SPATIAL LIGHT MODULATOR

This application claims the benefit of U.S. Provisional Patent Application: No. 60/662,823 filed March 18, 2005.

BACKGROUND OF THE INVENTION This invention relates to a display device, and more particularly to a spatial light modulator based on electrically switchable Bragg gratings.

Spatial Light Modulators (SLMs) are devices that generate light with a spatially modulated intensity. SLMs are commonly used in the fields of displays, image processing and optical communications networks. Several SLM technologies have been developed, including liquid crystals, micro-mechanical mirrors (MEMs), micro mechanical diffraction gratings and others. Liquid Crystal Displays (LCDs) are the most well known examples. LCDs rely on the bulk properties of the liquid crystal to achieve intensity modulation. LCDs are limited in the extent to which they can provide efficient SLMs by switching time, transmission losses and polarization sensitivity. Polymer Dispersed Liquid Crystal (PDLC) materials that rely on scattering by liquid crystal droplets have been considered as an alternative to bulk nematic liquid crystals. However, PDLC scattering materials suffer from slow switching speeds.

SLMs based on MEMs gratings, such as the devices manufactured by Silicon Light Machines Inc., are well known. However, MEMs gratings suffer from several problems. MEMs direct light into a multitude of different diffracted orders and are limited to grating periods in the region of 5 to10 microns, which limits the diffraction angles that may be used. The diffraction efficiencies of MEMs gratings are limited to 40%. In general, SLMs implemented in MEMs, whether based on micro mirrors or gratings, suffer from the problem that the technology is inherently reflective, requiring complex off-axis illumination schemes that compromise form factor, manufacturability and cost.

The most efficient method of illuminating SLMs is to present red green and blue illumination sequentially. The illumination may be provided by lasers or Light Emitting Diodes (LEDs). Colour sequential illumination requires that the SLM update rate is fast enough for the sequential single-color images to appear to the observer as a full color image.

Thus there exists a need for an improved SLM with a suitably fast update rate, a compact form factor and high optical transmission efficiency.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an improved SLM with a fast update rate, a compact form factor and high optical transmission efficiency.

The objects of the invention are achieved in a display device comprising: a light source for generating red, green, and blue bandwidth lights, a Spatial Light Modulator (SLM) containing at least one pixelated diffraction grating recorded in a electrically variable refractive index material; a control circuit coupled to the SLM; an image generator; and a light stop. Each source illuminates the SLM with red light at a first angle, green light at a second angle and blue light at a third angle. The third angle characterizes a viewing direction. The control circuit is configured to receive red, green, and blue pixel component signals from the image generator and generates red, green, and blue pixel component analog voltages in response to receiving the red, green, and blue pixel component signals, respectively. The magnitudes of the red, green, and blue pixel component analog voltages depend on the red, green, and blue pixel component signals, respectively. The SLM diffract portions of the red, green, and blue bandwidth lights, respectively, in response to receiving red, green, and blue pixel component analog voltages, respectively. The diffracted portions of the red, green, bandwidth light and

undiffracted portions of the blue light emerge from the SLM in the viewing direction. Undiffracted portions of the red, green blue bandwidth light and diffracted portions of the blue light are blocked by the stop after emerging from said SLM. The intensities of the diffracted red, green, and blue bandwidth output light are inversely proportional to the magnitudes of the red, green, and blue pixel component analog voltages, respectively. The intensities of the undiffracted red, green, and blue bandwidth output lights are proportional to the magnitudes of the red, green, and blue pixel component analog voltages, respectively.

Each SLM pixel diffracts light in the viewing direction when in a diffracting state. However, each said pixel is substantially transparent to light when not in a diffracting state.

Red, green and blue illumination is presented sequentially to the SLM. The SLM is updated with red, green and blue image pixel data in sequence with said illumination.

An output image may be viewed directly on the surface of the SLM or, alternatively, may be projected on to screen by means of a projection lens. The illumination may be provided by red, green and blue lasers. Alternatively illumination may be provided by Light Emitting diodes or other types of illumination source.

In preferred embodiments of the invention the electrically variable refractive index element is an ESBG comprising at least one Bragg grating recorded in Holographic Polymer Dispersed Liquid Crystal (HPDLC) sandwiched between transparent substrates to which transparent conductive coatings have been applied. At least one of said coatings is patterned to provide a two-dimensional array of independently switchable pixels.

In a first embodiment of the invention the SLM comprises a single ESBG into which three superimposed Bragg gratings have been recorded. Two of the gratings are configured such that red and green light incident at first and second incident angles is modulated and diffracted into the viewing direction. The third grating modulates and diffracts blue light incident parallel to the viewing direction into a direction away from the viewing direction. The red and green light, which is not diffracted, and the diffracted blue light are all trapped by a light- absorbing stop. Hence the light transmitted along the viewing direction comprises red and green diffracted light and blue non-diffracted light. The SLM is sequentially updated with red, green and blue image data. The relative durations of presentation of the red, green and blue image data are dictated by considerations of maximizing throughput, achieving an acceptable color gamut and satisfactory fusion of red, green and blue light in the viewed image.

In a second embodiment of the invention the SLM comprises a single ESBG configured such that red light at said first angle and green light at said second angle are both diffracted into said viewing direction and blue light at said third angle is diffracted away from the viewing direction. The SLM relies on the property of Bragg holograms that high diffraction efficiency can be provided in different incidence angle ranges for different wavelengths according to the Bragg diffraction equation. The red and green light, which is not diffracted, and the diffracted blue light are all trapped by a light-absorbing stop. Hence the light transmitted along the viewing direction comprises red and green diffracted light and blue non-diffracted light. The SLM is sequentially updated with red green and blue image data.

In a third embodiment of the invention the SLM comprises first and second SLM devices. The first SLM comprises an ESBG with single grating designed to modulate and diffract red light incident at a first angle into the viewing direction. The first SLM device also modulates and diffracts green light incident at a second angle into the viewing direction. The second SLM

device contains a single ESBG designed to modulate diffract blue light incident parallel to the viewing direction into an angle away from the viewing direction. The non-diffracted blue light continues to propagate into the viewing direction. The red and green light, which is not diffracted, and the diffracted blue light are all trapped by a light-absorbing stop. Hence the light transmitted along the viewing direction comprises red and green diffracted light and blue non- diffracted light. The first SLM device is sequentially updated with red and green image data. The second SLM device displays blue image data wavelength only. In a typical operational mode of the invention blue information may be displayed at the same time as either the red or green information.

In a fourth embodiment of the invention similar in operation to the third embodiment the first SLM device comprises a single ESBG into which superimposed red and green diffracting Bragg gratings have been recorded.

In further embodiments of the invention based on the third and fourth embodiments the first SLM device may comprise an ESBG designed to modulate and diffract one colour into the viewing direction and a second colour away from the viewing direction. In such embodiments the second SLM device would diffract a third colour into the viewing direction. In such embodiments the first SLM device may comprise an ESBG into which a single grating has been recorded. Alternatively the first SLM device may comprise an ESBG into which two superimposed gratings have been recorded.

In a further embodiment of the invention the SLM comprises separate red, green and blue SLM devices each comprising a single ESBG. Two of said ESBGs would be designed to diffract red and green incident light into directions substantially parallel to the viewing direction. The third ESBG would be designed to diffract blue light parallel to the viewing direction into a

direction away from the viewing direction. The red and green light, which is not diffracted, is trapped by a light-absorbing stop. The diffracted blue light is trapped by the stop. Hence the light transmitted along the viewing direction comprises red and green diffracted light and blue non-diffracted light. Red, green and blue illumination is presented sequentially to the SLM. Each SLM layer is updated with red, green and blue image data in sequence with said illumination.

In further embodiments of the invention an SLM which modulates both the S and P polarized components of the incident light is provided. The SLM comprises first and second ESBG groups separated by a half wave plate (HWP). Said first and second ESBG groups have substantially identical specifications. Said first and second ESBG groups may each comprise any of the above SLM embodiments. Identical image modulation data is applied to said first and second ESBG groups. The procedures for illuminating the SLM and updating the ESBG data are equivalent to those used in the above SLM embodiments.

The first ESBG group modulates and diffracts incident P-polarized red and green light into the viewing direction and modulates and diffracts incident P-polarized blue light away from the viewing direction. The portion of incident red and green light that is not diffracted continues to propagate away from the viewing direction. However the portion of P-polarized incident blue light, from which diffracted light has been subtracted, proceeds along the viewing direction. At the same time, the incident S-polarized blue light continues to propagate in the viewing direction and the incident S-polarized red and green light continues to propagate away from the viewing direction.

After propagation through the HWP said diffracted P-polarized red and green light is converted to S-polarized light and is therefore not diffracted by the second ESBG group.

However, said incident S-polarized red and green light that was not diffracted by the first ESBG

group is converted to P-polarized light and is therefore diffracted into the viewing direction by the second ESBG group, which has identical diffracting characteristics to said first ESBG group. The portion of incident red and green light that was not diffracted is converted to S-polarized light and proceeds without deviation through the second ESBG group and then onto a light absorbing stop.

After propagation through the HWP said diffracted P-polarized blue light is converted to S-polarized light and proceeds without deviation through the second ESBG group and then onto a light-absorbing stop. Said portion of P-polarized incident blue light from which diffracted light has been subtracted is converted into S polarized light by the HWP and propagates through the second ESBGH group into the viewing direction. Said incident S-polarized blue light is converted into P-polarized light by the HWP after which it is diffracted by the second ESBG group away from the viewing direction towards the light absorbing stop while the portion of blue light from which diffracted light has been subtracted by the second ESBG propagates in the viewing direction. Hence the output light from the SLM in the viewing direction comprises S and P polarized modulated diffracted red and green light. The output light from the SLM in the viewing direction further comprises and S and P polarized blue light from which modulated diffracted S and P blue light has been subtracted.

A preferred operational embodiment of the invention further comprises a condenser lens and a projection lens

A further operational embodiment of the invention further comprises a light control film disposed between the SLM and the viewer to block stray light that would otherwise reduce contrast and degrade color gamut. The light control film essentially transmits light incident in the viewing direction and blocks or absorbs light incident at any other angle.

A more complete understanding of the invention can be obtained by considering the following detailed description in conjunction with the accompanying drawings wherein like index numerals indicate like parts. For purposes of clarity details relating to technical material that is known in the technical fields related to the invention have not been described in detail.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG.1 A is a schematic side elevation view of a first embodiment of the invention.

FIG.1 B is a schematic side elevation view of a second embodiment of the invention. FIG.1 C is a schematic side elevation view of a third embodiment of the invention.

FIG.2A is a schematic side elevation view of a fourth embodiment of the invention.

FIG.2B is a schematic side elevation view of a fifth embodiment of the invention.

FIG.2C is a schematic side elevation view of a sixth embodiment of the invention.

FIG.3 is a schematic side elevation view of a seventh embodiment of the invention. FIG.4 is a schematic side elevation view of an eighth embodiment of the invention.

FIG.5 is a schematic side elevation view of a ninth embodiment of the invention.

FIG.6 is a schematic side elevation view of a light control film used in the invention.

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a Spatial Light Modulator (SLM) employing at least one pixelated diffraction grating recorded in an electrically variable refractive index medium. In preferred embodiments of the grating is an Electrically Switchable Bragg Grating.

ESBG devices are formed by recording a volume phase grating, or hologram, in a holographic polymer dispersed liquid crystal (HPDLC) mixture. Typically, ESBG devices are fabricated by first placing a thin film of a mixture of photopolymerizable monomers and liquid crystal material between parallel glass plates. Techniques for making and filling glass cells are

well known in the liquid crystal display industry. One or both glass plates support electrodes, typically transparent indium tin oxide films, for applying an electric field across the HPDLC layer. A volume phase grating is then recorded by illuminating the liquid material with two mutually coherent laser beams, which interfere to form the desired grating structure. During the recording process, the monomers polymerize and the HPDLC mixture undergoes a phase separation, creating regions densely populated by liquid crystal micro-droplets, interspersed with regions of clear polymer. The alternating liquid crystal-rich and liquid crystal-depleted regions form the fringe planes of the grating. The resulting volume phase grating can exhibit very high diffraction efficiency, which may be controlled by the magnitude of the electric field applied across the HPDLC layer. When an electric field is applied to the hologram via transparent electrodes, the natural orientation of the LC droplets is changed causing the refractive index modulation of the fringes to reduce and the hologram diffraction efficiency to drop to very low levels. Note that the diffraction efficiency of the device can be adjusted, by means of the applied voltage, over a continuous range from near 100% efficiency with no voltage applied to essentially zero efficiency with a sufficiently high voltage applied. U.S. Patent 5,942,157 and U.S. Patent 5,751 ,452 describe monomer and liquid crystal material combinations suitable for fabricating ESBG devices. A publication by Butler et al. ("Diffractive properties of highly birefringent volume gratings: investigation", Journal of the Optical Society of America B, Volume 19 No. 2, February 2002) describes analytical methods useful to design ESBG devices and provides numerous references to prior publications describing the fabrication and application of ESBG devices. HPDLC materials as described above are intrinsically faster than bulk liquid crystal materials and conventional PDLC materials.

An SLM is provided by patterning at least one of said conductive coatings to provide a two-dimensional array of independently switchable pixels. Each said pixel diffracts said red, green and blue light towards viewer when in a diffracting state and each said pixel is

substantially transparent to said light when in a non diffracting state. The portion of incident light diffracted by any given pixels is dependent on the electric field applied to the electrode covering said pixel. In conventional HPDLC materials the portion of incident light diffracted is inversely proportional to the applied electric field. Conversely the amount of light that is not diffracted which will be referred to as undiffracted light is proportional to the applied field. Each ESBG is operative to diffract at least one wavelength of red, green or blue light towards a viewer. The basic principles of the SLM are illustrated in the schematic side views of three embodiments of the invention in FIG.1. In each case light propagation through one pixel is shown. For the purposes of explaining the various embodiments of the invention it is assumed that three light source having red, green and blue wavelengths are provided. Although the invention has many applications in the field of displays, image processing and optical telecommunications, the basic principles of the invention will be discussed in relation to an image display device. The image may be viewed directly on the surface of the SLM or alternatively may be projected on to screen by means of a projection lens.

In the embodiment of the invention shown in the schematic side elevation view in FIG.1 A the SLM 10 comprises a first ESBG 101 and a second ESBG 102. Light from red, green and blue sources is incident in the ray directions 1000a, 1000b and 1000c respectively. It should be noted that 1000c defines the viewing direction. The red, green and blue light may be provided by lasers, LEDs, filtered white light sources or any other source of illumination. The blue light 1000c propagates along a direction normal to the SLM substrates, said direction corresponding to a viewing direction. The grating 101 is designed to diffract a portion of red light incident in the direction 1000a into the direction 2000a. The ESBG 101 is also operative to diffract a portion of a green light incident in the direction 1000b into the direction 2000b. The ESBG 101 relies on the property of Bragg holograms that high efficiency can be provided in different incidence angle ranges for different wavelengths according to the Bragg diffraction equation. The diffracted light

directions 2000a and 2000b are substantially parallel to the viewing direction. The red and green light which is not diffracted, represented by rays 3000a and 3000b, is trapped by a light- absorbing stop 20. In contrast the second ESBG 102 is designed to diffract blue light incident ir the direction 1000c into the direction 2000c. Blue light from which diffracted blue light has beer subtracted propagates in the direction 3000c. The diffracted blue light 2000c is trapped by the stop 20. Hence the light transmitted along the viewing direction comprises red and green diffracted light 2000a, 2000b and blue light 3000c.

The inventors have found that high diffraction efficiency is obtained when the first ESBG is designed to have incidence angles of approximately 40° for green light and approximately 50 ^c for red light. It should be noted that the red and green output from the SLM comprises light diffracted out of the incident red and green illumination. On the other hand the blue output from the SLM comprises blue light from which diffracted blue light has been subtracted.

The first ESBG is sequentially updated with red and green image data. The second

ESBG displays blue image data only. In a typical operational mode of the invention blue information may be displayed at the same times as the red or green information The relative durations of the red green and blue data are dictated by considerations of maximizing throughput, achieving an acceptable color gamut and satisfactory fusion of red, green and blue light in the viewed image.

In an alternative embodiment of the invention, which is also illustrated by FiG.1 A, the SLM layer 101 may comprise an ESBG into which two superimposed red and green Bragg gratings have been recorded. Such a configuration may allow more flexibility in the choice of incident angles. The basic principles of recording multiple superimposed gratings will be well known to those skilled in the art of holography and is discussed in textbooks such as Optical

Holography" by RJ. Collier, CB. Burkhardt and L.H. Lin published by Academic Press, New York (1971 ). However, as discussed in references such as Collier superimposed Bragg gratings suffer from reduced diffraction efficiency.

In an alternative embodiment of the invention the SLM comprises a single ESBG 11 into which three superimposed Bragg gratings have been recorded. Two of said gratings are designed to diffract red and green incident light directions 2100a and 2100b substantially parallel to the viewing direction. The third grating is designed to diffraction blue light parallel to the viewing direction into a direction 2100c away from the viewing direction. The red and green light, which is not diffracted, represented by rays 3100a and 3100b, is trapped by a light- absorbing stop 20. The diffracted blue light 2100c is trapped by the stop 20. Hence the light transmitted along the viewing direction comprises red and green diffracted light 2100a, 2100b and blue light from which diffracted blue light has been subtracted 3100c. As in the case of FIG.1B, this approach would suffer from the problem of reduced diffraction efficiency of superposed gratings. In the embodiment of FIG.1 B red, green and blue light would be presented sequentially. The display information would be updated in sequence with the illumination.

In an alternative embodiment of the invention which is also illustrated using the schematic side view of FIG.1 B the SLM comprises a single ESBG configured such that red light at said first angle and green light at said second angle are both diffracted into said viewing direction and blue light at said third angle is diffracted away from the viewing direction. The SLM relies on the property of Bragg holograms that high diffraction efficiency can be provided in different incidence angle ranges for different wavelengths according to the Bragg diffraction equation. The red and green light, which is not diffracted, and the diffracted blue light are all

trapped by a light-absorbing stop. Hence the light transmitted along the viewing direction comprises red and green diffracted light and blue non-diffracted light.

In an alternative embodiment of the invention shown in the schematic side view of

FIG.1C the SLM 12 comprises separate red green and blue ESBGs 121 ,122,123. Two of said ESBGs 121 ,122 are designed to diffract red and green incident light directions 2200a and 2200b substantially parallel to the viewing direction. The third ESBG 123 are designed to diffraction blue light parallel to the viewing direction into a direction 2200c away from the viewing direction. The red and green light, which is not diffracted, represented by rays 3200a and

3200b, is trapped by a light-absorbing stop 20. The diffracted blue light 2200c is trapped by the stop 20. Hence the light transmitted along the viewing direction comprises red and green diffracted light 2200a, 2200b and blue light from which diffracted blue light has been subtracted 3200c. Since this embodiment requires an additional ESBG, it is not a preferred embodiment. Red, green and blue light is presented simultaneously. The red, green and blue display data are presented simultaneously.

One of the well known attributes of transmission ESBGs is that the liquid molecules tend to align normal to the grating fringe planes. The effect of the liquid crystal molecule alignment is that ESBG transmission gratings efficiently diffract P polarized light (ie light with the polarization vector in the plane of incidence) but have nearly zero diffraction efficiency for S polarized light (ie light with the polarization vector normal to the plane of incidence. Hence in the embodiments of FIG.1 only P polarized red and green light is transmitted in the viewing direction while the blue light transmitted in the viewing direction will be S-polarized.

FIG 2 illustrates three embodiments of the invention that provide modulation of both the S and P components of the incident light. Essentially, the embodiments of FIGS.2A-2C are equivalent to the embodiments of FIGS.1A-1C respectively. In each case the FIG.1 embodiment is modified by adding a Half Wave Plate (HWP) layer and a second SLM of similar specification to the SLM shown in the FIG.1 embodiment. Identical image modulation data is applied to said first and second SLM devices. The procedures for controlling the SLMs and updating the image data displayed on the SLMs are equivalent to those used for the FIG.1 embodiments.

It is well known that HWPs rotate the polarization of incident light through ninety degrees thereby converted S-polarized light to P-polarized light and vice versa. In the following discussion the ray direction labels used in FIG.1 may be assumed to apply in FIG.2.

The embodiment of the invention shown in the schematic side elevation view of FIG.2A comprises first and second ESBGs 101a, 102a, identical to the ESBGs 101 ,102 of FIG.1 A, a half wave plate 30 and third and fourth ESBGs 101b, 102b, having substantially identical specifications to the ESBGs 101a, 101b respectively. The first ESBG group modulates and diffracts incident P-polarized red and green light 1000a, 1000c into the viewing direction and modulates and diffracts incident P-polarized blue light 1000b away from the viewing direction. The portion of incident P-polarized red and green light from which modulated diffracted S and P blue light have been subtracted, 3000a, 3000c, continues to propagate away from the viewing direction. Likewise, incident S-polarized red and green light continues to propagates away from the viewing direction along directions 3000a, 3000c.

However, the portion of P-polarized incident blue light, from which diffracted light has been subtracted and the S-polarize incident blue light, which has not been diffracted, proceeds along the viewing direction along direction 3000b. After propagation through the HWP said

diffracted P-polarized red and green light is converted to S-polarized light 4000a, 4000c and is therefore not diffracted by the second ESBG group.

Said incident S-polarized red and green light that was not diffracted by the first ESBG group is converted to P-polarized light along directions 5000a, 5000c until it is diffracted into the viewing direction 6000a, 6000c by the second ESBG group, which has identical diffracting characteristics to said first ESBG group. The portion of incident red and green light from which diffracted light has been subtracted is converted to S-polarized light and proceeds in the directions 5000a, 5000c without deviation through the second ESBG group and then onto a light absorbing stop. After propagation through the HWP said diffracted P-polarized blue light 3000b is converted to S-polarized light 4000b and proceeds without deviation through the second ESBG group and then onto the light-absorbing stop. Said portion of P-polarized incident blue light from which diffracted light has been subtracted, propagating in the direction 3000b, is converted into S polarized light by the HWP and propagates through the second ESBG group into the viewing direction 7000b. Said incident S-polarized blue light is converted into P-polarized light 4000b by the HWP after which it is diffracted by the second ESBG group away from the viewing direction in the direction 6000b towards the light absorbing stop while the portion of blue light from which diffracted light has been subtracted by the second ESBG propagates in the viewing direction indicated by7000b. Hence the output light from the SLM in the viewing direction comprises S and P polarized modulated diffracted red and green light The output light from the SLM in the viewing direction further comprises and S and P polarized blue light from which modulated diffracted S and P blue light have been subtracted.

The embodiment of the invention shown in the schematic side elevation view of FIG.2B comprises first ESBG 11a identical to the ESBG 11 of FIG.1 B, a half wave plate 30 and a second ESBG 11b, having substantially identical specifications to the ESBG 11a.

The first ESBG modulates and diffracts incident P-polarized red and green light 1000a, 1000c into the viewing direction and modulates and diffracts incident P-polarized blue light 1000b away from the viewing direction. The portion of incident P-polarized red and green light from which modulated diffracted S and P blue light have been subtracted, 3100a, 3100c, continues to propagate away from the viewing direction. Likewise, incident S-polarized red and green light continues to propagates away from the viewing direction along directions 3100a, 3100c.

However, the portion of P-polarized incident blue light, from which diffracted light has been subtracted and the S-polarize incident blue light, which has not been diffracted, proceeds along the viewing direction along direction 3000b.

After propagation through the HWP said diffracted P-polarized red and green light is converted to S-polarized light 4100a, 4100c and is therefore not diffracted by the second ESBG.

Said incident S-polarized red and green light that was not diffracted by the first ESBG is converted to P-polarized light along directions 5100a, 5100c until it is diffracted into the viewing direction 6100a, 6100c by the second ESBG, which has identical diffracting characteristics to said first ESBG. The portion of incident red and green light from which diffracted light has been subtracted is converted to S-polarized light and proceeds in the directions 5100a, 5100c without deviation through the second ESBG and then onto the light absorbing stop.

After propagation through the HWP said diffracted P-polarized blue light 3100b is converted to S-polarized light 4100b and proceeds without deviation through the second ESBG and then onto the light-absorbing stop. Said portion of P-polarized incident blue light from which diffracted light has been subtracted, propagating in the direction 3100b, is converted into S polarized light by the HWP and propagates through the second ESBG into the viewing direction 7100b. Said incident S-polarized blue light is converted into P-polarized light 4100b by the HWP after which it is diffracted by the second ESBG away from the viewing direction in the direction 6100b towards the light absorbing stop while the portion of blue light from which

diffracted light has been subtracted by the second ESBG propagates in the viewing direction indicated by7100b. Hence the output light from the SLM in the viewing direction comprises S and P polarized modulated diffracted red and green light The output light from the SLM in the viewing direction further comprises and S and P polarized blue light from which modulated diffracted S and P blue light have been subtracted.

The embodiment of the invention shown in the schematic side elevation view of FIG.2C comprises first second and third ESBGs 121a, 122a, 123a identical to the ESBGs 121 ,122, 123 of FIG.1 C, a half wave plate 30 and fourth, fifth and sixth ESBG 121 b, 122b, 123b, having substantially identical specifications to the ESBGs 121a, 122a, 123a respectively. The first ESBG group modulates and diffracts incident P-polarized red and green light 1000a, 1000c into the viewing direction and modulates and diffracts incident P-polarized blue light 1000b away from the viewing direction. The portion of incident P-polarized red and green light from which modulated diffracted S and P blue light have been subtracted, 3200a, 3200c, continues to propagate away from the viewing direction. Likewise, incident S-polarized red and green light continues to propagates away from the viewing direction along directions 3200a, 3200c.

However, the portion of P-polarized incident blue light, from which diffracted light has been subtracted and the S-polarize incident blue light, which has not been diffracted, proceeds along the viewing direction along direction 3000b. After propagation through the HWP said diffracted P-polarized red and green light is converted to S-polarized light 4200a, 4200c and is therefore not diffracted by the second ESBG group.

Said incident S-polarized red and green light that was not diffracted by the first ESBG group is converted to P-polarized light along directions 5200a, 5200c until it is diffracted into the viewing direction 6200a, 6200c by the second ESBG group, which has identical diffracting characteristics to said first ESBG group. The portion of incident red and green light from which

diffracted light has been subtracted is converted to S-po!arized light and proceeds in the directions 5200a, 5200c without deviation through the second ESBG group and then onto the light absorbing stop.

After propagation through the HWP said diffracted P-polarized blue light 3200b is converted to S-polarized light 4200b and proceeds without deviation through the second ESBG group and then onto the light-absorbing stop. Said portion of P-polarized incident blue light from which diffracted light has been subtracted, propagating in the direction 3200b, is converted into S polarized light by the HWP and propagates through the second ESBG group into the viewing direction 7200b. Said incident S-polarized blue light is converted into P-polarized light 4200b by the HWP after which it is diffracted by the second ESBG group away from the viewing direction in the direction 6200b towards the light absorbing stop while the portion of blue light from which diffracted light has been subtracted by the second ESBG propagates in the viewing direction indicated by7200b. Hence the output light from the SLM in the viewing direction comprises S and P polarized modulated diffracted red and green light The output light from the SLM in the viewing direction further comprises and S and P polarized blue light from which modulated diffracted S and P blue light have been subtracted.

FIG.3 shows an operational embodiment of the invention based on FIG.1 A, which operates on P-polarized light. The optical system comprises an SLM comprising the elements 101 ,102 according to the principles of FIG.1 A and a light stop 20, a condenser lens 40 and a projection lens 50 for directing light to a screen or viewing arrangement. The light propagation geometry is identically to that of FIG.1 A.

FIG.4 Shows one operational embodiment of the invention based on FIG.2A, which operates on both P-polarized and S-polarized light. The optical system comprises an SLM comprising the elements 101a, 102a, 101b, 102b according to the principles of FIG.2A, a light

stop 20, a condenser lens 40 and a projection lens 50 for directing light to a screen or some other viewing arrangement. The light propagation geometry is identically to that illustrated in FIG.2A.

FIG.5 Shows an operational embodiment of the invention based on FIG.4, which further comprises a light control film 60. The function of the light control film shown in is to block stray light that would otherwise reduce contrast and degrade color gamut. Since practical ESBGs do not achieve the 100% theoretical diffraction efficiency of Bragg gratings, the displayed imagery may be degraded by zero order (or non-diffracted light) and spurious diffracted light arising from the diffraction of more than one wavelength by the ESBGs in the illumination-directing device. Further, the diffraction efficiency versus incidence angle characteristic of transmission gratings will exhibit secondary diffraction maximum to both sides of the primary diffraction peak. While the peak diffraction efficiency of these secondary peaks will be small, effect may be sufficient to reduce the color purity of the display. One known means for providing a light control film is illustrated in a schematic side elevation view in FIG.6. The light control film 60 comprises an array of micro-sphere lenses such as 62 embedded in a light-absorbing layer 61. Each lens provides a small effective aperture 63 such that incident rays 8001 , substantially normal to the screen, are transmitted with low loss as a divergent beam 8001 while incident rays 8002, incident at an off axis angle, are absorbed.

Light control films such as the one illustrated in FIG.6 may be applied to any of the embodiments of the invention. Light control films of the type shown in FIG. 6 are manufactured by 3M Inc. (Minnesota). Other methods of providing a light control film, such as louver screens may be used as an alternative to the light control film described above.

The ESBG layers and HWP layers used in SLM devices illustrated in FIGS.1-4 are shown as physically separated elements for the purposes of explaining the invention. In preferred practical embodiments of the invention said layers would be combined in a single planar multiplayer device. The multilayer SLM devices may be constructed by first fabricating the separate ESBG devices and then laminating the ESBG devices using an optical adhesive. Suitable adhesives are available from a number of sources, and techniques for bonding optical components are well known. The multilayer structures may also comprise additional transparent elements, not shown in FIGS.1-4, to control the optical properties of the SLM

It should be noted that in order to ensure efficient use of the available light and a wide color gamut for the SLM, the ESBG devices should be substantially transparent when a voltage is applied, and preferably should diffract only the intended color without an applied voltage.

In each of the embodiments discussed above it is found that the procedure of introducing red and green illumination at different angles to the viewing direction and introducing blue light along the viewing direction is the most desirable in terms of providing high image contrast. However, it should be emphasized that the angles and illumination colors may be interchanged in alternative embodiments of the invention.

It should be emphasized that the Figures are exemplary and that the dimensions have been exaggerated. For example thicknesses of the ESBG layers have been exaggerated. In projection applications SLMs will require diffusion characteristics matched to the specification of the projection lens. The required diffusion characteristics may be built into the ESBG devices using procedures well known to those skilled in the art of HOEs.

Any of the embodiments described in the FIGS.1-5 may incorporate Computer Generated Holograms for the purpose of control the uniformity of the illumination at the surface of the SLMs.

In alternative embodiments of the basic invention further ESBGs and light sources may be added to allow light from multiple sources of a particular color to be modulated.

The switching of the pixelated SLM devices described above may be performed according to the basic principles of active matrix displays. However, other switching architectures and processes compatible with the illumination procedures describe earlier may be used.

The invention is not restricted to any particular type of HPDLC material. In the embodiments described above it is assumed the portion of incident light diffracted by an ESBG is inversely proportional to the applied electric field. However the invention also applies to ESBGs based on HPDLC material systems in which the portion of incident light diffracted is proportional to the applied electric field. In the embodiments described above it is assumed the ESBG diffracts P-polarized light.

However the invention also applies to alternative HPDLC material systems which diffract S- polarized light

The invention may use HPDLCs based on any type of liquid crystal material including nematic and chiral types.

Although the invention has been described in relation to what are presently considered to be the most practical and preferred embodiments, it is to be understood that the invention is not limited to the disclosed arrangements, but rather is intended to cover various modifications and equivalent constructions included within the spirit and scope of the invention.