System for controlling contrast in projection displays

An optical projection system has a display cell that is configured to project a display image in response to light striking the display cell. The system has a light source that is configured to project a beam of light and a lens array that is positioned to intercept the beam and generate multiple beamlets of the beam. An aperture assembly is positioned to intercept the beamlets and is configured to selectively control which portions of the images contained in the beamlets strike the display cell.

BACKGROUND OF THE INVENTION 
The invention relates to controlling contrast in an optical projection 
system which uses a display cell. 
Unlike an overhead projector which uses a transparency or slide to generate 
an image (e.g., a figure or photograph) on a screen, a typical liquid 
crystal projector generates the image via a liquid crystal display cell. 
The display cell is electrically controlled which allows, for example, a 
portable computer (interfaced to the display cell) to generate and revise 
the image during a presentation. 
The display cell may be reflective or transmissive. If transmissive, light 
used for illumination passes through the display cell to project the image 
upon the screen. If the display cell is reflective, the light is 
selectively reflected off a back surface of the display cell to project 
the image on the screen. 
Both display cells take advantage of the fact that light may either be 
scattered, absorbed, or reflected by subregions of the display cell, 
called pixels, to form the image. Each pixel is electrically placed in 
either a scattering state or a clear state. In the scattering state, the 
pixel scatters and/or absorbs the incident illumination light and appears 
dark in color. In the clear state, the pixel reflects the incident light 
(for a reflective display cell) or allows the incident light to pass (for 
a transmissive display cell) to appear bright in color. 
The light used for illumination typically is generated by an arc lamp which 
is located near a parabolic mirror. The mirror collimates the light 
generated by the lamp and directs the collimated light toward the display 
cell. A portion of the collimated light is intercepted by the glass arc 
tube of the arc lamp which creates a dark spot on the display cell. To 
remove the dark spot, the projector may pass the collimated light (with 
the dark spot) through an optical integrator to average the illumination 
distribution of the light and thus, eliminate the dark spot. 
The perceived contrast ratio of the image formed on the screen may be 
defined as the ratio of the brightest spot of the image to the darkest 
spot of the image, and the perceived contrast ratio is a function of the 
brightness of the room (where the image is being viewed) and the f-number 
(a measure of the amount of light introduced into the image by the 
projector) of the projector. The effect of the f-number is particularly 
important for projectors that utilize scattering to generate images or 
where unintentional scattering occurs. However, the perceived contrast 
ratio may be different than an actual contrast ratio (the contrast ratio 
of the image coming out of the projector) set by the projector due to the 
contribution of ambient light present in the viewing environment. 
Accordingly, it would be useful if the contrast ratio of the projector 
could be controlled. The present invention provides a system and method 
that provides such control. 
BRIEF SUMMARY OF THE INVENTION 
Among the advantages of the invention are one or more of the following. The 
projector has an adjustable f-number. The perceived contrast ratio may be 
maximized for a given room brightness. Maximum contrast is provided at 
high illumination efficiency. 
The details of one or more embodiments of the invention are set forth in 
the accompanying drawings and the description below. Other features, 
objects, and advantages of the invention will be apparent from the 
description and drawings, and from the claims. 
Thus, there is provided in a first embodiment of the invention an optical 
projection system, comprising: 
a light source configured to project a beam of light; 
a lens array optically positioned to intercept the beam of light and 
generate at least one image thereof; 
at least one aperture assembly, having at least one aperture optically 
positioned to intercept the at least one image and to control the amount 
of light passing therethrough; 
a display cell having plural individually switchable pixels from an 
optically scattering state to a less optically scattering state, for 
forming a display image in the at least one image; and 
a projection lens for projecting the display image onto a screen. 
In a second embodiment of the invention, there is provided an optical 
projection system, comprising: 
a light source configured to project a beam of white light; 
a lens array optically positioned to intercept the beam of white light and 
generate a set of multiple beamlets thereof; 
at least one aperture assembly having plural apertures and optically 
positioned to intercept the set of multiple images and control the amount 
of light passing therethrough; 
a beam splitter optically positioned to receive from the aperture assembly 
the set of multiple images and to split the set of multiple beamlets into 
three sets of multiple beamlets, each set containing a light beam within a 
different frequency band; 
three display cells, one for each set of multiple beamlets, each display 
cell having plural individually switchable pixels for forming a display 
image in the respective set of multiple beamlets; and 
a projection lens for projecting a display image onto a screen. 
In a third embodiment of the invention, there is provided a method for use 
in an optical projection system, comprising the steps of: 
forming a beam of light; 
generating a set of multiple beamlets of the beam; 
intercepting the set of multiple beamlets and attenuating the amount of 
light contained in the set using an aperture assembly; 
individually controlling pixels of a display cell to generate a display 
image; 
directing the beamlets from the aperture assembly to the display cell; and 
directing the beamlets from the display cell to a projection lens. 
In a fourth embodiment of the invention, there is provided a method for use 
in an optical projection system, comprising the steps of: 
forming a beam of light; 
generating a set of multiple beamlets of the beam; 
individually controlling pixels of a display cell to generate a display 
image; 
directing the set of multiple beamlets to the display cell; 
intercepting light emerging from the display cell; 
attenuating the amount of light so intercepted using an aperture assembly; 
and 
directing light from the aperture assembly to a projection lens. 
In a fifth embodiment of the invention, there is provided a method for use 
in an optical projection system, comprising the steps of: 
forming a beam of light; 
generating a set of multiple beamlets of the beam; 
intercepting the set of multiple beamlets and attenuating the amount of 
light contained in the set using a first aperture assembly; 
individually controlling pixels of a display cell to generate a display 
image; 
directing the set of multiple beamlets so attenuated to the display cell; 
intercepting light emerging from the display cell and attenuating the 
amount of light so intercepted using a second aperture assembly; and 
directing light from the second aperture assembly to a projection lens. 
In a sixth embodiment of the invention, there is provided a method for use 
in an optical projection system, comprising the steps of: 
forming a beam of light; 
generating a set of multiple beamlets of the beam; 
intercepting the set of multiple beamlets and attenuating the amount of 
light contained in the set using an aperture assembly; 
separating the light that passes through the aperture assembly into 
different frequency bands; and 
individually controlling pixels of a plurality of display cells to form a 
color display image in the respective light beams of the different 
frequency bands; and 
directing a superposition of the light of the different frequency bands 
towards a projection lens. 
In a seventh embodiment of the invention, there is provided an optical 
projection system, comprising: 
a light source configured to project a beam of light; 
a lens array optically positioned to intercept the beam of light and 
generate a set of multiple images thereof; 
at least one aperture assembly having plural apertures optically positioned 
to intercept the set of multiple images and to control the amount of light 
passing therethrough; 
a plurality of display cells having plural individually switchable pixels 
for forming a display image in each of respective ones of the set of 
multiple images; and 
a projection lens for projecting the display images onto a screen. 
In an eighth embodiment of the invention, there is provide a method for use 
with an optical projection system, comprising: 
configuring a light source to project a beam of light; 
optically positioning a lens array to intercept the beam of light and 
generate at least one image thereof; 
optically positioning at least one aperture assembly to intercept the at 
least one image to control the amount of light passing therethrough; 
positioning a display cell having plural individually switchable pixels to 
form a display image in the at least one image; and 
projecting the display image onto a screen with a projection lens.

DETAILED DESCRIPTION OF THE INVENTION 
The quality of an image formed on a screen is quite often judged by the 
perceived contrast ratio of the image (i.e., the ratio between the darkest 
spot of the image to the brightest spot of the image). As shown in FIG. 1, 
the perceived contrast ratio is in part a function of the ambient light in 
the room where the image is viewed. To maximize the perceived contrast 
ratio of the image for both dark room and bright room viewing (where the 
brightness of the room is measured in lumens), i.e., to have a sharp image 
for all viewing environments, the f-number of a projector 10 (a measure of 
introduced screen illumination, see FIG. 3) is adjustable according to the 
present invention to accommodate both environments. 
The perceived contrast ratio of the image does not always correspond to an 
actual contrast ratio of the image produced by the projector 10. For 
example, in order to view the image in a bright room (i.e., the bright 
room region 2), a small f-number (e.g., f/5) is desired. 
To maximize the perceived contrast ratio in a bright room, where the 
darkest areas of the image correspond to the brightness of the typical 
screen (in the case where the projector is off), the illumination of the 
screen by the projector 10 may be increased by lowering the f-number. 
To maximize the perceived contrast ratio in a dark room (i.e., the dark 
room region 1), a larger f-number (e.g., f/10) is desired. As shown in 
FIG. 2, the perceived contrast ratio increases (e.g., from 50:1 to 200:1) 
with an increasing f-number (e.g., from f/5 to f/10) in a dark room, as 
ambient room illumination cast by the projector 10 upon the screen 
degrades the quality of the darkest spots of the image. For a dark room, 
the perceived contrast ratio approximates the actual contrast ratio of the 
image coming out of the projector 10. 
As shown in FIG. 3, for purposes of generating light to illuminate the 
screen, the projector 10 has a light source such as an arc lamp 105. The 
arc lamp 105 is configured to convert the light into a cylindrically 
symmetric beam 113 by a reflector 100 such as a parabolic or an elliptical 
mirror. The beam 113 is directed toward, for example, a transmissive 
liquid crystal display cell 112 (e.g., a liquid crystal display cell 
having plural individually switchable pixels formed from a 
polymer-dispersed liquid crystal material). In order to remove the 
characteristic dark spot created by the arc lamp 105 (due to a glass arc 
tube of the arc lamp 105 intercepting the beam of light 113) and to 
efficiently convert the beam of light 113 into a rectangular illumination 
beam 133 conforming to the shape of the display cell 112, the beam 113 is 
passed through an optical integrator 11. 
The optical integrator 11 includes a rectangular lens array 102 which 
intercepts the beam 113 and converts it into multiple arc images formed in 
the plane of another lens array 122. A converging lens 104 is disposed 
between the lens arrays 102 and 122 to direct the multiple arc images 
toward the lens array 122. The converging lens may be omitted or its power 
reduced if the flat surface of the lens array 102, as drawn, was instead 
made convex. In general, the power of the converging lens 104 or a convex 
surface of the lens array 102 may be altered to take advantage of the 
degree of convergence produced by the reflector 100 in the case where the 
reflector 100 is an elliptical mirror. Referring to FIG. 4, the lens array 
102 may be formed from nine rectangular coplanar lenses 160 arranged in a 
3.times.3 pattern. The optical integrator 11 serves to intercept the 
multiple images of the beam, average the images together to form averaged 
beam images, and project the averaged beam images towards the display 
cell. 
Referring back to FIG. 3, just prior to being incident on the lens array 
122, an aperture assembly or filter 115, discussed in more detail below in 
connection with FIGS. 10-13, is disposed to controllably adjust the 
f-number of the projector 10. The aperture assembly 115 may be positioned 
before or after the display cell 112. There may be more than one aperture 
assembly, e.g., one may be located before and one after the display cell 
112. In this case, the image attenuated by a first aperture assembly and 
the image attenuated by a second aperture assembly may form conjugate 
images. 
After passing through the aperture assembly 115, the light is incident on 
the lens array 122. Referring to FIG. 5, the lens array 122 may be formed 
from eight coplanar lenses 152 arranged in a circular pattern about a lens 
150. Preferably, the number of lenses in the lens arrays 102 and 122 is 
equal. Lens 150 within the lens array 122 may have the shape of an 
octahedron and may form the center piece of the lens array 122. Eight 
other lenses 152 may surround the lens 150. Each of the lenses 152 may be 
wedge-shaped with two radial edges, each radial edge adjoining a radial 
edge of an adjacent lens 152. Each lens 152 radially extends from one of 
the edges of the lens 150 towards the outer edge of the lens array 122. 
Each lens of the lens array 122 projects an image of the incident arc 
(from the lens array 102) onto the display cell 112. In this case, as the 
lenses in the lens array 102 are rectangular, the images from the lens 
array 122 are also rectangular. A field lens 110 collimates the light from 
the lens array 122 prior to the light reaching the display cell 112. 
The optical technique employed by the projector 10 is now described. In 
general, by superposition and by the action of the aperture assembly 115, 
the combination of the images projected by the lens array 122 produces a 
substantially uniform rectangular illumination beam image on the display 
cell 112. 
In particular, referring to FIG. 6, the lens array 102 breaks up the beam 
into as many constituent beams 113 as there are lenses 160 (see also FIG. 
3). The constituent beams are referred to herein as "beamlets". It should 
be noted that the invention also encompasses the case where only one 
beamlet is present. FIG. 6, depicting the beam directly before the same is 
incident on the lens array 102, does not show this broken-up nature. It 
should further be noted that the beam profile, soon after exiting the lens 
array 102, does not differ very much from FIG. 6 as the multiple arc 
images only become strongly evident at and near the focus of the lens 
array 102. 
The aperture assembly 115 and the lens array 122 may be placed at 
approximately the focal point of the lens array 102. The aperture assembly 
115 is discussed in more detail below. Here it is noted that the aperture 
assembly 115 is designed to adjust the aperture (and thus the contrast) of 
each individual beamlet emerging from the lens array 102. Thus, the number 
of apertures in the aperture assembly 115 may be equal to the number of 
lenses in the lens array 102. A converging lens 104 is used to, in part, 
limit the size of the lens array 122. The size of the lens array 122 
determines the collection angle of the subsequent projection system (and 
thus the f-number) and thus is important in setting the contrast ratio. 
Each lens 152 in the lens array 122 accepts the beamlet 113 from the lens 
array 102 and transmits the beamlet 113 in response. At the lens array 
122, the beamlet 113 is in the form of an image of the arc lamp 105. Each 
image of the arc lamp 105 may be slightly different as the arc lamp 105 
has a finite extent, i.e., is not a point source, and thus each image is 
reflected in a slightly distorted manner by the reflector 100. Of course, 
due to imperfections in the reflector 100 and inaccuracies in the optical 
system, more than one beamlet 113 may occasionally be incident on the lens 
152. However, the predominantly bright image in any given lens 152 within 
the lens array 122 should be from only one beamlet 113 from the lens array 
102. Because the beamlet 113 exits from a rectangular aperture (lens 160), 
the transmitted beamlet 113 (and the image carried therein) is also 
rectangular (at a certain distance from the lens array 122). This is the 
situation shown in FIG. 7, where the images from the beamlets 113 appear 
roughly rectangular. It should be noted that FIG. 7 shows a superposition 
of images from the beamlets 113 on the lens array 122. Generally, images 
from the beamlets 113 do not appear rectangular at a point adjacent to the 
lens array 122; they only do so once they are more focused. In other 
words, the beamlets 113 form images of the rectangular apertures in the 
neighborhood of the display 112. 
Each lens 152 is tilted, at the time when the lens array 122 is mounted, in 
a controlled and determinable fashion such that the superposition of 
images from the beamlets 113 results in a uniform rectangular image. FIG. 
8 shows an intermediate stage in the superposition process, where a 
superposition image 119 is somewhat uniform. The image shown by FIG. 8 
would appear at a location between the lens array 122 and the display cell 
112. FIG. 9 shows the final stage of the superposition process, and more 
particularly how the superposition image 119 appears at the display cell 
112. As can be seen, the image 119 is highly uniform across the display 
cell 112. The amount of tilt for the lenses 152 may be determined, for 
example, by computer simulation. 
The aperture assembly 115 is now described in more detail. As shown in 
FIGS. 10 and 11, the aperture assembly 115 may be formed from two opaque 
aperture plates 129a and 129b. Each aperture plate has a circular center 
opening 142 and, for example, eight openings 141 radially extending from 
the center opening 142. The openings 141 and 142 are sized to match the 
arc images appearing in the plane of the lens array 122. When aligned 
(FIG. 10), the openings 141 and 142 of both plates 129a and 129b allow the 
entire arc images to pass, thereby setting a low f-number (a function of 
the amount of light that passes and the focal length of the collimating 
lens 110). Rotating the first plate relative to the second plate varies 
the amount of overlap and thus varies the sizes of the plural apertures. 
In particular, when one of the plates is rotated relative to the other 
plate (FIG. 11), the overlap of the openings 141 restricts the amount of 
light from each of the beamlets 113, thereby decreasing the f-number. The 
opaque portions of the aperture plates also block any stray light that is 
not a part of the beamlets formed in the plane of the lens array 122. 
As shown in FIGS. 12 and 13, the plates 129a and 129b may be replaced with 
plates 130a and 130b having elliptical apertures 143 or other shapes such 
as circular to compensate for optical aberrations and efficiently allow 
the desired parts of the images to pass through the plates. 
In order to form the image displayed on the screen, the pixels of the 
display cell 112 that are black scatter the illumination light that 
strikes the display cell 112. To prevent such scattered light from being 
projected onto the screen (which would degrade the quality of the darker 
portions of the image), the projector 10 may have, as noted above, another 
aperture assembly 116 of similar design to the aperture assembly 115 (see 
FIG. 3). The aperture assembly 116 is located on the other side of the 
display cell 112 from the arc lamp 105. A field lens 114 intercepts images 
from the beamlets projected by the display cell 112 and projects these 
images onto a plane containing the assembly 116. The assembly 116 may have 
the same angular orientation as the assembly 115 (i.e., the openings of 
the assemblies 115 and 116 are aligned) so that the images projected by 
the display cell 112 onto the aperture assembly 116 are conjugate to the 
arc images formed at the assembly 115. A mechanical mechanism (not shown) 
inside the projector 10 may be used to ensure that the openings 141 of 
both assemblies 115 and 116 close and open together. 
For purposes of forming the image on the screen, the projector 10 may also 
have a projection lens 118. The projection lens 118 intercepts the 
beamlets from the aperture assembly 116 and forms the image in the plane 
of the screen. 
As another example, as shown in FIG. 14, another projector 200 may have a 
reflective liquid crystal display cell 201. The components of the 
projector 200 are similar to the components of the projector 10, with the 
major differences being the absence of the lens 114 and the fact that the 
components of the projector are located along two optical axes. In other 
words, the lens arrays 102 and 122 are located along a first optical axis 
(extending to the cell 201), and the projection lens 118, the aperture 
assembly 116, and the screen are along another (second) optical axis (also 
extending to the cell 201, but at an angle to the first optical axis). 
Referring to FIG. 15, the above-described system may also be used in a 
color projector 202. In this arrangement, white light (passing through the 
aperture assembly 115) coming from the lens array 122 is separated into 
different frequency bands such as the red, green and blue primary colors 
by beam splitters such as dichroic plates 203. The red, green and blue 
beams strike reflective liquid crystal display cells 201c, 201a and 201b, 
respectively, for example, which create images corresponding to the 
respective color. The resultant three color images are combined near or in 
front of the aperture assembly 116. The display cells 201a, 201b and 201c 
may also be transmissive and may be formed from a polymer dispersed liquid 
crystal material. 
Of course, it should generally be noted that the device embodied in FIG. 3 
may form a subset of a larger color system. That is, the device embodied 
in FIG. 3 may be used to process one color of light, and that two or more 
other similar devices may be used to process other colors of light. 
As noted above, a preferred display cell comprises polymer-dispersed liquid 
crystal. Polymer dispersed liquid crystal may be made by emulsion or phase 
separation techniques. Illustrative disclosures relating to the 
preparation of polymer-dispersed liquid crystals and their use in display 
cells include Fergason, U.S. Pat. Nos. 4,435,047 (1984), 4,606,611 (1986), 
4,616,903 (1986), and 4,707,080 (1987); Pearlman et al., U.S. Pat. No. 
4,992,201 (1991); Wartenberg et al., U.S. Pat. No. 5,202,063 (1993); 
Reamey, U.S. Pat. No. 5,335,101 (1994); Reamey et al., U.S. Pat. No. 
5,405,551 (1995); Havens et al., U.S. Pat. No. 5,585,947 (1996); Wu et 
al., U.S. Pat. No. 4,671,618 (1987); West et al., U.S. Pat. Nos. 4,673,255 
(1987) and 4,685,771 (1987); Doane et al., U.S. Pat. No. 4,688,900 (1987); 
and Dainippon Ink and Chemicals, EP 0,313,053 (1989); the disclosures of 
which are incorporated herein by reference. 
A number of embodiments of the present invention have been described. 
Nevertheless, it will be understood that various modifications may be made 
without departing from the spirit and scope of the invention. Accordingly, 
other embodiments are within the scope of the following claims.