GB2518664A - Projector - Google Patents

Abstract

Projector comprising Spatial Light Modulator (SLM) 705 using first image data and second data representing a Programmable Fresnel Lens (PFL) that performs optical Fourier transform of first data to form holographic image reconstruction at replay field 709, wherein physical optical element 707 is arranged between SLM and replay field to form telecentric image plane at replay field 711. SLM pixels may be phase-delay elements arranged in two-dimensional array and spatially-modulated light may be spatial distribution of phase-delays. The distance between SLM and replay field may equal PFL focal length. Optical element may be plano-convex or a bi-convex, meniscus or aspheric lens. Data may be combined using vector addition, stored in a repository and processed in real time. Laser light source 701 may illuminate a Liquid Crystal On Silicon (LCOS) SLM. A phase-only hologram is provided in a compact system and a Gerchberg-Saxton algorithm may compute the data. Monochromatic or colour 2D holographic reconstruction may be provided using frame-sequential colour (FSC) or spatially separate colours (SSC). Telecentricity provides improved measurement accuracy due to reduced perspective error reduced distortion, increased resolution and increased illumination uniformity at the replay field. The holographic projector may be used for an in-vehicle heads-up display (HUD).

Description

PROJECTOR

Field of the invention

S The present disclosure relates to the field of projection. Embodiments disclosed herein generally relate to holographic image projection using a spatial light modulator and a method of operating a spatial light modulator.

Background

Light scattercd from an object contains both amplitude and phase information. This amplitude and phase information can be captured on, for example, a photosensitive plate by well-known interference techniques to fonn a holographic recording, or "hologram", comprising interference fringes. The "hologram" may be reconstructed 1 5 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 ftrther

I

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 "pixellated". 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-gencrated 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 IC 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 Gerchberg-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 in the Fourier domain may be calculated.

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

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 (111). 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 wavefi'ont (112) is applied to optics including a Fourier transform (F'!') lens (120), having its focus at a screen (125) placed in the focal plane of the FT lens.

The Fourier transfonn 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 Ia(x,y), in the planes A and B respectively, are known and IA(x,y) and IB(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, 4A(x,y) and Fu(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 Tp(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, for example.

Figure 2 shows a system in which a physical lens is used to perform an optical Fourier transform of the Fourier domain image data encoded th the spatially modulated light output from the SLM.

In more detail, figure 2 shows a laser 201 arranged to illuminate a collimating lens 203 to form collimated light. The collimated light is incident upon a SLM 205.

Figure 2 shows a transmissive SLM but the SLM may equally be reflective. SLM 205 outputs spatially modulated light in transmission. The spatially modulated light is received by a Fourier transform lens 207 having a focal length off and positioned at a distance f from the SLM. The Fourier transform lens 207 forms a replay field 209 at a thither distance off Notably, the Fourier transform lens is equidistance between SLM 205 and replay field 209. The distance from SLM 205 to replay field 209 is 2f The system may therefore be referred to as a "2f" system. As shown in magnified window 211, light arriving at replay field 209 is telecentric because of the symmetry in this arrangement.

The system shown in figure 2 is, however, relatively long. This is disadvantageous in practical situations in which compactness is important. It is, in fact, desirable to produce a so-called "I f'configuration without compromising in other areas.

The present disclosure aims to address these problems and provide an improved pro jcctor.

Summary of the invention

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

The present disclosure relates to a hybrid device which is more compact than known devices but provides teleeentrieity at the replay field for improved measurement accuracy at the replay field. This is achieved owing to a reduction/elimination of perspective error, a reduction in distortion, an increase in the image resolution and an increase in the illumination uniformity at the replay field. The inventors have recognised that the projector in accordance with the present disclosure may be used in applications requiring accuracy at the replay field without compromising on compactness. In particular, the inventors have provided a device which contains the same number of physical components as known devices but provides a telecentric replay field and a so-called "If" footprint. The device is considered to be a hybrid device because it comprises a programmable Fresnel lens encoded on the SLM anda physical optical element between the SLM and replay field for further processing the reconstruction light.

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 is a so-called 2f system with a physical Fourier transform lens equidistant

S between the SLM and replay field;

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 shows a pseudo-If system in which the Fourier transform lens is positioned close to the SLM; figure 6 shows a so-called lens-less if system comprising a programmable Frcsnel lens; figure 7 shows an embodiment of the present disclosure; and figure 8 is a schematic of a LCOS SLM.

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

Detailed description of the drawings

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 Gerehberg-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 jr[u,v] of the Fourier transform of the data set which gives rise to a known amplitude information T[x,yJ 362. Amplitude information T{x,y] 362 is representative of a target image (e.g. a photograph). The phase information iv[u,v] 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 in[brmnation 301 and phase information 303) and a complex wave output (also having amplitude information 311 and phase information 3 13). 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 ftinctions of the spatial coordinates (x,y) for the farfield 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 iji[u,vj 305.

The amplitude information from processing block 350 is set to a distribution representative of the light source but the phase information w11[u,vI 305 is retained, Phase information 305 is quantised by processing block 354 and output as phase information y[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 transi'orrn.

This produces a fourth data set R[x,yJ 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 R11x,yJ 311 is modified by subtraction from amplitude information T[x,y] 362 from the target image to produce an amplitude information 315 set, Sealed 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: I, [x,y] = F'{exp(içv,, [it, vI)} w.Lu.vJ = exp(IZR,,[x,y])} 1 =T[x,y]-cx(JR[x,yJ-T[x,y]) Where: F' is the inverse Fourier transfonu.

F if the forward Fourier transfonn.

Risthereplayfield.

Tisthetargetimage.

Z is the angular information.

F is the quantized version of the angular Information.

s is the new target magnitude, »= 0 aisagainelement-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 infonnation from the preceding iteration, the first iteration of the algorithm uses a random phase generator to supply random phase infonnation 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 MI 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. Embodiments relate to phase-holograms by way of example only and it may be appreciated that the present disclosure is equally applicable to amplitude holograms. Embodiments relate to calculating the phase-hologram using a Gerchberg-Saxton algorithm or modified Gerchberg-Saxton algorithni but the present disclosure is not limited to such holograms. The present disclosure relates to an S improved system for reconstructing the hologram regardless of how it was calculated.

Figure 5 shows a configuration in which the Fourier transform lens 507 is moved close to SLM 505 such that the overall length of the system has been reduced to approximately if. Such a system is advantageous in many applications owing to its compactness.

In more detail, figure 5 shows a laser 501 arranged to illuminate a collimating lens 503 to form collimated light. The collimated light is incident upon a SLM 505.

Figure 5 shows a transrnissivc SLM but the SLM may equally be reflective, SLM 505 outputs spatially modulated light in transmission. The spatially modulated light is received by a Fourier transform lens 507 having a focal length off and positioned close to SLM 505. The Fourier transform lens 507 forms a replay field 509 at a distance f from the Fourier transform lens 507.

However, the inventors have recognised that such a system is disadvantageous in other applications because the reduced-footprint optical configuration results in a non-telecentric replay field -as shown in magnified window 511. A non-telecentric replay field is disadvantageous in applications in which accurate measurements at the rcplay field are required. In particular, non-telecentricity introduces inaccuracies in measurements made at the replay field. By way of example only, telecentricity at the replay field may be important when the projector is used for optical correlation or coupled to another optical system, for example.

It has been recognised that the physical Fourier transform lens 507 may be replaced by a programmable Fresnel lens, PFL, encoded at the SLM. The PFL comprises Fourier domain data having a lensing effect. hi this way, the PFL may be arranged to perform optical processing. In fact, the PFL may replace the physical Fourier transform lens 507.

More specifically, data characterising the PFL may be combined with image data to simultaneously perform a Fourier transform of the image data when the SLM is illuminated. Fourier domain data representative of an image may be combined with Fourier domain data having a lcnsing effect by simple vector addition, br example. it is known in the art how to calculate a PFL and how to combine PFL data with image data to perform a Fourier transform without a physical lens.

Figure 6 shows a if lens-less system.

In more detail, figure 6 shows a laser 601 arranged to illuminate a collimating lens 603 to form collimated light. The collimated light is incident upon a SLM 605. The SLM 505 is arranged to spatially modulate the collimated light with data comprising PFL data and Fourier domain image data, Figure 6 shows a transmissivc SLM but the SLM may equally be reflective, SLM 605 outputs spatially modulated light in transmission, The spatially modulated light fomis a replay field 609 at a distance f, wherein f is the focal length of the PFL. Nevertheless, as shown in magnified window 611, light arriving at replay field 609 is non-telecentric.

In summary, the inventors have recognised that a telecentric lf system may be provided by using a novel hybrid system in which a physical lens is comb/nec/with a PFL.

Figure 7 shows an embodiment utilising both a PFL and a physical lens between the

SLM and replay field.

In more detail, figure 7 shows a laser 701 an'anged to illuminate a collimating lens 703 to form collimated light. The collimated light is incident upon a SLM 705. [he SLM 705 is arranged to spatially modulate the collimated light with data comprising PFL data and Fourier domain image data. Figure 7 shows a transmissive SLM but the SLM may equally be reflective. SLM 705 outputs spatially modulated light in transmission, The spatially modulated light forms a replay field 709 at a distance 1, wherein t' is the focal length of the PFL. Notably, however, the system further comprises a further physical optic 707 arranged to form a telecentric relay field.

The inventors have therefore provided a hybrid system in which additional physical components are not required but telecentricity is provided. This has been acheived by using a hybrid design utilising both a PFL and a physical optical element. In particular, improvements are provided by using an optical element arranged between the spatial light modulator and the replay field to form a telecentric image

plane/replay field.

Whilst the skilled person may know how to design and position an optic to achieve tclecentricity, it was only known in the art to use a PFL to replace the physical Fourier transform lens. For at least this reason, it was not obvious to use both a PFL and a physical optical element. The inventors have identified that by retaining the additional physical optical element but using it differently, a If telecentric system may be provided without requiring more physical components than shown in figures 2 and 5.

There is therefore provided a projector comprising a spatial light modulator, "SLM", comprising a plurality of pixels arranged to receive light and spatially modulate the light in accordance with data provided to the SLM to form spatially modulated light, wherein the data comprises first data representative of an image and second data representative of a programmable Fresnel lens, "PFL", arranged to perform an optical Fourier transform of the first data and thereby form a holographic rcconstruction of the image at a replay field; and an optical element arranged between the spatial light modulator and the replay field to form a telecentric image plane at the replay field.

Correspondingly, there is provided a method of operating a SLM, the method comprising: receiving data wherein the data comprises first data representative of an image and second data representative of a PFL; receiving light and spatially modulate the light in accordance with the data; performing an optical Fourier transform of the first data using the PFL; forming a holographic reconstruction of the image at a replay field; and providing teleccntricity at the reply field using an optical element arranged between the spatial light modulator and the replay field.

It may therefore be understood that the PFL is arranged to perform a Fourier transform of the first data when the SLM is illuminated. The Fourier transform of the first data produces a holographic reconstruction of an image. It may therefore be said that the first data is in the Fourier or frequency domain, as will be well understood by the person skilled in the art of Fourier holography, That is, in embodiments, the first data representative of an image is Fourier domain data representative of an image.

In embodiments, the hologram is a phase-only hologram and the SLM therefore encodes the incident light with a phase-delay distribution. Accordingly, in embodiments, the pixels (of the SLM) are phase-delay elements. In these embodiments, the spatially modulated light comprises a spatial distribution of phase-delays. However, the present disclosure is not limited to phase-only holography and, in other embodiments, the SLM comprises amplitude-only or amplitude and phase modulating elements.

It can be understood from the foregoing that the inventors have provided a more 1 5 compact optical system in which the overall distance from SLM to replay field is reduced from 2f to 1f wherein f is the focal length of the Fourier lens or PFL. In embodiments, the distance between the spatial light modulator and replay field is substantially equal to the focal length of the PFL.

In embodiments, the optical element is a bi-convex lens. In other embodiments, the optical element takes other forms such as meniscus, bi-convex, asphcric. In embodiments, the optical clement is computer optimised.

It is known how to combine PFL data with image data. In an embodiment, the data is formed by vector addition of the first data and second data.

In an embodiment, the plurality of pixels are arranged to form a two-dimensional array.

Optionally, the PFL data and/or image data may be stored in a data repository such as a database and sent to the SLM when required. That is, in an embodiment, the device Further comprises a repository of first and/or second data.

Alternatively or additionally, the first and/or second data may be calculated on-the-fly using a computational algorithm such as the Gerchberg-Saxton algorithm or a modified version of the Gerchberg-Saxton algorithm. For example, the system may further comprise image capture components and be arranged to calculate a hologram corresponding to a captured image for projection purposed. Therefore, in embodiments, the device further comprises a processor arranged to calculate the first and/or second data substantially in real-time.

In an embodiment, the device further comprises a light source arranged to illuminate the spatial light modulator. In embodiments, the SLM is illuminated with a plane wave of light or substantially plane wave of light. Although embodiments relate to light, the skilled person will understand that the present disclosure is equally applicable to radiation in other parts of the electromagnetic spectrum.

In accordance with the present disclosure, the light may be spatially modulated using a spatial light modulator such as a liquid crystal on silicon SLM. Tt can be understood that the holographic data is written to the SLM such that an incident plane wave of light is spatially modulated with the holographic data. In this respect, it may be considered that the pixels of the SLM "display" or "represent" the holographic data.

The quality of the reconstructed hologram may be affected by the so-called zero order problem which is a consequence of the diffractive 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.

However as the hologram contains three dimensional information, it is possible to displace the reconstruction into a different plane in space -see, for example, published PCT application WO 2007/131649 incorporated herein by reference, 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 ftrther divided up into a number 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 fiwther apart the reconstructed pixels are separated when the image is produced. The image is created within the zeroth diffraction 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 olan 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 usc fractions of a tile.

Although embodiments relate to variants of the Gerchberg-Saxton algorithm, the skilled person will understand that other phase retrieval algorithms may implement the improved method disclosed herein.

The skilled person will understand that the improved method disclosed herein is equally applicable to the reconstruction of a two-dimensional or a three-dimensional image.

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 known as "frame-sequential colour" (FSC). In an FSC system, three lasers are used (red, green and blue) and each laser is fired in succession at the SLM to produce each frame of the video, The colours are cycled (red, green, blue, red, green, blue, ctc.) 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 thc 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 fired 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 med 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 S 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 equal 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 1 0 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.

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

LCOS devices arc now available with pixels between 3jxni and 20 pin.

The structure of an LCOS device is shown in Figure 8.

A 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 (801a), 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 (801 a). By control of tile 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 thercon. 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 transniissive devicc 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 diffi action 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 devicc. 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.

The above device typically operates within a temperature range of 10°C to around 50°C, with the optimum device operating temperature being around 40°C to 50°C, depending however on the LC composition used..

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 SLMs.

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

Claims (11)

1. Claims 1. A projector comprising: a spatial light modulator, "SLM", comprising a plurality of pixels arranged to receive light and spatially modulate the light in accordance with data provided to the SLM to form spatially modulated light, wherein the data comprises first data representative of art image and second data representative of a programmable Fresnel lens, "PFL", arranged to perform an optical Fourier transform of the first data and thereby form a holographicreconstruction of the image at a replay field; andan optical clement arranged between the spatial light modulator and the replay field to form a telecentric image plane at the replay field.
2. 2. A projector as claimed in claim 1 wherein the first data representative of an image is Fourier domain data representative of an image.
3. 3. A projector as claimed in any preceding claim wherein the pixels are phase-delay elements,
4. 4. A projector as claimed in ally preceding claim wherein the spatially modulated light comprises a spatial distribution of phase-delays.
5. 5. A projector as claimed in any preceding claim wherein the distance between the spatial light modulator and replay field is substantially equal to the focal length of the PFL.
6. 6. A projector as claimed in any preceding claim wherein the optical element is a pIano-convex lens.
7. 7. A projector as claimed in any preceding claim wherein the data is formed by vector addition of the first data and second data.
8. 8. A projector as claimed in any preceding claim wherein the plurality of pixels are arranged to form a two-dimensional array.
9. 9. A projector as claimed in any preceding claim further comprising a repository of first and/or second data.
10. 10. A projector as claimcd in any preceding claim further comprising a processor arranged to calculate the first and/or second data substantially in real-time.
11. 11. A projector as claimed in any one of the preceding claims further comprising a light source arranged to illuminate the spatial light modulator.[2. A method of operating a SLM, the method comprising: receiving data wherein the data comprises first data representative of an image and second data representative of a PFL; receiving light and spatially modulate the light in accordance with the data; performing an optical Fourier transform of the first data using the PFL; forming a holographic reconstruction of the image at a replay field; and providing telecentricity at the reply field using a an optical element arranged betwecn the spatial light modulator and the replay field.13. A method of operating a SLM as claimed in claim 12 wherein the iirst data representative of an image is Fourier domain data representative of an image.14, A method of operating a SLM as claimed in claim 12 or claim 13 wherein the distance between the spatial light modulator and replay field is substantially equal to the focal length of the PFL.15. A projector or method of operating a SLM substantially as hereinbefore described with reference to the accompanying drawings.3509BO5v1