WO2010125367A1

WO2010125367A1 - Holographic display

Info

Publication number: WO2010125367A1
Application number: PCT/GB2010/050554
Authority: WO
Inventors: Lilian Lacoste; Dominik Stindt
Original assignee: Light Blue Optics Ltd
Priority date: 2009-04-29
Filing date: 2010-03-31
Publication date: 2010-11-04
Also published as: GB0907297D0

Abstract

This invention generally relates to displaying a substantially two-dimensional input image, and is particularly applicable for use in a head-up display. Display apparatus for displaying a substantially two-dimensional input image comprises: a first spatial light modulator configured to form a hologram on the basis of a first drive signal to phase modulate a beam of substantially coherent light; a second spatial light modulator comprising a plurality of pixels, the pixels configured to selectively amplitude modulate on the basis of a second drive signal the light modulated by the first spatial light modulator; and a controller configured to generate said second drive signal on the basis of said input image.

Description

Holographic Display

FIELD OF THE INVENTION

The invention relates to display apparatus for displaying a substantially two-dimensional input image, a head-up display comprising such display apparatus, and a method of displaying a substantially two- dimensional input image.

BACKGROUND TO THE INVENTION

Figure 1 shows a traditional approach to the design of a head-up display (HUD), in which lens power is provided by the concave and fold mirrors of the HUD optics in order to form a virtual image, typically displayed at an apparent depth ranging from Im to the infinite.

Figure 2, which is taken from WO/2009/156752, shows a virtual image display which provides imagery in which the focal point of the projected image is some distance behind the projection surface, thereby giving the effect of depth. A projector 200 is used as the image source, and an optical system 202 is employed to control the focal point at the viewer's retina 204, thereby providing a virtual image display.

We have previously described techniques for displaying an image holographically - see, for example, WO 2005/059660 (Noise Suppression Using One Step Phase Retrieval), WO 2006/134398 (Hardware for OSPR), WO 2007/031797 (Adaptive Noise Cancellation Techniques), WO 2007/110668 (Lens Encoding), WO 2007/141567 (Colour Image Display), and PCT/GB2008/050224 (Head Up Displays). These are all hereby incorporated by reference in their entirety.

Further background prior art can be found in:

http://www.dolby.com/uploadedFiles/zz- Shared Assets/Engrish PDFs/Professional/dolby- hdr-video-technical-overview.pdf;

WO03/077013A2;

US2008/0174614 A1;

US2007/0132956 A1;

US2008/0043034 A1;

US Patent 7,413,309 "High Dynamic Range display devices";

"A High-Dynamic Range Projection System", Andriy Pavlovych, Wolfgang Stuerzlinger, Dept. of Computer Science, York University, 4700 Keele Street, Toronto, Canada;

Norbert Fruehauf, Presentation at Euroforum 2009 in Munich; and

DisplaySearch, TFT LCD Materials Report 2007. SUMMARY OF THE INVENTION

According to a first aspect of the present invention, there is provided display apparatus for displaying a substantially two-dimensional input image, the display apparatus comprising: a first spatial light modulator configured to form a hologram on the basis of a first drive signal to phase modulate a beam of substantially coherent light; a second spatial light modulator comprising a plurality of pixels, the pixels configured to selectively amplitude modulate on the basis of a second drive signal the light modulated by the hologram; and a controller configured to generate said second drive signal on the basis of said input image.

Thus, an embodiment may perform two-stage modulation of coherent light. The first stage performed by the first spatial light modulator, may primarily modulate phase. Particularly advantageously, if the selective amplitude modulation dims principally portions of the light modulated by the first spatial light modulator corresponding to regions of the input region that are black, then the contrast ratio of the displayed image may be increased. This is of interest particularly in the field of head-up displays where a user, e.g., a driver, may need regions of the displayed image corresponding to black (or another background colour) regions of the input image to appear substantially transparent.

In any embodiment, the second spatial light modulator may be binary or M-ary, where M > 2. (The M- ary modulator may modulate M different arbitrarily distributed transmission states). In either case, the controller may be configured to generate the second drive signal on the basis of the input image (this case may correspond to masking the black parts). Such a controller may further be configured to generate said second drive signal on the basis of data determining said hologram, for example by performing a Fourier transform on said first drive signal (in order to calculate the image projected by the first holographic SLM). In particular, the controller may be configured to generate the second drive signal according to deviation between values derived from the hologram and corresponding values derived from the input image.

Regarding the selective amplitude modulation, in any embodiment, a pixel of the second spatial modulator may be configured to receive a portion of the light modulated by the hologram, the controller being configured to determine a target grey level on the basis of a region which is a part or substantially the whole of the input image, and the controller further configured such that the second drive signal causes the pixel to amplitude modulate the portion according to the target grey level. Determining the target grey level on the basis of substantially the whole of the input image may be advantageous for output uniformity. In particular, the controller may be configured to determine such target grey levels on the basis of colours of individual regions of the input image. For example, the controller may be configured to determine a target grey level on the basis of a colour of a region, such that the above amplitude modulation substantially fully blocks the light portion if the colour is a predetermined colour, e.g., black. (In this way, the second SLM may substantially fully block light over areas covered by "black" colour in the input image). In this case, the controller may further be configured to determine the target grey level such that the amplitude modulation blocks about 0% to about 20% of said light portion depending on the target grey level if the colour is not the predetermined colour. (In this manner, the selective amplitude modulation of the second SLM may block about 0% to about 20% of said light portion depending on the target grey level and 100% for the black level).

In any embodiment, the second spatial light modulator may be transmissive or reflective. This may further apply to the first spatial light modulator. Further in this regard, either or both of the first and second spatial light modulators may comprise liquid crystal on silicon (LCOS) or a MEMS-based device. Further still, either or both of the first and second spatial light modulators may comprise a microdisplay such as a ferroelectric LCOS microdisplay. Particularly to avoid mismatch between optical components of different sizes, for example in the case of using a microdisplay (which may be smaller than an equivalent non-microdisplay comprising substantially the same number of pixels), the display apparatus may further comprise magnifying and/or demagnifying optics. For example, magnifying optics may expand a beam according to an angle of divergence. Conversely, demagnifying optics in the form of a reverse telescope may narrow the beam according to an angle of convergence.

In particular, a controller in any embodiment may generate the first drive signal on the basis of the input image, e.g., by performing One Step Phase Retrieval on the basis of the input image. (In any embodiment, this particular controller may further be the controller that generates the second drive signal).

Any embodiment may further comprise a projection screen. In particular, light modulated by the second SLM may be projected onto the projection screen so that the image formed on the projection screen can be viewed by the user.

Any embodiment may further comprise a diffusing screen between the first spatial light modulator and the second spatial light modulator. Alternatively or additionally, a diffusing screen may be provided to receive light modulated by said second spatial light modulator, advantageously such that the distance between the diffusing screen and the second spatial light modulator is less than about 20% of the throw distance of a holographic projector, the first spatial light modulator being provided as part of the holographic projector. The provision of a diffusing screen may be of particular advantage where widening of the projected beam is required. Any embodiment may further comprise a laser configured to generate the beam of substantially coherent light. Where a colour display is required, a plurality of lasers of different colours may be provided to provide respective beams of substantially coherent light. In an embodiment, each such beam may be modulated by a respective hologram by means of the first spatial light modulator. Such beams may sequentially illuminate the first spatial light modulator.

According to a second aspect of the invention, there is provided a head-up display (HUD) comprising display apparatus of any embodiment of the first aspect. An embodiment of the second aspect may use a combiner and a projection optics to combine light projected from the second SLM with external light, e.g., from the environment so that the light combination may be received by the user's eye. (The combiner may be a surface onto which the light modulated by the second SLM is projected so that a user can view the two-stage modulated image. Such a combiner may operate by reflecting light of some wavelengths while allowing light of other wavelengths to pass through the combiner). Such a combiner may be provided externally, e.g., the windshield of a plane or automobile. Thus, a virtual image may be formed beyond the windshield so that the user can focus on the outside world and displayed information, e.g., symbology, without re- focussing.

According to a third aspect of the invention, there is provided a method of displaying a substantially two-dimensional input image, the method comprising: phase modulating on the basis of a first drive signal a beam of substantially coherent light, using a hologram comprised in a first spatial light modulator; selectively amplitude modulating on the basis of a second drive signal the light modulated by the hologram, using a second spatial light modulator; and generating said second drive signal on the basis of said input image.

Generally, the third aspect is a method broadly corresponding to the apparatus of the first aspect. Similarly, method embodiments may be provided corresponding to the various embodiments of the first aspect as described above.

Preferred embodiments are defined in the appended dependent claims.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the invention and to show how the same may be carried into effect, reference will now be made, by way of example, to the accompanying drawing, in which:

Figure 1 shows a conventional example of a head-up display;

Figure 2 shows a generalised optical system of a virtual image display using a holographic projector; Figures 3a to 3d show, respectively, a block diagram of a hologram data calculation system, operations performed within the hardware block of the hologram data calculation system, energy spectra of a sample image before and after multiplication by a random phase matrix, and an example of a hologram data calculation system with parallel quantisers for the simultaneous generation of two sub- frames from real and imaginary components of complex holographic sub-frame data;

Figures 4a and 4b show, respectively, an outline block diagram of an adaptive OSPR-type system, and details of an example implementation of the system;

Figure 5 shows a holographic image display system that may be used in any embodiment as a holographic projector comprising the first, holographic SLM;

Figure 6 shows a first embodiment of a High Dynamic Range (HDR) projector based on holographic projection;

Figure 7a shows a first configuration that may be implemented in any embodiment, wherein a diffusing screen is on the side of the second SLM (SLM2) opposite to the first SLM (SLMl);

Figure 7b shows a second configuration that may be implemented in any embodiment, wherein a diffusing screen is in between the first SLM (SLMl) and the second SLM (SLM2);

Figure 8 shows a functional representation of a HUD architecture which may comprise or represent any embodiment of the present invention;

Figures 9 to 11 show, respectively, a colour holographic image projection system, and image, hologram (SLM) and display screen planes illustrating operational features of an embodiment; and

Figure 12 shows a Fresnel diffraction geometry in which a hologram h(x,y) is illuminated by coherent light, and an image H(u,v) is formed at a distance z by Fresnel (or near- field) diffraction.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The following describes, in particular, holographic laser projection for dynamic backlighting. Such backlighting may achieve improved background light levels, for example in regions corresponding to black areas of an input image or areas where the input image does not have information content. Advantageously, an embodiment may allow to achieve this benefit preserving a high optical efficiency due to the non blocking nature of the holographic projection stage. Some example implementations of a holographic image display system employ an OSPR-type hologram generation technique, described later with reference to Figures 3, 4 and 9 to 11. However applications of the techniques we describe are not limited to this method of generating holograms. Figure 5, described later, shows an example of a holographic image projector which may be employed in embodiments of our dynamic backlighting techniques (although the techniques are also applicable to other holographic projectors).

Generally, embodiments may be described as "High Dynamic Range" (HDR) displays. A high dynamic range may provide high contrast between displayed image areas corresponding to "black" and "non-black" regions of an input image. (Throughout this specification, "black" is assumed to be the input image background colour. However, in any embodiment, the background colour may be a different colour, e.g., blue. Thus, references to black in this specification generally refer to the background colour of the input image, e.g., the colour of input image areas that do not comprise information). Thus, HDR displays may be able to display images where a high ratio between the element of highest luminance and the element of lowest luminance is desirable (typically > 1,000 and ideally over 100,000).

One application of an embodiment is a head-up display (HUD). In such a display, relatively little information may be displayed given the size of the projected image. Moreover, it may be advantageous to project black image areas (or a different background colour of the input image) such that a user, e.g., a driver, can still see the outside world properly through such areas. Thus, in a HUD, black image areas may correspond to areas of a projected image that are advantageously substantially transparent to the user. Such a requirement may be fulfilled by a contrast ratio of, e.g., about 400 to about 800.

Regarding the extinction level for HUD displays in particular, Figure 8 illustrates a basic principle of a HUD embodiment. An imager (10) produces an image that is projected by a projection optics (11) onto a semi transparent element called a combiner (2) so that the image emerging from the HUD (1) in the form of light beams (50) superimposes with the image perceived by the user (3) of the outside landscape. Finally, the beams emerging from the landscape (40) and from the HUD (50) add-up and are perceived together for one specific direction ((41)+(51)) behind the combiner.

Generally, any embodiment of the present invention, whether for a HUD display or general projection system, may use two-stage modulation. The first stage may be provided by means of a first spatial light modulator (SLM), which is holographic and may be comprised within a holographic image display system as shown in Figure 5. The second stage may be configured to provide selective amplitude modulation by means of a second SLM in the form of a light valve, e.g., an LCD. Particularly advantageously, FLCOS (ferroelectric liquid crystal on silicon), which may be reflective, may be used for the first and/or second SLM.

Such an embodiment is illustrated schematically in Figure 6, which shows the first embodiment of a High Dynamic Range (HDR) projector based on holographic projection. The HDR projector comprises a projection display 1, an LCD 2 and a holographic projector 3.

Figure 5 shows an example of a holographic image projector 1000 that may be used in any embodiment as a holographic projector comprising the first, holographic SLM. The example projector has a light engine comprising red R, green G, and blue B lasers, and the following additional elements:

• SLM is the hologram SLM (spatial light modulator). In embodiments the SLM may be a liquid crystal device. Alternatively, other SLM technologies to effect phase modulation may be employed, such as a pixellated microelectromechanical system (MEMS)-based piston actuator device, e.g., comprising an array of micromirrors.

• Ll, L2 and L3 are collimation lenses for the R, G and B lasers respectively (optional, depending upon the laser output).

• Ml, M2 and M3 are corresponding dichroic mirrors.

• PBS (Polarising Beam Splitter) transmits the incident illumination to the SLM. Diffracted light produced by the SLM - naturally rotated (with a liquid crystal SLM) in polarisation by 90 degrees - is then reflected by the PBS towards L4.

• Mirror M4 (optional) folds the optical path.

• Lenses L4 and L5 (optional) form an output telescope (demagnifying optics), as with holographic projectors we have previously described. The output projection angle is proportional to the ratio of the focal length of L4 to that of L5. In embodiments L4 may be encoded into the hologram(s) on the SLM, for example using the techniques we have described in WO2007/110668, and/or output lens L5 may be replaced by a group of projection lenses.

• Dl is an optional piezoelectrically-actuated diffuser to reduce speckle, as we have described, for example in GB0800167.9..

• A system controller 1012 performs signal processing in either dedicated hardware, or in software, or in a combination of the two, as described further below. Thus controller 1012 inputs image data (and optionally touch sensed data) and provides hologram data 1014 to the hologram SLM. The controller may also provide laser light intensity control data to each of the three lasers to control the overall laser power in the image. Not shown in Figure 5 is an exit pupil expander which may optionally be employed in a head up display (HUD), as described in GB0902468.8, "Optical Systems", filed on 16 February 2009, hereby incorporated by reference in its entirety. This may comprise a parallel sided waveguide into which light is injected at an angle and which multiply the exit pupil of an HUD by providing a plurality of output beams, tiling the exit pupils, the output beams emerging substantially parallel to one another and tilted with respect to a normal to the parallel sided waveguide.

The light engine of Figure 5 may be used as light source of any embodiment, e.g., the projector 3 of Figure 6. The lenses Ll - L3, mirrors Ml - M3 and PBS of Figure 5 in particular may be used as shown in Figure 5 to input the R,G,B laser light into the hologram SLM of any embodiment.

Processing used to drive the holographic projector on the basis of an input image (e.g., all-to-one Fourier transform conjugation) may allow the hologram to be driven to output an image that approximates the input image. Furthermore, the processing may allow calculation of deviations of the approximation relative to the input image. Such deviations may be considered as errors, which may correspond to residual non-black areas where full black would be desired on the basis of the input image. The light valve may advantageously be controlled to compensate for these errors or to mask such residual non-black areas on the basis of the calculated errors. The algorithm used to determine the driving of the amplitude modulation may vary depending on the choices made. For example, the amplitude modulation may optimize light efficiency at the expenses of uniformity. It can also be optimising uniformity at the expenses of light efficiency.

Thus, in an embodiment, the holographic projector may be controlled to produce an output corresponding to an input image. In some embodiments, the optimised output may however deviate relative to the input image dependent on factors such as processing speed and/or the algorithm used to generate the hologram SLM drive signal(s).

The above error calculation may provide an error for each image pixel of the amplitude modulator, so that the light valve may be used to correct such individual errors. In an embodiment, a transform such as Fourier transform may be applied to the signal(s) used to control the holographic projector, in order to obtain errors corresponding to noise in the holographic projector output.

One or more lasers may be used to obtain light of different preferred colours. Colour filters may then not be required on any of the SLM.

Regarding laser light colours used in an embodiment applied for a HUD, a duty cycle comprising blue, red and green portions may be adjusted to shorten or eliminate a certain colour such that, the other colours portion can be extended to enhance the display of the corresponding image content. In some embodiments, there may be a reduced number of lasers depending on the colours required in a particular use case.

The light valve may be, for example, a liquid crystal device (LCD) or MEMS-based device. Use of an LCD as the light valve may be particularly advantageous if polarized light is conveniently available, e.g, by using lasers. In this case, one polarizer, may be omitted at the light valve level allowing further cost reduction.

One parameter that may influence the efficiency and/or contrast ratio of a display system of an embodiment is the open aperture ratio (OAR). An LCD pixel may have an open aperture ratio (OAR), which be expressed as the ratio of the transmissive area to the total area of the pixel. The OAR may be determined by the proportion of the pixel covered by wiring, control device (e.g., Thin Film Transistor (TFT)) and masking material mounted on the pixel. The OAR may be about 50 - about 60% for a high density monochrome LCD, or about 40% for a colour LCD. (For a colour LCD, a greater proportion of the pixel may be covered by masking material due to additional coloured filters).

Producing the colours at the holographic projector level therefore saves the introduction of coloured filters in the LCD, advantageously making it : more transmissive and therefore more efficient; and/or less expensive (less manufacturing steps, less material required).

Some low performance very inexpensive LCD (e.g. passive matrix) may even be considered for such implementation. Of particular interest in this regard, high contrast ratios are generally not directly obtainable from such inexpensive LCDs. A high contrast ratio may nevertheless obtainable from an embodiment as described herein.

As an alternative to a standard-size LCD, a microdisplay which has relatively small pixels, may be used as the light valve, particularly if the microdisplay is followed by a relay or projection optics, a planar, parallel reflective waveguide or pupil expander.

The light valve may be binary, e.g., a binary LCD or a binary microdisplay such as a ferroelectric microdisplay. In alternative embodiments however an m-ary light valve may be used, where m>2.

Binary modulation may be sufficient in embodiments where the function of the light valve is primarily masking. In other words, the amplitude modulation performed by a light valve pixel uses two states, which respectively correspond to blocking and non-blocking. Such masking may advantageously block light transmission through all light valve pixels that correspond to input image areas not containing information. Light valve pixels corresponding to areas containing information may be controlled to be non- blocking, and optionally to have appropriate grey levels, depending on information present within these areas of the input image. The number of grey levels available may depend on the binary or m-ary nature of the light valve.

A target image to be projected by the holographic SLM on the basis of the input image may comprise some error modelled by a certain noise level. For a non- zero-value region, the NSR may be, e.g., 10- 20%. For a zero-value region, the presence of noise results in an infinite NSR (the signal being zero). The NSR may represent a ratio of averages over time where sub-frames are displayed successively, in particular where One Step Phase Retrieval (OSPR) is employed. A NSR of 10% may correspond to variation of ±10% and thus may be corrected for using only few grey levels distributed to pass most of the light (e.g. 80 to 100% of maximal transmission) in addition to the black level. This may be advantageous, since modulation percentages between, e.g., about 1 and about 80%, may otherwise result in an operating mode where significant light power is produced but is blocked to a large extent, so reducing the overall efficiency of the system.

A light valve such as an LCD may have a relatively narrow viewing angle. For example, the contrast in vertical viewing angles may be approximately halved at an angle of 30 degrees.

To use the light valve in its angle high contrast angles (e.g normally to the surface of the display) a lens may be used A Fresnel lens may advantageously be used to allow light from the holographic projector to address normally the light valve. In this configuration, a diffuser may also be used after the light valve to form the image with a desired angular distribution shape.

Alternatively or additionally, a curved LCD (e.g. an LCD on a flexible substrate) may be employed such that substantially all of the light output from holographic image projector impinges on the LCD normal (i.e., perpendicularly) to the LCD.

This embodiment is particularly suited to implementations where: the throw angle of the projector is not too important (LCDs are generally not meant to be working efficiently with such wide angles), and/or the distance between the LCD and the projection display is not too important (typically <20% of the unfolded distance from the holographic projector to the projection display).

This embodiment may further have some practical advantages in applications where the LCD is to be protected from some harsh environment. In particular, this may be because projection displays can be made much more resistant to some aggressions than some LCDs. One example of such an aggressive environment is in an automotive HUD for which external light entering the optical architecture ends up substantially concentrated on the image source, causing a heat concentration point. If the image source is directly the LCD, this situation may cause a permanent failure of the region hit by the external light, e.g., sunbeams. Keeping the LCD behind a diffuser that incorporates a heat drain material may make the image source resistant to this sort of environmental influence.

Considering further the use of a diffusing screen in any embodiment, two possible configurations are shown in Figs. 7a and 7b.

In a first configuration as shown in Figure 7a, the diffusing screen is on the side of first SLM 2 opposite to the first SLM 1. In this configuration, the holographic projector may be tuned so that the image plane is positioned at a distance between D_SLM2 and D_Dlffuser so that the pixels modulated by SLM2 appear sharp in the diffusing screen. This may particularly be possible due to the large depth of focus of the projector and preferably we aim at having D_Dlffuser not larger than 120% of D_SLM2 (also called throw distance). Note that this distance is along an optical path that can be folded in space. This configuration may be used in conjunction with a Fresnel lens. As mentioned previously, the viewing angles of the light valve not be compatible with the projection angles of the holographic projector.

In a second configuration as shown in Figure 7b, the diffusing screen is between the first SLM 1 and the second SLM 2. In this configuration, the image may be formed on the diffusing screen (1) that has a characteristic emission lobe as represented. This emission may be basically characterized by a certain diffusing angle (half gain angle may be used as a reference value). This means that the image modulated by SLM2 may be blurred compared to the one formed on the diffusing screen. The blurring is increasing with the diffusing power of the diffusing screen and with the distance between the diffusing screen and the second SLM 2 (i.e. D_SLM2 - D_Dlffilser). Practically speaking a setup may have the diffusing screen positioned within the size of an SLM2 pixel behind the SLM2. This may mean that D_SLM2 - D_Dlffiιser< 0.5mm.

Another constraint relating to the configuration of Figure 7b may be the preservation of polarization. The light emerging from the holographic projector (3) may be adequately polarized for use by the second imager (SLM2) in the case of polarization based imaging device like LC-based imagers. In this case, the diffusing screen is advantageously selected so that it preserves as much as possible the polarization of incoming light.

In any embodiment, a desired input image may be converted into sets of holograms by processing according to algorithms, and displayed on a hologram SLM such as a phase modulating microdisplay. Red, green and blue laser light may time-sequentially illuminate the hologram SLM. Light passed through the hologram SLM is passed through the light valve SLM. The projected image from the light valve may be a consequence of Fourier optics, such as the Fourier transform F [h(n)] (where n = 1,...,N) of the pattern on the hologram SLM. Since the image may be created in the far field (or Fraunhofer regime) rather than by means of converging rays, the image may remain in focus at all distances.

A holographic projector of an embodiment may thus have specific properties. In particular, where the image is in focus over a significant distance, an embodiment may have a projection screen and/or diffusing screen placed without undue positioning precision in the neighbourhood of the light valve (e.g., 0.5cm from the light valve rather than needing to be within, e.g. less than lmm of the light valve) without any intermediate optical system and further advantageously without significantly harming the quality of the image produced.

To compensate for mismatch between advantageous image and component sizes at any point in an embodiment such as a HUD, optical systems may be used at appropriate positions. Such optical systems may be a beam expander (magnifying optics) and/or reverse telescope (demagnifying optics) implemented using lenses.

However, regarding the image being in focus over a certain distance as mentioned above, there may be no need to provide optical components specifically to form a focussed image on the light valve and/or to (re-)focus to reimage on the screen.

A first specific embodiment may comprise a first, holographic Spatial Light Modulator (SLM) in the form of the holographic projector, followed by an optical system such as a lens, then a second, light valve SLM, and then projection optics and/or a diffuser. The first and second SLMs correspond to a holographic plane and an image plane. A light source provides light to the holographic SLM, which is controlled depending on an input image(s). The combination of the first and second SLMs and the intervening optical system may be functionally equivalent to a display.

In any embodiment, the holographic projector may be driven to produce video frames at a rate of, e.g., about 30Hz, and sub frames at a higher rate. Taking into account averaging of sub-frames by the eye, the light valve may be operated at the lower, frame rate, e.g., 30Hz rather than the higher sub-frame rate. This is particularly the case where the sub-frames are likely to contribute to the rendering of the same video frame. Thus, the light valve may not need to be operated at a very high frequency.

Brightness control may be implemented in any embodiment, such that the holographic projector determines the brightness of the image while the light valve refines the NSR and/or improves the contrast. Thus, the brightness may be controlled at the projector to produce an effect similar to a dimming backlight. In a particular embodiment, a holographic laser projection system for dynamic backlighting comprises a holographic projection engine that controls the amount of light projected onto a light valve. Each individual pixel of the light valve is controlled to pass a respective portion of light. On the basis of a received input image, a controller may provide a control signal comprising a low-resolution monochrome approximation of the input image. This control signal is used to control the holographic projection engine to modulate light, which may be obtained from one or more lasers, corresponding to the approximation. Furthermore, a controller may provide a control signal comprising differences, and optionally colour, between the approximation and the input image to output light. This control signal is used to control the pixels of the light valve to further intensity modulate the light outputted from the holographic projection engine. The intensity modulation may be binary or m-ary (m>2). Thus, the intensity modulation performed by the light valve may be coarser than the modulation performed by means of the holographic projection engine.

An embodiment comprises a HDR display based on a first stage of holographic projection illuminating a second stage of amplitude modulation. In the first stage, a holographic projection engine may control the amount of light passed from a light source such as laser(s) by a combination of all-to-one Fourier transform conjugation and laser driving. Considering the amplitude modulation stage, the second stage may be a light valve in the form of a transmissive display and/or an LCD display. Thus, an embodiment may use a holographic projector and a light valve such as an LCD display. Where the light valve is transmissive, the light projected by the HDR display system may not be diffused for direct observation at the light valve's level, but may instead be projected through the light valve to form a remote image, e.g., on a projection screen. Similarly, where the light valve is reflective, a combination of optical elements such as a polarisation beam splitter and/or folding mirrors may be used to project the light from the light valve to form a remote image.

A number of advantages may be derived from the first embodiment, such as:

The transmission of state of the art full-colour LCDs may be in the region of 7%. However, a laser-based holographic light source may increase the LCD transmission up to more than 50% (by removing the need for colour filters and addressing the LCD directly with polarized light).

A monochrome amplitude modulating LCD with a small number of greyscales (potentially non-linearly distributed) and low constrast figures may be used because the holographic light engine may produce colour and greyscales. This allows considering low cost LCD technologies.

Contrast levels of more than 10,000:1 (holographic projector 100:1 * LCD 100:1) may be achieved for symbology content (e.g. HUD applications). This may be particularly advantageous for HUD applications during night time conditions. For HUD applications (low image coverage), the overall system efficiency (Holographic laser light engine plus LCD) may be more than 10 times better than state of the art technology with a contrast ratio which is higher than what is currently achieved in existing HUDs.

The manufacturing cost of a monochrome LCD without any colour filters, backlighting and polarizer may be considerably lower for a given technology, additionally, alternative low performance technologies can be considered.

Any embodiment of the invention may use an OSPR-type hologram generation procedure (being referred to in the previous paragraph as the "all-to-one Fourier transform" hologram generation), and we therefore describe examples of such procedures below. However embodiments of the invention are not restricted to such a hologram generation procedure and may be employed with other types of hologram generation procedure including, but not limited to: a Gerchberg-Saxton procedure (R. W. Gerchberg and W. O. Saxton, "A practical algorithm for the determination of phase from image and diffraction plane pictures" Optik 35, 237-246 (1972)) or a variant thereof, Direct Binary Search (M. A. Seldowitz, J. P. Allebach and D. W. Sweeney, "Synthesis of digital holograms by direct binary search" Appl. Opt. 26, 2788-2798 (1987)), simulated annealing (see, for example, M. P. Dames, R. J. Dowling, P. McKee, and D. Wood, "Efficient optical elements to generate intensity weighted spot arrays: design and fabrication," Appl. Opt. 30, 2685-2691 (1991)), or a POCS (Projection Onto Constrained Sets) procedure (see, for example, C. -H. Wu, C. -L. Chen, and M. A. Fiddy, "Iterative procedure for improved computer-generated-hologram reconstruction," Appl. Opt. 32, 5135- (1993)).

OSPR

Broadly speaking, in a preferred embodiment, the first, holographic SLM is modulated with holographic data approximating a hologram of the image to be displayed. However this holographic data is chosen in a special way, the displayed image being made up of a plurality of temporal sub- frames, each generated by modulating the SLM with a respective sub-frame hologram, each of which spatially overlaps in the replay field (in embodiments each has the spatial extent of the displayed image).

Each sub-frame when viewed individually would appear relatively noisy because noise is added, for example by phase quantisation by the holographic transform of the image data. However when viewed in rapid succession the replay field images average together in the eye of a viewer to give the impression of a low noise image. The noise in successive temporal subframes may either be pseudorandom (substantially independent) or the noise in a subframe may be dependent on the noise in one or more earlier subframes, with the aim of at least partially cancelling this out, or a combination may be employed. Such a system can provide a visually high quality display even though each sub-frame, were it to be viewed separately, would appear relatively noisy.

The procedure is a method of generating, for each still or video frame I = I_xy, sets of N binary-phase holograms h⁽¹⁾... h^. In embodiments such sets of holograms may form replay fields that exhibit mutually independent additive noise. An example is shown below:

1. Let G_x ⁽"^] = I_xy exp ^φ^' ) where φ^ is uniformly distributed between 0 and 2π for 1 < n ≤ Ni l and 1 < x,y ≤ m

2. Let g^ = F^~l G_x"^ where F^~ represents the two-dimensional inverse Fourier transform operator, for 1 < n ≤ N 12

3. Let rn^ = ^{g^⁽ } ϊor \ ≤ n ≤ NI 2

4. Let m^;^+Λr/2) = 3 {gL^B)} for l ≤ n ≤ N/ 2 -l if m⁽ⁿ⁾ < O⁽ⁿ⁾ J^m where Q⁽ⁿ⁾ = median (m™ ) and 1 < n < N .

Step 1 forms N targets G_x" ^} equal to the amplitude of the supplied intensity target I_xy, but with independent identically- distributed (i.i.t), uniformly-random phase. Step 2 computes the N corresponding full complex Fourier transform holograms g^ . Steps 3 and 4 compute the real part and imaginary part of the holograms, respectively. Binarisation of each of the real and imaginary parts of the holograms is then performed in step 5: thresholding around the median of m _v" ensures equal numbers of -1 and 1 points are present in the holograms, achieving DC balance (by definition) and aallssoo mmiinniimmaall rreeccoonnssttrruuccttiioonn eerrrroorr,. The median value of m _v" may be assumed to be zero with minimal effect on perceived image quality.

Figure 3a, from our WO2006/134398, shows a block diagram of a hologram data calculation system configured to implement this procedure. The input to the system is preferably image data from a source such as a computer, although other sources are equally applicable. The input data is temporarily stored in one or more input buffer, with control signals for this process being supplied from one or more controller units within the system. The input (and output) buffers preferably comprise dual-port memory such that data may be written into the buffer and read out from the buffer simultaneously. The control signals comprise timing, initialisation and flow-control information and preferably ensure that one or more holographic sub-frames are produced and sent to the SLM per video frame period. The output from the input comprises an image frame, labelled I, and this becomes the input to a hardware block (although in other embodiments some or all of the processing may be performed in software). The hardware block performs a series of operations on each of the aforementioned image frames, I, and for each one produces one or more holographic sub-frames, h, which are sent to one or more output buffer. The sub-frames are supplied from the output buffer to a display device, such as the first, holographic SLM, optionally via a driver chip.

Figure 3b shows details of the hardware block of Figure 3 a; this comprises a set of elements designed to generate one or more holographic sub-frames for each image frame that is supplied to the block. Preferably one image frame, I_xy, is supplied one or more times per video frame period as an input. Each image frame, I_xy, is then used to produce one or more holographic sub-frames by means of a set of operations comprising one or more of: a phase modulation stage, a space-frequency transformation stage and a quantisation stage. In embodiments, a set of N sub-frames, where N is greater than or equal to one, is generated per frame period by means of using either one sequential set of the aforementioned operations, or a several sets of such operations acting in parallel on different sub- frames, or a mixture of these two approaches.

The purpose of the phase-modulation block is to redistribute the energy of the input frame in the spatial- frequency domain, such that improvements in final image quality are obtained after performing later operations. Figure 3c shows an example of how the energy of a sample image is distributed before and after a phase-modulation stage in which a pseudo-random phase distribution is used. It can be seen that modulating an image by such a phase distribution has the effect of redistributing the energy more evenly throughout the spatial-frequency domain. The skilled person will appreciate that there are many ways in which pseudo-random binary-phase modulation data may be generated (for example, a shift register with feedback).

The quantisation block takes complex hologram data, which is produced as the output of the preceding space- frequency transform block, and maps it to a restricted set of values, which correspond to actual modulation levels that can be achieved on a target SLM (the different quantised phase retardation levels may need not have a regular distribution). The number of quantisation levels may be set at two, for example for an SLM producing phase retardations of 0 or π at each pixel.

In embodiments the quantiser is configured to separately quantise real and imaginary components of the holographic sub-frame data to generate a pair of holographic sub-frames, each with two (or more) phase-retardation levels, for the output buffer. Figure 3d shows an example of such a system. It can be shown that for discretely pixellated fields, the real and imaginary components of the complex holographic sub-frame data are uncorrelated, which is why it is valid to treat the real and imaginary components independently and produce two uncorrelated holographic sub-frames. An example of a suitable binary phase SLM is the SXGA (1280x1024) reflective binary phase modulating ferroelectric liquid crystal SLM made by CRL Opto (Forth Dimension Displays Limited, of Scotland, UK). A ferroelectric liquid crystal SLM is advantageous because of its fast switching time. Binary phase devices are convenient but some preferred embodiments of the method use so- called multiphase spatial light modulators as distinct from binary phase spatial light modulators (that is SLMs which have more than two different selectable phase delay values for a pixel as opposed to binary devices in which a pixel has only one of two phase delay values). Multiphase SLMs (devices with three or more quantized phases) include continuous phase SLMs, although when driven by digital circuitry these devices are necessarily quantised to a number of discrete phase delay values. Binary quantization results in a conjugate image whereas the use of more than binary phase suppresses the conjugate image (see WO 2005/059660).

Adaptive OSPR

In the OSPR approach we have described above subframe holograms are generated independently and thus exhibit independent noise. In control terms, this is an open-loop system. However one might expect that better results could be obtained if, in any embodiment, instead, the generation process for each subframe took into account the noise generated by the previous subframes in order to cancel it out, effectively "feeding back" the perceived image formed after, say, n OSPR frames to stage n+1 of the algorithm. In control terms, this is a closed-loop system.

One example of this approach comprises an adaptive OSPR algorithm which uses feedback as follows: each stage n of the algorithm calculates the noise resulting from the previously-generated holograms H₁ to H_n.], and factors this noise into the generation of the hologram H_n to cancel it out. As a result, it can be shown that noise variance falls as 1/N². An example procedure takes as input a target image T, and a parameter N specifying the desired number of hologram subframes to produce, and outputs a set of N holograms H₁ to H_N which, when displayed sequentially at an appropriate rate, form as a far-field image a visual representation of T which is perceived as high quality:

An optional pre-processing step performs gamma correction to match a CRT display by calculating T(x, y)^{1 3}. Then at each stage n (of N stages) an array F (zero at the procedure start) keeps track of a "running total" (desired image, plus noise) of the image energy formed by the previous holograms H₁ to H_n.] so that the noise may be evaluated and taken into account in the subsequent stage:

F(x, y) := F(x, y) + ^[H_n-1 (x, y)]\ . A random phase factor φ is added at each stage to each pixel of the target image, and the target image is adjusted to take the noise from the previous stages into account, calculating a scaling factor α to match the intensity of the noisy "running total" energy F with the target image energy (T')². The total noise energy from the previous n - \ stages is given by <xF- (n

∑τ'(^χ,y)⁴

- l)(r') , according to the relation CC :=

and therefore the target energy at this stage is given by the difference between the desired target energy at this iteration and the previous noise present in order to cancel that noise out, i.e. (T') - [ocF - (n - l)(r') ] = n(T') + ocF. This gives a target amplitude \T"\ equal to the square root of this energy value, i.e. > oF

e

At each stage n, H represents an intermediate fully-complex hologram formed from the target T" and is calculated using an inverse Fourier transform operation. It is quantized to binary phase to form the output hologram H_n, i.e.

J l if Re[H (x,y)] > 0 [-1 otherwise

Figure 4a outlines this method and Figure 4b shows details of an example implementation, as described above.

Thus, broadly speaking, an ADOSPR-type method of generating data for displaying an image (defined by displayed image data, using a plurality of holographically generated temporal sub frames displayed sequentially in time such that they are perceived as a single noise-reduced image), comprises generating from the displayed image data holographic data for each sub frame such that replay of these gives the appearance of the image, and, when generating holographic data for a subframe, compensating for noise in the displayed image arising from one or more previous subframes of the sequence of holographically generated subframes. In embodiments the compensating comprises determining a noise compensation frame for a subframe; and determining an adjusted version of the displayed image data using the noise compensation frame, prior to generation of holographic data for a subframe. In embodiments the adjusting comprises transforming the previous subframe data from a frequency domain to a spatial domain, and subtracting the transformed data from data derived from the displayed image data.

More details, including a hardware implementation, can be found in WO2007/141567 hereby incorporated by reference. Colour holographic image projection

The total field size of an image scales with the wavelength of light employed to illuminate the SLM, red light being diffracted more by the pixels of the SLM than blue light and thus giving rise to a larger total field size. Naively a colour holographic projection system could be constructed by superimposed simply three optical channels, red, blue and green but this is difficult because the different colour images must be aligned. A better approach in any embodiment is to create a combined beam comprising red, green and blue light and provide this to a common SLM, e.g., the first, holographic SLM, scaling the sizes of the images to match one another.

Figure 9 shows an example colour holographic image projection system 1000, here including demagnification optics 1014 which project the holographically generated image onto a screen 1016. The system comprises red 1002, green 1006, and blue 1004 collimated laser diode light sources, for example at wavelengths of 638nm, 532nm and 445nm, driven in a time-multiplexed manner. Each light source comprises a laser diode 1002 and, if necessary, a collimating lens and/or beam expander. Optionally the respective sizes of the beams are scaled to the respective sizes of the holograms, as described later. The red, green and blue light beams are combined in two dichroic beam splitters 1010a, b and the combined beam is provided (in this example) to a reflective spatial light modulator 1012; the figure shows that the extent of the red field would be greater than that of the blue field. The total field size of the displayed image depends upon the pixel size of the holographic SLM but not on the number of pixels in the hologram displayed on the holographic SLM.

Figure 10 shows padding an initial input image with zeros in order to generate three colour planes of different spatial extents for blue, green and red image planes. A holographic transform is then performed on these padded image planes to generate holograms for each sub-plane; the information in the hologram is distributed over the complete set of pixels. The hologram planes are illuminated, optionally by correspondingly sized beams, to project different sized respective fields on to the display screen. Figure 11 shows upsizing the input image, the blue image plane in proportion to the ratio of red to blue wavelength (638/445), and the green image plane in proportion to the ratio of red to green wavelengths (638/532) (the red image plane is unchanged). Optionally the upsized image may then be padded with zeros to a number of pixels in the holographic SLM (preferably leaving a little space around the edge to reduce edge effects). The red, green and blue fields have different sizes but are each composed of substantially the same number of pixels, but because the blue, and green images were upsized prior to generating the hologram a given number of pixels in the input image occupies the same spatial extent for red, green and blue colour planes. Here there is the possibility of selecting an image size for the holographic transform procedure which is convenient, for example a multiple of 8 or 16 pixels in each direction.

Lens encoding

We now describe encoding lens power into the hologram by means of Fresnel diffraction, as may be implemented in any embodiment.

We have previously described systems using far-field (or Fraunhofer) diffraction, in which the replay field F and hologram h_uv are related by the Fourier transform:

F_xy = F[hJ (1)

In the near-field (or Fresnel) propagation regime, RPF and hologram are related by the Fresnel transform which, using the same notation, can be written as:

F_xy = FR[h_uv] (2)

The discrete Fresnel transform, from which suitable binary-phase holograms can be generated, is now introduced and briefly discussed.

Referring to Figure 12, the Fresnel transform describes the diffracted near field F(x, y) at a distance z , which is produced when coherent light of wavelength λ interferes with an object h(u,v) . This relationship, and the coordinate system, is illustrated in the Figure. In continuous coordinates, the transform is defined as:

where x = (x,y) and u = (u,v) , or

This formulation is not suitable for a pixellated, finite-sized hologram h_x , and is therefore discretised. This discrete Fresnel transform can be expressed in terms of a Fourier transform

where

and

In effect the factors /⁷^ and F*²⁾ in equation (5) turn the Fourier transform in a Fresnel transform of the hologram h. The size of each hologram pixel is A_x xΔ , and the total size of the hologram is (in pixels) NxM . In equation (7), z defines the focal length of the holographic lens. Finally, the sample spacing in the replay field is:

λz

K =

^NAx

(8) λz

A = NA,.

_λz_ y _λz_ so that the dimensions of the replay field are Δ Δ , consistent with the size of replay field in the Fraunhofer diffraction regime.

The above OSPR algorithm, which may be implemented in any embodiment, can be generalised to the case of calculating Fresnel holograms by replacing the Fourier transform step by the discrete Fresnel transform of equation 5. Comparison of equations 1 and 5 show that the near- field propagation regime results in different replay field characteristics. One advantage associated with binary Fresnel holograms is that the diffracted near-field does not contain a conjugate image. In the Fraunhofer diffraction regime the replay field is the Fourier transform of the real term h_uv , giving rise to conjugate symmetry. In the case of Fresnel diffraction, however, equation 5 shows that the replay field is the Fourier transform of the complex term F^h_uv . It can be seen from equation 4 that the diffracted field resulting from a Fresnel hologram is characterised by a propagation distance z ,so that the replay field is formed in one plane only, as opposed to everywhere where z is greater than the Goodman distance [J. W. Goodman, Introduction to Fourier Optics, 2nd ed. New York: McGraw-Hill, 1996, ch. The Fraunhofer approximation, pp. 73- 75] in the case of Fraunhofer diffraction. This indicates that a Fresnel hologram incorporates lens power (a circular structure can be seen in a Fresnel hologram). Further, the focal plane in which the image is formed can be altered by recalculating the hologram rather than changing the entire optical design.

There can be an increase in SNR when using Fresnel holograms in a procedure which takes the real (or imaginary) part of the complex hologram, because the Fresnel transform is not conjugate symmetric. However error diffusion, for example, may be employed to mitigate this - see our WO 2008/001137 and WO2008/059292. The use of near- field holography also results in a zero-order which is approximately the same size as the hologram itself, spread over the entire replay field rather than located at zero spatial frequency as for the Fourier case. However this large zero order can be suppressed either with a combination of a polariser and analyzer or, for example, by processing the hologram pattern (see C. Liu, Y. Li, X. Cheng, Z. Liu, et ah, "Elimination of zero-order diffraction in digital holography," Optical Engineering, vol. 41, 2002).

We now describe an implementation of a hologram processor to be used in any embodiment, in this example using a modification of the above described OSPR procedure, to calculate a Fresnel hologram using equation (5). Other OSPR-type procedures may be similarly modified.

Referring back to steps 1 to 5 of the above described OSPR procedure, step 2 was previously a two- dimensional inverse Fourier transform. To implement a Fresnel hologram, also encoding a lens, as described above an inverse Fresnel transform is employed in place of the previously described inverse Fourier transform. The inverse Fresnel transform may take the following form (based upon equation (5) above):

F -1 H.

F (⁽i^l)

F (2)

Similarly the transform shown in Figure 3b is a two-dimensional inverse Fresnel transform (rather than a two-dimensional FFT) and, likewise the transform in Figure 3d is a Fresnel (rather than a Fourier) transform. In the hardware a one- dimensional FFT block is replaced by an FRT (Fresnel transform) block and the scale factors F_xy and F_uv mentioned above are preferably incorporated within the block.

Aberration correction

The procedure of Figure 3d may be modified to perform aberration correction for a HDR display embodiment such as a HUD. The additional step is to multiply the hologram data by a conjugate of the distorted wavefront, which may be determined from a ray tracing simulation software package such as ZEMAX. In some preferred embodiments the (conjugate) wavefront correction data is stored in non- volatile memory. Any type of non- volatile memory may be employed including, but not limited to, Flash memory and various types of electrically or mask programmed ROM (Read Only Memory). There are a number of ways in which the wavefront correction data may be obtained. For example a wavefront sensor may be employed to determine aberration in a physical model of the optical system by employing a wavefront sensor such as a Shack-Hartman or interferogram-based wavefront sensor. By employing this data in a holographic image projection system broadly of the type previously described a display may also be tailored or configured for a particular user.

In some embodiments the wavefront correction may be represented in terms of Zernike modes. Thus a wavefront W = exp (i Ψ) may be expressed as an expansion in terms of Zernike polynomials as follows:

W = exp (i Ψ) = exp (H)

Where Z₇ is a Zernike polynomial and a_} is a coefficient of Z₇. Similarly a phase conjugation of the Ψ_c of the wavefront Ψ may be represented as:

Ψ_c = ∑C_J Z_J (12) j

For correcting the wavefront preferably Ψ_c » Ψ. Thus for (uncorrected) hologram data g_uv (although h_uv is also used above with reference to lens encoding), the corrected hologram data g_uv ^c can be expressed as follows:

g_a ^c _v = exp (i Ψ_c) g_av (13) For further details, reference may be made to our WO 2008/120015, hereby incorporated by reference.

No doubt many other effective alternatives will occur to the skilled person. It will be understood that the invention is not limited to the described embodiments and encompasses modifications apparent to those skilled in the art lying within the spirit and scope of the claims appended hereto. For example, the features shown in the figures may by present in any combination in an embodiment.

Claims

CLAIMS:

1. Display apparatus for displaying a substantially two-dimensional input image, the display apparatus comprising:

a first spatial light modulator configured to form a hologram on the basis of a first drive signal to phase modulate a beam of substantially coherent light;

a second spatial light modulator comprising a plurality of pixels, the pixels configured to selectively amplitude modulate on the basis of a second drive signal the light modulated by the hologram; and

a controller configured to generate said second drive signal on the basis of said input image.

2. Display apparatus of claim 1 , wherein said second spatial light modulator is binary.

3. Display apparatus of claim 1, wherein said second spatial light modulator is M-ary, where M > 2.

4. Display apparatus of any preceding claim, wherein said controller is configured to generate said second drive signal on the basis of said input image.

5. Display apparatus of claim 4, wherein said controller is further configured to generate said second drive signal on the basis of data determining said hologram.

6. Display apparatus of claim 5, wherein said controller is configured to generate said second drive signal by performing a Fourier transform on said first drive signal.

7. Display apparatus of any preceding claim, wherein: a said pixel of said second spatial modulator is configured to receive a portion of the light modulated by the hologram; said controller is configured to determine a target grey level on the basis of a region which is a part or substantially the whole of the input image; and said controller is configured such that said second drive signal causes said pixel to amplitude modulate said portion according to said target grey level.

8. Display apparatus of claim 7, wherein said controller is configured to determine said target grey level on the basis of a colour of said region, such that said amplitude modulation substantially fully blocks said light portion if said colour is a predetermined colour.

9. Display apparatus of claim 8, wherein said controller is further configured to determine said target grey level on the basis of said colour of said region such that said amplitude modulation blocks about 0% to about 20% of said light portion depending on the target grey level if said colour is a colour other than said predetermined colour.

10. Display apparatus of any preceding claim, wherein said second spatial light modulator is transmissive or reflective.

11. Display apparatus of any preceding claim, wherein said second spatial light modulator comprises liquid crystal on silicon or a MEMS array.

12. Display apparatus of any preceding claim, wherein said first spatial light modulator is transmissive or reflective.

13. Display apparatus of any preceding claim, wherein said first spatial light modulator comprises liquid crystal on silicon or a MEMS array.

14. Display apparatus of any preceding claim, wherein said first spatial light modulator comprises a microdisplay and/or said second spatial light modulator comprises a microdisplay.

15. Display apparatus of any preceding claim, wherein the display apparatus further comprises magnifying and/or demagnifying optics.

16. Display apparatus of any preceding claim, further comprising a controller configured to generate said first drive signal on the basis of said input image.

17. Display apparatus of claim 16, wherein said controller configured to generate said first drive signal is further configured to perform said first drive signal generation by One Step Phase Retrieval on the basis of said input image.

18. Display apparatus of any preceding claim, further comprising a laser configured to generate said beam of substantially coherent light.

19. Display apparatus of any preceding claim, further comprising a projection screen.

20. Display apparatus of any preceding claim, further comprising a diffusing screen between the first spatial light modulator and the second spatial light modulator.

21. Display apparatus of any preceding claim, further comprising a diffusing screen configured to receive light modulated by said second spatial light modulator.

22. Display apparatus of claim 21, further comprising a holographic projector, which comprises said first spatial light modulator and has a throw distance, wherein a distance between said diffusing screen and said second spatial light modulator is less than about 20% of said throw distance.

23. A head-up display comprising display apparatus of any preceding claim.

24. A method of displaying a substantially two-dimensional input image, the method comprising: phase modulating on the basis of a first drive signal a beam of substantially coherent light, using a hologram comprised in a first spatial light modulator; selectively amplitude modulating on the basis of a second drive signal the light modulated by the hologram, using a second spatial light modulator; and generating said second drive signal on the basis of said input image.