WO2012069811A2 - Systèmes holographiques - Google Patents

Systèmes holographiques Download PDF

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Publication number
WO2012069811A2
WO2012069811A2 PCT/GB2011/052258 GB2011052258W WO2012069811A2 WO 2012069811 A2 WO2012069811 A2 WO 2012069811A2 GB 2011052258 W GB2011052258 W GB 2011052258W WO 2012069811 A2 WO2012069811 A2 WO 2012069811A2
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Prior art keywords
light
slm
image
intensity
pixels
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PCT/GB2011/052258
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English (en)
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WO2012069811A3 (fr
Inventor
Adrian James Cable
Raul Benet Ballester
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Light Blue Optics Ltd
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Publication of WO2012069811A2 publication Critical patent/WO2012069811A2/fr
Publication of WO2012069811A3 publication Critical patent/WO2012069811A3/fr

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    • G—PHYSICS
    • G03—PHOTOGRAPHY; CINEMATOGRAPHY; ANALOGOUS TECHNIQUES USING WAVES OTHER THAN OPTICAL WAVES; ELECTROGRAPHY; HOLOGRAPHY
    • G03H—HOLOGRAPHIC PROCESSES OR APPARATUS
    • G03H1/00—Holographic processes or apparatus using light, infrared or ultraviolet waves for obtaining holograms or for obtaining an image from them; Details peculiar thereto
    • G03H1/22—Processes or apparatus for obtaining an optical image from holograms
    • G—PHYSICS
    • G03—PHOTOGRAPHY; CINEMATOGRAPHY; ANALOGOUS TECHNIQUES USING WAVES OTHER THAN OPTICAL WAVES; ELECTROGRAPHY; HOLOGRAPHY
    • G03H—HOLOGRAPHIC PROCESSES OR APPARATUS
    • G03H1/00—Holographic processes or apparatus using light, infrared or ultraviolet waves for obtaining holograms or for obtaining an image from them; Details peculiar thereto
    • G03H1/22—Processes or apparatus for obtaining an optical image from holograms
    • G03H1/2202—Reconstruction geometries or arrangements
    • G03H1/2205—Reconstruction geometries or arrangements using downstream optical component
    • G—PHYSICS
    • G03—PHOTOGRAPHY; CINEMATOGRAPHY; ANALOGOUS TECHNIQUES USING WAVES OTHER THAN OPTICAL WAVES; ELECTROGRAPHY; HOLOGRAPHY
    • G03H—HOLOGRAPHIC PROCESSES OR APPARATUS
    • G03H1/00—Holographic processes or apparatus using light, infrared or ultraviolet waves for obtaining holograms or for obtaining an image from them; Details peculiar thereto
    • G03H1/02—Details of features involved during the holographic process; Replication of holograms without interference recording
    • G—PHYSICS
    • G03—PHOTOGRAPHY; CINEMATOGRAPHY; ANALOGOUS TECHNIQUES USING WAVES OTHER THAN OPTICAL WAVES; ELECTROGRAPHY; HOLOGRAPHY
    • G03H—HOLOGRAPHIC PROCESSES OR APPARATUS
    • G03H1/00—Holographic processes or apparatus using light, infrared or ultraviolet waves for obtaining holograms or for obtaining an image from them; Details peculiar thereto
    • G03H1/04—Processes or apparatus for producing holograms
    • G03H1/0486—Improving or monitoring the quality of the record, e.g. by compensating distortions, aberrations
    • G—PHYSICS
    • G03—PHOTOGRAPHY; CINEMATOGRAPHY; ANALOGOUS TECHNIQUES USING WAVES OTHER THAN OPTICAL WAVES; ELECTROGRAPHY; HOLOGRAPHY
    • G03H—HOLOGRAPHIC PROCESSES OR APPARATUS
    • G03H1/00—Holographic processes or apparatus using light, infrared or ultraviolet waves for obtaining holograms or for obtaining an image from them; Details peculiar thereto
    • G03H1/04—Processes or apparatus for producing holograms
    • G03H1/08—Synthesising holograms, i.e. holograms synthesized from objects or objects from holograms
    • G—PHYSICS
    • G03—PHOTOGRAPHY; CINEMATOGRAPHY; ANALOGOUS TECHNIQUES USING WAVES OTHER THAN OPTICAL WAVES; ELECTROGRAPHY; HOLOGRAPHY
    • G03H—HOLOGRAPHIC PROCESSES OR APPARATUS
    • G03H1/00—Holographic processes or apparatus using light, infrared or ultraviolet waves for obtaining holograms or for obtaining an image from them; Details peculiar thereto
    • G03H1/22—Processes or apparatus for obtaining an optical image from holograms
    • G03H1/2294—Addressing the hologram to an active spatial light modulator
    • G—PHYSICS
    • G03—PHOTOGRAPHY; CINEMATOGRAPHY; ANALOGOUS TECHNIQUES USING WAVES OTHER THAN OPTICAL WAVES; ELECTROGRAPHY; HOLOGRAPHY
    • G03H—HOLOGRAPHIC PROCESSES OR APPARATUS
    • G03H1/00—Holographic processes or apparatus using light, infrared or ultraviolet waves for obtaining holograms or for obtaining an image from them; Details peculiar thereto
    • G03H1/26—Processes or apparatus specially adapted to produce multiple sub- holograms or to obtain images from them, e.g. multicolour technique
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04N—PICTORIAL COMMUNICATION, e.g. TELEVISION
    • H04N5/00—Details of television systems
    • H04N5/74—Projection arrangements for image reproduction, e.g. using eidophor
    • H04N5/7416—Projection arrangements for image reproduction, e.g. using eidophor involving the use of a spatial light modulator, e.g. a light valve, controlled by a video signal
    • G—PHYSICS
    • G03—PHOTOGRAPHY; CINEMATOGRAPHY; ANALOGOUS TECHNIQUES USING WAVES OTHER THAN OPTICAL WAVES; ELECTROGRAPHY; HOLOGRAPHY
    • G03H—HOLOGRAPHIC PROCESSES OR APPARATUS
    • G03H1/00—Holographic processes or apparatus using light, infrared or ultraviolet waves for obtaining holograms or for obtaining an image from them; Details peculiar thereto
    • G03H1/02—Details of features involved during the holographic process; Replication of holograms without interference recording
    • G03H2001/0208—Individual components other than the hologram
    • G03H2001/0224—Active addressable light modulator, i.e. Spatial Light Modulator [SLM]
    • G—PHYSICS
    • G03—PHOTOGRAPHY; CINEMATOGRAPHY; ANALOGOUS TECHNIQUES USING WAVES OTHER THAN OPTICAL WAVES; ELECTROGRAPHY; HOLOGRAPHY
    • G03H—HOLOGRAPHIC PROCESSES OR APPARATUS
    • G03H1/00—Holographic processes or apparatus using light, infrared or ultraviolet waves for obtaining holograms or for obtaining an image from them; Details peculiar thereto
    • G03H1/04—Processes or apparatus for producing holograms
    • G03H1/0486—Improving or monitoring the quality of the record, e.g. by compensating distortions, aberrations
    • G03H2001/0491—Improving or monitoring the quality of the record, e.g. by compensating distortions, aberrations by monitoring the hologram formation, e.g. via a feed-back loop
    • G—PHYSICS
    • G03—PHOTOGRAPHY; CINEMATOGRAPHY; ANALOGOUS TECHNIQUES USING WAVES OTHER THAN OPTICAL WAVES; ELECTROGRAPHY; HOLOGRAPHY
    • G03H—HOLOGRAPHIC PROCESSES OR APPARATUS
    • G03H1/00—Holographic processes or apparatus using light, infrared or ultraviolet waves for obtaining holograms or for obtaining an image from them; Details peculiar thereto
    • G03H1/04—Processes or apparatus for producing holograms
    • G03H1/08—Synthesising holograms, i.e. holograms synthesized from objects or objects from holograms
    • G03H1/0808—Methods of numerical synthesis, e.g. coherent ray tracing [CRT], diffraction specific
    • G03H2001/0825—Numerical processing in hologram space, e.g. combination of the CGH [computer generated hologram] with a numerical optical element
    • G—PHYSICS
    • G03—PHOTOGRAPHY; CINEMATOGRAPHY; ANALOGOUS TECHNIQUES USING WAVES OTHER THAN OPTICAL WAVES; ELECTROGRAPHY; HOLOGRAPHY
    • G03H—HOLOGRAPHIC PROCESSES OR APPARATUS
    • G03H1/00—Holographic processes or apparatus using light, infrared or ultraviolet waves for obtaining holograms or for obtaining an image from them; Details peculiar thereto
    • G03H1/22—Processes or apparatus for obtaining an optical image from holograms
    • G03H1/2202—Reconstruction geometries or arrangements
    • G03H1/2205—Reconstruction geometries or arrangements using downstream optical component
    • G03H2001/2213—Diffusing screen revealing the real holobject, e.g. container filed with gel to reveal the 3D holobject
    • G03H2001/2215—Plane screen
    • G—PHYSICS
    • G03—PHOTOGRAPHY; CINEMATOGRAPHY; ANALOGOUS TECHNIQUES USING WAVES OTHER THAN OPTICAL WAVES; ELECTROGRAPHY; HOLOGRAPHY
    • G03H—HOLOGRAPHIC PROCESSES OR APPARATUS
    • G03H1/00—Holographic processes or apparatus using light, infrared or ultraviolet waves for obtaining holograms or for obtaining an image from them; Details peculiar thereto
    • G03H1/22—Processes or apparatus for obtaining an optical image from holograms
    • G03H1/2202—Reconstruction geometries or arrangements
    • G03H2001/2244—Means for detecting or recording the holobject
    • G03H2001/2247—Means for detecting or recording the holobject for testing the hologram or holobject
    • G—PHYSICS
    • G03—PHOTOGRAPHY; CINEMATOGRAPHY; ANALOGOUS TECHNIQUES USING WAVES OTHER THAN OPTICAL WAVES; ELECTROGRAPHY; HOLOGRAPHY
    • G03H—HOLOGRAPHIC PROCESSES OR APPARATUS
    • G03H1/00—Holographic processes or apparatus using light, infrared or ultraviolet waves for obtaining holograms or for obtaining an image from them; Details peculiar thereto
    • G03H1/22—Processes or apparatus for obtaining an optical image from holograms
    • G03H1/2249—Holobject properties
    • G03H2001/2263—Multicoloured holobject
    • G03H2001/2271—RGB holobject
    • G—PHYSICS
    • G03—PHOTOGRAPHY; CINEMATOGRAPHY; ANALOGOUS TECHNIQUES USING WAVES OTHER THAN OPTICAL WAVES; ELECTROGRAPHY; HOLOGRAPHY
    • G03H—HOLOGRAPHIC PROCESSES OR APPARATUS
    • G03H2210/00—Object characteristics
    • G03H2210/10—Modulation characteristics, e.g. amplitude, phase, polarisation
    • G—PHYSICS
    • G03—PHOTOGRAPHY; CINEMATOGRAPHY; ANALOGOUS TECHNIQUES USING WAVES OTHER THAN OPTICAL WAVES; ELECTROGRAPHY; HOLOGRAPHY
    • G03H—HOLOGRAPHIC PROCESSES OR APPARATUS
    • G03H2210/00—Object characteristics
    • G03H2210/20—2D object
    • G—PHYSICS
    • G03—PHOTOGRAPHY; CINEMATOGRAPHY; ANALOGOUS TECHNIQUES USING WAVES OTHER THAN OPTICAL WAVES; ELECTROGRAPHY; HOLOGRAPHY
    • G03H—HOLOGRAPHIC PROCESSES OR APPARATUS
    • G03H2222/00—Light sources or light beam properties
    • G03H2222/10—Spectral composition
    • G03H2222/17—White light
    • G03H2222/18—RGB trichrome light
    • G—PHYSICS
    • G03—PHOTOGRAPHY; CINEMATOGRAPHY; ANALOGOUS TECHNIQUES USING WAVES OTHER THAN OPTICAL WAVES; ELECTROGRAPHY; HOLOGRAPHY
    • G03H—HOLOGRAPHIC PROCESSES OR APPARATUS
    • G03H2222/00—Light sources or light beam properties
    • G03H2222/35—Transverse intensity distribution of the light beam
    • G—PHYSICS
    • G03—PHOTOGRAPHY; CINEMATOGRAPHY; ANALOGOUS TECHNIQUES USING WAVES OTHER THAN OPTICAL WAVES; ELECTROGRAPHY; HOLOGRAPHY
    • G03H—HOLOGRAPHIC PROCESSES OR APPARATUS
    • G03H2223/00—Optical components
    • G03H2223/12—Amplitude mask, e.g. diaphragm, Louver filter
    • G—PHYSICS
    • G03—PHOTOGRAPHY; CINEMATOGRAPHY; ANALOGOUS TECHNIQUES USING WAVES OTHER THAN OPTICAL WAVES; ELECTROGRAPHY; HOLOGRAPHY
    • G03H—HOLOGRAPHIC PROCESSES OR APPARATUS
    • G03H2223/00—Optical components
    • G03H2223/50—Particular location or purpose of optical element
    • G03H2223/54—Filtering the holobject information
    • G—PHYSICS
    • G03—PHOTOGRAPHY; CINEMATOGRAPHY; ANALOGOUS TECHNIQUES USING WAVES OTHER THAN OPTICAL WAVES; ELECTROGRAPHY; HOLOGRAPHY
    • G03H—HOLOGRAPHIC PROCESSES OR APPARATUS
    • G03H2225/00—Active addressable light modulator
    • G03H2225/20—Nature, e.g. e-beam addressed
    • G03H2225/22—Electrically addressed SLM [EA-SLM]
    • G—PHYSICS
    • G03—PHOTOGRAPHY; CINEMATOGRAPHY; ANALOGOUS TECHNIQUES USING WAVES OTHER THAN OPTICAL WAVES; ELECTROGRAPHY; HOLOGRAPHY
    • G03H—HOLOGRAPHIC PROCESSES OR APPARATUS
    • G03H2225/00—Active addressable light modulator
    • G03H2225/20—Nature, e.g. e-beam addressed
    • G03H2225/24—Having movable pixels, e.g. microelectromechanical systems [MEMS]
    • G—PHYSICS
    • G03—PHOTOGRAPHY; CINEMATOGRAPHY; ANALOGOUS TECHNIQUES USING WAVES OTHER THAN OPTICAL WAVES; ELECTROGRAPHY; HOLOGRAPHY
    • G03H—HOLOGRAPHIC PROCESSES OR APPARATUS
    • G03H2225/00—Active addressable light modulator
    • G03H2225/30—Modulation
    • G—PHYSICS
    • G03—PHOTOGRAPHY; CINEMATOGRAPHY; ANALOGOUS TECHNIQUES USING WAVES OTHER THAN OPTICAL WAVES; ELECTROGRAPHY; HOLOGRAPHY
    • G03H—HOLOGRAPHIC PROCESSES OR APPARATUS
    • G03H2225/00—Active addressable light modulator
    • G03H2225/30—Modulation
    • G03H2225/31—Amplitude only
    • G—PHYSICS
    • G03—PHOTOGRAPHY; CINEMATOGRAPHY; ANALOGOUS TECHNIQUES USING WAVES OTHER THAN OPTICAL WAVES; ELECTROGRAPHY; HOLOGRAPHY
    • G03H—HOLOGRAPHIC PROCESSES OR APPARATUS
    • G03H2225/00—Active addressable light modulator
    • G03H2225/30—Modulation
    • G03H2225/32—Phase only
    • G—PHYSICS
    • G03—PHOTOGRAPHY; CINEMATOGRAPHY; ANALOGOUS TECHNIQUES USING WAVES OTHER THAN OPTICAL WAVES; ELECTROGRAPHY; HOLOGRAPHY
    • G03H—HOLOGRAPHIC PROCESSES OR APPARATUS
    • G03H2225/00—Active addressable light modulator
    • G03H2225/60—Multiple SLMs
    • G—PHYSICS
    • G03—PHOTOGRAPHY; CINEMATOGRAPHY; ANALOGOUS TECHNIQUES USING WAVES OTHER THAN OPTICAL WAVES; ELECTROGRAPHY; HOLOGRAPHY
    • G03H—HOLOGRAPHIC PROCESSES OR APPARATUS
    • G03H2240/00—Hologram nature or properties
    • G03H2240/50—Parameters or numerical values associated with holography, e.g. peel strength
    • G03H2240/51—Intensity, power or luminance

Definitions

  • This invention relates to apparatus and methods for generating an image holographically, and also to spatial light modulators for use in such techniques.
  • the light source used to illuminate the SLM can be characterised outside the complete system and then stabilised (through precise temperature and current control) so that it does not change with time, and the recorded illumination data can be used in any processing required to calculate data to display on the SLM.
  • a method of generating an image holographically comprising: displaying a hologram on a spatial light modulator (SLM) comprising a plurality of light modulating pixels; and illuminating said spatial light modulator with a beam of coherent light to generate said image from said displayed hologram by diffracting said beam of coherent light using said light modulating pixels; and wherein the method further comprises: providing at least one light sensing pixel on said SLM to sense an intensity of said beam of coherent light at a position of said light sensing pixel; and determining an intensity of said generated image by sensing said beam with said light sensing pixel.
  • SLM spatial light modulator
  • the light sensing is performed by replacing one or more of the light modulating pixels with one or more corresponding light sensing pixels.
  • a diffractive imaging system such as a holographic imaging system the displayed diffraction pattern or hologram can undergo significant degradation whilst preserving the displayed image, albeit with increased noise.
  • a number of light modulating pixels may be replaced by light sensing pixels without substantially affecting the displayed image, in particular in a system as described later in which multiple temporal sub-frames are averaged to provide the displayed image, thus reducing the noise in the replay field.
  • sensing in the SLM plane is desirable because this can significantly reduce the size of an optical assembly in a holographic image projection system because it is not necessary to split the beam to direct a portion of the beam to a light sensor or light sensor array. Sensing in the SLM plane also facilitates accurate measurement of light intensity because this measurement is made on the actual beam generating the hologram.
  • the invention provides a spatial light modulator (SLM) comprising a plurality of light modulating pixels for diffracting a beam of coherent light to generate an image from a hologram displayed on said SLM, wherein at least one of said light modulating pixels is replaced with a light sensing pixel to sense an intensity of said beam of coherent light at a position of said light sensing pixel.
  • SLM spatial light modulator
  • the light modulating pixels comprise MEMS (microelectromechanical systems) structures, fabricated on a silicon substrate, in which case a light sensing pixel may be fabricated in COMS (complementary metal oxide semiconductor) technology.
  • MEMS microelectromechanical systems
  • COMS complementary metal oxide semiconductor
  • preferably plurality of light sensing pixels is provided distributed over the plane of the SLM so that a spatial distribution of the intensity of the holographically generated image can be determined.
  • light sensing pixels may be employed to determine a more accurate average beam intensity.
  • the sensed (average) beam intensity may be employed in a feedback loop to control the brightness of the holographically generated image, in particular to compensate for brightness variations resulting from selective diffraction of light into illuminated portions of the image - unlike a system which directly displays an image on the SLM, in a holographic image display system light is selectively diffracted into illuminated regions of the image and thus if such a region is small it will appear correspondingly brighter unless brightness compensation is provided dependent on (proportional to) the coverage of the image.
  • the light-sensing SLM is employed as the first SLM of two SLMs in the system.
  • the first SLM displays a hologram which generates an image defining an illumination pattern on the second SLM, which displays an intensity image comprising a spatial pattern of intensity modulation, and this in turn is projected, preferably via a diffuser.
  • the intensity field which is seen by the eye of a viewer is in effect a product of the intensity field defined by the replay or reconstruction of the hologram on the first SLM, multiplied by the intensity modulation pattern on the second SLM, broadly speaking the second SLM adding the high frequency detail to the image. (The skilled person will appreciate that the eye in fact perceives the squared modulus of the intensity field).
  • the beam illuminating the first SLM tends to approximate a Gaussian beam, which has a quasi-low pass filtering effect (recalling that the first SLM effectively performs a Fourier transform).
  • the spatial pattern of intensity modulation on the second SLM can be adjusted in order to compensate for the non-uniformities introduced by the spatial beam intensity profile of the beam illuminating the first SLM.
  • the first SLM performs a Fourier transform of a product of the beam spatial amplitude profile and the hologram spatial phase profile (both in two dimensions, x and y).
  • the result of the Fourier transform is a complex reconstruction field, and the intensity perceived by the eye is the squared modulus of this field, modified by the spatial pattern and intensity modulation on the second SLM.
  • Using a plurality of spatially distributed light sensing pixels on the first SLM facilitates accurately determining the intensity field perceived by the eye, that is generating a displayed image which is an accurate representation of that desired.
  • the invention provides apparatus for generating an image holographically, the apparatus comprising: a spatial light modulator (SLM) comprising a plurality of light modulating pixels; an illumination source to illuminate said SLM with a beam of coherent light; a hologram data generator coupled to said SLM to provide hologram data to said SLM to display a hologram on said SLM to generate said image by diffracting said beam of coherent light using said light modulating pixels; wherein said SLM includes at least one light sensing pixel to sense an intensity of said beam of coherent light at a position of said light sensing pixel; and wherein the apparatus further comprises a data processor coupled to said at least one light sensing pixel to receive a signal from said at least one light sensing pixel and to determine an intensity of said generated image by sensing said beam with said light sensing pixel.
  • SLM spatial light modulator
  • an illumination source to illuminate said SLM with a beam of coherent light
  • a hologram data generator coupled to said SLM to provide hologram data to said SLM
  • the apparatus uses an SLM according to an aspect/embodiment of the invention as previously described.
  • Figure 1 shows a first example of a holographic image projection system for use with an SLM according to an embodiment of the invention
  • Figure 2 shows an improved holographic image projection system for use with an SLM according to an embodiment of the invention
  • Figures 3a to 3d show an example of a holographic image display system without aberration correction illustrating, 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;
  • Figures 5a and 5b show, respectively, a schematic illustration of a 25 x 25 SLM array with active (white) and photodiode (black) pixels according to an embodiment of the invention; and a schematic representation of four pixels from a MEMS piston SLM with one pixel replaced by a photodiode circuit;
  • Figure 6 shows a first example of a holographic image projection system including an SLM according to an embodiment of the invention
  • Figure 7 shows an improved holographic image projection system including an SLM according to an embodiment of the invention.
  • a spatial light modulator comprising an array of pixels in which one or more of the pixels is replaced with a photosensitive element, for example a photodiode.
  • a spatial light modulator may be employed in wavefront correction or other applications, but is particularly useful for diffractive image formation, and more specifically in a diffractive holographic projector.
  • Figure 1 shows an example optical layout for a first example of a holographic image projection system 100 to project a 2D image onto a screen 1 10.
  • a holographic image projection system 100 to project a 2D image onto a screen 1 10.
  • red R, green G, and blue B lasers In the full colour holographic image projector of Figure 1 there are red R, green G, and blue B lasers.
  • the system also includes the following additional elements:
  • SLM is the hologram SLM (spatial light modulator).
  • L1 , L2 and L3 are collimation lenses for the R, G and B lasers respectively (optional, depending upon the laser output).
  • ⁇ M1 , M2 and M3 are corresponding dichroic mirrors; they may be implemented as a prism assembly.
  • PBS Polyarising Beam Splitter
  • Lenses L4 and L5 form an output telescope (demagnifying optics).
  • the output projection angle is proportional to the ratio of the focal length of L4 to that of L5.
  • L4 may be encoded into the hologram(s) on the SLM, for example using the techniques we have described in WO2007/1 10668, and/or output lens L5 may be replaced by a group of projection lenses.
  • L5 may comprise a wide-angle or fisheye lens, mounted for translation perpendicular to the output optical axis (e.g left-right in Figure 1 ), to enable configuration of the output optical system as an off-axis system for table- down projection.
  • D1 is a diffuser located at intermediate image plane to reduce speckle. It may comprise a plastic plate, and optionally, may be piezoelectrically-actuated so that it can be moved rapidly in two orthogonal directions to reduce streaking (see our GB2456170).
  • the diffuser increases the etendue of the source, increasing safety and reducing speckle.
  • a processor 102 acts as a system controller and performs signal processing in either dedicated hardware, or in software, or in a combination of the two, as described further below. Thus processor 102 inputs image data and provides hologram data 104 to the SLM.
  • the different colours are time-multiplexed and the sizes of the replayed images are scaled to match one another, for example by padding a target image for display with zeros (the field size of the displayed image depends upon the pixel size of the SLM not on the number of pixels in the hologram).
  • the SLM may be a liquid crystal device.
  • other SLM technologies to effect phase modulation may be employed, such as a pixellated MEMS-based piston actuator device.
  • Figure 2 shows an optical architecture for a second example of a holographic image projection system 200, in which like elements to Figure 1 are indicated by like reference numerals. This is described in more detail later.
  • the SLM is modulated with holographic data approximating a hologram of the image to be displayed.
  • 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.
  • 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 pseudo-random (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.
  • sets of holograms may form replay fields that exhibit mutually independent additive noise. An example is shown below:
  • Step 1 forms N targets equal to the amplitude of the supplied intensity target l xy , but with independent identically-distributed (i.i.d.), 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 rrt ⁇ ensures equal numbers of -1 and 1 points are present in the holograms, achieving DC balance (by definition) and also minimal reconstruction error. The median value of may be assumed to be zero with minimal effect on perceived image quality.
  • Figure 3a 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 /, 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, /, 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 a SLM, optionally via a driver chip.
  • a ferroelectric liquid crystal SLM can be advantageous because of its fast switching time.
  • the SLM may be binary phase or multi-phase - binary phase devices can be convenient but binary quantization results in a conjugate image whereas the use of a multi-phase SLM suppresses this.
  • Figure 3b shows details of the hardware block of Figure 3a; 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, l xy , is supplied one or more times per video frame period as an input. Each image frame, l 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.
  • a set of N sub-frames 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.
  • phase-modulation block 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.
  • 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 ⁇ at each pixel, or more for a multi-phase SLM.
  • 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.
  • 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 better results can be obtained if, instead, the generation process for each subframe takes 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 H 1 to H n .
  • noise variance falls as ⁇ ( in comparison to the 1 / ⁇ / falloff for (non-adaptive) OSPR.
  • An example procedure takes as input a target image ⁇ , and a parameter N specifying the desired number of hologram subframes to produce, and outputs a set of N holograms H 1 to H N which, when displayed sequentially at an appropriate rate, form as a far-field image a visual representation of ⁇ which is perceived as high quality.
  • An optional pre-processing step performs gamma correction to match a CRT display by calculating T(x, y) 3 .
  • pre-processing may include colour space conversion and geometry correction (if projecting at an angle).
  • 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 H 1 to H n -i so that the noise ma be evaluated and taken into account in the subsequent stage:
  • F(x, y) y)]f A random phase factor 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 a 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 - 1 stages is given by F - (n - 1 )( 7 ⁇ ) 2 ,
  • /-/ represents an intermediate fully-complex hologram formed from the target 7 ⁇ " and is calculated using an inverse Fourier transform operation. It is quantized to binary phase to form the output hologram H n , i.e.
  • an ADOSPR-type method of generating data for displaying an image comprises generating from the displayed image data holographic data for each subframe 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.
  • 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.
  • 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.
  • Super-resolution ADOSPR The eye perceives not the field amplitude F but its intensity
  • Super- resolution can be implemented using an ADOSPR-type procedure to generate OSPR hologram sets of resolution M x M that form image reproductions at double (in each dimension) the resolution of that of the hologram, i.e. 2M x 2M (2M x /W for a binary phase modulator).
  • this uses a modified inter-pixel Fourier transform operation to evaluate the frequency components every half-sample, instead of every sample.
  • such a transform can be implemented by padding each M x M hologram up to 2M x 2/W by embedding it in a matrix of zeros.
  • a parameter c which defines the coverage of an image, that is the energy in the desired image as a proportion of the maximum available energy. The average contrast ratio in a
  • a very approximate value for c is employed, say 2 data bits, and a simplified expression may thus be used, for example without the squaring.
  • holographic image projection system architecture 200 of Figure 2. This employs dual SLM modulation - low resolution phase modulation and higher resolution amplitude (intensity) modulation. This can provide substantial improvements in image quality, power consumption and physical size.
  • the primary gain of holographic projection over imaging is one of energy efficiency.
  • the low spatial frequencies of an image can be rendered holographically to maintain efficiency and the high-frequency components can be rendered with an intensity-modulating imaging panel, placed in a plane conjugate to the hologram SLM.
  • diffracted light from the hologram SLM device (SLM1 ) is used to illuminate the imaging SLM device (SLM2).
  • the hologram SLM is preferably a fast multi-phase device, for example a pixellated MEMS-based piston actuator device.
  • SLM1 is the hologram SLM (spatial light modulator), for example a 160 ⁇ 160 MEMS or ferroelectric liquid crystal device with pixels of size ⁇ ; it may have physically small lateral dimensions, e.g ⁇ 1 mm.
  • L1 , L2 and L3 are the collimation lenses.
  • M1 , M2 and M3 are dichroic mirrors a implemented as prism assembly.
  • M4 is a turning beam mirror
  • SLM2 is the imaging SLM and has a resolution at least equal to the target image resolution (e.g. 854 ⁇ 480); it may comprise a LCOS (liquid crystal on silicon) panel.
  • target image resolution e.g. 854 ⁇ 480
  • LCOS liquid crystal on silicon
  • Diffraction optics 210 comprises lenses LD1 and LD2, forms an intermediate image plane on the surface of SLM2, and has effective focal length f such that fX / ⁇ covers the active area of imaging SLM2.
  • optics 210 perform a spatial Fourier transform to form a far field illumination pattern in the Fourier plane, which illuminates SLM2.
  • PBS2 (Polarising Beam Splitter 2) transmits incident light to SLM2, and reflects emergent light into the relay optics 212.
  • PBS2 preferably has a clear aperture at least as large as the active area of SLM2.
  • Relay optics 212 relay light to the diffuser D1.
  • M5 is a beam turning mirror
  • D1 is a diffuser to reduce speckle, as previously described.
  • Projection optics 214 project the object formed on D1 by the relay optics 212, and preferably provide a large throw angle, for example >90°, for angled projection down onto a table top (the design is simplified by the relatively low scattere from the diffuser).
  • a system controller and hologram data processor 202 performs signal processing in either dedicated hardware, or in software, or in a combination of the two, as described further below.
  • controller 202 inputs image data and provides low spatial frequency hologram data 204 to SLM1 and higher spatial frequency intensity modulation data 206 to SLM2.
  • the controller also provides laser light intensity control data 208 to each of the three lasers.
  • the hologram SLM size is M ⁇ M pixels.
  • the input image target amplitude, ⁇ is of size ⁇ ⁇ ⁇ pixels. Amplitude range for the input is between 0 (black) and 1 (white).
  • D is a diffraction efficiency boost parameter controlling the trade-off between reconstruction error and diffraction efficiency
  • a value of 1 .0 gives theoretically perfect reconstruction; larger values of D increase the optical efficiency at the expense of increasing the noise. (Simulations suggest using a value for D of approximately 1 .5).
  • a hologram set H of N holograms of size x from R.
  • the above ADOSPR algorithm optionally with super-resolution may be employed, optionally iteratively optimising the holograms, for example using a Gerchberg-Saxton procedure.
  • the relative laser power K used to display this frame is given by mID.
  • the image is projected by displaying F on the imaging SLM, while sequentially displaying the N hologram subframes on the hologram SLM.
  • WO2010/007404 hereby incorporated by reference.
  • a spatial light modulator comprising an array of optically-active pixels (which may modulate phase or amplitude, or some combination of both) wherein one or more pixels are replaced by photosensitive elements, such as photodiodes.
  • photosensitive elements such as photodiodes.
  • photosensitive elements include phototransistors, photovoltaic elements, and so forth.
  • phase modulation such as diffractive image formation
  • diffractive image formation is fundamentally robust to a small number of non- modulating pixels, since the relationship between SLM data and projected image is given by a Fourier transform, and it can be shown that a small number (up to around 1 %) of inactive SLM pixels does not produce visible defects in the projected image.
  • Such photodiode elements may be produced using the same standard CMOS fabrication technology as employed for a conventional device's existing backplane, making this a minimal impact change in terms of cost and process.
  • the photodiodes are distributed throughout the array in such a manner that the spatial intensity profile of the beam can be extracted with some accuracy.
  • single-mode laser diode illumination is substantially Gaussian in structure, given by 6 parameters (beam centre given by ⁇ ⁇ and ⁇ , beam variance given by ⁇ ⁇ 2 and ⁇ , beam covariance given by o xy , peak beam intensity given by /), and so 5 or preferably more (for improved accuracy) appropriately-positioned photodiodes may be employed to determine these parameters. Because photodiode response is fast, the beam profile (and total intensity of incident light) can be determined in real time with minimal computation.
  • FIG. 5a shows a schematic illustration of a spatial light modulator 500 with, in this example, an array of 25 x 25 square, light modulating pixels 502 in which 25 of these are replaced by photodiodes 504 for the purpose of beam sensing (profiling).
  • photodiodes 504 for the purpose of beam sensing (profiling).
  • the techniques we describe are applicable to arrays of different sizes and different pixel shapes, and that whilst for illustration Figure 5a depicts photodiodes which are equally spaced, in other arrangements the photodiode spacing may not be equal.
  • the photodiodes may be more densely or less densely packed towards the centre of the array, or otherwise arranged.
  • CMOS complementary metal-oxide-semiconductor
  • MEMS liquid crystal
  • FIG. 5b shows a cross-sectional representation of four pixels from a MEMS piston pixel device 600, comprising MEMS structures 602 deposited on top of a standard 4-metal-layer silicon CMOS backplane 604, with one of the active pixel driver circuits 606 being replaced by a photodiode 608, optionally in combination with an interface circuit.
  • the MEMS structures 602 are shown schematically and comprise an electrode 602a mechanically coupled to a mirror 602b.
  • the CMOS back plane 604 comprises a silicon substrate 604a and, in the illustrated example, four layers of Inter-Layer Dielectric (ILD) 604b and metal/ILD 604c. It will appreciated that many alternative layouts are possible including other numbers of layers, and it will also be appreciated that a backplane may be fabricated using a technology other than CMOS.
  • ILD Inter-Layer Dielectric
  • the beam size ( ⁇ ⁇ 2 , ⁇ ) and beam covariance o xy (corresponding to the rotation of the beam) are given by:
  • the total intensity of the beam (including the part that overfills the SLM) is given by /:
  • FIGS 6 and 7 show holographic image projection systems 600, 700 which broadly correspond to those shown in Figures 1 and 2 (like elements are indicated by like reference numerals). Each incorporates a spatial light modulator as described above providing beam profile data over a respective connection 604, 704 to the hologram data processor 102, 202. The system at Figure 6 may employ this information for example to determine an average beam intensity, which may then be used for image brightness control as previously described.
  • this data may be employed for controlling the relative laser power K (see above), but preferably Gaussian beam parameters determined from the profile data as described above are used to define a profile of beam amplitude over the hologram SLM (SLM 1 ) so that the illumination pattern on the second SLM (SLM 2) can be more accurately determined from:
  • This reconstruction intensity can be employed as a more accurate reconstruction intensity at step (3) of the above described dual modulation architecture procedure.
  • the pixels may be non-rectangular, of unequal width and height.
  • the pixels are configured to modulate the phase of light incident on the spatial light modulator.
  • the SLM may be a liquid crystal SLM, magneto-optic SLM, acousto-optic SLM or an optically-addressed SLM. Phase modulation may be effected using the Kerr or Pockels effect, or the SLM may be, for example, a MEMS SLM.
  • the MEMS pixels may move along an axis substantially perpendicular to the device surface or they may tilt around one or more axes substantially parallel to the device surface; in embodiments movement of the pixels of the SLM may be controlled by electrostatic means.

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Abstract

L'invention concerne un appareil et des procédés permettant de générer une image par holographie, ainsi que des modulateurs de lumière spatiaux que l'on utilise dans ces techniques. L'invention concerne tout particulièrement un modulateur de lumière spatial (SLM) comprenant plusieurs pixels de modulation de lumière afin de diffracter un faisceau de lumière cohérente et de générer ainsi une image à partir d'un hologramme affiché sur ledit SLM, l'un au moins des pixels de modulation de lumière étant remplacé par un pixel de détection de lumière afin de détecter une intensité dudit faisceau de lumière cohérente à la position du pixel de détection de lumière. Le SLM peut être inclus dans un assemblage optique pour un système de projection d'image holographique, ceci en combinaison avec un système d'éclairage laser afin d'éclairer le SLM et un système de sortie optique afin d'émettre une image.
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