Image projection arrangement

An image projection arrangement is disclosed, comprising a radiation source (1), an image display system having at least one image display panel (10) in which the polarisation direction of an incident beam is modulated with the image information, and a polarisation-sensitive beam splitter (2), arranged between the source and the image display system, for forming two mutually perpendicularly polarised sub-beams (b.sub.1, b.sub.2). By having the two sub-beams be modulated by the same image display system (10) and by thereafter combining these sub-beams again a very efficient use can be made of the available light without the necessity of a significant more complex design of the arrangement.

BACKGROUND OF THE INVENTION 
The invention relates to an image projection arrangement, comprising a 
radiation source, an image display system having at least one image 
display panel for generating an image to be displayed and wherein the 
direction of polarisation of the beam originating from the source is 
modulated in accordance with the image information, a projection lens 
system for projecting the image produced by the image display system onto 
a projection panel, a polarisation-sensitive beam splitter being arranged 
in the path of the beam coming from the source for producing two mutually 
perpendicular-polarised sub-beams which are each intended to be modulated 
with image information. 
The concept of image projection arrangement must be understood in a very 
general sense and comprises an arrangement for displaying, for example, a 
video image, a graphic image, numerical information or a combination 
thereof. The images may be both monochrome and colour images. In the 
latter case, the display system may have three colour channels for, for 
example, the primary colours red, green and blue, each channel including a 
display panel. 
Such an image projection arrangement for a colour image is disclosed in the 
U.S. Pat. No. 4,127,322. The display panels of the prior art arrangement 
are reflective light valves having as the active, or image producing, 
element a layer of liquid crystalline material of the so-called nematic 
type. This layer locally changes the polarisation direction of an incident 
light beam, in dependence on the image information. For that purpose the 
light beam must be linearly polarised, for which a polarisation-sensitive 
beam splitter is arranged in the path of the beam originating from the 
light source. This beam splitter splits the light beam into two, mutually 
perpendicular-polarised sub-beams. Only one of these beams is transmitted 
to a display system, so that approximately half of the light of the 
radiation source reaches this display system. 
So as to make a more efficient use of the light from the source, the U.S. 
Pat. No. 4,127,322 proposes the use of a second display system. Therein 
the first display system is exposed to the sub-beam polarised in a first 
direction and the second display system to the sub-beam polarised in the 
second direction. After modulation by the display systems the beams are 
combined by the same polarisation-sensitive beam splitter which also 
effected splitting according to polarisation direction. Thus, in 
principle, 100% of the light from the radiation source is used for the 
image projection. 
In the arrangement disclosed in the U.S. Pat. No. 4,127,322, the number of 
display valves is twice the number in more conventional arrangements. In 
addition, each display valve forms part of a complex, relatively large and 
expensive system. In this system an image is generated by means of a 
cathode-ray tube. The light beam emanating from this tube is incident onto 
a photo-conducting layer in which a charge pattern is produced in 
accordance with the image on the cathode-ray tube. As a result thereof, an 
electric field is formed, also in accordance with the image on the 
cathode-ray tube, across a layer of liquid crystalline material disposed 
between the photo-conductive layer and a second, counter, electrode. The 
varying electric field produces a variation of the birefringence within 
the liquid crystalline layer and consequently local differences in the 
shift of the polarisation direction of a projection beam incident on this 
layer. To enable proper operation this system must be provided with yet a 
number of additional layers. The arrangement disclosed in the U.S. Pat. 
No. 4,127,322 is intended for professional applications, and, because of 
its complex structure, size and cost price is not so suitable for consumer 
purposes. 
More suitable for consumer, and other, applications are the so-called 
matrix controlled display panels having a layer of liquid crystalline 
material between two electrodes. In the case of a passively controlled 
display panel, both electrodes are distributed into rows and columns, and 
in the case of an actively controlled display panel a matrix of electronic 
drive circuits are provided on one of the electrodes. In both cases the 
panel is divided by the electrode matrix into a large number of image 
elements. The electrode matrix is controlled by an electronic signal, for 
example a video signal. An image projection arrangement comprising display 
panels of this type is less complicated, cheaper and of smaller bulk than 
the arrangement disclosed in the U.S. Pat. No. 4,127,322. However, due to 
the construction of the display panels, the useful luminous flux is small. 
In, for example, the case of an actively controlled display panel, only 
approximately 10% of the light from the source is transmitted to the 
projection lens system via the display panel. 
OBJECTS AND SUMMARY OF THE INVENTION 
The present invention has for its object to provide a significantly more 
efficient use of the available light in an image projection arrangement 
having matrix-controlled display panels without the necessity of a 
significantly more complex structure of the arrangement. To that end the 
arrangement according to the invention, is characterized in that the two 
sub-beams are incident on a common system. 
The image display system may be a monochromatic system and be comprised of 
only one image display panel, but it may alternatively be a colour display 
system of variable design. 
In the image projection arrangement according to the invention the common 
display panel modulates both polarisation directions of the projection 
light, so in principle all the available light, so that an optimum use is 
made of this light without the need for additional image display panels. 
The most simple embodiment of the arrangement according to the invention, 
is characterized in that it comprises only one polarisation-sensitive beam 
splitter for both producing the two sub-beams and combining the sub-beams 
after they have been modulated by the image display system, and at least 
two reflectors which are both included in the radiation paths of the two 
sub-beams for re-directing the sub-beams emanating from the beam splitter 
to the beam splitter, via the image display system. 
In this arrangement the number of optical elements is minimal. 
This simple embodiment may be further characterized in that the image 
display panels are radiation-transmissive and that the sub-beams are 
incident on these panels substantially perpendicularly. 
This perpendicular angle of incidence contributes to satisfying the 
requirement for the path length for the differently coloured partial 
sub-beams in a colour image projection device to be as equal as possible, 
and prevents any angle-dependency of a liquid crystal display panel from 
affecting the quality of the projected image. 
Alternatively, this simple embodiment may be characterized, in that the 
display panels are reflective. The sub-beams can then, for example, be 
incident on these panels at an acute angle. 
A further embodiment of the image projection device according to the 
invention having a radiation-transmissive image display system and wherein 
a first polarisation-sensitive beam splitter is provided for splitting the 
beam coming from the radiation source into two mutually perpendicularly 
polarised sub-beams, is characterized by a second polarisation-sensitive 
beam splitter for combining the modulated sub-beams, a third 
polarisation-sensitive beam splitter and a plurality of reflectors, the 
radiation path between the first and second beam splitter for the first 
sub-beam comprises reflection from a first and a second reflector, and 
from the third beam splitter passage through the display system and 
reflection from a third reflector, whilst the said radiation path for the 
second sub-beam comprises passage through the image display system and 
through the third beam splitter and reflection from a fourth and fifth 
reflector. 
This embodiment has the advantage that the different reflectors need only 
to reflect beams having one polarisation direction and consequently these 
reflectors can be optimised for the relevant polarisation direction as 
regards their reflection power. 
In accordance with a further characteristic feature of this embodiment in 
the path portions of the first sub-beam which do not coincide with the 
path portions of the second sub-beam, polarisation-sensitive absorption 
filters are arranged which block light having a polarisation direction 
different from that of the first sub-beam. 
This enables an increase in the degree of polarisation of the sub-beams. 
The degree of polarisation is the quotient of the light having the desired 
polarisation direction and the total quantity of light in the beam. 
An embodiment of the image projection device according to the invention, 
which is important for practical use, is characterized in that the image 
display system is a colour image display system comprising 
colour-selective elements and a composite liquid crystal display panel 
whose image elements are divided into groups, each group generating a 
sub-image of a given colour corresponding to the colour of the 
colour-selective elements added to the relevant group of image elements. 
This colour image image system may have the further characteristic feature 
that the colour-selective elements are constituted by a plurality of 
colour-selective beam splitters, for splitting the sub-beams into three 
monochrome partial sub-beams of different colours and for combining the 
partial sub-beams modulated with monochrome image information to form a 
colour image information-modulated sub-beam, and that in the path of each 
of the partial sub-beams a separate image display panel is arranged whose 
collective image elements constitute one of said groups of image elements. 
Alternatively, this embodiment may have the further characteristic feature 
that the colour image display system has one image display panel whose 
image elements are divided into groups, each group generating a sub-image 
of a predetermined colour and that for each of the image elements a colour 
filter is provided which only transmits light having the colour 
corresponding to the colour of the sub-image to be generated by the group 
to which the relevant image element belongs. 
The embodiment of the alternative characteristic has the advantage that the 
colour image projection device comprises a minimum number of optical 
components and is very compact. 
The above-formulated invention achieves the following objectives: to 
provide an image projection device in which the available light is used as 
efficiently as possible and which it is as simple and cheap as possible 
can be satisfied to a still greater extent if this device has the further 
characteristic feature that the polarisation-sensitive beam splitter is 
formed by a set of two transparent elements having the same single index 
of refraction, two surfaces of which face each other, between which 
surfaces a layer of oriented liquid crystalline material is provided of 
which one refractive index is equal to that of the said elements while the 
other refractive index is less than that of the said elements. 
This beam splitter is cheaper than other polarisation-sensitive beam 
splitters and furnishes an appropriate polarisation separation over a 
relatively large range of wavelengths and for a large range of angles of 
incidence.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
In FIG. 1a, reference numeral 1 denotes a radiation source emitting a beam 
b. The beam comprises two beam components with mutually perpendicular 
polarisation directions. Of this beam only the chief ray is shown which, 
with the object of indicating the two differently polarised components, is 
split into two rays. In actual practice these rays coincide. The beam b is 
incident onto polarisation-sensitive beam splitter 2, for example 
constituted by two transparent prism portions 3 and 4 between which a 
polarisation-separating layer 5 is provided. The layer 5 reflects the beam 
component whose polarisation direction is parallel to the plane of 
incidence, the so-called p-component and transmits the component whose 
polarisation direction extends transversely of the plane of incidence, the 
so-called s-component. The plane of incidence is the plane formed by the 
incident ray and the normal on the layer 5. The reflected component, 
denoted by the sub-beam b.sub.1, is directed to the display panel 10 by a 
reflector 6. 
This panel has a layer of liquid crystalline material 17, for example of 
the nematic type, enclosed between two transparent, for example glass, 
plates 11 and 12. Each of these plates may comprise a transparent control 
electrode 13 and 14, which electrodes can be divided into a large number 
of rows and columns, thus defining a large number of image elements in the 
display panel. The different image elements can then be controlled by 
driving the matrix electrodes, as is shown schematically by means of the 
drive terminals 15 and 16. Thus, an electric field can be applied at the 
desired position across the liquid crystalline material 17. Such an 
electric field causes a change of the effective refractive index of the 
material 17, so that the light passing through a given image element is 
submitted or not submitted to a rotation of the polarisation direction, 
depending on whether a local electric field is present or not present in 
the region of the relevant image element. 
Instead of this what is commonly referred to as passively controlled image 
display panel an actively controlled panel may alternatively be used. In 
this latter type of image display panel one of the substrate plates is 
provided with an electrode whilst the other plate is provided with the 
semi-conductor drive electronics. Each of the image elements is now driven 
by its own active control element, such as, for example, a thin film 
transistor. 
Both types of directly driven image display panels are described in, for 
example, the European patent application no. 0,266,184. 
The sub-beam b.sub.1 passing through the image display panel 10 is directed 
to the polarisation-sensitive beam splitter 2 by a second reflector 7. 
FIG. 1a shows the situation in which the display panel is energised, that 
is to say the polarisation direction of the sub-beam b.sub.1 is not 
changed. The sub-beam b.sub.1 incident on the polarisation-sensitive beam 
splitter 2, having the p-polarisation direction is then reflected towards 
a projection lens system 20, which is denoted schematically by means of a 
single lens element. This lens system forms an enlarged image of the 
display panel enlarged on a projection panel 30, the optical path length 
between this panel and the lens system 20 being relatively long. So as to 
limit the dimensions of the arrangement the path between the lens 20 and 
the panel 30 can be folded with the aid of additional reflectors. 
FIG. 1b illustrates the situation in which the panel 10 is not energised so 
that the polarisation direction of the incident sub-beam b.sub.1 is 
shifted through 90.degree. and this beam leaves the panel as an 
s-polarised beam. This beam is then transmitted by the beam splitter 
towards the radiation source 1 and does not reach the projection panel 30. 
According to the invention, the sub-beam b.sub.2 denoted by double arrows, 
this sub-beam having the s-polarisation, denoted by means of the small 
circles in the beam, is directed to the image display panel via the 
reflector 7. After having passed through the panel 10 the polarisation 
direction of the sub-beam b.sub.2 has either remained constant or has been 
rotated through 90.degree., depending on whether the panel 10 was 
energised or not. After the sub-beam b.sub.2 has been reflected by the 
reflector 6 it is reflected by the beam splitter 2 towards the radiation 
source 1, or transmitted to the projection lens system 20. 
The beam splitter 2 does not only provide the formation of the two 
sub-beams, but also ensure that, after having been modulated by the image 
display panel, these sub-beams are again combined to one beam b'. In 
addition, the polarisation-sensitive beam splitter 2 ensures that the 
polarisation modulation of the sub-beams is converted into an intensity 
variation of these sub-beams. This beam splitter replaces, in this 
embodiment and further embodiments still to be described, a polariser and 
an analyser which in more conventional arrangements are disposed before 
and behind, respectively, the image display panel. Because of the many and 
various functions of the beam splitter 2 the number of elements can be 
limited to a minimum in the embodiment shown in the FIGS. 1a and 1b. 
It will be obvious that on projecting an image by means of an arrangement 
as illustrated in the FIGS. 1a and 1b, the two sub-beams will not be 
rotated in their totality as regards their direction of polarisation. Only 
those portions of the sub-beams originating from the non-energised image 
elements will be submitted to this polarisation rotation. 
In the embodiment described, the image elements across which no electric 
field is applied are shown as black elements on the projection panel 30. 
It is alternatively possible to energise an image element in such a 
manner, that is to apply such a field strength across it, that the 
polarisation direction of the incident linearly polarised light is not 
rotated through 90.degree. but that this linearly polarised light is 
converted into an elliptically polarised light. This light is split by the 
polarisation-sensitive beam splitter 2 into the p-and s-components, the 
p-component being reflected to the projection panel and the s-component 
being transmitted to the light source. The relevant beam element is then 
not shown as a black or a white element on the projection panel but as a 
grey element, the grey shade being adjustable. 
If use is made of an image display panel whose image elements rotate in the 
energised state the polarisation direction and do not rotate it in the 
non-energised state, an additional layer of liquid crystalline material 
which rotates the polarisation direction of the entire sub-beams through 
90.degree. can be arranged in series with the image display panel 10, so 
that the image on the projection panel has the same polarity as the image 
formed in an arrangement having an image display panel whose image 
elements do not rotate the polarisation direction in the energised state. 
An additional polarisation shifter, shown in FIGS. 1a and 1b by reference 
numeral 31, can alternatively be used if in an arrangement in which the 
image elements in the energised state do not rotate the polarisation 
direction one wants to have these image elements still appear as black 
elements on the projection panel, for example to obtain an increased 
contrast or to effect a decrease in the colour dependency of the 
arrangement, or in order to increase the switching rate of the display 
panel. 
Instead of a layer of liquid crystalline material it is alternatively 
possible to use a .lambda./2-plate, or for a reflective display panel a 
.lambda./4-plate, wherein .lambda. is the wavelength of the projection 
light, as an additional polarisation rotator 31. This polarisation rotator 
is also suitable for use in embodiments still further to be described 
hereinafter. 
In principle, the image display panel 10 can be provided in any arbitrary 
position between the reflectors 6 and 7 but alternatively also between the 
beam splitter 2 and the reflector 7 or between the reflector 6 and the 
beam splitter 2. Preferably however, the panel 10 is positioned such that 
the optical path length from the beam-splitting plane 5 to the panel 10 is 
the same for both sub-beams, so that for diverging sub-beams the 
cross-sections of these beams are identical in the region of the panel, 
and furthermore for both sub-beams the optical path length between the 
panel 10 and the projection lens is identical. 
The FIGS. 2a and 2b illustrate a second embodiment of the image projection 
arrangement, FIGS. 2a and 2b respectively, showing the radiation paths for 
the sub-beams b.sub.1 and b.sub.2 for an energised and nonenergised image 
display panel 10. In addition to the first polarisation-sensitive beam 
splitter 2, this arrangement includes a second polarisation-sensitive beam 
splitter 32 for combining the sub-beams after they have been modulated by 
the panel. Furthermore, a third polarisation-sensitive beam splitter 36 is 
provided, which is used as a polarisation-selective reflector, which, for 
example, reflects p-polarised light and transmits s-polarised light. The 
arrangement further comprises five reflectors 21, 22, 23, 24 and 25. 
The p-polarised sub-beam b.sub.1 originating from the beam splitter 2 is 
reflected by the reflectors 21 and 22 towards the polarisation-sensitive 
beam splitter 36 which in its turn reflects the sub-beam towards the 
display panel 10. The beam portions whose polarisation direction is not 
rotated (FIG. 2a) are reflected by the beam splitter 2 towards a reflector 
23 which reflects the beam portions to the beam splitter 32. The last beam 
splitter reflects the p-polarised beam portions towards the projection 
lens system 20. The portions of the sub-beam b.sub.1 whose polarisation 
direction is rotated, and which leave the panel 10 as s-polarised 
components (FIG. 2b) are transmitted by the beam splitter 2 towards the 
radiation source 1. 
The s-polarised sub-beam b.sub.2 originating from the beam splitter 2 first 
passes through the display panel 10. The portions of this sub-beam, whose 
polarisation direction is not rotated (FIG. 2a) pass through the third 
beam splitter 36 and are thereafter reflected towards the 
polarisation-sensitive beam splitter 32 by the reflectors 24 and 25. This 
last beam splitter transmits said portions of the sub-beam b.sub.2 to the 
projection lens system 20. The portions of the sub-beam b.sub.2 whose 
polarisation direction is indeed rotated, and which consequently leave the 
panel 10 as p-polarised components (FIG. 2b) are reflected, in this 
sequence, by the reflectors 22 and 21 and the beam splitter 2 to the 
radiation source 1. 
Since the light paths for the s- and p-polarisation components are 
spatially separated, the degree of polarisation of the sub-beams can be 
increased by incorporating in the separate path portions of these 
sub-beams additional elements which block light having an unwanted 
polarisation direction. As in practice the p-polarised light reflected by 
the beam splitter 2 will more easily be contaminated by s-polarised light 
than the s-polarised light passed by the beam splitter 2 is contaminated 
with p-polarised light, absorbing elements 37 and 38 which only absorb 
radiation of the s-polarisation are preferably, as is shown in FIG. 2a, 
arranged between the beam splitter 36 and the reflector 22 and between the 
beam splitter 2 and the reflector 23. 
In addition, in contrast to the reflectors in the embodiment of the FIGS. 
1a and 1b, the reflectors in the embodiment shown in the FIGS. 2a and 2b 
need only to reflect one polarisation direction. Only p-polarised light is 
incident on the reflectors 21, 22 and 23 and only s-polarised light on the 
reflectors 24 and 25. Consequently, the reflection power of these 
reflectors can be optimised, that is to say it can be at a maximum for the 
desired polarisation direction, which increases the light output of the 
arrangement. 
The invention is not only suitable for use in an image projection 
arrangement having a radiation-transmissive image display panel but also 
in an arrangement of that type having a reflecting image display panel. 
Such a panel, driven from a cathode-ray tube, is disclosed in said U.S. 
Pat. No. 4,127,322, whilst a directly driven reflective image display 
panel is disclosed in the U.S. Pat. No. 4,239,346. 
The FIGS. 3a and 3b illustrate an embodiment of an image projection 
arrangement according to the invention having a reflective image display 
panel. This panel 10 again has a layer of liquid crystalline material 17 
embedded between two substrate plates 11 and 18. The plate 11 is 
transparent whilst plate 18 is reflective. Control electronics is 
integerated in the plate 18. The control signal, for example a video 
signal, is applied to the driver electrodes 15 and 16 provided at the rear 
face of the plate 18. 
Apart from the fact that the image display panel 10 is reflective instead 
of transparent, the structure and mode of operation of the arrangement of 
FIGS. 3a and 3b correspond for a large part to that of the FIGS. 1a and 
1b. FIG. 3a again shows the situation in which the image display panel 
does not effect polarisation rotation and the sub-beams b.sub.1 and 
b.sub.2 after having passed through the path to and from the panel, reach 
the projection panel 30 via the beam splitter 2 and the projection lens 
system 20. In FIG. 3b the image display panel does rotate the polarisation 
direction of the sub-beams b.sub.1 and b.sub.2 and these sub-beams 
ultimately return to the radiation source 1 via the beam splitter 2. 
For a proper operation of the image display panel the sub-beams b.sub.1 and 
b.sub.2 must not be incident on this panel at excessive angles to the 
normal. So as to limit the angles of incidence, the image display panel 
can be positioned at a relatively large distance from the beam splitter. 
It is possible to "fold" the radiation path with the aid of additional 
reflectors between the reflector 6 and the panel 10 on the one hand and 
between this panel and the reflector 7 on the other hand, so that the 
dimensions of the projection arrangement can be of limited size. 
It should be noted that only those elements which are necessary to 
understand the invention are included in FIGS. 3a and 3b. In the practical 
embodiment of the arrangement shown in FIGS. 3a and 3b at least one 
additional reflector will be provided to achieve that the two sub-images 
are superimposed on each other with the same orientation, i.e. not 
mirror-inverted with respect to each other. 
FIG. 4 illustrates a further embodiment of an image projection arrangement 
having a radiation-transmissive display panel. This embodiment differs 
from the embodiment shown in the FIGS. 1a and 1b in that the 
polarisation-sensitive beam splitter 2 has a different construction, as a 
result of which the radiation source 1 and the projection lens system 20 
are arranged differently relative to each other. Two additional lenses 26 
and 27 are provided as extra elements in the embodiment of FIG. 4, on both 
sides of the display panel 10, which act as field lenses. These lenses 
reduce the divergence of the sub-beams b.sub.1 and b.sub.2 and provide 
that, at limited dimensions of the image projection arrangement, the 
highest possible amount of light from the source 1 is coupled into the 
pupil of the projection lens system 20. 
FIGS. 5a and 5b illustrate an embodiment of the image projection 
arrangement whose length in one direction is of a reduced size as a 
portion of the light path extends in a different direction. FIG. 5a is a 
side elevational view of the arrangement, or a cross-sectional view in the 
XZ plane, while FIG. 5b is a cross-sectional view in the YZ plane of the 
arrangement. Compared with the arrangement shown in the FIGS. 1a and 1b, 
the arrangement shown in the FIGS. 5a and 5b has two reflectors 8 and 9 
and two lenses 28 and 29 by way of additional elements. 
The projection beam b originating from the light source 1 is split by the 
polarisation-sensitive beam splitter 2 into a p-polarised sub-beam b.sub.1 
and into an s-polarised sub-beam b.sub.2. These sub-beams are incident on 
the reflectors 8 and 9, respectively. These reflectors are at such an 
angle with the YZ plane on the one hand and the XZ plane on the other hand 
that in principle they reflect the sub-beams b.sub.1 and b.sub.2 into the 
Y direction. The sub-beam b.sub.1 and b.sub.2, respectively, is thereafter 
directed to the radiation-transmissive display panel 10 by the respective 
reflectors 6 and 7. After having passed this panel, the sub-beam b.sub.1 
and the sub-beam b.sub.2, respectively, pass in the opposite direction 
through the same path the sub-beam b.sub.2 and the sub-beam b.sub.1, 
respectively, have passed through. The sub-beams b.sub.1 and b.sub.2 
finally reach the projection lens system 20 or the light source 1, 
depending on whether the polarisation direction of these beams has been 
shifted or not. 
The lenses 28 and 29 provide that the diverging sub-beams, going to the 
image display panel, are converted into less diverging or converging beams 
so that the dimensions of the optical elements in the arrangement may be 
of a limited size. These lenses are denoted as "relay" lenses or 
intermediate lenses. 
If a colour image is to be projected, it is possible to use instead of an 
image display system having one display panel a composite image display 
system having, for example, three display panels and a plurality of 
colour-selective beam splitters. FIG. 6 shows an embodiment of such a 
composite image display system, provided in an embodiment of an image 
projection arrangement similar to that shown in the FIGS. 1a and 1b. 
For the sake of clarity, only the radiation path of the sub-beam b.sub.1 is 
shown in this Figure. In a similar manner as shown in the FIGS. 1a and 1b, 
the sub-beam b.sub.2 passes through the same radiation path in the 
opposite direction. 
After having been reflected from the reflector 6, the sub-beam b.sub.1 is 
incident on a first colour-selective beam splitter 44, for example a 
dichroic mirror, which transmits, for example, red light and reflects blue 
and green light. The red beam b.sub.1,r is reflected to a first display 
panel 41 by a reflector 48 which may be a neutral reflector or a red 
reflector. In this panel the red sub-image is generated so that the beam 
b.sub.1,r is modulated with the red colour information. The light 
reflected by the beam splitter 44 is split by a second colour-selective 
beam splitter 45, which, for example reflects green light and transmits 
blue light, into a green beam b.sub.1,g and a blue beam b.sub.1,b. These 
beams are incident on the respective display panels 42 and 43 in which the 
green and the blue sub-image respectively, is generated. After having been 
passed through their image display panels, the sub-beams b.sub.1,r and 
b.sub.1,g are combined again with the aid of a third colour-selective beam 
splitter 46 which transmits the red beam and reflects the green beam. The 
blue beam emitted from the panel 43 is reflected to a fourth colour 
selective beam splitter 47 by a reflector 49, which may or may not be of a 
blue reflector. Said last beam splitter transmits the blue beam and 
reflects the combined red-green beam, so that all the light is combined 
again into one beam. This beam is conveyed by the reflector 7 to the 
polarisation-sensitive beam splitter 2 which reflects the light 
originating from driven image elements in the panel to the projection lens 
system, not shown, and allows the remaining light to pass to the source 1. 
The sequence of the colour-selective elements and the image display panels 
may of course also be chosen differently from the sequence shown in FIG. 
6. 
The FIGS. 7a and 7b show a compact embodiment of a colour image projection 
arrangement according to the invention, in which the colour separation of 
the sub-beams is effected with the aid of two intersecting dichroic 
mirrors, alternatively denoted dichroic cross. FIG. 7a is a perspective 
view of the arrangement, whereas FIG. 7b is a schematic plan view of the 
arrangement. For the sake of clarity only the radiation path for the green 
partial sub-beams b.sub.1,g and b.sub.2,g are shown in FIG. 7a. 
Of the projection beam b originating from the light source 1 the 
p-polarised sub-beam b.sub.1 is reflected by the beam separating plane 5 
of the polarisation-sensitive beam splitter 2 to the lower portion of the 
dichroic cross 50. This cross is formed by two dichroic mirrors 51 and 52 
of which the mirror 50 reflects red light and transmits blue and green 
light, whereas the mirror 51 reflects blue light and transmits red and 
green light. Consequently, of the sub-beam b.sub.1 only the green 
component b.sub.1,g is transmitted to a reflector 55. This reflector 
directs the component b.sub.1,g to the display panel 42 in which the green 
sub-image is generated. The sub-beam component b.sub.1,g modulated with 
the information of this sub-image is reflected by a reflector 56 and 
transmitted through the upper portion of the dichroic cross 50 to the 
polarisation-sensitive beam splitter 2. At the separating plane 5 the 
sub-beam component b.sub.1,g is reflected to the projection lens system 20 
or transmitted to the radiation source 1, depending on the fact whether 
the polarisation direction of the sub-beam component is rotated or not 
rotated by the display panel 42. 
The red beam component b.sub.1,r reflected by the dichroic mirror 51 and 
the blue beam component b.sub.1,b reflected by the dichroic mirror 52 pass 
through sub-systems which are similar to the sub-system for the green beam 
component. These sub-systems comprise reflectors 53 and 54 and a display 
panel 41, and reflectors 57 and 58 and a display panel 43, respectively, 
as is shown in the plan view in FIG. 7b. After having been modulated by 
the associated image display panels, the sub-beam components b.sub.1,r and 
b.sub.1,b are combined with the component b.sub.1,g by the dichroic cross 
50. Of the total colour image information-modulated sub-beam b.sub.1 the 
polarisation modulation is converted into an intensity modulation, by the 
polarisation-sensitive beam splitter 2 whereafter the sub-beam b.sub.1 is 
projected via the lens system 20 to the projection panel, not shown. 
Since the colour-separating properties of the dichroic mirrors 51 and 52 
depend on the direction of polarisation of the incident light, one of the 
reflectors 53 and 54 in the red channel is preferably a red reflector, 
whereas one of the reflectors 57 and 58 in the blue channel reflects only 
blue light. Also in this case said red and blue reflectors may be dichroic 
mirrors. 
To allow a compact design of the arrangement shown in FIGS. 7a and 7b, two 
field lenses which are similar to the field lenses 26 and 27 in the 
arrangement of FIG. 4 can be provided before and behind each image display 
panel 41, 42 and 43. 
Instead of providing two lenses in each colour channel the same object can 
also be achieved by using a total of only two lenses positioned in a 
similar manner in the arrangement as the intermediate lenses 28 and 29 in 
FIG. 5b, the dichroic cross together with the elements 41-43 and 53-58 
being provided at the position of the display panel 10 of FIG. 5b. Instead 
of only field lenses or intermediate lenses only combinations of such 
lenses can alternatively be used in the different embodiments. 
It will be obvious that the s-polarised sub-beam b.sub.2, which is not 
shown in the FIGS. 7a and 7b, can pass through the same path as the 
sub-beam b.sub.1 but then in the opposite direction. 
FIG. 8a shows schematically and in a plan view a colour projection 
arrangement having three colour channels 80, 81 and 82 for the primary 
colours red, green and blue, a separate radiation source 83, 84 and 85 and 
a separate polarisation-sensitive beam splitter being incorporated in each 
of the colour channels. 
FIG. 8b shows the green colour channel in greater detail. This channel 
includes a source 84 for green light. The beam b.sub.g produced by this 
source is incident on a polarisation-sensitive beam splitting prism 86. At 
the separating layer 87, the p-polarised sub-beam b.sub.g,1 is reflected 
to a first reflector 88, which directs the sub-beam b.sub.g,1 to the image 
display panel for the green sub-image. After having passed through this 
panel, the sub-beam b.sub.g,1 is incident on a second reflector 89 which 
reflects the sub-beam to the polarisation-sensitive beam splitter 86 where 
the polarisation modulation is converted into an intensity modulation. 
The s-polarised sub-beam b.sub.g,2 passes through the same path as the 
sub-beam b.sub.g,1 in the opposite direction. 
The colour channels 80 and 82 are of the same design as the colour channel 
81. The monochrome beams b.sub.g, b.sub.r and b.sub.b emanating from the 
colour channels are combined by, for example, a dichroic cross 50 into a 
coloured beam b which is projected by the projection lens system 20 onto a 
panel, not shown. 
In the arrangement shown in the FIGS. 8a and 8b, the elements 86, 88 and 89 
in the green colour channel 81 and the corresponding elements in the other 
colour channels 80 and 82 can be optimised for the relevant colour. 
The arrangement shown in FIGS. 8a and 8b can again be of a compact design 
by the use of field lenses similar to the field lenses 26 and 27 in FIG. 4 
or the use of intermediate lenses similar to the lenses 28 and 29 in FIG. 
5b or a combination of these lenses. 
Instead of transmission-display panels, the colour channels may 
alternatively include reflection panels. Then each colour channel may be, 
for example, of a design as shown in FIG. 3. 
Instead of using a separate light source in each one of the colour 
channels, it is alternatively possible to use one common light source. The 
beam produced by this source is split by colour-selective means into three 
beams of the respective primary colours red, green and blue which are then 
conveyed to the red, green and blue colour channels. An embodiment of the 
arrangement in which this has been realized is schematically shown in FIG. 
12. Said colour selective means may be constituted by a dichroic cross 
120. Reference numerals 80', 81' and 82' denote the red, green and blue 
channel without radiation sources. One of the colour beams, for example 
b.sub.g is directly incident into the associated colour channel, whereas 
the other colour beams b.sub.r and b.sub.b are conveyed to the associated 
channels 80' and 82' via additional reflectors 121, 122 and 123, 124, 
respectively. 
FIG. 13 schematically illustrates an embodiment of the arrangement which is 
based on the same concept as the embodiment shown in FIG. 8, but in which 
the beams b.sub.r, b.sub.g and b.sub.b are projected onto the screen 30 by 
separate projection lenses 131, 132 and 133, instead of being combined 
first and thereafter jointly projected by one projection lens. By tilting 
in the arrangement of FIG. 13 the picture display panels 41, 42, 43 or the 
colour channels 80, 81, 82 in their totality relative to each other, it 
can be ensured that the beams b.sub.r, b.sub.g and b.sub.b accurately 
coincide on the projection screen. 
The use of three projection lens systems has the advantage that each system 
can be optimized for the relatively narrow wavelengthband of the 
associated colour beam, so that the projection lens systems can be simpler 
and cheaper than a projection lens system for the projection of the three 
colour beams. 
A projection device having three projection lenses may alternatively use 
one radiation source instead of three separate radiation sources, as is 
shown schematically in FIG. 14. 
After the description of FIGS. 12 and 13, FIG. 14 does not require any 
further explanation. 
The inventive idea is also suitable for use in a colour image projection 
device in which only one display panel is used. This arrangement may be of 
a structure as shown in FIGS. 3a and 3b, in which the monochrome panel 10 
is replaced by a composite or colour panel. This colour panel then 
comprises a number of image elements which is, for example, three times as 
many as the number of image elements of a monochrome panel. The image 
elements of the colour panel are arranged in three groups, a red, green 
and blue sub-image beam being generated by these groups. As image element 
of each of the groups is always added to a image element on the projection 
panel. Each of the image elements is then, for example, preceded by an 
individual colour filter which only transmits the colour desired for the 
relevant image element. 
FIG. 9 shows an embodiment of an image projection arrangement in which, 
similar to the arrangement shown in FIGS. 2a and 2b, separate 
polarisation-sensitive beam splitters 90 and 96 are used for splitting on 
the one hand (beam splitter 90) the beam b into two sub-beams b.sub.1 and 
b.sub.2 having mutually perpendicular polarisation directions and on the 
other hand for combining (beam splitter 96) these sub-beams after they 
have been modulated by the image display panel 10. The beam splitter 96 
converts the polarisation modulation of the sub-beams b.sub.1 and b.sub.2 
into an intensity modulation. The sub-beams b.sub.1 and b.sub.2 now pass 
through the image display panel in the same direction. 
The p- and s-polarised sub-beams b.sub.1 and b.sub.2 emanating from the 
beam splitter 90 having polarisation separating plane 91 are reflected by 
their associated reflectors 92 and 93 to the same side of the image 
display panel 10. After having passed through this panel the sub-beams 
b.sub.1 and b.sub.2 are reflected by further reflectors 94 and 95 to the 
second beam splitter 96 having separating plane 97. The 
intensity-modulated beam b emanating from the beam splitter 96 is 
projected onto a panel, not shown, by the projection lens system 20. 
The projection arrangement shown in FIG. 9 can be made suitable for colour 
image projection by providing at the position of the image display panel 
10 a transmissive colour panel or a composite image display system having 
colour-splitting means and three monochrome panels, for example similar to 
FIGS. 6, 7a and 7b. 
It should be noted that in the arrangement whose basic circuit diagram is 
shown in FIG. 9, an additional reflector will in practice be provided in 
the path of one of the beams b.sub.1, b.sub.2 to achieve that the images 
formed by these beams on the projection screen will have the same 
orientation, i.e. they are not mirror-inverted with respect to each other. 
The arrangement shown in FIG. 9 again provides the possibility of arranging 
polarisation filters in positions in which the beams b.sub.1 and b.sub.2 
are spatially separated to increase the degree of polarisation of those 
beams. 
In the embodiments of the image projection device in which a 
polarisation-dependent beam splitter is only used to split the beam 
originating from the radiation source into two mutually 
perpendicular-polarised subbeams, this beam splitter can be accomodated in 
one housing, together with the radiation source and any beam shaping 
optical means. 
A polarisation-sensitive beam splitter in the image projection arrangement 
can be constituted in known manner by a Wollaston prism consisting of two 
cemented-together prisms of double-refractive material, the optical axes 
of the two prisms being perpendicular to each other. It is alternatively 
possible to utilise a what is commonly denoted a Glan-Thompson prism or a 
Glan-Taylor prism of double-refractive material, in which one beam 
component having one of the polarisation directions, p or s, is subject to 
total internal reflection at a prism face and the other component does not 
meet such an internal reflection. The two last-mentioned prisms as well as 
the Wollaston prism are expensive because of the double-refractive 
material to be used for them. 
Therefore, preference should be given to use in image projection 
arrangements, more specifically those intended for consumer usages, the 
beam splitter shown in FIG. 10. This beam splitter 100 is formed by two 
transparent prisms 101 and 102 of, for example, glass, with an 
intermediate layer 103. This layer is formed by a liquid crystalline 
material, and consequently has double refraction. The ordinary index of 
refraction n.sub.o of the material is substantially always equal to 
approximately 1.5, while the extraordinary index of refraction n.sub.e can 
have a value between 1.6 and 1.8 depending on the composition of the layer 
103. The prisms 101 and 102 are provided with what are commonly referred 
to as orientation layers 104 and 105 which ensure that the optical axis of 
the layer 103 is perpendicular to the plane of drawing. In FIG. 12 this 
axis is indicated by the circle 106. 
The beam b incident on the beam splitter has two polarisation components, 
the p- and s-polarised components. Measures have been taken to ensure that 
the refractice index of the prism material is equal to n.sub.e of the 
layer 103, for example 1.8. If the beam b is incident on the layer 103 at 
an angle of incidence .theta.i which exceeds or is equal to the critical 
angle .theta..sub.g the p-polarised beam component is subjected to a total 
reflection in the direction of the arrow 107 as the ordinary refractice 
index applies to this component. For the s-polarised beam component, whose 
direction of polarisation extend transversely of the angle of incidence, 
the extraordinary refractive index of the liquid crystalline material 
applies, so that this component does not "see" any refractive index 
difference on passing through the beam splitter, and consequently passes 
through the layer 103 and the prism 102 in the original direction. 
The refractive index difference .DELTA.n=ne-no of liquid crystalline 
material can be great, so that the beam splitter is suitable for a large 
range of angles of incidence. In addition it can be ensured, that the 
refractive index of the prism material and that of the layer 103 vary in 
the same way versus varying wavelength of the beam b, so that the beam 
splitter has a high polarisation efficiency for a large wavelength range. 
A very important advantage of the beam splitter of FIG. 10 is that it is 
cheap since no expensive double-refractive prism material need to be used 
and its production is relatively simple. 
There is no need for the prisms 101 and 102 to be solid, it is 
alternatively possible for these prisms to consist of glass, or other 
transparent, walls in which a transparent liquid or synthetic material 
having a high refractive-index equal, to n.sub.e of the layer 103 is 
applied. These walls must have the same refractive index as the liquid or 
synthetic resin material which shall not show any depolarising effects. 
By a change in the construction of the beam splitter shown in FIG. 10, the 
image projection arrangement can be given a more compact construction, as 
is shown in FIG. 11. Measures have been taken to ensure that the plane 110 
of prism 101 onto which the beam b originating from the source 1 is 
incident is perpendicular to this beam. In addition, the direction in 
which the beam is incident has been chosen such that at the position of 
the polarisation separating plane 103 the angle of incidence is less than 
the critical angle, so that the beam b is transmitted in its totality. The 
plane 113 of the prism 102 is coated with a reflecting layer 116 which 
reflects the beam b to the separating layer 103. At this second incidence 
on the layer 103 the angle of incidence exceeds the critical angle, as a 
result of which the desired separation according to polarisation direction 
occurs. The p-polarised beam b.sub.1 leaves the prism 102 via the plane 
114 and the s-polarised beam b.sub.2 leaves the prism 101 via the plane 
112. 
The beam splitter 100 can also be used for combining the sub-beams b.sub.1 
and b.sub.2 after they have been modulated with image information, for 
example in the case in which reflecting display panels are used. The 
modulated sub-beam b.sub.1 coming from the right enters the prism 101 via 
the face 112 and is reflected or not reflected at the separating plane 103 
to the projection lens 20, depending on whether its direction of 
polarisation has been rotated or not rotated. The modulated sub-beam 
b.sub.2 coming from the right enters the prism 102 via the face 114 and is 
transmitted or not transmitted to the projection lens system 20 by the 
separating plane 103 depending on whether its direction of polarisation 
has been shifted or not. The polarisation-sensitive beam splitter again 
converts the polarisation modulation of the sub-beam into an intensity 
modulation. 
FIG. 15 is a partial view of an embodiment of the image projection device 
in which, as also in FIG. 9, an only partially shown 
polarisation-sensitive beam splitter is exclusively used for splitting a 
beam produced by a radiation source, into two sub-beams. These sub-beams 
b.sub.1 and b.sub.2 are reflected by their associated reflectors 92 and 93 
to the same side of the image display panel 10, where they intersect. 
After passage through the panel 10, the beams b.sub.1 and b.sub.2 are 
deflected by a field lens 150 to the optical axis 00'. To approximately as 
far as the focal plane 151 the beams b.sub.1 and b.sub.2 partly coincide, 
thereafter they are fully separate. A first polarisation analyser 152 is 
disposed, behind the focal plane 151, in the path of only the beam 
b.sub.1, a second polarisation analyser 153 is disposed in the path of 
only the beam b.sub.2, which analysers convert the polarisation 
modulations of the beams b.sub.1 and b.sub.2 into intensity modulations. 
The intensity-modulated beams are projected onto the projection screen, 
not shown, by the projection lens system 20. The polarisation directions 
of the analysers 152 and 153 are perpendicular to each other so that the 
beams constituted by the beams b.sub.1 and b.sub.2 have the same polarity. 
Although the present invention is in the first place intended for an image 
projection arrangement having directly driven image display panels it is 
not limited thereto. Also when used in an image projection arrangement 
having indirectly driven display panels, for example by means of 
cathode-ray tubes, the invention can increase the luminous flux.