CA2233597C - High efficiency protection displays having thin film polarizing beam-splitters - Google Patents

Abstract

A projection display system includes a thin film polarizing beam splitter/combiner has first and second light transmissive substrate in the form of prisms and a plurality of thin film layers disposed between t he prisms. The thin film layers have predetermined thicknesses and refractive indices which allow unpolarized incident light to be separate d into s-polarized and p-polarized light beams. Both frustrated total internal reflection and thin film interference are simultaneously employed to allow s-polarized light to be transmitted and p-polarized light to be reflected over a wide range of wavelengths and angles of incidence. The device further includes two spatial light modulators for encoding an image to be projected. By providing a different signal to each spatial light modulator, a three-dimensional display results.

Description

High Efficiency Projection Displays Having Thin Film Polarizing Beam-Splitters Field of the Invention This invention relates generally to projection displays, and more particularly, to 2-D and 3-D projection display systems.
Background of the Invention Polarizers and polarizing beam-splitters (PBS) are optical components that are currently widely used in optical instruments, lasers, electro-optic displays, optical recording and in the fabrication of other optical components. There are several parameters 1o that can be used to describe the performance of a polarizer or polarizing beam-splitter.
These parameters are: the wavelength range over which the polarizer or polarizing beam-splitter is effecti~~e, the angular field of the incident light in which the pofarizer or polarizing beam-sputter is effective and the extinction ratio of the desired polarizLd light to the unwanted polarized light after the light passes through a polarizer or polarizing 15 beam-sputter.
Commonly available polarizers and polarizing beam-sputters can be divided into several types that depend upon different physical principles: pile-af~-plates polarizers, refection poiarizers. dichroic sheet polarizers, polarizers based on birefringent crystals, metallic grid polarizers, and thin film interference palarizers.
2o Pile-of plates polarizers consist of a stack of parallel transparent plates that are placed in series. 'they are mainly used in the ultraviolet and infrared parts of the spectrum.
Normally, light is incident at each interface at the Brewster angle such that all thep-polarized light and only some of the s-polarized light is transmitted. If a sufficient number of such plates are placed in series, the transmitted light can be highly polarized anal have a 25 high extinction ratio. Although these poiarizers act over a very broad spectral region, their angular field is limited.

Reflection polarizers are based on a similar principle but use light reflected from one or more surfaces to polarize a light beam. One advantage of this polarizes is that its performance is independent of the wavelength. However, its performance is very sensitive to the angle of the incident beam. An additional complication is that the reflected light .
propagates in a different direction from that of the incident light.
Dichroic poIarizers are both wide-angle and broad-band and are based on the absorption of light of one polarization. They can be very thin and are convenient to use.
They are made of plastic and can be produced in large sizes and at low cost.
However, at least 50% of light is lost by absorption in these polarizers and the extinction ratio is not l0 very high. Therefore, these polarizers are typically used in low power applications in which damage to the device due to light absorption is not a concern.
Polarizers based on birefringent materials also perform well over a broad band of wavelengths and a wide range of angles. These polarizers are based on the total internal reflection of light in birefringent crystals. Normally these polarizers are comprised of two birefringent crystal prisms that are in contact with each other. 'The optical axes of the two prisms are arranged in such a way that the refractive indices at both sides of the contact surface are the same for the ordinary light (polarized in one direction) and are higher in the first prism and lower in the second prism for the extraordinary light (polarized in the other direction), or vice versa. When unpolarized light is incident upon the interface between 2o the two prisms and if the angle of incidence is larger than the critical angle for the extraordinary light, the ordinary light will be substantially transmitted while the extraordinary light will be totally reflected. Therefore, a very high extinction ratio is achieved. Many different arrangements for such polarizers exist. However this type of polarizes is costly and cannot be made in large sizes because of the limited availability of 35 birefringent crystal materials.
Metallic grid polarizers transmit light whose electric field vector is parallel to the grid lines and reflect light whose electric field vector is perpendicular to the grid lines.
These polarizers are effective over a wide spectral region and a wide range of angles.
However, because of the difficulty of making large grids of very small spacings, their use 30 is confined to the infrared or longer wavelengths.

Multilayer thin film plate polarizers basically consist of quarterwave layers and generally have a high extinction ratio. Unfortunately, they operate over a narrow spectral ' region and have a small angular field. Another type of thin film polarizers invented by MacNeille (US. Patent No. 2,403,73 I ), makes use of the Brewster angle.
MacNeille polarizers are comprised of thin films embedded between two prisms and their extinction ratio increases with the number of layers. MacNeille polarizers operate over a very broad spectral region, but are only effective over a very narrow range of angles, usually the angular field measured in air is of the order of ~3°. In addition, the extinction ratios for the reflected and transmitted beams are different. There is some contamination of s-1 o polarized light in the transmitted p-polarized Light. Mouchart et. al. in a paper entitled "Modified MacNeille cube polarizers for a wide angular field," Appl. Opt. 28, ( I 989), have shown that it is possible to broaden the angular field of MacNeiIIe polarizers, but only at the expense of reducing the width of the spectral region over which they are effective.
D. Lees and P. Baumeister, in a paper entitled "Versatile frustrated-total-reflection polarizer for the infrared," Opt. Lett. 4, 66 ( I 979) describe a thin film infrared polarizer based on the principle of frustrated total internal reflection Fabry-Perot filters. In this device a high refractive index spacer layer is sandwiched between two low refractive index layers to form an etalon that is deposited onto the surface of a germanium prism. The light is incident at an angle that is greater than the critical angle - one that is chosen in such a way that the phase difference between s- and p-polarized light is 180°.
Therefore, if the Fabry-Perot etalon condition is satisfied for the p-polarization, all the p-polarized light will be transmitted while the s-polarized light is reflected. and vice versa. The phase difference between the s-polarized and p-polarized light depends on the refractive index of the substrate and the angle of incidence. Because of the limited range of refractive indices of materials for the visible part of the spectrum. such polarizers can only be constructed for the infrared. Furthermore, because these polarizers are essentially narrow band filters, the band-width is small. Another disadvantage of these polarizers is that they also have a ' small angular field. This is because the phase change on reflection and the optical 3o thickness of the spacer layer change with angle in opposite directions.

Polarizers are essential optical components in liquid crystal displays (LCDs).
Currently, most liquid crystal displays are based on twisted nematic liquid crystals that require the use of two polarizers. The principles and advantages/disadvantages of LCDs are described in the following prior art references. L. E. Tananas Jr., Flat panel displays and CRTs, Van Nostrand Reinhoid Company, New York, 1985. Terry J. Scheffer et al, "Supertwisted-Nematic (STN) LCDS." SID'95 Seminar Lecture Notes, 1995. Webster E.
Iioward, "Supertwisted-Nematic (STN) LCDS," SID'95 Seminar Lecture Notes, 1995.
Twisted nematic (TN) liquid crystal displays can be passive or active matrix addressed. A
basic TN LC cell or picture element (pixel) consists of a liquid crystal layer and two Io transparent electrodes, and they are is normally sandwiched by the first polarizer and the second poIarizer (also called analyzer). The transmitting axes of the two polarizers are perpendicular (or parallel} to each other. Unpolarized incident light passes through the first polarizer and becomes linearly polarized. When no voltage is applied to the liquid crystal layer, the liquid crystal acts as a halfwave plate and rotates the polarization of the f 5 incident polarized Light by 90°. The Light will therefore be transmitted by the second poiarizer; and, the pixel is said to in the"on" state. When a voltage is applied to a liquid crystal pixel, the liquid crystal molecules align themselves with the electric field and the light does not undergo rotation in the plane of polarization. In this instance, the light is blocked by the second polarizer and the pixel is in an "off' state.
2o Currently. dichroic sheet polarizers are the only choice for both flat panel and projection LCD displays due to their wide angular fields and broad band widths. The reason for this is that other currently available polarizers either have limited angular field, limited band width or are limited in size or are too expensive to use (birefringence crystals). however, dichroic sheet polarizers are based on the absorption of light of one 25 polarization in order to obtain polarized light. This absorption causes two problems: ~0-54% of the incident light is absorbed by the first polarizer and this makes the display less energy efficient; and, the light absorbed by the first polarizer and by the second polarizer (from the "off' pixels) is transformed into heat which can cause the polarizer's performance to deteriorate. This problem can be very serious in projection displays where 30 lugh power Light beams are applied.

Although LCDs are relatively more efficient than other types of flat panel displays such as those based on TFELs and on plasmas, the typical total transmission of light in an active matrix addressed direct-view flat panel LCD is still only about 3-7%.
The total transmission of LCD projection displays is higher because of the use of three panels. It follows from the above that both problems mentioned are due to the absorption of light in dichroic sheet polarizers and so their elimination or replacement by polarizers based on other principles is highly desirable.
To avoid the use of dichroic sheet polarizers, other types of LCDs that do not use the polarization scheme have been proposed. For example, polymer dispersed LCDs I o (PDLCDs} which use light scattering to distinguish between an "on" or "off pixel have been demonstrated. However, PDLCDs do not have high contrast and are still technically immature compared to TN LCDs.
Another way to overcome the light loss problem is to use a polarization recovery scheme; such schemes are proposed by Noriji Oishi, "Polarization forming optical device,"
is U.S. Patent No. 5,359,455, 1994., by Masso Imai, "LCD Projector," U.S.
Patent No.
5283600, 1994; and by GIueck, E. Ginter, E. Lueder and T. Kallfass, "Reflective TFT-Addressed LC Light-Valve TV Projectors with High Light effciency," SID'95, 235238( I 995). In these schemes, the polarization plane of the light with the unwanted polarization is rotated by 90° and the light is then redirected into the optical image system.
20 Therefore, no light is Lost. However, only NEC has thus far offered a commercial L,CD
projection display using a polarization recovery scheme. The above polarization recovery schemes require the use of thin film MacNeille polarizing beam-sputters which have a very small angular field (t2° in glass) and their extinction ratio is good for only one polarization and is not so good for the other polarization. Because of this, extra dichroic 25 sheet poiarizers are needed to clean up the beam. Furthermore, the above schemes do not solve the problem of the light absorption by the second polarizers.
Fuad E. Doany et al describes a way to form high performance projection display with two light valves, U.S. Patent No. ~, 5I7, 340, issued May I4, 1996. Thev use one polarizing beam-splitter to split light into two polarized beams and then direct the beams 30 to the corresponding light valves. The images from the light valves are reflected and then WO 98/07279 PCTICA97l00567 combined by the PBS to form a single image which is projected onto a screen.
In their display, they use polarizing beam-splitters(PBS) in a cube form that are based on conventional thin film PBSs. According to the limited performance of the conventional PBSs, displays based on this approach will suffer poor image contrast and low efficiency and can not be made practical. This is because of the low extinction ratios and limited angular field of the conventional PBSs.
To summarize, all proposed approaches to overcome problems associated with the use of dichroic poIarizers in LCDs will require the use of non-absorbing, broad-band and high extinction ratio polarizing beam-splatters. Unfortunately, none of the current to available polarizers or polarizing beam-splatters meet all the above requirements.
Lt I,i and J. A. Dobrowolski have proposed a method of designing of broad-band and wide-angle polarizing beam-splatters, see the paper "Visible broadband, wide-angle thin film multilayer polarizing beam splatter," Appl. Opt. Vol.
35,n.13,p2221(1996). The proposed polarizing beam-splatters are better than the conventional MacNeille polarizers;
t 5 however, extra dichroic sheet polarizers are still required.
Therefire an object of the invention is to provide a high efficiency projection display system that will work over a wide range of angles.
Su~ntnary of the Invention According to the present invention there is provided a projection display system 2o comprising a light source for generating an input beam of light; a polarizing beam discriminator employing frustrated total internal reflection and thin film interference to discriminate between s and p polarized light, said discriminator separating s and p polarized light beams from said input beam; spatial light modulator means for encoding an image onto said respective s and p polarized beams; means for combining said encoded s 25 and p polarized beams into an output beam, said combining means employing frustrated total internal reflection and interference; and means for focussing the output beam onto a display means.

_7_ The means for combining the encoded beams is preferably the same discriminator employed to separate the uncoded beams in to s- and p- polarized beams, in which case the encoded beams are reflected back into the device in accordance with a scheme that depends on the type of spatial modulators employed. However, this is not necessary and it is equally possible to combine the beams with a second similar polarizing beam discriminator downstream of the first. Instead of reflecting the encoded beams back into the first device, the encoded beams are transmitted to a second device where they are combined into the output beam.
The invention makes use of a thin film polarizing device of the type claimed per se to in our co-pending application no. PCT/CA96/00545. This device has a thin film arrangement for separating s-polarized light and p-polarized light by reflecting p-polarized light and transmitting s-polarized light. The device comprises first and second light transmissive substrates; and, a plurality of thin film layers disposed between the first and second light transmissive substrates. The thin flm layers comprise high refractive index 1 s layers and low refractive index layers, the high refractive index layers having one or more different refractive indices, and the low refractive index layers having one or more different refractive indices; the first and second light transmissive substrates, each in the form of a prism having a refractive index greater than the refractive index of each of the low refractive index layers; the prisms being shaped in such a manner as to allow the 2o incident light to be incident upon the thin f lm layers at a plurality of angles greater than or equal to the critical angle for the highest refractive index of the low refractive index layers;
the thickness of the low refractive index layers of the plurality of thin film layers being small enough so that light incident upon the thin film layers at an angle greater than the critical angle can be partially coupled out through the low refractive index layers such that ...+....+na +..+..t :_ .r_~ -_n__ ~~__ muauavcu m.ai ltCtttal tGllCCaiiJil ULC;LIIS, InuS pertniiting interference to take place between the light reflected at the interfaces of all the thin film layers, and in addition, the thicknesses of the thin film layers being such that the equivalent optical admittance of the plurality of the thin film layers for s-polarized light is substantially the same as the optical admittance of the substrate for s-polarized light for a wide range of angles of incidence and 30 a broad band of wavelengths when the incident light is incident upon the low refractive index layers at an angle greater that the critical angle, thereby allowing substantially all the -g_ incident s-polarized light to be substantially transmitted; the plurality of the thin film layers having an equivalent optical admittance forp-polarized light that is substantially different from the optical admittance of the substrate forp-polarized light for a wide-range of angles of incidence and a broad-band of wavelengths and thus substantially reflecting all the incident p-polarized light. This polarizing beam-sputter 30 lends itself to use in various embodiments of the novel 2-D and 3-D projection system claimed herein.
The invention makes provides a high efficiency projection display system that employs a non-absorbing, broad-band, wide-angle and high extinction ratio thin film polarizing beam-sputter. The display system that utilizes both s-polarized andp-polarized to light and projects both two-dimensional(2D) and three-dimensional(3D) images.
Aspects of this invention make use of a novel design of a very broad band {wavelength ratios as large as 50:1 ), very wide angular field (up to ~6 l ° in the infrared) thin film polarizing device. These thin film polarizing devices are based on frustrated total internal reflection. l Iowever, the design approach permits polarizers with varying 15 performance specifications to be produced for the ultraviolet, visible, infrared, far infrared to the microwave spectral regions.
Brief Description of the Drawings The present invention and exemplary embodiments of the invention will be described in accordance to the following drawings in which:
20 Fig. I . is a schematic view showing the beams of light transmitted and reflected by the interfaces of a single thin film structure;
Fig. 2. is a three-dimensional view of the phase change on reflection as a function of the angle of incidence and of the refractive index ratio for s-polarized light;
Fig. 3. is a three-dimensional view of the phase change on reflection as a function of the 'S angle of incidence and of the refractive index ratio fore-polarized Light, the refractive index ratio yis between 0.2 and 1;

WO 98/07279 PCT/CA97/0056'7 Fig. 4. is a three-dimensional view of the phase change on reflection as a function of the angle of incidence and of the refractive index ratio forp-polarized light, the refractive index ratio y is between 1 and 3;
' Fig. 5. shows another three-dimensional view of the phase change on reflection forp-polarized light as in Fig. 4;
Fig. 6. is a cross-sectional view of a basic thin film structure having low, high and low refractive index layers embedded between two substrates;
Fig. 7. is a cross-sectional view of another basic thin film structure having high. low and high refractive index Iavers embedded between two substrates;
1 o Fig. 8. shows the real and imaginary parts of the equivalent optical admittance of a basic syrrunetrical three-layer structure and the optical admittance of the substrate as a function of wavelength ratio for s-polarized light;
Fig. 9. shows the real and imaginary parts of the equivalent phase thickness of the same basic structure as in Fig. 8 for s-polarized Light;
Fig. I 0. shows the real and imaginary parts of the equivalent optical admittance of the same basic structure as in Fig. 8 and the optical admittance of the substrate as a function of wavelength ratio forp-polarized light;
Fig. 1 I . shows the real and imaginary parts of the equivalent phase thickness of the same basic structure as in Fig. 8 forp-polarized Light;
Fig. 12. shows the calculated reflectance curves for s-polarized light as a function of wavelength ratio for thin film systems having 1, 20, 40 and 60 periods of a three-layer basic structure as in Fig. 8. The angle of incidence 6b is 57.3°.
Fig. I 3. shows the calculated transmittance curves for s-polarized light as a function of wavelength ratio for the same thin film systems as shown in Fig. 12. The angle of incidence 6i~ is 57.3°.

Fig. 14. shows the calculated reflectance curves for p-polarized light as a fiznction of wavelength ratio for the same thin flm systems as shown in Fig. 12. The angle of incidence 6~ is 57.3 °. ' Fig. 15. shows the calculated transmittance curves forp-polarized light as a function of s wavelength ratio for the same thin film systems as shown in Fig. I2. The angle of incidence 6b is 57.3°;
Fig. 16. shows the calculated reflectance curves for s-polarized light as a function of wavelength ratio for a thin film system having 40 periods of the basic structure as in Fig. 8. The angles of incidences are 50°, 55°, 60°, 65° and 70°;
1o Fig. I7. shows the calculated transmittance curves fore-polarized light as a function of wavelength ratio for the same thin film system as in Fig. 16. The angles of incidences are 50°, 55°, 60°, 65° and 70°;
Fig. l 8. is a cross-sectional view of an extended basic symmetrical thin film structure having (2L-I) layers, L is larger than I;
t 5 Fig. I 9. shows the calculated reflectance curves for s-polarized Light as a function of wavelength ratio for thin film systems having 1, 50 and 100 periods ofa five-layer basic structure. The angle of incidence f~ is 57.3°;
Fig. 20. shows the calculated transmittance curves forp-polarized Light as a function of wavelength ratio for the same thin film systems as shown in Fig. I 9. The angle of 2o incidence b~ is 57.3°.
Fig. 21. shows the calculated reflectance curves for s-polarized light as a function of wavelength ratio for a thin film system having 100 periods of the basic structure as in 1~ig. 19. The angles of incidences are 50°, 55° and 60°;
Fig. 22. shows the calculated transmittance curves forp-polarized light as a function of ?s wavelength ratio for the same thin film system as in Fig. 19. The angles of incidences are 50°, 55°, and 60°;

-II
Fig. 23. is a cross-sectional view of a thin film system used in the thin film polarizing device in accordance with the present invention;
Fig. 24. is a cross-sectional view of one embodiment of a thin film polarizing device in accordance with the present invention. The device acts as a polarizer and only transmitted s-polarized light is used;
Fig. 25. is a cross-sectional view of another embodiment of a thin film polarizing device in accordance with the present invention. The device acts also as a polarizes and only re#Iectedp-polarized light is used;
Fig. 26. is a cross-sectional view of another embodiment of a thin film polarizing device in accordance with the present invention. The device acts as a polarizing beam-splitter and both transmitted p-polarized light and reflected s-polarized light are used;
Fig. 27. is a cross-sectional view of an embodiment of a thin film polarizing beam-splitter in accordance with the present invention. This thin film polarizing beam-splitter is designed intended for projection display application.
Fig. 28. shows the calculated reflectance curves for s-polarized light as a function of wavelength ratio for the embodiment VIS-1 at different angles of incidence;
Fig. 29. shows the calculated transmittance curves forp-polarized light as a function of wavelength ratio fox the embodiment VIS-1 at different angles of incidence;
Fig. 30. shows the calculated reflectance curves for s-polarized light as a function of 2o wavelength ratio for the embodiment V1S=2 at different angles of incidence;
Fig. 3I. shows the calculated transmittance curves forp-polarized light as a function of wavelength ratio for the embodiment VIS-2 at different angles of incidence:
Fig. 32. shows a Light element I with a white lamp;
Fig. 33. shows a Light element 2 u~ith a rotating colour filter wheel;
Fig. 34. shows a Light element 3 with colour LEI~s;

Fig. 35. shows a Light element 4 with colour lasers;
Fig. 36. shows a Imaging element I that is a monochrome, reflective, twisted nematic LC
panel;
Fig. 37. shows a Imaging element 2 that is a colour, reflective, twisted nematic LC panel;
s Fig. 38. shows a Imaging element 3 that is a monochrome, transmissive, twisted nematic LC
panel;
Fig. 39. shows a Imaging element 4 that is a colour, transmissive, twisted nematic LC panel;
Fig. 40. shows a Imaging element 5 that is a monochrome, reflective, polymer dispersive LC
panel with a quarterwave plate;
to Fig. 41. shows a Imaging element 6 that is a colour, reflective, polymer dispersive LC panel with a quarterwave plate;
Fig. 42. shows a Imaging element 7 that is a monochrome, transmissive, polymer dispersive LC panel;
Fig. 43. shows a Imaging element 8 that is a colour, transmissive, polymer dispersive LC
15 panel;
Fig. 44, shows a Imaging element 9 that is a monochrome digital micro-mirror device;
Fig. 45. shows a Imaging element l 0 that is a colour digital micro-mirror device;
Fig. 46. shows a Imaging system 1 in accordance with the present invention that uses one single transmissive imaging element made of TNLCs or PDLCs with quarterwave 2o plates and are capable of displaying high efficiency 2D images.
Fig. 47. shows a imaging system 2 in accordance with the present invention that uses two reflective imaging elements made of TNLCs or PDLCs and are capable of displaying high efficiency 2D and 3D images.

Fig. 48. shows a Imaging system 3 in accordance with the present invention that uses six reflective imaging elements made of TNLCs or PDLCs with quarterwave plates and ' are capable of displaying super high efficiency 2D and 3D images.
Fig. 49. shows a Imaging system 4 in accordance with the present invention that also uses six reflective imaging elements made of TNLCs or PDLCs with quarterwave plates and are capable of displaying super high efficiency 2D and 3D images.
Fig. 50. shows a Imaging system 5 in accordance with the present invention that uses three transmissive imaging element made of TNLCs or PDLCs and are capable of displaying super high efficiency 2D images.
Figs. 5 l a to 51 c show respectively a cross-sectional view along the line A-A in Figure 51 b, an underneath view, and a cross-sectional view along the line B-B in Figure 51 b;
and Fig. 52. is a pictorial view of a 3-D projection display system.
Detailed Description of the Invention The thin film polarizing device employed in the present invention is based not only on t 5 the principle of the fnistrated total internal reflection but also on the interference of light in thin films. Because of this, the thin flm polarizing devices are more versatile than devices obtained by conventional thin f lm design methods. General references to the design of thin film coatings including thin film polarizers or polarizing beam-splitters, can be found in the book entitled "Optical interference filters," written by H. A. Macleod (MacGraw HilI,1986, New York). The phenomenon of frustrated total internal reflection is described in the paper "Some current developments in multilayer optical filters," by A. F. Turner in J. Phys. Radium 1 l, 440(i950), and related applications can also be found in the paper "Optical tunneling and its applications in optical filters," by P. W. Baumeister in App. Opt. 6, 897(1967).
Clearly, the interference effect of light will not only depend on the reflection coefficients r~ and r2, but also on the phase difference ~tp between two adjacent reflected or transmitted beams. The phase difference Osa can be expressed by the following equations:

d~P = ~Pz ' ~P~ + ~, ~ ' Z~c ~ n,dt cosB, where ~, is the wavelength of the incident Light and ~, is the phase thickness of the Layer 4.
In conventional thin ftIm designs, either the angles of incidence of light are less than the critical angle or no critical angles exist. This means that for non-absorbing materials the phase changes on reflection gyp, and ~ for both s- andp-polarized light are either 0° or 180° as shown in Fig. 2 and Fig. 3. This is because r/o, rat and rf2 are real numbers.
Thus, once the coating materials have been chosen, phase changes on reflection at the interfaces have little effect on the thin film design. Only variations in the Layer thicknesses can be used to obtain a desired performance.
l0 However, when n~nl and when the angle of incidence 6~ is larger than the critical angle ~=siri ~(noln,), the'reflection coefficient r, at the interface 3 will no longer be a real number. As a consequence, the phase change on reflection will also be much more complicated. Three-dimensional diagrams of the phase changes on reflection for both s- andp-polarized light, respectively, at the interface 3, are plotted as a function of the angle of I5 incidence b6 and of the refractive index ratio y (y=nolnl ) for s-polarized light (Fig. 2) and p-poiarized light (Figs. 3, 4 and 5). These figures are particularly revealing of phenomena that are utilized in accordance with this invention:
1. As stated before, when the angle of incidence bb is less than the critical angle ~, the phase change on reflection for s-polarized light is 0° (Fig. 2). The phase change on reflection for 2o p-polarized Light is 0° when the angle of incidence ~ Lies between 0° and the Brewster angle ~=tan' (no/n1) (6~>BB), and it is 180° when the angle of incidence is between the Brewster angle ~ and the critical angle 6~ (Fig4).
2. When the angle of incidence 6~ is larger than the critical angle 6~, the phase change on reflection for s-polarized light changes from 0° to l 80° as the angle of incidence ~
25 increases from the critical angle ~ to 90° (Fig_ 2). Forp-polarized light, the phase change -IS-on reflection decreases from 180° to 0° as the angle of incidence 6b increases from the critical angle b~ to 90° (Figs. 4 and 5).
Some conclusions can be also drawn from the above diagrams. First, the phase change on reflection increases or decreases rapidly when the angle of incidence 6b is close to the s critical angle B° or when the refractive index ratio yis small. In other words, under those conditions, the dispersion of the phase change on reflection with the angle of incidence is very large. Second, if the thickness of the low refractive index layer 4 is small compared to the wavelength of the incident light, some light will leak out from the low refractive index layer 4 and enter the exit medium 6 even though the total internal reflection condition is satisfied.
I o This phenomenon is called frustrated total internal reflection. Third, and most importantly, the phase change on reflection characteristics for s- and p-polarized light are very different.
The above phenomena make it possible to design thin film coatings that are based not only on the interference effect of light in thin Elms but also on the frustrated total internal reflection of light in thin films. Therefore, in a multilayer structure consisting of alternating 15 low and high refractive index layers, phase changes on reflection at the interfaces as well as the Layer thicknesses contribute to the interference effects. This provides an extra degree of freedom for the design of optical thin film coatings. The thin film coatings can thus have very different requirements for s- and p-polarized light.
It is evident from Figs. 2, 4 and S that, when light passes from a high refractive index 20 layer to a low refractive index layer and when the angle of incidence is larger than the critical angle, the phase changes on reflection for the s- and p-polarized light are no longer 0° or 180°.
Since the phase changes on reflection are different, such multilayer structures are useful for the design of thin film polarizing devices in accordance with the present invention.
In multilayer structures of this type, low and high refractive index layers affect the 25 propagation of light very differently. When the angle of incidence is larger than the critical angle, for the low refractive index layer, the optical admittance rh and the phase thickness &, are imaginary. Hence, the layer thickness of the low refractive index therefore purely affect the amplitude of the reflected light. In other words, the low refractive index layer acts as an attenuator or behaves like a metal layer, but without the effect of Light absorption. If the thickness of the low refractive index layer is small compared to the wavelength of Iight, frustrated total internal reflection will occur at all the nHln~ interfaces and some of the light will leak out from the low refractive index layers and enter the adjacent layers. The amount of light leakage depends on the thickness of the low refractive index layer and also on the angle of incidence. On the other hand, at ni/nH interfaces, the total internal reflection condition of light is not satisfied, and therefore, the high refractive index layers act as phase adjusters.
Their layer thicknesses contribute to the phase changes on reflection and do not attenuate the light. All the light beams reflected from the nHlnL and nt/nH interfaces will interfere with each other and, as a result, modify the reflection and transmission of the light.
The above phenomena of the interference and frustrated total internal reflection in thin films is fully utilized in the thin film polarizing device in accordance with the present invention. Several approaches can be used for the design of such thin film polarizing devices.
For example, one can use one of various optimization methods described in the paper by J. A.
Dobrowolski and R. A. Kemp, Appl. Opt. 29, 2876(1990), or the needle design method 1 s described by S. A. Furman and A. V. Tikhonrovov in the book entitled "Optics of multilayer system, "published by Edition Frontiers in 1992, Gif sur-Yvette, or the equivalent optical admittance method as described in the book by 1. Tang and Q. Zhen, entitled "Applied thin film optics, "published by Shanghai Publishing House of Sciences and Technologies in 1984, Shanghai. Of these methods, the equivalent optical admittance method, applied to a symmetrical periodic layer structure is the best for a good understanding of the physics of the thin film polarizing device in employed in the present invention.
Fn Fig. 6, a basic thin f lm symmetrical structure 64 is comprised of a low refractive index layer 61, a high refractive index layer 62 and a low refractive index layer 63 and the layers are embedded between two identical substrates 60 and 65. The two low refractive index layers 61 and 63 are identical. Alternatively, as shown in Fig. 7, a basic thin film symmetrical structure 74 consists of a high refractive index layer 71, a low refractive index layer 72 and a high refractive index layer 73. Layers 7I and 73 are identical and all three layers are embedded between two identical substrates 73 and 75. The analysis for both structures 64 and 74 is similar. In order to simplify the explanation process, in the following text, the analysis for the structure 64 shown in Fig. 6 will be discussed in detail and the analysis for structure 74 is similar. The equivalent optical admittance Sand the equivalent phase thickness I"of the basic thin film symmetrical structure 64 can be calculated from the equations derived in the book "Applied Thin Film Optics" by J. Tang.
r/; (sin 2~, cossx + 2 ( r/L l r/x + r/f, l r~L ) cos2~L sin 8x - 2 ( r/L. . l ~7x - rlx l ~lr. ) sin 8x ) E=
(sin 2S, cos8x + 2 ( r~L l r/x + r/x l r/, ) cos2s,, sin~x + 2 ( r/,, l r/x -r/H l ~L ) sin8f, ) I-' = arccos(cos28, coscSx - ~ (r/L l r/x + r/x l r/, )sin28L sin8x) s (6) where rh, ~, r/o, ~, and ~ are given by:
~L - nG COSBL
r/x = nx cosBx, for s-polarized light (7) ~To = no cos Bo r~L = nL / cos8, r/x =nx /cos6x, forp-polarized light (8) ~lo =~o /COSBo (8L = 2nrzLdL cosBL / R
jll8x = 2~cnxdx cos9x / /1.
l0 When the angle of incidence ~ is larger than the critical angle for the low refractive index layer, rh and cos8,_, will be imaginary, and r~i and cosBi, will be real.
In order to transmit all the s-polarized light, the equivalent optical admittance E of the basic structure 64 should be equal to the optical admittance of the substrate r/o for the s-polarized light, i.e, Is Es =nocosBo (10) -IS-For a given angle of incidence 6b and a wavelength of incident light, it is possible to fnd solutions for the layer thicknesses d, and d2 that satisfy Eq. (10).
Because there are two variables and only one equation to satisfy, there will be an infinite number of solutions.
Therefore, it is possible to specify a second angle of incidence or a second wavelength in order to broaden the angular field or the band width of the thin film polarizing device.
It can be shown that a multilayer structure consisting of S periods of the above basic structure can be replaced by a single layer with the same equivalent optical admittance E and an equivalent phase thickness of S*I: The reflectance and transmittance of such muItilayer structure for s- andp-polarized light are best calculated using a computer program based on lo the matrix method as described in the book "Optical interference filters"
by H. A. Macleod.
To demonstrate the performance ofthe above basic thin film structure, a thin flm system with a n0 / aL l bH l aL / n0 structure has been designed. Here, a and b stand for the optical thicknesses of the low and high refractive index layers respectively, and no=2.35, n~=1.45, nH=4.0_ For an angle of incidence bb=57.3° and a wavelength ~?,o =4 urn, the optical 1 5 thicknesses a and ,3 were then calculated to be 17.9 nm and 31.3 nrn. The corresponding calculated equivalent optical admittance E and equivalent phase change on reflection hfor both s-polarized light andp-polarized light are plotted in Figs. 8 and 9 and Figs. 10 and I 1, respectively. It is clear from the above equations that both the equivalent optical admittance and the equivalent phase thickness T could be complex. Hence, both the real and the 20 imaginary parts of the two parameters are plotted in the above diagrams.
As shown in Fig. 8, the equivalent optical admittance for s-polarized light has only real part and is completely matched to the optical admittance of the substrate r/o over a very broad spectral region 0.2<g=,I,Q /~,<2. Therefore, very little s-polarized light is reflected and most of the s-polarized light is transmitted over this broad wavelength region. This is conf rmed by 2s the calculated reflectance and transmittance of the s-polarized light shown in Figs. 12 and 13.
As the number of periods of the basic structure S increases, the equivalent optical admittance is the same for s-polarized light. Therefore, the calculated reflectance does not show any significant change over the above broad wavelength region as is shown in Fig.
I2. Hence, the _ 19-performance for s-polarized light is essentially the same, regardless of the number of periods S.
As shown in Fig. 10, the equivalent optical admittance for the p-polarized light has only real part and it is different from the optical admittance of the substrate. Therefore, some of thep-polarized light will be reflected and the rest will be transmitted as shown in Figs. 14 and I 5 respectively. The equivalent phase thickness T has only positive imaginary part (Fig.
11 ), hence, the whole thin film structure acts like a amplitude attenuator for the p-polarized light. The transmittance or reflectance forp-polarized light will depend strongly on the number of periods S or the equivalent phase thickness of S*T, the imaginary part of the l o equivalent phase thickness Tas well as on the wavelength ~.. The larger S
or the shorter the wavelength is, the smaller the transmittance forp-polarized light is . It is possible to achieve almost any degree of attenuation in transmission for the p-polarized light by increasing S as shown in Fig. 15.
The calculated reflectance for s-polarized light and transmittance forp-polarized light is are also plotted in Figs. 16 and 17, respectively, for different angles of incidence 8a. in this case, the number of periods S was fixed to 40.
Clearly, the above thin film system acts as a very good polarizing beam-sputter over a very broad spectral region 0.2<g=,Z,o /a,<2 ~d a very wide range of angles of incidence from 50° to 70° measured within the substrate. The high to low wavelength ratio is about 10 and 2o the equivalent angular field in air is f24°. By comparison, a typical thin film MacNeille polarizer has a wavelength ratio of 2 and an angular field of~3° in air.
Thus, a thin film system having S periods of such a basic thin film structure can be used as a novel broad-band and wide-angle polarizing device. It can be used for separating s-polarized and p-polarized light by transmitting s-polarized light and reflecting p-polarized 25 light. This is contrary to a conventional thin film plate polarizes or a MacNeille polarizing beam-splitter that reflects s-polarized light and transmits p-polarized light.
Although the above explanations are based on a simple three layer basic structure, clearly, without departing from the spirit and scope of the present invention, the basic structure can be extended to consist of more than three layers of alternate high and low refractive index layers as shown as in Fig. 18. The basic thin f Im structure 186 has (2L-I ) alternatively low and high refractive index layers and the layers are embedded between two identical substrates 180 and 189. Clearly, the analysis and equations described for the three-layer basic structure in the above section can also be equally applied to the thin film structure described in Fig. 18.
Apparently, the center three layers 182 can be replaced by a single equivalent layer EQi. This equivalent layer EQ, together with the adjacent two layers marked as 184 in turn can be replaced by another equivalent layer EQ2. Repeating the same process, the whole thin film structure 186 can finally be replaced by a single equivalent layer EQL_j.
t o 'The apparent advantage having more than three layers in the basic structure in accordance with the present invention is that there are more parameters to choose in order to meet the requirements for different design wavelengths or angles of incidence.
According to Eq. ( 10), if the equivalent optical admittance of the equivalent layer EQ~_~
is equal to the optical admittance of the substrate, all the s-polarized light will be transmitted while some of t5 p-polarized light will be reflected. By increasing the number of periods in the basic structure, all the p-polarized Light can be reflected. If the refractive indices in the basic structure are fixed and the layer thicknesses are allowed to vary, there will be (2L-I) variables and only one equation to solve. Therefore, there will be much more freedom to choose the layer thicknesses to meet additional requirements. Thus, much wider band-width or wider angular field thin 2o film polarizing device can be obtained.
To illustrate the above principle, a thin film polarizing device having five layers in the basic structure ( n0 / aL l bH l cL l bH 1 aL / np ) has been designed. Here, a, b and c stand for the optical thicknesses of the layers, and no=2.35, nL=I .45, nn=4Ø The original design wavelength is 4l.em and the angle of incidence is 57.3°. The optical thicknesses a, b and c 25 were calculated to be I I .8 nm, I0.6 nm and 4I .8 nm, respectively. The calculated reflectance and transmittance curves are plotted as a function of the wavelength ratio in Figs. I9 and 20 respectively. The thin film polarizing devices are shown with I, 50 or 100 periods of the basic structure. The calculated reflectance and transmittance curves at different angles of incidence are also plotted in Figs. 21 and 22 for the case with S =100. Clearly, the thin film polarizing 3o device is effective over a very broad band of wavelengths. For the thin film polarizing device having 100 basic structures, the high to low wavelength ratio is as large as S0:1. The highest reflectance for s-polarized light is less than 3x10-5 and the highest transmittance forp-polarized light is about 1 x 10-I at X0.02 for angles of incidence between 50° to 60°. This highest value of transmittance for p-polarized light can be reduced by increasing the number of periods while the reflectance for s-polarized light will be essentially the same.
In addition, it is understood that each low refractive index layer in the basic structure can be replaced by a number of low refractive index layers having different refractive indices and each high index layer can be replaced by a number of high refractive index layers providing the critical angle condition is satisfied for each of the low refractive index layers. In addition, the performance of the initial thin film polarizing device based on the above symmetrical thin film structure can be further improved with the assistance of a computer optimization program wherein the layer thickness of each layer in the thin film system will be optimized. In the optimization process, the dispersion of the refractive indices can be taken into account. It is also possible to have two substrates having different refractive indices 15 providing the critical angle condition is satisfied for the substrate having the Lower refractive index. Normally, after the optimization procedure, the thin f Im system will not retain the symmetrical structure any more unless special steps are taken.
In principle, it is always possible to design a thin film polarizing device having a multilayer shown in Fig. 18, providing that the angle fib is larger than the critical angle for low 20 refractive index layers (nL<no<n~). However, the phase dispersion is very large when the angle of incidence is close to the critical angle. Thus, in practice, it is easier to design wide angle thin film polarizing devices in accordance with the present invention when the angle of incidence is not that close to the critical angle. In addition, the phase dispersion is large when the refractive index ratio nH/n~ is small. This is particularly the case in the visible part of the 25 spectrum because there the highest available refractive index ratio is about 1.75. In the infrared the refractive index ratio is of the order of 4Ø For this reason infrared broad band and wide angle polarizers have a much better performance than corresponding visible thin film polarizing devices.
In general, a novel thin film polarizing device employed in the present invention, as 3o shown in Fig. 23, is comprised of first and second light transmissive substrates 230 and 231, and a plurality of thin film layers 238 disposed between the first and second fight transmissive substrates. The thin film layers 238 consist of alternating high refractive index layers 233, 235, etc., and low refractive index layers 232, 234, etc.
Each high refractive index layer can include a number of high refractive index sub-layers having one or more different refractive indices, and each low refractive index layer can include a number of low refractive index sub-layers each having one or more different refractive indices. The first and second light transmissive substrates, each in the form of a prism, have a refractive index that is greater than the refractive index of each of the low refractive index layers. The thicknesses of the low refractive index layers of the plurality of thin film t o layers are small enough so that light incident upon the thin film layers at an angle greater than the critical angle can be partially coupled out through the low refractive index layers so that frustrated total internal reflection occurs. This permits interference to take place between the light reflected at the interfaces of the all thin film layers. In addition, the thicknesses of the thin film layers are such that the admittance of the plurality of the thin film layers for s-polarized light is substantially the same as the optical admittance of the substrate for s-polarized Light for a wide range of angles of incidence and a broad band of wavelengths when the incident light is incident upon the low refractive index layers at an angle greater that the critical angle. This permits substantially aII incident s-polarized light to be substantially transmitted. The plurality of the thin f lm layers have an 2o admittance forp-polarized light that is substantially different from the optical admittance of the substrate for p-polarized light for a wide-range of angles of incidence and a broad-band of wavelengths and thus they substantially reflect incident p-polarized light. The prism is shaped in such a manner as to allow the incident Light to be incident upon the thin film layers at a plurality of angles greater than or equal to the critical angle for the highest refractive index of the low refractive index layers.
Figs. 24, 25, 26 and 27 represent cross-sectional views of four arrangements for suitable thin film polarizing devices for use in the present invention. The multilayer thin film coatings are embedded between two prisms. The shapes of the prisms are selected in order to make the angle of incidence of the incident light at the hypotenuse larger than the critical angle 3o for the low index layers. The two prisms could be made of the same material. In the arrangement shown in Fig. 24, the shapes of the two prisms 240 and 244 are identical and the thin film coating 246 is deposited at the hypotenuse interface. The thin film polarizing device acts as a transmissive polarizes. Only the transmitted s-polarized light is used and the reflectedp-polarized light is absorbed by a light absorber 246. In the arrangement shown in Fig. 25, the shapes of the two prisms 250 and 254 are different. The thin film coating is also deposited at the hypotenuse interface. The thin film polarizing device is also configured as a reflective polarizes and only the reflected p-polarized light is used. The transmitted s-polarized light is absorbed by a light absorber 256. In the arrangements shown in Figs. 26 and 27, both devices are configured as polarizing beam-splitters. Therefore, the reflected p-poiarized light and the transmitted s-polarized are both used. In the .arrangement in Fig. 26, 1 o the two prisms 260 and 264 have different shapes while in the arrangement in Fig. 27 they are identical. The.advantage of this latter arrangement is that the unpolarized beam can be incident on either top-sides of the device. In fact, if a symmetrical layer system solution is found for the multiiayer 272, the light can be incident on any of the four sides of the prism arrangement.
Although the two prism substrates can have different refractive indices, normally in practice, they are made of the same material in order to reduce manufacturing costs. The two prisms may be joined together in various ways. For example, they can be joined together with optical cements that have refractive indices matching the refractive index of the substrate.
They can also be joined with a liquid that has a refractive index matching the refractive index of the substrate and the out-most edges of the two contact faces of the two prisms are then sealed. The two prisms, with coatings on one or both prisms, can also be brought together by using optical contact. This technique has been successfully developed for the construction of high laser damage threshold polarizers.
Because the thin film polarizing devices in accordance with the present invention are based on frustrated total internal reflection, the layer thicknesses are a fraction of the mean wavelength of the designed spectral region. Hence, the layers are very thin compared to those of conventional thin film polarizers and thus it should be less costly to manufacture such systems. This is especially important in the case of far infrared polarizers where normally the total layer thicknesses are very thick and require a very long deposition time.

The thin film coatings in the thin fim polarizing devices can be manufactured by conventional physical or chemical thin film deposition techniques, such as thermal evaporation, sputtering, ion-plating and plasma assisted evaporation. Those process can produce good quality thin films. Since no absorbing coating materials are used in the thin fzlm systems, the thin f lm polarizing devices are very durable both physically and chemically.
The thin film polarizing devices employed in the present invention are very broad-band and are effective over a very wide range of angles. By controlling the layer thiclcnesses, the extinction ratio in transmitted light can assume almost any value. The extinction ratio in reflected light is also very high when compared to that of conventional thin film polarizers. If t o necessary, another polarizing device of the same type can be placed in series to obtain an even higher extinction ratio. The novel thin f hn polarizing devices can be used in almost all applications where current polarizers or polarizing beam-sputters are used.
For example, in the visible and near infrared spectral regions, the novel thin film polarizing devices in accordance with this invention can be used to replace more expensive polarizing devices based 15 on birefringent crystals. In the infrared and far infrared regions they can replace metal grid polarizers.
Two embodiments of the thin film polarizing beam-splatters or thin f lm polarizing devices, also referred to as polarization beam discriminators, that will be used in the high effciency 2D/3D projection displays in accordance with the present invention have been 20 obtained. The layout of the thin film polarizing device is chosen as shown in Fig. 27 having two identical dove prisms and symmetrical thin film coatings. All four faces of the thin film polarizing device can be used to accept light. The two thin film coatings, VIS
1 and VIS 2, were designed for visible light having wavelength region of 0.4-0.8 l,un. For the visible thin film polarizing devices, substrates with refractive indices of I .75 and I.85 and coating 25 materials having refractive indices of I.38 and 2.35 were selected. Plots of the spectral performances of these thin f lm coatings V1S=I and VIS 2 at dii~erent angles of incidence are given in Figs. 28 and 29 (VIS I), and Figs. 30 and 3I (VIS-2), for s-polarized andp-polarized, respectively.
Clearly, in terms of band-width, angular field and extinction ratios, all two 3o embodiments are signifcantly better than conventional thin film polarizers or polarizing WO 98/07279 PCT/CA97l00567 -as-beam-splitters. The wavelength ratios of 2:1 and 1.5: l, the angular fields of ~11.4° and 115.8° in air, and the minimum extinction ratios of 1 x 104:1 and 1.5x 104:1 are obtained for VIS-1 and VIS-2 respectively.
An error analysis has shown that random errors of the order of t 1 or f2% in the thicknesses of the layers of polarizers of this type will not unduly affect the performance of the devices. This, of course, relaxes the manufacturing tolerances for these devices. Several prototype of the above polarizing beam-sputters have been fabricated in standard thin film deposition systems. The measured performances showed very good agreements with the calculated performances. They exceed even the most strict requirements for PBSs in very high l0 performance projection displays.
Numerous other thin film polarizers based on interference and frustrated total internal reflection may be employed without departing from the spirit and scope of the invention. Far example, other prism materials and coating materials can be used also. In the infrared, the prism material may be made of ZnS, ZnSe, Si, Ge, etc. In the visible, the 15 substrates may be made of various glasses and various plastics. The coating materials can be selected from the common materials used in conventional thin film coatings, such as MgF2, ThF4, Si02, A1203, Zr02, Ti02, Ta~05, Nb205, Si, ZnS, ZnSe, Si, Ge, etc.
The preferred embodiments of this invention include the above novel thin film polarizing beam-sputter described heretofore. Since the thin film polarizing beam-splitter 2o works in a different manner from conventional beam-splitters, different configurations for high effciency 2-D and 3-D projection displays are provided.
A high e~ciency projection display system in accordance with the principles of the present invention generally comprises a lighting element, a novel thin film polarizing device, at least one imaging element, a projection lens, and an optional screen. The 25 lighting element provides tight that is used to form images. The imaging elements, also referred to as spatial light modulators, consists of plurality of small pixels that can be addressed individually to form images. The thin film polarizing device as designed above splits the unpolarized incident light beam into two polarized beams and project them onto the corresponding imaging elements, and then combine the images from the imaging -2b-elements. The projection Ions projects the combined image onto a screen which then is viewed based on the type of imaging elements, the lighting element can take different forms as shown in Figs- 32-35. In Fig. 32, the first lighting element shown in Figure 32 comprises a lamp 300, a reflecting mirror 301, a cold mirror 302 to reraove unwanted infrared and ultraviolet light from the light beam and a collimating lens 303_ The lamp emits white light that contains three primary colours of blue, green and red in order to form black arid white, or colour images. In Fi,g.33, the second lighting element consists of a lanop 300 which emits vvhitc light, a reflecting mirror 301, a cold mirror 302, a focusing 14 lens 313, a colour filter wheel 310 mounted with R, G, B colour filters, and a collimating lens 303. The colour filter wheel rotates at a fast speed that farms a sequential-colour light which is synchronized with the imaging eleraent. In Fig. 34, R, G and B colour light emitting diodes (LEDs) 312 are used in combination with the collimating lens.
The three LEDs can be turned on sinoultaneously to give a white colour or sequentially to forYn time-sequential colour which is syncluonized with the imaging element. The lighting element shown in Fig. 35 is the same as the lighting element shown in Figure 34, except that the LEDs are replaced by three Rcd, Green, and Blue Iasezs.
The imaging elerraents shown in Figs. 36 to 39 are all based on twisted nematic liquid crystal displays_ In Fig. 36, the first imaging element is a monochrome reflective LCD. When no voltage is applied to the liquid crystal cell or pixel 320, the ,polarization of an incident polarized light is rotated 90°. When a,roltage is applied to the liquid cell or pixel, no polarisation rotation occurs. The incident beam will retain is original polarization. To view an image, a second polarizes is needed. The second imaging element shown in Fig. 37 is similar to imaging element shown in Figure 34 except that R, G and B colour filters 321, 322, 323 are added to eaclx individual liquid crystal pixel to only allow the intended colour light to be reflected. The imaging elements shown in Figs.
38, 39, are the transmissive versions of imaging elements the imaging elernents shown in Figures 35 and 36 respectively. Incident light is transmitted, instead of reflected by the imagimg elements. The imaging elements shown in Figs. 40-43, are based on polymer 3o dispersive liquid crystal displays (PALCs). 1n the imaging element shown in Fig. 40, r.. . ~- .~..

when no voltage is applied to the pixel 420, the incident light will be scattered and very little light will be collected by era optical system to form images, this pixel is said to be "off", l~Vhen voltage is applied to the cell or pixel, all the light will be reflected. To use PDLCs in combination with the thin film polarizing beam-splitters, an extra duarterwave s plate 330 is added. The puzpose of it is to rotate the polarization of the "on" pixels by 90°
as in the case oftho imaging element shown in Figures 35, 36, 38 and 39. The imaging elenraent shown in Fig. 41 is the colour version of the imaging element shown ixl Figure 39.
Additional R, Ki, and ~ flters are added to each individual liquid crystal pixel to form colour pixels. The imaging elements shown in Figures 42 and 43 are the transmissive versions of imaging elements shown in Figures 39 and 40. Instead of reflecting light, these imaging elements transmit light. The imaging elements shown in Figs. 44-4S are based an digital micro-mirror devices. Each individual pixel is a mirror that can reflect the iz~cidont beam and can be individually rotated by electronic means to a "on"
gosition in which the reilactcd light can be collected by an optical system, or to a "off' position in which the reflected tight is out of the collecting angle of the optical system and is absorbed by an absorber. The imaging element shown in Fig. 44 consists of only monochrome mirrors while the imaging element shown in Fig. 4S consists of colour mirrors as described in T 3S patent no. 5,619,059 issued April $, 1992.
Different embodiments of high eff ciency display systems can be formed by combining the above lighting elements, the thin filmy polarizing devices and the different imaging elemCntS.
In the first embodinmnt, shown in Fig. 46, the display system consists of a thin flm polarizing device 400 in accordance with the present invention, a lighting elem~x~t 401, x transmissive imaging element 402 that can be selected from the imaging elements shown in Figure 38 (TNLC) or Figure 42 (PDLC) to form monochrome images, and the imaging elements shown in Figure 39 (TNLC) or Figure 43 (PD)rC) to form colour images, two mirrors 403, 404, a projection lens 44S and an optional screen 406. in operation, unpolarized light front the lighting element x01 that an be selected $om lighting elements shaven in Figures 32 to 3~ is launched into the end face 407 of the hexagonal thin film 3o polarizing device 400 at an angle of incidence greater than the critical angle at the thin film surface and is split into two polarized light beams by the thin film coating.
The p-polarized light is reflected from the thin film layers to the end face 408;
the s-polarized light is transmitted.through the thin film layers and exits the end face 409.
The reflectedp-polarized tight exiting the end face 408 is then incident upon the first mirror 403 and is directed to the transmissive imaging element 402. ror the "on"
pixels, the polarization of the light is not changed by the imaging element;
therefore thep-polarized light retains its polarization. After being reflected by the second mirror 404, the transmitted Iight from the "on" pixels hits the thin fllm polarizing device 440 through the end face 409, it is reflected again by the thin film layers. The light then exits tho and face 410 and is projected onto the screen by the projection lens 40$. For the "off' pixels, the polarization of thep-polarized light is rotated 90° by the imaging element 402 and becomes s-polarized- Therefore, the light is trFUismitted back off the thin film layers toward the lighting element 401 and is not proj ected onto the screen.
The transmitted s-polarized light exiting the ez~d face 409 is first incident upon the second mirror 404 and is directed to the transmissive imasing element 402_ For the "on"
pixels, the polarization of the light is not changed and the s-polarised light remains its polarization and is subsequently transmitted again by the thin film polarizing device 400 and projected onto the screen. The light from the "off' pixels is rotated 90° and is sent back toward the lighting element 401.
2o In this embodiment, to form colour images, a white lighting element 1 in combination with colour TNLC or PDLC can be used, yr the valour lighting element shown in Figures 33-35 in combination with monochrome TNIrC or PDLC can be used.
Since both s-polarized and p-polarized light arc completely used for imaging, no light loss is present due to Choir polarization. Therefore, very high efficiency 2D
projection display 2s can be obtained. In addition, this embodiment is very cornpaet and can be made in low cast.
The display system shbwn in Fig. 47 consists of a thin film polarising device 400, a lighting element 401, two identical reflective imaging elements 424 and 421 that can be selected from the imaging elements shown in Figure 36 (TNL,C} or Figure 4U
(PDLC} to _29_ farm monochrome images, and the imaging elements shown in Figure 37 (TNLC) or Figure 41 (PDLC) to form colour images, a projection lens 405 and an optional screen 406_ In operation., unpolarized light from the lighting element 401 is lauanched into the end face 407 of the thin film polarizing device 400 and is split into two polazized light beams by the thin film coating. Thep-polarized light is reflected fmm the thin film layers to the end face 408; the s-polarized light is transmitted through the thin film Jayers and exits the end face 409.
The reflected p-polarized light exiting the end face d08 is then incident upon the first reflective imaging element 420. For the "on" pixels, the polarization of the light is rotated 90° by the imaging element; therefore the p polarized Jight becomes as s-polarized.
When the reflected light from the "on" pixels again hits the thin film lrolarizing device 400 through the end face 408, it is transmitted through by the than film layers and exits at the end face 410 and then is projected onto the screen by the projection lens 405.
For the "off' pixels, the polarization of the Jight is not changed by the imaging element 420 and, l s therefore, the light is reflected Back off the thin film layers toward the lighting element 401 and is not projected onto the screen- In this case, the project$d image coming from the imaging device 420 only consists of s-polarized light.
The transmitted s-polariz8d light exiting the end face 409 is then incident upon the second reflective imaging element 421. For the "on" pixels, the polarization of the light is 2o rotated 90° by the imaging element, therefore the s-polarized light becomes as p-polarized.
When the reflected ligkat from the "on" pixels again hits the thin iihn polarizing device 400 through the end face 409, it is reflected by the thin film layers and then projected onto the screen by the projection leas 405. Far the "ofF' pixels, the polarization of the light is not changed by the imaging element 421 Gild, therefore, the light is transmitted back off the zs thin film layers toward the lighting element 401 and is not projected onto the screen_ In this case, the projected image only consists ofp-polarized light.
In this embodiment, to form colour images, a white lighting element of the type shown in Fig, 32 in combination with the colour TNLC or PDLC can be used, or colour lighting element of the type shown in Figures 33 to 35 in combination with monochrome 3o TNLC or PDLC can be used. Clearly, if the images on the imaging elements 403 and 404 ...._".~ _.... _ _._ e.....".~..

are identical and arc mirror images of each other, the combined images will be identical too, and a single 2-D image is seen on the screen- In addition, if the images on the imaging el4,ncnta 420 and 421 represent right-eye and Ieft-eye images (also mirror images in one case), the combined images can be viewed through a polarizing glass and three-s dimensional effects can be perceived. Since both s-polarized and p-polarized light are fully used for imaging, no light lost is present due to their polarization.
Therefore the . display is very efficient and compact when compared to conventional >JCDs.
Therefore, very high efficiency projection display that is capable of displaying 2D and 3D images can be obtained. 2D and 3D display mode can be easily switched by just changing the input lo image signals to the 'imaging elements. No optical reconfiguration is required for displaying 3D images except that viewers have to wear polarizing glasses. This embodiment are specially suitable to home theater.
The display system shown in shown in Figure 48 is a super high efficiency projection display. 1t consists of one thin film polarizing device 400, a lighting element t5 401 that emits white light, six identical reflective imaging elements 440, 441, 442, 4.43, 444, and 44.5 that can be selected from monochrome ima,&ing elements shown in Figure 3G ('TNarC) or Piguro 40 (PDL.C), two identical "X" shape colour separation cubes or plates d46, 447, aprojection lens 405 and an optional screen 40fi. In operation, unpolarized light from the lighting element 401 is launched into the end face 407 of the 20 thin film polarizing device 400 and is split into two polarized light beams by the thin film coatis. Thep polarized light is refleetcd from the thin :01m Layers to the end face 408; the s-polarized light is transmitted through the thin hlm layers and exits the end face 409.
The refleetedn-polazi2ed light exiting the end face 408 is then incident upon the first "X" colour cube 446. The coating 450 reflects red light and transmits groan ligbt_ 2s The coating 451 reflects blue light and transmits green lift_ The red, green and blue light beams are separated by the first "X° colour cube 44.6 and are incident upon the three corresponding monochrome reflective imaging elements 4-40, 441 and 442 respectively.
These monochrome reflective imaging elements are fed with the corresponding colour images_ For "on" pixels, the polarization of the light is rotated 90°
by al l three ima~g 3o elements, therefore ihep-polarized light becomes as s-polarized. 'The three colour images _31 _ from the three imaging elements are combined by the "x" cuhe again, in the same way as it separates the colours, to form a full colour image. When the reflected light from the "on"
pixels again hits the thin film polarizing device through the end face 408, it is transmitted through by the thin f lm layers and then projected onto the screen by the projection lens 405. For the "off' pixels, the polarization of the light is not changed by the imaging element 440, 441, 442 and, therefore, the light is reflected buck off the thin fl lm layers toward the lighting element 401 and is not projected onto the screen. 1n this case, the projected image only consists ofs-polarized Light.
The transmitted s-polarized light exiting the end face 409 is then incident upon the to second "X" colour cube 447. The coating 46o reflects red light and transmits green light.
The coating 461 reflects blue tight and transmits green light. The rod, green and blue light beams are separated by the seeond''X" colour cube 447 and are incidc-mt upon the three corresponding monochrome reflective imaging elements 443, 444, and 445 respectively.
These monochrome reflective imaging elcmcats are fed with the corresponding colour is images. For "on" pixels, the polarization of the light is rotated 90° by all throe imaging elements, therefore the s polarized light becomes asp-polarized.. T'he three colour images from the th,~ee imaging elements are combined by the "X" cube again, in the same way as it separates the colours, to form a full colour image. When the reflected light from the "on"
pixots again hits the thin film ~polariaing device 400 through the end face 409, it is 20 reftectcd by the thin filin layers and then projected onto the screen by the projection lens 405. For the "ofF' pixels, the polarization of the light is not changed by the imaging element 443, 444, 445 and, therefore, the light is transmitted back off the thin film Layers toward the lighting element and is not projected onto the screen. In this case, the projected image only consists ofp-polarized light.
2s Clearly, if the colour images on atl the imaging elements 440, 445, 441 and 444, 442 and 443 are identical and are mirror image of their counter-parts, the combined full images will be identical too, and a single 2-D image will be seen on the screen. In addition, if the images on the imaging elements 440, 44t, 442 sand 443, 444, 445 represent right-eye and left-eye images (also nc~irror images in one case), the connbined images can 3t~ be viewed through a polarising glass and three-dimensional effects can be perceived. 2D

and 3D display mode can be easily inter changed by just changing the input image signals to the imaging elements. No optical reconfiguration is required for displaying 3D images except that viewers have to wear polarizing glasses.
In this embodiment, the vsrhite light is first separated into two polarized beams, then each polarized beam is split to three primary colours which are directed to the corresponding imaging elements. No light is lost at all either due to polarization or due to colour. Therefore, super high efficiency projection display can be obtained.
This embodiment are specially suitable to entertainment applications such as movie theaters.
The display system shown in Figure 49 is also a super high efficiency 2DI3D
to projection display_ It consists ofthree thin ~lrx~ polarizing devices 400', 400, and 4003, a lighting element X101 that emits white light, six identical reflective imaging elements 420, 421', 4202, 4212, 4203, and 421 ~ that can be selected from monochrome imaging elements shown in Figure 36 fTNLG) or Figure 40 (fDLC), two sets dichroic colour filter 470, 471, a projection lens 405 and an optional screen 406. The operation of this embodiment is t S very similar to that of the display system shown in Fi~ue 4? except that the light incident upon the thin film polarized devices is already separated into colours. In addition, the operation of the display system shown in Figure 49 has some similarity to the display system shown in Figure 48. In this latter system, the sequence of operation is like this;
white light ~ separating polarization --3 separating colow --~ foaming cblOUr ilnage5 -~
20 combining colour images -~ combining polarizations. In the display system shown it1 Figure 49, the order is Changed to the white Ixght --~ separating colours --~
separating polarizationS --~ fOnniilg colour images ~ Combining polaTiZatlOns --~
combining GOICUrS.
It is clear, the advantages that the advantages of the display system shown in >~igure 47 also apply the display system shown in Figure 49. Super high efficiency projection z5 displays that are capable ofdisplaying 2DI3p images can also be obtained in this embodiment.
An alternative embodiment of the above display system is shown in Figure S0.
In operation, this embodiment is very similar to the display system shown in 1~igure 46 except that the light incident upon the thin f lm polarized devices is already separated into colours, tt is also similar to the display system shown in Figure 49 except that instead of using six reflective imaging elements, it uses only three transmissive imaging elements 402', ~022, 4Q2;. It is capable bf displaying super high efficiency 2D images.
Another display system, which is suitable for projection high eFhciency 2D and images with two AMDs, is shown in Figs. 51 a to 51 e. Fig. 5 Ib is a top view.
It consists of two identical tlun film polarizing devices 400 and 4002 in accordance with the present invention, a lighting element 401, two identical imaging elements 490 and 491, ono far separating the ungolarized light and one for combining the polarized light, made of DMDs (Digital Micro Mirrors), a projection Lens 445 and an optional screen 406. In operation, t0 unpolarized tight from the Lighting element 401 is launched the first thin film polarizing device 400' and is split into two polarized light beams. The p-polarized light is reflected and the s-polarized light is transmitted.
The reflectedp-polarised light exiting the end face 408' is then incident upon the first DMD 490. For the "on" pixels, th,e light from DMDs is reflected at an "on" angle about 20° from the incident beam and is directed to the second polarizing device 4002.
Since the polarization of the light is not changed by the DMD, the light is reflected by the second thin iilna polarizing device 400Z is projected onto a screen 406. For "ofF' pixels, the light from the DMD is reflected at an "off' angle about 60° from the incident beam and is out of the collecting angle of the projection lens 405 and is absorbed by an absorber. In 2o this case, the projected image only consists. ofp-polarized light.
The transtnittod s-polarized light exiting from the first thin film polarizing device -400I is then incident upon the second DMD 491. For the "on" pixels, the light From DMDs is reflected at an "on" angle about ~0° frorra the incident beam and is directed to the second polarising device 400_ Sirtce the polarization is not changed by the DMD and 2s Light is reflected again bytho second thin film polarizing device 4002 and is projected to a screen 406. For "ofF' pixels, the light from the DMD is reflected at an "off' angle about 60° from the incident bum anal is out afthe collecting angle ofthc projection lens and is absorbed by an absorber, In this case, the projected image only consists ofs-polarized light.

Clearly, if the images on the imaging elements 490 and 491 are identical but mirror images of each other, the combined image is identical too and a 2D image is formed. In addition, if the images on the imaging elements represent right-eye and left-eye images (also mirror images in one case), the corobinad image is a 3D image and can be viewed through a polarizing glasses. in this embodiment, 2D and 3D diaplay mode carx be easily inter-changed by just changing the input image signalx to the imaging elements. No optical reconfiguration is required for displaying 3D images except that viewers have to WGaipOlaT7zing glasSCS.
To form full colour images, colour lighting systems in combination with monochrome DMDs can be used. lior example, a colour Tier wheel with red, green, and blue filters as in the lighting system shown in Figure 33 can be used to form a colour image. In this case, only one colour is displayed at any given time. The perceived colour will bt the time-integrated result of the thrv;e primary colours.
.Alternatively, a white lighting system in combination with colour DMDs can be used. For example, two l7MDs 13 (digital micro mirrors) with integrated colour filters disclosed in US
patent no_ 5,619,0S9 dated April 8, 1997 by LiLi, J.A.Dobrowolslci, P.D. Grant and ~.T.Sullivan entitled "Colour Deformable Mirror Dcvico having Optioal Thin fihm Interference Colour Coatings". In addition, more complicated and super high efficiency display systems that use more than three DMDs can be formed by expanding the display system shown in Figures 51a to 51 c in the same way as in the display systems shown in Figures 48 and 49.
As previously stated, in the above display systems shown in Figures 47, 48, 49, and S I a to 51 c, the polarization of the light beams reriu'ning from the two spatial light modulators are different; vr~e beam isp polarized and the other is s-polarized.
Advantageously, this is an ideal situation for forming 3-D images. To do so, it is z5 necessary for each of the images of the two panels to be recorded by each of two cameras representing the left and right eyes, respectively, as demonstrated in pictorial view Fig. 52.
To provide a 3-D image, a scene is recorded by each of two cameras which are disposed at different, o$'set locations. Thus a first camera records a scene fr6m a first location and a second camera, offset from the first camera, records essentially the same scene from a different Location. Preferably, the offset is approximately a distance between the eyes of an average person or a distance related thereto and to the nature of the camera lenses employed. In ardor for a viewer Io perceive 3-D effect the seine recorded by the first camera is viewed by only one of the viewer's eyes. Furthermore, the sosn~e recorded by the second camera is viewed by the other of the viewer's eyes. ~T'he configuration shown and described with reference to Fig. 52 lends itself to this ~-D recording and viewing.
The extension of the invention from two dimensions to three dimensions is possible in many display configurations. 'Therefore, the above configurations can be used equally well for both 2-D and 3-D images.
The invention uses both s and p-polarization to provide 2-I~ images which are twice as bright as conventional systems that utilize only one polarization state.
Furtl~orrnore, the invention provides a navel method of providing and displaying 3D
images.
To summarize, in all tho described preferred high efficiency projection display embodiments, the thin film polarizing device acts both as the first and as the second ~5 polarizer. It bas high transmittance and reflectance for the desired polarization, broad bandwidth, larger angular field and very high extinction ratios for both transmitted and reflected. light. All light is used for imaging and no light is lost due to undesired polarization and by the thin film polarizing device. Therefore, it solves the problems associated with the use of dichroic sheet polarizers in conventional liquid crystal displays.
The preferred display systems are high efficient and can employ high power light sources because of the use of both planes of polarization and the non-absorbing nature of the thin film polarizing devices. In addition, they can also be made very compact since the same thin film polarizing device can act as both the first and socond polarizers_ Furthermore, high contrast images can be obtained because of the high extinction ratio of the thin film polarizing device. Also, lame aperture lighting and projecting optics can be used duo to the wide angular field of the thin film polarizing device, which in turn, also enhances the efl~cieney of the display systems. Finally, most of tha display systems are capable of displaying high efficiency 2D and 3D images.

WO 98/07279 PCTlCA97/00567 The invention uses both s and p-polarization to provide 2-D images which are twice as bright as conventional systems that utilize only one polarization state.
Furthermore, the invention provides a novel method of providing and displaying images.
To summarize, in all the described preferred high efficiency projection display embodiments, the thin film polarizing device acts both as the first and as the second polarizer. It has high transmittance and reflectance for the desired polarization, broad bandwidth, larger angular field and very high extinction ratios for both transmitted and reflected light. All light is used for imaging and no light is lost due to undesired l o polarization and by the thin film polarizing device. Therefore, it solves the problems associated with the use of dichroic sheet polarizers in conventional liquid crystal displays.
The preferred display systems are high efficient and can employ high power light sources because of the use of both planes of polarization and the non-absorbing nature of the thin film polarizing devices. In addition, they can also be made very compact since the same 15 thin film polarizing device can act as both the f rst and second polarizers. Furthermore, high contrast images can be obtained because of the high extinction ratio of the thin film polarizing device. Also, large aperture lighting and projecting optics can be used due to the wide angular field of the thin film polarizing device, which in turn, also enhances the eff ciency of the display systems. Finally, most of the display systems are capable of 2o displaying high efficiency 2D and 3D images.
Numerous embodiments of high efficiency display systems that use different configurations and other types of imaging elements can be envisaged without departing from the scope of the invention. As pointed out above, as one skilled in the are would appreciate, it is possible to employ a second thin film polarizer for combining the encoded 25 beams, although the preferred embodiment employs a common device.

Claims (27)

Claims:

1. A projection display system comprising:
a light source for generating an input beam of light;
a polarizing beam discriminator employing frustrated total internal reflection and thin film interference to discriminate between s and p polarized light, said discriminator separating s and p polarized light beams from said input beam;

spatial light modulator means for encoding an image onto said respective s and p polarized beams;

means for combining said encoded s and p polarized beams into an output beam, said combining means employing frustrated total internal reflection and thin film interference; and means for focussing the output beam onto a display means.

2. A projection display system as claimed in claim 1, wherein said means for combining said s and p encoded polarized beams into an output beam is provided by said polarizing beam discriminator.

3. A projection display system as claimed in claim 1, wherein said means for combining said s and p encoded polarized beams into an output beam is provided by a second polarizing beam discriminator employing frustrated total internal reflection and thin film interference to discriminate between s and p polarized light.

4. A projection display system as claimed in any one of claims 1 to 3, wherein said spatial light modulator means creates an encoded output beam wherein respective image and non-image components of the beam have orthogonal planes of polarization.

5. A projection display system as defined in any one of claims 1 to 4, wherein the or each polarizing beam discriminator comprises first and second light transmissive substrates, and a plurality of thin film layers disposed between the first and second light transmissive substrates and having predetermined thicknesses and refractive indices for allowing unpolarized incident light to be separated into s-polarized and p-polarized light beams, wherein frustrated total internal reflection and thin film interference are simultaneously employed to allow s-polarized light to be transmitted and p-polarized light to be reflected over a wide range of wavelengths and angles of incidence.

6. A projection display system as defined in any one of claims 1 to 5, wherein the or each polarizing beam discriminator comprises a thin film polarizing device, for separating s-polarized light and p-polarized light by reflecting p-polarized light and transmitting s-polarized light comprising first and second light transmissive substrates, and a plurality of thin film layers disposed between the first and second light transmissive substrates, the thin film layers comprising high refractive index layers and low refractive index layers, the high refractive index layers having one or more different refractive indices, and the low refractive index layers having one or more different refractive indices, and the first and second light transmissive substrates, each in the form of a prism having a refractive index greater than the refractive index of each of the low refractive index layers, the prisms being shaped in such a manner as to allow the incident light to be incident upon the thin film layers at a plurality of angles greater than or equal to the critical angle for the highest refractive index of the low refractive index layers, the thickness of the low refractive index layers of the plurality of thin film layers being small enough so that light incident upon the thin film layers at an angle greater than the critical angle can be partially coupled out through the low refractive index layers such that frustrated total internal reflection occurs, thus permitting interference to take place between the light reflected at the interfaces of all the thin film layers, and in addition, the thicknesses of the thin film layers being such that an equivalent optical admittance of the plurality of the thin film layers for s-polarized light is substantially the same as an optical admittance of the substrate for s-polarized light for a wide range of angles of incidence and a broad band of wavelengths when the incident light is incident upon the low refractive index layers at an angle greater that the critical angle, thereby allowing substantially all the incident s-polarized light to be substantially transmitted, the plurality of the thin film layers having an equivalent optical admittance for p-polarized light that is substantially different from the optical admittance of the substrate for p-polarized light for a wide-range of angles of incidence and a broad-band of wavelengths and thus substantially reflecting the incident p-polarized light.

7, A projection display system as defined in any one of claims 1 to 6, wherein the polarization beam discriminator has input and output end faces forming an inside angle greater than 90 degrees.

8. A projection display system as defined in any one claims 1 to 7, wherein said spatial light modulator means comprises a spatial light modulator for each of said polarized beams, each said spatial light modulators being provided with a signal representative of a different image.

9. A projection display system as defined in claim 8, further comprising means for providing information corresponding to two images in the form of a recorded output signal.

10. A projection display system as defined in claim 9, wherein the means for providing information corresponding to two images are in the form of a video output signal.

11. A projection display system as defined in any one of claims 1 to 4, wherein said polarizing beam discriminator comprises a prism having first and second pairs of obliquely inclined first and second active faces, said first active face of said first pair receiving the input beam and said second active face of said first pair providing the output beam, said first active face of the second pair providing a first beam of one plane of polarization and said second active face of the second pair providing a second beam having a plane of polarization orthogonal to said first beam.

12. A projection display system as defined in claim 11, wherein said spatial light modulator means comprises a transmissive imaging element through which said p and s polarized beams are transmitted, said imaging element encoding an imago onto said polarized beams in such a way that the image component of the transmitted beam has the same plane of polarization as the input beam, and mirrors are provided to reflect each modulated light beam back into the prism through the; other active face of the pair from which it emerged.

13. A projection display system as defined in claim 11, wherein said spatial light modulator means comprises a pair of reflective imaging elements corresponding to said respective p and s polarized light beams, said imaging elements encoding; an image onto said polarized beams in such a way that the image component of the reflected beam has the opposite plane of polarization as the input polarized beans, said reflective imaging elements returning each modulated light beam through the active face from which it emerged.

14. A projection display system as defined in claim 12 or 13, wherein said light source is a white light source and the pixels of the imaging elements are provided with different colour filters to provide a colour display.

15. A projection display system as defined in claim 12 or 13, wherein each polarized beam emerging from the polarizing beam discriminator is input to a colour separating/combining prism for directing each primary colour to a respective reflective imaging element, said separating/combining prism combining the encoded primary colours and returning the encoded beam to the active face from which it emerged.

16. A projection display system as defined in any one of claims 1 to 13, further comprising filter means for separating said input beam into three primary colours, a said polarizing beam discriminator for each said primary colour and spatial light modulator means for each primary colour, and means for combining the output beams from the polarizing beam discriminators into a common output beam for focussing onto said display means.

17. A projection display system as defined in claim 16, wherein said polarizing beam discriminator comprises a prism having first and second pairs of obliquely inclined first and second active faces, said first active face of said first pair receiving the input beam and said second active face of said first pair providing the output beam, said first active face of the second pair providing a first beam of one plane of polarization and said second active face of the second pair providing a second beam having a plane of polarization orthogonal to said first beam, and wherein the spatial light modulator means for each primary colour comprises a pair of reflective imaging devices, each imaging device of each pair being associated with the respective active face from which the beam it encodes emerges from the prism.

18. A projection display system as defined in claim 16, wherein said polarizing beam discriminator comprises a prism having first and second pairs of obliquely inclined first and second active faces, said first active face of said first pair receiving the input beam and said second active face of said fast pair providing the output beam, said first active face of the second pair providing a first beam of one plane of polarization and said second active face of the second pair providing a second beam having a plane of polarization orthogonal to said first beam, wherein the spatial light modulator means for each primary colour comprises a transmissive imaging device, and mirrors are provided to reflect each polarized beam emerging from the prism through the imaging device and back into the prism through the other active face from which it emerged.

19. A projection display system as defined in claim 11 or 13, wherein a different image corresponding to the left and right eye view is encoded onto each polarized beam so as to create a 3-dimensional image that can be viewed with polarized glasses.

20. A projection display system as defined in any of claims 1 to 13, wherein said light source generates three primary colours which are sequentially transmitted to said spatial light modulator means.

21. A projection display system as defined in claim 20, wherein said light source comprises a source of white light and a colour wheel for sequentially transmitting each colour to the spatial light modulator means, said spatial light modulator means operating in synchronism with said colour wheel to generate a colour image.

22. A projection display system as defined in claim 21, wherein said light source comprises three light emitting diodes of different colour for sequentially transmitting each colour to the spatial light modulator means, said spatial light modulator means operating in synchronism with said colour wheel to generate a colour image.

23. A projection display system as defined in claim 21, wherein said light source comprises three lasers of different colour for sequentially transmitting each colour to the spatial light modulator means, said spatial light modulator means operating in synchronism with said colour wheel to generate a colour image.

24. A projection display system as defined in any one of claims 1 to 23, wherein said spatial light modulator menus comprise polymer dispersive liquid crystal displays (PDLCDs).

25. A projection display system as defined in claim 24, wherein said polymer dispersive liquid crystal displays (PDLCDs) associated with a quarterwave plate to rotate the plane of polarization of reflected light.

26. A projection display system as defined in any one of claims 1 to 23, wherein said spatial light modulators comprise twisted nematic liquid crystal displays.

27. A projection display system as defined in any one of claims 1 to 23, wherein said spatial light modulators comprise digital micromirrors, and the second said polarizing beam discriminator combines s and p polarized light reflected by said respective digital micromirrors into said output beam for focussing onto said display means.