JP4331873B2 - Hologram display - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a hologram display which can display a hologram moving image three-dimensional image which does not require observation glasses and has little visual fatigue.
[0002]
[Prior art]
Many methods have been proposed for conventional stereoscopic display. Among these, as a general stereoscopic display method, there is a binocular stereoscopic display. This method has the feature that 3D display can be easily performed with a relatively small amount of information, but visual fatigue due to the difference between the eye convergence distance and the focus adjustment distance is a problem when viewing 3D images. .
[0003]
Hologram display, which is a method for reproducing a stereoscopic image in space, is effective for displaying a stereoscopic image with little visual fatigue. As a conventional hologram moving image display method, there is a method using an acousto-optic element (AOM). The computer calculates image information when an object having a simple structure is moved, and inputs the image information as a video signal to the AOM for driving. Light is incident on the beam, and the beam is diffracted to generate a horizontal image signal. The vertical direction is scanned while vibrating a galvanometer mirror, and an image is synthesized and displayed. Although color display is possible, an arithmetic processing device with extremely high processing capability is required, the display object has a simple shape, the image size is small, and a real image cannot be stereoscopically displayed.
[0004]
As another hologram display method, there is a method using an electrically driven liquid crystal panel. With the advancement of high-definition liquid crystal display manufacturing technology, it has become possible to display interference fringes and diffract light to display holograms, but it is extremely difficult to display fine interference fringes corresponding to the wavelength of light. is there. In addition, since the display is performed mainly based on data calculated by a computer, a three-dimensional display of a live-action image cannot be performed.
[0005]
[Problems to be solved by the invention]
As described above, the problems with the prior art are that a 3D video display of live-action images cannot be performed, and a high-resolution hologram display device necessary for expressing interference fringes cannot be used. This is something that cannot be expected.
[0006]
Accordingly, an object of the present invention is to provide a hologram display in which the above problems are solved.
[0007]
[Means for Solving the Problems]
In order to solve the problems of the conventional methods described above, the present invention proposes a hologram display to which a new hologram display method is applied. First, in the present invention, a light writing type spatial light modulation element having a higher resolution characteristic than an electrically driven spatial light modulation element that is widely used as a liquid crystal TV (television) is used. In addition, based on the depth distance information of the display object so that a real subject can be stereoscopically displayed, a depth sampled image, which is an image sampled for each depth, is created, and these images are displayed in a holographic manner. By introducing a high-definition hologram display device, a bright and large display image is possible, and a 3D image is reconstructed from the subject's color image and depth distance information. Display is also possible.
[0008]
According to a first aspect of the present invention, a depth sampled image generation apparatus that generates a depth sampled image that is an image of each depth position of the object based on color information and depth distance information of the object to be displayed, and the depth sample Synchronized with a transmission type two-dimensional image display device for sequentially displaying each depth sampled image from the sampled image generation device, and a display switching of a plurality of depth sampled images displayed sequentially on the transmission type two-dimensional image display device An optical path length modulation optical system for electrically changing the optical path length of coherent reference light having a condensed or diffusing light beam, and a two-dimensional image obtained by coherent light incident on the transmission type two-dimensional image display device. An interference optical system that generates an interference fringe between the object light that has been intensity-modulated and the reference light from the optical path length modulation optical system, and the interference fringe generated by the interference optical system on one surface At the same time, an image read out on the other surface in response to the interference fringes is reproduced by the reflected light of the coherent reproduction light input to the other surface, and an image that is volumetrically displayed in the depth direction is realized. The optical path length modulation optical system can be switched between horizontal polarization and vertical polarization by electrical control of the polarization direction of coherent light polarized vertically or horizontally. A polarization switching element, a polarization beam splitter disposed after the polarization switching element, and an output direction of light transmitted through the polarization switching element and the polarization beam splitter. 1 and a first mirror disposed behind the first quarter-wave plate, and the polarization switch A second quarter-wave plate having a fast axis of about 45 degrees with respect to the incident polarized light is disposed in the output direction of the light transmitted through the element and reflected from the polarizing beam splitter, and the second quarter-wave plate And a polarization mirror that has a second mirror disposed behind the polarization mirror, and a distance between the polarization beam splitter and the first mirror is different from a distance between the polarization beam splitter and the second mirror. The optical path length corresponding to twice the difference between the distance between the first mirror and the polarizing beam splitter and the distance between the second mirror and the polarizing beam splitter is changed by switching the polarization direction of the light. The optical path length modulation optical system includes N (natural number) optical systems, and has an optical system that changes the optical path length of 2 N stages.
[0009]
According to a second aspect of the present invention, in the first aspect, the hologram recording display element includes a first transparent electrode, a photoconductive layer that is stacked on the first transparent electrode, and whose impedance changes according to the intensity of incident light. A light absorption layer laminated on the photoconductive layer, a dielectric multilayer mirror laminated on the light absorption layer and reflecting all or part of the visible light spectrum, and laminated on the dielectric multilayer mirror. An optical writing type spatial light comprising a light modulation layer and a second transparent electrode laminated on the light modulation layer and driven by an alternating voltage applied between the first transparent electrode and the second transparent electrode It is a modulation element.
[0010]
According to a third aspect of the present invention, in the second aspect, the light modulation layer is formed of any one of a nematic liquid crystal, a cholesteric liquid crystal, a smectic liquid crystal, a ferroelectric liquid crystal, or a mixed liquid crystal of these liquid crystals.
[0013]
DETAILED DESCRIPTION OF THE INVENTION
FIG. 1 is a block diagram showing an embodiment of a hologram display according to the present invention.
[0014]
This apparatus includes a three-dimensional information input apparatus 1 including a camera that detects a distance to a subject together with a color image of the subject, and a device that generates a color and a distance of an object to be displayed by calculation. In addition, a sampled image generation device 2 that can generate a depth sampled image that is a color image for each depth distance from the information of the color and distance of the display object output from the three-dimensional information input device 1 is provided. Yes.
[0015]
As an example of the three-dimensional information input device 1 used in the present invention, there is one having a conventional TV camera and a device (range finder) for measuring a distance to a subject. As the range finder, an apparatus using a distance detection method such as a triangulation method, a light time-of-flight measurement method, a moire method, a focusing method, or an optical interference method can be applied. As a characteristic required for the distance detection function, it is possible to detect a moving subject at a sufficiently high speed so that it can be detected, and further, it is possible to detect distance information of the entire subject with a fineness corresponding to a TV image. As a three-dimensional information input device that realizes these, a three-dimensional camera that combines intensity-modulated illumination light and a short-time imaging system (examples of this three-dimensional camera are described in the following documents:
1. Japanese Patent Application Laid-Open No. 2000-121339 “Solid Information Detection Method and Apparatus”
2. Hebei, Iizuka, Kikuchi, Fujikake, Yoneuchi, Iwata: A method of three-dimensional imaging using a combination of high-speed shutter and modulated illumination, IEICE Technical Report, EID 98-51, pp. 19-24 (1998)
3. Hebei, Iizuka, Iwata, Kikuchi, Fujikake, Yoneuchi, Takizawa: Development of three-dimensional imaging device Axi-Vision Camera, three-dimensional image conference '99, 6-1, pp. 151-156 (1999)
4). M.M. Kawakita, K .; Iizuka, T .; Aida, H.M. Kikuchi, H.K. Fujikake, J. et al. Yonai and K. Takizawa: Axi-Vision Camera (Real-Time Distance-Mapping Camera), Applied Optics, Vol. 39. pp3931-3939 (2000)
5. M.M. Kawakita, K .; Iizuka, T .; Aida, H.M. Kikuchi, H.K. Fujikake, J. et al. Yonai and K. Takizawa: Axi-Vision Camera: a three-dimension camera, Proc. SPIE, Vol. 3958, pp. 61-70 (2000)) is effective.
[0016]
A specific example of the three-dimensional information input apparatus 1 is shown in FIG. An illumination modulation signal 62 is output from the signal generator 61 and input to the illumination driving device 63, and an intensity modulation illumination device 64 that emits near-infrared light whose output light intensity changes with time is driven. Similarly, a high-speed shutter trigger signal 65 is output from the signal generator 61 and input to the high-speed shutter driving device 66 to operate the high-speed shutter 67. Near-infrared light 68 output from the intensity-modulated illumination device 64 is irradiated onto the subject 71 using an optical system such as a mirror 69 or a dichroic mirror 70. Of the reflected light component from the subject 71, the visible light 72 passes through the dichroic mirror 70 and enters the color camera 73 to obtain a color image 74. Further, near-infrared reflected light from the subject reflected by the dichroic mirror 70 and the mirror 69 enters the distance detection camera 75. A high-speed shutter captures reflected light for a short time, captures an image with a CCD (Charge Coupled Device) camera 76, calculates a distance with a signal processing circuit 77, and calculates a distance value from the camera to the subject. A distance image 78 expressed in the light and dark is obtained. If this apparatus is used, the distance information of the entire subject can be detected at a high speed equivalent to the video rate, and the distance can be obtained in units of pixels, which is suitable as the three-dimensional information input apparatus of the present invention.
[0017]
On the other hand, FIG. 7 shows an example of the configuration of the sampled image generation apparatus 2 that creates a depth sampled image from the color image and distance image of the subject obtained by the three-dimensional information input apparatus. First, channel 1 (CH1) will be described. An upper limit threshold value 79 and a lower limit threshold value 80 of the image luminance level range corresponding to the image cutout distance range are input to the memory 81. This is preset in each of the plurality of channels CH1 to CHm (each channel has the same configuration). The upper limit threshold value and the luminance signal level of the distance image are compared by the comparator 82, and at the same time, the lower limit threshold value and the luminance signal level of the distance image are compared by the comparator 83. This is performed in each channel for the distance image, and the selector switch circuit 85 selects and outputs the output of the gate 84 of the corresponding channel in response to the switching signal synchronized with the comparison timing of each channel. As a result, when there is a distance image signal within the range of the upper and lower thresholds set for each channel, the gate 84 is turned on, and a color image of that portion is output, and a sampled image with a specific depth distance is output. Can be output from the changeover switch circuit 85.
[0018]
In FIG. 1, each depth sampled image output from the sampled image generation device 2 is sequentially displayed on the transmission type two-dimensional image display device 3 with time. The transmissive two-dimensional image display device 3 is illuminated by the coherent light 4 from the back, and the intensity of the light is modulated. As a result, the display image is output as a two-dimensional light image. Enter.
[0019]
As this transmissive two-dimensional image display device, for example, a transmissive liquid crystal display device can be used. On the other hand, the coherent light 6 having a condensed or diffused light beam is used as the reference light. In the optical path of the reference light 6, an optical path length changing optical system 7 that changes the distance 14 between the hologram recording display element 8 and the condensing point 13 of the reference light is disposed.
[0020]
The depth sampled images are sequentially displayed on the transmission type two-dimensional image display device 3, and in synchronization therewith, the distance 14 between the hologram recording display element 8 and the condensing point 13 of the reference light is changed while interfering, and the hologram Interference fringes are written on the recording display element 8. Coherent light having a fixed optical path length is input to the hologram recording display element 8 as reproduction light 9, and a three-dimensional image 11 is reproduced from the modulated reflected light 10 and presented to the observer 12.
[0021]
Here, the 3D display principle of the present invention will be described. FIG. 2 shows a case where the condensing position 13 of the reference light is close to the hologram recording display element 8 ((a) in FIG. 2) and a case where it is far ((b) in FIG. 2). It is assumed that images 14 and 15 are displayed in each case, and the object light 5 is interfered with the reference light 6 and recorded on the hologram recording display element 8. When the reproduction light 9 is incident on the hologram recording display element 8 and an image is read out from the hologram recording display element 8, the reproduction image 16 in the case where the condensing position of the reference light is close to the hologram recording display element and the reference light The position reproduced in the depth direction differs from the reproduced image 17 when the condensing position is far from the hologram recording display element. Similarly, when a plurality of images are recorded on a hologram while sequentially shifting the emission point of the reference light, the reproduced images are sequentially reproduced at different positions in the depth direction, and as a result, displayed in a volumetric manner in the depth direction. It becomes a three-dimensional image.
[0022]
FIG. 3 shows an example of a light writing type spatial light modulation element as a hologram recording display element used in the apparatus of the present invention. This optical writing type spatial light modulation element includes a first transparent electrode 19 attached to a transparent substrate 18 and a photoconductive layer 20 which is laminated on the first transparent electrode 19 and whose impedance changes according to the intensity of incident light. A light absorption layer 21 laminated on the photoconductive layer 20, a dielectric multilayer film mirror 22 laminated on the light absorption layer 21 and reflecting all or part of the visible light spectrum, and the dielectric multilayer film The light modulation layer 23 is laminated on the mirror 22, and the light modulation layer 23 is laminated with the second transparent electrode 25 attached to the transparent substrate 24. The first transparent electrode 19, the second transparent electrode 25, It is driven by the AC voltage applied during When writing light 28 that induces the photoelectric effect of the photoconductive layer 20 is incident from the left side of FIG. 3 in a state where an AC voltage is applied from the driving AC power supply 27 via the lead wire 26, the intensity of the writing light is increased. The light modulation layer 23 is driven accordingly, and when the reading light 29 is irradiated from the right side of FIG. 3, the written light image is emitted as display light (reflected light).
[0023]
Both the first transparent electrode 19 and the second transparent electrode 25 are thin films such as In ₂ O ₃ : Sn that are in close contact with one surface of the photoconductive layer or one surface of the transparent electrode by a technique such as vapor deposition. Connected to each voltage output terminal of the driving AC power supply.
[0024]
The photoconductive layer 20 has CdS, CdSe, Se, SeTe, GaAs, GaP, Bi ₁₂ SiO ₂₀ , Bi ₁₂ GeO ₂₀ , Si, amorphous Se, and amorphous whose electrical impedance is greatly reduced with respect to the incidence of optical writing light. It is a layer composed of a material such as SeAs.
[0025]
The light absorbing layer 21 is a CdTe film, a diamond-like carbon film, one or more films selected from the group consisting of amorphous films substantially composed of silicon, carbon, and germanium, or inorganic pigments, organic pigments And a resin composite in which one or more materials selected from the group consisting of carbon and dye are dispersed in a resin.
[0026]
The dielectric multilayer mirror 22 includes an SiO ₂ film, a TiO ₂ film, an HfO ₂ film, a Ta ₂ O ₅ film, a ZnS film, an Al ₂ O ₃ film, an Na ₂ AlF ₆ film, an MgF ₂ film, an LaF ₃ film, and a GdF. _{One of the three} films, the SmF ₃ film, the CeF ₃ film, the ZrO ₂ film, and the CeO ₂ film, or a multilayer film in which two or more films selected from these films are stacked.
[0027]
The light modulation layer 23 is a liquid crystal, and one of a nematic liquid crystal, a cholesteric liquid crystal, a smectic liquid crystal, a ferroelectric liquid crystal, or a mixed liquid crystal of these liquid crystals is used.
[0028]
The transparent substrates 18 and 24 are portions that serve as substrates of the optical writing type spatial light modulation element, and are made of a highly transparent and excellent flatness glass or a synthetic resin such as acrylic.
[0029]
The operation of the optical writing type spatial light modulation element will be described. When this spatial light modulation element is driven, an AC voltage is applied between the first transparent electrode 19 and the second transparent electrode 25. Interference fringes created by reference light and object light are incident from the photoconductive layer side. On the other hand, the reading light is incident from the light modulation layer side, undergoes phase modulation by the light modulation layer 23, is reflected by the dielectric multilayer mirror 22, and is read and displayed as a phase hologram.
[0030]
At this time, when the writing interference fringes are extremely weak or not incident, most of the drive voltage is applied to the photoconductive layer 20 side having a higher electrical impedance than the light modulation layer 23. On the other hand, when writing light having a sufficient intensity is incident, the electrical impedance of the photoconductive layer 20 decreases, and a part of the voltage distributed to the photoconductive layer 20 moves to the light modulation layer 23. At this time, the alignment of the liquid crystal molecules is aligned with the electric field direction at once. Therefore, when an interference fringe is input and reproduced, a hologram image is displayed.
[0031]
FIG. 4 shows a configuration diagram of an optical path length modulation optical system for reference light used in the apparatus of the present invention.
[0032]
Light 30 that is vertically or horizontally polarized and is condensed or diffused other than parallel light is incident on a polarization direction switching cell 31 that electrically rotates the plane of polarization by 90 °. As this cell, a liquid crystal sandwiched between transparent electrodes and rotating the plane of polarization using the electric birefringence effect and the optical rotation effect of the liquid crystal can be used. A polarization beam splitter (PBS) 32 is disposed after the polarization direction switching cell 31. The first quarter-wave plate 33 and the first mirror 34 are disposed in the direction in which the light is transmitted through the PBS 32, and the second quarter-wave plate 35 is disposed in the direction in which the light is reflected by the PBS 32. A mirror 36 is arranged. An optical path difference 37 is provided between the distance between the first mirror 34 and the PBS 32 and the distance between the second mirror 36 and the PBS 32.
[0033]
First, when the polarization direction of light is controlled horizontally by the polarization direction switching cell 31 (left side in FIG. 4), the light incident on the PBS 32 is transmitted and transmitted through the first quarter-wave plate 33 to be circularly polarized. Then, the light is reflected by the first mirror 34 and again passes through the first quarter-wave plate 33 to become vertically polarized light, which is incident on the PBS 32 again, reflected by 90 degrees, and output.
[0034]
On the other hand, the case where the polarization direction of the light transmitted through the polarization direction switching cell 31 is controlled to be vertical (right side in FIG. 3) will be described. The light incident on the PBS 32 is reflected by 90 degrees, passes through the second quarter-wave plate 35 and becomes circularly polarized light, is reflected by the second mirror 36, and passes through the second quarter-wave plate 35 again. The light becomes horizontally polarized light, and is incident on the PBS 32 again and transmitted.
[0035]
As described above, when the polarization direction switching cell operates, the optical path length corresponding to twice the difference 37 between the distance between the first mirror 34 and the PBS 32 and the distance between the second mirror 36 and the PBS 32. The difference 38 can be modulated.
[0036]
If the optical path length modulation optical system is a single stage, the optical path length can be switched in two stages. However, the optical path length modulation optical system can be switched in more stages by providing multiple stages. For example, the optical path length modulation optical system is made up of N stages, and the difference between the distance between the first mirror and the PBS and the distance between the second mirror and the PBS in each optical path length modulation optical system is set to a different value. The number of stages can be modulated.
[0037]
<Experimental example>
FIG. 5 shows an experimental optical system of the hologram display of the present invention.
[0038]
The experimental optical system is roughly divided into a writing optical system 40 for inputting interference fringes to a spatial light modulator (SLM) 39 and a reading optical system 41 for reproducing an image from the SLM 39.
[0039]
First, the writing optical system will be described. As the light source, light having a sensitivity in the photoconductive layer of the SLM 39 and having a long coherence distance is used. In this experiment, since an amorphous silicon film is used as the photoconductive layer of the SLM 39, light having a wavelength of 632.8 nm of the He—Ne laser 42 is used. The output light from the laser 42 passes through the wave plate 43 and is split into two parts by the PBS 44 into light 45 toward the reference light and light 46 toward the object light. The intensity ratio of the distribution is adjusted by changing the ratio of the vertical and horizontal polarization components by the wave plate 43.
[0040]
In the optical system on the object light side, the polarization direction is rotated 90 degrees by the half-wave plate 47, the wavefront is aligned by the spatial filter 48, and collimated light is incident on the two-dimensional image display device. Although a moving image display can be expected by using a liquid crystal panel as a two-dimensional image display device, in this experimental system, first, a still image displayed on an OHP film 49 was used to verify the principle of stereoscopic display. The object light from the film 49 is incident on the SLM 39 through the lens 60 having a focal length of 150 mm. The distance between the film 49 and the lens 60 is 150 mm, the distance between the lens 60 and the SLM 39 is also 150 mm, and the Fourier transform hologram is written.
[0041]
On the other hand, in the optical system on the reference light side, the light 45 enters the optical path length modulation optical system 51 after passing through the spatial filter 50. The optical path length modulation optical system 51 uses a twisted nematic liquid crystal cell 52 as a polarization switching element. The optical path length modulation optical element shown in FIG. Thus, the optical path length was changed in 16 steps at intervals of 7 mm.
[0042]
The SLM 39 uses an amorphous silicon film as a photoconductive layer, has a thickness of 1 μm, and uses a homogeneous alignment nematic liquid crystal as a light modulation layer, and has a thickness of 1.5 μm and an effective area of 80 × 100 mm. In order to improve the resolution, the thickness of each layer constituting the SLM needs to be thin. Here, the wavelength of the writing light used is a photoconductive layer with a high sensitivity, the reading light has a low sensitivity, and the light absorbing layer is not provided. Further, the liquid crystal is laminated on the photoconductive layer, and the modulated light is read out by reflection at the interface between the liquid crystal and the photoconductive layer, and the dielectric multilayer mirror layer is omitted. The resolution of the SLM 39 is 115 [lp / mm] at a diffraction efficiency of 0.1%.
[0043]
The readout optical system uses a laser 53 having a wavelength of 532 nm, enters the SLM 39, and reads the image by reflection. An image is formed with a lens 54 having a focal length of 300 mm, and zero-order light is cut with an aperture stop 55 to observe a reproduced image. As a result of displaying a hologram image with a square with a side of 2 mm as an input image, a three-dimensional image with a reproduction image size of 4 × 4 mm, a depth distance of the entire image of 72 mm, and a viewing zone angle of 11.6 ° was displayed.
[0044]
【The invention's effect】
According to the present invention, a hologram display having the following features can be realized.
(1) Natural stereoscopic vision without visual fatigue that occurs in a stereoscopic display device using conventional parallax images is possible.
(2) A three-dimensional hologram display can be performed even on a subject of a live-action image.
(3) No special glasses are required.
(4) Stereoscopic viewing in both the horizontal and vertical directions is possible.
(5) Since the information necessary for the stereoscopic display is a two-dimensional color image and depth distance information, a moving image can be captured from the subject using these information. Moreover, since the amount of information is small, transmission is easy, and application to a stereoscopic TV system is possible.
[Brief description of the drawings]
FIG. 1 is a configuration diagram showing an embodiment of a stereoscopic display device according to the present invention.
FIG. 2 is an explanatory diagram of a stereoscopic display principle according to the present invention.
FIG. 3 is a diagram showing an example of a hologram recording display element according to the present invention.
FIG. 4 is a diagram showing an example of an optical path length modulation optical system used in the present invention.
FIG. 5 is a diagram showing an experimental system of a hologram display according to the present invention.
FIG. 6 is a diagram illustrating a specific example of a three-dimensional information input device.
FIG. 7 is a diagram illustrating a configuration example of a sampled image generation apparatus.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 3D information input device 2 Sampling image generation device 3 Transmission type 2D image display device 4, 6 Coherent light 5 Object light 7 Optical path length modulation optical system 8 Hologram recording display element 9 Reproduction light 10 Reflected light 11 Stereo image 12 Observation 13 Reference light condensing position 14, 15 Image 16, 17 Reconstructed image 18, 24 Transparent substrate 19, 25 Transparent electrode 20 Photoconductive layer 21 Light absorbing layer 22 Dielectric multilayer mirror 23 Light modulation layer 26 Lead wire 27 Driving AC power supply 28 Writing light 29 Reading light 30 Condensing or diffusing light 31 Polarization switching cell 32 Polarizing beam splitter (PBS)
33 First quarter-wave plate 34 First mirror 35 Second quarter-wave plate 36 Second mirror 37 Distance difference 38 Optical path length difference 39 SLM
40 Writing optical system 41 Reading optical system 42 Laser 43 Wave plate 44 PBS
45 Reference light side light 46 Object light side light 47 Half-wave plate 48, 50 Spatial filter 49 Film 51 Optical path length modulation optical system 52 Twisted nematic liquid crystal cell 53 Laser 54 Lens 55 Aperture stop

Claims (3)

Based on the color information and depth distance information of the object to be displayed,
A depth sampled image generation device that generates a depth sampled image that is an image of each depth position of the object;
A transmissive two-dimensional image display device for sequentially displaying each depth sampled image from the depth sampled image generation device;
Optical path length modulation that electrically changes the optical path length of coherent reference light having a condensed or diffused light beam in synchronization with display switching of a plurality of depth sampled images that are sequentially displayed on the transmission type two-dimensional image display device Optical system,
Interferometric optics for generating interference fringes between two-dimensional intensity-modulated object light obtained by coherent light incident on the transmission type two-dimensional image display device and reference light from the optical path length modulation optical system The system,
The interference fringes generated by the interference optical system are written on one surface, and at the same time, the image read out on the other surface in response to the interference fringes is reproduced by the reflected light of the coherent reproduction light input to the other surface. And a rewritable hologram recording display element that forms in real time an image that is volumetrically displayed in the depth direction,
The optical path length modulation optical system includes a polarization switching element capable of switching a polarization direction of vertically or horizontally polarized coherent light between horizontal polarization and vertical polarization by electrical control, and a polarization disposed after the polarization switching element. A first quarter-wave plate having a fast axis of about 45 degrees with respect to incident polarized light is arranged in the output direction of the light transmitted through the beam splitter and the polarization switching element and the polarization beam splitter, and the first A first mirror disposed behind the quarter-wave plate and having a fast axis of about 45 degrees with respect to the incident polarized light in the output direction of the light transmitted through the polarization switching element and reflected by the polarization beam splitter; And a second mirror disposed behind the second quarter-wave plate, and the polarization beam splitter and the first mirror. The distance between the over, the different distance between the polarizing beam splitter and the second mirror, the polarization direction switching of the light of the polarization switching element, between the first mirror and the polarization beam splitter the distance, the optical path length corresponding to twice the difference in distance between the second mirror and the polarization beam splitter is changed,
A hologram display comprising an optical system that changes the optical path length of 2 N stages by using N (natural number) optical path length modulation optical systems. In claim 1,
The hologram recording display element includes a first transparent electrode, a photoconductive layer laminated on the first transparent electrode, the impedance of which varies according to the intensity of incident light, and a light absorption layer laminated on the photoconductive layer. And a dielectric multilayer mirror that is laminated on the light absorption layer and reflects all or part of the visible light spectrum, a light modulation layer that is laminated on the dielectric multilayer mirror, and a laminate on the light modulation layer. A hologram display comprising: a second transparent electrode; and a light writing type spatial light modulation element driven by an alternating voltage applied between the first transparent electrode and the second transparent electrode. In claim 2,
The holographic display, wherein the light modulation layer is made of any one of a nematic liquid crystal, a cholesteric liquid crystal, a smectic liquid crystal and a ferroelectric liquid crystal, or a mixed liquid crystal of these liquid crystals.

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