Hologram recording method, hologram reproduction method and holography device

Abstract

To provide a hologram recording method, hologram reproduction method and holography device that enable multirecording of a hologram and reproduction thereof by a simple method using incoherent light.SOLUTION: A hologram recording method by incoherent light is configured to cause at least one of a subject and a phase modulation pattern to move in parallel to a center optical axis in an optical system in single picture-taking by an image sensor; and thus spatially divide a plurality of interference fringes at a different position of one piece of hologram to record.SELECTED DRAWING: Figure 19

Description

[Technical field]

[0001] The present invention relates to a hologram recording method using incoherent light, a hologram reconstructing method, and a holography device. [Background technology]

[0002] Holography is a technology that utilizes the interference phenomenon of light to record and reconstruct light waves from a three-dimensional object into a hologram. Conventionally, coherent light has been used to increase the coherence of light waves when recording a hologram. However, coherent light sources such as lasers are expensive, and in digital holography, which uses coherent light to reconstruct a hologram captured on an image sensor and then reconstructed in a computer, there is a problem that the high coherence of coherent light causes spot-like noise called speckle in the reconstructed image, which greatly limits the objects that can be captured.

[0003] Therefore, in recent years, in order to eliminate restrictions on the subject that can be photographed, research has been progressing on incoherent digital holography, which uses inexpensive incoherent light sources such as LEDs or halogen lamps to capture a hologram on an image sensor by utilizing the self-interference of incoherent light, and then reconstructs the image from the hologram within a computer.

[0004] As shown in Patent Document 1, in incoherent digital holography, a phase shift method has been established as a technique for removing unnecessary light such as direct light and conjugate light from a reproduced image in order to improve the image quality of the reproduced image. In detail, in the digital holography device of Patent Document 1, the phase of one of two orthogonal directional components in a single optical path is spatially linearly modulated to generate multiple types of phase shift amounts in the one directional component. In detail, for example, a polarizer array is used to generate a spatially and periodically changing distribution in the phase shift amount of phase-modulated light relative to wavefront modulated light, which is a part of the object light incident on an image sensor, and a hologram having multiple interference fringes with different phase shift amounts is recorded for each pixel on the imaging surface of the image sensor, and a reproduced image can be obtained using the multiple interference fringes with different phase shift amounts in a reproduction device by the parallel phase shift method. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] JP 2015-1726 A (pages 13 to 14, Figures 9 to 11) Summary of the Invention [Problem to be solved by the invention]

[0006] In the digital holography device of Patent Document 1, since a single optical path can be used to record multiple types of interference fringes with different phase shift amounts in one shooting by an image sensor, it is possible to use an image sensor to shoot a video of a hologram containing three-dimensional object information, but there is a problem that each polarization area of ​​the polarizer array needs to correspond to each pixel on the imaging surface, and therefore extremely high accuracy is required in the design of the optical system. In addition, in the digital holography device of Patent Document 1, a reproduced image is reconstructed from a hologram recorded in a state in which multiple types of interference fringes with different phase shift amounts are superimposed, so there is also a problem that the calculation time becomes enormous when processing a large number of interference fringes.

[0007] The present invention has been made in light of these problems, and has as its object to provide a hologram recording method, a hologram reproduction method, and a holography device that enable multiplexed recording and reproduction of holograms in a simple manner by using incoherent light. [Means for solving the problem]

[0008] In order to solve the above problems, the holographic recording method of the present invention comprises the steps of: A method for recording a hologram using incoherent light, comprising the steps of: A feature of this method is that in a single image capture by the image sensor, at least one of the subject and the phase modulation pattern is translated relative to the central optical axis of the optical system, thereby recording multiple interference fringes in different positions on a single hologram in a spatially divided manner. According to this feature, in one shooting by the image sensor, by translating at least one of the object and the phase modulation pattern with respect to the central optical axis of the optical system, the interference fringes can be translated while maintaining correlation, and multiple interference fringes can be easily recorded in a single hologram by spatially dividing them. Therefore, high-speed shooting of holograms by incoherent light at a speed exceeding the performance of the image sensor is possible.

[0009] It is characterized in that only the phase modulation pattern is translated. According to this feature, by translating the phase modulation pattern, it is possible to translate the interference fringes with high precision.

[0010] The phase modulation pattern is translated by switching the display on the liquid crystal type phase modulation means. According to this feature, the phase modulation pattern displayed on the liquid crystal type phase modulation means can be translated at high speed and with high precision by switching the phase modulation pattern.

[0011] The phase modulation pattern is translated so that the next interference fringe is positioned so as not to overlap with the interference fringes recorded in the hologram. According to this feature, it is possible to record many interference fringes in a single hologram by spatially dividing them, while protecting the interference fringes necessary for forming a reconstructed image.

[0012] The hologram reproducing method of the present invention comprises the steps of: A hologram reproducing method in which, in a single shooting by an image sensor, at least one of an object and a phase modulation pattern is translated relative to a central optical axis in an optical system to reproduce a reproduced image from a plurality of interference fringes that are spatially divided and recorded at different positions on a single hologram, using a hologram recording method using incoherent light, The method is characterized in that the average of all pixel values ​​is subtracted from the pixel values ​​of the hologram, and the portion of the hologram where interference fringes exist is cut out and reconstructed. According to this feature, it is possible to reduce the direct light component in the hologram and thereby enhance the contrast of the interference fringes, and also to efficiently remove the noise component in the hologram.

[0013] The reconstructed image is translated in accordance with the position of the interference fringes recorded in a spatially divided manner in the hologram. According to this feature, it is possible to correct the positional deviation of the reconstructed image that occurs when interference fringes are translated and recorded on a single hologram.

[0014] The holographic device of the present invention comprises: A holography device comprising an optical system that enables self-interference by incoherent light, and an image sensor that records interference fringes formed by the self-interference, The optical system is characterized by having a phase modulation means for translating the phase modulation pattern with respect to the central optical axis of the optical system. According to this feature, in one shooting by the image sensor, by translating at least one of the object and the phase modulation pattern with respect to the central optical axis of the optical system, the interference fringes can be translated while maintaining correlation, and multiple interference fringes can be easily recorded in a single hologram by spatially dividing them. Therefore, high-speed shooting of holograms by incoherent light at a speed exceeding the performance of the image sensor is possible.

[0015] The phase modulation pattern is translated by switching the display on the liquid crystal type phase modulation means. According to this feature, the phase modulation pattern displayed on the liquid crystal type phase modulation means can be translated at high speed and with high precision by switching the phase modulation pattern.

[0016] The phase modulation means translates the phase modulation pattern so that the next interference fringe is located at a position that does not overlap with the interference fringes recorded on the image sensor. According to this feature, it is possible to record many interference fringes in a single hologram by spatially dividing them, while protecting the interference fringes necessary for forming a reconstructed image. [Brief description of the drawings]

[0017] [Figure 1] FIG. 1 is a schematic diagram showing a holography device using an optical system that uses FISCH in an embodiment of the present invention. [Diagram 2] 1 is a diagram illustrating the principle of generation of interference fringes due to self-interference of incoherent light. [Diagram 3] FIG. 1 is a schematic diagram of a reconstruction optical system in incoherent digital holography. [Figure 4] 1A is a pinhole used in a verification experiment in the embodiment, and FIG. 1B is a resolution test chart of the same pinhole. [Diagram 5](a) is a hologram obtained by photographing a pinhole, (b) is an enlarged view of the frame part in the hologram (a) where interference fringes appear, and (c) is a reconstructed image obtained by performing a Fourier transform on the hologram (a). [Figure 6] (a) is a hologram obtained by photographing a resolution test chart, (b) is an enlarged view of the frame area in the hologram (a) where interference fringes appear, and (c) is a reconstructed image obtained by performing a Fourier transform on the hologram (a). [Figure 7] FIG. 2 is a simplified diagram showing the propagation of light waves from a point light source. [Figure 8] 13(a) to 13(d) are diagrams showing the results of image quality improvement processing performed when reproducing a hologram obtained by photographing a pinhole. [Figure 9] 13(a) to 13(d) are diagrams showing the results of image quality improvement processing performed when reproducing a hologram obtained by photographing a resolution test chart. [Figure 10] FIG. 10 is a graph showing the pixel value distribution of pixel rows indicated by straight lines in FIGS. 9(a) to (d). [Figure 11] 1 is a simplified diagram showing the propagation of light waves when a phase modulation pattern is translated relative to a central optical axis of an optical system. [Figure 12] FIG. 13 is a diagram showing an OHP sheet used as a subject to be photographed. [Figure 13] FIG. 1A is a diagram showing the phase distribution of the phase modulation pattern used in the verification before translation, and FIG. 1B is a diagram showing the phase distribution after translation. [Figure 14] (a) is a hologram when the phase modulation pattern is not translated, (b) is an enlarged view of the frame area in the hologram of (a) where interference fringes appear, and (c) is a reconstructed image reconstructed from the hologram of (a). [Figure 15] (a) is a hologram when the phase modulation pattern is translated, (b) is an enlarged view of the framed area in the hologram (a) where interference fringes appear, and (c) is a reconstructed image from the hologram (a). [Figure 16] 14(c) and 15(c) are graphs showing pixel value distributions of pixel rows along line (1) in FIG. 14(c) and line (3) in FIG. 15(c). [Figure 17] 14(c) and 15(c) are graphs showing pixel value distributions of pixel rows along line (2) in FIG. 14(c) and line (4) in FIG. 15(c). [Figure 18] FIG. 13 is a diagram showing an example of control of a phase modulation pattern displayed on an SLM in verification of space division multiplexing recording of a hologram using incoherent light. [Figure 19] (a) is a hologram when the phase modulation pattern is translated, (b) is a reconstructed image reconstructed from interference fringes (1) in the hologram of (a), and (c) is a reconstructed image reconstructed from interference fringes (2) in the hologram of (a). [Figure 20] 19(b) and the pixel value distribution of the pixel string of the straight line (3) in FIG. 19(c) are graphed. [Figure 21] 19(b) and the pixel value distribution of the pixel string of the straight line (4) in FIG. 19(c) are graphed. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0018] After a trial and error study, the inventors have found that it is possible to translate the interference fringes while maintaining correlation by translating at least one of the subject and the phase modulation pattern with respect to the central optical axis in an optical system that allows self-interference by incoherent light. Based on this finding, in one shooting by an image sensor, by translating at least one of the subject and the phase modulation pattern with respect to the central optical axis in an optical system that allows self-interference by incoherent light and recording a plurality of interference fringes in different positions of one hologram in a spatially divided manner, it is possible to give one hologram in one shooting the interference fringes that were obtained by conventional multiple shootings, and further, it is possible to perform high-speed shooting of a hologram by incoherent light at a speed exceeding the performance of an image sensor.

[0019] Hereinafter, an embodiment of the present invention will be described. However, the present invention can be embodied in many different forms, and is not limited to the following embodiments and examples.

[0020] In this embodiment, an optical system using Fourier incoherent single channel holography (FISCH) (see FIG. 1) is used, which is capable of capturing an image by using incoherent light emitted from a light source such as an incandescent light bulb, LED, halogen lamp, fluorescent lamp, etc., in one image capture by an image sensor, by utilizing the self-interference of the incoherent light to separate object light, direct light, and conjugate light. Note that "self-interference" refers to the interference of light waves by utilizing the characteristic that the optical path difference of light waves emitted from the same light source is short.

[0021] Furthermore, in this embodiment, the optical system used to record the hologram (interference fringes) is not limited to an optical system using FISCH, but may be an optical system using other technologies such as Fresnel incoherent autocorrelation holography (FINCH) or coded aperture correlation holography (COACH), as long as it enables self-interference by incoherent light.

[0022] Here, the principle of the generation of interference fringes due to self-interference of incoherent light will be explained with reference to FIG. 2. As shown in FIG. 2, when the object (subject) to be photographed is considered as a collection of point light sources, the spherical wave emitted from the point light source is first aligned in its polarization direction by passing through the first polarizer, and then enters the birefringent element. The fast axis and slow axis of the birefringent element are arranged to be parallel to the s-polarized component (see solid arrow) and the p-polarized component (see dotted arrow), respectively. When the light waves 1 and 2, whose polarization direction is aligned by passing through the first polarizer, pass through the birefringent element, a phase difference occurs between the s-polarized component and the p-polarized component, and the light waves are separated into two light waves 1 and 2 whose polarization directions are orthogonal to each other. At this time, the two separated light waves 1 and 2 have orthogonal polarization directions, so no interference occurs between them. However, when the light waves pass through the second polarizer, their polarization directions are aligned, and as a result, self-interference occurs and interference fringes are formed on the image sensor. In addition, the transmission axes of the first polarizer and the second polarizer in FIG. 2 are inclined at 45 degrees with respect to the fast axis and the slow axis of the birefringent element, respectively.

[0023] In the case of incoherent light, light waves emitted from different point light sources do not form interference fringes, but only light waves emitted from the same point light source undergo self-interference to form interference fringes. In other words, on the image sensor, interference fringes are formed by the light waves emitted from each point light source, and the sum of these fringes is recorded as a hologram.

[0024] The holography device in this embodiment will be described with reference to Fig. 1. As shown in Fig. 1, the holography device uses an optical system using FISCH, and is mainly composed of a white light source that emits incoherent light, a transmission-type shooting target as a subject, a bandpass filter that transmits only specific wavelengths, a first polarizer, a lens L0, a spatial phase modulator (SLM) as a phase modulation means, a mirror, a beam splitter, two lenses L2, a second polarizer, and an image sensor that records a hologram (interference fringes). The two lenses L2 have the same focal length f2, which will be described later. In this embodiment, the SLM performs the same function as the birefringent element in Fig. 2.

[0025] The phase modulation means in this embodiment uses an electrically controllable liquid crystal type such as an SLM or a liquid crystal display (LCD), which makes it easy to change the configuration of the phase modulation pattern according to the focal length, and in addition, by switching the display of the phase modulation pattern on the phase modulation means, the phase modulation pattern can be translated at high speed and with high precision relative to the central optical axis of the optical system. Furthermore, since the phase modulation pattern can be translated by switching the display of the phase modulation pattern on the phase modulation means, there is no need to translate the phase modulation means mechanically, and the holography device can be configured simply.

[0026] In this embodiment, by translating at least one of the object and the phase modulation pattern with respect to the central optical axis in the optical system using FISCH, it is possible to translate the interference fringes while maintaining correlation. Note that when the object is translated, even if the object is moved slightly, the movement distance of the interference fringes becomes large and difficult to control, whereas when the phase modulation pattern is translated, the movement distance of the phase modulation pattern and the movement distance of the interference fringes become approximately the same. For this reason, by translating only the phase modulation pattern, it is possible to improve the controllability of the position where the interference fringes appear.

[0027] In addition, when using self-interference due to incoherent light, it is difficult to form interference fringes on the image sensor, so it is preferable to align the central optical axis of the optical system as much as possible. As an example of the procedure for aligning the central optical axis of the optical system, first, with only the SLM and mirror arranged, a laser light source adjusted to be horizontal is made incident on the SLM, and the angles of the SLM and the mirror are adjusted, and the irradiated laser is set as the central optical axis of the optical system. Next, a beam splitter is arranged to ensure an optical path from the SLM to the image sensor, and each lens is arranged according to the focal length, and the position is adjusted so that the laser set as the central optical axis of the optical system passes through the center of the lens. Finally, the first polarizer, the second polarizer, the bandpass filter, the transmission type shooting target (subject), and the white light source are arranged.

[0028] As shown in FIG. 1, the positional relationship of each element constituting the optical system, specifically, the distance z between the transmission-type photographing target and the lens L0, s The focal length f0 of the lens L0, the focal length f1 of the Fresnel lens L1 (phase modulation pattern) displayed on the SLM, the focal length f2 of the lens L2, the distance d0 between the lens L0 and the SLM, and the distance d2 between the lens L2 and the image sensor are arranged to have a predetermined positional relationship. s = f0.

[0029] Next, the propagation of light waves in a holography device will be explained with reference to FIG. 1. As shown in FIG. 1, in a holography device, light waves emitted from a white light source pass through a transmission-type imaging target, and then pass through a bandpass filter, whereby only specific wavelengths are transmitted, enhancing the monochromaticity of the light waves. The light waves then pass through a first polarizer, whereby the polarization direction is aligned. The phase of the light waves with aligned polarization direction changes when passing through lens L0. Lens L0 is a convex lens, and the phase conversion action of the lens focuses the incident light of a plane wave at a position of a specified focal length and enters the SLM. The light waves that enter the SLM are separated into two light waves depending on the polarization direction. For more details, by displaying a phase modulation pattern of a specified focal length on the SLM, the SLM acts as a Fresnel lens for s-polarized light waves and as a mirror (plane mirror) for p-polarized light waves (see "Modulation by SLM" at the bottom right of FIG. 1). The s-polarized light wave modulated by the SLM and the p-polarized light wave not modulated by the SLM are reflected by a mirror arranged at an appropriate angle and enter the SLM again. The two light waves then pass through lens L2, the second polarizer, and lens L2 in that order on the optical path to the image sensor secured by the beam splitter, and reach the image sensor, where they form interference fringes due to self-interference.

[0030] Here, as in the embodiment described later, when interference fringes are recorded on an image sensor using coherent light for a shooting target having a size of 1 mm×1.5 mm (see FIG. 4(b) and FIG. 12), the interference fringes appear on the entire hologram, whereas when interference fringes are recorded on an image sensor using self-interference due to incoherent light as in the present embodiment, the interference fringes appear only on a part of the hologram (see FIG. 5 to FIG. 7, etc.). That is, when interference fringes are recorded on an image sensor using self-interference due to incoherent light as in the present embodiment, the interference fringes are recorded on the hologram in a smaller size than when interference fringes are recorded on an image sensor using coherent light. In addition, since the information of the shooting target is recorded as interference fringes, the parts of the hologram other than the interference fringes cause noise in the reproduced image. In addition, the inventors have confirmed that the interference fringes of the hologram change slightly when the phase modulation pattern is translated.

[0031] For these reasons, when using the self-interference of incoherent light to spatially divide and record a plurality of interference fringes at different positions on one hologram, the next interference fringe is translated so that it is positioned at a position that does not overlap with the interference fringes previously recorded on the hologram, thereby making it possible to record a large number of interference fringes on one hologram by spatial division while protecting the interference fringes necessary for forming a reconstructed image. In this way, in this embodiment, by utilizing the characteristics of the interference fringes when using the self-interference of incoherent light, it is easy to spatially divide and record a plurality of interference fringes on different positions on one hologram without overlapping.

[0032] Next, a method for reproducing a hologram obtained by the hologram recording method of this embodiment will be described.

[0033] A hologram recorded by an optical system using FISCH is considered to be simply the sum of contributions from all point light sources, and since the object light is included in an inverse Fourier transformed form, a Fourier transform must be performed to reconstruct the hologram. Because a Fourier transform is performed, in addition to the object image, a conjugate image identical to the object image appears in a point-symmetrical position in the reconstructed image, and the direct light component appears in the center of the image, making it possible to separate it from the object light component, and degradation of image quality due to direct light can be suppressed. As in this embodiment, when the distance z between the transmission-type shooting target and the lens L0 in the holography device is s and the focal length f0 of the lens L0 is the same plane (z s For z = f0), a reconstructed image can be obtained simply by performing a Fourier transform. s When f0 is not equal to the distance z between the target and the lens L0, s The reconstructed image is obtained by using the diffraction integral at

[0034] As shown in Fig. 3, the reconstruction optical system in incoherent digital holography is composed of a lens L r The reconstructed image is obtained by using the Fourier transform of the lens L. r The Fourier transform action of is that the distribution of light waves on the front focal plane is Fourier transformed and appears as a distribution of light waves on the back focal plane. r This shows that the object image and the conjugate image appear point-symmetrically at the position of the hologram. Therefore, if you want to adjust the reconstruction plane, you can obtain a reconstruction image at any position by appropriately calculating the propagation of the reconstruction light from the hologram using the angular spectrum method.

[0035] In addition, in this embodiment, a hologram reproducing method uses a hologram recording method that utilizes self-interference caused by incoherent light, and in order to improve the quality of a reproduced image reproduced from a plurality of interference fringes recorded in different positions of a single hologram by spatially dividing the hologram, the average of all pixel values ​​is subtracted from the pixel values ​​of the hologram, and only the portion of the hologram with interference fringes is cut out and reproduced. This reduces the direct light component in the hologram, strengthens the contrast of the interference fringes, and efficiently removes noise components in the hologram.

[0036] Furthermore, in this embodiment, as a hologram reproduction method, the reconstructed image reconstructed from multiple interference fringes recorded on a single hologram moves in parallel according to the distance the interference fringes are translated. Therefore, by translating the reconstructed image in parallel according to the positions of the interference fringes recorded in a spatially divided hologram, the positional shift of the reconstructed image can be corrected and it can be reproduced at a reference position. EXAMPLES

[0037] Here, the holography device according to the embodiment will be specifically described below.

[0038] Table 1 shows the positional relationship of each element constituting the optical system using FISCH in this embodiment (see FIG. 1).

[0039] [Table 1]

[0040] The SLM (HOLOEYE PLUTO-2.1 SLM) used in the optical system in this embodiment has a pixel count of 1920×1080 and a pixel interval of 8.00 μm. The image sensor (Baumer TXG50) used in the optical system in this embodiment has a pixel count of 2448×2050 and a pixel interval of 3.45 μm. The bandpass filter A (Edmund Optics BPF86365 for fluorescence) used in the optical system in this embodiment has a central transmission wavelength of 525 nm and a bandwidth of 15 nm, and the bandpass filter B (Edmund Optics BPF86995 for fluorescence) has a central transmission wavelength of 640 nm and a bandwidth of 14 nm. The white light source used was LUXYR-LED PICO manufactured by K-Able.

[0041] [Verification experiment] First, using the optical system of this embodiment, a pinhole (pinhole diameter 0.3 mm) shown in FIG. 4(a) and a resolution test chart (NegativeNBS 1963A manufactured by Edmund Optics) shown in FIG. 4(b) are photographed as subjects to verify whether photographing using an optical system using FISCH is possible. In more detail, by photographing a pinhole, it is verified whether a hologram can be recorded when the subject is the most basic single point light source. In addition, by photographing the character "6" with a size of 1 mm x 1.5 mm on the resolution test chart, it is verified whether a hologram can be recorded when the subject can be considered as a collection of point light sources.

[0042] In this embodiment, the pinhole was photographed using band pass filter A, and the resolution test chart was photographed using band pass filter B. The subject was placed at the focal length f0 of lens L0.

[0043] As shown in the hologram in FIG. 5(a) and its enlarged view in FIG. 5(b), it can be confirmed that small interference fringes appear in a part of the hologram obtained by photographing the pinhole. Also, as shown in the reconstructed image in FIG. 5(c), it can be confirmed that two bright spots appear in the reconstructed image. In this embodiment, after the Fourier transform to obtain a reconstructed image from the hologram, the pixel value due to the direct light at the center was too large and the object image was not visible. Therefore, a threshold was set so that the object image could be seen, and all pixel values ​​above the threshold were adjusted to the threshold before the reconstructed image was reconstructed.

[0044] Here, we will explain the propagation of light waves from a point light source in an optical system using FISCH. As shown in Figure 7, the light waves emitted from a point light source placed on the focal plane of lens L0 are converted into parallel light by lens L0, and then split into two light waves by the polarization direction when they enter the SLM. The two light waves reflected by the mirror are then modulated and converted into parallel light by entering the SLM again, and finally enter the image sensor as parallel light at different angles to form interference fringes by passing through two lenses L2 arranged on the optical path from the SLM to the image sensor. In addition, it is speculated that the two light waves are each incident on the sensor surface of the image sensor at an incident angle other than 0 degrees, and become in a state similar to a so-called off-axis hologram, so that the object light can be separated from the direct light and the conjugate light and reproduced. In addition, the range in which interference fringes are formed on the image sensor is limited to the area surface where the two light waves overlap on the image sensor. Furthermore, since interference occurs when the difference in optical path length between the two light waves does not exceed the coherence length of the light source used, it is assumed that small interference fringes from the point light source appear only in a part of a circular range (see dotted line area) centered on the point where the difference in optical path length is small, as shown in Figure 5(b).

[0045] As shown in Figures 6(a) and (b), in the hologram obtained by photographing the resolution test chart, it can be seen that small interference fringes appear in a part of the hologram, just like the pinhole photograph mentioned above. If we consider the character "6" on the resolution test chart, which is the subject of the photograph, to be a collection of point light sources, the hologram can be considered to be a superposition of interference fringes according to the positions of the point light sources, so the interference fringes appear over a wider area than the interference fringes that appeared in the pinhole photograph (see Figure 5(b)). In addition, as shown in Figure 6(c), because a Fourier transform is performed during reconstruction, the reconstructed image has the same conjugate image as the object image, but in a point-symmetrical position, and it can be seen that the direct light component appears in the center.

[0046] [Image quality improvement processing for reproduced images] In the above-mentioned verification experiment, interference fringes appeared only partially in the recorded hologram. Since the information of the object to be photographed is recorded as interference fringes, the parts other than the interference fringes cause noise in the reconstructed image. In addition, when using the self-interference of incoherent light, the light intensity on the image sensor is low, so the contrast of the interference fringes is weakened. In order to solve these problems and improve the image quality of the reconstructed image, the hologram obtained in the above-mentioned verification experiment was subjected to a process of subtracting the average value of all pixel values ​​from the pixel values ​​of the hologram recorded on the image sensor (hereinafter referred to as "image quality improvement process A"), and a process of cutting out only the parts with interference fringes (hereinafter referred to as "image quality improvement process B").

[0047] As shown in Figures 8(b) and 9(b), it can be seen that the noise and direct light in the center are reduced in the reconstructed image that has only undergone image quality improvement process A, compared to the reconstructed image without any processing (see Figures 8(a) and 9(a)). This is presumably because image quality improvement process A reduces the direct light component on the hologram, enhancing the contrast of the interference fringes.

[0048] As shown in Figures 8(c) and 9(c), the reconstructed image that has undergone only image quality improvement process B has less noise than the reconstructed image without any processing, and in particular, the noise that spread concentrically from the center has been significantly reduced. This is presumably because image quality improvement process B removes parts of the hologram other than the interference fringes that caused noise, improving the image quality of the reconstructed image.

[0049] Furthermore, as shown in Figures 8(d) and 9(d), it can be seen that the reconstructed images that have undergone both image quality improvement processes A and B not only have noise removed, but also reproduce the object light components more clearly.

[0050] Here, in order to evaluate the image quality of the reconstructed images, the pixel value distribution of the pixel rows in the parts indicated by the straight lines in Figures 9(a) to (d) was graphed and the results are shown in Figure 10. As shown in Figure 10, when comparing a reconstructed image without any processing (see Figure 9(a)) with a reconstructed image that has been subjected to image quality improvement processing A only (see Figure 9(b)), it can be seen that while there are some parts where noise has been reduced, there are also other parts where noise has increased. This is presumably because image quality improvement processing A has strengthened the contrast of the interference fringes as well as the noise contrast.

[0051] Furthermore, when comparing the reconstructed image without any processing (see FIG. 9(a)) with the reconstructed image subjected to only image quality improvement processing B (see FIG. 9(c)), it can be confirmed that the noise reduction is reflected as a change in pixel values.

[0052] Furthermore, when comparing a reconstructed image which has been subjected to only image quality improvement process B (see FIG. 9(c)) with a reconstructed image which has been subjected to both image quality improvement processes A and B (see FIG. 9(d)), it can be seen that the pixel values ​​of the pixels in which the object light appears are larger, and the contrast is stronger. From these facts, it can be seen that by performing both image quality improvement processes A and B on a hologram captured by the optical system using FISCH of this embodiment, it is possible to reduce direct light and noise on the reconstructed image and effectively improve image quality.

[0053] [Space-division multiplexed recording of holograms using FISCH] In the optical system using FISCH in this embodiment, an SLM is used to split the light waves into two, and therefore the phase modulation pattern (Fresnel lens pattern) displayed on the SLM is shifted, i.e., the phase modulation pattern is translated parallel to the central optical axis of the optical system, thereby translating the interference fringes of the hologram, thereby performing spatial division multiplexing recording of a hologram using incoherent light.

[0054] As shown in Figure 11, it can be seen that the light waves that are not modulated on the SLM (see dotted arrow) follow the same optical path before and after the phase modulation pattern is translated in parallel. On the other hand, it can be seen that the light waves modulated on the SLM (see solid arrow) and the light waves modulated by the translated Fresnel lens (phase modulation pattern) (see dashed arrow) have their optical axes translated in parallel with respect to the image sensor. As a result, the position where the interference fringes appear is translated on the image sensor. The extent to which the interference fringes translate in parallel with the translation distance of the phase modulation pattern varies depending on the positional relationship of each element that makes up the optical system and the position of the subject to be photographed.

[0055] In order to verify that the interference fringes can be translated to any location on the hologram by translating the phase modulation pattern displayed on the SLM, a holography device using an optical system using the same FISCH as in the verification experiment described above was used to capture an OHP sheet with the letter "T" printed on it as shown in Figure 12, using the bandpass filter B described above. The phase modulation pattern displayed on the SLM after translation is translated 480 pixels to the left from the position in Figure 13(a), as shown in Figure 13(b). Note that the translation distance of 480 pixels on the SLM is 8.00 μm because the pixel spacing of the SLM is 8.00 μm, so the translation distance is 8.00 μm x 480 = 3.84 mm.

[0056] Comparing Figures 14(a) and (b) with Figures 15(a) and (b), it can be seen that the position where the interference fringes are formed is also translated by translating the phase modulation pattern displayed on the SLM. Note that the pixel interval of the image sensor is 3.45 μm, and the position of the interference fringes is translated by approximately 1000 pixels, so that a translation distance of approximately 3.45 mm is performed. That is, it can be seen that in this embodiment, the difference between the translation distance of the phase modulation pattern displayed on the SLM and the movement distance of the interference fringes on the hologram is small, at approximately 0.39 mm.

[0057] As shown in Figure 14(c) and Figure 15(c), there is almost no difference in image quality between the reconstructed images. Note that, in reconstructing the reconstructed images, both of the above-mentioned image quality improvement processes A and B are performed.

[0058] Also, while the pixels at the peaks of the pixel value distributions of lines (1) and (3) in Fig. 16 are approximately the same, compared to the pixel at the peak of the pixel value distribution of line (2) in Fig. 17, the pixel at the peak of the pixel value distribution of line (4) has shifted approximately 25 pixels to the left, in other words, it can be confirmed that the reconstructed image has shifted in the left-right direction. This is presumably because the positional relationship between the lens and the subject has changed due to the translation of the phase modulation pattern displayed on the SLM.

[0059] From these findings, it was confirmed that it is possible to translate the interference fringes on the hologram by translating the phase modulation pattern displayed on the SLM, without affecting the image quality of the reconstructed image. Furthermore, when photographing using the method of this embodiment, the reconstructed image itself translates in a predetermined direction and a predetermined distance depending on the direction and distance to which the interference fringes are translated, so that after reconstructing the hologram, it is preferable to translate the reconstructed image and adjust it to return it to a reference position.

[0060] Next, the spatial division multiplexing recording of a hologram using incoherent light is verified. In this embodiment, a computer connected to the SLM of a holography device (see FIG. 1) using an optical system using FISCH controls the phase modulation pattern displayed on the SLM. In detail, switching control is performed so that n types of Fresnel lens patterns (phase modulation patterns) are displayed for T / n times during the exposure time T of the image sensor (camera) (see FIG. 18). As a result, if the center of the Fresnel lens pattern displayed on the SLM is appropriately translated, n types of interference fringes can be photographed at spatially different positions of one hologram during the exposure time T. In addition, n types of interference fringes can be cut out from one hologram in which they are spatially multiplexed and reproduced, thereby obtaining n reproduced images from one hologram.

[0061] Here, the exposure time of the image sensor is set to 1 second, n=2 types of Fresnel lens patterns are displayed on the SLM, and the result of spatial division multiplexing recording of a stationary subject (an OHP sheet with the above-mentioned "T" printed on it) is shown in Figure 19. Note that both of the above-mentioned image quality improvement processes A and B are performed in reconstructing the reconstructed image. As shown in Figure 19(a), it can be seen that two interference fringes (1) and (2) that are translated are recorded on one hologram. Also, as shown in Figures 19(b) and (c), it can be seen that the printed "T" is reproduced in the reconstructed image reconstructed from the above-mentioned interference fringes (1) and (2).

[0062] As explained above, in a holography device using an optical system using FISCH, space-division multiplexing recording of a hologram can be easily performed by controlling (translating) the phase modulation pattern on the SLM. In addition, by using multiple reconstructed images reconstructed from multiple interference fringes recorded in the hologram in space-division multiplexing, it is possible to improve the quality of the reconstructed image (still image) and expand the field of view.

[0063] Furthermore, the spatial division multiplexing recording of holograms demonstrated in this example has demonstrated that high-speed shooting of holograms using incoherent light at speeds that exceed the performance (frame rate) of image sensors is possible, which can be applied to shooting dynamic objects, i.e., video shooting.

[0064] As shown in Figs. 19(b) and (c), in the spatial division multiplexing recording of the hologram of this embodiment, the brightness of the reconstructed image is lower than the photographing results when the interference fringes shown in Figs. 14 and 15 are translated. Also, the pixel values ​​are lowered when comparing the graph of Fig. 20 showing the pixel value distribution in the pixel row of the straight line (1) part of Fig. 19(b) and the straight line (3) part of Fig. 19(c) and the graph of Fig. 21 showing the pixel value distribution in the pixel row of the straight line (2) part of Fig. 19(b) and the straight line (4) part of Fig. 19(c) with the graphs of Figs. 16 and 17. It is presumed that such a decrease in the image quality of the reconstructed image is due to the fact that the contrast of the interference fringes is weakened by the reduction in exposure time, which increases the influence of direct light and noise on the object light. For this reason, it is possible to improve the image quality by redesigning the system, such as by changing the position of each element in the optical system constituting the holography device or by changing the type of incoherent light source, and thereby enhancing the contrast of the interference fringes.

[0065] In this embodiment, the spatial division multiplexing recording of a hologram is explained using incoherent light with a transmission type object, but the spatial division multiplexing recording of a hologram is also possible with incoherent light with a reflection type object. Furthermore, the object is not limited to a two-dimensional object, but may be a three-dimensional object.

[0066] Although the embodiments of the present invention have been described above with reference to the drawings, the specific configuration is not limited to these embodiments, and the present invention also includes modifications and additions that do not deviate from the gist of the present invention.

[0067] For example, the incoherent light is not limited to light emitted from light sources such as incandescent light bulbs, halogen lamps, fluorescent lights, and LEDs, but may also be natural light such as sunlight.

[0068] In addition, there is no limit to the number of multiplexes in the space division multiplexing recording of the hologram, and it can be appropriately designed based on the balance between the size of the hologram and the size of the interference fringes. In the above embodiment, the embodiment using the shooting target of 1 mm x 1.5 mm in size has been described, but the size of the shooting target may be freely selected, and the larger the size of the shooting target, the larger the size of the interference fringes recorded in the hologram. In addition, the larger the size of the image sensor, the larger the size of the hologram. [Industrial Applicability]

[0069] The present invention can record a plurality of interference fringes at different positions on a single hologram in a single shooting by an image sensor by translating at least one of the subject and the phase modulation pattern with respect to the central optical axis in an optical system that enables self-interference by incoherent light, by spatially dividing the fringes, and can obtain a still image with high accuracy by using a plurality of reconstructed images reconstructed from the plurality of interference fringes. It also enables high-speed shooting of a hologram by incoherent light at a speed exceeding the performance of the image sensor, and therefore can be applied to shooting of dynamic objects, i.e., video shooting, and has industrial applicability. In addition, it becomes possible to record and reconstruct many shooting objects at high speed using incoherent light emitted from an inexpensive light source, thereby realizing a reduction in the cost of a holography device.

Claims

1. A method for recording a hologram using incoherent light, comprising the steps of: A hologram recording method characterized in that, in a single shooting by an image sensor, at least one of the subject and the phase modulation pattern is translated relative to the central optical axis of the optical system, thereby spatially dividing and recording a plurality of interference fringes at different positions on a single hologram.

2. 2. The hologram recording method according to claim 1, wherein only the phase modulation pattern is translated.

3. 3. The hologram recording method according to claim 2, wherein the phase modulation pattern is translated by switching the display on a liquid crystal type phase modulation means.

4. 4. The hologram recording method according to claim 2, further comprising translating the phase modulation pattern so that the next interference fringe is located at a position that does not overlap with the interference fringe recorded in the hologram.

5. A hologram reproducing method in which, in a single shooting by an image sensor, at least one of an object and a phase modulation pattern is translated relative to a central optical axis in an optical system to reproduce a reproduced image from a plurality of interference fringes that are spatially divided and recorded at different positions on a single hologram, using a hologram recording method using incoherent light, A hologram reconstructing method comprising subtracting an average of all pixel values ​​from the pixel values ​​of said hologram, and cutting out and reconstructing a portion of said hologram having interference fringes.

6. 6. The hologram reproducing method according to claim 5, further comprising the step of translating the reproduced image in accordance with the position of interference fringes recorded in a spatially divided manner in the hologram.

7. A holography device comprising an optical system that enables self-interference by incoherent light, and an image sensor that records a hologram having interference fringes formed by the self-interference, A holography device comprising: a phase modulation means for translating a phase modulation pattern with respect to a central optical axis of the optical system.

8. 8. The holographic device according to claim 7, wherein the phase modulation pattern is translated by switching the display on the liquid crystal type phase modulation means.

9. 9. The holography device according to claim 7, wherein the phase modulation means translates the phase modulation pattern so that the next interference fringe is positioned so as not to overlap with the interference fringes recorded on the image sensor.

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