Classifications

• • 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
• • 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
• • 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
  • G03H1/04—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
  • G03H2001/0833—Look up table
• • 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
  • G03H2001/221—Element having optical power, e.g. field lens
• • G—PHYSICS
  • G03—PHOTOGRAPHY; CINEMATOGRAPHY; ANALOGOUS TECHNIQUES USING WAVES OTHER THAN OPTICAL WAVES; ELECTROGRAPHY; HOLOGRAPHY
  • G03H—HOLOGRAPHIC PROCESSES OR APPARATUS
  • G03H2210/00—Object characteristics
  • G03H2210/30—3D object
• • G—PHYSICS
  • G03—PHOTOGRAPHY; CINEMATOGRAPHY; ANALOGOUS TECHNIQUES USING WAVES OTHER THAN OPTICAL WAVES; ELECTROGRAPHY; HOLOGRAPHY
  • G03H—HOLOGRAPHIC PROCESSES OR APPARATUS
  • G03H2223/00—Optical components
  • G03H2223/24—Reflector; Mirror

Definitions

• This invention relates to improvements to three-dimensional (3D) displays, and their associated image generation means. More specifically, it relates to a way f-impr ⁇ ving the image quality of Diffraction Specific computer generated holograms (CGH) by means of a novel way of representing and calculating data relating to the image.
• CGH computer generated holograms
• Holographic displays can be seen as being potentially the best means of generating a realistic 3D image, as they provide depth cues not available in ordinary two dimensional displays or many other types of 3D display.
• the accommodation depth cue for example, is a cue that the brain receives when a viewer's eye focuses at different distances and is important up to about 3 meters in distance. This is, of course a cue that is used when looking at real objects, but of the 3D display technologies currently available, only true holograms provide 3D images upon which the eye can use its accommodation ability. It is a desire to be able to produce reconfigurable holographic displays electronically, such that an image can be generated from computer held data. This gives flexibility to produce holographic images of existing objects or nonexistent objects without needing to go through the time consuming and expensive steps normally associated with their production.
• DS CGH Diffraction Specific
• a DS CGH is a true CGH (as opposed to a holographic stereogram variant) but has a lower computational load than Interference Based true CGH algorithms. The reason for this is that the DS . algorithm is currently the most effective in terms of controlling the information content of CGH and avoiding unnecessary image resolution detail that cannot be seen by the human eye.
• a key concept of the DS algorithm is the quantisation of the CGH in both the spatial and spectral domains. This allows control of the amount of data, or the information content of the CGH, that in turn reduces the computational load.
• the CGH is divided up into a plurality of areas, known as hogels, and each hogel has a plurality of pixels contained within it.
• the frequency spectrum of each hogel is quantised such that a hogel has a plurality of frequency elements known as hogel vector elements.
• the system constraints that are present using the methods of the prior art are: a) Plane waves from more than 1 hogel must enter the eye pupil. This provides a constraint on the hogel aperture. Therefore, if the hogel is smaller then light from more of them can enter the eye. b) The number of lateral image volume points (and therefore the number of hogel vector components) must not exceed the number of pixels in a hogel divided by 2. This means that a large number of pixels per hogel is needed to give a good quality image. c) The point-spread function (the fineness to which a point can be focussed) of an image volume point is related to the distance the point is from the focal plane and the size of the hogel aperture. A larger hogel will give a sharper focussed point.
• a computer generated holographic display comprising at least a light diffraction plane notionally divided into a plurality of hogels, an image volume space and image calculation means, wherein image data is created by the steps of
• the coefficients are used to produce a diffraction pattern across the hogel such that light diffracted by the hogel produces a curved wavefront, this wavefront going on to produce at least one point in the image volume.
• the present invention allows each hogel in the system to generate curved waveforms, as opposed to the plane waves as generated in the prior art. It does this by sampling an imaginary wavefront coming from each point in the 3D volume at a plurality of points over the hogel, as opposed to the single point of the prior art. These samples are used to produce a set of complex Fourier coefficients that can be used to approximate the original waveform.
• Each hogel has contained within it a plurality of pixels.
• the dimensions of the hogel in terms of pixels, defines certain properties of the 3D image that is produced by the system.
• a full parallax system allows a viewer of the projected image to "see around" the image both horizontally and vertically. This type of system would have hogels that have a plurality of pixels in two dimensions.
• HPO horizontal parallax only
• the current invention is equally applicable to both systems.
• the dimensions of the hogels will be different, and the HPO system will save on computing power as the processing required for each hogel is only one dimensional, and cylindrical as opposed to spherical coordinates may be used.
• Anamorphic optics can also be used to replay such a hologram.
• n the number of Fourier components used to represent the wavefront is limited to 0 ⁇ m ⁇ n/2 to avoid undersampling of the wavefront and loss of information.
• m coefficients represent the magnitude of the first m possible grating frequencies in the hogel, and are the hogel vector components that are stored in the diffraction table.
• a method of producing a computer generated hologram on a display comprising at least a light diffracting panel notionally divided into a plurality of hogels, and image calculation means, where the method comprises the steps of :
• a first wavefront is mathematically projected from a point in the image volume through the optical system to a hogel, the wavefront being distorted by any aberrations in the optical system;
• the distortions added to the first wavefront by the optical system are used to generate a real, pre-distorted second wavefront emanating from the hogel, such that as the second wavefront passes through the distorting optics the pre-distortions on the second wavefront are removed.
• the distortions present in a particular system need only be measured or calculated once, and the data so obtained can be stored for later use with any image to be displayed.
• the distortion information is used to compute a pre- compensation in the diffraction table and is stored as more advanced form of diffraction table.
• Patent application WO 00/75733 provides a full description of correcting aberrations by distorting the wavefront.
• the current invention provides a particularly efficient means with which such an aberration correction method may be implemented, as the information regarding the required pre-distortions is stored in the diffraction table, and the calculations are hence done off-line.
• the light diffraction plane, or CGH will comprise of a. spatial light modulator, but any device capable of being addressed with a diffraction pattern may be used.
• the current invention may be implemented as a computer program running on a computer system.
• the program may be stored on a carrier, such as a hard disk system, floppy disk system, or other suitable carrier.
• the computer system may be integrated into a single computer, or may contain distributed elements that are connected together across a network.
• Figure 2 illustrates in diagrammatic form a CGH showing the division of the area into hogels.
• Figure 3 illustrates in diagrammatic form a typical hogel vector.
• Figure 4 illustrates in diagrammatic form a series of planar wavefronts emitted from a hogel of the prior art.
• Figure 5 illustrates in diagrammatic form the curved wavefronts that can be emitted from a hogel of the present invention.
• Figure 6 illustrates in diagrammatic form (a) a point in the image volume being formed using multiple hogels from the prior art, and (b), a point being formed by a single hogel using the curved wavefronts of the current invention.
• Figure 7 illustrates in diagrammatic form the process of decoding a hogel vector
• Figure 8 illustrates in diagrammatic form a system of the prior art using multiple hogels to create an approximation to a curved wavefront.
• Figure 9 illustrates in diagrammatic form the principle of the first stage of calculation of the diffraction table.
• Figure 10 shows a calculated point spread function for a hogel containing 500 pixels and subjected to certain constraints.
• Figure 11 illustrates in diagrammatic form the distortions that can arise in a practical system, and how, by pre-distorting a waveform from a hogel, these distortions can be compensated for.
• diffraction panel is used to describe this panel in this specification before diffraction information is written to it, although once the diffraction panel is written with diffraction information, it can be interchangeably termed a CGH.
• the DS algorithm comprises the following stages.
• the 3D image is made up by the diffraction of light from the hogels.
• the diffraction process sends light from one of the hogels in a number of discrete directions, according to which basis fringes are selected, as described below.
• a basis fringe represents part of the hogel vector spectrum, and when many basis fringes are accumulated into a hogel a continuous spectrum is formed.
• the basis fringes are calculated once for a given optical geometry, and are independent of the actual 3D image to be displayed. They can therefore be calculated offline, before the CGH is calculated and displayed.
• a given image must have the correct basis fringes selected in the appropriate hogels in order to properly display the image components.
• a diffraction table allows this selection to be done correctly.
• the diffraction table maps locations in the image volume to a given hogel, and to hogel vector components of that hogel. These locations, or nodes, are selected according to the required resolution of the 3D image. More nodes will give a better resolution, but will require more computing power to generate the CGH. Having control of the nodes therefore allows image quality to be traded for reduced processing time.
• the hogel vector selects and weights which basis fringes are required by a given hogel in order to construct the 3D-image information.
• the hogel vectors themselves are generated from data based on the 3D object or scene to be displayed.
• a geometric representation of the object is stored in the computer system.
• the geometric information is rendered using standard computer graphics techniques in which the depth map is also stored.
• the rendering frustum is calculated from the optical parameters of the CGH replay system.
• the rendered image and the depth map are used to define, in three dimensions, which parts of the 3D object geometry that the given hogel must reconstruct.
• a hogel vector can then be calculated from a combination of this information and the diffraction table to produce the hogel vector.
• the hogel vectors are used to select the appropriate basis fringes needed to make up the image.
• the hogel vector is decoded by accumulating the appropriate basis fringes into the hogel. This is a linear process and is repeated for each hogel vector element. The result is a complete hogel that is part of the final CGH.
• the wavelength of the light used to read the resultant hologram is a parameter to be considered when calculating the hogel vector components that are stored in the diffraction table.
• that wavelength may be anything suitable for a given application.
• Off-line recalculation of the diffraction table is all that is necessary if the wavelength needs to be changed.
• the diffraction table can be enlarged to include hogel vector components that are calculated for more than one wavelength simultaneously. In this way, the system is able to quickly change between different readout wavelengths, or to create holograms for multiple wavelength readout.
• Figure 1 illustrates the replay optics of a general CGH system, including a system capable of implementing the current invention.
• the diffraction panel 1 is shown transmitting a set of plane waves 7, encompassed by a diffraction cone 5 through a Fourier lens 3, where the waves 7 get refracted towards an image volume 2. It can be seen that the extent of diffraction of the plane waves, given by the cone 5 defines the size of the image volume 2.
• a conjugate image volume 6 is also formed adjacent the image volume 2.
• Figure 1 only shows plane waves 7 radiating from one area of the panel 1 , but of course in practice, each hogel on the panel 1 will be radiating such waves.
• the diffraction panel 1 is written correctly with appropriate basis fringe data for a given hologram, a viewer in the viewing zone 4 will see a true 3D image in the image volume 2, and the image conjugate in the volume 6.
• the conjugate image volume 6 is usually masked out.
• the distance of separation between the Fourier lens 3 and the diffraction panel 1 is kept as short as possible to simplify the processing.
• the steps involved in calculating the hogel vector components as shown below assume that this distance is zero.
• FIG. 2 shows the spatial quantisation of the diffraction panel 1 into a 2D array of hogels.
• Each hogel for example 8) is shown having a plurality of pixels in two dimensions. Therefore, a diffraction panel 1 so divided would be suitable for implementing a full parallax system.
• the number of pixels shown present in each of the hogels is shown figuratively only. In practice there would be approximately 2000 to 4000 pixels in each hogel dimension. In a HPO system, each hogel would have only one pixel in the vertical dimension, but approximately 2000 to 4000 in the horizontal dimension. The current implementation is restricted to a HPO system, to ease computing requirements.
• Figure 3 shows the spectral elements 9 of a typical hogel vector that is stored for each hogel.
• Each component of the vector represents a spatial frequency present in the image as viewed from the hogel in question.
• Figure 4 shows light 11 being diffracted from a single hogel in a number of discrete different directions, symmetrical about the normal 10. This is the method of the prior art.
• the particular angle of diffraction, and hence direction of each of the plane waves 11 is chosen for the particular image that is desired to be displayed in the image volume 2. It is the presence of a particular basis fringe that decides the angle of diffraction of a particular plane wave.
• Figure 5 shows light emanating from a hogel that produces curved wavefronts 12. This is the current invention, and results in an image of better quality.
• the curved wavefronts 12 are produced by a multiple sampling technique, as discussed later.
• Figure 6 shows the different methods in which the optical systems of both the prior art and the present invention display a point in the image volume. Fourier optics are represented , but the idea is equally applicable to other optical arrangements.
• the hogel aperture size was constrained by the need to make it smaller so that as many hogels as possible are used to make up the point, but also to make it larger so that the point can be sharply focussed by the eye.
• Figure 7 shows the process of decoding a hogel vector to produce a continuous output spectrum.
• the vector similar to that shown in Figure 3, is multiplied with a basis fringe 15 to produce a smooth output spectrum 16 as shown in Figure 7b.
• a vector of the present invention will have more coefficients 9 than one of the prior art, as a wavefront is sampled at multiple points across the hogel 8, allowing a hogel to produce curved wavefronts 12 as described below.
• Figure 8 shows plane waves 11 emanating from four hogels that converge at a point 13 that represents a point in the image volume. These rays 11 go on to diverge, before entering the eye of an observer. This observer will see the waves 11 as a point in space. It will be appreciated that the more plane waves 11 that go on to make the point 13, the more well defined the point 13 will be. This, of course, means that more hogels are required to define a point satisfactorily. It will be recalled from Figure 5 that only one hogel is needed to define a point in space 14 with the current invention, due to the curved wavefront 12 that can be emanated from it. This changes the constraints on hogel size etc such that a better quality image may be produced.
• the multiple point sampling of the wavefront across the hogel is illustrated with the aid of Figure 9.
• the first action in the calculation of the hogel vectors of the current invention is to notionally transmit a wavefront from each point in the image volume 3.
• P is at one of the nodes in the diffraction table (the node spacings can be decided a priori and can be chosen to vary nonlinearly and continuously over a wide range).
• the hogel vector components 9 which are associated with the point P the following procedure can be used, where the optics of the system are assumed to be well approximated by a thin, ideal Fourier lens:
• r ( ⁇ - ⁇ [ + (z-z J and / is the wave vector magnitude, r A is the point amplitude.
• the wavefront at the output of the lens is given by the product of the wavefront falling on the lens and the lens transmission function. If the hogel is not in contact with the lens, a further propagation step needs to be made to calculate the wavefront across the hogel.
• the m hogel vector components associated with P can now be calculated. This may be done using an FFT technique or numerical integration technique to determine the first m complex coefficients of the Fourier series of the wavefront.
• the theoretical number of frequency components required in each hogel vector is estimated from the rate of change of the wavefront across the hogel.
• a centrally positioned hogel will have a wavefront across it that has a lower rate of change than a hogel on the extremities of the diffraction panel, where the phase terms of the wavefront will vary much more quickly.
• the largest value of m is proportional to ⁇ m ax /2 ⁇ , where ⁇ max is the maximum phase deviation of the waveform across the hogel.
• the diffraction table may therefore be of varying dimensionality.
• the hogel vectors for the centrally positioned hogels will need fewer frequency components to represent the " wavefront than those on the extremity of the diffraction panel.
• the number of frequency components of the Fourier transform can be reduced if this estimate derived from the paragraph above is lower than half the number of pixels across the hogel, thus saving computing time.
• Figure 11 shows, in a simplified form, (a) the effects on a wavefront being emitted from a system that adds no compensation correction, and (b) the current invention being used to pre-distort a wavefront to compensate for aberrations present in the optical system.
• the wavefronts from the hogel are intended to focus to as sharp a point as possible in the image volume.
• Figure 11a shows a wavefront 18 being emitted from a hogel and passing through an optical system comprising two mirrors 19, 20. This system, like all optical systems, will not be perfect, and hence distortions will be introduced. After reflecting from the mirror 19 the wavefront will be distorted, shown as 18'.
• FIG. 11 b shows how the present invention can correct for the aberrations present in the mirror. As the present invention allows the waveforms emitted from the hogel to be curved in a controlled manner, then the waveform can be pre-distorted with the inverse distortions that are present in the optics. This is shown as the passage of waveform 17.
• the pre-compensation distortions are present as the waveform is first emitted from the hogel, and these pre- compensation distortions are gradually removed by the aberrations in the optical system.
• the waveform at 17' is less distorted, and the waveform at 17" less distorted still. This results in the wavefront coming to a much sharper point 22.
• the invention thus gives control over the desired image quality.
• the current implementation of the invention is a HPO system, with the light diffraction plane comprising 20 hogels in the horizontal dimension, where each hogel has 1024 pixels. This is capable of providing 512 lateral resolution points across the image volume.
• the current invention has been implemented on an Active-Tiling® Computer Generated Hologram display system.
• the computer system used to produce the CGH can be a standalone unit, or could have remote elements connected by a network.
• the Active Tiling system is a means of producing holographic moving images by rapidly replaying different frames of a holographic animation.
• the Active Tiling system essentially comprises a system for directing light from a light source onto a first spatial light modulator (SLM) means and relaying a number of SLM subframes of the modulated light from the first high speed SLM means onto a second spatially complex SLM.
• SLM spatial light modulator
• the full CGH pattern is split up into subframes in which the number of pixels is equal to the complexity of the first SLM. These frames are displayed time- sequentially on the first SLM and each frame is projected to a different part of the second SLM. The full image is thus built up on the second SLM over time.
• the first SLM means comprises an array of the first SLMs that each tile individual subframes on the second SLM over their respective areas.
• the first SLM of such a system is of a type in which the modulation pattern can be changed quickly, compared to that of the second SLM. Thus its updating frame rate is greater than the readout frame rate of the second SLM.
• the Active Tiling system has the benefit that the image produced at the second SLM, which is addressed at a rate much, slower than that of the first SLM array, is effectively governed by the operation of the first SLM. This permits a trade off between the temporal information available in the high frame rate SLMs used in the SLM array and the high spatial resolution that can be achieved using current optically addressed SLMs as the second SLM. In this way, a high spatial resolution image can be rapidly written to an SLM using a sequence of lower resolution images.

Landscapes

• Physics & Mathematics (AREA)
• General Physics & Mathematics (AREA)
• Holo Graphy (AREA)

Applications Claiming Priority (5)

Publications (1)

Family

ID=26245243

Family Applications (1)

Country Status (5)

Families Citing this family (13)

Family Cites Families (3)

• 2001
  • 2001-11-02 EP EP01982588A patent/EP1332410A1/de not_active Withdrawn
  • 2001-11-02 JP JP2002541454A patent/JP2004516498A/ja not_active Withdrawn
  • 2001-11-02 AU AU2002214131A patent/AU2002214131A1/en not_active Abandoned
  • 2001-11-02 WO PCT/GB2001/004855 patent/WO2002039192A1/en active Application Filing
  • 2001-11-02 CA CA002428149A patent/CA2428149A1/en not_active Abandoned

Non-Patent Citations (3)

Also Published As

Similar Documents

Legal Events