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
  • G03H1/32—Systems for obtaining speckle elimination
• • 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
  • G03H1/2286—Particular reconstruction light ; Beam 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
  • G03H1/22—Processes or apparatus for obtaining an optical image from holograms
  • G03H1/2286—Particular reconstruction light ; Beam properties
  • G03H2001/2289—Particular reconstruction light ; Beam properties when reconstruction wavelength differs form recording wavelength
• • 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
  • G03H2001/2297—Addressing the hologram to an active spatial light modulator using frame sequential, e.g. for reducing speckle noise
• • 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
  • G03H1/2645—Multiplexing processes, e.g. aperture, shift, or wavefront multiplexing
  • G03H2001/266—Wavelength multiplexing
• • 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
  • G03H2222/13—Multi-wavelengths wave with discontinuous wavelength ranges

Definitions

• the invention relates to a method for reducing speckle patterns of a three-dimensional holographic reconstruction of a three-dimensional scene and a holographic display device for carrying out the method.
• Field of application of the invention are methods with which the storage and reconstruction of complex wavefronts of a three-dimensional scene (3D scene) by holography using preferably laser light in real time or near real time in holographic playback devices and in which the reconstruction can be seen from a virtual viewer window ,
• Holography enables the recording and the optical reproduction of a three-dimensional object or a moving 3D scene with wave-optical methods.
• the 3D scene is encoded in a light modulator that serves as the carrier medium.
• each point of the coded 3D scene forms a starting point of light pauses that interfere with each other and spatially reconstruct the 3D scene as resulting lightwave front as though it were due to light propagation from the actual object in space.
• the holographic reconstruction of the object or of the 3D scene preferably takes place with a projection device and / or a reconstruction optical unit by illuminating the carrier medium with normally sufficiently coherent light.
• the 3D scene is reconstructed in a holographic display with a viewer window, here a visibility area in a periodicity interval of the complex-valued wavefront in the back focal plane of a reconstruction means in a viewer space.
• a viewer window here a visibility area in a periodicity interval of the complex-valued wavefront in the back focal plane of a reconstruction means in a viewer space.
• the observer window is preset in size in front of a reproduction means and generally corresponds at least to the size of an eye pupil.
• the observer window is formed either as the direct or inverse Fourier transform or Fresnel transform of a hologram encoded in a carrier medium or as the image of a wavefront encoded in a carrier medium in a viewer space, the observer window comprising only a single diffraction order of a periodic reconstruction ,
• the hologram or the wavefront are calculated from the 3D scene in such a way that within the one diffraction order used as the visibility region, any crosstalk of other diffraction orders into the viewer window, which usually occurs in reconstructions using light modulators, is prevented.
• Spatial light modulators such as LCD, LCoS, etc., which modulate the phase and / or the amplitude of the incident light, serve as carrier or recording media for holograms or complex-valued wavefronts of a 3D scene.
• the refresh rate of the carrier medium must be sufficiently large.
• the values coded in the carrier medium in regularly arranged pixels can originate from a real object or be a computer-generated hologram (CGH).
• Viewing the reconstruction of the 3D scene can be done by the viewer looking directly at the carrier medium. This is referred to in this document as a direct view setup. Alternatively, the viewer can look at a screen onto which either an image or a transform of the values coded in the carrier medium is projected. This is called projection construction in this document.
• the designation screen is used both for the screen in the projection setup and for the carrier medium in the direct-view setup. Due to the discrete recording, the reconstruction of the hologram is only possible within a periodicity interval of the reconstruction of a wavefront given by the resolution of the carrier medium, as a result of diffraction. In the adjacent periodicity intervals, the reconstruction is repeated, usually with disturbances.
• speckle pattern a granulation-like interference pattern that results from the interference of many light waves with statistically irregularly distributed phase differences.
• the speckle patterns are detrimental.
• the hologram calculation is preceded by a discrete sampling of the 3D scene, since only a discrete recording is possible on the carrier medium.
• Certain coding methods in which the information about the 3D scene is suitably stored in the carrier medium allow, in principle, a reconstruction in which, at the location of the sampling points, even the reconstruction completely coincides with the scanned object.
• the physical reconstruction again results in a continuous course between the sampling points.
• the reconstruction contains speckle patterns that reduce the quality of the reconstruction. This is the case, in particular, if the calculation of the hologram is carried out with a random phase of the object points, which however is advantageous for certain other reasons.
• Reduction of the speckle patterns in the reconstruction of the 3D scene can in principle be achieved by temporal and / or spatial averaging, wherein the reconstruction is generated from values of a 3D scene coded into an external medium or from appropriately calculated hologram values.
• the eye of the beholder always averages several reconstructions with different speckle patterns that are shown to him, whereby a reduction of this disturbance is perceived.
• a rotating rectangular glass plate is brought into the beam path. It rotates with a tuned to the frequency of a detector frequency, causing the speckle no longer disturbing.
• the object of the invention is to significantly reduce the speckle patterns occurring in the reconstruction of a 3D scene in a holographic display device with a virtual viewer window and to provide a process operating close to the real-time, in which a carrier medium with a conventional refresh repetition frequency can be used.
• the invention is generally based on a method in which a controllable light modulator, in which a hologram of a 3D scene is coded, is illuminated by sufficiently coherent light, in which a reconstruction optic transforms modulated light into a viewer window or an eye position of a viewer's room and in FIG a reconstructed space reconstructs the 3D scene, and in which the illumination is controlled by a control means.
• the observer window on which the invention is based for reconstructing the SD scene can here also be set equal to the eye position as the location in the observer's room at which different light distributions of the complex wavefronts of the encoded hologram are generated. In order to see the reconstructed 3D scene, the eyes of an observer must be in this eye position.
• control means influences the coherent light in at least one property in such a way that a plurality of complex-value wavefronts with different wavelengths pass through the light modulator, wherein they are modulated with the coded log ram values, and the modulated complex-valued wavefronts are transformed by the reconstruction optics into the eye position and produce in the reconstruction space at the same location several reconstructions of the 3D scene with mutually slightly different speckle patterns, averaged from the eye position as a single speckle-reduced reconstruction of the 3D scene become.
• an illumination means controlled by the control means generates a temporally fast sequence of light pulses for illuminating the reconstruction optics and the light modulator, the light pulses differing slightly in their wavelengths,
• the temporally fast sequence of light pulses passes through the light modulator, whereby the complex valued wavefronts of the light pulses are modulated with the coded hologram values, and
• a plurality of illumination means simultaneously emit coherent light, which is influenced by the control means so that several complex-valued wavefronts with illuminate the reconstruction optics and the light modulator at slightly different wavelengths at the same time,
• the complex-valued wavefronts having slightly different wavelengths pass through the light modulator simultaneously, being modulated with the coded hologram values, and
• lasers are advantageously used as illumination means which are arranged in a spatial arrangement so that the coherent light of a respective illumination means with a separate imaging optics of an imaging agent is imaged in each case a separate optical fiber and then in a single optical fiber for simultaneously illuminating the reconstruction optics and the Light modulator is merged.
• a lighting means is created in a simple manner, which realizes coherent light with mutually slightly different wavelengths for the simultaneous illumination of the light modulator.
• the methods according to the invention can be carried out separately for each of a right and a left eye of a viewer, for example in chronological succession.
• the different wavelengths in the said methods are changed by the control means to each other in a defined manner or subjected to a random fluctuation within predetermined limits.
• a holographic display device for carrying out the method according to claim 2 contains in the light direction the following means:
• an illumination means which emits coherent light pulses in temporally rapid succession whose wavelengths differ slightly from each other, for
• a reconstruction optics for transforming a temporally fast sequence of modulated complex wavefronts into an eye position in one Observer space and to generate multiple reconstructions of the same 3D scene in rapid time series at the same location in a reconstruction space
• a coding means in the form of a light modulator, in which a hologram of a 3D scene is coded, and a control means for controlling the illumination means, the coding means and the reconstruction optics.
• Another embodiment of the holographic display device according to the invention contains the following means according to claim 9 for carrying out the method in the light direction:
• a reconstruction optical system for simultaneously transforming a plurality of modulated complex wavefronts of a hologram into an eye position of a hologram
• a coding means in the form of a light modulator, in which the hologram of the 3D scene is coded, an imaging means, comprising imaging optics having at least one dimensionally adjacently arranged imaging means, for imaging the coherent light of the illumination means into a plurality of optical fibers and
• control means for controlling the illumination means, the coding means and the reconstruction optics.
• An essential feature of the invention is also that the slight change in the wavelengths is in the range of a few nanometers. Even such a change of the wavelengths to each other is sufficient to produce a plurality of slightly different reconstructions of the same 3D scene with mutually changed speckle patterns in the reconstruction space.
• the respective eye of the observer averages from the eye position or from the viewer window over the speckle pattern and sees only a single speckle-reduced reconstruction of the original 3D scene.
• a holographic display device for reducing the speckle pattern is, for example, a holographic display.
• a holographic display with viewer window differs significantly from an ordinary Fourier hologram or a Fresnel hologram with respect to the wavelength dependence of the holographic reconstruction.
• the lateral position of a reconstructed object point of the 3D scene does not change with the wavelength, in contrast to a normal Fourier hologram or Fresnel hologram.
• the individual object points are encoded in the hologram as lenses.
• the wavelength is included in this coding.
• An encoded lens that has a certain focal length at one wavelength changes its focal length inversely proportional to the wavelength. A change in the wavelength therefore causes the depth of the reconstructed object point to change.
• a speckle reduction using different wavelengths thus takes place in a holographic display with viewer window via a change in the depth of the reconstruction with the wavelength.
• the change in the depth with the wavelength outside the center of the observer window results in a parallax effect.
• the observer then sees from his eye position the reconstruction of the different wavelengths next to each other.
• a speckle reduction results especially when this parallax is at least in the range of speckle size.
• the speckle reduction by using different wavelengths thereby increases from the center to the edge of the viewer window.
• the described effect of speckle reduction is smaller than that in a conventional Fourier hologram.
• the change of wavelengths must be in the range of several nanometers. Typical sizes can be 10 or even 20 nanometers.
• a wavelength range that is so large that it leads to a significant smearing of the reconstruction in a normal Fourier hologram and thus reduces the quality can thus lead to a good reconstruction quality with reduced speckle pattern in a holographic display with viewer window.
• the display can be realized either as a projection display or as a direct-view display.
• illumination means both lasers and LEDs can be used in the various embodiments of the invention.
• An inherently broadband light source such as an LED can also contribute to the reduction of speckle patterns through its spectrum.
• the laser has the advantages To be approximately a point light source and to deliver a higher power.
• the hologram is coded only once in contrast to the prior art and does not have to be recalculated several times, resulting in a saving of computing time.
• Fig. 1 is a schematic plan view of a holographic
• Fig. 2 is a schematic plan view of a holographic
• Direct view display in a second embodiment.
• the observer window for reconstructing the 3D scene on which the invention is based here corresponds to the visibility region with an eye position as the location in the observer's room, to which a plurality of slightly different ones from one another
• FIG. 1 shows a first possible embodiment of a holographic direct-view display schematically and simplified in plan view.
• a laser as reconstruction optics RO a transformation fin, followed by a pixelized light modulator SLM, arranged.
• a reconstruction space with a conical cross section between the light modulator SLM and an eye position PE a reconstruction of a 3D scene is shown.
• this Eye position PE off which lies in the rear focal plane of the transformation lens, the reconstruction of the 3D scene for a viewer's eye is completely visible.
• the lighting and thus also the light touched components in the beam path are controlled by a control means CM.
• An externally controlled by the control means CM laser illuminates the light modulator SLM and arranged in front of the transformation lens with sufficiently coherent light.
• the propagation direction of the light is indicated by an arrow.
• the control means CM causes by very fast turning on and off of the laser, that this generates a temporally fast sequence of coherent light pulses, each pulse corresponds to a complex wavefront and the light pulses have mutually different wavelengths.
• the light pulses are shown by several intensity curves on the arrow line in Fig. 1.
• the wavelengths of the individual light pulses differ only slightly from each other and can be changed defined by appropriate program specifications in the control means CM or be exposed within predetermined limits of a random fluctuation. It makes sense that the change of the wavelengths in the range of a few nanometers, so that the subsequent reconstructions and the corresponding speckle patterns for the means are not too large differences.
• the temporally fast succession of light pulses with the coded hologram values of a 3D scene is modulated and transformed in a temporally fast sequence into the rear focal plane BE of the transformation lens arranged in front of the light modulator SLM, wherein the transformation lens is also the reconstruction optics RO at the same time.
• the rear focal plane BE of the reconstruction optics RO is located in a viewer's room, in which the eye position PE always lies.
• the modulated complex-valued wavefronts generate in rapid succession several reconstructions of the same 3D scene with slightly different speckle patterns at the same location in the reconstruction space. From the eye position PE, a viewer's eye sees the reconstructions as a single reconstruction of the 3D scene with averaged bacon pattern. Even if temporally fast sequences of light pulses are generated, the same hologram with conventional image refresh rate can advantageously always be displayed on the light modulator and the hologram calculation must then be performed only with this image refresh rate.
• the number of reconstructions of the 3D scene without using additional components can be arbitrarily increased in order to further reduce the occurring speckle pattern.
• a second possible embodiment of a holographic direct-view display is shown schematically and simplified in plan view.
• illumination means L1, L2 and L3 are three juxtaposed lasers, as imaging means AM three one-dimensional adjacently arranged imaging optics AO, as reconstruction optics RO a transformation lens followed by a pixelated light modulator SLM provided.
• imaging means AM three one-dimensional adjacently arranged imaging optics AO
• reconstruction optics RO a transformation lens followed by a pixelated light modulator SLM provided.
• three slightly different reconstructions with mutually slightly different speckle patterns for averaging for one eye can be generated.
• the number of lasers and corresponding imaging optics can be arbitrarily increased in order to generate a larger number of reconstructions simultaneously and to improve the speckle reduction by averaging.
• the reconstruction of the 3D scene is shown. From the eye position PE, which lies in the rear focal plane of the transformation lens, the reconstruction of the 3D scene for a viewer's eye is completely visible.
• the lighting and thus also the light-affected components in the beam path are controlled by a Steuermtttel CM.
• three lasers emitting light of slightly different wavelengths emit sufficiently coherent light, which in each case has an associated imaging optical system AO in FIG
• an optical fiber is imaged.
• Both the laser and the imaging optics AO are arranged one-dimensionally adjacent to one another. With a larger number of lasers, however, they can also be arranged two-dimensionally as a composite.
• a suitable imaging means for two-dimensional imaging of the two-dimensional composite of lasers is then advantageously designed as a matrix-shaped lens array.
• the light of the optical fibers is concentrated in a single optical fiber LLF and, under the control of the control means CM, programatically illuminates the transformation lens and the light modulator SLM with combined light having three slightly different wavelengths.
• the transformation lens transforms the different wavelength light into its rear focal plane BE into the eye position PE. If there is a viewer's eye at this point, then three complex-valued wavefronts with mutually different wavelengths are present simultaneously within the eye pupil, from which the transformation lens simultaneously generates three reconstructions of the 3D scene. Because the three reconstructions with mutually slightly different speckle patterns are created and overlaid simultaneously at the same location in the reconstruction space, the eye averages over these reconstructions and sees only a single reconstruction of the reduced speckled 3D scene.
• the Fourier transformation is preferably used, since it can be easily implemented in terms of programming technology and can be realized very precisely by optical systems.
• the hologram coding can be variably specified in the exemplary embodiments in FIGS. 1 and 2 so that the reconstructions of the 3D scene can be seen in front of and / or behind the screen.
• the light modulator SLM also fulfills the function of the screen at the same time.
• the position data of a viewer's eye are usually determined in FIG. 1 and FIG. 2 by a position detection system (not shown) and taken over by the control means CM, which need not be discussed in greater detail here.
• the method for reducing speckle in the reconstruction of a 3D scene described on the basis of exemplary embodiments of a holographic direct-view display can also be used according to the invention in a holographic projection display.

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