Video hologram and apparatus for reconstructing a video hologramThe present invention is a divisional application of the application having the title "video hologram and apparatus for reconstructing video hologram" with the application number of 200810096741.9, the application date of which is 11/2003. The application with application number 200810096741.9 is a divisional application based on the application with application date 11/2003, application number 200380103105.X (PCT/DE2003/003791), invention name "video hologram and apparatus for reconstructing video hologram".
Technical Field
The invention relates to a video hologram and to a device for reconstructing video holograms comprising an optical system, comprising at least one light source, lenses and a hologram carrier medium consisting of cells arranged in a matrix or other regular pattern and each cell being provided with at least one opening, the phase and amplitude of which can be controlled, and a viewing plane located in the image plane of the light source.
Background
Devices for reconstructing video holograms using an acousto-optical modulator (AOM) are known from the prior art (Stephen a. benton, Joel s. kollin: three dimensional display system, US 5172251). Such an acousto-optical modulator converts an electrical signal into a light wave front, which is recombined into a video frame using a deflection mirror to form a two-dimensional holographic region. Other optical elements are used to reconstruct the scene visible to the observer from the individual wavefronts. The optical means used, such as lenses and deflection elements, have the dimensions of the scene reconstructed. Due to their large depth, these elements are bulky and heavy. It is difficult to narrow them, so that their application range is limited.
The use of Computer Generated Holograms (CGH) offers another possibility to produce large video holograms by the so-called "tiling method". In this method, it is known from WO00/75698a1 and US6,437,919B1 to combine small CGHs with a small pitch by means of an optical system. To this end, in a first step, the required information is written into small-spaced fast matrices (typically EASLMs (electronically addressable spatial light modulators)), which are then replicated onto appropriate hologram media and combined to form large video holograms. Typically, an Optically Addressable Spatial Light Modulator (OASLM) is used as the hologram medium. In a second step, the combined video hologram is reconstructed using coherent light in transmission and reflection.
In CGHs with controllable openings arranged in a matrix or other regular pattern, it is known, for example from WO01/95016A1 or from "Eye-position tracking type electro-logical display using liquid crystal devices" of Fukaya et al, Proceedings of EOS clinical Meeting Diffractive Optics, 1997, to code a scene with diffraction on small openings. The wavefronts coming out of the opening converge on the object points of the three-dimensional scene before they reach the observer. The smaller the pitch and thus the smaller the opening in the CGH, the larger the diffraction angle, i.e. the viewing angle. Accordingly, resolution can be improved using these known methods of enlarging the viewing angle.
As is generally known, in a fourier hologram, the scene is reconstructed as a direct fourier transform or an inverse fourier transform of the hologram in a plane. The reconstruction is continued periodically with a periodic interval, the extent of which is inversely proportional to the spacing in the hologram.
If the reconstruction size of the fourier hologram exceeds the period interval, adjacent diffraction orders will overlap. As the resolution is gradually reduced, i.e. as the aperture pitch is increased, the reconstructed edges will be gradually deformed by overlapping higher diffraction orders. The usable range of reconstruction is then progressively limited.
If a larger periodic spacing, and thus a larger viewing angle, is to be obtained, the required spacing in the hologram is closer to the wavelength of the light. Therefore, the CGH must be large enough to be able to reconstruct a large scene. Both of these conditions require a large CGH with a large number of openings. However, this is not feasible in displays with a controllable opening form (see EP0992163B 1). CGHs with controllable openings are only one to a few inches and the pitch is also substantially greater than 1 μm.
The two parameters, the pitch and the hologram size, are characterized by the so-called Space Bandwidth Product (SBP) as the number of openings in the hologram. If a CGH reconstruction with a controllable aperture having a width of 50cm is to be produced such that the observer can observe the scene at a distance of 1m and with a horizontal viewing window of 50cm width, the SBP in the horizontal direction is approximately 0.5 x 106. This corresponds to 500,000 openings in the CGH at a distance of 1 μm. Assuming an aspect ratio of 4: 3, 375000 openings are required in the vertical direction. Accordingly, if three color sub-pixels are considered, the CGH includes 3.75 × 1011An opening. If it is considered that a CGH with a controllable opening generally only allows the amplitude to be influenced, the number is tripled. The phase is encoded using the so-called meander phase effect, which requires at least three equidistant openings per sample point. SLMs with such a large number of controllable openings have not been known so far.
Hologram values have to be calculated from the scene to be reconstructed. Assuming a depth of 1 byte for each of the three primary colors and a frame rate of 50Hz, the information flow rate required for CGH is 50X 1012＝0.5×1014Bytes per second. The fourier transform of such a large data stream is beyond the capabilities of today's computers and thus is not capable of computing holograms on a local computer basis. However, transmitting such large amounts of data over a data network is currently not feasible for ordinary users.
In order to reduce the amount of computation, it has been proposed not to compute the entire hologram, but only those parts that can be directly seen by the observer, or those parts that change. Such holograms consisting of addressable subregions, such as the above-mentioned "tiled holograms", are disclosed in the above-mentioned patent document WO01/95016a 1. The starting point for the calculation is the so-called effective exit pupil, the position of which may coincide with the eye pupil of the observer. As the viewer position changes, the image is tracked by continually recalculating the hologram portions that produced the image for the new viewer position. However, this cannot reduce the amount of calculation in part.
The disadvantages of the known method can be summarized as follows: the structure with the acousto-optic modulator is too bulky and cannot be reduced to the dimensions known from the flat displays of the prior art; video holograms produced using the tiling method are a two-stage process that requires a great deal of technical effort and cannot be easily reduced to desktop size; and the structures based on SLMs with controllable openings are too small to reconstruct large scenes. There are currently no large controllable SLMs with extremely small pitch, which is desirable, and the technology is further limited by computer performance and the data network bandwidth currently available.
Disclosure of Invention
It is an object of the present invention to circumvent the above drawbacks and provide extended real-time reconstruction of large-view video holograms.
This object is achieved in an inventive manner according to the invention by a video hologram and a device for reconstructing a video hologram having the features of claim 1. Preferred embodiments of the invention are claimed in claims 2 to 10.
According to the invention, a video hologram with a controllable opening and a device for reconstructing the video hologram are characterized in that: in the viewing plane, at least one viewing window is formed in the periodic interval as a direct fourier transform or an inverse fourier transform of the video hologram, said viewing window allowing the viewer to view a reconstruction of the three-dimensional scene. The maximum extent of the video window corresponds to the period interval in the inverse fourier transform plane in the light source image plane. A frustum extends between the hologram and the viewing window, the frustum shown containing the entire three-dimensional scene as a fresnel transformation of the video hologram.
The viewing window in the present invention is approximately limited and positioned relative to one eye of the viewer, the gaze distance, or relative to another suitable area.
Now, in the present invention, a further viewing window for the other eye of the observer is provided. This is achieved by placing the observed light source in another suitable location, or adding a second actual or virtual sufficiently coherent light source to form a pair of light sources in the optical system. This configuration allows the use of two eyes to see a three-dimensional scene through two associated viewing windows. The content of the video hologram may be changed, i.e. re-encoded, in dependence on the eye position in synchronization with the start of the second viewing window. If several observers are watching the scene, a plurality of observation windows can be generated by switching on further light sources.
In a further aspect of the invention relating to an apparatus for reconstructing video holograms, the optical system and the hologram carrying medium are arranged such that the higher diffraction orders of the video hologram have a zero point of the first viewing window or have a minimum intensity at the location of the second viewing window. This prevents the viewing window of one eye from cross-talking to the viewer or other viewers. The light intensity reduction of the higher diffraction orders is then utilized, because of the limited opening width of the hologram carrying medium and/or the minimum of the intensity distribution. The intensity distribution of the rectangular opening is, for example, sinc2Function whose amplitude decreases rapidly and forms sin which decreases with increasing distance2A function.
The number of openings in the display determines the maximum number of values that must be calculated for the video hologram. The transmission of data from a computer or over a network to a display representing a video hologram is limited to the same number of values. The dataflow rate does not differ substantially from the dataflow rate known in typical displays used today. This is now described by way of example.
For example, if the viewing window is reduced from 50cm (horizontal) by 3.75cm (vertical) to 1cm by selecting a sufficiently low resolution display, the number of openings in the hologram may drop to 1/1875. The required bandwidth is reduced in the same way during data transmission through the network. A video hologram created using known methods requires 1012An opening, and in this example the number is reduced to 5 x 108And (4) pixels. The scene can be viewed through the rest of the viewing windows. These requirements on pitch and hologram size in terms of the spatial bandwidth product have been achieved by displays available today. This allows large real-time video holograms to be economically implemented on displays with large spacing for large viewing windows.
The viewing window is tracked by mechanically or electronically moving the light source, by using a movable mirror, or by using a light source that can be positioned sufficiently in any other way. The viewing window is moved in accordance with the movement of the light source image. If the observer moves, the light source is moved spatially so that the viewing window follows the eyes of the observer. This ensures that as the observer moves, they can also see the reconstructed three-dimensional scene so that their free movement is not restricted. Various systems are known for detecting the position of an observer, for example, magnetic sensor-based systems can advantageously be used for this.
The invention also allows efficient color reconstruction of video holograms. Here, the reconstruction is performed using at least three apertures per cell (representing three primary colors), the amplitude and phase of which are controllable and which are encoded separately for each primary color. Another possibility for color reconstructing a video hologram is to perform at least three reconstructions, i.e. for the individual primary colors, one after the other using the device of the invention.
The invention allows for the efficient generation of hologram reconstructions of spatially extended scenes in real time by controllable displays, such as TFT flat panel screens, and provides large viewing angles. These video holograms can be advantageously used in TV, multimedia, gaming and design applications, in the medical and military fields, as well as in many other fields of economy and society. The three-dimensional scene may be generated by a computer or other means.
Drawings
Embodiments of the invention will be described and explained with reference to the accompanying drawings, in which:
FIG. 1 is a general diagram of a video hologram and apparatus for reconstructing the video hologram showing the generation of diffraction orders and the location of a viewing window;
FIG. 2 is a general diagram of an apparatus for reconstructing a video hologram, showing a three-dimensional scene viewable through a viewing window;
FIG. 3 is a general diagram of an apparatus for reconstructing a video hologram, showing the encoding of a portion of a three-dimensional scene of the video hologram;
FIG. 4 is a diagram showing the light intensity distribution in the observation plane according to the diffraction order; and
fig. 5 is a general diagram of an apparatus for reconstructing a video hologram showing the position of the viewing windows for both eyes of an observer with respect to the diffraction orders to prevent cross-talk.
Detailed Description
The apparatus for reconstructing a video hologram comprises: a hologram carrying medium, a real or virtual substantially coherent point or line light source, and an optical system. The video hologram carrying medium itself is composed of cells arranged in a matrix or other regular pattern, each cell having at least one opening whose phase or amplitude is controllable. The optical system for reconstructing the video hologram may be implemented by an optical imaging system as known in the art, which comprises a point or line laser, or a sufficiently coherent light source.
Fig. 1 shows a general structure of a video hologram and its reconstruction. The light source 1, the lens 2, the hologram carrying medium 3 and the viewing plane 4 are arranged one after the other, seen in the propagation direction of the light rays. The viewing plane 4 corresponds to the fourier plane of the inverse transform of the video hologram with diffraction orders.
The light source 1 is imaged onto the viewing plane 4 by an optical system represented by a lens 2. If a hologram carrying medium 3 is inserted, it is reconstructed as an inverse fourier transform in this viewing plane 4. The hologram carrier medium 3 with periodic openings generates alternating diffraction orders at moderate distances in the viewing plane 4, where holograms encoded into higher diffraction orders occur, for example by the so-called detour phase effect. Since the light intensity of higher diffraction orders is reduced, 1stAnd-1stServes as an observation window 5. If not otherwise explicitly stated, 1stThe diffraction orders will be the basis for further description of the present invention.
The reconstructed dimensions are here chosen to correspond to 1 in the viewing plane 4stThe size of the periodic interval of diffraction orders. Accordingly, higher diffraction orders are assigned without forming a pitch and without overlapping.
As Fourier transform, selected 1stThe diffraction orders form a reconstruction of the hologram carrying medium 3. However, it does not represent the actual three-dimensional scene 6. It simply serves as a viewing window through which a three-dimensional scene 6 (see fig. 2) can be viewed. With 1stThe actual three-dimensional scene 6 is represented in the form of a circle (circle) within the ray bundle of diffraction orders. The scene is then located within a reconstruction frustum extending between the hologram carrying medium 3 and the viewing window 5. The scene 6 is treated as a fresnel transformation of the hologram-carrying medium 3, while the viewing window 5 is a partial fourier transformation.
Fig. 3 shows the corresponding hologram encoding. The three-dimensional scene is composed of discrete points. A pyramid with the observation window 5 as the base and a selected point 7 in the scene 6 as the apex extends through this point and is projected onto the hologram-carrying medium 3. A projection area 8 is created on the hologram-carrying medium 3, in which the points are holographically encoded. The distance between the point 7 and the elements of the hologram carrying medium 3 can be determined to calculate the phase value. This reconstruction allows the size of the observation window 5 to be limited by the period interval. However, if the dots 7 are encoded throughout the hologram-bearing medium 3, the reconstruction will exceed the period interval. The viewing areas from adjacent diffraction orders overlap which will result in the viewer seeing a periodic continuation of the spot 7. The contour of the coded surface is thus blurred by the multiple overlaps.
Cross-talk with other viewing windows (cross-talking) is suppressed by using the fact that the intensity decreases with increasing diffraction order. Fig. 4 schematically shows the light intensity distribution on the diffraction orders, which distribution is determined by the width of the opening in the CGH. The abscissa shows the diffraction order. 1stThe diffraction orders represent a viewing window 5 for the left eye, i.e. a left viewing window, through which the three-dimensional scene can be viewed. Crosstalk into the right eye viewing window is suppressed by the decrease in light intensity with higher diffraction orders and, in addition, by the zero point of the intensity distribution.
Of course, the observer can observe the scene 6 of the hologram 3 using both eyes (see fig. 5). For the right eye, -1 from the light source 1' is selectedstRight viewing window 5' in the representation of diffraction orders. As can be seen from the figure, this light affects the left eye with very low intensity. Here, it corresponds to-6thThe diffraction order.
For the left eye, 1 corresponding to the position of the light source 1 is selectedstThe diffraction order. The left viewing window 5 is likewise formed. According to the invention, the corresponding three-dimensional scenes 6 and 6 '(not shown) are reconstructed using the light sources 1 and 1' in a fixed position relative to the eye. Thus when the light sources 1 and 1' are switched on, the hologram 3 will be re-encoded. Alternatively, the two light sources 1 and 1 'may reconstruct the hologram 3 simultaneously in the two viewing windows 5 and 5'.
If the observer moves, the light sources 1 and 1 'are tracked so that the two viewing windows 5 and 5' remain located on the eyes of the observer. The same applies to movements in the normal direction, i.e. perpendicular to the direction of the video hologram.
Further, if additional viewing windows are created by turning on additional light sources, several observers can see the three-dimensional scene.