WO2010023444A1

WO2010023444A1 - Laser display incorporating speckle reduction

Info

Publication number: WO2010023444A1
Application number: PCT/GB2009/002074
Authority: WO
Inventors: Milan Momcilo Popovich; Jonathan David Waldern; Robert Hess
Original assignee: Milan Momcilo Popovich; Jonathan David Waldern; Robert Hess
Priority date: 2008-08-27
Filing date: 2009-08-26
Publication date: 2010-03-04

Abstract

A laser projection display comprises in sequence: a laser source; beam expansion and collimation optics; first and second ESBG arrays; a microdisplay; a projection lens; and a screen. Each ESBG array is recorded in a Holographic Polymer Dispersed Liquid Crystal sandwiched between transparent substrates patterned to provide a two-dimensional array of independently switchable ESBG pixels. Each ESBG pixel is characterized by a first unique speckle state under said first applied voltage and a second unique speckle state under said second applied voltage. Each ESBG pixel diffracts incident collimated light into an illumination patch in proximity to the active surface of said microdisplay.

Description

LASER DISPLAY INCORPORATING SPECKLE REDUCTION

CROSS REFERENCE TO RELATED APPLICATIONS The present application claims priority to U.S. provisional patent application 61/136,309 filed on 27 August 2008, entitled LASER DISPLAY INCORPORATING SPECKLE

REDUCTION.

The present application incorporates by reference in its entirety a PCT US2006/043938 filed 13 November 2006, claiming priority to U.S. provisional patent application 60/789,595 filed on 6 April 2006, entitled METHOD AND APPARATUS FOR PROVIDING A

TRANSPARENT DISPLAY.

The present application incorporates by reference in its entirety a PCT

PCT/IB2008/001909 filed 22 July 2008 entitled LASER ILLUMINATION DEVICE.

BACKGROUND OF THE INVENTION

The present invention relates to an illumination device, and more particularly to a laser illumination device based on electrically switchable Bragg gratings.

Miniature solid-state lasers are currently being considered for a range of display applications. The competitive advantage of lasers in display applications results from increased lifetime, lower cost, higher brightness and improved colour gamut. As lasers are polarized, they are ideally suited to Liquid Crystal on Silicon (LCoS) or High Temperature Poly Silicon (HTPS) projectors. In contrast to incoherent sources, lasers do not result in light from unwanted polarization states being discarded. Laser displays suffer from speckle, a sparkly or granular structure seen in uniformly illuminated rough surfaces. Speckle arises from the high spatial and temporal coherence of lasers. Speckle reduces image sharpness and is distracting to the viewer.

Several approaches for reducing speckle contrast have been proposed based on spatial and temporal decorrelation of speckle patterns. More precisely, speckle reduction is based on averaging multiple (M) sets of speckle patterns from a speckle surface resolution cell with the averaging taking place over the human eye integration time. The speckle resolution cell is essentially the smallest area of the image that the eye can resolve. Under optimal conditions speckle contrast is reduces from unity to the square root of M. The value of M should be as large as possible. However, the value of M is limited by the numerical aperture of the imaging optics. In other words the minimum cell size is approximately equal to the laser wavelength divided by the numerical aperture.

Speckle may be characterized by the parameter speckle contrast which is defined as the ratio of the standard deviation of the speckle intensity to the mean speckle intensity. Temporally varying the phase pattern faster than the eye temporal resolution destroys the light spatial coherence, thereby reducing the speckle contrast.

The basic statistical properties of speckle are discussed by J. W. Goodman in a first paper entitled "Some Fundamental Properties of Speckle" ( J. Opt. Soc. Am. 66, pp. 1145-1149, 1976) and a second paper entitled "Statistical Properties of Laser Speckle Patterns" (Topics in Applied Physics volume 9, edited by J. C. Dainty, pp. 9-75, Springer- Verlag, Berlin Heidelberg, 1984). There are two types of speckle: objective and subjective speckle. As noted in an article by D. Gabor in the IBM Journal of Research and Development, Volume 14, Number 5, Page 509 (1970) "Objective" speckle arises from the uneven illumination of an object with a multiplicity of waves that interfere at its surface. "Subjective" speckle arises at rough objects even if they are illuminated evenly by a single wave." In practical terms, objective speckle results from scattering in the illumination system while subjective speckle occurs at the projection screen. As its name implies objective speckle is not influenced by the viewer's perception of the displayed image. A photographic emulsion spread over the surface of the object would record all of the key characteristics of objective speckle. Even a perfect optical system cannot do better than to reproduce it exactly. Subjective speckle on the other hand arises by a diffraction effect at the receiving optics or, more exactly, by the limitation of the amount of light admitted into receiving optics (the eye, in the case of a display). The only remedy for to subjective speckle is to widen the aperture of the receiving optics or to perform an equivalent optical process. This is due to fundamental information theory limitations and not any practical optical consideration. In general, subjective speckle dominates in front projections systems while objective speckle is more important in rear projection displays.

The characteristics of objective and subjective speckle may be illustrated by considering a typical projection system. The illumination and beam shaping optics (for example components such as diffusers or fly's eye integrators) generates scattering that eventually creates a speckle pattern onto the microdisplay panel surface. The projection lens images this pattern onto the screen giving the objective speckle pattern. The screen takes the objective speckle pattern and scatters it into the viewing space. The human eye only collects a tiny portion of this light. Since the objective speckle acts like a coherent illumination field, the diffusion of the screen produces a new speckle pattern at the retina with a different speckle grain. This is the subjective speckle pattern. The subjective speckle pattern will be influenced by screen diffuser materials and lenticular structures and other features commonly used in screens. Since a well designed projection lens usually collects most of the light transmitted through or reflected by the microdisplay panel, the objective speckle pattern generated is well reproduced at the screen, allowing for some modification due to optical aberrations. The cumulative speckle seen by the eye is the sum of the objective and subjective speckles.

Removing the objective speckle is relatively easy since the speckle pattern is well transferred from the illumination to the screen: any change in the illumination will be transferred to the screen. Traditionally, the simplest way has been to use a rotating diffuser that provides multiplicity of speckle patterns while maintaining a uniform a time-averaged intensity profile. This type of approach is often referred to as angle diversity. Note that, if the objective speckle is suppressed at the screen, it will be suppressed at every plane between the projection lens and the screen.

Suppression of subjective speckle is more difficult. Because of large disparity between the projection optics and eye optics numerical apertures (or F-numbers), the objective speckle grain is much larger than the subjective speckle grain. Therefore, the objective speckle provides a relatively uniform illumination to the screen within one resolution cell of the eye regardless of the position of the rotating diffuser or other speckle reduction means in the illumination path. For the purposes of quantifying the subjective speckle it is convenient to define the speckle contrast as the ratio of the resolution spots of the eye and the projection optic at the screen.

The characteristics of speckle depend on whether it is observed in the near or far field. The far field of an optical system is the angular spectrum of the plane waves traversing or generated by the optical system. In case of a diffractive optical element such as a Computer Generated Hologram (CGH), the far field is a series of points located in the two dimensional angular spectrum, each point representing the intensity of a specific plane wave diffracted, refracted, reflected or diffused at a specific angle. If only one beam strikes the optical element, no overlap of plane waves occurs, each plane wave being spatially demultiplexed in the far field. This is not the case for the near field. The far field effectively at infinity, which according to Rayleigh-Sommerfeld theory is any distance after a specific finite distance, which is a function of the size of the beam (that is, the effective aperture of the CGH), the wavelength, the size of the microstructures in the element (amount of beam deflection), and other factors. Therefore, in order to change the speckle pattern of an individual beamlet in the far field, it is best to use phase diversity. Angular diversity would not produce good results, since none of the wave fronts would be overlapping and interfering. However, phase diversity would create a different phase pattern on a single beamlet and this would change the speckle. Speckle patterns in the far field are characterized by very small-grained speckle structures.

In the near field (that is any location closer than the Rayleigh-Sommerfeld distance), many different wave fronts are interfere and overlap resulting in a very large amount of local wave front interference and hence speckle. Therefore, in order to reduce speckle in the near field, it is advantageous to make slight variations to the angles of the overlapping beamlets. In other words, angular diversity despeckling schemes will be the most effective. Speckle in the near field is characterized by larger grains. The different grain structure in the near and far fields can lead to the erroneous conclusion that Fresnel CGH (near field) give less speckle than Fourier CGHs (far field). This is not the case; the nature of the speckle is different in the two cases.

The extent to which speckle can be corrected in the near and far fields has implications for the type of despecklers to be used in specific projector applications. In the case of a laser projector using traditional projection imaging apparatus, the image of a microdisplay is not in the far field of the despeckler, and thus angular diversity would be the most effective solution. In the case of a laser projector using diffractive imaging, the image is actually the far field of the microdisplay itself, and very close to the far field of the despeckler. Therefore, it is best to use a combination of angular diversity and phase diversity.

Techniques for speckle reduction are commonly classified into the categories of angular, phase and wavelength diversity according to the optical property used to generate the speckle patterns. Angular diversity typically relies on the used of rotating diffusers or vibrating screens. Phase diversity is typically provided by electrically controlled phase modulators. Wavelength diversity is provided by multiple laser sources or tuneable single laser sources. In the case of laser arrays, speckle reduces as the inverse of the square root of the number of die. Mechanical methods of suppressing speckle suffer from the problems of noise, mechanical complexity and size.

, It is known that speckle may be reduce by using an electro optic device to generate variation in the refractive index profile of material such that the phase fronts of light incident on the device are modulated in phase and or amplitude. The published Internal Patent Application No. WO/2007/015141 entitled LASER ILLUMINATOR discloses a despeckler based on a new type of electro optical device known as an Electrically Switchable Bragg Grating (ESBG).

An ESBG in its most basic form is formed by recording a volume phase grating, or hologram, in a polymer dispersed liquid crystal (PDLC) mixture. Typically, ESBG despeckler 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 PDLC 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 PDLC 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 despeckler 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 despeckler devices and provides numerous references to prior publications describing the fabrication and application of ESBG despeckler devices.

The ESBHG despeckler embodiments disclosed in WO/2007/015141 do not address the problem of overcoming subjective speckle. In front projection it is estimated that subjective accounts for 85-90% of observed speckle. Therefore at best the old D-ILC can only account for 10-15%. "Subjective Speckle" is difficult to eliminate in front projection. To the best of the inventors' knowledge there is no relevant prior art (apart from "vibrating Screens", which are not viable for portable projectors). The patented and published despeckler schemes known to the inventors are essentially directed at solving the problem of objective speckle even where this is not made explicit.

There is a requirement for an ESBG despeckler device that can overcome the problems of subjective laser speckle.

There is a requirement to provide an ESBG despeckler device that can overcome the problems of objective and subjective laser speckle simultaneously.

There is a further requirement to provide a compact, efficient laser display incorporating an ESBG despeckler device that can overcome the problem of objective and subjective laser speckle.

SUMMARY OF THE INVENTION

It is a first object of the present invention to provide an ESBG despeckler device that can overcome the problems of subjective laser speckle.

It is a first object of the present invention to provide an ESBG despeckler device that can overcome the problems of objective and subjective laser speckle simultaneously. It is a second object of the present invention to provide a compact, efficient laser display incorporating an ESBG despeckler device that can overcome the problem of objective and subjective laser speckle.

In one embodiment of the invention there is provided a laser projection display comprising in sequence: a laser source; beam expansion and collimation optics; an ESBG despeckler device comprising first and second ESBG arrays; a microdisplay; a projection lens; and a screen. Each said ESBG array is recorded in a Holographic Polymer Dispersed Liquid Crystal sandwiched between transparent substrates to which transparent conductive coatings have been applied. At least one of the coatings is patterned to provide a two-dimensional array of independently switchable ESBG pixels. An electrical control circuit is operative to apply at least first and second voltages across each ESBG pixel. Each ESBG pixel is characterized by a first unique speckle state under the first applied voltage and a second unique speckle state under thesecond applied voltage. The first and second speckle states occur during the integration time of the human eye. Each ESBG pixel diffracts incident collimated light into an illumination patch in proximity to the active surface of the microdisplay. The illumination pOches of each pixel substantially overlap.

In one embodiment of the invention the ESBG despeckler device comprises identical first and second ESBG arrays and the voltages applied to overlapping pixels of the first and second ESBG arrays operate in anti-phase.

In one embodiment of the invention the ESBG pixels are holograms of refractive microlenses. In one embodiment of the invention the ESBG pixels are holograms of beam-shaping diffusers.

In one embodiment of the invention the ESBG pixels are holograms of diffractive Fresnel lenses. In one embodiment of the invention the ESBG pixels are holograms of orthogonal cylindrical diffractive lenses.

In one embodiment of the invention the ESBG pixels are holograms of Fresnel computer generated holograms.

In one embodiment of the invention the objective has a relative aperture numerically smaller than F/2.4.

In one embodiment of the invention the objective has a relative aperture numerically smaller than F/2.0.

In one embodiment of the invention the objective has a relative aperture numerically smaller than F/1.5.

In one embodiment of the invention the objective has a relative aperture not greater than F/1.0.

In one embodiment of the invention the said first and second voltages are points on a time varying voltage characteristic.

In one embodiment of the invention the ESBG despeckler device comprises identical first and second ESBG elements and ESBG pixels from the first and second ESBG elements substantially overlap in the illumination beam cross section.

In one embodiment of the invention the ESBG despeckler device comprises identical first and second ESBG elements and ESBG pixels from the first and second ESBG elements are offset by a fraction of the ESBG element width in at least one of the vertical or horizontal array axes in the illumination beam cross section.

In one embodiment of the invention an ESBG array is fabricated using the following steps: providing a first transparent substrate having a first surface to which an anti reflection coating has been applied and a second surface to which a transparent electrode layer has been applied; removing portions of the transparent electrode layer to provide a patterned electrode layer including an first ESBG pixel pad; depositing a layer of UV absorbing dielectric material over the patterned electrode layer; removing the portion of the UV absorbing dielectric material overlapping the first ESBG pixel pad; providing a second transparent substrate having a first surface to which an anti reflection coating has been applied and a second surface to which a transparent electrode layer has been applied; removing portions of the transparent electrode layer of the second substrate layer to provide a second patterned electrode layer including a second ESBG pixel pad substantially identical to and spatially corresponding with the first ESBG pixel pad; combining the substrates to form a display cell with the transparent electrode coated surfaces of the two substrates aligned in opposing directions and having a small separation; filling said display cell with a PDLC mixture; and illuminating the first cell face by crossed UV laser beams, and simultaneously illuminating the second cell face formed by an incoherent UV source.

The antireflection coated surface of said first substrate forms a first cell face and the antireflection coated surface of the second substrate forms a second cell face. The step of illuminating the first cell face by crossed UV laser beams, and simultaneously illuminating the second cell face formed by an incoherent UV source forms an ESBG confined to the region between the first symbol pad and the second ESBG pixel pad.

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.l is a schematic side elevation view of a laser display according to one embodiment of the invention.

FIG.2 is a schematic diagram illustrating aspects of speckle formation. FIG.3 is a schematic diagram illustrating further aspects of speckle formation. FIG.4 is a schematic diagram illustrating further aspects of speckle formation. FIG.5 is a schematic side elevation view of one embodiment of the invention. FIG.6 is a schematic side elevation view of one embodiment of the invention. FIG.7A is a schematic side elevation view of an ESBG despeckler device according to one embodiment of the invention.

FIG.7B is a schematic side elevation view of an ESBG despeckler device according to one embodiment of the invention.

FIG.8A is a schematic side elevation view of an ESBG despeckler device according to one embodiment of the invention.

FIG.8B is a schematic side elevation view of an ESBG despeckler device according to one embodiment of the invention.

FIG.9 is a schematic side elevation view of an ESBG despeckler device according to one embodiment of the invention.

FIG.10 is a schematic side elevation view of an ESBG despeckler device according to one embodiment of the invention.

FIG.l 1 is a schematic side elevation view of an ESBG despeckler device according to one embodiment of the invention. FIG.12 is a schematic side elevation view of a laser display according to one embodiment of the invention.

FIG.13 is a schematic side elevation view of a laser display according to one embodiment of the invention.

FIG.14 is a schematic side elevation view of an aspect of an ESBG despeckler device according to one embodiment of the invention.

FIG.15 is a schematic side elevation view of an aspect of an ESBG despeckler device according to one embodiment of the invention.

FIG.16 is a schematic side elevation view of a laser display according to one embodiment of the invention.

FIG.17 is a schematic side elevation view of a laser display according to one embodiment of the invention.

FIG.18 is a schematic side elevation view of an ESBG despeckler device according to one embodiment of the invention.

FIG.19 is a schematic side elevation view of an ESBG despeckler device according to one embodiment of the invention.

FIG.20 is a schematic side elevation view of a detail of an ESBG lens array.

FIG.21 is a schematic side elevation view of a laser display according to one embodiment of the invention.

FIG.22A is a chart showing a first ESBG applied voltage characteristic.

FIG.22B is a chart showing a second ESBG applied voltage characteristic.

FIG.23 is a schematic plan view of a laser display according to one embodiment of the invention.

FIG.24 is a schematic plan view of a laser display according to one embodiment of the invention.

FIG.25 is a schematic side elevation view of an ESBG array in one embodiment of the invention. FIG.26 is a schematic side elevation view of an ESBG array in one embodiment of the invention. FIG.27 is a schematic side elevation view of an ESBG array in one embodiment of the invention. FIG.28 is a schematic side elevation view of an ESBG array in one embodiment of the invention. FIG.29 is a schematic side elevation view of an ESBG array in one embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION It is a first object of the present invention to provide an ESBG despeckler device that can overcome the problems of subjective laser speckle.

It is a second object of the present invention to provide an ESBG despeckler device that can overcome the problems of objective and subjective laser speckle simultaneously.

It is a second object of the present invention to provide a compact, efficient laser display incorporating an ESBG despeckler device that can overcome the problem of objective and subjective laser speckle.

To assist in clarifying the basic principles of the despeckler device the invention will be described in relation to a practical laser display in which there is provided a laser source comprising one or more red, green or blue laser die, a flat panel microdisplay and projection optics. It will be clear that the despeckler embodiments to be discussed are not restricted to application in laser displays of the types described herein. For the purposes of explaining the invention an ESBG despeckler device will be understood to comprise at least one ESBG layers or cells each comprising an ESBG encapsulated between parallel transparent glass walls according to the principles to be discussed below. An ESBG array will refer to an ESBG with switching electrodes patterned such that individual ESBG pixels can be switched selectively.

It will be apparent to those skilled in the art that the present invention may be practiced with only some or all aspects of the present invention as disclosed in the following description. For the purposes of explaining the invention well-known features of laser technology and laser displays have been omitted or simplified in order not to obscure the basic principles of the invention.

Unless otherwise stated the term "on-axis" in relation to a ray or beam direction refers to propagation parallel to an axis normal to the surfaces of the despeckler and microdisplay and lenses described in relation to the embodiments of the invention. A fast objective refers to an objective having a numerically small F-number.

Parts of the following description will be presented using terminology commonly employed by those skilled in the art of optics and laser displays in particular.

It should also be noted that in the following description of the invention repeated usage of the phrase "in one embodiment" does not necessarily refer to the same embodiment.

FIG.l is a schematic side elevation view of one embodiment of the invention in which a laser display comprises a laser source 1, an Electrically Switchable Bragg Grating (ESBG) despeckler device 2, which is disposed along the laser beam path, a flat panel display 4 and a projection lens 5. The laser source 1 comprises at least a single laser emitter die providing monochromatic light. The ESBG drive electronics are indicated by 3. The laser and ESBG despeckler device form part of an apparatus for illuminating an electronic display to provide a viewable image. The projection lens forms a magnified image on the surface of a projection screen 6. In FIG.1 a lens 11 is used to convert diverging laser emission light 1100 into a collimated beam 1200. Alternatively, where the laser provides a collimated output beam two or more lenses may be used to expand and collimate the beam. The collimated beam is diffracted into a direction 1300 by the ESBG despeckler device. The projection lens forms a diverging beam 1400, which illuminates the screen.

The flat panel display may be an LCD, a DLP or other type of display commonly used in projection displays. Desirably the flat panel display is a microdisplay. The apparatus may further comprise other optical components such as relay optics for coupling the ESBG despeckler device to the display panel, filters, prisms, polarizers and other optical elements commonly used in displays. The details of the projection optical system do not form part of the invention. The invention is not restricted to any particular type of display configuration. At least one viewable surface illuminated by the laser light exhibits laser speckle. Said viewable surface may be at least one of the projection screen or an internal optical surface within the projection optical system. The speckle at the projection screen may be characterised substantially as subjective speckle whereas the speckle resulting from reflections from surfaces within the projector may be characterised as objective speckle. Although a rear projection screen is illustrated in FIG.l the invention may also be used in front projection.

An ESBG despeckler device according to the principles of the invention comprises two ESBG elements. Each ESBG layer has a diffracting state and a non-diffracting state. Typically, the ESBG element is configured with its cell walls perpendicular to an optical axis. An ESBG element diffracts incident off-axis light in a direction substantially parallel to the optical axis when in said active state. However, each ESBG element is substantially transparent to said light when in said inactive state. An ESBG element can be designed to diffract at least one wavelength of red, green or blue light. In the embodiments to be discussed in the following description of the invention each ESBG layer in the ESBG despeckler device is configured as an array of selectively switchable ESBG pixels. The preferred method for fabricating an ESBG array is discussed later in the description.

ESBG despeckler devices for reducing speckle according to the principles of the present invention are configured to generate set of unique speckle patterns within an eye resolution cell by operating on the angular and/or phase characteristic of rays propagating through the ESBG despeckler device. It should be emphasized that the ESBG despeckler devices disclosed herein may be used to overcome both objective and subjective speckle.

The basic principles of speckle reduction using angular diversity are illustrated schematically in FIG.2. The projection beam axis and the eye line of sight are assumed to lie on a common optical axis indicated by 1201. The exit pupil of the projection systems is indicated by 1202 and the entrance pupil of the eye is indicated by 1203. The diameters of the projection and eye pupils are Di and D₂ respectively and the projection and eye pupils are located at distances of Ri and R₂ respectively from a transmissive screen 5. The projection light indicated by 1204 is provided by an optical system such as the one illustrated in FIG.1. The light detected by eye indicated by 1203 is imaged onto the retina. In order for the eye to detect the optimum speckle reduction the eye must resolve the laser illuminated area into resolution spots having a resolution spot size indicated by 1501 which is greater than or approximately equal to a speckle surface resolution cell such as the one indicated by 1502 For light of wavelength λ the diameter of the eye resolution spot is given by the Airy point spread function diameter 2.44λ R₁ZD₁. The diameter of the speckle resolution cell such as 1501 is given by 2.44λ R₂/D₂. Temporally varying the phase pattern faster than the eye temporal resolution destroys the light spatial coherence, thereby reducing the speckle contrast.

Varying the electric field applied across the ESBG despeckler device varies the optical effect of the ESBG despeckler device by changing the refractive index modulation of the grating. Said optical effect could be a change in the amplitude and phase of light waves interacting with the grating . The optical effect of the ESBG despeckler device is varied from zero to a predetermined maximum value at a high frequency by applying an electric field that varies in a corresponding varying fashion. Said variation may follow sinusoidal, triangular, rectangular or other types of regular waveforms. Alternatively, the electrical waveform may have random characteristics. Each incremental change in the applied voltage results in a unique speckle phase cell. A human eye 5 observing the display of FIG.1 integrates speckle patterns to provide a substantially de-speckled final image.

The inventors have discovered that subjective speckle can be overcome in a front projection display by illuminating the microdisplay panel with light from an extended diffuse source and using a fast objective to project an image of the display panel onto the screen. The basic principle of the despeckler can be demonstrated using a simple a illumination system comprising a laser beam expander and objective lens for projecting a patch of light onto a screen. The light projected onto the screen exhibits both objective and subjective speckle. In a front projection laser display it is estimated that subjective speckle accounts for 85-90% of observed speckle. The inventors have found that the objective speckle can be substantially removed by introducing an electro optic despeckler based on the principles disclosed in a co-pending patent application PCT/IB2008/001909 filed 22 July 2008 entitled LASER ILLUMINATION DEVICE. However it is found that the 909' apparatus leaves the subjective speckle substantially unchanged. Furthermore, contrary to what might be expected from basic theoretical considerations, the inventors have found that combining the 909' apparatus with a fast objective, even one with an F-number as high as F/l .0 does not make a significant impact on the subjective speckle. However, he inventors have found that inserting a passive diffuser such as a ground glass screen near to the objective lens focal plane at which the microdisplay would normally be located is very effective at reducing the subjective speckle with the virtually complete elimination of subjective speckle being achieved with an F/l .0 objective. The inventors propose that the physical mechanism for the reduction of the subjective speckle is the use of an extended diffuse source near to the focal plane combined with a fast projection objective. The dimensions of the extended diffuse source should be such that its area exceeds that of the microdisplay by the amount required to provide illumination rays filling the light acceptance cone of the microdisplay. Clearly the size of the diffuse source will depend on the distance of the source from the microdisplay.

The present invention is directed at translating the above experimentally observed principle into a despeckler that simultaneously removes objective and subjective speckle. However, a solution based on combining a despeckler based on the principles of the 909' apparatus with a passive diffuser of the type discussed above may suffer from the problems of size reduce transmission and optical complexity. Desirably, the despeckler should combine objective and subjective speckle in a single compact, high transmission device.

Advantageously, the ESBG despeckler device comprises single electro optical module. In extending the principle to a practical projector it is necessary to address two issues. Firstly, most practical microdisplay technologies such as LCoS and DLP impose a limit on the range of incident ray angles at the display surface. Typically the range of angles is characterised by an F- number without F/2.0 being a typical value. Secondly, since a despeckler directed at removing objective and subjective speckle will incorporate pixelated structure it is desirable that the despeckler is disposed away from the microdisplay to avoid flicker from the switched ESG pixels interfering with the viewer's perception of the microdisplay image.

It is proposed that in one embodiment of the invention the problem of flicker can be overcome by an ESBG device configured to form a virtual image of the diffuse source using recording procedures to be discussed below.

It is proposed that in one embodiment of the invention the problem of flicker can be overcome by using an ESBG despeckler device using high resolution SBG arrays.

Some insight into the experimentally observed effects described above may be provided by considering FIGS.3-4. FIG.3 shows a schematic illustration of a portion of front projection system in which there is provided a despeckler 21 a microdisplay 4 an objective lens 5 and a screen 6 observed by an eye 7. The details of the illumination system up to the despeckler are not shown. The illumination system provides collimated light as indicated by 1370. The despeckler 21 provides an extended diffuse source 81 by means to be made clear in the following description of the embodiments. The position of the physical despeckler device within the specie generally indicated by the numeral 21 in FIG.3 depends on the embodiment as will be explained in the following description. A given point on the diffuser provides a divergent ray bundle containing the limiting rays 1371,1372 which illuminates the surface of the microdisplay. The rays from the diffuse source illuminated the microdisplay in such a fashion that at any point on the surface of the microdisplay there is a convergence of rays where each incident ray at said surface point originates at a unique point on surface of the diffuse source. For example the rays 1371,1373,1374,1375 contribute to the converging ray bundle generally indicated by 1376 converging onto the display point indicated by 41. Desirable as wide a range of ray angles as possible is provided and as large an area of the diffuse source as possible is used. For a typical LCoS panel the range of ray incidence angles should be confined a cone defined by a F-number of around F/2.0. Display light such as the divergent light indicated by 1377 is projected onto the screen by the objective 5 as light 1378 forming a resolution spot 1379. The screen is observed by an eye7 which receives light from an eye resolution cell 1380 along the beam path indicated by 1381 The subjective speckle contrast is given by the ratio of the F-numbers of the projection system to the F-number of the eye. An equivalent measure of the speckle contrast is given by the ratio of the diameters of the resolution spots of the eye 1361 and the projection optics 1362 at the screen.

Referring to FIG.4, the eye resolution cell diameter D_eye is approximately λRey_e/d_eye were is the distance of the eye from the screen and is the wavelength of the light. The projector resolution cell diameter is given by D_prOj approximately equal to λR_pro_j/dpr_Oj where Rp_rOj is the projector throw distance and d_Pr_Oj is the proctor objective pupil diameter. The speckle contrast is defined as the ratio of the eye and projector resolution cell sizes in other words as Dp_r0J /D_eye x 100%. In general, fast, at least F/l, objective is required to provide low speckle contrast. In one embodiment of the invention the despeckler provides a virtual extended diffuse source. In such an embodiment the diffuse source is located near to then objective focal plane. The principles of such an embodiment will be discussed with reference to the FIGS.5-6 which provide schematic side elevation views of two modes of operation of one embodiment of the invention directed at providing objective and subjective despeckling from a single device. A despeckler according to the principles of the invention comprises a HOE 22 a first ESBG array 21 A and a second ESBG array 2 IB. The two ESBG arrays operate in anti phase.. The despeckler is illuminated by collimated on-axis laser light generally indicated by 1312. The ESBG 21 A diffracts the light 1312 to forms the first virtual diffuser 82 A while the ESBG 2 IB diffracts light 1312 to forms the second virtual diffuser 82B.

The basic principles of recording the SBG array are illustrated in the schematic side elevation views of FIG.7. First a quartz binary diffuser 8 and an electrode patterned cell containing HPDLC mixture are provided. Desirably, the quartz diffuser is a multilevel device with three or more levels to provide diffraction efficiencies greater than 80%. The quartz diffuser converts incident collimated light 1300 into diffuse light 1301. Using standard holographic recording procedures familiar to those skilled in art a HPDLC cell is exposed to light 1301 from the diffuser and a collimated off axis reference beam 1302. Turning now to FIG.7B we see that the resulting ESBG 21 provides a virtual image of the diffuser 81 under exposure by the reference beam 1302. The diffuser 81 has diffusion ray distribution 1303 substantially the same as that of the quartz diffuser. The process illustrated in FIGS.7A-7B is repeated to provide a second ESBG of similar specification to the first.

In an alternative embodiment of the invention the extended diffuser surface may be a real image recorded using the procedures illustrated in FIGS8A-8B. Again a quartz binary diffuser 8 and an electrode patterned cell containing HPDLC mixture are provided. The quartz diffuser converts incident collimated light 1300 into diffuse light 1301. The cell is exposed to the diffuser light 1301 and a collimated off axis reference beam 1302 with a lens 8 being used to from an image of the diffuser. Turning now to FIG.8B we see that the resulting ESBG 21 provides a real image of the diffuser 81 under exposure by the reference beam 1302. It should be noted that the term "virtual" is applied to any diffuser resulting from using the recording arrangements in FIG.7 or FIG.8 or any equivalent means regardless of whether the resulting images is virtual or real in purely optical sense.

Advantageously, a despeckler should be configured with its optical surfaces normal to the optical axis. However, ESBGs require off axis illumination for high diffraction efficiency. This problem is solved by providing a passive diffractive element such as a HOE for converting incident on-axis light into off-axis light at the optimal ESBG incidence angle. The HOE 22 is typically recorded in a photopolymer of the type manufacture by DuPont. Desirably, the HOE has a diffraction efficiency of at least 99%. The HOE typically diffracts incident collimated light at normal incident into a direction at 30 degrees to said normal incidence direction. The output angle of the HOE provides the off axis launch angle for the ESBGs. The invention does not rely on any particular value of the launch angle. However, the inventors have found that typical launch angles are in the range 30-50 degrees. In other embodiments of the invention the HOE may be replaced by another diffractive device suitable for performing the required beam steering such that each ESBG elements diffracts light incident at some specified launch angle into a direction normal to the surfaces of the ESBG elements. In FIG.9 there is illustrated a despeckler comprising a first ESBG array 21 A and a second ESBG array 2 IB. There is further provided a Diffractive Optical Element (DOE) 22. Said DOE may be a holographic element such as a Bragg hologram. Said DOE may be a ESBG. The DOE directs off axis incident laser light 1100 A into a direction 1101 A normal to the surfaces of the ESBG arrays. The light emerging from the ESBG arrays is emitted in the average ray direction 1102A. The direction 1102 A may be substantially the same as the ray direction 1101 A. FIG.10 shows an alternative embodiment of the invention similar to that of FIG.9 in which incident light 1103 A is substantially normal to the surfaces of the ESBG arrays. A DOE 22 is used to deflect the light away from the incident light direction in the direction 1104 A. The ESBG arrays then deflect light into an average ray direction 1105 A substantially parallel to the incident light direction 1 103 A. In most cases the arrangement of FIG.9 is the most practical one and will therefore be assumed in the following discussion of the invention.

It will be clear from consideration of FIGS.9- 10 that an equivalent arrangement of the HOE and the ESBG arrays is provided by disposing the HOE 22 between the first and second ESBG arrays. In other embodiments of the invention the ESBG elements may be tilted at a suitable angle with respect to the illumination eliminating the need for the HOE 22.

Turning again to FIGS.5-6 we see that that each point on the surface of each diffuser provides a divergent ray bundle which illuminates the surface of the image plane 85 at which the microdisplay would normally be located. For example referring to FIG.6 we see that the virtual diffuser 82B provides ray bundles such as the ones indicated by 1313 and 1314 where each bundle illuminates a substantial portion of the image plane 85. Hence, as explained in FIG. , every point on the image plane is illuminated by a bundle of converging of rays where each ray in said bundle originates at a unique point on surface of the diffuse source. The virtual diffuser 82A also provides ray bundles but with points of origins displaced a small distance parallel to the optical axis. The effect of adding ray bundles such as 1313 and 1314 at the image plane 85 is to provide angular diversity. The effect of adding ray bundles such as 1314 and 1315 at the image plane 85 is to provide angular diversity. In practice depending on the precise optical configuration the despeckler may provide a combination of angular and phase diversity. Since the two SBG arrays operate in anti phase the ray distributions at the image plane can be varied. However at any time the entire aperture of the despeckler diffuses light.

In one embodiment of the invention the first and second virtual diffusers may be separated as shown in FIG.6 . Since the extended diffuser is virtual it does not affect the overall dimensions of the optics. However, it may be necessary to incorporate a further lens between the D-ILC to provided telecentric illumination at the microdisplay. Assuming a diffusion angular range of ±14° based on an F/2.0 microdisplay angular acceptance limit the width of the virtual source size may be estimated as X=2 Z tan(14°) +S where S is the display panel width.

In one embodiment of the invention the first and second virtual diffuser may be substantially coplanar.

In one embodiment of the invention the two physically separated ESBG arrays shown in FIGS.5-6 provide a phase diversity scheme for overcoming objective speckle. FIG.l 1 is a schematic side elevation view of a detail of the despeckler showing the ESBG layer 21A,21B and the HOE layer 22. The substrates 23,24,25 sandwich the SBG layers. The despeckler is illuminated by off axis collimated light generally indicated by 1321 which is directed into the required SBG diffraction angle indicated by 1322 by the HOE 22. Two ray paths are illustrated, as first path indicated by 1323 1325 traverses the active SBG element 26 A in the array 21 A and the inactive SBG 26A in the array 2 IB. A second path 1324,1326 traverses the inactive SBG elemetn26B in the array 21 A and the active SBG 26B in the array 2 IB. It will be clear from the consideration of the first and second ray paths in FIG.l 1 that a large number of different ray path phase differences may be integrated by switching different combinations of pixels in the two arrays.

Referring to FIGS.12-13 a laser projector employing the despeckler of FIGS.5-6 further comprises a laser source 1, beams expansion and collimation optics 11,12, a microdisplay 4 and a projection objective 5. The microdisplay is located in proximity to the illumination surface 85 of FIGS. 5-6. Light from the laser 1 indicated by 1310 is expanded and collimated by the lenses 11,12 as represented by the rays 1311 1312. The details of the despeckler are as indicated in FIGS.5-6

As indicated earlier in the description the diffuser surface may be a real image recorded using the procedures such as the one illustrated in FIGS8A-8B. The principles of such an embodiment are illustrated in FIGS.14- 15 which provide schematic side elevation views of two modes of operation of one embodiment of the invention directed at providing objective and subjective despeckling from a single device. A despeckler according to the principles of the invention comprises a HOE 22 a first ESBG array 26 A and a second ESBG array 26B. The two ESBG arrays operate in anti phase. The despeckler is illuminated by collimated on-axis laser light generally indicated by 1312. The ESBG 26A diffracts the light 1312 to forms the first virtual diffuser 82A while the ESBG 26B diffracts light 1312 to forms the second virtual diffuser 82B. Each diffuser illuminates the surface of the image plane 86 at which the microdisplay would normally be located. For example referring to FIG.14 we see that the diffuser 82B provides ray bundles such as the ones indicated by 1317 and 1318 where each bundle illuminates a portion of the image plane 86. Hence, every point on the image plane is illuminated by a bundle of converging of rays where each ray in said bundle originates at a unique point on surface of the diffuse source. As shown in FIG.15 the virtual diffuser 82 A also provides ray bundles but with points of origins displaced a small distance parallel to the optical axis. The effect of adding ray bundles such as 1317- 1319 at the image plane 86 is to provide angular diversity. In practice depending on the precise optical configuration the despeckler may provide a combination of angular and phase diversity. Since the two SBG arrays operate in anti phase the ray distributions at the image plane can be varied. However at any time the entire aperture of the despeckler diffuses light.

Referring to FIGS.16-17 a laser projector employing the despeckler of FIGS.14-15 further comprises a laser source 1, beams expansion and collimation optics 11,12, a microdisplay 4 and a projection objective 5. The microdisplay is located in proximity to the illumination surface 86 of FIGS. 14-15. Light from the laser 1 indicated by 1310 is expanded and collimated by the lenses 11,12 as represented by the rays 1311 1312. The details of the despeckler are as indicated in FIGS.14- 15

In one embodiment of the invention the extended diffuse source principle is implemented using SBG elements configured as microlens arrays. The embodiment of the despeckler shown in the schematic side elevation views of FIGS.18- 19 comprises two arrays of SBG microlens27A,27B and a HOE 22. The HOE performs an identical function to the one shown in FIGS.5-6 and FIGS.14-15. The ESBGs are illuminated by collimated axial light. Each lens element in each array provides a divergent beam that fills the display panel. For example, the arrays 27A,27B provide divergent beams such as the one indicated by 1350,1351 from the upper portions of the arrays that overlap at the illumination surface 87. The illumination surface is disposed in proximity to the microdisplay. As indicated in FIG.19 which shows beams 1352,1353 from the lower portions of the arrays 27A,27B, any point on the illumination surface receives rays from each element of each SBG lens array. The array dimensions and array to illumination surface distance are constrained to limit the size of the angle θ representing the range of incidence angles at any point on the microdisplay surface. The incident trays should fall inside an F/2.0 cone with an axis normal to the illumination surface.

The SBG arrays are made by fabricating a CGH with the required optical properties and recording the CGH into the SBGs. (essentially forming a hologram of the CGH). The lenses have anamorphic prescription to match the lens apertures to the microdisplay aspect ratio.

It will be clear from consideration of FIGS.18-19 that the prescriptions of the microlenses will require tilt factors providing progressively greater lens tilt angles towards the edge of the array for optimal overlap of light patches at the illumination plane 86. Only the lens near to the centre of the array will have approximately axisymmetric characteristics. FIG.20 is a schematic representation of a detail of the SBG lens array showing lens elements 86A-86D with axes of symmetry disposed along the directions 1357A-1357B respectively and rotation axes lying in the array plane indicated by 87. Advantageously, in addition to the tilt required to achieve the above beam aiming the prescription of each lens may be further comprise a random "tilt" to improve the angular diversity of the despeckler. Desirably the random tilts should not exceed 0.5deg to avoid creating a "fuzzy edge" to the illumination patch. Advantageously, the lens arrays have the same aspect ratio as the microdisplay. However, this is not essential since the anamorphic characteristics of the microlenses can be designed to match the microdisplay aspect ratio.

Advantageously, the SBG lens arrays should have a resolution of at least 60x60 to overcome flicker.

Referring to FIGS.12-13 a laser projector employing the despeckler of FIGS.18-19 further comprises a laser source 1, beams expansion and collimation optics 11,12 , a microdisplay 4 and a projection objective 5. The microdisplay is located in proximity to the illumination surface 87 of FIGS. 18-19. Light from the laser 1 indicated by 1310 is expanded and collimated by the lenses 11 , 12 as represented by the rays 1311 1312. The details of the despeckler are as indicated in FIGS.18-19.

The large area diffuse illumination patch formed by the despeckler meets the conditions of a large speckle source coupled with a fast lens to overcome subjective speckle. The embodiment of FIGS.18-21 provides a despeckler that uses a combination of angular diversity and phase diversity. Individual lenses are selectively switched to implement an objective despeckling scheme.

The two SBG lens arrays operate in anti-phase such that the entire aperture is diffusing at any time. The individual lenses in each array are selectively switched to implement an objective despeckling scheme. It should be noted that since the ESBG lens arrays are driven in anti- phase only one ESBG element is active at any time along a give ray path through the ESBG arrays.

In one embodiment of the invention the ESBG lens arrays are offset by a fraction of the ESBG element width in at least one of the vertical or horizontal array axes. In some cases the ESBGs may be offset by an ESBG element width in at least one of the vertical or horizontal axes.

The ESBG lens arrays are driven in a random anti -phase fashion by means of an ESBG controller which is not illustrated... The optical effect of each ESBG despeckler array element is varied from zero to maximum value at a high frequency by applying an electric field that varies in a corresponding varying fashion. Each incremental change in the applied voltage results in a unique speckle phase cell. Referring to FIG.22A which is a chart showing voltage versus time applied to the ESBG arrays it will be seen that there is a phase lag between the voltages 1001,1002 applied across the ESBG arrays. The effect of applying such waveforms is that the average intensity 1003 of the speckle phase cells remains substantially constant, thereby satisfying the statistical requirements for speckle reduction. Other types of waveforms may be applied, for example sinusoidal, triangular, rectangular or other types of regular waveforms. Alternatively, it may be advantageous in statistical terms to use waveforms based on a random stochastic process such as the waveforms 2001,2002 illustrated in the chart of FIG.22B. The effect of applying such waveforms is that the average intensity 2003 of the speckle phase cells remains substantially constant satisfying the statistical conditions for speckle reduction. The invention may be applied to any projection display architecture. FIG.23 shows a plan schematic view of one operational embodiment of the invention for providing a color laser display using three microdisplay panels. There are provided red green and blue transmitting microdisplays 4R,4G,4B, separated red, green and blue laser illuminations modules 15R,15G,15B providing despeckled illumination light 1330R, 1330G, 1330B respectively according to the principles of the present invention. Each module comprises at least one laser source, beam expansion and collimation lens system represented an ESBG despeckler device according to the principles of the present invention. The light transmitted by the microdisplay is combined by an X-cube 16 and projected onto a screen by the objective 5.

FIG.24 shows a plan schematic view of one operational embodiment of the invention for providing colour sequential red green and blue laser display using a single microdisplay. There are provided separated red, green and blue laser modules. The red module comprises at least one laser source IR, beam expansion and collimation lens system represented by 2R, and an ESBG despeckler device. The lens 2R forms the collimated beam generally indicated by 101 OR. The despeckled beam at the output of the red module is generally indicated by 1020R. The green module comprises at least one laser source IG, beam expansion and collimation lens system represented by 2G, and an ESBG despeckler. The lens 2G forms the collimated beam generally indicated by 1010G. The despeckled beam at the output of the blue module is generally indicated by 1020B. The blue module comprises at least one laser source IB, beam expansion and collimation lens system represented by 2B, and an ESBG despeckler device. The lens 2B forms the collimated beam generally indicated by 101 OB. The despeckled beam at the output of the blue module is generally indicated by 1020B. A mirror 5R reflects the red beam along an optical axis to provide a beam 1030R. A green reflecting dichroic mirror 5 G reflects the green beam along an optical axis to provide a beam 1030G. A blue reflecting dichroic mirror 5B reflects the blue beam along an optical axis to provide a beam 1030G. A lens system generally indicated by 14 directs the beams 1030R, 1030G, 1030B towards a display panel 4. A projection lens 5 projects an image of the display panel onto a screen, which is not shown. ESBG array optical configurations

Examples of ESBG arrays that may be used in the invention are provided in FIGS.25-29. In each case only one array is illustrated.

In one embodiment of the invention shown in the schematic side elevation view of FIG.25 the SBG array 28 A comprises an array of Fourier type SBG diffusers and beam shapers. The apparatus further comprises the Fourier transform lens 14. The SBG array and lens forms a focused beam patch 89 at the lens focal distance F. Such a despeckler offers the benefits of ease of design and mastering and very good control of illumination patch uniformity and aspect ratio. A practical implementation would require an additional lens to bring the far field into the near field. This embodiment also suffers from a zero order hot spot in the center of the illumination patch.

In one embodiment of the invention shown in the schematic side elevation view of FIG.26 the SBG array 28B comprises an array of refractive microlenses recorded in ESBGs. The ESBG lenses focus incident light into an array of focal spots at a focal plane indicated by F. The illumination patch may be formed at any distance Z. The despeckler forms a non focused patch 89. The embodiment of FIG,26 has the benefit that suitable refractive microlens arrays are readily available. There is no need for an additional lens. There is no zero order light. Such an embodiment suffers from the problems of difficulty of recording the microlenses into ESBGs, very low efficiency and the formation of ripples in the illumination patch. Illumination shaping and randomization is also very difficult

In one embodiment of the invention shown in the schematic side elevation view of FIG.27 the SBG array 28C comprises an array of diffractive Fresnel lenses recorded in ESBGs. The ESBG lenses focus incident light into an array of focal spots at a focal plane indicated by F. The illumination patch may be formed at any distance Z. The despeckler forms a non focused patch 89. The embodiment of FIG.27 has the benefits that it is relatively easy to calculate and fabricate a master DOE. There is no need for an additional lens. Zero order light exists but the effect is minimized since the illumination patch is not focused. Beam shaping may be carried out with the aid of anamorphic lenses. Randomization of the illumination is relatively easy. The embodiment of FIG.27 suffers from illumination ripples which can be smooth out by the randomization process.

In one embodiment of the invention shown in the schematic side elevation view of FIG.28 the SBG array 28D comprises arrays of orthogonal cylindrical diffractive lenses recorded in recorded in ESBGs. The ESBG lenses focus incident light into an array of focal spots at a focal plane indicated by F. The illumination patch may be formed at any distance Z. The despeckler forms a non focused rectangular beam patch 89. Since orthogonal cylindrical lens arrays have different foci in orthogonal planes it is possible to shape the ESBG array into a rectangle of any aspect ratio required in imaging applications. The embodiment of FIG.28 has the benefits that it is relatively easy to calculate and fabricate a master DOE. The ESBG process is also relatively easy. There is no need for an additional lens. Zero order light exists but the effect is minimized since the illumination patch is not focused. Beam shaping may be carried out with the aid of anamorphic lenses. Randomization of the illumination is relatively easy. The embodiment of FIG.27 suffers from illumination ripples which can be smooth out by the randomization process.

In one embodiment of the invention shown in the schematic side elevation view of FIG.29 the SBG array 28E comprises arrays diffractive Fresnel computer generated holograms recorded into ESBGs. The illumination patch may be formed at the focal distance F. The benefits of the embodiment of FIG.29 are that there is no need for an additional lens, the zero order light is not focused, beam shaping and homogenization are relatively easy, there are no lens type illumination ripples and ESBG recording process is relatively simple.

The ESBG arrays used in the present invention may employ the passive matrix addressing schemes disclosed in the inventors co pending application PCT/IB2008/001909 filed 22 July 2008 entitled LASER ILLUMINATION DEVICE which is incorporated herein by reference in its entirety.

The invention may be applied to any of the edge illuminated despeckler embodiments disclosed in PCT/IB2008/001909.

The ESBG arrays used in the present invention may be fabricated using the methods disclosed in the co pending applicationPCT US2006/043938 filed 13 November 2006, claiming priority to U.S. provisional patent application 60/789,595 filed on 6 April 2006, entitled METHOD AND APPARATUS FOR PROVIDING A TRANSPARENT DISPLAY which is incorporated herein by reference in its entirety.

Although the above application is not directed at despecklers it will be clear from consideration of the process steps described therein that it may be applied to any type of ESBG device including the ESBG devices disclosed in the present applications. In summary an ESBG arrays for use in any of the embodiments described in the present invention is fabricated using the following steps: a) providing a first transparent substrate having a first surface to which an anti reflection coating has been applied and a second surface to which a transparent electrode layer has been applied; b) removing portions of said transparent electrode layer to provide a patterned electrode layer including an first ESBG element pad; c) depositing a layer of UV absorbing dielectric material over said patterned electrode layer; d) removing the portion of said UV absorbing dielectric material overlapping said first ESBG element pad; e) providing a second transparent substrate having a first surface to which an anti reflection coating has been applied and a second surface to which a transparent electrode layer has been applied; f) removing portions of the transparent electrode layer of said second substrate layer to provide a second patterned electrode layer including a second ESBG element pad substantially identical to and spatially corresponding with said first ESBG element pad; g) combining the substrates to form a display cell with the transparent electrode coated surfaces of the two substrates aligned in opposing directions and having a small separation, wherein the antireflection coated surface of said first substrate forms a first cell face, wherein the antireflection coated surface of said second substrate forms a second cell face; h) filling said display cell with a PDLC mixture; i) illuminating said first cell face by crossed UV laser beams, and simultaneously illuminating said second cell face formed by an incoherent UV source, j) wherein said step of illuminating said first cell face by crossed UV laser beams, and simultaneously illuminating said second cell face formed by an incoherent UV source forms an ESBG confined to the region between said first symbol pad and said second ESBG element pad.

In step j one of the UV laser beams will have its phase and amplitude modulated by a CGH element designed with the optical characteristics of the ESBG. The terms ESBG element may refer to a diffusing pixel or a lens element.

The present invention does not assume any particular process for fabricating ESBG despeckler devices. The fabrication steps may be carried out used standard etching and masking processes. The number of steps may be further increased depending on the requirements of the fabrication plant used. For example, further steps may be required for surface preparation, cleaning, monitoring, mask alignment and other process operations that are well known to those skilled in the art but which do not form part of the present invention.

In one embodiment of the invention the ESBG arrays illustrated in FIGS.14- 15 may encode further optical properties for optimizing the optical characteristics of the beamlets. For example, in further embodiments of the invention the ESBG arrays may encode diffusing characteristics. In yet further embodiments of the invention the ESBG arrays may encode keystone correction. Desirably the illumination at the microdisplay should be telecentric to ensure even illumination of the image. In one embodiment of the invention compensating lenses may be used in the recording set-up to optimise virtual source size and location and provide telecentric illumination at the microdisplay

The invention is not restricted to any type of projection lens. Although the invention is directed at front projection displays, in certain applications of the invention the image displayed on the flat panel display may be viewed by means of an eyepiece as used in, for example, a wearable display.

In the embodiments of the invention discussed above the ESBG despeckler device is located in the illumination path leading up to the flat panel display. In alternative embodiments of the invention the ESBG despeckler device may be located in the optical train after the flat panel display.

In one embodiment of the invention in which the ESBG despeckler device is located after the flat panel display the ESBG despeckler device forms part of the projection lens.

In a further embodiment of the invention a despeckler according to the principles of the invention may further comprise an electrically controllable phase modulator cell. The phase modulator is any optical device that can provide a phase retardation in the range from 0 two pi radians. The invention is not limited to any particular phase modulator. Desirably, the phase modulator may be based on a ESBG despeckler devices which encodes a sub wavelength grating. It will be clear from the above description of the invention that the ESBG despeckler embodiment disclose here may be applied to the reduction of speckle in a wide range of laser displays including front and rear projection displays, wearable displays, scanned laser beam displays and transparent displays for use in viewfinders and HUDs.

In preferred practical embodiments of the invention the ESBG layers continued in an ESBG despeckler device would be combined in a single planar multilayer device. The multilayer ESBG despeckler devices may be constructed by first fabricating the separate ESBG and then laminating the ESBGs 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 members, if needed, to control the optical properties of the illuminator.

The invention is not limited to any particular type of HPDLC or recipe for fabricating HPDLC. The HPDLC material currently used by the inventors typically switches at 170us and restores at 320us. The inventors believe that with further optimisation the switching times may be reduced to 140 microseconds.

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 greatly exaggerated.

The ESBGs may be based on any crystal material including nematic and chiral types. In particular embodiments of the invention any of the ESBG arrays discussed above may be implemented using super twisted nematic (STN) liquid crystal materials. STN offers the benefits of pattern diversity and adoption of simpler process technology by eliminating the need for the dual ITO patterning process described earlier.

The invention may also be used in other applications such as optical telecommunications.

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.

Claims

CLAIMS What is claimed is:

1. A laser projection display comprising in sequence: a laser source [I]; beam expansion and collimation optics [H]; an ESBG despeckler device [2] comprising first and second ESBG arrays [21A,21B; 26A,26B;27A,27B]; a microdisplay [4]; a projection lens [5]; and a screen [6] wherein each said ESBG array is recorded in a Holographic Polymer Dispersed Liquid Crystal sandwiched between transparent substrates to which transparent conductive coatings have been applied, characterized in that at least one of said coatings is patterned to provide a two- dimensional array of independently switchable ESBG pixels , wherein an electrical control circuit [3] is operative to apply at least first and second voltages across each said ESBG pixel, wherein each said ESBG pixel is characterized by a first unique speckle state under said first applied voltage and a second unique speckle state under said second applied voltage, wherein said first and second speckle states occur during the integration time of the human eye, wherein each said ESBG pixels diffracts incident collimated light into an illumination patch in proximity to the active surface of said microdisplay wherein illumination matches of each pixel substantially overlap.

2. The apparatus of Claim 1 wherein said first and second ESBG arrays are identical and the voltages applied to overlapping pixels of said first and second ESBG arrays operate in anti-phase.

3. The apparatus of Claim 1 wherein said ESBG pixels are holograms of refractive microlenses.

4. The apparatus of Claim 1 wherein said ESBG pixels are holograms of beam-shaping diffusers.

5. The apparatus of Claim 1 wherein said ESBG pixels are holograms of diffractive Fresnel lenses.

6. The apparatus of Claim 1 wherein said ESBG pixels are holograms of orthogonal cylindrical diffractive lenses.

7. The apparatus of Claim 1 wherein said ESBG pixels are holograms of Fresnel computer generated holograms.

8. The apparatus of Claim 1 wherein said objective has a relative aperture numerically smaller than F/2.4.

9. The apparatus of Claim 1 wherein said objective has a relative aperture numerically smaller than F/2.0.

10. The apparatus of Claim 1 wherein said objective has a relative aperture numerically smaller than F/ 1.5.

11. The apparatus of Claim 1 wherein said objective has a relative aperture not greater than F/1.0.

12. The illumination device of Claim 1 wherein said first and second voltages are points on a time varying voltage characteristic.

13. The apparatus of Claim 1 wherein said ESBG despeckler device comprises identical first and second ESBG elements and ESBG pixels from said first and second ESBG elements substantially overlap in the illumination beam cross section.

14. The apparatus of Claim 1 wherein said ESBG despeckler device comprises identical first and second ESBG elements and ESBG pixels from said first and second ESBG elements are offset by a fraction of the ESBG element width in at least one of the vertical or horizontal array axes in the illumination beam cross section.