Chromatic light separator and picture projector using a chromatic light separator

This invention concerns chromatic light separators, and is particularly intended to reduce their dimensions. A chromatic separator in accordance with the invention contains at least two selective wave length mirrors. According to one characteristic of the invention, the two selective mirrors are nested holographic mirrors. The invention is particularly suitable for three-color rear-projection type picture projectors using liquid crystal screens.

This invention concerns a chromatic light separator of the type that can 
form at least two monochrome beams each designed to be spatially 
modulated. It also concerns a picture projector using this type of 
chromatic separator. 
As part of the development of high definition televisions, an attempt is 
made to obtain large picture sizes (of the order of one meter diagonal) in 
the 16/9 format (length to height ratio) and containing a large number, 
for example 1 million, pixels. 
In order to generate this type of video picture, the current trend is 
towards picture projection devices using spatial light modulation 
techniques. In these projection devices, the polychrome picture is 
obtained by superimposition on a projection screen of three monochrome 
pictures, one red, one green and one blue. Each monochrome picture is 
firstly formed on the surface of a spatial light modulating screen called 
the "imager" throughout the rest of this description. 
The imager usually consists of an L.C.D. (Liquid Crystal Display) type 
screen containing liquid crystal cells in a matrix layout, with each cell 
corresponding to a pixel. A beam of monochrome and polarized light is 
spatially modulated by the imager, and the projected picture is the result 
of this modulation. 
In general, the different monochrome beams (green, red and blue) are 
obtained using a chromatic light separator device illuminated by a beam of 
white light. Note that the expression "monochrome beam" refers to a beam 
of colored light particularly a so-called primary color such as blue, 
green or red, unlike a beam of white light that contains several primary 
colors. Obviously such a monochrome beam could equally well be a 
relatively wide spectral band or a narrow spectral band of the 
monochromatic radiation type. 
Pictures formed by the three imagers are projected onto the projection 
screen by means of a single objective common to the three monochrome 
pictures, or using three objectives, or in forward projection or in 
rear-projection. 
This type of projector has an important advantage over cathode tubes in 
that it is more compact. However they are still relatively large, such 
that all manufacturers are attempting to reduce their size, particularly 
for rear-projection type projectors (overhead projectors) for which the 
entire length of optical paths between imagers and the projection screen 
lies within the total size. 
Note that in these projectors using one or more LCD imagers to spatially 
modulate a monochrome beam, a large part of the overall size is due to the 
illumination device, namely the optical chromatic separation and light 
transport system, from the illumination source to each LCD imager. 
This type of rear-projector is well known, and typical examples have been 
published particularly in the following two documents: 
"100--in Extra-Slim Liquid-Crystal Rear-Projection Display", Fukuda et al. 
(Hitachi), SID 1991 Digest, p. 423; 
"High Definition Liquid Crystal Projection TV", Noda et al. (Matsushita), 
Japan Display 1989, p. 256. 
Another disadvantage of these picture projectors is that their light 
efficiency is low, particularly due to differences between the shape of 
the imager to be illuminated and the shape of the section of the light 
beam from the light source. Illumination sources normally used (for 
example arc lamp associated with a reflector) generally produce a light 
beam with a circular cross section. If it is required to illuminate a 
rectangular shape LCD imager with a circular beam, the rectangle must be 
inscribed within the circular cross section. In this case the ratio 
between the area of the rectangle and the area of the disk is 0.54 at the 
most. Therefore practically half of the energy is lost. 
A French patent published under n.sup.o 2 642 927 describes how to reduce 
the size of this type of picture projector using holographic mirrors 
instead of ordinary mirrors in the light transport optical system. The 
optical system is practically the same as in the first publication 
mentioned above: holographic mirrors are used instead of ordinary (metal) 
mirrors on the trajectory of the imager illumination beams. The resulting 
advantage is due to the fact that in this configuration, a holographic 
mirror does not respect Descartes law, in other words it can deviate light 
rays by 90.degree. without necessarily being inclined at 45.degree. from 
the incident beam. Compared with ordinary mirrors, holographic mirrors can 
therefore be inclined further for the same angle of deviation, thus 
reducing the dimensions of these mirrors. 
Holographic elements or components in particular performing the functions 
of mirrors are well known in themselves. They are obtained by interference 
and can carry out complex optical functions in films such as described in 
document: "L. Solymar, D. J. Cooke, Volume Holography and Volume 
Gratings"-- Academic Press (1981). 
Properties of these holographic components in terms of angular or spectral 
selectivity or polarization are described through coupled wave formalism 
in a document by H. Kogelnik, Bell Syst. Tech. J. 48, p. 2909 (1969). 
A holographic mirror such as that used in French patent n.sup.o 2 642 927 
mentioned above, consists of a volume hologram recorded in a 
photosensitive material. 
FIG. 1 shows a classical method of recording a hologram. The photosensitive 
material M with refraction index n is deposited on a transparent plane 
support St, and one or several networks with periodic refraction indexes 
are recorded in this material using two beams F1, F2 with parallel rays. 
These two beams F1, F2 are output from the same laser source with a wave 
length .lambda..sub.0 (measured in air), and they are propagated along two 
directions making an angle A between them in the medium with index n. 
When these two beams intersect, light interference fringes Fi appear 
parallel to each other. The common direction of these fringes is the 
bisector of the two propagation directions of beams F1, F2. The light 
intensity in a direction perpendicular to fringes Fi and in a medium with 
a refraction index equal to n, is modulated by a sinusoidal profile with a 
period P given by the following relation: 
##EQU1## 
In order to record a holographic mirror, the photosensitive material and 
its support are placed in the interference fringes field. Two types of 
photosensitive material are frequently used: dichromated gelatines and 
photopolymers. Exposure to light modifies the material refraction index. 
Interference fringes, alternatively light and dark following a sinusoidal 
profile, modulate the refraction index between the values n.sub.max and 
n.sub.min. This modulation may be sinusoidal or may be of a different 
shape, for example square or trapezoidal as a function of the response of 
the photosensitive material (as a function of its recording resolution). 
This modulation may be fixed into the material by a chemical or 
photochemical fixing process. Note on FIG. 1 that the transparent support 
St may be inclined with respect to the fringes Fi. 
FIG. 2 shows that after recording the index modulation in material M, if 
beam F2 is eliminated and the hologram is illuminated by beam F1, the 
recorded index fringes FI behave like multiple mirrors. Beam Fi is 
deflected into a beam F'1 that is identical to beam F2 in FIG. 1. 
Let R be one of the rays of beam F1. Each portion of a hologram containing 
a continuous index variation between n.sub.min and n.sub.max behaves like 
a mirror for ray R, and produces the reflected ray R'. The angle between 
rays R and R' (in the medium with index n) is equal to A. A simple 
variation of the index between n.sub.min and n.sub.max is not sufficient 
to completely deflect the ray R. Part of the energy continues to be 
propagated through the hologram and encounters other variations n.sub.min 
and nma.sub.x, thus creating a multitude of rays R'. On the scale of beam 
F1, if there are enough fringes recorded in the hologram, the entire 
energy of beam F1 will be reflected into a beam F'1 which is the 
reconstitution of beam F2. 
In practice, the result obtained-is that a beam of parallel rays can be 
deviated by an angle A using a pseudo-mirror, or holographic mirror, for 
which the angle of inclination from the incident beam is not necessarily 
A/2 (this would necessarily be the case for a conventional mirror). There 
is therefore a potential gain in size compared with a conventional mirror. 
One of the interesting properties of the holographic mirror is 
anamorphosis. We can see in the drawing in FIG. 2 that the width L' of 
beam F'1 is narrower than width L of beam F1. This is not the case in a 
direction perpendicular to the plane of the figure. Therefore a beam with 
a square cross-section can be transformed into a beam with a rectangular 
cross section, or a beam with a circular cross section can be transformed 
into a beam with an elliptical cross section. 
French patent application n.sub.o 90 14620 describes the use of holographic 
mirrors in a liquid crystal video projector, and states that by 
construction, these holographic mirrors perform an anamorphosis of the 
beam that can thus be adapted to the 16/9 format. 
Holographic mirrors have another known property, namely that they are 
selective in wave length. 
Holograms are recorded with laser light in order to produce interference s 
fringes. If the laser beam F1 is replaced on restitution by a beam of 
white light with the same geometry, a reflecting beam is obtained with a 
spectral distribution centered around .lambda..sub.0 : consequently the 
holographic mirror is also a dichroic mirror. By means of three holograms 
recorded with red, green and blue lasers, it is possible to manufacture an 
optical system that separates the three primary colors generally used for 
picture display. Note that for some photosensitive materials, one wave 
length is recorded and the index modulation is transformed by chemical 
treatment so that the mirror works at a different wave length. 
Finally, note also the polarization property of the light presented by 
holograms. The reflection coefficient of a mirror depends on the direction 
of polarization of the beam of incident light. In particular at the 
Brewster angle of incidence, the reflection coefficient cancels out for 
the polarization direction parallel to the incidence plane. The reflected 
radiation is therefore polarized in the direction perpendicular to the 
plane of incidence. For a ray incident to an air-mirror interface, and if 
the mirror medium has a refraction index equal to, for example, 1.5 (the 
index for air being equal to 1), the Brewster angle is equal to 56.degree. 
40'. 
Photosensitive materials also have a refraction index significantly equal 
to that of glass, namely 1.5. When the index fringes are recorded in the 
material, it is done by index variations around this average value. A 
typical maximum modulation value is n.sub.max -n.sub.min =0.14. 
A holographic mirror does not follow Descartes laws, but a ray incident on 
the layer of photosensitive material will follow the laws of refraction. 
In other words, a ray cannot reflect on the fringes network until after it 
has penetrated into the medium with index n=1.5. Thus reflection on the 
network of index fringes must be considered as a reflection of a ray 
coming from a medium with index n=1.5 onto a medium with an average index 
n=1.5. These considerations are also applicable when rays firstly cross 
the transparent support (glass index 1.5) of the holographic mirror before 
entering the photosensitive material. 
We can show that if the refraction indexes of the two media of an 
interface-mirror are equal, the Brewster angle is equal to 45.degree. . An 
angle of incidence of 45.degree. (in the medium with index 1.5) therefore 
produces a reflected beam that is polarized in a direction perpendicular 
to the index plane. This polarizing property of holographic mirrors is 
particularly useful for the illumination of liquid crystal type imagers. 
The invention concerns an optical light chromatic separation device called 
a light chromatic separator, and its objective is to make very compact 
separators of this type. 
For this purpose, the invention proposes to use holographic mirrors 
recorded in volume, firstly in order to create a highly compact assembly, 
and secondly in order to take advantage of all the other properties of 
this type of mirror, particularly anamorphosis of the section of light 
beams in order to improve the light energy efficiency. 
According to the invention, a chromatic light separator illuminated by a 
beam of white light called the primary beam containing at least two 
mirrors with selected wave lengths each reflecting a monochrome beam, is 
characterized in that at least two of the selected mirrors are holographic 
mirrors nested inside each other. 
The term "nested inside each other" refers to a layout in which the planes 
of the holographic mirror intersect, in other words are crossed. 
This type of nesting is useful, particularly in that it makes it easier to 
reduce the size of the mirrors, gives perfect symmetry of the trajectory 
of monochrome beams and is capable of simultaneously using all properties 
of holographic mirrors. 
The invention also concerns a picture projector using this type of light 
chromatic separator. It has a particularly useful (but not exclusive) 
application in projectors with rear-projection, in which it is easier than 
with forward projection, to project each monochrome picture onto the 
projection screen using an objective dedicated to each imager.

FIG. 3 schematically shows chromatic light separator SC in accordance with 
the invention. The chromatic separator SC is formed in a parallelepiped 
shaped prism p containing several mirrors Mh1, Mh2. 
According to one characteristic of the invention, at least two of these 
mirrors Mh1, Mh2 are holographic mirrors nested within each other. 
FIGS. 4a and 4b are perspective views showing the prism parallelepiped p in 
order to better situate the positions of the two nested holographic 
mirrors Mh1 and Mh2. 
Parallelepiped p in the example has two large opposite square faces FE, FS; 
its other four faces, referred to as side faces FL1, FL2, FL3 and FL4 are 
rectangular and it may be useful to make them with a 16/9 format, as 
explained in more detail later in the description. 
The corners of the parallelepiped are identified A to H. If the 
parallelepiped is cut by two planes, one of which passes through corners 
A, C, E, G and the other passes through corners B, D, H, F, the first and 
second diagonal surfaces are defined as S1 and S2 and are materialized on 
FIG. 4a and 4b respectively. 
These two diagonal surfaces S1 and S2 intersect each other, and each 
represents the position of a holographic mirror Mh1, Mh2 in a chromatic 
separator SC in accordance with the invention. 
Referring again to FIG. 3, the figure shows the parallelepiped p in a top 
view, in other words holographic mirrors Mh1, Mh2 extend perpendicularly 
to the plane of the figure. The two holographic mirrors Mh1, Mh2 are 
nested, and their faces S1, S2 form diagonal surfaces that intersect with 
each other, in the same way as surfaces S1, S2 on FIGS. 4a, 4b. 
Parallelepiped p is made of a transparent material, for example glass or 
plastic. One of its large square faces FE, called the entry face, is 
illuminated by a beam of white light called the primary beam FP. The 
primary beam FP is collimated along the direction of its propagation axis 
X. The propagation axis X is normal to the plane of the entry face FE and 
passes through the center of it, and therefore through the line formed by 
the intersection of the two nested mirrors Mh1, Mh2. 
On FIG. 3, holographic mirrors Mh1 and Mh2 are shown simply by a thick 
line, although obviously each includes a transparent support on which a 
layer of photosensitive material is deposited (forming the entire surface 
of the mirror S1, S2), in which holograms are traditionally recorded with 
the required characteristics of these mirrors, using known techniques. 
Consequently interference fringes (not shown) similar to the interference 
fringes Fi shown in FIGS. 1 and 2 have been recorded in the photosensitive 
layers in the two holographic mirrors Mh1, Mh2. 
Thus each holographic mirror Mh1, Mh2 may be made such that, under the 
effect of illumination by the primary beam FP, it reflects a monochrome 
beam FB, FR of a different color to that of the beam reflected by the 
other mirror: for example blue for the first monochrome beam FB from the 
first holographic mirror Mh1, and red for the second beam FR from the 
second holographic mirror Mh2. 
In the non-limitative example described, the holograms defining mirrors 
Mh1, Mh2 have been recorded such that the two monochrome beams FB, FR are 
reflected in opposite directions located on the same -Y +Y axis 
perpendicular to the X propagation axis of the primary beam FP; this is 
true for a symmetrical inclination of these mirrors with respect to the -Y 
+Y axis, at an angle of inclination b1, b2; the -Y +Y axis is divided by 
the X axis into a +Y part and a -Y part that represents the propagation 
axis of the blue monochrome beam FB and the propagation axis of the red 
monochrome, beam FR, respectively. 
Under these conditions, for each holographic mirror Mh1, Mh2, the index 
fringes (not shown on FIG. 3) have a general orientation (shown by dashed 
lines marked OF1, OF2 in FIG. 3) such that they are inclined symmetrically 
at angles c1, c2 from 45.degree. from the monochrome beam -Y, +Y 
propagation axes: more specifically the index fringes for the first 
holographic mirror Mh1 bisect +Y and +X, (where +X is the part of the X 
propagation axis located on the side opposite the entry face FE for the 
monochrome beams FB, FR +Y, -Y propagation axes); the index fringes for 
the o second holographic mirror Mh2 bisect -Y and +X. 
If angle b1, b2 at which the holographic mirrors Mh1, Mh2 are inclined is 
not equal to the angle c1, c2 of orientation of their index fringes, an 
anamorphosis will be made. 
Thus for example, assuming that the primary beam FP has a circular cross 
section with a diameter D1 equivalent to the diagonal of the entry face FE 
of the parallelepiped p, and this face is square, the maximum amount of 
light energy can thus be extracted from the primary beam FP. And if also 
holographic mirrors Mh1, Mh2 work as described above for angles of 
inclination b1, b2 equal to 29.36.degree., an anamorphosis will be made 
from a square to a 16/9 format rectangle. This type of anamorphosis makes 
it possible to keep all energy taken from the primary beam FP by 
holographic mirrors Mh1 and Mh2 in each of the monochrome beams FB, FR, 
and each of these beams will have a rectangular section which in the 
non-limitative example described will be in the 16/9 format. 
In this configuration, the section of each monochrome beam FB, FR is 
rectangular, with a dimension equal to the length L1 of one side of the 
entry face FE in a plane perpendicular to the plane of the figure, and a 
smaller dimension in the plane of the figure corresponding to the 
thickness Ep of the prism p. In fact, the section of monochrome beams FB, 
FR corresponds to the shape of the opposite side faces FL1, FL2 through 
which these beams exit from the prism p. 
Another advantage of this configuration, in which holographic mirrors Mh1, 
Mh2 reflect monochrome beams FB, FR in directions of 90.degree. from the 
direction of the incident primary beam FP, without being oriented at 
45.degree., is that the size of mirrors Mh1, Mh2 parallel to the thickness 
Ep of the prism is reduced. 
In the configuration shown in FIG. 3, if the light in the primary beam FP 
has no specific polarization direction, monochrome beams FB, FR acquire a 
single polarization direction P1 perpendicular to the planes of incidence, 
and which is shown on FIG. 3 perpendicular to the plane of the figure. In 
fact reflections are polarizing, because firstly the angle cl, c2 of 
incidence of rays in the primary beam FP on the index fringes of 
holographic mirrors Mh1, Mh2 is 45.degree., and secondly the average 
refraction index of each fringe grating is equal to the index of the 
glass. 
Since holographic mirrors have determined spectral passbands, they are 
calculated such that the second mirror Mh2 reflects a red component at 
90.degree. in the -Y direction, and that the first Mh1 reflects a blue 
component at 90.degree. in the +Y direction. These components are 
polarized perpendicularly to the planes of incidence. The complementary 
red and blue components, in other words for which the polarization 
directions P2 are parallel to the plane of incidence (and therefore shown 
on FIG. 3 parallel to the plane of incidence) pass through the 
parallelepiped p along the X axis without being affected. Similarly, light 
belonging to the spectral band in which mirrors Mh1 and Mh2 are not 
sensitive, crosses through the parallelepiped p without being affected and 
emerges from it through the second square face or exit face FS opposite to 
the entry face FE. 
A chromatic separator in accordance with the invention with two nested 
holographic mirrors as shown in FIG. 3, can be applied to all illumination 
devices using at least two monochrome beams obtained from white light. 
Obviously, the operation described above is applicable regardless of the 
color of monochrome beams reflected by the two nested holographic mirrors, 
if the two beams have different colors. 
Also, in order to make one or several other beams in addition to the two 
monochrome beams FB, FR, it is only necessary to process the light that 
exits from the prism p through its exit face, and select the required 
components. 
Color separation and/or anamorphosis can be done by the chromatic separator 
SC in the invention, independently of selection of the polarization 
direction, in other words the separator SC in the invention may be 
illuminated by a previously polarized radiation, preferably along the 
polarization direction P1. The invention can therefore be associated with 
any radiation polarization system, particularly of the type that separates 
the two orthogonal polarization components and rotates one of them by 
90.degree., as described for example in the French patent application 
n.sup.o 90 13942. 
FIG. 5 contains a perspective view schematically showing a picture 
projector 1 with three primary colors, using a chromatic separator SC in 
accordance with the invention in order to illuminate three rectangular 
imagers I1, I2, I3 located in the same plane. 
The chromatic separator SC is made as in the example in FIG. 3, using a 
prism or parallelepiped p in which the first and second holographic 
mirrors Mh1, Mh2 are nested. 
The entry face FE of the separator SC is illuminated by a white light beam 
or primary beam FP (marked by an arrow drawn in a thick line), moving 
along the X axis. The first and second holographic mirrors Mh1, Mh2 
reflect the first and second monochrome beams FB, FR that exit from the 
parallelepiped p through the first and second side faces FL1, FL2 
respectively, and along the +Y, -Y propagation axis, these two beams FB, 
FR having the same polarization direction P1 as explained above with 
reference to FIG. 3. 
Among other components, light that exits from the parallelepiped p through 
the exit face F S contains the primary green component that is selected by 
a 90.degree. reflection on a third holographic mirror Mh3. This mirror Mh3 
reflects a third monochrome beam RG, with wave length corresponding to the 
green, along a propagation axis Z3 which is vertical in the example in 
FIG. 5. The first, second and third monochrome beams FB, FR and FG 
correspond to blue, red and green respectively, and will be modulated by 
the first, second and third imagers I1, I2, I3 respectively. Each imager 
I1 to I3 is a screen capable of spatially modulating light, in itself 
using known techniques. In the non-limitative example described, these 
imagers I1, I2 and I3 are liquid crystal matrix screens with the same 
format as for side faces FL1, FL2, FL3, namely rectangular in the example, 
and particularly in the 16/9 format. 
In the non-limitative example described, the three imagers I1, I2 and I3 
are located in the same plane (or in a parallel and close plane) as the 
plane containing the third side face FL3 of parallelepiped p; this third 
side face is the face which, in the top of the parallelepiped p, connects 
the first and second side faces FL1, FL2 through which the first and 
second monochrome beams FB, FR exit. 
The third holographic mirror Mh3 sensitive to the green component reflects 
this component along the Z3 axis in the form of a third monochrome beam 
towards the top of the figure at 90.degree. from the X axis along which 
the primary beam FP propagates. For this purpose, the third holographic 
mirror Mh3 may be made so that it can be inclined with respect to the 
parallelepiped exit face FS by an angle dl of the same value as the angle 
of inclination b1, b2 (shown in FIG. 3) in order to limit the size, and 
also to make the square--&gt;rectangle anamorphosis: the square corresponds 
to the exit face FS, and the rectangle corresponds to the shape of imagers 
I1 to I3, or more precisely to the third imager I3 which is inserted on 
the trajectory of the third monochrome beam FG. 
Concerning the two imagers I1, I2 to be used to modulate the first and 
second monochrome beams FB, FR respectively, these imagers are placed on 
each side and as close as possible to the third side face FL3 of prism p, 
such that they extend it, in other words their length is added to the 
length of the third side face; a third side (corresponding to the length) 
of this third side face is bounded by the third imager I3. 
The first and second monochrome beams FB (blue) and FR (red) are reflected 
by 90.degree. with respect to their +Y, -Y propagation axis in the 
direction of the first and second imagers I1, I2 respectively, by means of 
fourth and fifth mirrors M4, M5 respectively. 
These fourth and fifth mirrors may be either ordinary mirrors or 
holographic mirrors because there is no anamorphosis or component 
selection to be made since these functions have been done by the first and 
second holographic mirrors Mh1, Mh2. 
After reflection on mirrors M4, M5, the first and second monochrome beams 
FB, FR propagate in the direction of imagers I1, I2 along axis Z1, Z2 
parallel to the Z3 axis along which the third monochrome beam FG 
propagates in the direction of the third imager I3. 
Each of the monochrome beams FB, FR, FG passes through an imager I1, I2, I3 
which may possibly modulate it to enable it to become a picture carrier, 
from which it emerges to be propagated in the direction of a projection 
objective 01, 02, 03, in itself conventional. Each projection objective 
01, 02, 03 is also assigned to a single monochrome beam FB, FR, FG, that 
it focuses using known techniques onto a projection screen (not shown) 
common to the three modulated beams. 
Obviously, imagers I1, I2 and I3 may be laid out in various ways: for 
example the first and second imagers I1, I2 may be placed directly on side 
faces FL1, FL2 through which the first and second monochrome beams FB, FR 
exit from the prism or parallelepiped p. But it is useful to place these 
imagers or LCD screens I1, I2, I3 in a plane that contains the third side 
face FL3, in order firstly to have an equal distance between each imager 
I1, I2, I3 and the projection objective 01, 02, 03 with which it is 
associated; and secondly so that the distance between imager and objective 
is as short as possible. 
In order to restrict the number of interfaces (air-transparent material) 
that cause losses of light by reflection, it is useful to design a single 
piece structure: 
the fourth and fifth mirrors M4, M5 may each be made by metallizing one 
face of an additional prism p2, p3 at 45.degree. (to prevent any 
confusion, the first prism formed by the parallelepiped p is called "main 
prism" in the rest of this description). The supplementary prisms p2, p3 
are placed one in contact with the first side face FL1, and the other in 
contact with the second side face FL2. 
the third holographic mirror Mh3 may consist of a third supplementary prism 
made of transparent material on one face, inclined by an angle dl with 
respect to the exit face FS from the first prism p, and is covered with a 
layer of photosensitive material (not shown) in which the hologram forming 
the third holographic mirror Mh3 is recorded. 
There is thus continuity of the refraction index between the entry face FE 
of the first prism p and the position of each imager or LCD screen I1, I2, 
I3. Due to this index continuity, reflection on the third holographic 
mirror Mh3 is polarizing: only the green component polarized vertically 
with respect to the plane of incidence on the third mirror Mh3 is 
reflected by this mirror to make the third monochrome beam FG. 
The three monochrome beams FB, FR, FG are thus made by blue, red and green 
components respectively polarized along the same direction P1 parallel to 
the +Y propagation axis. 
Spatial modulation screens or LCD type imagers generally have two 
polarizers (not shown): the first polarizer located at the side of their 
illumination face F1, filters the light in order to allow only light with 
a first given polarization direction to pass; and the second polarizer 
located at the side of the display face opposite to the illumination face, 
displays polarization direction rotation caused by modulation of the LCD 
screen. 
In the projector according to the invention, since the three blue, red and 
green components (forming the monochrome beams FB, FR, FG)illuminating the 
screens or LCD imagers I1, I2, I3 are polarizing, the first of the 
polarizers mentioned above on each LCD screen may possibly be eliminated. 
This is an advantage since normal polarizers usually increase the 
temperature of the LCD due to the fact that they operate on the principle 
of absorbing undesirable radiation. However if it is required to keep 
these first polarizers in a projector according to the invention, for 
example in order to increase picture contrast, the temperature increase 
problem is not serious since incident radiation on these first polarizers 
is already polarized. 
If the polariization direction forced on monochrome beams FB, FR, FG is not 
optimum for the performances of imagers or LCD screen I1, I2, I3, a 
conventional .lambda./2 retarding plate may be placed on the trajectory of 
these beams, for example immediately in front of the LCD screen adjacent 
to its illumination face F1, particularly adapted to the color of the 
monochrome beam FB, FR, FG modulated by this LCD screen (for example a 
piece of double refraction plastic material). Thus, the polarization 
direction of monochrome beams or FB, FR, FG illumination beams may be 
rotated to be in a particularly favorable direction for operation of the 
LCD screens I1, I2, I3. 
Note that optical paths between the entry face FE and imagers I1, I2, I3 
are not necessarily exactly equal to each other, and it may be necessary 
to insert a lens made by known techniques on the trajectory of one or 
several components, to assist in concentrating light in the rectangular 
aperture of the imager. FIG. 6 is a schematic view of a picture projector 
1 in accordance with the invention, and illustrates its compactness. FIG. 
6 shows a view of the projector in FIG. 5 along a section parallel to the 
first and second side faces FL1, FL2 of the main prism p, the plane of 
this section passing through the intersection line of the two nested 
holographic mirrors Mh1, Mh2. Consequently compared with FIG. 5, FIG. 6 
does not show the first and second imagers I1, I2 (blue and red) in 
particular, which are in a deeper and less deep plane respectively than 
that shown in FIG. 5. 
The main prism p or parallelepiped appears in the form of a rectangle that 
corresponds to the first or second side face FL1, FL2, whereas the entry 
and exit faces FE, FS extend perpendicularly to the plane of the figure. 
The entry face FE is illuminated by the white light beam or primary beam 
FP. The primary beam FP results from emission of light by a light source 
SL, for example made up of an arc lamp Lrc placed at the focal point of a 
parabolic reflector RP. Light emitted by the source SL forms a beam of 
white light FP' propagating in the direction of a sixth mirror M6 along a 
propagation axis Z' parallel to the entry and exit faces FE, FS of the 
main prism p. 
The sixth mirror M6 may be a "cold type mirror": firstly it reflects the 
useful part of the spectrum, in other words visible light forming the 
primary beam FP, by 90.degree. along the X propagation axis; secondly, by 
transmission along the Z' axis, it eliminates infrared and ultra violet 
radiations. 
The primary beam FP encounters the chromatic separator SC containing the 
main prism p in which the first and second holographic mirrors Mh1, Mh2 
are nested (not shown on FIG. 5). The two holographic mirrors Mh1, Mh2 
reflect the first and second monochrome beams FB, FR (not shown on FIG. 5) 
along the +Y, -Y propagation axis perpendicular to the plane of the 
figure. 
The rest of the light forming the primary beam FP then exits from the main 
prism p through its exit face F S, and penetrates into the third 
supplementary prism p3 supporting the third holographic mirror Mh3, 
sensitive to the green in the example. 
The third holographic mirror Mh3 reflects the third monochrome beam FG 
composed of the green component, polarized along the polarization 
direction P1 perpendicular to the plane of the figure as already described 
with reference to FIG. 5. The third monochrome beam FG is reflected along 
the propagation axis Z3 parallel to the entry and exit faces FE, FS in the 
direction of the third imager I3. After passing through imager I3, the 
third monochrome beam carries a picture and forms a third modulated beam 
FGm that propagates along axis Z3 in the direction of the OG projection 
objective. 
FIG. 6 shows a field lens LC placed in front of the third imager I3 on the 
trajectory of the third monochrome beam FG. The field lens LC converges 
the rays of the third monochrome beam FG, FGm into the aperture of the 
third projection objective OG. Note in the case of a picture projector 
with three LCD type screen imagers, in general the three imagers are each 
associated with a field lens of this type. Consequently for this entire 
description, the term imager or LCD screen refers to a field lens plus LCD 
screen assembly. 
Also, FIG. 6 shows a .lambda./2 retarding plate LR, placed between the 
field lens LC and the third imager I3; it should also be considered that 
this type of retarding plate is also placed in front of the other imagers 
I1, I2. However the presence of this retarding plate LR is not always 
compulsory, it may be added in order to rotate the polarization direction 
as previously explained, so that this polarization direction is the 
optimum for operation of an LCD screen type imager. 
With this type of layout, the size of projector 1 in the invention is 
particularly small along the X propagation axis of the primary beam FP, 
and this is the axis along which the size of a complete rear-projector is 
appreciated. Along the X axis, the size D2 reduces to the width La1 of the 
third imager I3 plus the thickness Ep of the main prism p plus the width 
of the parabolic reflector RP, due to the fact that the dimension of 
elements useful to the first and second monochrome beams FB, FR is 
measured in a plane perpendicular to the plane in FIG. 5. 
Another important advantage resulting from this type of layout is that for 
each channel or color, the distance between the projection objective and 
its imager may be minimum since there is no optical component placed on 
the trajectory between these two elements. 
It is well known that liquid crystal screen or LCD type imagers are 
characterized by a "solid acceptance angle". If we wish that the contrast 
of an LCD screen remains above a given value, incident angles of rays 
illuminating the LCD screen must be restricted; they must be contained 
within a given solid angle. 
In the general case, this solid angle does not have any axial symmetry; it 
is extended along a particular direction. This direction bisects the 
directions of the two polarizers (mentioned previously) of an LCD screen. 
The geometric acceptance extent of the LCD is equal to the product of its 
area and its solid acceptance angle. 
Another advantage of the invention is that it can match the solid angle of 
illumination rays to the solid acceptance angle. The square--&gt;rectangle 
anamorphosis is accompanied by a deformation of the solid angle of 
illumination rays. Thus in the case of a beam passing through a square 
surface with a solid angle with axial symmetry, as in the case of a 
primary beam FP passing through the square shaped entry face FE, if all 
the energy removed from this beam by an anamorphosizing holographic mirror 
(such as mirrors Mh1, Mh2 or Mh3) is forced through a rectangular surface 
(such as side faces FL1, FL2), there will be a reduction in the diaphragm 
dimension in one direction. Consequently there will be an increase in the 
solid angle for rays in this direction such that the geometric extent of 
the beam is maintained. 
In the examples shown in FIGS. 4 and 5, we can see that it is possible to 
illuminate rectangular surfaces (those for imagers I1, I2, I3) with a 
polarization direction parallel to the length of the rectangles. However 
the solid angle of the rays is extended in the direction of the width of 
the rectangles. Thus if the solid acceptance angle of an LCD screen is to 
be extended in the direction of its width, the radiation illuminating it 
must be polarized at 45.degree.. 
Consequently a .lambda./2 retarding plate could be placed in front of 
imagers I1 to I3, in the same way as the retarding plate LR is placed in 
front of the third imager I3, in order to make the radiation polarization 
direction coincide with the optimum polarization direction for the LCD 
screen, while adapting the solid illumination angle to the solid 
acceptance angle of the LCD screen. Obviously each retarding plate thus 
inserted in front of an imager I1, I2, I3 must be adapted to the color of 
the monochrome beam thus treated. 
Note that the dimension of the picture projector along the X axis may be 
reduced even further, at the expense of somewhat more complexity in 
manufacturing of the first main prism or parallelepiped p. This is done by 
including the third holographic mirror Mh3 in the first principal prism p, 
in a position such that it would lie in this prism in a plane parallel to 
the plane that it would occupy when placed outside as shown in FIG. 5. 
An example of this construction is shown in FIG. 6, by means of a dotted 
line marked Mh3' which symbolizes this new position of the third 
holographic mirror Mh3. Obviously in this configuration, the assembly 
formed by the field lens LC, the retarding plate LR, the third imager I3 
and the third projection objective O3, would itself also be moved in order 
to be placed above the third side face FL3 on the new reflection axis Z' 
of the third holographic mirror Mh3'. The characteristics and properties 
of the third holographic mirror Mh3 as described previously remain 
unchanged in the new position of this mirror. This new layout of the third 
holographic mirror does not modify the position of elements that are used 
to treat the two monochrome beams FB, FR. 
FIG. 7 shows the parallelepiped or first prism p, in order to illustrate 
the construction method in which the third holographic mirror Mh3' is 
contained in this first prism. 
A third surface S3 is materialized within parallelepiped p, representing 
the s "reflecting" face of the third mirror Mh3'. This third surface S3 is 
delimited by corners D, C, E and F of parallelepiped p. Consequently in 
this situation, the third surface S3 is nested with the first and second 
surfaces S1, S2, in other words the three holographic mirrors Mh1, Mh2, 
Mh3' are nested with each other. 
Nested holographic mirrors Mh1, Mh2, Mh3 may be made, for example, by 
breaking down the parallelepiped p along the planes of each of the 
different mirrors that it contains. 
FIG. 8 illustrates a non-restrictive example showing the main prism or 
parallelepiped p being cut along two nested planes, producing four 
secondary prisms PS1 to PS4 in order to make the first and second nested 
holographic mirrors Mh1, Mh2. 
Layers C1 to C4 of this photosensitive material are placed on the inner 
faces of these secondary prisms, in order to be able to reconstitute the 
two diagonal surfaces S1, S2 (shown on FIGS. 4a, 4b) intended to form the 
first and second holographic mirrors Mh1, Mh2. Each diagonal surface S1 
and S2 is formed using two layers C1, C2 and C3, C4, located so as to be 
extensions of each other, in other words they are in the same plane when 
the parallelepiped is formed. 
In the non-restrictive example described, each diagonal surface S1, S2 is 
obtained by two layers C1, C2 and C3, C4 placed on the inner surfaces of 
opposite secondary prisms PS1 to PS4: 
the first diagonal surface S1 is formed by a first and second secondary 
prism PS1, PS2 placed opposite to each other, and which in the example are 
those with external faces forming side faces FL1, FL2. 
the second diagonal surface S2 is formed using a third and fourth secondary 
prism PS3, PS4 placed opposite to each other, and which in the example 
have external faces corresponding to entry and exit faces FE, FS. 
Obviously, either or both diagonal surfaces may be obtained in different 
ways, for example they could be formed from photosensitive layers 
deposited on adjacent secondary prisms. 
The two secondary prisms forming a diagonal surface S1, S2 are then placed 
in the position that they will occupy when they form the chromatic 
separator SC, and they are exposed to interferences produced by two beams 
in order to record the hologram corresponding to a holographic mirror Mh1, 
Mh2. 
FIG. 9 schematically shows the first and secondary prisms PS1, PS2 exposed 
to two recorded beams FB1, FB2 in order to make the first holographic 
mirror Mh1. 
In the example, since this first mirror Mh1 is sensitive to the blue, the 
two recording beams FB1, FB2 originate from the same laser source (not 
shown) emitting in the blue. 
The first recording beam FB1 is propagated in the direction of the two 
secondary prisms PS1, PS2 along the X' axis. The two secondary prisms PS1, 
PS2 are placed such that the first diagonal surface S1 (formed by layers 
C1, C2) are oriented with respect to the X' axis in the same way as the 
first holographic mirror Mh1 is oriented with respect to the propagation 
X-axis of the primary beam FP (see FIG. 3). 
The second recording beam FB2 is propagated in the direction of the two 
secondary prisms PS1, PS2, along a propagation axis Y' perpendicular to 
the X' axis. This second recording beam is incident to the first secondary 
prism PS1, on one of its external faces forming the second side face FL2. 
The hologram corresponding to the first holographic mirror Mh1 is thus 
recorded using principles already described in the preamble. 
FIG. 10 schematically shows the third and fourth secondary prisms PS3, PS4 
exposed to a third and fourth recording beam FR3, FR4 in order to make the 
second holographic mirror Mh2. These two recording beams are emitted by 
the same laser source (not shown), which is red in the example. 
The third recording beam FR3 is propagated in the direction of the two 
secondary prisms PS3, PS4 along the same propagation axis X' as shown in 
FIG. 9. These two prisms PS3, PS4 are laid out such that the second 
diagonal surface S2 (formed by the third and fourth layers C3, C4) is 
oriented with respect to the X' axis in the same way as the second mirror 
Mh2 is oriented with respect to the propagation X-axis of the primary beam 
FP. Consequently, the third recording beam FR3 is incident on an external 
face of the third secondary prism PS3, that corresponds to the entry face 
FE of parallelepiped p. 
The fourth recording beam FR4 is propagated in the direction of the two 
secondary prisms PS3, PS4 along a propagation axis Y' perpendicular to the 
X' axis of the third recording beam FR3. The fourth recording beam FR4 
actually propagates along the same axis as the second recording beam FB2 
(shown in FIG. 9), but in the opposite direction. 
The hologram corresponding to the second holographic mirror Mh2 is thus 
recorded using known techniques described in the preamble. 
When holographic mirrors Mh1, Mh2 have been recorded, the four secondary 
prisms PS1 to PS4 are assembled in order to make up the main prism or 
parallelepiped. If necessary, an index matching material may be used for 
this assembly between two prisms. When parallelepiped p is reconstituted, 
it forms a chromatic separator SC for two colors. A supplementary prism p3 
may be added to this basic chromatic separator supporting the third 
holographic mirror Mh3 in order to separate a third component, as shown on 
FIGS. 5 and 6. 
This chromatic separator SC may also be added to as shown in FIGS. 5 and 6 
in order to return each component to a spatial modulating screen or 
imager. 
Note that it is also possible to record holograms on a flexible support, 
particularly made of plastic. After fixing holograms by a classical 
photographic process, they may be transferred (adhered) for example to the 
surface of a glass prism. 
Another method may consist of using photosensitive materials that are only 
sensitive to some spectral ranges. Photopolymers are known that are only 
sensitive in the red, and there are others that are only sensitive in the 
green-blue range. Parallelepiped p may therefore be assembled coated with 
layers of different photosensitive materials on each separating surface, 
and parallelepiped p may be exposed successively to different wave lengths 
of laser light.