Illumination system for non-imaging reflective collector

A first concave mirror collects the light emitted by a light source in a first half space and forms a first image of the light source at a point proximate thereto. A second larger concave mirror opposite the first mirror forms second and third images from the respective source and first image. The second and third images are formed at the input aperture of a non-imaging reflector, which in turn emits the light uniformly through the output aperture thereof.

BACKGROUND OF THE INVENTION 
The present invention relates to an illumination system which images the 
arc of a light source into the input aperture of a non-imaging reflector, 
and more particularly to an illumination system for use in projection 
displays having a light valve in the form of a liquid crystal display 
illuminated by light emitted from the output aperture of the non-imaging 
reflector. 
U.S. patent application Ser. No. 137,049, and a continuation-in-part 
thereof filed Dec. 9, 1988, which are hereby incorporated by reference, 
disclose an illumination system including a light source and a first 
concave mirror positioned to collect substantially all the luminous flux. 
emitted by the light source within a first half space and to reflect it 
toward a second opposite half space. The system further includes a 
non-imaging reflective collector having an input aperture positioned to 
receive substantially all of the light emitted by the light source in the 
second half space as well as substantially all the light reflected by the 
first concave mirror into the second half space. 
The first concave mirror disclosed in application Ser. No. 137,049 is 
contiguous with the input aperture of the non-imaging reflective collector 
and reflects the luminous flux from the light source directly into the 
input aperture of the collector. Since the size of the modulation device 
is fixed and it is desired to keep the emission angle of the beam exiting 
the output aperture of the collector to a minimum, the input aperture will 
generally be relatively small. For example, for maximum emission angles 
.theta.x, .theta.y of .+-.15 degrees in air, and a rectangular light valve 
with a 4:3 aspect ratio and a diagonal of 48 mm, the input aperture must 
have a diagonal of 12 mm. This calculation is more fully discussed in Ser. 
No. 137,049 and the continuation-in-part thereof. 
SUMMARY OF THE INVENTION 
It is the primary object of the invention, to eliminate the size 
constraints imposed on the light source while optimizing the efficiency of 
light collection and limiting the divergence of light emitted from the 
non-imaging reflector. 
According to the invention, therefore, the first concave mirror is profiled 
to form a first image of the light source at a point proximate thereto. 
The system further includes a second concave mirror positioned to collect 
all the light emitted by the light source in the second half space as well 
as all the light reflected by the first concave mirror through the first 
image into the second half space. The second concave mirror is profiled to 
form a second image of the light source at a point remote therefrom and to 
form a third image at a point proximate to the second image, the third 
image being formed from the first image. The input aperture of the 
non-imaging reflector receives substantially all the light from the second 
and third images of the light source. 
In the illumination system according to the invention, the light source 
does not suffer the size constraints imposed when the first concave mirror 
is contiguous with the input aperture of the non-imaging reflector and 
maximum collection efficiency and limited divergence from the non-imaging 
reflector are to be obtained. The output aperture of the first concave 
mirror may thus be considerably larger than the input aperture of the 
non-imaging reflector, so that it can readily accommodate the envelope of 
the available arc lamps and image same with good image quality, which 
results in maximum collection efficiency at the input aperture of the 
non-imaging reflector. The larger mirror also eliminates heating problems 
which arise when the envelope is accommodated too closely. 
According to a preferred embodiment, the first and second mirrors are 
profiled so that the second and third images are accommodated by the input 
aperture of the non-imaging reflector. 
The first concave mirror may be a spherical with the light source located 
proximate to the center of the curvature thereof. The second concave 
mirror may also be a spherical mirror having a center of curvature offset 
from the center of the curvature of the first concave mirror and toward 
the input aperture by a given distance. The second and third image are 
then focussed near a point approximately twice the given distance from the 
light source. Preferably, the light source, the centers of curvature, and 
all of the images lie substantially in a common plane. 
According to a further preferred embodiment, the second concave mirror is 
an elliptical mirror having a first focus proximate to the light source 
and a second focus proximate to the input aperture of the non-imaging 
reflector. The first concave mirror may also be an elliptical mirror 
having a first focus at the light source and a second focus at the first 
image. Where both mirrors are elliptical their major axes are at least 
substantially coincident.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIGS. 1-3 illustrate the evolution of the preferred embodiment of the 
illumination system. The system incorporates a light source 10, a first 
concave mirror 20, a second concave mirror 30 and a non-imaging reflector 
40. Referring to FIG. 1, the first concave mirror 20 is spherical, the 
light source 10 being located at the center of curvature 25 thereof. The 
light source 10 emits luminous flux within a first half space 15 to form a 
first image 21 which is likewise at the center of curvature 25. The light 
source 10 also emits luminous flux in a second half space 16 and toward 
the second concave mirror 30 which is also spherical. The mirror 30 forms 
a second image 32 of the light source 10 at the input aperture 42 of 
non-imaging reflector 40, which collects the light for emission from 
output aperture 44 toward a modulating dence such as LCD 50. Note that the 
center of curvature 25 of the first mirror 20 as well as the second image 
32 are both the same distance from the center of curvature 35 of the 
second mirror 30, and further that the images and centers of curvature are 
generally coplanar. The problem with the geometrically simple embodiment 
of FIG. 1, however, is that the first image 21 is coincident with the 
light source 10 and thus may not itself be fully imaged to the reflector 
40; the light source 10 effectively absorbs some of its own light. It 
should be noted that there are light source which can be imaged through 
themselves, but these are not readily available. 
FIG. 2 again depicts first and second concave mirrors 20, 30 which are 
spherical in shape, but here the light source 10 is offset from the center 
of curvature 25 so that first image 21 is formed opposite the center of 
curvature 25 from the light source 10. Since the light source 10 will 
generally comprise an arc 12 surrounded by a cylindrical glass envelope 13 
(FIG. 4), the spacing of the arc from the center of curvature 25 is 
slightly more than the radius of the envelope. The first image 21 is thus 
clear of the light source 10 and is itself imaged by the second mirror 30 
to form a third image 33. Since the light source 10 and first image 21 are 
offset from the center of curvature 25, the second and third images 32, 33 
are indistinctly formed in proportion to the degree of offset. However, 
the distinctness of the images 32, 33 is not especially important since 
the non-imaging reflector 40 is by definition not concerned with 
transmitting an image for efficiency it is only important that as much of 
the luminous flux as possible be transmitted form the light source 10 to 
the input aperture 42 of the reflector 40. The luminous flux (light) will 
in turn be transmitted from the output aperture 44 and through the 
modulator 50 within minimum deviation angles .theta.x, .theta.y according 
to the teaching of U.S. application Ser. No. 137,049 and the 
continuation-in-part thereof. 
The embodiment of FIG. 3 further improves the efficiency of the embodiment 
of FIG. 2 by utilizing concave mirrors 20, 30 which are elliptical in 
shape. The first concave mirror 20 has a first focus 26 and a second focus 
27, while the second concave mirror 30 has a first focus 36 and a second 
focus 37. The light source 10 is placed at first focus 26 so that the 
first image 21 is formed at second focus 27. The foci 26, 27 are spaced on 
either side of focus 36 at a distance which is just sufficient for image 
21 to be formed without interference from the lamp. Since the light source 
10 and first image 21 thereof are both proximate first focus 36 of second 
mirror 30, they will be imaged thereby to the respective second and third 
images 32, 33. The images 32, 33 are proximate to the second focus 37 at 
the input aperture 42 of the non-imaging reflector 40. The major axes of 
the ellipses are at least substantially coincident, so that all foci 26, 
27 36, 37 lie in a substantially common plane. 
The foregoing is exemplary and not intended to limit the scope of the 
claims which follow.