JP2012181615A - Information holding medium and display device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an information holding medium for holding image information capable of being read out with light having a specific wavelength, and provide a display device for reading out and displaying the image information held in the information holding medium.SOLUTION: In a sheet-like information holding medium for holding image information represented by plural gradations, plural pixel units are arranged in an in-plane direction, and each of the plural pixel units has a birefringence phase difference Δ for transmitting light with a predetermined wavelength at a transmittance corresponding to the image information.

Description

  The present invention relates to an information holding medium and a display device.

  In recent years, information security technology using optical technology has attracted attention as a technology that can achieve both convenience and safety. In particular, the visual decryption encryption technique is highly secure because secret information is distributed among a plurality of physical keys, and is excellent in convenience because no computer operation is required for decryption. As a visual decoding type encryption technique, “an encryption technique for expressing pixels with a spatial code pattern” and “a technique for expressing an image by superimposing multilayered polarization phase shifters” have been proposed.

  The “encryption technique for expressing pixels with a spatial code pattern” is based on a concept called optical array logic, which is composed of a pair of random dot-like transmissive pixels or light-shielded pixels. When a decryption mask is placed in front of the encrypted display image and the encrypted image is viewed through the decryption mask, the viewpoint position corresponding to the one-to-one relationship between the display pixel and the decryption mask is limited in three dimensions. (See Non-Patent Documents 1 and 2).

  "Method of expressing an image by superimposing multiple polarization polarizers" is a method of laminating two quarter-wave plates with their birefringent main axis orientations rotated appropriately and laminated in a mosaic pattern. Thus, the direction of the polarization axis of incident polarized light is controlled to create an image. Although the confidential information cannot be displayed with only one encryption film, the confidential information can be displayed by overlaying another encryption film (see Non-Patent Document 3).

Tanida and Ichioka, "Optical logic array processor using shadowgrams", J.Opt.Soc.Am.A 73,800-809 (1983) Yatagai, "Optical space-variand logic-gate array based on spatial encoding technique", Opt. Lett. 11, 260-262 (1986) Imagawa, Suyama and Yamamoto, "Visual cryptography using polarization-modulation films", Jpn.J.Appl.Phys. 48, No.9, 09LC02-09LC02-5 (2009)

  However, the “encryption technique for expressing pixels with a spatial code pattern” requires a precise alignment at the time of observation and has a problem of lack of simplicity. In addition, the “method of expressing an image by superimposing multilayer polarization phase shifters” has a problem that secret information is easily displayed after a plurality of physical keys are prepared.

  The present invention has been made in view of the above-described problems of the prior art, and an object of the present invention is to hold an information holding medium that holds image information that can be read by light of a specific wavelength, and the information holding medium. And a display device for reading out and displaying the image information.

  In order to achieve the above object, the invention described in each claim has the following configuration.

  The invention according to claim 1 is a sheet-like information holding medium that holds image information expressed in a plurality of gradations, wherein a plurality of pixel portions are arranged in an in-plane direction, and the plurality of pixel portions Each of these is an information holding medium having a birefringence phase difference Δ that transmits light of a predetermined wavelength at a transmittance according to the image information.

The invention according to claim 2 is the information holding medium according to claim 1, wherein the birefringence phase difference Δ is defined by the following formula (1).

Δ = 2πN + φ Equation (1)
In the above formula (1), N represents the birefringence order (N is an integer). φ represents a fraction of 2π or less.

  A third aspect of the present invention is the information holding medium according to the second aspect, wherein the value of the birefringence order N is randomly changed in each of the plurality of pixel portions.

  The invention according to claim 4 is the information holding medium according to claim 2 or 3, wherein each of the plurality of pixel portions has the birefringence order N of the fourth order or higher.

According to a fifth aspect of the present invention, the information holding medium holds image information expressed as a bright and dark image with two gradations, a bright pixel portion that transmits light of a predetermined wavelength, and a predetermined wavelength And a dark pixel portion that attenuates or blocks light, and one of the bright pixel portion and the dark pixel portion has a birefringence phase difference Δ represented by the following formula (2), and the bright pixel portion and the dark pixel portion: 5. The information holding medium according to claim 1, wherein the other has a birefringence phase difference Δ represented by the following formula (3).
Δ = 2πN + π (φ = π) Equation (2)
Δ = 2πN + 0 (φ = 0) Equation (3)

  According to a sixth aspect of the present invention, the information holding medium is composed of a plurality of sheet-like members, and when the plurality of sheet-like members are superimposed, The information holding medium according to any one of claims 1 to 5, wherein the birefringence phase difference Δ is set so that light having a predetermined wavelength is transmitted with a transmittance according to the image information.

  In the seventh aspect of the invention, the information holding medium holds a plurality of pieces of image information, and different wavelengths are determined in advance corresponding to each of the plurality of pieces of image information. Each of the plurality of pieces of image information has the birefringence phase difference Δ so that light having a predetermined wavelength is transmitted with a transmittance according to the corresponding image information. The information holding medium according to Item 1.

  According to an eighth aspect of the present invention, there is provided a display device for reading out and displaying image information held in the information holding medium according to the first to seventh aspects, wherein the light having a predetermined wavelength is displayed on the information holding medium. And a pair of polarizers arranged to be separated from each other so as to sandwich the information holding medium therebetween.

  The invention according to claim 9 further includes a wavelength filter that is disposed between an observation point set outside the display device and the light source, and selectively transmits light having a predetermined wavelength. Item 9. The display device according to Item 8.

  According to the information holding medium of the present invention, it is possible to hold image information that can be read with light of a specific wavelength. Further, according to the display device of the present invention, the image information held on the information holding medium can be read and displayed.

It is a perspective view which shows an example of a structure of the information holding medium which concerns on embodiment of this invention. It is a perspective view which shows the structure of the apparatus which measures the transmitted light intensity of a birefringent body. It is sectional drawing which shows an example of a structure of a high-order birefringent body. It is a graph which shows an example of the spectral transmittance of a high-order birefringent body. (A) is a figure which defines birefringence phase difference (DELTA) of a pixel part, (B) is a figure which shows birefringence phase difference (DELTA) of each pixel part of an information holding medium. (A) is a graph showing the spectral transmittance of the pixel portion with phase φ = π, and (B) is a graph showing the spectral transmittance of the pixel portion with phase φ = 0. It is a disassembled perspective view which shows an example of a structure of the display apparatus which concerns on embodiment of this invention. It is a figure which shows the read image in each wavelength when irradiating the light of a some wavelength to an information holding medium. It is a disassembled perspective view which shows another example of a structure of the display apparatus which concerns on embodiment of this invention. (A) And (B) is a perspective view which shows an example of a structure of the information holding medium (information dispersion | distribution type) which concerns on the 2nd Embodiment of this invention. (A) is a figure which shows birefringence phase difference (DELTA) of each pixel part of a 1st member, (B) is a figure which shows birefringence phase difference (DELTA) of each pixel part of a 2nd member, (C ) Is a diagram showing a birefringence phase difference Δ of each pixel portion of the information holding medium constituted by the first member and the second member. (A) is a figure which shows the read image from the 1st member, (B) is a figure which shows the read image from the information holding medium comprised by the 1st member and the 2nd member. It is a graph which shows the design principle of the information holding medium (wavelength multiplexing type) based on the 3rd Embodiment of this invention. It is a figure which shows birefringence phase difference (DELTA) of each pixel part of the information holding medium which concerns on the 3rd Embodiment of this invention. (A) It is a figure which shows the read image in 1st key wavelength (lambda) 1, (B) is a figure which shows the read image in 2nd key wavelength (lambda) 2. (A) is a graph which shows an example of the spectral transmittance of a high-order birefringent body. (B) is a figure which shows the birefringence wavelength dispersion of the high order birefringent body obtained from the spectral transmittance shown to (A). It is a figure which represents the distribution of the transmitted light intensity in a predetermined wavelength range with a light-dark image with respect to the conversion order when the birefringence phase difference Δ is converted into the order. It is a graph which shows the spectral transmission factor of four types of high-order birefringent bodies which comprise each pixel part of the information holding medium (wavelength multiplexing type) which concerns on a design example. It is a graph which codes and shows the transmitted light intensity | strength of four types of high-order birefringent bodies in wavelength 560nm and wavelength 600nm. It is a figure which shows birefringence phase difference (DELTA) of each pixel part of the information holding medium (wavelength multiplexing type) which concerns on a design example.

  Hereinafter, an example of an embodiment of the present invention will be described in detail with reference to the drawings.

<First Embodiment>
(Structure of information holding medium)
First, the information holding medium according to the first embodiment of the present invention will be described. FIG. 1 is a perspective view showing an example of the configuration of the information holding medium according to the embodiment of the present invention. As shown in FIG. 1, the information holding medium 10 is a sheet-like information holding medium in which a plurality of pixel units 12 are arranged in the in-plane direction. In the illustrated example, nine pixel portions 12 having a rectangular shape in plan view are arranged in a matrix of 3 rows and 3 columns. A pixel portion arranged in m rows and n columns is displayed as “pixel portion 12 _mn ”. In addition, when it is not necessary to distinguish these, they are collectively referred to as “pixel portion 12”.

  The information holding medium 10 holds image information expressed by a plurality of gradations. For example, binary image information expressed in two gradations or multi-value image information expressed in multiple gradations may be used. Hereinafter, in order to simplify the description, a case where binary image information configured by a bright pixel portion and a dark pixel portion is held will be described.

Each of the plurality of pixel units 12 has a birefringence phase difference Δ that transmits light of a predetermined wavelength (hereinafter referred to as “key wavelength”) with a transmittance according to image information. The birefringence phase difference Δ of the pixel portion 12 _mn is displayed as “birefringence phase difference Δ _mn ”. As will be described later, the image information held in the information holding medium 10 is read by transmitting light of the key wavelength through each pixel unit 12 with a predetermined transmittance. In addition, the information held by the birefringence phase difference Δ is not destroyed by a magnetic field or an electric field.

  For example, the birefringence phase difference Δ is set in each of the plurality of pixel portions 12 so as to maximize the transmittance of the light having the key wavelength in the bright pixel portion and to minimize the transmittance of the light having the key wavelength in the dark pixel portion. Is granted. Although the read principle will be described later, image information that can be read with light of a key wavelength is given to the information holding medium 10 by giving a birefringence phase difference Δ to each of the plurality of pixel units 12 according to the image information. Can be held.

(Birefringence phase difference Δ)
Next, the birefringence phase difference Δ will be described. The birefringence phase difference Δ is a physical quantity representing the magnitude of birefringence. In the present embodiment, the birefringence phase difference Δ is defined by the following formula (1).
Δ = 2πN + φ Equation (1)
In the above formula (1), N represents the birefringence order (N is an integer). φ represents a fraction of 2π or less. Hereinafter, the fraction φ is referred to as “phase φ”.

  The relationship between the birefringence phase difference Δ, the birefringence order N, and the phase φ can be described in relation to the transmitted light intensity I of the birefringent body. FIG. 2 is a perspective view showing the configuration of an apparatus for measuring the transmitted light intensity of the birefringent body. As shown in FIG. 2, the measurement apparatus 20 includes a light source 22, a pair of polarizers 24 and 26, and a photodetector 28. On the light irradiation side of the light source 22, a polarizer 24 having a transmission axis 24A, a polarizer 26 having a transmission axis 26A, and a photodetector 28 are arranged in this order from the light source 22 side. The pair of polarizers 24 and 26 is a parallel Nicol optical system arranged so that the transmission axis 24A and the transmission axis 26A are parallel to each other.

  The birefringent body 30 is disposed so as to be sandwiched between the pair of polarizers 24 and 26. The birefringent body 30 is disposed with the main axis direction 30A inclined by 45 ° with respect to the polarizer direction (transmission axis 24A and transmission axis 26A). The birefringent body 30 sandwiched between the pair of polarizers 24 and 26 is irradiated with light from the light source 22. Light that has passed through the polarizer 24, the birefringent body 30, and the polarizer 26 is detected by a photodetector 28. The transmitted light intensity I changes depending on the birefringence phase difference Δ of the birefringent body 30, and the transmitted light intensity I of the birefringent body 30 is measured.

  The transmitted light intensity I of the birefringent body is expressed by the following formula (4) using the birefringent phase difference Δ of the birefringent body. Further, the following formula (4) can be rewritten to the following formula (5) or the following formula (6).

  In the above formulas (5) and (6), λ represents a wavelength, and k represents a wave number. The wave number k and the wavelength λ satisfy the relationship k = 1 / λ. δn represents the amount of birefringence per unit thickness of the birefringent body, and d represents the thickness of the birefringent body. [Δn · d] is the birefringence amount Δn of the birefringent body.

  As can be seen from the above formulas (5) and (6), the transmitted light intensity I varies depending on the wavelength λ or the wave number k of the irradiated light. As can be seen from the above equation (6), the transmitted light intensity I changes in a cosine wave shape with respect to the wave number k, and its frequency is equal to the birefringence amount [δn · d] of the birefringent body. Therefore, the birefringence phase difference Δ can be expressed by N times 2π. For example, when N is limited to an integer, the relationship (Δ = 2πN + φ) of the above formula (1) is satisfied. The transmitted light intensity I is expressed by the following formula (7) using the birefringence order N and the phase φ.

  For example, assuming that the wavelength corresponding to 2π is 560 nm, the birefringence phase difference Δ of 1400 nm has a birefringence order N of “2” and a phase φ of “π (= 280 nm)”. Further, as can be seen from the above equation (7), the transmitted light intensity I with respect to light of a specific wavelength λ does not depend on the birefringence order N and increases or decreases according to the value of the phase φ. The birefringence phase difference Δ can also be expressed as a “converted order” by converting the phase φ into an order. For example, when the birefringence order N = 2 and the phase φ = π, the converted order is “2.5”.

  In the measurement apparatus 20 described above, the pair of polarizers 24 and 26 are arranged to constitute a “parallel Nicol optical system”. In the parallel Nicol optical system, the sign of “cosΔ” in the above formula (4) is “positive”. Accordingly, the transmitted light intensity I is maximum when φ = 0 and is minimum when φ = π. That is, the key wavelength is 560 nm. When φ = 0, the light transmittance of the key wavelength is maximized, and when φ = π, the light transmittance of the key wavelength is minimized.

  On the other hand, when the pair of polarizers 24 and 26 are arranged so as to constitute an “orthogonal Nicol optical system”, the sign of “cosΔ” in the above formula (4) changes to “negative”. Therefore, the transmitted light intensity I is maximum when φ = π, and is minimum when φ = 0. That is, when φ = π, the light transmittance of the key wavelength is maximized, and when φ = 0, the light transmittance of the key wavelength is minimized.

(Birefringent)
Next, a birefringent body will be described. Each of the plurality of pixel portions 12 is composed of a birefringent material. The birefringent body may be any anisotropic material having birefringence, and a polymer material such as a polymer alignment film, a liquid crystal material such as a liquid crystal polymer, a crystal material such as an optical crystal, and the like can be used. Further, different materials may be used in combination. For example, a retardation film can be used. The retardation film is an optical film that has been subjected to stretching processing such as biaxial stretching, and imparts a predetermined retardation between polarized components of a specific wavelength that are orthogonal to each other.

  In addition, each of the plurality of pixel units 12 is formed of a birefringent material that is transparent to light in a predetermined wavelength band. Therefore, the information holding medium 10 is also transparent as a whole with respect to light of a predetermined wavelength band. Here, “transparent” means transmitting light in a predetermined wavelength band. For example, a birefringent body that transmits light in the visible region (visible light) looks visually transparent. On the other hand, a birefringent body that transmits light in the infrared region (infrared light) appears visually black. In addition, a birefringent body that transmits light in a band of 600 nm to 900 nm looks visually red.

  The image information held in the information holding medium 10 is read out when the key wavelength light is transmitted. Therefore, the transmission wavelength band of the birefringent body is determined so that the key wavelength is included. In the case where image information is read using light having a key wavelength as infrared light, each pixel unit may be formed of a birefringent material that is transparent to infrared light. When the confidential information is retained in the information retaining medium 10, the confidentiality of the retained information is improved by setting the wavelength outside the visible region such as the infrared region or the ultraviolet region as the key wavelength. In the following, a case where each pixel unit is formed of a birefringent material that is transparent to visible light will be described.

  Further, as the birefringent body constituting the plurality of pixel portions 12, it is preferable to use a “high-order birefringent body” having a birefringence order N of 4th order or higher. Although details will be described later, when the plurality of pixel portions 12 are formed of a high-order birefringent body, an image of only the bright pixel portion is obtained when white light is irradiated. That is, the image information held in the information holding medium 10 can be read with the light of the key wavelength and cannot be read with the white light. When confidential information is retained in the information retaining medium 10, the confidentiality of the retained information is improved.

  Below, the example which comprises a high-order birefringent body using retardation film is demonstrated. FIG. 3 is a cross-sectional view showing an example of the configuration of the high-order birefringent body. The birefringent body 30 includes a full wave plate 32 that generates a phase difference of 2π and a quarter wave plate 34 that generates a phase difference of π / 2. A commercially available retardation film can be used as the full wave plate 32 and the quarter wave plate 34. By stacking N full-wave plates 32 on the two quarter-wave plates 34, a high-order birefringent body having a birefringence order N and a phase φ = π can be obtained.

  In the example shown in FIG. 3, a retardation film having a birefringence amount Δn of 560 nm is used as the full wavelength plate 32, and a retardation film having a birefringence amount Δn of 140 nm is used as the quarter wavelength plate 34. Ten full-wave plates 32 are laminated on two quarter-wave plates 34 to obtain a high-order birefringent body with N = 10 and φ = π. FIG. 4 is a graph showing an example of the spectral transmittance of the high-order birefringent body shown in FIG. As shown in FIG. 4, the birefringent body 30 transmits light having a key wavelength of 560 nm with a minimum transmittance.

  Further, the laminate of N full-wave plates 32 and the laminate obtained by laminating N full-wave plates 32 on two quarter-wave plates 34 are both transparent to visible light and visually. Cannot be distinguished. That is, in the example in which a high-order birefringent body is configured using a retardation film, information on the phase φ provided by two quarter-wave plates 34 is obtained by stacking N full-wave plates 32. Can be obscured.

  In the example shown in FIG. 3, the example in which the birefringent body 30 is configured by the full wave plate 32 and the quarter wave plate 34 has been described. However, a desired birefringence order N and phase φ can be obtained. What is necessary is just to design, and the characteristic and combination of retardation film may be changed suitably. For example, in order to generate a phase difference of π, one half wavelength plate may be used instead of the two quarter wavelength plates 34.

(Retention of image information)
Next, a method for holding image information will be described. FIG. 5A is a diagram defining the birefringence phase difference Δ of the pixel portion, and FIG. 5B is a diagram showing the birefringence phase difference Δ of each pixel portion of the information holding medium. As shown in FIGS. 5A and 5B, each of the plurality of pixel units 12 is provided with a birefringence phase difference Δ represented by a birefringence order N and a phase φ, thereby the information holding medium 10. Holds image information. The image information is binary image information, and each of the plurality of pixel portions 12 is a “bright pixel portion” or a “dark pixel portion”. Hereinafter, a case where the “key wavelength” is 560 nm will be described.

  As shown in FIG. 5B, each of the plurality of pixel portions 12 is given a phase φ according to image information. Here, assuming that a pair of polarizers of the display device constitutes an “orthogonal Nicol optical system”, φ = π in the bright pixel portion and φ = 0 in the dark pixel portion. As described above, the transmitted light intensity I with respect to the light of the key wavelength does not depend on the birefringence order N, and is maximum when φ = π and minimum when φ = 0. That is, by providing the phase φ (that is, the birefringence phase difference Δ) according to the image information, the image information held in the information holding medium 10 can be read with high contrast by the light of the key wavelength. .

  6A is a graph showing the spectral transmittance of the pixel portion with φ = π, and FIG. 6B is a graph showing the spectral transmittance of the pixel portion with φ = 0. As shown in FIG. 6 (A), even if the birefringence phase difference Δ (converted order) changes in the range of “3.5 to 8.5”, the transmitted light intensity I with respect to light having a wavelength of 560 nm is φ Maximum when π. As shown in FIG. 6B, even if the birefringence phase difference Δ (converted order) changes in the range of “2.0 to 7.0”, the transmitted light intensity I with respect to light having a wavelength of 560 nm is , The minimum value when φ = 0.

  Further, as shown in FIG. 5B, each of the plurality of pixel portions 12 is given a birefringence order N so that the value of the birefringence order N changes randomly. When the birefringence order N is changed randomly, the birefringence phase difference Δ changes. Therefore, even if the phase φ is fixed to “π or 0” with respect to the light of the key wavelength, the birefringence phase difference Δ differs when the birefringence order N is different, and the phase φ is different from the light of the wavelength other than the key wavelength Does not become “π or 0”. That is, by randomly changing the birefringence order N, it becomes impossible to read the image information held in the information holding medium 10 with light having a wavelength other than the key wavelength.

  In the example shown in FIG. 5B, the distribution range of the birefringence order N is “6 to 12”. If the distribution range of the birefringence order N is narrow, that is, if the randomness of the birefringence order N is low, an “inverted image” in which light and dark are inverted at wavelengths other than the key wavelength may be displayed. For this reason, it is preferable that the distribution range of the birefringence order N is wider, that is, the randomness of the birefringence order N is higher.

  As shown in FIG. 5B, the birefringence order N is an integer of 4 or more for each of the plurality of pixel portions 12. As shown in FIG. 4, the spectral transmittance curve of the high-order birefringent body has a plurality of maximum points and a plurality of minimum points in the visible region. This means that light of wavelengths other than the key wavelength is also transmitted. Accordingly, when white light is irradiated, an image of only the bright pixel portion is obtained. When the brightness of these bright pixel portions becomes substantially uniform, the plurality of pixel portions 12 cannot be distinguished. That is, by configuring each of the plurality of pixel units 12 with a high-order birefringent material, the image information held on the information holding medium 10 cannot be identified with white light.

  As described above, image information that can be read with light having a key wavelength can be obtained by applying a phase φ (that is, a birefringence phase difference Δ) to each of the plurality of pixel units 12 according to the image information. It can be held on the holding medium 10. Further, by randomly changing the birefringence order N, image information held in the information holding medium 10 cannot be read with light having a wavelength other than the key wavelength. Further, by configuring each of the plurality of pixel units 12 with a high-order birefringent material, it is impossible to read image information held in the information holding medium 10 with white light.

  In other words, by randomly changing the birefringence order N of the plurality of pixel units 12, image information that can be read only with light having a key wavelength can be held in the information holding medium 10. In the case where confidential information is held in the information holding medium 10, the confidentiality of the held information is improved if the image information held by light having a wavelength other than the key wavelength cannot be read. Furthermore, by configuring each of the plurality of pixel units 12 with a high-order birefringent material, image information that cannot be identified with white light can be held in the information holding medium 10, and the confidentiality of the held information can be maintained. Is further improved.

(Reading image information)
Next, reading of image information will be described. FIG. 7 is an exploded perspective view showing an example of the configuration of the display device according to the embodiment of the present invention. As shown in FIG. 7, the display device 40 includes a monochromatic light source 42 that emits light having a key wavelength and a pair of polarizers 44 and 46. On the light irradiation side of the monochromatic light source 42, a polarizer 44 with a transmission axis 44A and a polarizer 46 with a transmission axis 46A are arranged in this order from the light source 42 side. The pair of polarizers 44 and 46 is an “orthogonal Nicol optical system” in which the transmission axis 44A and the transmission axis 46A are arranged to be orthogonal to each other.

  When reading out image information, the information holding medium 10 is disposed so as to be sandwiched between a pair of polarizers 44 and 46. The information holding medium 10 sandwiched between the pair of polarizers 44 and 46 is irradiated with light having a key wavelength from the monochromatic light source 42. The light having the key wavelength transmitted through the polarizer 44, the information holding medium 10, and the polarizer 46 is observed on the light emission side (observation side) of the polarizer 46. Here, a state in which transmitted light is projected onto the virtual projection surface 48 is illustrated.

  As described above, each of the plurality of pixel units 12 of the information holding medium 10 has φ = π in the bright pixel unit and darkness on the assumption that the pair of polarizers of the display device 40 constitutes an “orthogonal Nicol optical system”. By setting φ = 0 in the pixel portion, binary image information is held. The key wavelength light emitted from the monochromatic light source 42 passes through the bright pixel portion (φ = π) with the maximum transmittance, and the bright pixel 48H is projected onto the projection surface 48. Further, the light of the key wavelength emitted from the monochromatic light source 42 is transmitted through the dark pixel portion (φ = 0) of the information holding medium 10 with the minimum transmittance, that is, attenuated or blocked, and dark pixels on the projection surface 48. 48L is projected.

  In this example, each of the plurality of pixel portions 12 has a birefringence phase difference Δ so that a letter “T” is formed by the bright pixel portion (φ = π) and the dark pixel portion (φ = 0). Has been granted. When the information holding medium 10 is irradiated with light having a key wavelength, a letter “T” is formed on the projection surface 48 by the bright pixels 48H and the dark pixels 48L. That is, binary image information (information representing the character “T”) held in the information holding medium 10 is read out with high contrast and displayed on the projection plane 48.

  FIG. 8 is a diagram showing read images at each wavelength when the information holding medium is irradiated with light having a plurality of wavelengths. As shown in FIG. 8, when the wavelength of light emitted from the monochromatic light source 42 is changed from 400 nm to 710 nm in units of 10 nm, the image displayed on the projection plane 48 also changes. Information representing the letter “T” is displayed with the highest contrast at a wavelength of 560 nm, which is a key wavelength. On the other hand, at a wavelength other than the key wavelength, an image that cannot be recognized as to whether or not the information represents the character “T” is displayed.

  Further, image information may be read out through a wavelength filter. FIG. 9 is an exploded perspective view showing another example of the configuration of the display device according to the embodiment of the present invention. As shown in FIG. 9, the display device 50 includes a white light source 52 that emits white light, a pair of polarizers 54 and 56, and a wavelength filter 58 that transmits only light having a key wavelength. On the light irradiation side of the white light source 52, a polarizer 54 with a transmission axis 54A, a polarizer 56 with a transmission axis 56A, and a wavelength filter 58 are arranged in this order from the light source 52 side. The pair of polarizers 54 and 56 is an “orthogonal Nicol optical system” in which the transmission axis 54A and the transmission axis 56A are arranged to be orthogonal to each other.

  When reading image information, the information holding medium 10 is disposed so as to be sandwiched between a pair of polarizers 54 and 56. White light is irradiated from the white light source 52 to the information holding medium 10 sandwiched between the pair of polarizers 54 and 56. The light transmitted through the polarizer 54, the information holding medium 10, the polarizer 56, and the wavelength filter 58 is observed on the light emission side (observation side) of the wavelength filter 58.

  The white light emitted from the white light source 52 includes light having a key wavelength. Light having a wavelength other than the key wavelength also passes through the polarizer 54, the information holding medium 10, and the polarizer 56, but is removed by the wavelength filter 58. Accordingly, only the light having the key wavelength is observed on the light emitting side of the wavelength filter 58, as in the case where the light having the key wavelength is irradiated. When the information holding medium 10 is irradiated with the light of the key wavelength, the light pixel 58H and the dark pixel 58L form the letter “T” on the light emission side of the wavelength filter 58. That is, information representing the letter “T” held in the information holding medium 10 is read out with high contrast and displayed on the surface of the wavelength filter 58 on the light emission side.

  In the example illustrated in FIG. 9, the example in which the wavelength filter 58 is disposed on the observation side has been described. However, the position of the wavelength filter 58 is not limited to this. The wavelength filter 58 may be disposed between the observation point set outside the display device 50 and the white light source 52. For example, it may be disposed between the white light source 52 and the polarizer 54, or may be disposed between the polarizer 54 and the polarizer 56.

<Second Embodiment>
Next, an information holding medium according to the second embodiment of the present invention will be described. FIGS. 10A and 10B are perspective views showing an example of the configuration of an information holding medium (information distribution type) according to the second embodiment of the present invention. As shown in FIG. 10A, the information holding medium 60 is composed of a sheet-like first member 60A and a sheet-like second member 60B that is used while being superimposed on the first member 60A.

  The information holding medium 60 holds the image information separately for the first member 60A and the second member 60B so that the image information can be read only when the first member 60A and the second member 60B are overlapped. ing. In this way, by configuring the information holding medium from a plurality of members, it is possible to realize a visual decryption type encryption device that can achieve both high secrecy and simple handling. That is, even if a plurality of physical keys are prepared, confidential information cannot be read unless the key wavelength is known.

In the first member 60A, a plurality of pixel portions 62A having a rectangular shape in plan view are arranged in a matrix of 3 rows and 3 columns. A pixel portion arranged in m rows and n columns is displayed as “pixel portion 62A _mn ”. In addition, when it is not necessary to distinguish these, they are collectively referred to as “pixel portion 62A”. Further, in the second member 60B, a plurality of pixel portions 62B are arranged in a matrix of 3 rows and 3 columns corresponding to the plurality of pixel portions 62A of the first member 60A. A pixel portion arranged in m rows and n columns is displayed as “pixel portion 62B _mn ”. In addition, when it is not necessary to distinguish these, they are collectively referred to as “pixel portion 62B”.

As shown in FIG. 10B, when the first member 60A and the second member 60B are overlapped, the pixel portion 62A and the corresponding pixel portion 62B are overlapped. The information holding medium 60 is an information holding medium in which a plurality of pixel units 62 are arranged in a matrix in a state where the first member 60A and the second member 60B are overlapped. Each of the plurality of pixel units 62 has a birefringence phase difference Δ that transmits light having a key wavelength with a transmittance according to image information. A pixel portion arranged in m rows and n columns is displayed as “pixel portion 62 _mn ”. In addition, when it is not necessary to distinguish these, they are collectively referred to as “pixel unit 62”. Further, the birefringence phase difference Δ of the pixel portion 62 _mn is displayed as “birefringence phase difference Δ _mn ”.

  Also in the second embodiment, a phase φ (that is, a birefringence phase difference Δ) is given to each of the plurality of pixel units 62 according to image information, so that the information holding medium 60 has a key wavelength. Image information that can be read by light can be held. Further, by randomly changing the birefringence order N, the image information held in the information holding medium 60 cannot be read with light having a wavelength other than the key wavelength. Furthermore, by configuring each of the plurality of pixel units 62 with a high-order birefringent body, it is impossible to read image information held in the information holding medium 60 with white light.

  Next, the image information holding method will be described with a specific example. 11A is a diagram showing the birefringence phase difference Δ of each pixel portion of the first member, and FIG. 11B is a diagram showing the birefringence phase difference Δ of each pixel portion of the second member. FIG. 11C is a diagram showing the birefringence phase difference Δ of each pixel portion of the information holding medium composed of the first member and the second member. As shown in FIGS. 11A to 11C, each of the first member 60A, the second member 60B, and the information holding medium 60 is represented by a birefringence order N and a phase φ in each of the plurality of pixel portions. The birefringence phase difference Δ is given.

  As shown in FIG. 11C, each of the plurality of pixel portions 62 of the information holding medium 60 is given a birefringence order N and a phase φ according to image information. The information holding medium 60 holds the same image information (information representing the character “T”) as the information holding medium 10 shown in FIG. Therefore, the value of the birefringence phase difference Δ (birefringence order N, phase φ) given to each of the plurality of pixel units 62 is also the same as that of the information holding medium 10.

  Further, as shown in FIG. 11A, a phase φ is given to each of the plurality of pixel portions 62A of the first member 60A according to the dummy image information. Here, on the premise that a pair of polarizers of the display device constitutes an “orthogonal Nicol optical system”, φ = π for the bright pixel portion and for the dark pixel portion for the light of the key wavelength By setting φ = 0, binary image information is held. Further, a high-order birefringence order N is randomly given to the plurality of pixel portions 62A.

  The “dummy image information” is image information different from the “true image information” held in the information holding medium 60. In this example, dummy image information representing the character “L” is held in the first member 60A. When confidential information is stored in the information storage medium 60, displaying “dummy image information” makes it difficult to determine the authenticity of “true image information”, and improves the confidentiality of the stored information.

  In addition, as shown in FIG. 11B, each of the plurality of pixel portions 62B of the second member 60B is overlapped with the first member 60A and the second member 60B in FIG. 11C. A birefringence phase difference Δ (birefringence order N, phase φ) designed in advance is given so as to be the information holding medium 60 shown. The birefringence phase difference Δ when the pixel portion 62A and the pixel portion 62B are superposed is obtained by adding the values of the birefringence order N and the phase φ. Since the birefringence phase difference Δ has a periodicity of 2π, the addition of the birefringence phase difference Δ is an exclusive OR (XOR) in which φ = 0 is “0” and φ = π is “1”. It becomes an operation.

For example, the pixel portion 62A ₃₃ of the first member 60A has a birefringence order N = 8 and a phase φ = π. Further, the pixel portion 62B ₃₃ of the second member 60B has the birefringence order N = 1 and the phase φ = π. Accordingly, the birefringence phase difference Δ ₃₃ of the corresponding pixel portion 62 ₃₃ of the information holding medium 60 is birefringence order N = 10 and phase φ = 0. Thus, the birefringence order N and phase φ of each pixel portion 62A of the first member 60A and the birefringence order N and phase φ of each pixel portion 62B of the second member 60B are the pixel portion of the information holding medium 60. It is determined according to the birefringence phase difference Δ of 62.

  FIG. 12A shows a read image from the first member 60A, and FIG. 12B shows a read image from the information holding medium 60 composed of the first member 60A and the second member 60B. FIG. When reading out image information, the display device shown in FIG. 7 or FIG. 9 is used. Here, a case where image information is read using the display device shown in FIG. 7 will be described.

  Only the first member 60 </ b> A is disposed so as to be sandwiched between the pair of polarizers 44 and 46, and the first member 60 </ b> A is irradiated with light having a key wavelength from the monochromatic light source 42. The light having the key wavelength transmitted through the polarizer 44, the first member 60A, and the polarizer 46 is observed on the light emission side (observation side) of the polarizer 46. As shown in FIG. 12A, when the first member 60A is irradiated with light having a key wavelength, a letter “L” is formed on the projection surface 48 by the bright pixels 48H and the dark pixels 48L. That is, the “dummy image information” held in the first member 60 is read and displayed on the projection plane 48.

  On the other hand, the first member 60 </ b> A and the second member 60 </ b> B are overlapped to form the information holding medium 60, and are arranged so as to be sandwiched between the pair of polarizers 44 and 46. Irradiate light of key wavelength. The light having the key wavelength transmitted through the polarizer 44, the information holding medium 60, and the polarizer 46 is observed on the light emission side (observation side) of the polarizer 46. As shown in FIG. 12B, when the information holding medium 60 is irradiated with light having a key wavelength, a letter “T” is formed on the projection surface 48 by the bright pixels 48H and the dark pixels 48L. That is, “true image information” held in the information holding medium 60 is read and displayed on the projection plane 48.

  In the second embodiment, the example in which the information holding medium is configured by the first member and the second member has been described. However, the number of members is not limited to two. Image information may be distributed and held in three or more members. When used as a visual decryption type encryption, the confidentiality of the stored image information improves as the number of distributed sheets increases.

  In the second embodiment described above, an example in which “dummy image information” is read with light of a key wavelength has been described, but image information is read only with some members such as the first member and the second member. You may make it out. Even when “dummy image information” is read, the wavelength of the read light is not limited to the “key wavelength”. The “dummy image information” may be read with light having a wavelength other than the key wavelength.

  In the second embodiment, the phase φ of each pixel portion of the first member and the second member is “0” or “π”, but the phase φ of each pixel portion in each member is “0”. ”Or“ π ”. In an “information holding medium” obtained by overlapping a plurality of sheet-like members, the phase φ of each pixel portion may be “0” or “π”.

For example, the pixel unit _{62A 11} of the first member 60A, the birefringent order N = 6, the phase φ = π / 2. Further, the pixel portion _{62B 11} of the second member 60B, birefringence order N = 1, the phase φ = π / 2. Thereby, the birefringence phase difference Δ ₁₁ of the corresponding pixel portion 62 ₁₁ of the information holding medium 60 becomes the birefringence order N = 7 and the phase φ = π. As described above, for a plurality of members constituting the “information holding medium”, the phase φ of each pixel portion may be a value other than “0” and “π”.

<Third Embodiment>
Next, an information holding medium according to the third embodiment of the present invention will be described. FIG. 13 is a chart showing the design principle of the information holding medium (wavelength multiplexing type) according to the third embodiment of the present invention. FIG. 14 is a diagram showing the birefringence phase difference Δ of each pixel portion of the information holding medium according to the third embodiment of the present invention.

As shown in FIG. 14, the information holding medium 70 is a sheet-like information holding medium in which a plurality of pixel portions 72 having a rectangular shape in plan view are arranged in a matrix of 3 rows and 3 columns. Each of the plurality of pixel units 72 has a birefringence phase difference Δ that transmits light having a key wavelength with a transmittance according to image information. A pixel portion arranged in m rows and n columns is displayed as “pixel portion 72 _mn ”. In addition, when it is not necessary to distinguish these, they are collectively referred to as “pixel portion 72”.

  The information holding medium 70 holds a plurality of pieces of image information on one information holding medium. In this embodiment, “a plurality of key wavelengths” having different wavelengths are set, and different image information can be read for each key wavelength. In other words, it is only necessary to be able to read different image information for each key wavelength, and there is no limit to the number of image information held on one information holding medium. In the following, for the sake of simplicity, the first image information is read by irradiating the light of “first key wavelength λ1”, and the light of “second key wavelength λ2” is irradiated. A case where the image information 2 is read will be described. In order to set two key wavelengths and read out two types of image information, it is necessary to prepare at least four types of birefringent bodies having a birefringence phase difference Δ as shown in FIG.

  As in the first embodiment, each of the plurality of pixel units 72 is given a phase φ (that is, a birefringence phase difference Δ) according to image information, whereby the information holding medium 70 is Image information that can be read with light of a key wavelength can be held. Further, by randomly changing the birefringence order N, it becomes impossible to read the image information held in the information holding medium 70 with light having a wavelength other than the key wavelength. Furthermore, by configuring each of the plurality of pixel units 72 with a high-order birefringent material, the image information held in the information holding medium 70 cannot be read with white light.

  The four types of birefringence phase differences Δ are birefringence phase differences “Δ1” that are “bright” with respect to “first key wavelength λ1” and “dark” with respect to “second key wavelength λ2”, A birefringence phase difference “Δ2” which is “dark” with respect to “first key wavelength λ1” and “light” with respect to “second key wavelength λ2”, and “first key wavelength λ1” with “ Birefringence phase difference “Δ3” that is “dark” and “dark” with respect to “second key wavelength λ2”, “bright” with respect to “first key wavelength λ1”, and “second key wavelength λ2” Is a birefringence phase difference “Δ4” that is “bright”.

  When a pair of polarizers of the display device constitutes an “orthogonal Nicol optical system”, φ = π in the bright pixel portion and φ = 0 in the dark pixel portion with respect to the light having the key wavelength. On the other hand, when the pair of polarizers of the display device constitutes a “parallel Nicol optical system”, φ = 0 for the bright pixel portion and φ = π for the dark pixel portion with respect to the light of the key wavelength.

  As shown in FIG. 14, each of the plurality of pixel portions 72 of the information holding medium 70 is given any one of birefringence phase differences Δ1 to Δ4 according to image information. When reading image information from the information holding medium 70, the display device shown in FIG. 7 or FIG. 9 is used. Here, a case where image information is read using the display device shown in FIG. 7 will be described. FIG. 15A shows a read image at the first key wavelength λ1, and FIG. 15B shows a read image at the second key wavelength λ2.

  The information holding medium 70 is disposed so as to be sandwiched between a pair of polarizers 44 and 46 constituting the “orthogonal Nicol optical system”, and the information holding medium 70 is moved from the monochromatic light source 42 to the “first key wavelength λ1. Is irradiated. The light having the “first key wavelength λ 1” transmitted through the polarizer 44, the information holding medium 70, and the polarizer 46 is observed on the light emission side (observation side) of the polarizer 46. As shown in FIG. 15A, when the information holding medium 70 is irradiated with light of “first key wavelength λ1”, the letter “T” is formed on the projection surface 48 by the bright pixels 48H and the dark pixels 48L. Is formed. That is, the “first image information” held in the information holding medium 70 is read and displayed on the projection surface 48.

  On the other hand, the information holding medium 70 disposed between the pair of polarizers 44 and 46 is irradiated with light of “second key wavelength λ2” from the monochromatic light source 42. The light having the “second key wavelength λ 2” transmitted through the polarizer 44, the information holding medium 70, and the polarizer 46 is observed on the light emission side (observation side) of the polarizer 46. As shown in FIG. 15B, when the information holding medium 70 is irradiated with light of “second key wavelength λ 2”, the letter “L” is formed on the projection surface 48 by the bright pixels 48 H and the dark pixels 48 L. Is formed. That is, the “second image information” held in the information holding medium 70 is read and displayed on the projection surface 48.

  Here, as shown in FIG. 13, a specific design method of selecting four types of birefringent bodies having a birefringence phase difference Δ in order to set two key wavelengths and read out two types of image information will be described. To do. By laminating ten retardation films (full wave plates) having a birefringence amount Δn of 560 nm, a high-order birefringent body having N = 10, φ = 0, and converted order “10.0” can be obtained. FIG. 16A is a graph showing an example of the spectral transmittance of this higher-order birefringent body. The pair of polarizers of the measuring device constitutes a “parallel Nicol optical system”, and this high-order birefringent body transmits light having a key wavelength of 560 nm with a maximum transmittance.

  The birefringence wavelength dispersion of the high-order birefringent body, that is, the birefringence amount Δn at an arbitrary wavelength is expressed by the following formula (9) using the “Cauchy dispersion formula” expressed by the following formula (8). The Hereinafter, the following formula (9) is referred to as a “Cauchy dispersion formula in a birefringent body”.

In the above formula (8), n is a refractive index, λ is a wavelength, and A and B are coefficients. Further, in the above formula (9), n _e is the extraordinary light component of the refractive index, n _o is the ordinary component of refractive index. λ is a wavelength, and ΔA and ΔB are coefficients.

  From the graph (actually measured value) shown in FIG. 16A, for a high-order birefringent body, a plurality of wavelengths at which the transmittance is maximized or minimized are known. When the values of the coefficients ΔA and ΔB are obtained by fitting (regression calculation) with the “Cauchy dispersion formula in the birefringent body” of the above formula (9) so as to satisfy these multiple conditions, Birefringence wavelength dispersion can be obtained. For example, as shown in FIG. 16B, a chromatic dispersion curve in which the birefringence amount Δn is plotted with respect to the wavelength λ can be obtained from the actually measured values shown in FIG.

  [Δn · d] in the above formula (6) is the birefringence amount Δn of the birefringent body. For example, at the time of fitting, as shown in the following formula (10), the parameter “ΔA” and the parameter “ΔA · ΔB” can be obtained as a function of the wave number k that is easy to fit. At this time, the thickness “d” of the birefringent body corresponds to the number of retardation films (full wave plates), that is, “birefringence order N”. That is, the birefringence amount Δn increases in proportion to the value of the birefringence order N.

  When birefringence wavelength dispersion is obtained by the above method for a plurality of high-order birefringent bodies having different birefringence orders N and phase φ values, as shown in FIG. 17, transmission in a wavelength range predetermined for the birefringence orders is performed. The light intensity distribution can be represented by a bright and dark image. Note that the birefringence order in FIG. 17 is expressed as “converted order” representing the birefringence phase difference Δ. Whether a birefringent body having a “specific birefringence phase difference Δ” becomes a “bright pixel portion” or a “dark pixel portion” with respect to a “specific wavelength” is expressed by a straight line representing a “specific conversion order” and “ It can be determined whether the intersection with the straight line representing the “specific wavelength” is “bright” or “dark”.

  Next, a design example of the wavelength multiplexing type information holding medium 70 is shown. FIG. 18 is a graph showing the spectral transmittances of four types of higher-order birefringent bodies that constitute each pixel portion of the information holding medium (wavelength multiplexing type) according to the design example. FIG. 19 is a chart showing encoded light intensity of four types of higher-order birefringent materials at wavelengths of 560 nm and 600 nm. FIG. 20 is a diagram showing the birefringence phase difference Δ of each pixel portion of the information holding medium (wavelength multiplexing type) according to the design example.

  As shown in FIG. 18, four types of high-order birefringent bodies having different spectral transmittance characteristics are prepared. These four types of higher-order birefringent materials have different conversion orders of “5.0”, “6.5”, “11.5”, and “12.0”, and have different birefringence phase differences Δ. is doing. As described based on FIG. 13, the two key wavelengths of the first key wavelength λ1 and the second key wavelength λ2 are set by four kinds of higher-order birefringent bodies having different birefringence phase differences Δ, Two types of image information can be read out.

  The birefringence phase differences Δ1 to Δ4 shown in FIG. 13 are “N = 5, φ = 0”, “N = 6, φ = π”, “N = 11, φ = π”, “N = 12, φ = 0 ”. Based on the relationship shown in FIG. 19, a birefringence phase difference Δ (birefringence order N, phase φ) corresponding to image information is given to each of the plurality of pixel portions 72 of the information holding medium 70 as shown in FIG. Give. Accordingly, when reading is performed using a display device (see FIG. 7) including a pair of polarizers constituting an “orthogonal Nicol optical system”, as shown in FIG. 15A, the first key wavelength λ1 (wavelength 560 nm), the first image information can be read out. As shown in FIG. 15B, the second image information can be read out at the second key wavelength λ2 (wavelength 600 nm).

<Other variations>
In the first to third embodiments described above, the information holding medium having nine pixel units arranged in a matrix has been described. However, it is sufficient that a plurality of pixel units are arranged in the in-plane direction. The number, arrangement, shape, etc. of the pixel portions are not limited to this. For example, the number of pixel portions may be increased according to image information. The arrangement of the pixel portions may be a regular arrangement such as a honeycomb structure in which regular hexagons are arranged without gaps, or may be an irregular arrangement such as random dots. The shape of the pixel portion may be another polygonal shape (triangle, quadrangle, pentagon, octagon, etc.) in plan view, circular shape (perfect circle shape, ellipse shape, oval shape) in plan view, or indefinite shape Good. In addition, each of the plurality of pixel portions does not have to have the same shape, and may have different sizes.

  In addition, the information holding medium of the present invention can hold image information that can be read with light of a key wavelength, and the display device of the present invention reads and displays the image information held on the information holding medium. be able to. These basic principles are excellent in affinity with other information concealment technologies and can be used in combination with other information concealment technologies. By combining with existing information concealment technology, it is possible to prevent the leakage of personal information, the impersonation of others, and the forgery of documents. For example, the technique of the present invention can be used in combination with a QR code or the “spatial code pattern”.

  The QR code is known to be able to describe a large amount of information in a small area by two-dimensionally arranging black and white random patterns. For example, secret information can be safely stored by creating a QR code as image information that can be read with light of a key wavelength. The spatial code pattern method is characterized by high secrecy with respect to “peeps from the side”, but the viewpoint position is limited three-dimensionally. For example, by creating a spatial code pattern as image information that can be read with light of a key wavelength, information can be easily read and displayed by a simple device such as a light source and a pair of polarizers. The property is remarkably improved.

  The information holding medium and the display device of the present invention can be used in the information security field. For example, IC card authentication alternative technology, application of personal authentication to ID cards, storage of confidential documents, use as communication media, encryption media for computer authentication (for identity verification), credit cards, cash cards, etc. Anti-counterfeiting and authentication media are examples of application fields.

DESCRIPTION OF SYMBOLS 10 Information holding medium 12 Pixel part 20 Measuring device 22 Light source 24 Polarizer 26 Polarizer 28 Photo detector 30 Birefringent body 30A Main axis direction 32 Full wave plate 34 1/4 wave plate 40 Display device 42 Monochromatic light source 44 Polarizer 46 Polarization Child 48 Projection surface 48H Bright pixel 48L Dark pixel 50 Display device 52 White light source 54 Polarizer 56 Polarizer 58 Wavelength filter 58L Dark pixel 58H Bright pixel 60 Information holding medium 60A First member 60B Second member 62 Pixel portion 62A Pixel portion 62B Pixel unit 70 Information holding medium 72 Pixel unit

Claims (9)

A sheet-like information holding medium that holds image information expressed in multiple gradations,
A plurality of pixel portions are arranged in the in-plane direction,
Each of the plurality of pixel units is an information holding medium having a birefringence phase difference Δ that transmits light having a predetermined wavelength at a transmittance according to the image information. The information holding medium according to claim 1, wherein the birefringence phase difference Δ is defined by the following formula (1).
Δ = 2πN + φ Equation (1)
In the above formula (1), N represents the birefringence order (N is an integer). φ represents a fraction of 2π or less.   The information holding medium according to claim 2, wherein each of the plurality of pixel units has a value of the birefringence order N that randomly changes.   4. The information holding medium according to claim 2, wherein each of the plurality of pixel units has the birefringence order N of the fourth order or higher. The information holding medium holds image information expressed as a light and dark image with two gradations,
A bright pixel portion that transmits light of a predetermined wavelength and a dark pixel portion that attenuates or blocks light of a predetermined wavelength, and one of the bright pixel portion and the dark pixel portion is expressed by the following formula (2). The birefringence phase difference Δ, and the other of the bright pixel portion and the dark pixel portion has a birefringence phase difference Δ expressed by the following formula (3). The information holding medium described.
Δ = 2πN + π (φ = π) Equation (2)
Δ = 2πN + 0 (φ = 0) Equation (3) The information holding medium is composed of a plurality of sheet-like members,
When the plurality of sheet-like members are overlaid,
Each of the plurality of pixel portions has the birefringence phase difference Δ so that light of a predetermined wavelength is transmitted with a transmittance according to the image information.
The information holding medium according to any one of claims 1 to 5. The information holding medium holds a plurality of pieces of image information,
Corresponding to each of the plurality of image information, a different wavelength is predetermined,
Each of the plurality of pixel portions has the birefringence phase difference Δ so that each of the plurality of pieces of image information transmits light having a predetermined wavelength with a transmittance according to the corresponding image information.
The information holding medium according to any one of claims 1 to 6. A display device that reads and displays image information held in the information holding medium according to claim 1,
A light source for irradiating readout light including light of a predetermined wavelength on the information holding medium;
A pair of polarizers spaced apart from each other so as to sandwich the information holding medium;
A display device comprising:   The display device according to claim 8, further comprising a wavelength filter that is disposed between an observation point set outside the display device and the light source and selectively transmits light having a predetermined wavelength.