Dark field projection display

A dark field projection display includes a light source, a spatial light modulator, and an optical system for directing light from the light source onto the spatial light modulator. The optical system includes a plurality of optical subsystems and a plurality of reflectors. Each of the optical subsystems has an input aperture and is arranged to image light from the light source, the subsystems forming a spatial distribution of source images whose relative positions are different from the relative positions of the input apertures. Each of the optical subsystems further includes a plurality of reflectors disposed at the relative positions of the source images. Each of the reflectors is arranged to reflect light from a respective one of the subsystems onto the spatial light modulator.

The present invention relates to a dark field projection display. 
The term "optical extent" as used herein is defined to mean the product of 
the size and optical divergence of a beam of light. 
Certain optical systems are inherently one dimensional in that they 
effectively integrate the optical intensity in one dimension and are 
therefore tolerant to optical extent in the orthogonal dimension. Examples 
of such systems include projection systems, for instance projection 
television systems, which modulate light by deflecting it in one dimension 
and which require light sources which are physically small in that 
dimension. Known systems of this type use small light sources which have 
lifetime restriction making them unsuitable or undesirable for commercial 
products. Metal halide lamps with relatively large electrode gaps are 
efficient in terms of conversion of electricity into light output and have 
relatively long lifetimes. Such lamps are therefore suitable for use in 
home consumer projection systems. However, it is desirable to provide 
smaller light sources with longer lifetimes so as to improve the 
performance of projection displays. 
H. Roder, H. J. Ehrke, R. Gerhard-Multhaupt, E. Ipp and Imenzel, "Full 
Color Diffraction-based Optical System for Light Valve Projection 
Displays" Jn. Display vol. 16 No.1 1995 pp 27-33 and David Armitage 
"Design Issues in Liquid Crystal Projection Displays", pp.41-51 SPIE 
Proceedings Vol. 2650 Projection Displays II Editor(s): Ming H. Wu, 
Hamamatsu Corp., Bridgewater, N.J., USA. ISBN: 0-8194-2024-7, 308 pages, 
published 1996 disclose a known type of system generally referred to as a 
Schlieren or dark field optical system. Such a system is suitable for use 
with projection displays, for instance as disclosed in "The Grating Light 
Valve: revolutionising Display Technology" D. M. Bloom, Photonics 
West/Electronic Imaging '97 SPIE and in EP 0 811 872. However, such 
systems suffer from the disadvantages described hereinbefore of requiring 
a small light source of high efficiency. 
Examples of other devices which require illumination sources which are 
small in one dimension are disclosed in "Digital Light Processing for 
Projection Displays: A Progress Report" Larry J. Hornbeck, Proceedings 
Society of Information Display 16 th International Display Research 
Conference 1009 pp 67-71 and H. Hamada et al "A New Bright Single Panel 
LC-Projector System without a Mosaic Color Filter" IDRC '94 Proceedings, 
422 (1994) and C. Joubert, B. Loiseaux, A. Delboulbe and Huignard J-P 
"Dispersive Holographic Microlens Matrix for LCD Projection" SPIE 
Proceedings Vol. 2650 Projection Displays II Editor(s): Ming H. Wu, 
Hamamatsu Corp., Bridgewater, N.J., USA. ISBN: 0-8194-2024-7, 308 pages 
(published 1996). 
U.S. Pat. No. 3,296,923, U.S. Pat. No. 4,497,015 and U.S. Pat. No. 
5,594,526 disclose arrangements for improving the uniformity of 
illumination provided by a light source in the form of a light emitter and 
a light gathering reflector such as an elliptical mirror. In such light 
sources, the light emitter itself partially obscures light directed into 
the output beam so that the output beam has an annular intensity 
distribution. The above-mentioned patents attempt to make the distribution 
more uniform by sampling the light beam from the light source in a 
plurality of optical systems to produce images with the same relative 
positions as the input apertures of the optical systems. The outputs of 
the optical systems are then recombined so as to overlap each other by a 
further optical device which is generally a lens. 
Although systems of this type are capable of producing an output light beam 
with a substantially flat intensity distribution at an image plane of the 
illumination source, they are not capable of altering the optical extent, 
ie: the product of the area of the light source and the radiating solid 
angle. 
Systems which are capable of altering or redistributing optical extent are 
disclosed in GB 2 125 983, GB 1 391 677, GB 1 353 739, EP 0 660 158, EP 0 
527 084, EP 0 493 365, EP 0 395 156, EP 0 343 729, EP 0 201 306, WO 
96/41224, WO 95/18984, WO 95/00865, U.S. Pat. No. 5,463,497 and U.S. Pat. 
No. 5,005,969. 
U.S. Pat. No. 5,662,401 and U.S. Pat. No. 5,418,593 each discloses an 
optical system for illuminating a spatial light modulator. The optical 
system re-distributes the extent of the light source, so as to improve the 
uniformity of illumination of the spatial light modulator. The optical 
systems disclosed in these documents consist of two arrays of refracting 
lenses. 
According to the invention, there is provided a dark field projection 
display comprising a light source, a spatial light modulator, and an 
optical system for directing light from the light source onto the spatial 
light modulator, characterised in that the optical system comprises; a 
plurality of optical subsystems, each of which has an input aperture and 
is arranged to image light from the light source, the sub-systems forming 
a spatial distribution of source images whose relative positions are 
different from the relative positions of the input aperture; and a 
plurality of reflectors disposed at the relative positions of the source 
images, each of the reflectors being arranged to reflect light from a 
respective one of the sub-systems on to the spatial light modulator. 
A "dark field" optical system is an optical system that normally appears 
dark. Only areas of the optical system that scatter or diffract light past 
an edge (or a "stop") appear bright. For example, when the optical system 
shown in FIG. 12 is in the OFF state (that is, when the LCD panel 7 is not 
in the diffractive state), the LCD panel will act as a mirror. Light from 
the lamp 1 that is incident on the LCD panel 7 will be reflected back 
towards the lamp 1 and not towards the projection lens 17, so that the 
projection lens will see a dark field. Light is directed to the projection 
lens 17 only when the LCD panel 7 is in the diffractive state (ie, when 
the optical system is in the ON state), so that light is diffracted by the 
LCD panel 7 past the second array of reflectors 4 to the projection lens 
17. 
The reflectors may be disposed between the spatial light modulator and a 
projection optic for receiving light deflected by the spatial light 
modulator. 
A field lens may be disposed between the reflectors and the spatial light 
modulator. 
Each of the reflectors may include or may be associated with imaging means 
for forming overlapping images of the input apertures at the spatial light 
modulator. 
The overlapping images may be substantially superimposed on each other. 
Each reflector may comprise a plane reflector and may be associated with an 
image forming device. Each image forming device may be a converging lens. 
Each light deflecting element may comprise a concave reflector. Each 
reflector may comprise a mirror whose reflecting surface has the shape of 
part of a paraboloid. 
The input apertures may be arranged as a two dimensional array and the 
spatial distribution of source images may comprise a one dimensional 
array. 
Each sub-system may comprise an optical imaging element. Each of the 
imaging elements may comprise a converging lens. The converging lenses may 
comprise relatively displaced portions of a Fresnel lens. As an 
alternative, the converging lenses may comprise an array of microlenses. 
The microlenses may have rectangular apertures. 
The light source may comprise a light emitter and a concave reflector and 
the optical imaging element may comprise relatively displaced portions of 
the concave reflector. The reflector may be of ellipsoidal shape. 
It is thus possible to provide a dark field projection display having a 
compact optical system and with substantially uniform illumination 
intensity at the spatial light modulator. Also, compared with known 
projection systems, increased image brightness at a screen of the 
projection system can be provided by virtue of the optical extent 
modification.

Like reference numerals refer to like parts throughout the drawings. 
The dark field projection display illustrated in FIGS. 1 to 3 comprises a 
light source in the form of a lamp 1, such as an arc lamp, and a parabolic 
reflector 2. The light source produces a substantially collimated beam 
which is incident on a split Fresnel lens 3. The lens 3 comprises two 
semi-circular halves 3a and 3b with the half lens 3a being displaced 
vertically upwardly with respect to the half lens 3b. 
The half lenses 3a and 3b form images of the light source at an optical 
imaging and combining system 4 in the form of two mirrored comer cubes 5 
and microlenses 6. As shown in FIG. 2, each mirrored comer cube 5 
comprises half of a cube of transparent material, such as glass or 
plastics, having an inclined surface 5b which is made reflective, for 
instance by silvering. Light from the split lens 3 is incident on a 
surface 5a through which it passes and is reflected at the silvered 
surface 5b. The microlens 6 is formed at the output surface 5c of the 
mirrored corner cube 5. 
The images of the light source 1, 2 formed by the half lenses 3a and 3b are 
disposed at the respective mirrored corner cubes 5, in particular, at the 
reflecting surfaces 5b. The microlenses 6 in turn form images of the 
apertures of the half lenses 3a and 3b on a spatial light modulator (SLM) 
7 via a field lens 16. The images are overlapped as shown at 8. Light from 
picture elements (pixels) of the SLM are selectively deflected, for 
example by reflection or diffraction, is directed to a projection lens 17 
which projects an image corresponding to image data supplied to the SLM 17 
onto a front or back projection screen (not shown). 
The split lens 3 forms a plurality of optical sub-systems, namely the half 
lenses 3a and 3b. Each sub-system 3a, 3b has an input aperture which, in 
this embodiment, comprises the shape of the half lens in the optical plane 
thereof, and images light from the light source to form source images at 
the surface 5b of the respective mirrored corner cube 5. The sub-systems 
(half lenses 3a, 3b ) form a spatial distribution (vertically spaced) of 
the source images whose relative positions are different from the relative 
positions (horizontally spaced) of the input apertures. 
The light source 1, 2 produces an intensity distribution illustrated at 9 
in FIG. 3. The output beam of the light source 1, 2 is substantially 
collimated and comprises a cylindrical light beam of cylindrically 
symmetrical intensity distribution. The lamp 1 partially obscures light, 
for instance as a result of an electrode 1a disposed between the lamp 1 
and the split lens 3. As a result, the light distribution has an annular 
peak and is reduced at the centre or axis of the light source and towards 
the edges of the output beam. 
The two half lenses 3a and 3b divide the extent of the light source 1, 2 
into two beams which are individually imaged at the SLM 7 by the two 
mirrored comer cubes 5 and microlenses 6. The resulting images overlap 
and, as shown in FIGS. 1 and 3, may be substantially superimposed on each 
other. As a result of incoherent mixing of the images formed by the 
microlenses 6, the illumination has a substantially more homogeneous or 
uniform intensity distribution as illustrated at 10 in FIG. 3. Further, 
the extent of the light source 1, 2 is modified to provide an extent at 
the SLM 7 which is reduced in one dimension and increased in the 
orthogonal dimension. The illumination may therefore be made more "one 
dimensional" and is particularly suitable for illuminating, for instance, 
SLMs of the type disclosed in EP 0 811 872. In particular, the display 
brightness and contrast ratio may be increased and the improved uniformity 
of intensity distribution provides a more evenly illuminated image. 
FIG. 4 illustrates a dark field projection display which differs from that 
shown in FIG. 1 to 3 in that the reflector 2 is ellipsoidal, cylindrical 
lenses 18 and 19 are disposed between the light source 1,2 and the split 
lens 3, and the optical combining system 4 comprises a split plane mirror. 
Light from the lamp 1 and reflector 2 is collimated by the cylindrical 
collimating lenses 18 and 19. The focal length f" of the lens 19 is equal 
to twice the focal length f of the lens 18. Each of the lenses 18 and 19 
alters the divergence in one dimension with a corresponding alteration in 
that dimension of the image size. This equalises the divergence of the 
light source and allows a uniform illumination cone to be produced. This 
has the effect of producing an oval image of the circular source when 
imaged by the split Fresnel lens 3. Offsetting the images produced by the 
lens halves results in two images having their extents reduced by 
approximately a half in the horizontal direction. The split mirror 4 at 
the image plane adjusts the direction of optical propagation from these 
two images and produces a near uniform illumination region via the field 
lens 16 at the SLM 7. 
The display shown in FIG. 5 differs from that shown in FIGS. 1 to 3 in that 
the mirrored corner cubes 5 and microlenses 6 are replaced by an array of 
off-axis parabolic mirrors 11 which form the optical imaging and combining 
system. The array comprises two mirrors 11 for imaging the two half lenses 
3a and 3b of the split Fresnel lens 3. Each mirror 11 performs the 
deflecting and imaging function of one of the mirrored corner cubes 5 and 
one of the microlenses 6. 
FIG. 6 illustrates a display which differs from that shown in FIG. 1 in 
that the Fresnel lens 3 is divided into four quadrants 3c, 3d, 3e, 3f 
which are vertically shifted or displaced relative to each other so as to 
form four spots 22, comprising respective images of the light source 1, 2, 
at different heights. Also, the system 4 comprises four mirrored comer 
cubes and microlenses such that the four spots 22 are formed at the 
reflecting surfaces 5b of the four mirrored corner cubes 5, respectively. 
FIG. 7 illustrates at 9 a conventional intensity distribution, for instance 
as would be formed by the light source 1, 2 alone. As shown above the 
distribution 9, the SLM panel 7, for instance of a liquid crystal spatial 
light modulator, is illuminated by a beam 15 which is substantially larger 
than the panel 7. Thus, a substantial amount of light is lost and this 
represents inefficient use of light produced by a conventional light 
source. The four overlapped quadrant intensity profiles 8 are illustrated 
in FIG. 7 above the intensity distribution 10 produced at the SLM 7 of the 
display illustrated in FIG. 6. The intensity distribution 10 is very 
uniform and closely approaches the ideal distribution, which comprises a 
flat top with vertical edges. This arrangement allows a square or 
rectangular profile to be achieved which matches the size of the panel 7 
so as to make efficient use of light from the light source 1, 2. 
The display shown in FIG. 8 differs from that shown in FIG. 1 in that the 
split Fresnel lens 3 is omitted and the parabolic reflector 2 is replaced 
by a split ellipsoidal reflector 2. The reflector 2 is shown as being 
split into two sections 2a and 2b but may be split into any desired number 
of sections according to the number of optical sub-systems required. The 
sections 2a and 2b may be vertically displaced with respect to each other 
or may be angularly displaced about a horizontal axis through the focus at 
which the lamp 1 is located. The sections 2a and 2b constitute two optical 
sub-systems which form images of the light source 1, 2 with a spatial 
distribution different from that of the input apertures of the 
sub-systems, which are effectively the output apertures of the mirror 
sections 2a and 2b. 
Each of the sections 2a, 2b performs the combined function of gathering 
light from the lamp 1 and imaging the light onto the reflecting surface of 
the corresponding mirrored corner cube 5. Otherwise, the display shown in 
FIG. 8 operates in the same way as that shown in FIG. 1. 
FIG. 9 illustrates a display which differs from that shown in FIG. 1 in 
that the split Fresnel lens 3 is replaced by an array 3' of microlenses 
with rectangular apertures. Each of the microlenses images the light 
source 1, 2 onto a respective mirrored corner cube 5 and microlense 6 of 
the system 4 which, in turn, images the rectangular aperture of the 
microlense of the array 3 via the field lens 16 on the SLM 7. 
FIG. 10 illustrates the geometry involved in correctly aligning the 
reflecting surface 5b or mirror of the mirrored comer cube 5. In order for 
the mirror to be correctly oriented, the angles .theta. (x) and .theta. 
(z) of the normal n vector, having components (x.sub.n,y.sub.n, z.sub.n) 
must be determined. Components and angles are measured with respect to a 
three dimensional Cartesian co-ordinate system (x, y, z). 
The mirror normal is defined by: 
EQU n=(1/2)*{a-c+(b-c)*.vertline.a-c.vertline./.vertline.b-c.vertline.} 
The components (x.sub.n,y.sub.n,z.sub.n) can then be used to determine the 
angles as follows: 
EQU .theta.(z)=a tan(y.sub.n /x.sub.n) 
and 
EQU .theta.(x)=a tan(z.sub.n /(x.sub.n.sup.2 +y.sub.n.sup.2).sup.1/2) 
where the vectors a, b, c, d are defined as follows: 
a=(a1,a2,a3) represents the vector at the centre of the lens rectangular 
section; 
b=(b1,0,0) represents the vector at the centre of the device; 
c=(d1,d2-F,d3) represents the vector at the centre of the mirror; and 
d=(d1,d2,d3) represents the vector at the optical centre of the lens 
section. 
The focal length f.sub.m of the microlense attached to the mirrored comer 
cube may then be calculated from the following expression: 
EQU 1/f.sub.m =(1/F)+(1/L) 
where F is the focal length of the microlenses of the array 3 and L is the 
distance between the mirror plane and the device plane. 
For example, with the following values in millimeters: 
a=(7.5, 70,13.25) 
b=(0, 60, 0) 
c=(0, 0, 7.5) 
F=70 
L=60 
the following values are calculated: 
EQU .theta.(z)=38.7.degree. 
EQU .theta.(x)=-2.6.degree. 
EQU f.sub.m =32.3 mm 
The displays described hereinbefore have the mirrored corner cubes 5 and 
microlenses 6 or parabolic mirrors 11 arranged as one dimensional arrays. 
However, other arrangements are possible and FIG. 11 illustrates a display 
which differs from that shown in FIG. 9 in that the mirrored corner cubes 
5 and microlenses 6 are arranged as a two dimensional array forming the 
optical imaging and combining system 4. 
FIG. 12 illustrates a display using an array 3' of microlenses with 
rectangular apertures as shown in FIG. 11. However, the display of FIG. 12 
differs from that of FIG. 11 in that the number of microlenses is 
increased and the system comprises a two dimensional array of off-axis 
parabolic mirrors. 
It is thus possible to provide a dark field display having substantially 
improved uniformity of image illumination. The illumination can be 
accurately matched to a SLM panel size. For instance, the illumination may 
be made more one dimensional by reducing the extent in the appropriate 
direction so as to improve the performance of displays which require one 
dimensional illumination. For SLMs having two dimensional apertures 
requiring even illumination, the illumination can be more accurately 
matched to the shape and size of the SLM so as to reduce the amount of 
light wasted around the periphery of the SLM Thus, in addition to 
improving the uniformity of illumination, an increase in the brightness of 
the display may be achieved.