GB2526275A - Display for reducing speckle - Google Patents

Abstract

There is provided display comprising a diffuser, projector and driver. The diffuser comprises a liquid crystal operable in a light scattering mode in which the liquid crystal is arranged to have a spatially-variant refractive index. The projector is arranged to project an image onto the diffuser when the diffuser is operated in the light scattering mode. The driver is arranged to dynamically change the spatially-variant refractive index of the liquid crystal to provide a de-speckling effect to the image on the diffuser. The diffuser may also be operable in a transmissive mode and it may be switchable between the light scattering mode and the transmissive mode with the light scattering mode being selectively induced by voltage. The liquid crystal diffuser may comprise nematic or smetic-A liquid crystals. The image may be a holographic reconstruction and the display may form part of a Heads-up display (HUD) unit.

Description

DISPLAY FOR REDUCING SPECKLE

Field of the invention

The present disclosure relates to the field of image display. The present disclosure also relates to the field of reducing speckle. The present disclosure also relate to the field of head-up displays and viewing systems for head-up displays.

Background

Light scattered from an object contains both amplitude and phase information. This amplitude and phase infomrntion can be captured on, for example, a photosensitive plate by well-known interference techniques to form a holographic recording, or "hologram", comprising interference fringes. The "hologram" may be reconstructed by illuminating it with suitable light to form a holographic reconstruction, or replay image, representative of the original object.

It has been found that a holographic reconstruction of acceptable quality can be formed from a "hologram" containing only phase information related to the original object. Such holographic recordings may be referred to as phase-only holograms.

Computer-generated holography may numerically simulate the interference process, using Fourier techniques for example, to produce a computer-generated phase-only hologram. A computer-generated phase-only hologram may be used to produce a holographic reconstruction representative of an object.

The term "hologram" therefore relates to the recording which contains information about the object and which can be used to form a reconstruction representative of the object. The hologram may contain information about the object in the frequency, or Fourier, domain.

It has been proposed to use holographic techniques in a two-dimensional image projection system. An advantage of projecting images using phase-only holograms is the ability to control many image attributes via the computation method -e.g. the aspect ratio, resolution, contrast and dynamic range of the projected image. A further advantage of phase-only holograms is that no optical energy is lost by way of amplitude modulation.

A computer-generated phase-only hologram may be "pixelated". That is, the phase-only hologram may be represented on an array of discrete phase elements. Each discrete element may be referred to as a "pixel". Each pixel may act as a light modulating element such as a phase modulating element. A computer-generated phase-only hologram may therefore be represented on an array of phase modulating elements such as a liquid crystal spatial light modulator (SLM). The SLM may be reflective meaning that modulated light is output from the SLM in reflection.

Each phase modulating element, or pixel, may vary in state to provide a controllable phase delay to light incident on that phase modulating element. An array of phase modulating elements, such as a Liquid Crystal On Silicon (LCOS) SLM, may therefore represent (or "display") a computationally-determined phase-delay distribution. If the light incident on the array of phase modulating elements is coherent, the light will be modulated with the holographic information, or hologram.

The holographic information may be in the frequency, or Fourier, domain.

Alternatively, the phase-delay distribution may be recorded on a kinoform. The word "kinoform" may be used generically to refer to a phase-only holographic recording, or hologram.

The phase delay may be quantised. That is, each pixel may be set at one of a discrete number of phase levels.

The phase-delay distribution may be applied to an incident light wave (by illuminating the LCOS SLM, for example) and reconstructed. The position of the reconstruction in space may be controlled by using an optical Fourier transform lens, to form the holographic reconstruction, or "image", in the spatial domain.

Alternatively, no Fourier transform lens may be needed if the reconstruction takes

place in the far-field.

A computer-generated hologram may be calculated in a number of ways, including using algorithms such as (}erchberg-Saxton. The Gerchberg-Saxton algorithm may be used to derive phase information in the Fourier domain from amplitude information in the spatial domain (such as a 2D image). That is, phase information related to the object may be "retrieved" from intensity, or amplitude, only information in the spatial domain. Accordingly, a phase-only holographic representation of an object may be calculated.

The holographic reconstruction may be formed by illuminating the Fourier domain hologram and performing an optical Fourier transform, using a Fourier transform lens, for example, to form an image (holographic reconstruction) at a reply field such as on a screen.

Figure 1 shows an example of using a reflective SLM, such as a LCOS-SLM, to produce a holographic reconstruction at a replay field location, in accordance with the

present disclosure.

A light source (110), for example a laser or laser diode, is disposed to illuminate the SLM (140) via a collimating lens (Ill). The collimating lens causes a generally planar wavefront of light to become incident on the SLM. The direction of the wavefront is slightly off-normal (e.g. two or three degrees away from being truly orthogonal to the plane of the transparent layer). The arrangement is such that light from the light source is reflected off a mirrored rear surface of the SLM and interacts with a phase-modulating layer to form an exiting wavefront (112). The exiting wavefront (112) is applied to optics including a Fourier transform lens (120), having its focus at a screen (125).

The Fourier transform lens (120) receives a beam of phase-modulated light exiting from the SLM and performs a frequency-space transformation to produce a holographic reconstruction at the screen (125) in the spatial domain.

In this process, the light-in the case of an image projection system, the visible light-from the light source is distributed across the SLM (140), and across the phase modulating layer (i.e. the array of phase modulating elements). Light exiting the phase-modulating layer may be distributed across the replay field. Each pixel of the hologram contributes to the replay image as a whole. That is, there is not a one-to- one correlation between specific points on the replay image and specific phase-modulating elements.

The Gerchberg Saxton algorithm considers the phase retrieval problem when intensity cross-sections of a light beam, IA(x,y) and I(x,y), in the planes A and B respectively, are known and IA(x,y) and Ia(x,y) are related by a single Fourier transform. With the given intensity cross-sections, an approximation to the phase distribution in the planes A and B, DA(x,y) and cD(x,y) respectively, is found. The Gerchberg-Saxton algorithm finds solutions to this problem by following an iterative process.

The Gerchberg-Saxton algorithm iteratively applies spatial and spectral constraints while repeatedly transferring a data set (amplitude and phase), representative of IA(x,y) and IB(x,y), between the spatial domain and the Fourier (spectral) domain.

The spatial and spectral constraints are IA(x,y) and IB(x,y) respectively. The constraints in either the spatial or spectral domain are imposed upon the amplitude of the data set. The corresponding phase information is retrieved through a series of iterations.

A holographic projector may be provided using such technology. Such projectors have found application in head-up displays for vehicles.

The use of head-up displays in automobiles is becoming increasing popular. Head-up displays are broken down in to two main categories, those which use a combiner (a free standing glass screen whose purpose is to reflect a virtual image in to the driver's line of sight) and those which utilise the vehicle's windscreen to achieve the same purpose.

Figure 2 shows an example head-up display comprising a light source 206, a spatial light modulator 204 arranged to spatially modulate light from the light source with holographic data representative of an image for projection, a Fourier transform optic 205, a diffuser 203, a freeform mirror 201, a windscreen 202 and a viewing position 207. Figure 2 shows a so called "indirect view" system in which a real image of the holographic reconstruction is formed at a replay field on the diffuser 203. A holographic reconstruction is therefore projected on the diffuser 203 and may be viewed from viewing position 207 by focusing on the diffuser 203. The projected image is viewed via a first reflection off freeform mirror 201 and a second reflection off windscreen 202. The diffuser acts to increase the numerical aperture of the holographic system, fully illuminating the freefomi mirrors thereby allowing the virtual image to be viewed by a driver, for example.

Such display systems need to use a fixed diffuser or similar component to increase the viewing angle. This diffuser serves as a key component in the imaging system; its distance from the projection optic (normally a freeform mirror) determines the virtual image distance from the viewer's eye.

Alternatively, the holographic reconstruction may be viewed directly. Using "direct view" holography does enable information to be presented in 3D, however as the name suggests direct view requires the viewer to look at the hologram directly without a diffuser between the viewer and the light source. This type of 3D display has a number of problems, firstly the current generation of phase modulators have a relatively small diffraction angle and therefore to create a sufficiently large viewing area (eye-box) requires the use of complex and expensive optics parts. Secondly and more importantly, this type of configuration required the viewer to be directly exposed to laser radiation. There are very strict regulations surrounding the use of lasers and providing a sufficiently robust safety system that will ensure that the eye is never exposed to dangerous levels to laser radiation significantly increases the system complexity.

The use of a diffuser between the viewer and the projection engine mitigates both of the issues highlighted above. However, the structure of the diffuser in conjunction with the use of coherent light makes the object appear speckled owing to a complex interference process occurring at the image plane. The present disclosure aims to address this problem and improve the quality of the virtual image of the object.

Summary of the invention

Aspects of an invention are defined in the appended independent claims.

There is provided an improved imaging device that incorporate one or more switchable screens or diffusers comprising liquid crystal selectively diffuse elements.

Each liquid crystal selectively diffuse element can be switched between; i) exhibiting a uniform refractive index, in which it appears as a transparent window, and ii) a randomly varying local refractive index, in which case it scatters light and becomes a screen onto which an image might be projected.

Each selectively diffuse element is provided with an electrical driver or controller arranged to modify its light scattering properties by, for example, applying an appropriate voltage.

The displayed real image may be viewed using a lens. No image is present if the display screen is switched into a transparent state.

The level of speckle perceived in the image depends in part on the nature of the light scattering by the liquid crystal layer. If the refractive index variations are continuously changing at a fast enough rate, then the speckle visible in the image is greatly reduced.

A property of the liquid crystal is a switchable randomly variable refractive index.

The image speckle changes somewhat with the static light scattering properties of the display screen. However it can only be greatly reduced by rapidly switching between different light scattering states.

There is therefore disclosed a liquid crystal based diffuser operable in a first state in which light is scattered such that the diffuser may act as a display medium. In the first state, the diffuser may act as a screen because the liquid crystal is structurally arranged is have a refractive index distribution. That is, a locally varying refractive index. The present disclosure relates to rapidly changing that locally varying refractive index so that a depeckling effect is provided to the real image seen on the diffuser.

Although embodiments describe the element as being a "diffuser", it may be understood that in examples, the disclosed diffuser is operated in a non-diffusive mode such as a transmissive mode. The diffuser may be considered an element with a selectable, controllable or switchable diffuse state or mode.

Brief description of the drawings

Embodiments will now be described to the accompanying drawings in which: figure 1 is a schematic showing a reflective SLM, such as a LCOS, arranged to produce a holographic reconstruction at a replay field location; figure 2 shows a so-called "indirect view" holographic projector for a head-up display of a vehicle; figure 3 shows an example algorithm for computer-generating a phase-only hologram; figure 4 shows an example random phase seed for the example algorithm of figure 3; figure 5 is a virtual imaging schematic; figure 6 is multi-diffuser schematic in accordance with the present disclosure; and figure 7 is a schematic of a LCOS SLM.

In the drawings, like reference numerals referred to like parts.

Detailed description of the drawings

The present disclosure relates to an improved so-called "indirect view" system in which a viewer views a virtual image of a holographic reconstruction visible on a diffuser. However, the viewed "object" need not necessarily be a holographic reconstruction. In other words, the present disclosure is equally applicable to other display systems such as more conventional LED backlit liquid crystal display projectors and the like. Embodiments describe a method of computer-generating a hologram by way of example only. In summary, the present disclosure relates to despeckling the image visible on the diffuser. Accordingly, any virtual image of the image on the diffuser is also despeckled.

Holographically-generated 2D images are known to possess significant advantages over their conventionally-projected counterparts, especially in terms of definition and efficiency.

Modified algorithms based on Gerchberg-Saxton have been developed -see, for example, co-pending published PCT application WO 2007/131650 incorporated herein by reference.

Figure 3 shows a modified algorithm which retrieves the phase information w[u,vI of the Fourier transform of the data set which gives rise to a known amplitude information T[x,y] 362. Amplitude information T[x,y] 362 is representative of a target image (e.g. a photograph). The phase information w[u,vJ is used to produce a holographic representative of the target image at an image plane.

Since the magnitude and phase are intrinsically combined in the Fourier transform, the transformed magnitude (as well as phase) contains useful information about the accuracy of the calculated data set. Thus, the algorithm may provide feedback on both the amplitude and the phase information.

The algorithm shown in Fig. 3 can be considered as having a complex wave input (having amplitude information 301 and phase information 303) and a complex wave output (also having amplitude information 311 and phase information 313). For the purpose of this description, the amplitude and phase information are considered separately although they are intrinsically combined to form a data set. It should be remembered that both the amplitude and phase information are themselves functions of the spatial coordinates (x,y) for the farfleld image and (u,v) for the hologram, both can be considered amplitude and phase distributions.

Referring to Fig. 3, processing block 350 produces a Fourier transform from a first data set having magnitude information 301 and phase information 303. The result is a second data set, having magnitude information and phase information wEu,vI 305.

The amplitude information from processing block 350 is set to a distribution representative of the light source but the phase information lpn[UY] 305 is retained.

Phase information 305 is quantised by processing block 354 and output as phase information w[u,v] 309. Phase information 309 is passed to processing block 356 and combined with the new magnitude by processing block 352. The third data set 307, 309 is applied to processing block 356 which performs an inverse Fourier transform.

This produces a fourth data set R4x,y] in the spatial domain having amplitude information 311 and phase information 313.

Starting with the fourth data set, its phase information 313 forms the phase information of a fifth data set, applied as the first data set of the next iteration 303'.

Its amplitude information R[x,y] 311 is modified by subtraction from amplitude information T[x,y} 362 from the target image to produce an amplitude information 315 set. Scaled amplitude information 315 (scaled by a) is subtracted from target amplitude information T[x,yJ 362 to produce input amplitude information [x,y] 301 of the fifth data set for application as first data set to the next iteration. This is expressed mathematically in the following equations: [x, yJ = F {expQ yç [u, v])} w[u,v] = ZF{ij. exp(iZR[x,y])) = T[x,y]-c4]R{x,y]J-T[x,y]) Where: F' is the inverse Fourier transform; F if the forward Fourier transform;

R is the replay field;

T is the target image; Z is the angular information; W is the quantized version of the angular information; c is the new target magnitude, c»= 0; and a is a gain element -1.

The gain element a may be predetermined based on the size and rate of the incoming target image data.

In the absence of phase information from the preceding iteration, the first iteration of the algorithm uses a random phase generator to supply random phase information as a starting point. Figure 4 shows an example random phase seed, In a modification, the resultant amplitude information from processing block 350 is not discarded. The target amplitude information 362 is subtracted from amplitude information to produce a new amplitude information. A multiple of amplitude information is subtracted from amplitude information 362 to produce the input amplitude information for processing block 356. Further alternatively, the phase is not fed back in full and only a portion proportion to its change over the last two iterations is fed back. Accordingly, Fourier domain data representative of an image of interest may be formed.

In summary, there is provided an improved display comprising a selectively diffuser element arranged to function as a screen when operated in a light scattering mode. In this mode, an image -such as a projected image -may be visible on the selectively-diffuse element. The device comprises a driver or controller arranged to change to structure of the liquid crystal of the diffuser during display of the object so as to despeclde the image shown in the diffuser. The device therefore provides a simple and convenient means for improving the quality of the image.

The display may incorporate one or more switchable screens comprising liquid crystal selectively diffuse elements.

Each liquid crystal selectively diffuse element can be switched between: i) exhibiting a uniform refractive index, in which it appears as a transparent window, and ii) a randomly varying local refractive index, in which case it scatters light and becomes a screen onto which an image might be projected.

By dynamically changing the scattering state of the selectively diffusing element during the display of an image, the complex integration associated with the use of coherent light sources in projected displays, leading to a speckled image, are effectively smoothed or averaged out.

Figure 5 shows an embodiment of the present disclosure in which a virtual projection optic 505 forms a virtual image 501 of an object 503. The object 503 is visible on a diffuser. The virtual image 501 is seen from observation plane 507.

The virtual image distance is set by placing the object inside the focal length of the imaging optic, the apparent virtual distance may then be calculated.

For the optical schematic shown above, the virtual image distance (i) is determined by the following equation: 1= fo A linear change in the object distance causes a non-linear change in the virtual distance.

The virtual image has to be sufficiently far from the eye so that the eye refocusing time from infinity, which is the normal fccal length when driving, to the display information is small thereby reducing the blind flight time. However, the virtual image distance must also be sufficiently close so that the information presented to the driver is clearly legible. These two competing factors normally result in the virtual image distance being configured such that the essential driving information is presented at a distance of 1.5 to 3.5m, optionally 2.5m, from the driver's eye.

There is therefore provided a display comprising: a diffuser comprising a liquid crystal operable in a light scattering mode in which the liquid crystal is arranged to have a spatially-variant refractive index; a projector arranged to project an image onto the diffuser; and driver arranged to dynamically change the spatially-variant refractive index of the liquid crystal. Accordingly, speckle in the image is reduced and the perceived quality of the image is improved. In other words, the present disclosure provides convenient means for despeckling the image visible on the screen.

The driver/controller is arranged to dynamically switch the appropriate selectively diffuse elements of the liquid crystal at a rate which is faster than the frame rate of image projected onto the diffuser in order to minimise speckle. In an embodiment, the driver is arranged to dynamically change the spatially variant refractive index of the liquid crystal at a rate of greater than 50 Hz. That is, the spatially variant refractive index of the liquid crystal is changed at least every 20 ms during display.

Any single diffuser might be operated in either or both the static or dynamic modes.

However, not all liquid crystal diffusers are capable of both modes of operation.

In an embodiment, the diffuser is also operable in a transmissive mode. Accordingly, the diffuser may be switched between a mode in which it functions as a screen/display and a mode in which it does not. The object may therefore be selectively visible on the diffuser. That is, the diffuser is switchable between the scattering mode and the transmissive mode. In an embodiment, the light scattering state is selectively-induced by voltage.

In embodiments, the image is a holographic reconstruction. However, it is not essential that the image is a holographic reconstruction and the present disclosure is equally suitable to any method of projecting an image on the diffuser.

In an embodiment, the display further comprises a projection optic arranged to form a virtual image of the image visible on the diffuser when the diffuser is operated in the light scattering mode. In accordance with the present disclosure, the image visible on the diffuser and the virtual image are both despeckled.

In embodiment, the diffuser comprises at least one selected from the group comprising cholesteric liquid crystals, polymer dispersed liquid crystals, nematic liquid crystals and smectic-A liquid crystals.

In an advantageous embodiment, the device comprises a plurality of further diffusers positioned at different distances from the projection optic. In an embodiment, each difThser is switchable between a light scattering mode and a light transmissive mode and only one difThser is operated in the light scattering mode at any one point in time.

The perceived depth of an image of the object may be controlled by determining which diffuser to display the object on.

Figure 6 shows an embodiment of the present disclosure in which the distance from the viewer 620 to the virtual image 651, 652, 653, 654, formed by virtual projection optic 630, is changed by selecting which diffuser 601, 602, 603, 604 the object is visible on. In this embodiment, diffusers 601, 602, 603, 604 are substantially parallel and stacked together. In this embodiment, the diffusers are spaced by glass substrate 610. If the object is visible on diffuser 601, the virtual image 651 will appear at a first depth to viewer 620. If the image is visible on difThser 602, the virtual image 652 will appear at a second depth to viewer 620, and so on.

Accordingly, the effective position of the image may be changed by selected which diffuser the image is visible, or "displayed", on.

In an embodiment, each diffuser is independently switchable between a scattering mode and a transmissive mode. A diffuser operating in the transmissive mode will transmit the projected object but a diffuser operating in the scattering mode will effectively "display" the image. That is, the image will be visible on the diffuser operating in the scattering mode. By stacking a plurality of diffusers together and arranging them such that each is at a different distance from the virtual projection optic, the parameter "o" in equation I may be varied. Accordingly, the distance from the observation plane to the viewed virtual image is changed. It may therefore be understood that the perceived depth of the displayed infonnation may be changed by selecting the diffuser.

Advantageously, the present disclosure also provides a convenient means for despeckling the variable position virtual image.

Light scattering states can be induced in thin liquid crystal layers by a number of mechanisms. In each case the refractive index of the liquid crystal varies from point to point with a magnitude and spatial frequency sufficiently close to the wavelength of light so as to result in strong scattering.

Some of these liquid crystal electro-optic effects are static and some are dynamic (consisting of turbulent motion). The static effects may be transient (only present when a voltage is maintained), bistable (a single scattering state that can be switched on and off with a voltage pulse) or multistable (a number of different stable scattering states that can be switched on and off with voltage pulses. That is, in an embodiment, the scattering state is selectively-induced by voltage.

Embodiments use liquid crystals selected from the group comprising: (1) Cholesteric liquid crystals (also called chiral nematic phases) with a suitably small cholesteric pitch can be driven into transparent and light scattering states by dielectric re-orientation. Polymer materials may be added to these materials to stabilise the textures. The textures are static and can exhibit bi-stability, (but not multi-stability). For more information concerning this type of liquid crystal, the reader is referred to Gruebel. W., U. Wolff, and H. Kruber., "Electric field induce texture changes in certain nematica! cholesteric liquid crystal mixtures"., Mol. Cryst.

Liq. Cryst, Vol. 24, 1973, pp 103-109 and V. G. Chigrinov, "Liquid Crystal devices, Physics and Applications"., ISBN 0-89006-895-4, Published by Artech House, 1999, ppl34-l48.

(2) Films of nematic liquid droplets in a polymer matrix (polymer dispersed liquid crystal or "PDLC5") can exhibit light scattering and can be switched into a clear state by dielectric re-orientation. This is a static texture and relaxes back to a clear state on removal of the drive voltage, i.e. PDLCs are not usually bi-stable. Some bi-stability can be induced by using a chiral nematic liquid crystal (i.e. a cholesteric liquid crystal) instead of the nematic phase in the droplets. For more information concerning this type of liquid crystal, the reader is referred to Coates D., "Polymer dispersed Liquid Crystals", J. Mater. Chem., Vol. 5, No. 12., 1994, pp2063-2072 and Doane, J. W., eta!., "Wide-angle View PDLC Displays". SID 90 Digest, 1990, pp224-226.

(3) Neinatic liquid crystals can be doped with an ionic dopant and the application of a low frequency waveform (< 1 KEIZ) induces a turbulent dynamic light scattering state by an electro-chemical process known as dynamic scattering'. The scattering state is in continuous motion and reverts to a clear slate when the voltage is removed. For more information, the reader is referred to G. Heilmeier, L. A. Zanoni, and L. Barton, Proc. IEEE 56, 1162 (1968).

(4) Dynamic scattering can also be electro-chemically induced in the liquid crystal smectic A phase, which are more ordered that nematic phases. The application of a low frequency voltage produces a turbulent dynamic scattering state resembling that occurring in nematic liquid crystals. However when the voltage is removed, the scattering state does not relax back to clear state, but remains as a semi-permanent static texture. It can however then be removed by applying a higher frequency voltage (>1KHz). This scattering state is multi-stable' in that different degrees of scattering can be induced and they are all stable in the absence of voltage. High voltages (around 1 OOV) are required for smectic dynamic scattering. For more information concerning this type of liquid crystal, the reader is referred to: D. Coates, W. A. Crossland, J. H. Morrissy, and B, Needham, J. Phys. D. 11, 1(1978); and Crossland W. A., Davey A. B., Chu D., Clapp T. V., "Smectic A Memory Displays", in Handbook of Liquid Crystals: 7 Volume Set, Second Edition. Edited by J. W. Goodby, P. J. Collings, T. Kato, C. Tschierske, H. Gleeson, and P. Raynes..

Published 2013 by Wiley-VCH Verlag GmbH & Co. KGaA. Chapter 7, pp1-39.

The inventors have identified which of these liquid crystals are suitable in accordance with the present disclosure. These possibilities are summarised in the table below.

Dynamic Switchable BI-stability of Multi-stability scattering static diffusers scattering of scattering ________________________ (turbulent state) ______________ state state CholestCric liquid crystals no yes yes rio PDLCs no yes possible no Nematic dynamIc -yes no -no rio: scattering -I _____________ _______________ Smectic dynamic scattering yes yes yes yes

Table 1

In one example, nematic dynamic scattering is used for the liquid crystal selectively diffuse element. The rise and fall time for the scattering may take much longer than milliseconds (with a typical drive voltage of about 15 Volts applied). However the continuous dynamic local variations in refractive index (caused by the voltage induced turbulence whilst the voltage is applied) are fast enough to result in significant speckle reduction. The voltage must be maintained continuously to sustain the scattering; if the voltage is removed the liquid crystal layer reverts to the transparent state.

In a second example, smectic dynamic scattering is driven into a light scattering state by means of a low frequency pulsed voltage (e.g 120 Vat 50Hz) and into a transparent state by means of a high frequency pulsed voltage (e.g. 100 V at> 1 KHz). Driving into the transparent state can be relatively fast (C 10 ms) but setting up the scattering is much slower e.g. > 20 ms. As with nematic dynamic scattering, whilst a low frequency voltage is applied a dynamic scattering condition is maintained, which scatters light and is in continuous motion. This can also act as a dynamic despeckling screen, however it may be difficult to maintain continuously dynamic scattering for long periods of time for electro-chemical reasons.

In smectic dynamic scattering, once the drive voltage pulses have been applied the liquid crystal remains is either a clear or a transparent state indefinitely (until another pules is applied). It is therefore highly suitable for switchable static difffising screens.

The quality of the scattering state and the clarity of the transparent state are both excellent, making it suitable for switchable static difThser screens As shown in figure 6, in an embodiment, the first and second ditThser are substantially parallel andlor positioned on a common optical axis.

Although figure 6 shows an arrangement of four diffusers, it may be understand that any number of diffusers may be employed depending on the resolution required. That is, in an embodiment, the device further comprises a plurality of further diffusers positioned at different distances from the projection optic. All the diffusers may be substantially parallel and/or positioned on a common optical axis. In an embodiment, the diffusers are on a common optical axis with the virtual projection optic.

In an optional embodiment, the image is visible on the chosen diffuser because the diffusor is scattering and the image is projected onto the diffuser by a projector. In an embodiment, the image is projected onto the diffuser by a holographic projector and the object is a holographic reconstruction of a predetermined object. An example holographic projector has been previous described in, for example, WO 2013/153354 incorporated herein by reference.

In an embodiment, the projector is a holographic projector arranged comprising a spatial light modulator arranged to apply a phase-delay distribution to incident light, wherein the phase-delay distribution comprises phase-only data representative of the image and, optionally, phase-only data representative of a phase-only lens.

It is known in the art how a phase-only programmable lens may be combined with phase-only object data such that, when reconstructed by reverse Fourier transform (e.g. optically), a focused holographic reconstruction is formed at a chosen depth of replay field. In embodiments, the data is combined by simple vector addition.

In a further embodiment, the holographic projector further comprises Fourier transform means arranged to perform an optical Fourier transform of phase modulated light received from the spatial light modulator to form the object. In an embodiment, the Fourier transmforin is a physical lens arranged to perform an optical Fourier transform. In a further embodiment, the Fourier transform optic utilised by the holographic projector is not a physical optic but, instead, a further phase only lens implemented using the same holographic techniques.

In a yet further embodiment, the holographic projector is further arranged to select the focal length of the lens so that the projected object is substantially focused on the diffuser.

Advantageously, the above system may be employed in conjunction with the phase only holographic projector, to provide fine-tuning to the position of the focal plane of the image.

In embodiment, the display system is a head-up display although it may be understood the disclosed imaging device is equally applicable to other display systems and projection systems.

There is also provided a method of despeckling an image, the method comprising: projecting an image onto a diffuser comprising liquid crystal when the diffuser is operated in a light scattering mode in which the liquid crystal is arranged to have a spatially-variant refractive index; dynamically changing the spatially-variant refractive index of the liquid crystal.

In an embodiment, the step of projecting the image onto the diffuser comprises: applying a phase-delay distribution to incident light wherein the phase-delay distribution comprises phase-only data representative of the image; and performing a Fourier transform of phase modulated light reôeived from a spatial light modulator to form the image on the diffuser.

In a further embodiment, the phase-delay distribution further comprises phase-only data representative of a lens and the method further comprises selecting the focal length of the lens so that the image is substantially focused on the diffuser.

It can be understood that a head-up display may display a variety of infonnation as known in the art. Holograms corresponding to all the possible displays may be therefore be pre-calculated and stored in a repository, or calculated in real-time. In an embodiment, the projector further comprises a repository of Fourier domain data representative of a plurality of 2D images.

Embodiments described herein relate to Fourier holography by way of example only.

The present disclosure is equally applicable to Fresnel holography in which Fresnel transform is applied during calculation of the hologram.

The quality of the reconstructed hologram may be affect by the so-called zero order problem which is a consequence of the diffiactive nature of the reconstruction. Such zero-order light can be regarded as "noise" and includes for example specularly reflected light, and other unwanted light from the SLM.

This "noise" is generally focussed at the focal point of the Fourier lens, leading to a bright spot at the centre of a reconstructed hologram. Conventionally, the zero order light is simply blocked out however this would clearly mean replacing the bright spot with a dark spot.

Alternatively and angularly selective filter could be used to remove only the collimated rays of the zero order. Other methods of managing the zero order may also be used.

Whilst embodiments described herein relate to displaying one hologram per frame, the present disclosure is by no means limited in this respect and more than one hologram may be displayed on the SLM at any one time.

For example, embodiments implement the technique of "tiling", in which the surface area of the SLM is further divided up into a munber of tiles, each of which is set in a phase distribution similar or identical to that of the original tile. Each tile is therefore of a smaller surface area than if the whole allocated area of the SLM were used as one large phase pattern. The smaller the number of frequency component in the tile, the further apart the reconstructed pixels are separated when the image is produced. The image is created within the zeroth diffiaction order, and it is preferred that the first and subsequent orders are displaced far enough so as not to overlap with the image and may be blocked by way of a spatial filter.

As mentioned above, the image produced by this method (whether with tiling or without) comprises spots that form image pixels. The higher the number of tiles used, the smaller these spots become. If one takes the example of a Fourier transform of an infinite sine wave, a single frequency is produced. This is the optimum output. In practice, ifjust one tile is used, this corresponds to an input of a single cycle of a sine wave, with a zero values extending in the positive and negative directions from the end nodes of the sine wave to infinity. Instead of a single frequency being produced from its Fourier transform, the principle frequency component is produced with a series of adjacent frequency components on either side of it. The use of tiling reduces the magnitude of these adjacent frequency components and as a direct result of this, less interference (constructive or destructive) occurs between adjacent image pixels, thereby improving the image quality.

Preferably, each tile is a whole tile, although it is possible to use fractions of a tile.

Embodiments relate to variants of the Gerchberg-Saxton algorithm by way of

example only.

The skilled person will understand that the improved method disclosed herein is equally applicable to the calculation of a hologram used to form a three-dimensional reconstruction of an object.

Equally, the present disclosure is not limited to projection of a monochromatic image.

A colour 2D holographic reconstruction can be produced and there are two main methods of achieving this. One of these methods is knom as "frame-sequential colour" (FSC). In an FSC system, three lasers are used (red, green and blue) and each laser is fifed in succession at the SLM to produce each frame of the video. The colours are cycled (red, green, blue, red, green, blue, etc.) at a fast enough rate such that a human viewer sees a polychromatic image from a combination of the three lasers. Each hologram is therefore colour specific. For example, in a video at 25 frames per second, the first frame would be produced by firing the red laser for 1/75th of a second, then the green laser would be fired for 1/75th of a second, and finally the blue laser would be fifed for 1/75th of a second. The next frame is then produced, starting with the red laser, and so on.

An alternative method, that will be referred to as "spatially separated colours" (SSC) involves all three lasers being fired at the same time, but taking different optical paths, e.g. each using a different SLM, or different area of a single SLM, and then combining to form the colour image.

An advantage of the frame-sequential colour (FSC) method is that the whole SLM is used for each colour. This means that the quality of the three colour images produced will not be compromised because all pixels on the SLM are used for each of the colour images. However, a disadvantage of the FSC method is that the overall image produced will not be as bright as a corresponding image produced by the SSC method by a factor of about 3, because each laser is only used for a third of the time. This drawback could potentially be addressed by overdriving the lasers, or by using more powerful lasers, but this would require more power to be used, would involve higher costs and would make the system less compact.

An advantage of the SSC (spatially separated colours) method is that the image is brighter due to all three lasers being fired at the same time. However, if due to space limitations it is required to use only one SLM, the surface area of the SLM can be divided into three parts, acting in effect as three separate SLMs. The drawback of this is that the quality of each single-colour image is decreased, due to the decrease of SLM surface area available for each monochromatic image. The quality of the polychromatic image is therefore decreased accordingly. The decrease of SLM surface area available means that fewer pixels on the SLM can be used, thus reducing the quality of the image. The quality of the image is reduced because its resolution is reduced.

In embodiments, the SLM is a Liquid Crystal over silicon (LCOS) device. LCOS SLMs have the advantage that the signal lines, gate lines and transistors are below the mirrored surface, which results in high fill factors (typically greater than 90%) and high resolutions.

LCOS devices are now available with pixels between 2,Sjsm and 15 p.m.

The structure of an LCOS device.is shown in Figure 7.

An LCOS device is formed using a single crystal silicon substrate (802). It has a 2D array of square planar aluminium electrodes (801), spaced apart by a gap (801 a), arranged on the upper surface of the substrate. Each of the electrodes (801) can be addressed via circuitry (802a) buried in the substrate (802). Each of the electrodes forms a respective planar mirror. An alignment layer (803) is disposed on the array of electrodes, and a liquid crystal layer (804) is disposed on the alignment layer (803). A second alignment layer (805) is disposed on the liquid crystal layer (404) and a planar transparent layer (806), e.g. of glass, is disposed on the second alignment layer (805).

A single transparent electrode (807) e.g. of ITO is disposed between the transparent layer (806) and the second alignment layer (805).

Each of the square electrodes (801) defines, together with the overlying region of the transparent electrode (807) and the intervening liquid crystal material, a controllable phase-modulating element (808), often referred to as a pixel. The effective pixel area, or fill factor, is the percentage of the total pixel which is optically active, taking into account the space between pixels (801a). By control of the voltage applied to each electrode (801) with respect to the transparent electrode (807), the properties of the liquid crystal material of the respective phase modulating element may be varied, thereby to provide a variable delay to light incident thereon. The effect is to provide phase-only modulation to the wavefront, i.e. no amplitude effect occurs.

A major advantage of using a reflective LCOS spatial light modulator is that the liquid crystal layer can be half the thickness than would be necessary if a transmissive device were used. This greatly improves the switching speed of the liquid crystal (a key point for projection of moving video images), A LCOS device is also uniquely capable of displaying large arrays of phase only elements in a small aperture. Small elements (typically approximately 10 microns or smaller) result in a practical diffiaction angle (a few degrees) so that the optical system does not require a very long optical path.

It is easier to adequately illuminate the small aperture (a few square centimetres) of a LCOS SLM than it would be for the aperture of a larger liquid crystal device. LCOS SLMs also have a large aperture ratio, there being very little dead space between the pixels (as the circuitry to drive them is buried under the mirrors). This is an important issue to lowering the optical noise in the replay field.

Using a silicon backplane has the advantage that the pixels are optically flat, which is important for a phase modulating device.

Whilst embodiments relate to a reflective LCOS SLM, the skilled person will understand that any SLM can be used including transmissive or MEMs based SLMs.

The invention is not restricted to the described embodiments but extends to the full scope of the appended claims.

Claims (18)

1. Claims 1. A display comprising: a diffuser comprising a liquid crystal operable in a light scattering mode in which the liquid crystal is arranged to have a spatially-variant refractive index; a projector arranged to project an image onto the diffuser; a driver arranged to dynamically change the spatially-variant refractive index of the liquid crystal.
2. 2. A display as claimed in claim 1 wherein the diffuser is also operable in a transmissive mode.
3. 3. A display as claimed in any preceding claim wherein the image is a holographic reconstruction.
4. 4. A display as claimed in any preceding claim thither comprising a projection optic arranged to form a virtual image of the image visible on the diffuser when the diffuser is operated in the light scattering mode;
5. 5. A display as claimed in any preceding claim wherein the diffuser is switchable between the light scattering mode and the transmissive mode.
6. 6. A display as claimed in claim 5 wherein the light scattering state is selectively-induced by voltage.
7. 7. A display as claimed in any preceding claim wherein the diffuser comprises nematic liquid crystals.
8. 8. A display as claimed in any preceding claim wherein the diffuser comprises smectic-A liquid crystals.
9. 9. A display as claimed in any preceding claim further comprising a plurality of ftrther diffusers positioned at different distances from the projection optic.
10. 10. A display as claimed in any preceding claim wherein the projector is a holographic projector comprising a spatial light modulator arranged to apply a phase-delay distribution to incident light, wherein the phase-delay distribution comprises phase-only data representative of the image.
11. 11. A display as claimed in any preceding claim wherein the phase-delay distribution further comprises phase-only data representative of a lens.
12. 12. A display system as claimed in claim 11 wherein the projector is further arranged to select the focal length of the phase only lens so that the image is substantially focused on the diffuser.
13. 13. A display as claimed in any one of claims 10 to 12 wherein the projector further comprises Fourier transform means arranged to perform a Fourier transform of phase modulated light received from the spatial light modulator to form the image.
14. 14. A display as claimed in any preceding claim wherein the display is a head-up display.
15. 15. A method of despeckling an image, the method comprising: projecting an image onto a diffuser comprising liquid crystal when the diffuser is operated in a light scattering mode in which the liquid crystal is arranged to have a spatially-variant refractive index; dynamically changing the spatially-variant refractive index of the liquid crystal.
16. 16. A method of despeckling an image as claimed in claim 14 wherein the step of projecting the image onto the diffuser comprises: applying a phase-delay distribution to incident light wherein the phase-delay distribution comprises phase-only data representative of the image; and perfonning a Fourier transform of phase modulated light received from a spatial light modulator to form the image on the diffuser.
17. 17. A method of despeckling an image as claimed in claim 16 wherein the phase-delay distribution further comprises phase-only data representative of a lens and the method further comprises: selecting the focal length of the lens so that the image is substantially focused on the diffuser.
18. 18. A display or method of despeckling an image substantially as hereinbefore described with reference to the accompanying drawings.