JPH0478977B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present invention relates to a multiplex recording device and a multiplex recording method for recording a large amount of image information with high contrast and high gradation directly or in the form of a hologram, etc. In particular, the present invention relates to a new projection type multiplex recording device and multiplex recording method that record a series of video signals on a hologram and enable clear three-dimensional image synthesis.

BACKGROUND ART In recent years, information recording and reproducing systems using various electronic, magnetic, optical, or composite media have been developed. Among these, hologram memory, which can record image information with high quality and high density, has attracted particular attention since the 1960s, and its application to computer memory and video memory has been actively researched. Research and development of three-dimensional display technology is also expanding into the fields of art, education, and medicine. For example, a holographic stereogram has been developed in which a three-dimensional object is photographed from each direction and then combined into a sheet-like hologram that can be displayed as a three-dimensional image with parallax. Reproduction using a non-laser light source (white light source is also possible) achieves considerable image quality.

The holographic stereogram will be described below. FIG. 5 is a diagram schematically showing a hologram recording optical system for a conventional holographic stereogram. First, as shown in Figure 5 a, 360
A projector 3 displays a series of two-dimensional images of a three-dimensional object on a stage 1 that can be rotated by a small angle in different viewing directions.
Take a photo with Next, the photographed film 4 is irradiated with a laser beam 5 as shown in FIG.
The hologram is projected onto a screen 9 having slit openings through a hologram 8, and a hologram is recorded on a film-like recording medium 10 provided on the back surface of the screen 9. Here, reference light is made to enter from a reference light source 11 located above. Note that the beam passing through the recording medium 10 converges at the position of the straight line 12. FIG. 6 is a diagram showing the principle of image synthesis by forming the recording medium 10 on which the hologram array obtained in this way is recorded into a cylindrical shape. FIG. 6a shows a method for reproducing a three-dimensional image from a hologram 14 using a diverging laser beam 13. When an observer looks at the position indicated by a straight line 15, the reproduction is incident on both eyes 16 and 17 of the observer. The images are different, and a three-dimensional image 18 is obtained depending on the effect of parallax. FIG. 6b shows a similar 3
A method for performing dimensional image reconstruction is shown. In this case, the eye position is slightly behind the hologram 14, and the red, green, and blue diffraction image components 20 to 2 with chromatic dispersion are
2, for example, the central green image 21).

Problems to be Solved by the Invention In conventional holographic stereograms, the process of photographing an actual three-dimensional object once on a photographic film is essential, and after the film is developed, each frame of the photograph is transformed into a hologram. A process of conversion was necessary. Furthermore, the procedure used consisted of two completely separated optical systems. Therefore, real-time recording is of course impossible, and the uses of holographic stereograms are also limited. In addition, today, many parts of image processing are processed and recorded as video information, especially video signals, which are directly converted into holograms via spatial modulation elements (hereinafter abbreviated as SLM) and reproduced. There is a strong desire for technology that can effectively utilize images. However, in order to record and convert input images into high-quality holograms one after another using SLM,
Various issues arose, including how to achieve high-performance SLM and a method for recording high-quality images such as high contrast and high gradation.

Means for Solving the Problems In the inventions according to claims 1 and 4, in order to solve the above-mentioned problems, a planar image is displayed as a shading pattern whose transmittance or reflectance is modulated by an external signal such as a video signal. By using a spatial modulation element that applies temporal pulse width modulation according to the signal level of each pixel, and recording temporally different image information overlappingly on the same part of the recording medium at appropriate times, contrast can be improved. - Provides a projection type multiplex recording device and multiplex recording method for recording holograms with excellent image quality in terms of gradations and the like.

Furthermore, the invention according to claim 2 uses an active matrix type liquid crystal display device in which an a-SiTFT or the like is formed in each pixel as a spatial modulation element, a liquid crystal display device using ferroelectric liquid crystal, etc. While the contrast and gradation of the spatial modulation element itself can be obtained by making monochromatic light such as a laser beam into almost parallel light, it is also possible to obtain multiple images obtained by such a device from the same part of the recording medium at appropriate time intervals. The object of the present invention is to provide a projection type multiplex recording device that records a hologram having performance superior to the contrast and gradation of the spatial modulation element itself by superimposing the recording on the spatial modulation element itself.

Furthermore, the invention as set forth in claim 6 provides a method for correcting an image obtained from a hologram recorded on a recording medium using light transmitted or reflected by the spatial modulation element in response to an external signal input to the spatial modulation element. A hologram is recorded by feeding back a conversion function.

Operation Let us assume that a spatial modulation element with a contrast of Cs and a gradation of Ks is used, and a hologram is recorded on a recording medium corresponding to the lightness and darkness of a certain pixel of the spatial modulation element. As a recording medium,
If photographic film is used, the diffraction efficiency of the hologram is controlled according to the shading of each pixel by dividing the total amount of incident light into each pixel by time-dividing the appropriate amount of light, and adjusting the hologram's diffraction efficiency to the desired shading of the original image. The corresponding image will be played. It is assumed that the gradation of the image to be recorded or reproduced is Kf, and each pixel of the spatial modulation element transmits (or reflects) a desired amount of light at each time interval t ₀ . Required number of times to perform this operation N
By repeating the time Nt ₀ on the film
The desired gradation information is recorded as the sum of the amounts of light accumulated during this period. In this way, the contrast Cf of the recorded image on the film is Cf=
Cs, and the tone is Kf>ks. Therefore,
It is now possible to record and reproduce images with much greater gradation and better contrast than before.

Embodiments Hereinafter, a projection multiplex recording method and a multiplex recording apparatus according to embodiments of the present invention will be described with reference to the drawings. FIG. 1 is a perspective view showing a schematic configuration of a holographic stereogram recording apparatus according to the present invention. As can be understood by comparing the conventional example with FIG.
is being irradiated. With this arrangement, a uniform and large hologram can be recorded without increasing the aperture of the imaging lens 26. In FIG. 1, 7 to 11 have the same structure as the conventional example shown in FIG.
and a reference light source. The SLM 24 displays a gray pattern that is two-dimensional image information by changing the light transmittance for each pixel electrode using a signal indicating two-dimensional image information from an external electric signal source 27. As the SLM 24, an active matrix liquid crystal panel shown in FIG. 2 is used. In the figure, 28 and 29 are polarizing plates whose optical axes are substantially orthogonal to each other. On the polarizing plate 29, a nonlinear element (switching element) 30 is placed at the intersection of pixel electrodes arranged two-dimensionally in a matrix. A glass substrate 35 is provided on which a glass substrate 33 and an alignment film 34 are formed on one principal surface. This glass substrate 35
A narrow space is formed by a glass substrate 38 having a counter electrode 36 and an alignment film 37, and a seal 39, and a liquid crystal 40 such as twisted nematic (TN) liquid crystal is sandwiched in the space. Incidentally, reference numerals 41 and 42 indicate laser beams that are incident and transmitted and are collimated almost in parallel, and when using a linearly polarized laser, the polarizing plate 29 is not necessary.

As is well known to those skilled in the art, the performance of active matrix liquid crystal display devices is determined by contrast,
Regarding gradation, it currently exhibits the best characteristics among the liquid crystal display devices proposed so far. Under parallel monochromatic light, the contrast is more than 100,
A gradation of 16 or more is obtained. (References: Tanaka et al. “3-inch color LCD TV using a-Si TFT”, National Technical Report,
Vol.33, No.1, 1987, pp64-75, etc.).

FIG. 3 is a characteristic diagram schematically showing the dependence of the transmittance of the above-mentioned transmission active matrix liquid crystal display device on the input signal Vrms. Tmax is the maximum transmittance of the display device and is shown normalized to 1. Tmin indicates the minimum transmittance of the display device, and the amount of light leaking out during time t ₀ is W ₀
It is. The contrast of the display device in this case is
It is expressed as Cs=Tmax/Tmin. This value is 100
In this case, the contrast Cs of the display device is defined as 100. Maximum transmittance Tmax of this display device
is divided into 8 equal parts and the transmitted light amount per t ₀ time is W. In order to transmit the light amount of n×W per t ₀ time, Sn of S ₁ , S ₂ to S ₈ is used as an input signal. All you have to do is input the signal. Note that n is a positive number.
t ₀ To set the minimum amount of light per hour (W ₀ ),
Input S ₀ signal. In this embodiment, in order to simplify understanding, a case will be considered in which the density pattern of an image to be recorded (or recorded/reproduced) changes in proportion to the amount of incident light. In holography, since the recording method uses a carrier wave, such an approximation is generally allowed. Furthermore, in the present invention, an image is defined as
Think of it as a collection of pixels of various shading.

The following description focuses on one pixel of an image to be recorded (or recorded/reproduced).

FIG. 4 is a diagram for explaining the multiplex recording method of the present invention, and shows how the transmittance of one pixel of a display device is temporally controlled. Vertical axis B
represents the amount of transmitted light per unit time of the display device, in other words, the amount of light per unit time that enters the recording medium. The horizontal axis shows time. Assume that the recording medium is exposed for a time of t=8×t ₀ . Note that the signal input to the display device is changed every _t0 .

When exposed at the maximum transmittance of the display device, a total amount of light of 64×W is incident on the recording medium in a time of 8×t ₀ as shown in the figure. The black recording level is achieved when the transmittance of the display device is minimized, and the total exposure amount in that case is 8×W ₀ . As mentioned above, the contrast of the display device is Cs=8・W/W ₀ =
100, the maximum contrast ratio (contrast Cf) of the total exposure amount is Cf=(64・W)/(8・W ₀ )=
8.W/W ₀ =Cs=100, and the contrast Cf of the image recorded on the recording medium matches the contrast Cs of the display device.

On the other hand, the maximum brightness for the gradation of the image you want to record is 64.
If n is n/8=n1...remainder
At n2, the transmittance of the time display device for n1 × t ₀ is set to the maximum 8W, the transmittance for the period n2 × W for t ₀ , and the transmittance of the display device for the remaining period (8-n1-1) × t ₀ . The input signal is controlled so that the In this case, the total exposure amount to the recording medium is n・W+α・W ₀ (α≦8−n1−
1). That is, when each pixel is controlled by an external signal as described above, eight grayscale patterns are displayed every _t0 using a display device that displays only eight shades of transmittance per unit time, and the remaining exposure time is 8.
By exposing for t ₀ , approximately 64
A gradation pattern of gradations will be recorded.
Or the contrast of the recorded image at that time.
Cf matches that of the display device, Cs.

Next, a multiplex recording method according to a second embodiment of the present invention will be explained. This embodiment is also structurally the same as the first embodiment. However, in the first embodiment, the shading to be recorded (recorded/reproduced) changes in proportion to the amount of incident light, and the shading of the stored (stored/reproduced) image is approximately proportional to the shading pattern of the recording medium. I mentioned the case. However, a recording medium with such good characteristics that the shading of a recorded/reproduced image and the shading of a recording medium forming a hologram are directly proportional to each other cannot be easily and inexpensively obtained. In practice, special photographic film is often used. Furthermore, images recorded and reproduced holographically are not simply proportional to the density of the recording medium. Therefore, the shading of the recorded (recorded/reproduced) image in such a case is not simply proportional to the amount of incident light, but changes in proportion to a certain function f. Therefore, as a second embodiment, a case will be described in which the density y of a recorded (recorded/reproduced) image changes with respect to the amount of incident light x in the relationship y=f(x). First, the conversion function f upon recording is measured in advance. In this measurement, light is incident on a film as a medium, and a function f is determined based on the resulting shading. According to this function f, by overlapping exposure of 8 gradation patterns every t ₀ for a period of 8·t ₀ , a reproduced image with a gradation nearly eight times the gradation of the display device (SLM) is realized. It is. In other words, when recording the brightness of n out of 64 equal parts of the display form from bright to dark as the degree of image shading to be recorded (recorded/played), the exposure time is 8·t _0. At every _t0 , the input signal of the display device is controlled so that the transmittance is as explained below. In this case, the density of the recorded (recorded/reproduced) image when exposed for a time of 8·t ₀ so that the transmittance of the display device is minimized is y ₀ = f
(8・W ₀ ). As a display device, the transmittance is changed in eight gradations (8·W). record (record/
Since the amount of change in density of the reproduced image is y=f(x),
The transmittance of the display device is controlled by an external signal by time modulation so that the total amount of light incident on the recording medium becomes x=f ^-1 (y). For example, evaluate in advance how y varies with x. 8・t ₀ period,
When exposed at the maximum transmittance set by the display device, the density of the recording medium becomes ymax=f ^-1 (64W).
Therefore, if f ^-1 (64W) is set so that the recording density is close to the maximum of the recording medium, the contrast Cf of the recorded (recorded/played) image will be Cf
= f ^-1 (64·W)/f ^-1 (8·W ₀ ). There are threshold values and nozzle levels for recording and reproducing photographic film and holograms, and it is easy to set them so that no exposure occurs or the density of the reproduced image does not change at an exposure amount of 8.W ₀ . Contrast Cf is the contrast of the film itself or the hologram recording.
This becomes the contrast of the playback itself, and nearly 100 possibilities are possible. Therefore, Cf=f ^-1 (64・W)/f ^-1
(8・W ₀ )>Cs=64・W/8・W ₀ =100 can be realized. If you want to obtain the light and shade of n tones out of 64 tones, read xn=f ^-1 (n) from the table and write x・n/8・
W = n1 × t ₀ Calculate the remainder n2 × W, maximize the transmittance of the display device for the time n1 × t ₀ , and for the period t ₀ ,
The transmittance is set to the recording density of n2×W, and the input signal is controlled so that the transmittance of the display device is minimized during the remaining period (8-n1-1)× _t0 . When this is done, the density of the recording medium is f(8×n1×W+n2×W
+α×W ₀ )=f(xn+α×W ₀ ) to n, which means that n tones out of 64 tones have been expressed. That is, using a display device that displays only 8 tones of transmittance per unit time, eight transmittance (or reflectance) patterns of the same or different display devices are recorded every t ₀ for an exposure time of 8t ₀ . By exposing the medium to light, an image with approximately 64 shades of light and shade is recorded on the recording medium. Further, the contrast Cf of the recorded image at that time can be made larger than the contrast Cs of the display device.

The recording apparatus and recording method of the present invention have been described above, focusing on an active matrix type transmissive liquid crystal display device that can provide relatively good gradations as a recording medium (SLM). Spatial modulation elements (SLM), such as reflective type,
Liquid crystal display devices other than BSO (BiSiOx)-based SLMs, PROM-SLMs, and active matrix types can also be used. In addition, in the explanation of the embodiment, the explanation was not limited to holographic recording, but if the same recording medium is irradiated with a reference beam that is optically branched from the laser beam used as the incident beam, holographic recording can be made. Needless to say.

Effects of the Invention As described above, according to the present invention, even if a spatial modulation element capable of displaying only about 8 tones is used,
By overlapping and recording temporally identical or different images on the same part of a recording medium at appropriate intervals, a hologram is formed by utilizing only the gradations inherent in the spatial modulation element in terms of contrast, gradation, etc. It is possible to record holograms with superior image quality. Furthermore, the effects of the present invention are enhanced by using an active matrix liquid crystal display device with excellent gradation as a spatial modulation element.
Furthermore, by making the light incident on the liquid crystal display device more like parallel light, the contrast and gradation characteristics of the display device are improved, which is very effective.

[Brief explanation of the drawing]

FIG. 1 is a perspective view showing the configuration of a projection multiplex recording device according to an embodiment of the present invention, FIG. 2 is a sectional view of an active matrix liquid crystal display device used as a spatial modulation element of the projection multiple recording device, and FIG. Figure 3 is a characteristic diagram showing the input signal dependence of transmittance of an active matrix liquid crystal display device, Figure 4 is a diagram showing time and exposure conditions for explaining the multiplex recording method of the present invention, and Figure 5 a. , b is a perspective view schematically showing the recording system of a conventional holographic stereogram;
Figures a and b are perspective views schematically showing a conventional holographic stereogram reproduction system. 7, 8, 23, 26... Lens, 10... Recording medium, 24... SLM, 25... Parallel monochromatic light, 2
7... Image signal generator.

Claims (1)

[Claims] 1. A spatial modulation element that displays a two-dimensional gradation pattern as a pattern of transmittance or reflectance based on an external input image signal, a light source that irradiates light to the spatial modulation element, and a light source that emits light from the spatial modulation element. Using an optical system that projects transmitted or reflected light and a recording medium that records the light projected by this optical system, one of the two-dimensional gray patterns is displayed at a time ti, which is obtained by dividing the time T to be displayed. By recording the plurality of grayscale patterns displayed by the spatial modulation element at each time ti in a superimposed manner on the same part of the recording medium,
A projection multiplex recording method characterized in that the gradation of the recording pattern accumulated on the recording medium has a gradation larger than the limit of the gradation of gradation that can be displayed by the spatial modulation element. 2. The projection multiplex recording method according to claim 1, wherein the spatial modulation element is a liquid crystal display device. 3. Claim 2, wherein the spatial modulation element is an active matrix liquid crystal display device.
The projection type multiplex recording method described in . 4 As the degree of shading of an image recorded on a recording medium or a reproduced image pixel k of a shading pattern recorded on the recording medium, the brightness of n out of the total m divisions between the bright and dark display states is indicated. A projection multiplex recording method in which the total amount of light transmitted or reflected and reaching the recording medium over the entire exposure time Xmax is determined by the maximum transmittance or reflectance set by the spatial modulation element and dividing the exposure time into each time ti, inputting an external signal so that the pixel s on the spatial modulation element corresponding to the pixel k exhibits a predetermined transmittance or reflectance for each time ti, The pixel s
The external signal input to the spatial modulation element is controlled so that the total amount x of light that is transmitted or reflected at a predetermined transmittance or reflectance and reaches the recording medium during the entire exposure time is n/m. The recording medium is characterized in that a plurality of temporal shading patterns displayed by the spatial modulation element as a set of pixels s exhibiting shading according to each external signal are superimposed and recorded on substantially the same portion of the recording medium. The projection type multiplex recording method according to claim 1. 6. As the degree of shading of a pixel k recorded on a recording medium having a characteristic in which the exposure amount is expressed by a specific function f with respect to the input light amount, n is calculated by dividing the display state from bright to dark into m equal parts. The projection multiplex recording method records the brightness of the pixel s on the spatial modulation element corresponding to the pixel k for each divided time ti of the total exposure time, and the pixel s on the spatial modulation element has a predetermined transmittance or reflectance. An external signal is input so that the pixel s during the entire exposure time is the predetermined transmittance or reflectance, and the total amount of light that is transmitted or reflected and reaches the recording medium is x, and the spatial modulation is When the maximum transmittance or reflectance of the element is defined as the total amount of light that is transmitted or reflected during the exposure time and reaches the recording medium, fx/f
The external signals input to the spatial modulation element are controlled so that the relationship (Xmax)=n/m is established, and the temporal 2. The projection multiplex recording method according to claim 1, wherein a plurality of grayscale patterns are recorded in substantially the same area on the recording medium in a superimposed manner.