eliminated by slightly tilting the
PBS cube 5A through a small angle "a" using rotation and/or
tilting independently the mirror 9A attached to the PBS
cube through rotation of angle "b". Rotation of the PBS
cube 5A must be performed while simultaneously all optics
along that axis are rotated so that the optical axis
alignment between the lamp, collector, LPI, relay optics
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and PBS is maintained. The image of the LPI exit pupil
formed on the image gate is made of two halves generated by
the PBS half-wave plate, mirror assembly. These two halves
can be adjusted to have any amount of overlap desired. If
the overlap is too bright, it can be diffused using a strip
of neutral density filter between the overlap and the field
lens 6A.
The functioning of one type of a PBS is shown in detail
in Figure 31. Light 30A entering the PBS cube 5A is
randomly polarized (unpolarized). The reflected beam 31A
has an S-polarization, the transmitted beam 32A has a P-
polarization. The cube 5A is not a true "cube" in the
sense that all of its faces are squares. Its input face
36A and its output faces 37A and 38A have the aspect ratio
1/2 W:H where W and H are the aspect Width and Height of
the LCD panel. For example, faces 36A, 37A and 38A have an
aspect ratio of 2:3 and dimensions of 12 mm - Width and
18 mm - Height.
The incoming beam 30A is reflected by the coating 35A
which is at a suitable angle to the input face 36A of cube
5A. The coating 35A reflects the S-component of the beam
30A to form outgoing beam 31A and transmits the P-component
of beam 30A. The coating 35A may be a multi-layer coating
formed by laminating alternating coatings of a high
refractive material, such as Ti02 and MgO, and a low
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refractive material,such as Si02 and MgF2, on the face of a
right-angle prism which is then cemented to another right-
angle prism to form the cube 5A. For example, cube 5A may
be of BK-7 glass with a one-half wave coating 35A of MgF
632.8 nm. Alternatively, the coating 35A may be a
birefringent adhesive layer, for example, an adhesive of
liquid-crystalline diacrylate and a polyamide orientation
layer (thickness 50 mm) which is rubbed. A suitable cube
5A (cubic polarizer) is described in connection with Figure
11 of U.S. Patent 2,578,680, and an alternative PBS cube is
described at Figure 2 of U.S. Patent 5,570,209, both
incorporated by reference. In this case, the P-polarized
transmitted beam 32A is passed through a half-wave retarder
plate 33A (polarizer rotator) and rotated 90° to line up
with the polarization axis of the reflected S-polarized
component. The beam 34A which is transmitted through
retarder plate 33A has an S-polarization. A half-wave
retarder plate 33A may be a layer which is coated, or
cemented, on the face 37A of cube 5A, for example, a
birefringent adhesive layer on face 37A. For example, the
one-half wave retarder plate 33A may be a suitable film on
the face of the cube of polyvinyl alcohol, polycarbonate or
polystyrene. In principle, one could choose to rotate the
S-polarized beam and end up with two P-polarized beams.
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In place of cube 5A one may employ a special design of
a PBS prism which is a modification of a Glan-Thompson
prism. It uses only a thin slab of birefringent material,
such as a liquid crystal layer, or a birefringent adhesive
layer, between two glass prisms, and is disclosed in U.S.
Patent 5,601,351 and the DeVaan article (1995) cited above.
That prism operates by total internal reflection and is
therefore suitable for all visible wavelengths and has a
large angular acceptance. The geometry of the PBS is
optimized for LCD projection. The functioning of this PBS
prism is considerably improved when used in conjunction
with the aconic collector/LPI system disclosed in the
present patent application. This improvement relates
primarily to the aspect of the invention where the two
similarly polarized beams recombine. It is more efficient
to recombine two rectangular shaped beams rather than two
circular beams, as in the system of the DeVaan article.
Generally the LCD panel (Figure 30) has an aspect ratio
of 4:3 (width-to-height). Consequently, the beam from the
PBS cube 5A, and its mirror 9A of Figure 30, would have the
same aspect ratio of 4:3 and would be rectangular, as shown
in Figure 32. The LPI 3A at its exit pupil 26A has a
cross-section which is one-half of the aspect ratio of 2:3;
however, it may be slightly larger in width to prevent a
gap between the side-by-side beams. A high quality large
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color LCD panel is 6.4 inches (22.7 cm) diagonal. It has a
4:3 aspect ratio so its dimensions are 5.12" wide and 3.84"
high. The exit pupil of the LPI would be .84" wide and
1.26" high. The exit pupil of the LPI is then magnified
through the relay optics.
In the embodiment of Figure 35 the LPI 50 is solid
(clear glass or plastic) and is integral with one-half of
the cube 5A. It terminates in a 45° face 51A which is
coated with the coating of face 36, and a 45° prism 52A
adhered thereto. This is a low-cost system, although less
efficient than a hollow tube LPI. The LCD panel may be an
active matrix system, a time-division matrix system, a
monochrome panel, a three-color or four-color panel or
other type of LCD panel using either S or P polarized
light.
A problem with the polarizer doubler embodiment of
Figures 30, 31 and 35 is that the two optical paths of the
light, after it exits from the light pipe, are not equal.
One path, for example S, will be shorter as it goes
directly through the cube 5A of Figure 30. The other path
P will be longer as it is reflected from the mirror 9A.
This has an adverse effect as the beam with the longer path
P will be larger than the beam with the shorter path. The
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image of the two halves of the light pipe exit will be
unequal and consequently will not fit properly into the
target area 9A (the LCD plate).
The path lengths are equal in the prism of U.S. Patent
5,601,351, although there will be a loss of light due to
the passage of light through the glass, or plastic prism.
A number of alternative optical systems are presented
in Figures 36-41 to make path lengths of the two beams
equal to each other. These PD embodiments may be used in
conjunction with the light pipe embodiments of Figure 30 or
with other embodiments of light pipes.
In the PD embodiment of Figure 36 the length of the
path of beam S is made longer by reflecting it from several
mirrors 50A, 51A. The P beam is converted to S by PBS cube
54A (the converted beam is labeled "CP") and reflected from
two mirrors 52A, 53A. The mirror 52A,~for CP, is closer to
the PBS cube 54A than is mirror 50A, for path S, making the
CP and S paths equal over their entire lengths. This
embodiment uses a first relay lens 55A, a second relay lens
56A, a light pipe 57A, an LCD plate 58A and a PBS cube 54A
of the type shown in Figure 31. Unfortunately, the mirrors
may increase the size, cost and complexity of the optical
system: In addition, in Figure 36, both beams CP and S
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have a long path, which may cause the light in both paths
to expand beyond the boundaries of the lens apertures,
requiring larger lenses.
In the embodiments of Figures 37 and 38, the mirrors
are held in air and are not surfaces on a glass cube or
prism. The mirror 63A is a coating, like coating 35A, but
which passes the P beam and reflects the S beam and mirror
64A reflects the S beam through half-wave retarder plate
65A.
In the embodiment of Figure 37, P path 61A and S path
62A are made equal to each other in length by placing the
focus of P before the LCD plate 60A and the focus of S
after the LCD plate 60A.
In the embodiment of Figure 38, a positive lens 70A
(convex-convex) is placed in S path 68A. In Figure 39 a
negative lens (concave-concave) is placed in P path 66A.
The lenses 66A, 67A provide a focus effect which provides
the same effective optical effect as equal path lengths for
the P and S beams.
In another embodiment (not shown) the angles in the S
path are shifted to reduce separation by making a cross-
section of the S beam non-round, i.e., an ellipse. This
may be accomplished by tilting the mirror in the S path.
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In the embodiment of Figure 40, the P path 75A is
shortened by having it pass through glass. The S path is
reflected from coating 76A, on the top angled surface of
glass prism 77A. The P path passes through the glass prism
78A and is reflected by mirror 78A, held in air. The
optical path in units of glass is..~ x M = -a x 1.5, which
is 0.5 more than in air, e.g., the optical path through
glass has the same effective length as a longer measured
path through air. The path lengths Z of P and S should be
equal, that is: 8p = 8s. If.5 - 2u (units),,.2 = 2/.5u=L/u,
the amount of glass (width of glass) in the path length Zp
would be about 4 cm. The coating 76A (reflects S) and
mirror 78A (reflects P) are at 45o angles to the beams S
and P and a half-wave plate 65A is in the front of mirror
78A to convert the P beam to an S beam.
The embodiment of Figure 41 is another polarizer
doubler. It includes a metal housing which holds the four
glass sheets in air. The housing is about 9.7 cm long
(back to front), 6 cm wide (W), 4.5 high (H), and the PBS
is sold by Philips. It divides the incoming unpolarized
light beam 81A from the light pipe into two P beams which
exit at the exit face 86A of the housing. The path lengths
of the two P beams are practically equal. As shown in
Figure 41, a beam 81A of random polarized light having an
aspect ratio of 1/2 W:H is the incoming beam.
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The S component if beam 81 is reflected from the
mirrors 82A and 83A which pass the P component. The S
component is reflected from the mirror 84A to pass through
the one-half wave plate 85A which is held by a metal clip.
The P and S components exit the holder face 86A as parallel
(upper and lower) bands both with P polarization. The
mirrors 82A, 83A and 84A are parallel to each other, and
they are angled with respect to the exit face 86A. The
mirrors 82A and 83A are similar to coating 35A (Figure 31),
except they transmit the P component and reflect the S
component. The face plate 86A (Figure 31) is a
polarization rotator (half-wave retarder plate) which
rotates the S component 90° and converts it into P
component.
If the PD of Figure 41 is used, then the exit pupil of
the light pipe should match its entry face in size (without
a relay lens) and in shape (with or without a relay lens).
In the case of an aspect ratio of the image gate, i.e., LCD
panel of Width: Height ratio of 4:3 the PD input face (1/2
W) and light pipe exit pupil will have a 2:3 ratio. For
example, a light pipe would have an entry pupil which is
flat and square (28 mm x 28 mm) and an exit pupil which is
flat and rectangular (24 mm x 18 mm).
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It is believed that the position of a polarizer doubler
within a light pipe is novel because of the expectation
that the light pipe, by its reflections, would mix the
unipolarized light and cause it to become unpolarized.
However, the inventors' experiments and computer analysis
indicates that most of the polarized light exiting the PD
within the light pipe remains polarized despite its
reflections between the output face of the PD and the exit
pupil of the LPI.
In the PD embodiments of Figures 44 and 45 the PBS
(polarizing filter) comprises two plates 120,121 forming a
wedge at a 90° angle to each other, the plates passing P
and reflecting S. Two mirrors 124,125, forming wings, are
parallel to plates 120,121 and, respectively, reflect S
(which had been reflected from plates 120,121) through the
one-half wave retarder plates 122,123 (polarizer rotator).
Alternatively, and not shown, the two plates 120,121 may
instead be four such plates formed as a four-sided pyramid
with four mirrors parallel to the four plates and with four
retarder plates formed as a frame (front view) around the
pyramid.
This type of PD may be positioned at the exit pupil of
the LPI, as shown in Figure 44, or within the LPI, as shown
in Figure 45. The angles ~ and B may be varied and the
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areas covered by the edge and wings may also be varied.
The wing area may be reduced to increase the light per unit
area in the wings and aid distribution.
A study was conducted of the usefulness of those
polarizes doublers (PD) which increased the geometric
extent (GE) with various sizes of LCD panels and various
commercially available bulbs (lamps). In general, because
available bulbs have a minimum arc gap of 1 mm (Philips
UHP-X under development), it was found that an LPI, without
a PD, produced more light to small size pixels (1.3 inch
diagonal and below) than using a PD. For example, the
bulbs UHP 100, UHP 120 (Philips) can be used without a PD
for a 1.3 inch panel. However, fox larger panels (over 1.8
inches diagonal), the combination of LPI and PD produced
the most effective light to the LCD panel.
However, when the PDs of Figures 43A-43C are used
within a light pipe, they do not increase the geometric
extent. Therefore, those PDs are particularly useful with
small panels (under 1.8 inches diagonal).
The embodiments of polarizes doublers of Figures 43A-
43C are especially useful when positioned within a hollow
light pipe (LPI). Unlike other PDs, they do not increase
the cross-sectional area (the geometric extent), e.g., the
areas of their input faces are the same as the areas of
their output faces. This is especially useful when the PD
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is within the light pipe as the light pipe need not double
its cross-sectional area (cross-section perpendicular to
optical axis) at the PD.
In the embodiment of Figures 43A the light pipe (LPI)
100 is hollow and has a rectangular cross-section 101. The
unpolarized light 102 from the LPI entry pupil is polarized
into P polarization by the polarizer doubler 103, which
consists of angled coated plates 104,105 (PBS-passing P and
reflecting S) and one-half wave retarder plate (polarizer
rotator) 106 which converts S into P.
In the embodiment of Figures 43C the light pipe
rectangular cross-section 101 has an angled PBS plate 110
(polarizing filter) passing P and reflecting S, and a one-
fourth wave plate 111 which reflects S and converts it into
P.
In the embodiment of Figure 43B a polarizing beam
splitter plate 130 (PBS) reflects S and passes P. The S
component is reflected from one-quarter wave plates 131,132
which converts S to P. There would be four such quarter-
wave plates on the four inner walls of the light pipe
rectangular section.
