High contrast illumination system for video projector

A high contrast illumination system particularly suitable for use with light valves that have an array of reflective pivotable pixels. The light valve is arranged in light path which extends from a projection lamp to a projection lens. By the action of the pivotable pixels the light is directed either into the projection lens (ON) or away from the projection lens (OFF) to modulate the light with video information. An asymmetric aperture is disposed in the light path with its longitudinal axis located along the pivot axis of the pixels. This configuration provides maximum brightness without adversely affecting contrast. The system may be used in either a monochrome or color (RGB) system and in single or multiple light valve systems.

BACKGROUND AND SUMMARY OF THE INVENTION 
This invention relates to projection video systems and specifically to 
projection video systems utilizing a light valve of the type known as a 
deformable mirror device. 
Most commercially available projection video systems utilize separate 
projection paths for each of the three primary colors. The systems thus 
require red, green and blue light valves and optical paths which must be 
accurately converged on the screen which adds to complexity and expense. 
Recently, projection video systems utilizing only a single light valve 
have been developed. One such system is a color field sequential system, 
in which the normal video field, 1/60th of a second, is broken into three 
parts, or color subfields of 1/180th of a second. 
During the three color sub-fields, the light valve is illuminated with red, 
green and blue light sequentially. While the light valve is illuminated 
with any given color, the video data corresponding to that color is 
displayed on the light valve. The eye then fuses the three color 
sub-fields into a single, full color field. The eye also fuses successive 
video fields into full motion, full color video. This system requires a 
device for sequentially illuminating the surface of the light valve with 
the three colors. The simplest of such devices is a color wheel which 
changes the color of a white projection lamp as it rotates. 
Recently, improved light valves particularly suitable for use in projection 
television systems have become available. One such device is a so-called 
deformable mirror device (sometimes called a digital mirror device or DMD) 
which is illustrated in U.S. Pat. No. 5,079,544 (the disclosure of which 
is hereby incorporated by reference as if fully set forth herein) and 
patents referenced therein, in which the light valve consists of a array 
of tiny pivotable mirror-like pixels for deflecting a beam of light either 
to the display screen (on) or away from the display optics (off). This 
device is particularly suitable for use in a field sequential system 
because its pixels are capable of being switched very rapidly. By 
additional rapid switching of the pixels a grey scale is generated. 
There is always a desire for greater brightness in projection video systems 
so that the brightness approaches or exceeds that of direct view (CRT) 
systems. In addition to a bright image, the image must also have good 
contrast so that the projected image does not appear "washed out". The 
present invention is directed towards providing an illumination system 
that has both increased brightness and increased contrast. A usual method 
of increasing brightness in a projection video system is to increase the 
aperture of the system. However, increasing the aperture generally 
increases the complexity and expense of the optical system and may reduce 
system contrast because the increased aperture permits more spurious light 
to enter the system. Where high contrast is of paramount importance many 
projection designs have decreased the aperture to increase the contrast, 
but at the expense of reduced brightness. 
The present invention is adapted to the unique geometry of the DMD system 
in order to increase the brightness of the system without adversely 
affecting the contrast of the system. As in any projection system, the 
illumination system comprises a light path extending from a projection 
lamp to the projection lens. In this invention, an asymmetric aperture is 
located at one or more places along the light path. The asymmetric 
aperture is oriented such that the longitudinal axis of the asymmetric 
aperture is aligned with the pivot axis of each of the pixels (the 
longitudinal axis of the asymmetric aperture is thus orthogonal to the 
switching direction of the 10 pixels). If greater contrast enhancement is 
desired, the aperture can be further arranged so that it is narrower in 
the direction in which light is diffracted or scattered from the structure 
of the DMD. The asymmetric aperture can be disposed in the light path 
between the lamp and the DMD or after the DMD (i.e., in the projection 
lens) or at both places. The invention is useable in color or monochrome 
systems as well as systems using multiple light valves and multiple 
projection lamps.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 illustrates a light valve 20 in the form of a deformable mirror 
device (hereinafter referred to as "DMD") having an array of reflective 
pixels 22. Each pixel 22 is mounted so as to be pivotable about torsion 
hinges 24 which are located at two diagonal corners of each pixel 22. In 
operation when an appropriate ON voltage is applied to pixel 22, the upper 
lefthand corner 26 of pixel 22 will move upwardly from the plane of the 
DMD 20 while the lower righthand corner 28 will move downwardly. 
Similarly, when an OFF voltage is applied, pixel 22 will pivot about 
torsion hinges 24 so that corner 26 moves downwardly and corner 28 moves 
upwardly. Thus the ON and OFF positions comprise two distinct movements of 
each pixel 22. Generally, pixels 22 can be pivoted by 10.degree. to either 
side of planar. In the ON position, the incident illumination is reflected 
into the aperture of a projection lens. In the OFF position, the incident 
illumination is reflected outside the aperture of the projection lens and 
thus does not reach the viewing screen. This operation is illustrated in 
FIG. 2. The present invention is also applicable to DMD's of the so called 
"hidden hinge" configuration in which the hinges for the individual pixels 
are disposed underneath the pixels. 
FIG. 2 illustrates the light path resulting from the switching of pixels 
22. In FIG. 2, the dotted line 30 illustrates the position of pixel 22 
when pivoted into its 0N position and dotted line 32 illustrates the 
position of pixel 22 when pivoted into its OFF position. The vertical 
solid line illustrates the planar position of pixels 22 (neither ON nor 
OFF). An illumination system schematically illustrated at 34 provides a 
beam of incident light 36 upon the surface of each pixel 22. When pixel 22 
is in ON position 30, the incident beam 36 will be reflected as a beam 38 
to a projection system 40 which will project beam 38 onto the viewing 
screen. When pixel 22 is activated and pivoted into its OFF position 32, 
incident beam 36 will be reflected to an "OFF" beam 42 which is outside of 
the angle of acceptance of the projection system 40, so that no light will 
reach the projection screen from the OFF position of pixel 22. 
In actuality, ON beam 38 and OFF beam 42 are not the only beams reflected 
from DMD 20. Additionally, there is a specular beam 44 which is reflected 
from the window covering DMD 20 as well as from the interpixel structure. 
Specular beam 44 has no information content and represents optical system 
"noise". Projection system 40 and illumination system 34 should be 
designed so as to reduce the effect of specular beam 44 which will raise 
the black level, thus reducing overall contrast in the projection system. 
The projection system must separate each of the four beams 36, 38, 44 and 
42 in order to provide adequate contrast, thus the angular acceptance of 
projection system 40 is generally limited to less than .+-.10.degree.. If 
one attempts to limit the angle of acceptance of the projection system to 
a greater degree, system brightness will be reduced. 
FIG. 3 is a schematic representation of a video projection system 48 in 
accordance with the present invention. The system illustrated in FIG. 3 is 
a single panel full color (RGB) system. However, the principles of the 
present invention are not limited to single panel color systems and may be 
used in monochrome systems and multiple panel color systems as well. As 
shown in FIG. 3, a projection lamp 50 is used to provide a source of white 
light. Projection lamp 50 may include an integral reflector or be used in 
conjunction with a separate reflector. The light emitted from lamp 50 
passes to a converging lens, or lens system 52 which is used to focus a 
beam 54 towards a color wheel 56. Color wheel 56 comprises a series of 
segments, or spokes, of red, green and blue transmission filters. Color 
wheel 56 is rotated about its axis so as to have its colored segments 
sequentially intercept light beam 54. After exiting color wheel 56, the 
now colored light beam 58 is acted upon by an imaging lens 60 and in turn 
passes to a condenser lens 62 and an integrator 63. 
After exiting integrator 63 beam 64 is directed to an aperture plate 66 
having an aperture 68 which has a configuration and alignment as is 
described in detail below. Also disposed at aperture plate 66 is a further 
integrator plate 70, the beam of light is then focussed towards a prism 
illuminator system 72. In prism system 72 light is reflected from a 
surface 74 which directs the light originating from lamp 0 onto the 
surface of DMD 20. In turn, prism system 72 is arranged so that the light 
which is reflected off the surface of DMD 20, and thus contains the 
modulated video information, is directed to a projection lens 76 for 
projection onto a viewing screen or surface (not shown). Prism system 72 
may be of the configuration shown in U.S. Pat. No. 4,969,730 to van den 
Brandt. Integrators 63, 70 which form a uniform illumination beam may be 
one of those shown in U.S. Pat. No. 5,098,134 to van den Brandt et al. The 
disclosures of U.S. Pat. Nos. 4,969,730 and 5,098,134 are hereby 
incorporated by reference as if fully set forth herein. 
FIG. 4 illustrates aperture configurations for use in DMD projection 
systems such as that illustrated in FIG. 3. In most optical systems, the 
optical system is designed to have an angle of acceptance of 
.+-.15.degree. or less. Optical design, particularly that of the 
projection lens, becomes more difficult as the aperture increases. Thus to 
simplify the optical system, the aperture is limited to a practical value. 
However in devices such as DMDs, the DMD itself further limits the 
aperture in the switching direction, generally to that of .+-.8.degree.. 
Generally in projection systems, the apertures are circular. A circular 
aperture having a beam acceptance of .+-.8.degree. is shown as dotted line 
90 in FIG. 4. 
However, the angular acceptance of a DMD is limited only in the directions 
in which pixels 22 pivot. Thus, the aperture of the system can be 
increased in a direction orthogonal to the direction of tilt (in a 
direction parallel to the hinges 24 of pixels 22). In FIG. 4, an 
asymmetric aperture 92 having a 15.degree. acceptance angle along the 
hinge direction with an 8.degree. angle of acceptance along the mirror 
pivot (hinge) direction is shown. In FIG. 4, arrow 94 is oriented in the 
direction of pivoting of pixels 22 and arrow 96 shows the direction of the 
hinges 24 of individual pixels 22. Thus, it is seen that the longitudinal 
axis of asymmetric aperture 92 is oriented along the hinge axis. Put 
another way, symmetric aperture 90 has been elongated along the pivot 
axis. 
FIG. 5 is a view along the optical axis of the projection system 48 and 
illustrates the orientation of asymmetric aperture 92 with respect to DMD 
20 and pixels 22. As is seen in FIG. 5, asymmetric aperture 92 is aligned 
along the axis of the hinges 24 of each pixel 22 and thus is tilted at an 
angle of 45.degree. with respect to the horizon of the image formed by DMD 
20. In general, the brightness of the image is proportional to the area of 
the aperture and the area of asymmetric aperture 92 is 2.3 times the area 
of symmetric aperture 90. The actual increase in brightness realized 
depends upon the characteristics of the light source and the collection 
optics. 
In FIG. 4, the system aperture has been extended in the direction of hinges 
24 (orthogonal to the mirror tilt direction). In a DMD system, this 
aperture configuration may lead to a reduced contrast ratio because a 
larger aperture may accept more spurious light in the OFF state. 
Furthermore, the distribution of the spurious light may be such that 
further shaping of the aperture is desirable. One contribution to loss in 
contrast is light which is diffracted from the pixel structure into the 
projection system. This diffracted light is oriented along the direction 
of the pixel edges which are at 45.degree. to the mirror tilt direction. 
Accordingly, in order to improve contrast ratio, the system aperture may 
be shaped as shown in FIG. 6. In FIG. 6, another embodiment 98 of an 
asymmetric aperture is shown. In FIG. 6, asymmetric aperture 98 has been 
narrowed in the direction 100 towards the center of specular beam 44 as 
shown in FIG. 2. This will reduce the possibility of diffraction of light 
from the specular beam into the projection beam thus raising the contrast 
of the system. As is shown in FIG. 7 the edge 102 of aperture 98 will be 
aligned parallel to the side edges of pixels 22, while the edge 104 will 
be parallel to the upper and lower edges of pixels 22. As is seen in FIG. 
6, edges 102, 104 extend at a 45.degree. angle to upper wall 106 of 
aperture 98. 
As noted above, the described embodiments are not limited to color systems 
and are equally applicable to monochrome systems. For example, a 
monochrome system similar to that shown in FIG. 3 would simply omit the 
color wheel and the circuitry used to drive the light valve for the three 
colors. Similarly, in color systems, many other methods, other than color 
wheels, may be used to generate three sequential colors of light. An RGB 
system may also be formed with three lamps, one for each primary color. 
The use of an aperture configured in accordance with the invention is also 
applicable to this type of illumination system as well, with an aperture 
disposed in each of the three illumination paths. Anamorphic lenses may 
also be used to shape the illumination beam into an asymmetric shape. In 
order to control the aperture of an optical system, a physical stop such 
as aperture plate and aperture 68 of the desired shape is placed at the 
system aperture stop or an image thereof. In addition, it is desirable 
that projection lens 76 also have an internal stop shaped to the desired 
form. As with most projection systems, the aperture stop of the 
illumination system should be imaged into the internal stop of the 
projection lens. 
Although the present invention has been described as applied to a 
deformable mirror devices, the concept can be applied to any type of light 
valve. For example, the contrast ratio produced by a liquid crystal 
display (LCD) varies as a function of the incidence angle. Generally, this 
function is not circularly symmetric about normal incidents. Thus, an 
improved system brightness and/or an improved contrast ratio can be 
obtained by using a system aperture that is not circular symmetric. 
The above-described arrangement is merely illustrative of the principles of 
the present invention. Numerous modifications and adaptations thereof will 
be readily apparent to those skilled in the art without departing from the 
spirit and scope of the present invention.