JP2008217990A - Hologram recording apparatus, hologram playback apparatus, information encoding method, method of recording, and information playback method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a hologram recording and playback apparatus that reduces error bits. <P>SOLUTION: The hologram recording and playback apparatus 100 has a spatial light modulator 112 that modulates a light flux spatially and an objective lens 106 that collects object light and reference light for recording to a recording medium, in order to perform hologram recording of information. The spatial light modulator 112 is controlled by a control section that generates a two-dimensional digital pattern which is formed with n items of first subsets comprising sixteen elements and which corresponds to the information. A first subset includes four pairs of second subsets, each second subset includes four elements. Out of the four pairs of second subsets, in three pairs of second subsets, the control section makes one element out of the elements forming the second subsets a first state, makes the three elements a second state, and makes four elements forming the remaining one pair of second subsets out of the four pairs of second subsets, a second state. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a technique used in a holographic memory for storing information using light.

  Using a light source such as a laser, interfere with non-scattered light (referred to as “reference light” or “pump light”) from the same light source as light scattered by the object (referred to as “object light” or “signal light”), The interference draft (referred to as “hologram”) is recorded on a storage medium such as a photographic plate capable of optical recording. A technique for reproducing the scattered light from the object by irradiating the recorded interference manuscript only with the reference light during reproduction is called holography.

  Further, when the depth of the storage medium is sufficiently longer than the wavelength of the recording light, a plurality of holograms can be recorded in the same medium. This technique is called volume multiplex holography. Certain materials, such as photorefractive crystals and photosensitive photopolymers, change their refractive index when irradiated with light. For this reason, these materials are used as recording media for volume multiplex holography.

  On the other hand, in the recording of digital information, the object light is passed through a two-dimensional spatial light modulator such as a liquid crystal panel, and the object light has a two-dimensional digital pattern information by giving the object light an intensity distribution or phase distribution. give. The object light after passing through the two-dimensional spatial light modulator is condensed using a lens and irradiated by a recording medium. A system that records and reproduces information using a hologram that gives two-dimensional digital pattern information to object light in this way is called a holographic memory.

  As a method for giving digital pattern information to object light by a two-dimensional spatial light modulator, Patent Document 1 reduces the medium saturation caused by the continuation of on (bright) bits, and reduces the recording density, storage capacity, and data transfer of the device. A method for improving speed is disclosed.

  In the method disclosed in Patent Document 1, digital pattern information in which two types of information bits, on (bright) and off (dark), are two-dimensionally arranged is generated. Here, n × n bits (where n is a predetermined integer of 3 or more) is a unit code block, and the number of on bits s in the unit code block is an integer of n−1. Then, m is selected so that the value of 2 to the power of m (where m is an integer) takes a maximum value that does not exceed the total number of patterns in which the number of on bits in the unit code block is s, and the unit code block It represents m-bit information.

FIG. 13 is a diagram illustrating an example of a two-dimensional digital pattern in the case of n = 3. In FIG. 13, the on (bright) bit is indicated by white, and the off (dark) bit is indicated by diagonal lines. As shown in FIG. 11, when n = 3, the number of on bits s is 2 (= 3-1).
JP 2001-75463 A

  However, in the method disclosed in Patent Document 1, when luminance unevenness due to crosstalk or an optical system occurs in reproduction light, one page which is a two-dimensional digital pattern included in one object light beam is set as one threshold value. An error may occur when performing binarization processing with.

  As a specific error, for example, when there is luminance unevenness in a part of the reproduced image information, a data bit that should be processed as “1” due to the luminance unevenness is erroneously detected because the luminance value is less than the threshold value. It may be recognized as “0”. Conversely, the luminance value of a block that should be processed as “0” originally exceeds the threshold value, so that it may be erroneously recognized as “1”. Also, when the area per page is large, error bits are likely to occur by performing binarization processing with one threshold value due to unevenness in the intensity distribution of the reproduction light.

  The present invention has been made to solve the above-described problems, and an object of the present invention is to provide a hologram recording / reproducing apparatus capable of reducing error bits.

  According to one aspect of the present invention, there is provided a hologram recording apparatus that records information by optical interference fringes, and generates a two-dimensional digital pattern corresponding to information by a first subset n of 16 elements. A modulator and its control means, wherein the first subset includes four second subsets of four elements, and the control means comprises three sets of the four second subsets For one of the four elements constituting the second subset, the means for setting the three elements to the second state, and the remaining of the four sets of the second subset Means for bringing all four elements into a second state in a second subset of sets.

  Preferably, the spatial light modulator includes a display device for displaying a two-dimensional digital pattern, and the display device includes a reference light generation region for generating recording reference light from a light beam incident on the spatial light modulator; And an object light generation region for generating object light from a light beam incident on the spatial light modulator, and the control means displays a two-dimensional digital pattern corresponding to the information on the object light generation region.

  Preferably, the first state is an on (bright) bit and the second state is an off (dark) bit.

  Preferably, the first state is an off (dark) bit and the second state is an on (bright) bit.

  According to another aspect of the present invention, there is provided a hologram reproducing device for reading information from a recording medium on which optical interference fringes are recorded, the means for detecting the reproducing light from the recording medium, and the two-dimensional digital contained in the reproducing light. Means for dividing the pattern into a plurality of first subsets, wherein the first subset includes four sets of second subsets respectively arranged at predetermined positions in the first subset. , The second subset has four elements, the elements being in either the first state or the second state, and for each of the plurality of first subsets, the first Means for dividing the subset into a plurality of second subsets, and detecting a second subset in which all four elements are in the second state from the plurality of second subsets.

  Preferably, the apparatus further includes means for correcting the luminance of the plurality of second subsets based on the luminance value of the second subset in which all four elements are in the second state.

  Preferably, the information corresponds to a two-dimensional digital pattern composed of n first subsets of 16 elements, and in three sets of second subsets of the four sets of second subsets, Of the four elements constituting the second subset, one element is in the first state, three elements are in the second state, and among the four sets of second subsets, In the remaining one set of the second subset, all four elements constituting the second subset are in the second state.

  Preferably, the second subset in which all four elements are in the second state has the lowest luminance value among the plurality of second subsets, and the first subset is divided into the first subset for each of the plurality of first subsets. Means for ranking each element constituting the subset based on the luminance value, and means for performing luminance correction has the luminance values of the top three of the 16 elements constituting the first subset The elements are corrected to the first state, and the remaining elements are corrected to the second state.

  The second subset in which all four elements are in the second state has the highest luminance value among the plurality of second subsets. For each of the plurality of first subsets, the first subset Means for ranking each element that constitutes the element based on the luminance value, and the means for performing luminance correction includes elements having luminance values up to the third lowest rank among the 16 elements constituting the first subset. The state is corrected to 1 and the remaining elements are corrected to the second state.

  Preferably, the second subset in which all four elements are in the second state has the lowest luminance value among the plurality of second subsets, and the means for performing luminance correction includes four elements All elements included in the second subset, all in the second state, are corrected to be off (dark) bits, and four elements of the second subset included in the first subset For each second subset excluding the second subset, all of which are in the second state, the element having the maximum luminance value among the elements included in each second subset is set to the on (bright) bit, and the luminance The elements other than the element with the maximum value are corrected so as to be an off (dark) bit.

  The second subset in which all four elements are in the second state has the highest luminance value among the plurality of second subsets, and the means for performing luminance correction is that all four elements are in the second state. So that all elements included in the second subset in the state are on (bright) bits, and all four elements of the second subset included in the first subset For each of the second subsets excluding the second subset in the state 2, the element having the maximum luminance value among the elements included in each second subset is set to the off (dark) bit, and the luminance value is the maximum. The other elements are corrected so as to be on (bright) bits.

  According to still another aspect of the present invention, there is provided an information encoding method for displaying information as an 8n-bit two-dimensional digital pattern for each 16n-bit pattern, wherein the two-dimensional digital pattern includes a first 16-bit pattern. The first subset includes four second subsets respectively arranged at predetermined positions in the first subset, and each second subset includes 4 subsets. One of four bit patterns constituting the second subset in the second subset of the three sets out of the four second subsets. In the step of setting the bit pattern to the first state and the three bit patterns to the second state, and the remaining second set of the second subset of the four sets of the second subset, 4 pieces constituting a subset All the bit patterns and a step of the second state.

  Preferably, the step of setting all four bit patterns to the second state determines 2n bits out of 8n bits.

  Preferably, among the four bit patterns, the step of setting one bit pattern to the first state and the three bit patterns to the second state determines 6n bits out of 8n bits.

Preferably, the information is displayed as a two-dimensional digital pattern for hologram recording.
According to still another aspect of the present invention, there is provided a recording method for writing a digital pattern on a recording medium using an information encoding method for displaying information as an 8n-bit two-dimensional digital pattern for each 16n-bit pattern. The dimensional digital pattern is composed of n first subsets of 16-bit patterns, and the first subsets are four sets of second parts respectively arranged at predetermined positions in the first subset. Each of the second subsets has four bit patterns, and among the four second subsets, in the second set of three sets, the second subset Of the four bit patterns that constitute, the step of setting one bit pattern to the first state and the three bit patterns to the second state, and the remaining two of the four second subsets 1 In a second subset of, comprising the steps of a four second all bit patterns condition constituting the second subset, and writing to the four sets of the second subset of the recording medium.

Preferably, the recording medium is a hologram memory.
According to another aspect of the present invention, there is provided an information reproduction method for reading a two-dimensional digital pattern recorded using an information encoding method for displaying information as an 8n-bit two-dimensional digital pattern for every 16n-bit pattern, A step of detecting a two-dimensional digital pattern; and means for dividing the two-dimensional digital pattern into a plurality of first subsets, each of the first subsets being arranged at a predetermined position in the first subset. The second subset has four bit patterns, and the element is in either the first state or the second state The first subset is divided into a plurality of second subsets for each of the plurality of first subsets, and all four bit patterns from the plurality of second subsets are in the second state. A second one Further comprising the step of detecting a partial set.

  Preferably, the method further includes a step of correcting the luminance of the plurality of second subsets based on the luminance value of the second subset in which all four bit patterns are in the second state.

  Preferably, the information corresponds to a two-dimensional digital pattern composed of n first subsets of 16-bit patterns, and in three sets of second subsets out of four sets of second subsets. Of the four bit patterns constituting the second subset, one bit pattern is in the first state, three digital patterns are in the second state, and four sets of second parts In the remaining second subset of the set, all four bit patterns constituting the second subset are in the second state.

  Preferably, the second subset in which all of the four bit patterns are in the second state has the lowest luminance value among the plurality of second subsets, and the second subset is in each of the plurality of first subsets. Each bit pattern constituting one subset is ranked based on the luminance value, and among the 16-bit patterns constituting the plurality of first subsets, the ranking results based on the luminance value Correcting the three bit patterns from the highest to the first state and correcting the remaining bits to the second state.

  Preferably, the second subset in which all of the four bit patterns are in the second state has the lowest luminance value among the plurality of second subsets, and the second subset is in each of the plurality of first subsets. Each bit pattern constituting one subset is ranked based on the luminance value, and among the 16-bit patterns constituting the plurality of first subsets, the ranking results based on the luminance value Correcting the three bit patterns from the lowest to the first state and correcting the remaining bit patterns to the second state.

  Preferably, the second subset in which all four bit patterns are in the second state has the lowest luminance value among the plurality of second subsets, and all four bit patterns are in the second state. Correcting all the bit patterns included in the second subset to be off (dark) bits, and four bit patterns of the second subset included in the first subset For each second subset excluding the second subset, all in the second state, the bit pattern having the maximum luminance value among the bit patterns included in each second subset is turned on (bright) bit, And a step of correcting a bit pattern other than the bit pattern having the maximum luminance value to be an off (dark) bit.

  Preferably, the second subset in which all four bit patterns are in the second state has the lowest luminance value among the plurality of second subsets, and all four bit patterns are in the second state. Correcting all the bit patterns included in the second subset to be on bits, and four bit patterns of the second subset included in the first subset are For each of the second subsets excluding the second subset, all of which are in the second state, the bit pattern having the maximum luminance value among the bit patterns included in each second subset is set to an off (dark) bit. And a step of correcting a bit pattern other than the bit pattern having the maximum luminance value to be an on (bright) bit.

  Preferably, information recorded in the hologram memory is read out.

  According to the present invention, a two-dimensional digital pattern can be recorded with redundancy. Thereby, error bits can be reduced.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. In the following description, the same parts are denoted by the same reference numerals. Their names and functions are also the same. Therefore, detailed description thereof will not be repeated.

(First embodiment)
FIG. 1 is a diagram showing a recording mode of the hologram recording / reproducing apparatus according to the first embodiment of the present invention.

  With reference to FIG. 1, the configuration of the hologram recording / reproducing apparatus according to the present invention and the operation during recording will be described.

  In FIG. 1, a hologram recording / reproducing apparatus 100 includes a spatial light modulator 112 controlled by a control device 210, a beam splitter 102, relay lenses 103a and 103b, a mirror 104, a quarter-wave plate 105, The objective lens 106, the recording medium 107, the photodetector 110, and the actuator 111 are included. Note that the recording medium 107 includes a photosensitive material 108 and substrates 109a and 109b, and interference fringes (= holograms) are formed by refractive index modulation. In addition, a reflective film is provided on the substrate 109a which is the substrate opposite to the side on which light is incident.

  During recording by the hologram recording / reproducing apparatus 100, a coherent light beam emitted from a light source (not shown) is spatially modulated into a light beam 101 including object light and reference light by transmitting or reflecting the spatial light modulator 112. Is done.

FIG. 2 is a diagram schematically showing the structure of the spatial light modulator 112.
As shown in FIG. 2, the spatial light modulator 112 has an object light display area 201 for generating object light, and reference light display areas 202a, 202b, 202c, and 202d for generating reference light. Controlled by the control device 210.

  The control device 210 converts the information to be recorded into a two-dimensional digital pattern, and controls the object light generation processing unit 204 to give to the object light display area and the spatial light modulator to generate the reference light for recording. Recording reference light generation processing unit 206 and a reproduction reference light generation processing unit 208 for controlling the spatial light modulator to generate reproduction reference light. The coherent light beam is amplitude-modulated as necessary in each region. As the spatial light modulator 112, for example, a transmissive liquid crystal element or the like can be used. The object light generation processing unit 204 designates transmission or blocking for each pixel of the liquid crystal element in the object light display area 201. Thereby, 1-bit data is associated with each pixel.

  Returning to FIG. 1, the relay lenses 103 a and 103 b are a pair of lens groups, and form an image displayed on the spatial light modulator 112 as a real image again. The mirror 104 is an optical element that directs the traveling direction of the recording light and the reproducing light toward the objective lens 106. The quarter-wave plate 105 is a phase plate that changes the optical path difference of polarized light that vibrates in directions perpendicular to each other by a quarter wavelength. P-polarized light is changed to circularly polarized light by the quarter-wave plate 105, and when the circularly polarized light passes through the quarter-wave plate 105, it is changed to S-polarized light. The objective lens 106 converges the recording light and the reproduction light toward the recording medium 107, and the recording light and the reproduction light are applied to a predetermined position of the recording medium 107 by the objective lens 106. become.

  FIG. 3 is a diagram showing a form during reproduction of the hologram recording / reproducing apparatus according to the first embodiment of the present invention.

  With reference to FIG. 3, the operation at the time of reproduction of the hologram recording / reproducing apparatus according to the first embodiment of the present invention will be described.

  At the time of reproduction, the same light beam as the reference light used at the time of recording is irradiated on the hologram recorded on the recording medium 107 to generate reproduction light. The reproduction light is reflected to the objective lens 106 side by the reflective film of the substrate 109a, and propagates as a light beam having a polarization direction different from that at the time of recording by the quarter-wave plate 105. The reproduction light is reflected by the beam splitter 102 toward the photodetector 110 side. The photodetector 110 receives reproduction light and reproduces recorded information, and has a large number of light receiving elements arranged in a lattice pattern. A CCD array employing a CCD (Charge Coupled Device) as a light receiving element, a CMOS sensor employing a CMOS (Complementary Metal-Oxide Semiconductor), or the like can be used.

  In this embodiment, an optical system as shown in FIGS. 1 to 3 is used. However, the present invention is not limited to this, and an optical system as shown in the following figure may be used.

  FIG. 4 is a diagram showing an optical system different from those shown in FIGS. 1 to 3 of the hologram recording / reproducing apparatus according to the present invention.

  As shown in FIG. 4, in this optical system, the optical paths of the object light 401 and the reference light 402 are separated. The object light 401 is irradiated to the recording medium 400 as a spherical wave by the lens 403 and the reference light 402 is irradiated as a plane wave. Therefore, in the case of the optical system of FIG. 4, the object light 401 is condensed on the recording medium by the lens 403. An angle multiplexing type optical system that performs multiple recording by changing the incident angle θ of the reference beam 402 may be used.

  Next, with respect to information (hereinafter referred to as “page data”) given to a coherent light beam in the object light display region 201 of the spatial light modulator 112 provided in the hologram recording / reproducing apparatus as described above, first, an encoding method is described. After that, a recording method and an information reproducing method will be described. In the following, for simplicity, an 8-bit two-dimensional digital pattern using 4 × 4 pixels will be described as an example.

  In the information encoding method according to the present invention, in an 8-bit two-dimensional digital pattern expressed using 4 × 4 pixels, 2 bits out of 8 bits are used, and a reference of luminance value in 4 × 4 pixels is set. Indicates the region. Then, in the information reproduction process, the luminance is adjusted every 16 pixels based on the luminance value of the reference region. As a result, error bits caused by luminance unevenness in the reproduced image can be reduced.

  FIG. 5 is a diagram for explaining a method of expressing information indicated by 2 bits out of 8 bits.

  As shown in FIG. 5, in the information encoding method according to the present invention, a region where light is always blocked of 2 × 2 pixels among the 4 × 4 pixels of the spatial light modulator is determined. That is, 2 bits out of 8 bits are used to define a region where light is always blocked. In FIG. 5, the shaded area indicates a light blocking area, and the white area indicates a light transmitting area.

  For example, if the first 2 bits of the 8-bit bit string are “00”, the upper left 2 × 2 pixel of 4 × 4 pixels always blocks light as shown in FIG. 5A. And If the first 2 bits are “11”, as shown in FIG. 5B, the upper right 2 × 2 pixel of 4 × 4 pixels is a region where light is always blocked. Further, if the first 2 bits are “01”, the lower left 2 × 2 pixel of the 4 × 4 pixels is a region that always blocks light as shown in FIG. 5C. If the first 2 bits are “10”, the rule that the lower right 2 × 2 pixels of the 4 × 4 pixels always block light as shown in FIG. give.

  Thus, in the information encoding method according to the present invention, the position of the region where light is always blocked in 4 × 4 pixels represents 2 bits of 8-bit information. Here, each pixel does not necessarily have to be one pixel of the spatial light modulator, and may be a set of a plurality of pixels. In the above description, the first two bits are used as a criterion for determination.

  Next, a method of expressing information indicated by the remaining 6 bits out of 8 bits will be described.

  The remaining 6 bits in the information encoding method according to the present invention are expressed by dividing the area other than the shaded area shown in FIG. 5 into 3 areas by 2 bits. The remaining 6 bits are a pattern in which the region other than the shaded portion shown in FIG. 5 is advanced from the upper left to the right end, moved to the left end when reaching the right end, and displayed so as to go down to the right end again by 2 pixels. I will decide. Since the region where all four pixels block light is known, the display pattern is determined by skipping.

  FIG. 6 is a diagram for describing a method of expressing information indicated by the remaining 6 bits out of 8 bits.

  As shown in FIG. 6, the remaining 6 bits can be divided into three areas and displayed in three areas by using the area other than the shaded area shown in FIG. The remaining 6 bits are divided into 3 areas of 2 bits for display. The pattern consists of 2 × 2 pixels, only one of which transmits light, and the other 3 pixels block light. Use a pattern. There are four cases of which pixels transmit light, that is, the minimum pattern corresponds to 2 bits.

  For example, if the 2 bits are “00”, as shown in FIG. 6A, one pixel at the upper left of the 2 × 2 pixels is set as a region that always transmits light. If the 2 bits are “11”, as shown in FIG. 6B, the upper right pixel of the 2 × 2 pixels is a region that always transmits light. Further, if the 2 bits are “01”, as shown in FIG. 6C, one pixel at the lower right of the 2 × 2 pixels is a region that always transmits light. If the 2 bits are “10”, as shown in FIG. 6D, a rule is given so that the lower left one pixel of the 2 × 2 pixels is always a light transmitting region. In FIG. 6, the shaded area indicates a light blocking area, and the white area indicates a light transmitting area.

  As described above, 2 bits are represented by 2 × 2 pixels, so that the connection of bright bits (pixels that transmit light) in one direction is long and 4 in 4 × 4 pixels. As a result, the concentration of bright bits in the two-dimensional digital pattern can be avoided.

  In the above description, the shaded area is a light blocking area and the white area is a light transmitting area.

  Here, the information encoding method according to the present invention that complies with the above-described rules will be described using a specific example.

FIG. 7 is a diagram for describing encoding of 8-bit information.
FIG. 7A shows 8-bit information, and FIG. 7B shows a two-dimensional digital pattern obtained by encoding 8-bit information. The shaded area indicates a light blocking area, and the white area indicates a light transmitting area.

  As shown in FIG. 7A, the first 2 bits of the 8-bit bit string are “00”. For this reason, in FIG. 7B, the 2 × 2 pixels 710 are regions that block light as shown in FIG. The next 2 bits are “10”. For this reason, as shown in FIG. 6D, the 2 × 2 pixel 712 is a region through which one pixel at the lower left transmits light. Further, the next 2 bits “01” correspond to 2 × 2 pixels 714, and the next 2 bits “11” correspond to 2 × 2 pixels 716.

  In the present embodiment, for simplicity, the amount of information included in the page data is expressed as 8 bits using 4 × 4 pixels, but is not limited thereto. For example, 4 × 8 pixels are used as page data, and 8-bit information is generated for each 4 × 4 pixel as described above to express 16-bit information, or each 8 × 8 pixel is used. A method of generating 8-bit information for each 4 × 4 pixel as described above and expressing 32-bit information, or using 16n pixels, an 8-bit information for each 4 × 4 pixel as described above. Information may be recorded by increasing the amount of information included in the page data using a method of generating information and expressing 8n-bit information.

  Next, a description will be given of a procedure in which information to be recorded by the control device 210 is encoded into a two-dimensional digital pattern as described above, given to the object light display area 201, and recorded on the recording medium 107. For the sake of simplicity, a procedure for recording 8-bit information will be described here.

  FIG. 8 is a flowchart showing a procedure for recording on the recording medium 107 by the hologram recording / reproducing apparatus 100.

  A procedure for recording information by the hologram recording / reproducing apparatus 100 will be described with reference to FIG.

In step S800, the control device 210 reads information to be recorded.
Next, in step S802, the control device 210 determines a block corresponding to the first 2 bits among the 8 bits. Here, as shown in FIG. 5, the area | region which interrupts | blocks light is determined.

  Subsequently, in step S804, the control device 210 determines the remaining 6-bit digital pattern. Here, 6 bits are divided into 2 bits, and a digital pattern corresponding to each 2 bits is determined as shown in FIG. Then, a region other than the region determined in step S802 is advanced from the upper left to the right end, moved to the left end when reaching the right end, and a pattern to be displayed is defined so as to go down to the right end again by two pixels.

  In step S806, the control device 210 gives the generated two-dimensional digital pattern to the object light display region 201 to generate object light.

  Finally, in step S808, the lens 106 condenses the object light and the recording reference light on the recording medium 107. The object light and the recording reference light interfere to record interference fringes between the object light and the recording reference light.

  Note that the procedure from step S800 to step 804 corresponds to the process using the information encoding method according to the present invention.

  Next, a processing method for reproducing information by the photodetector 110 of the hologram recording / reproducing apparatus 100 according to the first embodiment of the present invention will be described. Here, for the sake of simplicity, an 8-bit two-dimensional digital pattern processing method in which 4 × 4 pixels are one block will be described.

  In the reproduction method according to the present invention, even if luminance unevenness occurs in the reproduced image, since it is encoded in advance as described above, the reproduced image is corrected based on the luminance value of the region serving as a reference for the reproduction process. It becomes possible. Thereby, error bits can be reduced.

  FIG. 9 is a flowchart showing a procedure of reproducing information recorded on the recording medium 107 by the hologram recording / reproducing apparatus 100.

  With reference to FIG. 9, the procedure in which the hologram recording / reproducing apparatus 100 reproduces information will be described.

  In step S900, the photodetector 110 divides the detected two-dimensional digital pattern into 2 × 2 pixel sub-blocks.

  Next, in step S902, the photodetector 110 calculates the luminance value of each sub-block divided in step S900. For example, the sum of the luminance values of four pixels constituting each sub block is set as the luminance value of the sub block.

  Subsequently, in step S904, the photodetector 110 detects a sub-block serving as a reference for reproduction processing. For example, the difference detection is performed for each sub block from the sum of the luminance values of each sub block obtained in step S902, and the sub block having the minimum luminance value is set as a reference sub block.

  In step S906, the photodetector 110 corrects the luminance value of each sub block by subtracting the luminance value of the reference sub block obtained in step S904 as an offset amount.

  Finally, in step S908, the photodetector 110 binarizes the luminance distribution in each sub-block by threshold processing, and then compares the information with the rules shown in FIG. 5 and FIG. Play.

Here, a method of reproducing information as described above will be described using a specific example.
FIG. 10 is a diagram showing an example of a reproduced two-dimensional digital pattern 800.

  FIG. 10A shows a two-dimensional digital pattern 800, and FIG. 10B shows 8-bit information obtained by decoding the two-dimensional digital pattern. In FIG. 10A, the shaded area is an area that blocks light, and the white area is an area that transmits light.

  The two-dimensional digital pattern 800 is detected by a photodetector such as a CCD array or CMOS as described above. In the two-dimensional digital pattern shown in FIG. 10A, black and white are separately applied. In an ideal reproduction image, reproduction light exists only in the white area, and reproduction is performed in the shaded area. There is no light.

  However, in an actual reproduced image, unevenness occurs in the white area, or noise is partially present in the shaded area due to the influence of stray light.

  In the information reproducing method of the hologram recording / reproducing apparatus according to the present invention, as shown in FIG. 10A, a two-dimensional digital pattern 800 that is a reproduced image of 4 × 4 pixels is converted into 2 × 2 pixel pixels 810, 812, and 814. , 816 and processing.

  Next, the luminance value of each 2 × 2 pixel 810, 812, 814, 816 is obtained. Then, a difference in luminance value between the 2 × 2 pixels 810, 812, 814, and 816 is detected, and an area having the lowest luminance value is obtained. Specifically, in each region of 2 × 2 pixels 810, 812, 814, and 816, the sum of the luminance values of the four pixels constituting each region is obtained. The sum of luminance values is obtained, and difference detection is performed for each region of 2 × 2 pixels 810, 812, 814, and 816. In FIG. 10A, the shaded area is an area that blocks light, and the white area is an area that transmits light, so that the area having the minimum luminance value is determined by difference detection.

  In FIG. 10A, the luminance values in the three regions of the 2 × 2 pixels 810, 812, and 816 where the reproduction light is present are the diagonal lines of the diffracted light of the noise 2 × 2 pixels 810, 812, and 816. Even if it exists in the area of the part, it is higher than the area of 2 × 2 pixels 814. From this, it is found that the area of 2 × 2 pixels 814 has the minimum luminance value. Therefore, according to the rules shown in FIG. 5, it is found that the first two bits are “10” in the reproduced two-dimensional digital pattern 800.

  Similarly, for example, when the 2 × 2 pixel 810 has the lowest luminance value, the first two bits are “00”. When the 2 × 2 pixel 812 has the lowest luminance value, the first 2 bits are “11”. Furthermore, when the 2 × 2 pixel 816 has the lowest luminance value, the first two bits are “01”.

  In this embodiment, the region serving as the reference for the information reproduction process is the region having the lowest luminance value, but the region serving as the reference for the information reproduction process uses the region having the highest luminance or the lowest luminance. It is possible to facilitate the detection of the region to be.

  Subsequently, the bit discrimination of the remaining 810, 812, and 816 having a luminance value higher than that of the 2 × 2 pixel 814 will be described.

  As described above, it has been found that 2 × 2 pixels 814 have the lowest luminance value in FIG. Therefore, the luminance value constituting the 2 × 2 pixel 814 is obtained, and the luminance value of the 2 × 2 pixel 814 is offset from the luminance value of the remaining 2 × 2 pixels 810, 812, and 816 in the two-dimensional digital pattern 800. Subtract as.

  Thereafter, the luminance distribution of the 2 × 2 pixels 810, 812, and 816 is binarized by threshold processing, and then collated with the rules shown in FIG. 6 to check the two-dimensional digital pattern 800 that is a 4 × 4 pixel reproduced image. This bit is judged. As a result, even when noise occurs in the 2 × 2 pixels 810, 812, and 816, bit determination can be performed.

  As described above, even if luminance unevenness occurs in the reproduced image, the reproduced image can be corrected every 16 pixels according to the luminance value of the region serving as a reference for the information reproducing process. The bit determination of 2 × 2 pixels 810, 812, and 816 proceeds from the upper left to the right end, and when reaching the right end, moves to the left end, moves down by two pixels and proceeds to the right end again, and the luminance value is found to be the lowest. The determined 2 × 2 pixels 814 are skipped and determined. If the above rules are followed, the bits shown in FIG.

  In this embodiment, for simplicity, the 4 × 4 pixel two-dimensional digital pattern is restored to 8-bit information. However, the present invention is not limited to this.

FIG. 11 is a diagram showing a 4 × 8 pixel reproduced image and divided blocks.
FIG. 12 is a diagram showing an 8 × 8 pixel reproduced image and divided blocks.

  With reference to FIGS. 11 and 12, restoration of a two-dimensional digital pattern other than 4 × 4 pixels will be described.

  For example, in the case of a reproduced image of a two-dimensional digital pattern 900 composed of 4 × 8 pixels as shown in FIG. 11A, every 4 × 4 pixels as shown in FIGS. 11B and 11C. The reproduced image is divided into two blocks 901 and 902, and each block 901 and 902 is divided into four sub-blocks 910, 912, 914, 916 and 920, 922, 924, 926 of 2 × 2 pixels. . Then, in blocks 901 and 902, the difference in luminance value between the four sub-blocks 910-916 and 920-926 is detected, and the luminance value of the sub-block serving as a reference for image processing in the blocks 901 and 902 is obtained. The luminance value of the reference sub-block is used as an offset amount and is subtracted from the luminance values of the remaining sub-blocks. Thereafter, in blocks 901 and 902, binarization of the block is executed by threshold processing, respectively, and bit determination is performed.

  Thus, in the case of a reproduced image of the two-dimensional digital pattern 900 composed of 4 × 8 pixels, the reproduced image is divided into two blocks 901 and 902 for each 4 × 4 pixel, and bit determination is performed in each block. Thus, 16-bit information can be restored.

  Similarly, in the case of an 8 × 8 pixel two-dimensional digital pattern 1000 as shown in FIG. 12 (A), as shown in FIGS. 12 (B) to 12 (E), four reproduced images are obtained every 4 × 4 pixels. The blocks are divided into blocks 1001 to 1004, and each block 1001 to 1004 is further divided into four sub-blocks of 2 × 2 pixels as described above. A series of processing is performed in each of the blocks 1001 to 1004 to restore 32-bit information. Alternatively, in the case of a reconstructed image of 16n pixels, the reconstructed image may be divided into n blocks every 16 pixels, and processing may be performed as described above to restore 8n-bit information. Therefore, even if the diameter of the light beam of the object light is increased and the capacity of the page data is increased, image processing that is not affected by noise or the like becomes possible.

  According to the hologram recording / reproducing apparatus according to the present embodiment, one set of sub-blocks composed of at least 4 pixels out of 16 pixels constituting a two-dimensional digital pattern is in a state of blocking light (or As a transparent state). For this reason, even if luminance unevenness occurs in the two-dimensional digital pattern, the luminance adjustment for each 16 pixels can be performed by using the luminance value of the region that always blocks light in each of the 16 pixels as a reference. it can. In addition, in the two-dimensional digital pattern, information is represented by the position of the sub-block where all four pixels block light and the state of the four pixels that make up the remaining sub-blocks. The error can be reduced.

  Further, according to the hologram recording / reproducing apparatus according to the present embodiment, the bright bits are continuous in two directions in the two-dimensional digital pattern, and two bright bits can be avoided. Therefore, the low frequency component of the bit pattern is suppressed, and the concentration of the light amount on the low frequency component in the Fourier plane is alleviated. As a result, image degradation due to medium saturation is suppressed, and a reproduced image having a high SN can be obtained. Further, the dynamic range of the recording medium can be effectively used, and as a result, a large number of holograms can be multiplex-recorded.

  Further, according to the hologram recording / reproducing apparatus according to the present embodiment, it is possible to provide redundancy to the two-dimensional digital pattern and increase the number of bright bits in the two-dimensional digital pattern. Therefore, the light use efficiency can be increased. Furthermore, since one of the four sets of sub-blocks is a region that always blocks light (or a region that transmits light), the luminance is the lowest (or higher). It becomes easy to detect the sub-block area.

  Further, according to the hologram recording / reproducing apparatus according to the present embodiment, the two-dimensional digital pattern reproduced from the hologram is divided into a plurality of blocks, and the block is further divided into a plurality of sub-blocks. As a result, even if luminance unevenness occurs in the reproduced image, it becomes possible to correct the reproduced image for each block by the luminance value of the sub-block.

(Second Embodiment)
Next, a second embodiment of the present invention will be described. The hologram recording / reproducing apparatus according to the present embodiment is different from the hologram recording / reproducing apparatus according to the first embodiment in the information reproducing method as described below.

  The configuration of the hologram recording / reproducing apparatus according to the second embodiment is the same as that of the first embodiment, and will not be repeated. Also, the page data encoding method and the method for recording information on the recording medium are the same as the method described in the first embodiment, and thus will not be repeated.

  A procedure for reproducing information by the hologram recording / reproducing apparatus 100 according to the second embodiment will be described with reference to FIG. FIG. 14 is a flowchart showing a procedure for reproducing information recorded on the recording medium 107 by the hologram recording / reproducing apparatus according to the second embodiment. In the following description, it is assumed that the information is encoded by a block composed of three bright bits and 13 dark bits.

  The processing performed from step S1400 to step S1406 is the same as the processing performed from step S900 to step S906 in FIG.

  In step S1408, the photodetector 110 ranks the luminance value of each pixel constituting the block. Here, since each sub-block is composed of 16 pixels, 16 luminance values are ranked in descending order.

  Subsequently, in step S1410, the photodetector 110 determines that the pixel having the luminance value up to the top three of the pixels ranked in luminance in step S1408 (the pixel having the highest luminance value, the luminance value is next). Higher pixels and pixels with the next highest luminance value) are extracted. Thereafter, a binarization process is performed in which the extracted three pixels are corrected to bright bits and the remaining 13 pixels are corrected to dark bits. Since the number of bright bits included in a 4 × 4 pixel is determined to be up to 3 by the above encoding method, appropriate binarization is performed for one block by the above processing. I can expect that.

  Finally, in step S1412, the photodetector 110 checks the rules shown in FIGS. 5 and 6 and performs bit determination to reproduce information.

  In the above, the reproduction of information encoded using a block consisting of three bright bits and 13 dark bits has been described. However, information is encoded using a block consisting of three dark bits and 13 bright bits. If it is, the photodetector 110 extracts pixels having luminance values up to the third lowest rank in step S1410.

(Third embodiment)
Next, a third embodiment of the present invention will be described. The hologram recording / reproducing apparatus according to the present embodiment is also different from the hologram recording / reproducing apparatus according to the first embodiment in the information reproducing method.

  The configuration of the hologram recording / reproducing apparatus according to the third embodiment is the same as that of the first embodiment, and will not be repeated. Also, the page data encoding method and the method for recording information on the recording medium are the same as the method described in the first embodiment, and thus will not be repeated.

  A procedure for reproducing information by the hologram recording / reproducing apparatus 100 according to the third embodiment will be described with reference to FIG. FIG. 15 is a flowchart showing a procedure for reproducing information recorded on the recording medium 107 by the hologram recording / reproducing apparatus according to the third embodiment. It is assumed that the information is encoded in a block composed of 3 bright bits and 13 dark bits.

  In step S1500, the photodetector 110 divides a block included in the detected two-dimensional digital pattern into 2 × 2 pixel sub-blocks.

  Next, in step S1502, the photodetector 110 calculates the luminance value of each sub-block obtained in step S1500. For example, the sum of the luminance values of four pixels constituting each sub block is set as the luminance value of the sub block.

  In step S1504, the photodetector 110 detects a sub-block having the lowest luminance value.

  In step S1506, the photodetector 110 corrects the luminance value of the pixel included in the sub-block having the lowest luminance value to 0.

  Subsequently, in step S1508, the photodetector 110 extracts the pixel having the maximum luminance value for each of the remaining three sub-blocks, and sets the luminance values of the pixels other than the pixel having the maximum luminance value to 0.

  Finally, in step S1510, the photodetector 110 compares the rules shown in FIGS. 5 and 6 and performs bit determination to reproduce information.

  FIG. 16 is a diagram illustrating evaluation results of errors that occur in the reproduction method according to the third embodiment and the reproduction method using the binarization process with a uniform threshold. This experimental data is obtained by reproducing a two-dimensional digital pattern obtained by encoding page data of 192 pixels × 192 pixels. In addition, two-dimensional digital patterns are recorded on the recording medium, and the evaluated two-dimensional digital pattern is one of them.

  The number of errors and the bit error rate (BER) when reproduced by the method of the present invention were 120 and 0.004, respectively. On the other hand, the number of errors and the bit error rate (BER) when the binarization process is performed using a uniform threshold for all the pixels in the block and the reproduction process is performed are 167 and 0, respectively. 0.006. Here, as the threshold value, the luminance value between the valleys of the distribution of the bright bits and the dark bits in the histogram of the reproduced image detected is used. From this experimental result, it can be seen that the number of errors can be reduced by the reproduction method of the present invention.

(Summary)
As described above, the hologram recording apparatus according to the present invention irradiates the recording medium with the object light representing information and the recording reference light, and the interference fringes generated by the object light and the recording reference light are applied to the recording medium. Information is recorded by writing, and a spatial light modulator for spatially modulating a light beam to perform hologram recording of information, and a first subset of 16 elements corresponding to information and a control means for controlling the spatial light modulator based on the two-dimensional digital pattern, and generating a two-dimensional digital pattern composed of n pieces, wherein the first subset is included in the first subset 4 sets of 2nd subsets each arranged at a predetermined position, each 2nd subset has 4 elements, and the control means includes 4 sets of 2nd subsets. 3 sets of 2nd subset And means for setting one element to a first state and three elements to a second state among four elements constituting the second subset, and four sets of second parts Means for bringing all four elements constituting the second subset into the second state in the remaining second subset of the set, the object beam and the recording reference beam. And a lens for focusing on the recording medium.

  The hologram reproducing apparatus according to the present invention reads information from a recording medium on which interference fringes between an object beam representing information and a recording reference beam are recorded, and irradiates the recording medium with the reproducing reference beam. Means for detecting the reproduction light bearing the two-dimensional digital pattern generated by this, and means for dividing the two-dimensional digital pattern included in the reproduction light into a plurality of first subsets, the first subset Includes four sets of second subsets respectively arranged at predetermined positions in the first subset, and the second subset has four elements, and the elements include the first subset Or the second state, and for each of the plurality of first subsets, the first subset is divided into a plurality of second subsets, and the plurality of second subsets The second part in which all four elements are in the second state It means for detecting a set, on the basis of the luminance values of the four elements second subset all in the second state, and means for performing brightness correction of the plurality of second subset.

  The embodiment disclosed this time should be considered as illustrative in all points and not restrictive. The scope of the present invention is defined by the terms of the claims, rather than the description above, and is intended to include any modifications within the scope and meaning equivalent to the terms of the claims.

It is a figure which shows the form at the time of the recording of the hologram recording / reproducing apparatus which concerns on the 1st Embodiment of this invention. 3 is a diagram schematically showing the structure of a spatial light modulator 112. FIG. It is a figure which shows the form at the time of reproduction | regeneration of the hologram recording / reproducing apparatus which concerns on the 1st Embodiment of this invention. It is a figure which shows the optical system different from FIGS. 1-3 of the hologram recording / reproducing apparatus based on this invention. It is a figure for demonstrating the expression method of the information shown by 2 bits among 8 bits. It is a figure for demonstrating the expression method of the information shown by remaining 6 bits among 8 bits. It is a figure for demonstrating encoding of 8-bit information. 3 is a flowchart showing a procedure for recording on the recording medium 107 by the hologram recording / reproducing apparatus 100 according to the first embodiment. 3 is a flowchart showing a procedure for reproducing information recorded on a recording medium 107 by the hologram recording / reproducing apparatus 100 according to the first embodiment. It is a figure which shows an example of the reproduced two-dimensional digital pattern 800. It is a figure which shows the reproduced image of 4x8 pixel, and the divided | segmented block. It is a figure which shows the reproduced image of 8x8 pixel, and the divided | segmented block. It is a figure which shows the example of the two-dimensional digital pattern in the case of n = 3. 6 is a flowchart showing a procedure for reproducing information recorded on a recording medium 107 by the hologram recording / reproducing apparatus 100 according to the second embodiment. 10 is a flowchart showing a procedure for reproducing information recorded on a recording medium 107 by a hologram recording / reproducing apparatus 100 according to a third embodiment. It is a figure which shows the evaluation result of the error which arises in each of the reproducing | regenerating method in 3rd Embodiment, and the reproducing | regenerating method using the binarization process by a uniform threshold value.

Explanation of symbols

  100 hologram recording / reproducing apparatus, 102 beam splitter, 103a, 103b relay lens, 104 mirror, 105 quarter wave plate, 106 objective lens, 107 recording medium, 108 photosensitive material, 109a, 109b substrate, 110 photodetector, DESCRIPTION OF SYMBOLS 111 Actuator, 112 Spatial light modulator, 120 processing apparatus, 121 division | segmentation part, 122 detection part, 123 correction | amendment part, 124 determination part, 125 ranking part, 201 object light display area, 202a, 202b, 202c, 202d Reference light display Area, 204 object light generation processing unit, 206 reference light generation processing unit for recording, 208 reference light generation processing unit for reproduction, 210 control device, 400 recording medium, 401 object light, 402 reference light, 403 lens.

Claims (25)

A hologram recording apparatus for recording information by optical interference fringes,
A spatial light modulator that generates a two-dimensional digital pattern corresponding to the information by n first subsets of 16 elements and its control means;
The first subset consists of four sets of second subsets of 2 × 2 elements,
For the three sets of the four second subsets, the control means sets one of the four elements constituting the second subset to the first state, and sets the three elements to the second state. Means for making the state of
A hologram recording apparatus comprising: means for bringing all four elements into the second state in the remaining second subset of the four sets of second subsets. The spatial light modulator includes a display device for displaying the two-dimensional digital pattern;
The display device
A reference light generation region for generating recording reference light from a light beam incident on the spatial light modulator;
An object light generation region for generating object light from a light beam incident on the spatial light modulator,
The hologram recording apparatus according to claim 1, wherein the control unit displays a two-dimensional digital pattern corresponding to the information on the object light generation region.   The hologram recording apparatus according to claim 1, wherein the first state is an on (bright) bit, and the second state is an off (dark) bit.   The hologram recording apparatus according to claim 1, wherein the first state is an off (dark) bit, and the second state is an on (bright) bit. A hologram reproducing apparatus for reading information from a recording medium on which optical interference fringes are recorded,
Means for detecting reproduction light from the recording medium, and means for dividing a two-dimensional digital pattern included in the reproduction light into a plurality of first subsets,
The first subset is composed of four sets of second subsets respectively arranged at predetermined positions in the first subset;
The second subset has 2 × 2 elements;
The element is in either the first state or the second state;
For each of the plurality of first subsets, the first subset is divided into a plurality of the second subsets, and all the four elements of the plurality of second subsets are the first subsets. A hologram reproducing apparatus, further comprising means for detecting a second subset in the state of 2.   6. The hologram reproduction according to claim 5, further comprising means for correcting the luminance of the plurality of second subsets based on the luminance value of the second subset in which all of the four elements are in the second state. apparatus. The information corresponds to a two-dimensional digital pattern composed of n first subsets of 16 elements,
Of the four sets of second subsets, in three sets of second subsets, one element out of the four elements constituting the second subset is in the first state, The three elements are in the second state,
The four elements constituting the second subset in the remaining second subset of the four sets of the second subset are all in the second state. Hologram reproducing device. The second subset in which the four elements are all in the second state has the lowest luminance value among the plurality of second subsets,
Means for ranking each element constituting the first subset based on a luminance value for each of the plurality of first subsets;
The means for performing luminance correction corrects an element having a luminance value of the top three of the 16 elements constituting the first subset to the first state, and sets the remaining elements to the second state. 6. The hologram reproducing apparatus according to claim 5, wherein correction is performed. The second subset in which the four elements are all in the second state has the highest luminance value among the plurality of second subsets,
Means for ranking each element constituting the first subset based on a luminance value for each of the plurality of first subsets;
The means for performing luminance correction corrects elements having luminance values up to the third lowest rank among the 16 elements constituting the first subset to the first state, and sets the remaining elements to the second state. 6. The hologram reproducing apparatus according to claim 5, wherein correction is performed. The second subset in which the four elements are all in the second state has the lowest luminance value among the plurality of second subsets,
The means for performing luminance correction corrects all the elements included in the second subset in which the four elements are in the second state to be the off (dark) bits,
For each of the second subsets excluding the second subset in which all of the four elements of the second subset included in the first subset are in the second state, the second subset 6. The element having the maximum luminance value among the elements included in the subset is corrected to the on (bright) bit, and the elements other than the element having the maximum luminance value are corrected to the off (dark) bit. The hologram reproducing apparatus as described. The second subset in which the four elements are all in the second state has the highest luminance value among the plurality of second subsets,
The means for performing luminance correction corrects all the elements included in the second subset in which the four elements are all in the second state to be the on (bright) bits,
For each of the second subsets excluding the second subset in which all of the four elements of the second subset included in the first subset are in the second state, the second subset 6. The element having the maximum luminance value among the elements included in the subset is corrected to the off (dark) bit, and the elements other than the element having the maximum luminance value are corrected to the on (bright) bit. The hologram reproducing apparatus as described. An information encoding method for displaying information as an 8n-bit two-dimensional digital pattern every 16n-bit pattern,
The two-dimensional digital pattern is composed of a first subset n of 16-bit patterns,
The first subset is composed of four sets of second subsets respectively arranged at predetermined positions in the first subset;
Each said second subset has 2 × 2 bit patterns;
Of the four sets of second subsets, one of the four bit patterns constituting the second subset in the second set of three sets has one bit pattern as the first state. Placing three bit patterns in a second state;
A step of setting all four bit patterns constituting the second subset to the second state in the remaining second subset of the four sets of the second subsets. Information encoding method.   13. The information encoding method according to claim 12, wherein the step of setting all the four bit patterns to the second state determines 2n bits among the 8n bits.   The step of setting one bit pattern to the first state and the three bit patterns to the second state among the four bit patterns determines 6n bits of the 8n bits. The information encoding method described.   The information encoding method according to claim 12, wherein the information is displayed as the two-dimensional digital pattern for hologram recording. A recording method for writing a digital pattern on a recording medium using an information encoding method for displaying information as an 8n-bit two-dimensional digital pattern every 16n-bit pattern,
The two-dimensional digital pattern is composed of a first subset n of 16-bit patterns,
The first subset is composed of four sets of second subsets respectively arranged at predetermined positions in the first subset;
Each said second subset has 2 × 2 bit patterns;
Of the four sets of second subsets, one of the four bit patterns constituting the second subset in the second set of three sets has one bit pattern as the first state. Placing three bit patterns in a second state;
A step of setting all four bit patterns constituting the second subset to the second state in the remaining second subset of the four sets of the second subset;
And a step of writing the four sets of second subsets on the recording medium.   The recording method according to claim 16, wherein the recording medium is a hologram memory. An information reproduction method for reading a two-dimensional digital pattern recorded by using an information encoding method for displaying information as an 8n-bit two-dimensional digital pattern every 16n-bit pattern,
Detecting the two-dimensional digital pattern;
Dividing the two-dimensional digital pattern into a plurality of first subsets,
The first subset is composed of four sets of second subsets respectively arranged at predetermined positions in the first subset;
The second subset is
It has 2x2 bit patterns,
The element is in either the first state or the second state;
For each of the plurality of first subsets, the first subset is divided into a plurality of second subsets, and all of the four bit patterns from the plurality of second subsets are An information reproducing method, further comprising detecting a second subset that is in a second state.   19. The information according to claim 18, further comprising the step of performing luminance correction of the plurality of second subsets based on luminance values of the second subset in which all the four bit patterns are in the second state. Playback method. The information corresponds to a two-dimensional digital pattern composed of n first subsets of 16-bit patterns;
Of the four sets of second subsets, in the second set of three sets, one bit pattern of the four bit patterns constituting the second subset is in the first state. Yes, the three digital patterns are in the second state,
19. In the remaining second subset of the four sets of the second subsets, all four bit patterns constituting the second subset are in the second state. The information reproduction method described. The second subset in which the four bit patterns are all in the second state has the lowest luminance value among the plurality of second subsets,
For each of the plurality of first subsets, ranking each bit pattern constituting the first subset based on a luminance value;
Of the 16-bit patterns constituting the plurality of first subsets, three bit patterns are corrected to the first state from the highest luminance according to the ranking result based on the luminance value, and the remaining bits The information reproducing method according to claim 18, further comprising a step of correcting to a second state. The second subset in which the four bit patterns are all in the second state has the highest luminance value among the plurality of second subsets,
For each of the plurality of first subsets, ranking each bit pattern constituting the first subset based on a luminance value;
Of the 16-bit patterns constituting the plurality of first subsets, three bit patterns are corrected to the first state from the lowest luminance according to the ranking result based on the luminance value, and the remaining bits The information reproducing method according to claim 18, further comprising a step of correcting the pattern to the second state. The second subset in which the four bit patterns are all in the second state has the lowest luminance value among the plurality of second subsets,
Correcting all the bit patterns included in the second subset in which the four bit patterns are all in the second state to be the off (dark) bits;
For each of the second subsets excluding the second subset in which the four bit patterns are all in the second state among the second subsets included in the first subset, the second subset The bit pattern having the maximum luminance value among the bit patterns included in the subset is corrected to the on (bright) bit, and the bit pattern other than the bit pattern having the maximum luminance value is corrected to the off (dark) bit. The information reproducing method according to claim 18, further comprising the step of: The second subset in which the four bit patterns are all in the second state has the highest luminance value among the plurality of second subsets,
Correcting all the bit patterns included in the second subset in which the four bit patterns are all in the second state to be the on (bright) bits;
For each of the second subsets excluding the second subset in which the four bit patterns are all in the second state among the second subsets included in the first subset, the second subset Among the bit patterns included in the subset, the bit pattern having the smallest luminance value is corrected to the off (dark) bit, and the bit pattern other than the bit pattern having the smallest luminance value is corrected to the on (bright) bit. The information reproducing method according to claim 18, further comprising the step of:   The information reproducing method according to claim 18, wherein the information recorded in the hologram memory is read out.

Images