RECONFIGURABLE SPATIAL LIGHT MODULATORS
This invention relates to reconfigurable spatial light modulators (SLM) incorporating a scatter plate. Computer generated diffraction patterns or holograms may be loaded on the (SLM) either as a single frame or as a series of frames for observation by an observer. In a preferred embodiment both an electrically addressable spatial light modulator (EASLM) and an optically addressable spatial light modulator (OASLM) are used. The (OASLM) may be formed of several smaller (OASLMs) arranged in a matrix format. The faster (EASLM) forms a light pattern on the sub-areas of the large (OASLM) in turn to give a large display. The scatter plate is arranged at the output of the (SLM) nearest an observer. This scatter plate has a known characteristic and serves to increase the field of view and/or reduce the number of pixels required to give a holographic or two dimensional displays. Prior to producing a display, the diffraction patterns, holograms, or image in the computer is modified to take account of the properties of the scatter plate; a modified computer generated diffraction pattern, hologram, or image is then displayed to an observer. The system may also be used for optical switching.
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This application is a divisional of U.S. application Ser. No. 10/528,670, filed Mar. 22, 2005, titled Reconfigurable Spatial Light Modulators, which is a national phase application of PCT/CB2003/004284, filed Oct. 5, 2003, titled Reconfigurable Spatial Light Modulators which is a PCT application of foreign application no. GB 0223119.9 filed Oct. 5, 2002, titled Reconfigurable Spatial Light Modulators, the disclosures of which are herein incorporated by reference in entirety.FIELD OF INVENTION
This invention relates to reconfigurable spatial light modulators (SLM) on which computer generated diffraction patterns or holograms may be loaded either as a single frame or as a series of frames.BACKGROUND
Re-configurable SLMs based on liquid crystal (and other types of) devices are widely used for controlling and manipulating optical beams. In diffractive mode they may be used for three dimensional (3D) imaging [BROWN, C V and STANLEY, M, UK Patent Application GB2330471, Production of Moving Images for Holography] and for routing optical signals in telecommunications networks [See for example ROSES (Re-configurable Optical Switches) website, http://www-g.eng.cam.ac.uk/photonics/rose1.html].
The SLM.modulates the complex amplitude of an incoming wave front (i.e. changes its phase and/or amplitude), which causes it to propagate in the desired manner. The SLM generally comprises a liquid crystal panel containing a number of individually addressed pixels, onto which a diffraction pattern or Computer Generated Hologram (CGH) is written [CAMERON, C D et al, SPIE Conference on Critical Technologies for the Future of Computing (San Diego, USA), July-August 2000, Computational Challenges of Emerging Novel True 3D Holographic Displays].
CGH 3D display systems typically use a computer to generate and/or store electronic copies of the hologram. This hologram is then replayed on an SLM which is switched to modulate (in transmission or reflection) light from a source which then passes through suitable replay optics, thereby providing a visible three-dimensional image to observers.
For many image generation applications, especially holographic 3D image generation, it is important to maximise the image size and/or the range of angles over which the image can be viewed. Conventionally, this is achieved by increasing the spatial frequency content of the hologram, which increases the achievable diffraction angle of the modulated beam and/or increasing the number of pixels in the SLM. In order to produce satisfactory images, SLMs containing of the order of 10.sup.10 pixels may be required. An increased spatial frequency is also desirable for many other applications utilising diffractive SLMs, such as optical switching.
HAINES and BRUM [Proceedings of the IEEE (Letters), p 1512-3, August 1967, A Technique for Bandwidth Reduction in Holographic Systems] proposed using a ground glass scatter plate as a means of enhancing the viewing angle without having to increase the spatial frequency content. The principle was successfully demonstrated using a conventional fixed, photographically recorded hologram of a point source hologram (note: the scatter plate is used in both the recording and re-construction stage). KOMAR [SPIE Vol 120, p127, 1977, Progress on the Holographic Movie Process in the USSR] discussed the use of the scatter-plate technique as a means of producing holographic “movies” in which the viewing angle was big enough for a large audience to see the 3D images. He concluded that the technique caused unacceptable degradation in the image quality.
The main factor preventing the introduction of reconfigurable computer generated holograms (CGH) in many applications is the number of addressable, reconfigurable pixels required in such devices. This is particularly important in the area of 3D image generation.
To ensure adequate fields of view (FOV) and image sizes, conventional approaches typically need a CGH having a pixel count several orders of magnitude higher than that required to produce image resolutions that the human visual system can perceive. The simple relation FOV.1.about.n.lamda./4 (1) (where n is the number of pixels across the display in the plane where the FOV is specified, I is image width, and A is the wavelength of light generating the real image) shows that, for typical applications (e.g. FOV=.+−.30.degree., I=0.05 m), .about.10.sup.10 pixels are required to be addressed. This number is enormous. Any method allowing a pixel count reduction, without significantly compromising perceived image qualities, will have great effect on the practicality of such systems.STATEMENT OF INVENTION
The above problem of a very large pixel count is reduced, according to the present invention, by the use of a scatter plate of known feature composition together with a computer generated diffractive pattern or hologram calculated to pre-compensate for effect of the scatter plate.
According to this invention, a reconfigurable spatial light modulator system comprises:
a controller for holding a compensated pattern;
a first spatial light modulator having a plurality of addressable pixels controlled by the controller each pixel being capable of modulating incident light and collectively replicating the compensated pattern;
a scatter plate of known characteristics for scattering light from the first spatial light modulator;
optical means for directing light scattered by the scatter plate and presenting a pattern to a receiver;
the compensated pattern being related to both the scatter plate characteristics and to the pattern presented to the receiver.
The scatter plate may be a simple binary phase pseudo random diffuser and may have a number of pixels much greater than the number of pixels on the first SLM. Other formats may be used. For example multi level pseudo random devices and diffusers with periodic structures.
The first spatial light modulator may be an electrically addressable liquid crystal spatial light modulator (EASLM) operable either in transmissive or reflective mode.
The optical means may be a lens system, e.g. a Fourier Transforming lens.
The system may include a second SLM which receives modulated light from the first SLM; the scatter plate 5 is associated with the second SLM in such systems. This second SLM may be optically addressable (OASLM) having substantially the same or different number of addressable pixels as the first SLM, and may be of greater dimensions to give a magnified image or smaller dimensions to provide increased spatial frequency. Alternatively, the second SLM may be formed by a number of individual OASLMs arranged in an x,y matrix or tiled configuration. When the second SLM has a tiled configuration, then scanning means are included for directing scattered light to different tiled areas in turn.
The controller may be a computer having memory for storing basic images and calculating means for providing compensated pattern from basic images to take account of scattering by the scatter plate. The basic images may be obtained by scanning of actual objects, in a manner analogous to forming holographic images in film material, conventional holography as in U.S. Pat. No. 6,078,392, direct to digital holography, or by calculation as in computer aided design (CAD) processes. Alternatively the computer may have a memory unit for storing images previously processed elsewhere, and then read out into the SLM as required.
The system may display holographic images directly to one or more observers. Alternatively, the system may further include a receiver, which may be an array of detector elements, a bundle of optical fibres, a screen onto which an image is projected for observation by one or more observers. The light source may be ambient light, or light from a single or multiple light sources such as one or more optical fibres or lasers.
According to this invention a method of providing a holographic image to an observer includes the steps of:
providing a holographic engine for storing a computer generated hologram pattern of an image to be displayed
providing a spatial light modulator having a large number of addressable pixels each capable modulating light under the control of the engine
controlling the spatial light modulator so that the observer receives a holographic image; CHARACTERISED BY
providing a light scattering plate of known characteristics to modify light from the spatial light modulator;
calculating and providing a compensated computer generated hologram pattern of an image to compensate for the known characteristics of the scattering plate so that an observer receives a holographic image.
The compensated CGH pattern may be calculated using direct binary search algorithm.
According to another aspect of this invention a method of increasing the range of diffraction angles from a computer designed diffraction structure includes the steps of:
providing a holographic engine for storing a computer generated pattern of a structure to be displayed;
providing a spatial light modulator having a large number of addressable pixels each capable of modulating light under the control of the engine; CHARACTERISED BY
providing a light scattering plate of known characteristics to modify light from the spatial light modulator;
calculating and providing a compensated computer generated hologram pattern of a diffractive structure to compensate for the known characteristics of the scattering plate so that an increased range of diffraction angles are obtained.BRIEF DESCRIPTION OF DRAWINGS
The invention will now be described, by way of example only, with reference to the accompanying drawing of which:
As shown in
A computer 4 controls the signals applied to the SLM 3 and may contain electronic copies of images to be displayed in both original format and modified format as discussed below.
In front of the SLM 3 is a scatter plate 5. This is shown as separated from the SLM 3, but in many cases it will be in contact with the SLM and may be formed on the output wall of the liquid crystal cell forming the SLM 3. In another embodiment, a lens or optical element (not shown) may be located between the SLM 3 and scatter plate 5.
Part of one example of scatter plate 5 is shown in
A lens system 6 transforms light from the scatter plate 5 to present a three dimensional (3-D) holographic image 7 to an observer 8. This image 7 appears to float in space in front of the observer 8 at an enlarged size in comparison with a system without a scatter plate 5.
Some benefits of the scatter plate 5 can be seen by comparing
The value of .phi. is given by the grating equation as: sin.phi.=.lamda./(2 Mp) (2) where p is the pixel pitch on the SLM, and M is given below.
The above shows that larger diffraction angles .phi. can be produced for a given number of addressable pixels (or, alternatively, the same value of FOV.I product for a smaller number of addressable pixels) by combining the reconfigurable modulator 3 (SLM) with a fixed scattering plate (
If the scattering plate 5 has spatial frequencies of M times the SLM 3, then, for the same number of addressable pixels n across the SLM: FOV.I=Mn.lamda./4 (3)
The number of pixels that have to be addressed for a given FOV.I product is thus reduced by M.sup.2, assuming a unity aspect ratio SLM. So, for M=10 and the FOV.I product in the example above, only 10.sup.8 addressable pixels in the SLM would be required.
A scatter plate design may be used to increase the size of a 2D image generated from a e.g. binary phase, Fourier CGH. For example the scatter plate of
Step 1. Calculate scatter plate geometry, e.g. to double available range of diffraction angles, the scatter plate requires a pixel spatial frequency of twice that of the associated spatial light modulator.
Step 2. Specify the required fraction of pixels in state 0
Step 3. Starting at the first pixel element, randomly assign a state of 0 or .pi. using a probability specified in step 2.
Step 4. Repeat step 3 for all locations on the scatter plate
- n.sub.sp=number of pixels across the scatter plate
- n.sub.sim=number of pixels across the SLM
- p.sub.sp=pixel pitch of scatter plate
- P.sub.sim=pixel pitch of SLM
- M=required ratio of pixel spatial frequencies
When the scatter plate 5 of
Step 1. Specify intensity distribution of the image to be generated by CGH/scatter plate combination (the “target” intensity distribution, e.g.
Step 2. Specify system parameters (wavelength of operation, focal length of Fourier lens system 6, pixel pitch of CGH)
Step 3. Specify scatter plate characteristics e.g. a pseudo-random, binary phase distribution, each scatter plate pixel having a retardation of .pi. or 0 at the design wavelength, with a pixel pitch of half that of the CGH in the x and y directions (see
Step 4. Use DBS (direct binary search) algorithm to optimise the CGH pixel distribution, such that when replayed through the scatter plate 5, the system delivers the required image having twice the lateral size (4.times. the area) of that which would be possible from the CGH alone. DBS as follows:
Step 5. Start with random binary phase pattern on CGH.
Step 6. Assuming on axis replay of the CGH with a plane wave, and that the CGH is in contact with the scatter plate, determine the electric field distribution at the output surface of the scatter plate, sampled at the centre of each scatter plate pixel.
This can be done using scalar transmittance theory, for example, if the pixel spacings are large compared to the wavelength of light used, and the CGH and scatter plate are sufficiently close enough together. The number of sample points (and complex electric field values) will be 4.times. the number of pixels in the CGH, in this example.
Step 7. Take the fast Fourier transform of the array of electric field values at the output of the scatter plate. This gives the electric field distribution in the image plane. The modulus squared of this is the intensity in the image plane. Compare this intensity distribution in the region of interest to the target intensity distribution, using some numerical metric e.g. normalised mean squared error (DBS reference above). Call this the error e.
Step 8. Flip the top left pixel of the CGH distribution e.g. if it was of value .pi., change its value to 0, if it was of value 0, change it to .pi.. Repeat steps 6 and 7 to determine a new error value e.sub.new. If e.sub.new<e, then accept the pixel flip and set e=e.sub.new. Otherwise flip the CGH pixel back to its original value.
Step 9. Flip the next pixel (in e.g. lexicographical order) in the CGH and repeat step 8.
Step 10. Repeat step 9 and continually cycle through all the CGH pixels in the same manner.
Step 11. Repeat steps 8 to 10 until there are no more accepted pixel flips.
Step 12. The resulting CGH pattern will be a local solution to the problem of determining the CGH pattern required to give the target image distribution when replayed through the scatter plate.
Incorporating the scatter plate 5 produces advantages as follows:
The image size can be increased without increasing the number of pixels in the SLM, which increases the flexibility of the system.
There is some perceived degradation in image quality, but this can be controlled, therefore the technique is appropriate for high quality imaging applications.
The number of pixels needed to achieve a certain image size can be reduced, therefore simpler more cost effective SLMs may be employed.
For a given image size, the data storage and data bandwidth requirements for re-configurable CGHs are reduced, thereby making them more practical to implement.
For a given data storage and bandwidth capacity, the image size available from re-configurable CGHs is maximised, thereby producing a more flexible system.
Whilst calculating a compensated pattern, the computer may also provide correction or compensation for known aberrations in the various optical components.
When light from the EASLM is incident upon a photosensitive area, a voltage is generated to apply an electrical field across the liquid crystal layer under that area, thereby modifying its transmission properties. Thus a pattern of light from the EASLM is transmitted through the OASLM 11 and scatter plate 5 combination to an observer 8 at a greatly increased size. As before the scatter plate 5 is shown separate from the OASLM 11 but in practice will be close to or formed as part of the front face of the OASLM. In another embodiment, the scatter plate may be on the light receiving face of the OASLM.
A description of EASLMs and OASLMs used with holographic displays is described in patent application EP-1064651, PCT WO-00/2350472, GB98/04996.
One potential advantage of the OASLMs 11, 14 are that they may modulate light at wavelengths other than that of the light source 1. For example the light source 1 may generate light of a narrow wavelength range to control the EASLM 3 and hence the OASLMs 11, 14. These OASLMs 11, 14 may then transmit or reflect light from additional sources (not shown), either white light or red, blue and green light to provide a colour display e.g. in a frame or line sequential manner.
The scatter plate 5 of
The examples in
When used for two dimensional displays, larger images can be shown without increased pixellation or the same size displayed with reduced pixel numbers in the SLM 3. The enlarged image may be directed onto a reflective (or transmissive) screen for viewing by several observers; or the enlarged image viewed by an observer 8 as in
In the optical switching device shown in
1. A method comprising:
- generating a hologram pattern of a holographic image to compensate for known characteristics of a light scattering plate;
- transmitting incident light from a light source to a hologram device including a spatial light modulator and the light scattering plate;
- controlling a transmission of the incident light through the spatial light modulator having a number of addressable pixels;
- modulating the incident light to replicate the hologram pattern; and
- presenting the holographic image.
2. The method of claim 1 including modifying an allowable range of diffraction angles of the incident light through the light scattering plate.
3. The method of claim 1 wherein the allowable range of diffraction angles is increased by the light scattering plate.
4. The method of claim 1 wherein the available range of diffraction angles is proportional to a ratio of a pixel spatial frequency of the light scattering plate to a pixel spatial frequency of the spatial light modulator
5. The method of claim 1 wherein the compensated pattern is calculated using a direct binary search algorithm.
6. The method of claim 1 wherein the hologram pattern further compensates for the holographic image presented to a receiver.
7. The method of claim 1 including storing an electronic copy of both the hologram pattern and the holographic image.
8. A method comprising:
- storing a computer generated pattern of a diffractive structure to be displayed;
- controlling a transmission of incident light through a spatial light modulator having a number of pixels;
- modulating the incident light according to the computer generated pattern to compensate for a diffractive effect of the incident light through the spatial light modulator including a scattering plate; and
- displaying the diffractive structure as an enlarged image compared to an image displayed as a result of a transmission of incident light through the spatial light modulator without a scattering plate.
9. The method of claim 8 including compensating for the diffractive effect according to a known surface characteristic of the scatter plate.
10. The method of claim 9 wherein the known surface characteristic includes a number of pixels of the scatter plate.
11. The method of claim 9 wherein the known surface characteristic includes an electric field distribution.
12. The method of claim 9 wherein a size of the enlarge image may be increased without increasing the number of pixels in the spatial light modulator.
13. The method of claim 8 including storing an electronic copy of the diffractive structure.
14. A system comprising:
- a holographic engine capable of storing a hologram pattern of an image;
- one or more spatial light modulators having a number of addressable pixels capable of passing light under the control of the engine;
- a light scattering plate capable of modifying the light that passes through the one or more spatial light modulators; and
- a processor configured to control at least one of the one or more spatial light modulators to compensate for a scattering effect of the light scattering plate in order to replicate the hologram pattern.
15. The system of claim 14 wherein the light scattering plate has a number of surface features greater than the number of addressable pixels of the one or more spatial light modulator.
16. The system of claim 14 including displaying the image, wherein the image displayed is larger than an image generated by a system including the one or more spatial light modulators without the light scattering plate.
17. The system of claim 14 wherein the hologram pattern compensates for a scattering effect of the light scattering plate and further compensates for a known optical aberration of one or more components of the system.
18. The system of claim 14 wherein the holographic engine is a computer with storage and means for calculating both a computer generated hologram image and a compensated hologram pattern for each image to be displayed.
19. The display system of claim 14 including an electrically addressable liquid crystal spatial light modulator (EASLM).
20. The system of claim 19 wherein the system comprises at least two spatial light modulators arranged such that light from the EASLM is directed towards an optically addressable spatial light modulator.
International Classification: G02B 21/06 (20060101);