Computer-assisted holographic display apparatus

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BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a holographic-display apparatus, and more particularly to an apparatus for forming holographic three-dimensional (3-D) optical images corresponding to input object image information. Further, the present invention specifically relates to a computer-assisted holographic display system for calculating wave front (diffraction image) data obtained on the hologram plane based on sampling data of a 3-D object, calculating interference pattern data between the calculated diffraction pattern data and reference light data, and forming an interference pattern corresponding to the interference pattern data on a spatial light modulator such as a liquid crystal spatial light modulator. Further, the present invention specifically relates to a computer assisted holographic-display system for calculating wave front (diffraction image) data obtained on the hologram plane based on sampling data of a 3-D object, and forming a phase-modulation pattern corresponding to the calculated wave front data on a spatial light modulator such as a liquid crystal spatial light modulator.

2. Description of the Related Art

The computer holography is a technique for forming an optical image of a 3-D object on a plane medium (which is normally called "holographic plate") with the assistance of a highly advanced computer. With the recent development of the digital equipment, the computer hologram technique becomes increasingly important in the application field of 3-D image data process, measurement and display thereof, for example.

Unlike the existing purely optical hologram devices, the computer-hologram apparatus produces by computation a holographic image pattern to record a resultant computer-generated holographic image on a recording medium of selected type. Since the computer can create any desired 3-D object images including imaginary graphic images, the computer hologram is excellent in flexibility and wide in applicability for the industrial use. The presently available computer hologram apparatus, however, suffers from the fact that the efficiency of computation remains low. An increased amount of repetitive computations should be required to produce a computer hologram. The necessity of such repetitive computations forces the total processing time to increase, which necessitates the use of a large-scale computer system. This reduces the production efficiency of 3-D object hologram which is required to be recorded to maintain the high quality of a reproduced image.

Until today, several techniques have been proposed for reducing in amount the image information to be processed, thereby to attain an increased computation efficiency in the art of computer hologram. One of the techniques may be found in what is called the "Lohmann type" computer hologram apparatus as is well known among those skilled in the art. This computer hologram apparatus generates a hologram by computing the diffraction pattern of an object. The computation algorithm is described, for example, in "Precision Machine", Vol. 47, No. 12, Supplement, (Dec. 6, 1981) at pp. 101-105, wherein a computer-generated hologram is formed by (1) inputting an object data to the computer, (2) deriving the wave of an object on the hologram plane by computation of the diffraction image to produce a binary-coded recording pattern, (3) forming an original picture drawing, and (4) reducing the original picture by photographing (completion of the hologram).

To reduce the amount of information to be computed, a hologram is created by dividing the hologram plane into a large number of small picture points (called "cells"), computing a diffraction pattern at the representative point of each cell to derive the complex amplitude and phase of each point, and giving an opening to each cell according to the computation results. The opening given to each cell is determines as follows: the height of opening is determined in accordance with the computed value of the complex amplitude of a corresponding cell, whereas the positional relation (distance) between the center of the opening and the cell center is determined in accordance with the value of the phase. The method of determining the size and position of the opening for each cell is described in detail in A. W. Lohmann & D. P. Paris "Binary Fraunhofer Holograms generated by Computer" Appl. Oct., Vol. 6, No. 10 (October 1967) at pp. 1739-1748. An original object image can be optically reconstructed or reproduced by applying a coherent reconstructing light such as laser light to the recorded hologram. A resultant reproduced image obtained from the computer hologram, however, is not satisfactory in the image quality. This is because the center of the cell opening is positionally deviated from the representative point used as the basis for computations of phase.

Another method of forming a computer hologram is also known which is based on the computation of a fringe-shaped interference pattern. The interference computation type computer-generated holography is conceptually similar to a conventional optical hologram forming scheme in that a reference light emitted from a laser source is superposed on the diffraction image of an object of interest to derive an interference pattern therebetween. The recording of a hologram is performed so that the transmissivity or the density may vary on a photographic plate in accordance with the intensity of a resulting fringe-like interference pattern.

According to the interference computation type computer-generated holography, unlike the aforementioned diffraction computation type (i.e., Lohmann type) computer-generated holography, the phase information of a holographic image is recorded in the interference fringe form. The phase error can thus be minimized, which leads to enhancement of the image quality. However, the interference computation type holography suffers from the decreased computation efficiency due to the fact that the decisive means for reducing the amount of information used to compute the interference pattern has not been accomplished yet. Extra large-capacity semiconductor memories are necessary to execute the computation for an enormous amount of information. This results in that the scale of the hologram recording system is increased unwantedly, which makes almost impossible the accomplishment of a high-speed computation process with the use of a smaller computer system. This is a serious bar to the industrial spread of the computer hologram recording system.

Further, as a method for creation of the computer hologram, a method (Kinoform) for deriving phase data of the diffraction wave of an object and directly recording the data on a phase modulation type medium is known. In the hologram of a type which is based on the diffraction wave of the object as described above, since the spatial frequency of information to be calculated can be made lower than that of the interference fringe type, the amount of information to be processed or the amount of calculations can be reduced. However, even in this case, since a relatively large amount of error components of the wave front may remain if the light modulation is effected by use of a less amount of information, unnecessary diffraction light will occur, thereby causing a problem that an image to be displayed becomes dark or the quality of the image is lowered. In order to solve the above problem, it is necessary to increase an amount of information to be processed and effect a large amount of calculations, and as a result, it becomes difficult to construct a practical system.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide a new and improved computer-assisted holographic-display technique.

It is another object of the invention to provide a new and improved computer-assisted holographic-display technique capable of enhancing the computation efficiency while maintaining high quality of a reproduced image.

In accordance with the above objects, the present invention is drawn to a specific computer-assisted holographic-display apparatus which comprises a first computation section for receiving an input image signal representing an object and computing corresponding diffraction pattern data with a first sampling density. A second computation section is connected to the first computation section to subject the diffraction pattern data to the interpolation process so as to create interpolated diffraction pattern data with a second sampling density which is thus increased. A third computation section is connected to the second computation section to compute interference pattern data between the interpolated diffraction pattern and the reference wave. A display section is connected to the third computation section to form an interference pattern on a display device based on the interference pattern data and to form a wave front corresponding to an image of the object by illuminating a reconstructing light to the interference pattern.

In accordance with the above objects, the present invention is drawn to a specific computer-assisted holographic-display apparatus which includes a first computing section for receiving an input image signal expressing an object and calculating corresponding diffraction pattern data with first sampling density. A second computing section is connected to the first computing section to subject the diffraction pattern data to the interpolation process so as to create interpolated diffraction pattern data having second sampling density. A display section is connected to the second computing section to create a phase modulation pattern corresponding to the diffraction pattern on a previously selected display device by use of the diffraction pattern data, modulate a reconstructing light by use of the phase modulation pattern and create wave fronts of light corresponding to the object image.

Additional objects and advantages of the present invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the present invention. The objects and advantages of the present invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out in the appended claims.

BRIEF DESCRIPTION OF THE DRAWING

The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate presently preferred embodiments of the present invention and, together with the general description given above and the detailed description of the preferred embodiments given below, serve to explain the principles of the present invention in which:

FIG. 1 is a block diagram schematically showing the whole construction of a computer-assisted holographic-display apparatus in accordance with one preferred embodiment of the present invention;

FIG. 2 is a diagram showing the internal construction of a diffraction-image computation section contained in the computer-assisted holographic-display apparatus of FIG. 1;

FIG. 3 is a diagram showing the internal construction of a main computation unit contained in the diffraction image computation section of FIG. 2;

FIG. 4 is a diagram showing the internal construction of an intermediate page memory unit contained in the diffraction image computation section of FIG. 2;

FIG. 5 is an illustration modeling a method of performing the internal division for a memory space of the intermediate page memory of FIG. 4;

FIG. 6 is an illustration partially indicating the diffraction pattern which is interpolated in the main scanning direction X and the sub-scanning direction Y;

FIG. 7 is a block diagram showing the internal construction of an interpolation processor of FIG. 1;

FIG. 8 is a block diagram showing the internal construction of an interference fringe pattern generator of FIG. 1;

FIG. 9 is a block diagram showing the internal circuit construction of a reference light wave generator of FIG. 8;

FIG. 10 is a block diagram showing the internal circuit configuration of a gradient corrector of FIG. 1;

FIG. 11 is a block diagram showing a first example of the internal construction of a image display section of FIG. 1;

FIG. 12 is a block diagram showing a second example of the internal construction of a image display section of FIG. 1;

FIG. 13 is a block diagram showing a third example of the internal construction of a image display section of FIG. 1;

FIG. 14 is a block diagram showing an example of the internal construction of a display subunit of FIG. 1; and

FIG. 15 is a block diagram schematically showing the whole construction of a computer-assisted holographic-display apparatus in accordance with another preferred embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to FIG. 1, a computer-assisted holographic-display apparatus in accordance with one preferred embodiment of the present invention is generally designated by the numeral 10. The computer-assisted holographic-display apparatus 10 includes a diffraction image computation section 12, an interference image computation section 14 and an image displaying section 16. The diffraction image computation section 12 has a function of computing a diffraction image (diffraction pattern) information data based on a sampled input image data representing an object of interest (20) in the hologram forming process. The interference image computation section 14 and image displaying section 16 compute interference image information data indicative of an interference pattern between the resultant diffraction pattern data and a reference light data, and display the same on a preselected displaying device.

The diffraction image computation section 12 includes an input image acquisition section 18 for receiving a sampling image data being externally supplied thereto. The input image acquisition section 18 receives a sampling object signal Sob, which is produced by the photoelectric conversion to represent an object of interest 20 by means of an external photoelectric converting image photographing unit (not shown). Alternatively, the input image acquisition section 18 may be connected to an external computer graphics creation equipment (not shown); in such a case, the equipment internally produces a graphic image signal Sob that represents the object 20 without requiring any optical photographing process of the object 20. In either case, the input image acquisition section 18 contains a semiconductor page memory for temporarily storing the input image signal Sob therein. The page memory will be designated by the numeral 52 in FIG. 2.

The diffraction image computation section 12 also includes a diffraction image processing section 22 for computing a diffraction image or pattern and an intermediate page memory 24 which is bi-directionally communicative with the diffraction image processor 22. The intermediate memory 24 temporarily stores therein a computed diffraction pattern information that is output by the diffraction image processor 22 therein. The above constituents 18, 22 and 24 are connected to an input/diffraction image control section 26.

As shown in FIG. 1, the interference image computation section 14 includes (1) an interpolation processing section 28 coupled to an output of the intermediate page memory 24, (2) a processing section 30 connected to the interpolation processor 28 to produce an interference image or pattern, and (3) a gradient correcting section 32 connected to the interference image generator 30. The constituents 28, 30, 32 are connected to an interference image generation control section 34 and operate under the control of the controller 34. The intermediate memory 24 of the diffraction image computation section 12 is also connected to the controller 34. The controllers 26, 34 are associated with a main controller 36. An input/output console section 38 is connected to the main controller 36. The I/O console 38 includes a known keyboard unit and display terminal such as cathode-ray tube (CRT) display terminal or a flat-panel display device, as a man-machine interface.

As shown in FIG. 1, the image display section 16 includes a display subunit 40, light source 42, and display control section 44. The display section 16 forms an interference pattern on a spatial light modulator of the display subunit 40 based on finally obtained interference pattern data, modulates an output light of the light source 42 and displays the object image.

As shown in FIG. 2, the input image acquisition section 18 includes an image data input unit 50 and a page memory unit 52 connected to the output of the image input 50. The page memory 52 includes a memory area 54 and a memory controller 55 associated therewith. The diffraction image processor 22 includes a diffraction pattern computation unit 56, coordinate control unit 58 and adder 60. The computation unit 56 computes a two-dimensional (2-D) diffraction pattern which is obtained on the hologram recording surface with respect to each cell of the sampling input image data stored in the page memory 52. The diffraction pattern thus computed is sequentially supplied to a first input of the adder 60 under the control of the coordinate controller 58. The adder 60 has an output connected to a memory area 62 of the intermediate page memory 24. The page memory area 62 is provided in the intermediate page memory 24 together with a memory controller 63. The intermediate page memory area 62 has a plurality of outputs (for example, four outputs 92, 96a, 96b and 96c), one of which (92) is fed back to a second input of the adder 60.

The diffraction pattern computation unit 56 fetches data of one input cell read out from the input page memory 52 and computes the two-dimensional (2-D) diffraction pattern thereof. The result of computation is supplied to the first input of the adder 60. At this time, the second input of the adder 60 is supplied with data read out from the intermediate page memory 24. Data items supplied to the first and second inputs of the adder 60 are added together and the updated result of computation appears on the output of the adder 60. The updated result of computation is written into the intermediate page memory 24 again. Thus, diffraction patterns derived by the adder 60 for respective picture elements (PEL) or cells of the input image are added together to create a diffraction pattern which is obtained as the result of addition at every picture element and which is kept stored in the intermediate page memory 24 for later use.

Each picture element (cell) of diffraction image data essentially consists of two multivalued gradient data segments which are the real part and imaginary part of a complex number. The operations of writing the diffraction pattern data into the intermediate page memory 24 and of reading the data from the memory 24 are carried out sequentially and alternately. The 2-D coordinate position control in the image space for a series of memory access operations is effected in accordance with coordinate data generated by the coordinate controller 58. A cell position whereat the input image is read and a cell position of the diffraction pattern are determined under the control of the controller 58; then, the positional superposition of each of the diffraction patterns in the intermediate page memory 52 is effected. As a result, final diffraction pattern data is written into the memory area 62 of the intermediate page memory 24.

The computation algorithm for deriving diffraction patterns of the hologram may be determined differently depending on the type of the hologram to be formed. For example, in a Fresnel's hologram used when the distance between an object and the hologram plane is relatively short, the approximate computation for a Fresnel diffraction image can be made by use of Fresnel integrals. Alternatively, in a Fourier transform hologram used when the distance between the object and the hologram plane is relatively long, the approximate computation of Fraunhofer diffraction image can be applied by use of the Fourier transform. Further, in the image hologram, rainbow hologram, holographic stereogram, or the like, diffraction calculations effected in a direction opposite to the actual light traveling direction may be used. To flexibly cope with such different types of holograms, the diffraction pattern data computation unit 56 of this embodiment is constructed by use of a micro-processing unit (MPU) 64 as shown in FIG. 3.

More specifically, the MPU 64 is connected to an internal system bus 66 of the diffraction pattern computation unit 56 together with an input-stage data input/output port (IOP) 68, an output-stage data IOP 70, a control-signal IOP 72, a random access memory (RAM) 74 and a programmable read-only memory (PROM) 76. The input IOP 68 receives an input image data and supplies the same to the system bus 66. The output IOP 70 receives the result of computation by the diffraction pattern computation unit 56 appearing on the system bus 66 and sends forth the same to the adder 60 of FIG. 2. The control IOP 72 is used to receive various kinds of control information signals that are supplied from a host control machine such as a host computer (not shown). The PROM 76 stores therein one or a plurality of computation algorithm software programs externally supplied via the system bus 66. In the case of the plurality of different kinds of algorithm software programs being stored in the ROM 76, one of the algorithms which is optimum for the type of a presently selected hologram can be made active in reply to an instruction from the control IOP 72. Further, various parameters of hologram models including the positional relation between the hologram and the object, wavelength and the like can be externally set in the control IOP 72. The MPU 64 performs processing operations according to the current parameter setting state in the control IOP 72.

By way of an example, assume that the Fresnel's hologram of a two-dimensional (2-D) object image is computed. The following algorithm is used to derive Fresnel diffraction image data. Firstly, the 2-D image is sampled and a set of sampled values is derived. The sampling density at this time may be determined depending on the performance of a system hardware actually used and/or the required quality of an image reproduced from the hologram; in practice, the sampling density may be so selected as to offer the resolution of approximately eight to ten dots per one millimeter. In the above sampling process, the operation of enlarging or reducing the 2-D image may be additionally effected; in this case, the hologram recording apparatus 10 is so designed to have the hologram enlarging/reducing function.

A 2-D Fresnel diffraction pattern is then computed which is formed on the hologram plane by a light component emitted from one of a large number of resultant sampling points is computed. A similar diffraction pattern computation is repeatedly executed with respect to each of the remaining sampling points. The diffraction pattern computation may be made under an assumption that the wavefront of light emitted from a point light source is computed. The diffraction pattern computation may alternatively be made