JP2004184555A - Reversible image display medium and image forming method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a color reversible image display medium and a photoirradiation method which require only one light shielding layer which selectively transmits or shields light and only two light sources as light sources for selective coloring, with respect to a reversible image display medium capable of repeating writing and erasure of color information by photoirradiation applicable to rewritable media, digital paper, etc. <P>SOLUTION: In the reversible image display medium formed by stacking photosensitive layers separately containing photochromic compounds different from one another in hue of coloring to perform multicolor display, at least one of the photosensitive layers comprises a photosensitive layer material capable of reversibly changing coloring threshold wavelength of a radiation on a photochromic compound which causes transition from an achromatic state to a chromatic state by light absorption, in accordance with stimuli from the outside. <P>COPYRIGHT: (C)2004,JPO&NCIPI

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a reversible image display medium, and more particularly, to a reversible image display medium capable of repeating writing and erasing of color information by light irradiation that can be used for rewritable media, digital paper, and the like.
[0002]
[Prior art]
With the increase in paper consumption in offices, research on a reversible image display medium capable of repeatedly recording and erasing images has been attracting attention as a medium replacing paper. Among them, some reports have been made on a color reversible image display medium capable of rewriting a multicolor image.
[0003]
As a conventional example of the present invention, there is provided a rewritable color image recording medium and an image forming method using the same, wherein the recording material comprises a cholesteric liquid crystal compound having a molecular weight of 2000 or less and a glass transition temperature of 35 ° C. or more or a mixture thereof. In Japanese Patent Application Laid-Open No. H11-163, there is known a technique in which the reflected color can be stored at room temperature for a long period of time by rapidly cooling from a cholesteric liquid crystal state, and can be repeatedly recorded by returning to a liquid crystal state. .
[0004]
Further, as a multi-color photochromic composition that causes a reversible color change by light irradiation, a mixture of three types of photochromic diarylethene compounds that emit yellow-orange, red, and blue-violet colors having different absorption wavelength ranges is used. A technique for producing a multicolor reversible display medium by irradiating three different types of ultraviolet light to a composition comprising the same is known (for example, see Patent Document 2).
[0005]
Further, as a color image material and a color image forming method using the same, a color reversible image display medium using three types of photochromic fulgide compounds that emit yellow, magenta, and cyan is colored with one type of ultraviolet light to form all colors. After that, a technique of displaying a color image by selectively decoloring with visible light is known (for example, see Patent Document 3).
[0006]
[Patent Document 1]
JP-A-11-24027
[Patent Document 2]
JP-A-5-271649
[Patent Document 3]
JP-A-7-199401
[0007]
[Problems to be solved by the invention]
The above-mentioned conventional technique using a photochromic compound which displays a color by absorbing light proposed in JP-A-5-271649 and JP-A-7-199401 is disclosed in Japanese Patent Application Laid-Open No. 11-24027. This method has excellent characteristics as image display characteristics, such as a higher white reflectance and a higher display color density than a method using a cholesteric liquid crystal compound that displays colors by selective reflection.
[0008]
However, the method disclosed in Japanese Patent Application Laid-Open No. Hei 5-271649, in which three types of photochromic compounds are independently colored by ultraviolet light of a specific wavelength, has the following problems. That is, the light having the longest wavelength in the wavelength range where the photochromic compound is changed from the decolored state to the colored state, that is, the threshold wavelength at which light absorption occurs in the decolored state of the photochromic compound differs depending on the photochromic compound. However, most of the light having a wavelength shorter than the threshold wavelength is colored by light irradiation. Therefore, the method disclosed in Japanese Patent Application Laid-Open No. 5-271649, which utilizes color development only in a specific ultraviolet wavelength region, is not practical.
[0009]
On the other hand, the method proposed in Japanese Patent Application Laid-Open No. 7-199401, in which all colors are colored with one kind of ultraviolet light and then selectively erased with visible light, does not have the above problem, but it is used as a substitute for paper, for example. Most of the documents created in the office are written in black letters on a white background, and the average manuscript ratio (the ratio of the character area to the entire paper) is 10% or less. Irradiation of visible light to most areas will cause the color to be erased. Therefore, there is much waste of energy for displaying an image.
[0010]
As a method of avoiding the waste of the above energy, visible light for selectively coloring only pixels displaying an image with ultraviolet rays instead of full-color coloring with ultraviolet rays, and then decoloring each pixel according to the color density of the pixels. May be sequentially irradiated with light in the sensitivity wavelength region of each photochromic compound. However, even in this method, when the background of the image is set to a specific color and characters are displayed thereon, it is necessary to irradiate most of the area with visible light other than the background color and then erase the color after the entire medium is colored. There is a problem that there is a lot of waste of energy.
[0011]
Therefore, the present invention has been made to solve the above problems, and an object of the present invention is to provide only one kind of light-shielding layer for selectively transmitting or blocking light, and a light source for selectively coloring. An object of the present invention is to provide a color reversible image display medium and a light irradiation method that require only two types of light sources.
[0012]
[Means for Solving the Problems]
In order to solve the above-mentioned problems, in the invention according to claim 1, in a reversible image display medium that performs multicolor display by laminating photosensitive layers individually containing a plurality of types of photochromic compounds having different color hues, Among the types of photosensitive layers, at least one type of photosensitive layer has a color development threshold wavelength of irradiation light to a photochromic compound that transitions from a decolored state to a colored state by light absorption, from the outside to the one type of photosensitive layer. The most important feature is a reversible image display medium composed of a photosensitive layer material that can be reversibly changed by stimulation.
[0013]
According to the second aspect of the present invention, in a reversible image display medium for performing multicolor display by laminating photosensitive layers individually containing a plurality of types of photochromic compounds having different coloring hues, the laminated lowermost photosensitive layer and the Between the photosensitive layer located in or between the uppermost photosensitive layer and the photosensitive layer located thereunder, a polarizing filter layer having a function of transmitting only a specific polarization plane of ultraviolet light is provided. The feature of the reversible image display medium is the most important feature.
[0014]
According to a third aspect of the present invention, in the second aspect, at least one of the plurality of types of photosensitive layers has a color development threshold wavelength according to the first aspect reversibly by an external stimulus in a decolored state. The main feature is a reversible image display medium which is a photosensitive layer composed of a variable photosensitive layer material.
[0015]
According to the fourth aspect of the present invention, in the reversible image display medium, a coloration threshold wavelength λ of irradiation light to the lowermost photosensitive layer is used.₁Is the color development threshold wavelength λ of the photosensitive layer on the top₃Wavelength between the uppermost photosensitive layer and the middle photosensitive layer.₁And λ₃At least some wavelengths in the intermediate wavelength range of₃A light-shielding layer having a property of shielding light of at least a part of the wavelength in a shorter wavelength region is provided, and only a specific polarization plane of ultraviolet light is provided between the lowermost photosensitive layer and the photosensitive layer located in the middle. A polarizing filter layer having a function of transmitting light is provided, and the photosensitive layer located in the middle between the uppermost and lowermost photosensitive layers has a wavelength of λ due to external stimulation.₃Longer, wavelength λ₁Shorter wavelength X₁For the wavelength X₁Longer color development threshold wavelength and wavelength X₁The reversible image display medium according to any one of claims 1 to 3, which is a photosensitive layer capable of being reversibly changed between a state of a shorter color development threshold wavelength and a state of a shorter wavelength.
[0016]
According to a fifth aspect of the present invention, in the fourth aspect, the color development threshold wavelength of the photosensitive layer located at the intermediate portion is set to the wavelength X.₁Longer than the wavelength X having a polarization plane that does not transmit through the lowermost photosensitive layer and the polarizing filter layer located in the middle.₁A first light irradiation step of irradiating only the photosensitive layer located in the intermediate portion by irradiating the ultraviolet ray, the color development threshold wavelength of the photosensitive layer located in the intermediate portion is set to the X₁Shorter than the wavelength X having a polarization plane transmitting through the ultraviolet polarization filter layer.₁A second light irradiation step of irradiating the ultraviolet light to only develop a color in the photosensitive layer located at the lowermost part;₃Light X in a shorter wavelength range than₂Irradiates the photosensitive layer selectively containing three types of photochromic compounds by the three light irradiation steps of the third light irradiation step in which only the uppermost photosensitive layer is colored by irradiating the photosensitive layer. The main feature is the forming method.
[0017]
According to a sixth aspect of the present invention, in the reversible image display medium according to any one of the first to third aspects, the color formation threshold wavelength λ of the lowermost photosensitive layer is set.₁Is the coloration threshold wavelength λ of the photosensitive layer located in the middle₂Longer than the wavelength, between the uppermost photosensitive layer and the photosensitive layer located in the middle there is provided an ultraviolet polarizing filter layer having a function of transmitting only a specific polarization plane of ultraviolet light, the lowermost photosensitive layer and Λ between the photosensitive layers located in the middle₁Shorter than λ₂Transmits light of at least some wavelengths within a longer wavelength range than λ₂A light-blocking layer having a property of blocking light of at least a part of the wavelength in a shorter wavelength region is provided, and the uppermost photosensitive layer has a wavelength of λ.₂Wavelength X shorter than₃The color development threshold wavelength is X₃Longer state and X₃The main feature of the present invention is a reversible image display medium that reversibly changes during a shorter state due to an external stimulus.
[0018]
According to a seventh aspect of the present invention, in the sixth aspect, the color development threshold wavelength of the uppermost photosensitive layer is set to the X value.₃Shorter than the wavelength X having a polarization plane transmitting the polarization filter layer.₃The first light irradiation step of irradiating only the photosensitive layer located in the intermediate portion by irradiating the light, the color development threshold wavelength of the photosensitive layer located in the uppermost portion is X₃Longer than the wavelength X having a polarization plane that does not transmit through the ultraviolet polarization filter layer.₃A second light irradiation step of irradiating shorter ultraviolet rays to develop a color only in the uppermost photosensitive layer;₁Shorter than λ₂Light X in a longer wavelength range than₄Irradiates the photosensitive layer selectively containing three types of photochromic compounds by the three light irradiation steps of the third light irradiation step in which only the lowermost photosensitive layer is colored by irradiating the photosensitive layer. The main feature is the forming method.
[0019]
According to the invention described in claim 8, the photosensitive layer containing the photochromic compound having the characteristic that the color development threshold wavelength used in claim 1 to 7 is reversibly changed is incorporated into the photosensitive layer by the external stimulus. The main feature is a reversible image display medium containing a compound causing an action.
[0020]
According to the ninth aspect of the present invention, in the eighth aspect, the reversible image display medium is characterized in that the compound that chemically interacts with the photochromic compound is an electron accepting compound or an electron donating compound.
[0021]
According to a tenth aspect of the present invention, the main feature of the eighth aspect is a reversible image display medium in which the compound that chemically interacts with the photochromic compound is an electron-accepting compound or an electron-donating compound.
[0022]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. As means for performing color reversible image display with low light irradiation energy, the present applicant has previously disclosed in Japanese Patent Application No. 2002-171415 a technique for selectively developing a photosensitive layer individually containing three types of photochromic compounds having different color hues. Has filed.
[0023]
Japanese Patent Application No. 2002-171415 discloses that in a reversible image display medium that performs multicolor display by laminating photosensitive layers individually containing three types of photochromic compounds having different color hues, the three types of photochromic compounds in a decolorized state are in a colored state. The photochromic compound is different between the lowermost photosensitive layer and the photosensitive layer located thereon, and between the uppermost photosensitive layer and the photosensitive layer located therebelow, each having a different threshold wavelength for transition to And a light-shielding layer that shields light for producing a color.
[0024]
First, the principle of the invention of Japanese Patent Application No. 2002-171415 will be described with reference to FIGS. The compound (1) is a photochromic compound having a maximum absorption wavelength in the range of 600 nm or more and less than 700 nm in the color-developing state, and the compound (2) is a photochromic compound having a maximum absorption wavelength in the range of 500 nm or more and less than 600 nm in the coloration state. A photochromic compound having an absorption wavelength in the range of 400 nm or more and less than 500 nm is referred to as compound (3).
[0025]
At this time, in the decolored state of the compounds (1), (2) and (3), the color transition threshold wavelengths of the light that changes to the color development state by light absorption are respectively different, and the color development threshold wavelength λ of the compound (1) is different.₁Is the coloration threshold wavelength λ of compound (2)₂Longer than the color development threshold wavelength λ of compound (2).₂Is the color development threshold wavelength λ of compound (3)₃Longer than
[0026]
Here, in the decolored state, the color development threshold wavelength of the light that transitions to the color development state by light absorption is the wavelength of the longest light among the wavelengths of light that transits to the color development state by irradiating the photochromic compound in the decolored state. Is defined. In other words, when light having a wavelength shorter than the coloration threshold wavelength is applied to the decolored state of the photochromic compound, the photochromic compound transitions to a colored state, and light having a wavelength longer than the coloration threshold wavelength is irradiated to the decolored state of the photochromic compound. The photochromic compound does not transition to a colored state. The photochromic compound has light absorption in the decolored state for the wavelength of light that transitions from the decolored state to the colored state due to light absorption, while the wavelength of light that transitions from the colored state to the decolored state by light absorption. Needless to say, the photochromic compound has light absorption in the color-developed state.
[0027]
FIG. 1 shows the configuration of the image display medium of the prior application. A photosensitive layer 2 containing a photochromic compound (1) is formed on a substrate 1 holding a multilayer photosensitive layer, and a light-shielding layer 3 for blocking light having a wavelength in step 15 described later is formed thereon. Further, a photosensitive layer 4 containing a photochromic compound (2) is formed thereon, and a light-shielding layer 5 for blocking light having a wavelength of a step 16 described later is formed thereon, and the photochromic compound (3) is further provided thereon. The photosensitive layer 6 containing the photosensitive layer 6 is formed.
[0028]
A process for selectively coloring a medium having the above configuration will be described with reference to FIG. The light amount corresponding to the concentration after the final color development of the photochromic compound (1) is λ₁Shorter than λ₂Step 14 of irradiating a light beam having a longer wavelength than the photosensitive layer 6 which is the outermost surface for each pixel of the image display medium is performed first. By this step, a desired coloring density is given to the photochromic compound (1). Since the photochromic compounds (2) and (3) have no color sensitivity to the light having the wavelength used in step 14, they do not develop color even when they pass through the photosensitive layers 6 and 4.
[0029]
Next, at a light amount corresponding to the concentration of the photochromic compound (2) after the final coloring,₂Shorter than λ₃Step 15 of irradiating each pixel of the image display medium with a light beam having a longer wavelength and a wavelength absorbed by the light shielding layer 3 is performed. By this step, a desired coloring density is given to the photochromic compound (2). The photochromic compound (3) does not develop color with respect to the light having the wavelength used in step 15 because it has no color development sensitivity. In addition, the photochromic compound (1) is not affected and does not develop color because the light in step 15 is shielded by the light shielding layer 3.
[0030]
Finally, the light amount corresponding to the concentration of the photochromic compound (3) after the final coloring is λ.₃A step 16 of irradiating each pixel of the image display medium with a light beam having a shorter wavelength and a wavelength absorbed by the light shielding layer 5 is performed. By this step, a desired coloring density is given to the photochromic compound (3). The photochromic compounds (1) and (2) do not develop color because the light in step 16 is blocked by the light blocking layer 5.
[0031]
With the configuration of the photochromic compound having the above characteristics, and the step of coloring, photochromic compounds having three different hues can be selectively colored, and the decoloring process is not required when performing image formation. An image can be formed with less waste of energy.
[0032]
The configuration and the image forming method of the prior application have been described above, but the following two problems can be cited as problems of the prior application. One is to provide two types of light blocking layers for selectively transmitting or blocking light between the three types of photosensitive layers. The light-shielding layer is generally provided by applying an organic molecule that absorbs specific light, and a material having a small half-value width of a spectral wavelength absorbed by the organic molecule has a spread of about 20 nm. Therefore, in the case where the two types of light-shielding layers are provided with different absorption wavelengths and the photosensitive layer is selectively colored, at least the central wavelength of the irradiated light must differ by 30 nm or more.
[0033]
For this reason, in the case of irradiating three types of light having different wavelengths to selectively form a color, the wavelength difference between the light having the longest wavelength and the light having the shortest wavelength is required to be 60 nm or more in the configuration of the prior application. In other words, assuming that there is no coloring in the visible light region in the decolored state of the photochromic compound, it is necessary to use ultraviolet light having a wavelength of 400 nm or less as a coloring light source. Three types of ultraviolet light sources of 400 nm are required.
[0034]
In recent years, the development of semiconductor lasers has been advanced and semiconductor lasers that emit ultraviolet light have been reported, but semiconductor lasers having an oscillation wavelength of 350 nm or less have not yet been put to practical use. Therefore, when an inexpensive semiconductor laser is used as the light source, the configuration of the prior application has many problems regarding the light source.
[0035]
Another problem of the prior application is that, as described above, three types of light sources having different wavelengths are required to selectively form a photosensitive layer. It is clear that an inexpensive device can be constructed with as few light sources as possible.
[0036]
In the present invention, in a reversible image display medium that performs multicolor display by laminating photosensitive layers individually containing three types of photochromic compounds having different color hues, the two types of photochromic compounds in a decolored state are threshold wavelengths at which a transition to a colored state occurs. The remaining one type of photochromic compound is characterized in that in the decolored state, the color development threshold wavelength of light that transitions to the colored state by light absorption changes reversibly by external stimulus such as heat.
[0037]
A light-shielding layer for selectively transmitting or blocking light of different wavelengths is provided at one position of the three types of laminated photosensitive layers, and only a specific polarization plane of ultraviolet light is provided at another position of the photosensitive layers. An ultraviolet polarizing filter layer having a function of transmitting light is provided.
[0038]
Next, an example of the configuration of the present invention will be described with reference to FIGS. The compound (1) is a photochromic compound having a maximum absorption wavelength in the range of 600 nm or more and less than 700 nm in the color-developing state, and the compound (2) is a photochromic compound having a maximum absorption wavelength in the range of 500 nm or more and less than 600 nm in the coloration state. A photochromic compound having an absorption wavelength in the range of 400 nm or more and less than 500 nm is referred to as compound (3).
[0039]
At this time, the coloring threshold wavelengths of the compounds (1) and (3) are different from each other, and the coloring threshold wavelength λ of the compound (1) is different.₁Is the color development threshold wavelength λ of compound (3)₃Longer than The photochromic compound (2) is λ₃Longer wavelength λ₁Wavelength X shorter than₁, The color development threshold wavelength is X₁Longer states α and X₁It is characterized in that it changes reversibly during the shorter state β by an external stimulus.
[0040]
FIG. 3 shows the configuration of the image display medium of the present invention. A photosensitive layer containing a photochromic compound (1) is formed on a substrate holding the photosensitive layer, and only a specific polarization plane of ultraviolet light is formed on the photosensitive layer so that the lowermost photosensitive layer is not affected when the intermediate photosensitive layer is colored. An ultraviolet polarizing filter layer having a function of transmitting light is provided. Further, an intermediate photosensitive layer 8 having a variable coloring threshold containing a photochromic compound (2) having a reversible coloring threshold wavelength is formed thereon, and X is formed thereon.₁X which transmits light having a wavelength of₂A light-shielding layer 19 for blocking light having a wavelength of is formed, and a photosensitive layer containing the photochromic compound (3) is further formed thereon.
[0041]
A process for selectively coloring a medium having the above configuration will be described with reference to FIG. The color development threshold wavelength of the photochromic compound layer 17 of the intermediate layer is set to the irradiation wavelength X.₁In a shorter state, the photochromic compound (1) is parallel to the polarization plane of the ultraviolet polarizing filter layer so that light passes through the photochromic compound (1) even if the filter layer is present at a light amount corresponding to the density after color development. Wavelength X having polarization plane₁Is performed for each pixel of the image display medium. By this step, a desired coloring density is given to the photochromic compound (1). The photochromic compounds in the intermediate photosensitive layer and the uppermost layer do not develop color with respect to the light having the wavelength used in the first step because they do not have color development sensitivity.
[0042]
Next, the color development threshold wavelength of the photochromic compound in the intermediate photosensitive layer was set to the irradiation wavelength X.₁In a longer state, a wavelength X having a polarization plane orthogonal to the polarization plane of the ultraviolet polarizing filter layer with an amount of light corresponding to the final density of the photochromic compound of the intermediate photosensitive layer after color development.₁A second step of irradiating each pixel of the image display medium with this light is performed. This step gives a desired color density to the photochromic compound in the intermediate photosensitive layer. Since the photochromic compound (1) is not irradiated with the light having the wavelength used in the second step by the ultraviolet polarizing filter, the photochromic compound (1) is not affected. In addition, the photochromic compound (3) does not form a color because it has no coloring sensitivity.
[0043]
Finally, the light amount corresponding to the concentration of the photochromic compound (3) after the final coloring is λ.₃Shorter than the wavelength X and absorbed by the light shielding layer 19.₂Is performed for each pixel of the image display medium. By this step, a desired coloring density is given to the photochromic compound (3). The photochromic compounds (1) and (2) have a wavelength X₂Is blocked by the light-shielding layer 19, so that no color is generated.
[0044]
With the configuration of the photochromic compound having the above characteristics and the step of coloring, only one type of light-shielding layer is required, and the photochromic compound having three different hues can be selectively colored with two types of light. it can.
[0045]
Another configuration example of the present invention will be described with reference to FIGS. The compound (1) is a photochromic compound having a maximum absorption wavelength in the range of 600 nm or more and less than 700 nm in the color-developing state, and the compound (2) is a photochromic compound having a maximum absorption wavelength in the range of 500 nm or more and less than 600 nm in the coloration state. The photochromic compound (3) having an absorption wavelength in the range of 400 nm or more and less than 500 nm is defined.
[0046]
At this time, the coloring threshold wavelengths of the compounds (1) and (2) are different from each other, and the coloring threshold wavelength λ of the compound (1) is different.₁Is the coloration threshold wavelength λ of compound (2)₂Longer than The photochromic compound (3) has a color development threshold wavelength of X₃Longer state γ and color development threshold wavelength X₃It is characterized in that it changes reversibly during the shorter state δ by an external stimulus.
[0047]
FIG. 5 shows the configuration of the image display medium of the present invention. A photosensitive layer 2 containing a photochromic compound (1) is formed on a substrate holding the photosensitive layer, and the X₄X of light having a wavelength of₃A light-shielding layer 20 that shields light having a wavelength of? Further, a photosensitive layer 4 containing a photochromic compound (2) is formed thereon, and an ultraviolet polarizing filter layer 7 having a function of transmitting only light having a specific polarization plane of ultraviolet light is formed thereon. A photosensitive layer 9 having a variable color development threshold including a photochromic compound (3) having a reversible color development threshold wavelength is formed.
[0048]
A process for selectively coloring a medium having the above configuration will be described with reference to FIG. The photosensitive layer 9 containing a photochromic compound capable of reversibly changing the coloring threshold wavelength is irradiated with the irradiation wavelength X₃In a state in which the color is developed at a shorter wavelength, the wavelength X having a polarization plane parallel to the polarization plane of the ultraviolet polarization filter layer with an amount of light corresponding to the final concentration of the color of the photochromic compound (2) after the color development.₃Is performed for each pixel of the image display medium. By this step, a desired coloring density is given to the photochromic compound (2). For the light having the wavelength used in the first irradiation step, the photosensitive layer 9 containing the photochromic compound capable of reversibly changing the coloring threshold wavelength does not develop color because it is in a state where it has no coloring sensitivity. Further, the photochromic compound (1) is not irradiated with light by the light shielding layer 20, so that the photochromic compound (1) does not develop color.
[0049]
Next, the coloration threshold wavelength of the photosensitive layer 9 containing the photochromic compound capable of reversibly changing the coloration threshold wavelength is changed to the irradiation wavelength X.₃In a state where color is formed at a longer wavelength, a wavelength X having a polarization plane orthogonal to the polarization plane of the ultraviolet polarizing filter layer with an amount of light corresponding to the density after the final color development of the photosensitive layer 9 is obtained.₃A second irradiation step of irradiating each pixel of the image display medium with this light is performed. In this step, the photosensitive layer 9 is irradiated, but the photochromic compounds (1) and (2) are not irradiated with light by the polarizing filter, so that the photochromic compounds (1) and (2) are not affected.
[0050]
Finally, the wavelength X is determined by the amount of light corresponding to the concentration of the photochromic compound (1) after the final color development.₄A third irradiation step of irradiating the light is performed. By this step, a desired coloring density is given to the photochromic compound (1). The photochromic compound (2) and the photosensitive layer 9 have a wavelength X₄It does not develop color because it does not have color development sensitivity to light.
[0051]
According to the configuration of the photochromic compound having the above characteristics and the step of coloring, photochromic compounds having three different hues can be selectively colored with two kinds of light.
[0052]
In the description of the above two configurations, an example in which three light irradiation steps are sequentially performed is given, but the order of the steps may be changed. Such a case is also included in the scope of the present invention.
[0053]
Many of the documents used in offices are documents containing only black and white characters or documents having many black and white characters. In the case of such a document, it is less necessary to simultaneously form the three photosensitive layers in one light irradiation process than to perform the above-described three processes to form black color by superimposing the three processes. Less worry about color misregistration. For this purpose, as a step different from the above-mentioned three light irradiation steps, a short wavelength light having a color sensitivity to all the photochromic compounds through the light-shielding layer and irradiating each pixel of the image display medium with the three photosensitive layers is provided. May be simultaneously performed.
[0054]
The present applicant has previously shown that the decoloring sensitivity of a photosensitive layer containing a photochromic compound and an electron-accepting compound can be reversibly changed by a heat treatment method. Further, as described in the example of the earlier application, it has been found that the decoloring sensitivity changes reversibly, and the maximum absorption wavelength in the color-developed state changes reversibly.
[0055]
The present applicant has studied the photosensitive layer containing the photochromic compound and the electron-accepting compound described in the prior application, and as a result, has found that the coloring threshold wavelength also reversibly changes depending on the heat treatment method. The material containing the photochromic compound and the electron-accepting compound described in the prior application can be used for the photosensitive layer used in the present invention, in which the color-forming threshold wavelength of the light that changes to the color-forming state is reversibly changed by an external stimulus.
[0056]
Although not described in the prior application, the substance that interacts with the photochromic compound is not limited to an electron-accepting compound, and it goes without saying that an electron-donating compound may be used.
[0057]
Examples of the color reversible image display medium of the present invention include those having the following configurations. The surface of the supporting substrate is preferably white for a white display with high reflectance. However, it may be colored according to the application. The support substrate is preferably a relatively thin medium such as paper or film, but is not limited to this.
[0058]
The photosensitive layer on the supporting substrate contains a photochromic compound which becomes colored by irradiation with ultraviolet light and becomes decolored by irradiation with visible light. Photochromic compounds include P-type compounds, which are thermally stable and change color only by light, and T-type compounds, which are unstable to heat and change color not only by light but also by heat. However, in the present invention, it is particularly desirable to use a P-type compound. Representative P-type compounds include fulgide-based compounds and diarylethene-based compounds.
[0059]
When a full-color image is to be recorded, a compound that develops the three primary colors of yellow, magenta, and cyan is important.
"2- [1- (3,5-dimethyl-4-isoxazolyl) ethylidene] -3-isopropylidene succinic anhydride", "2- [1- (5-methyl-2-phenyl-4-oxazolyl) ethylidene" ] -3-isopropylidene succinic anhydride "and" 2- [1- (2-phenyl-5-methyl-4-oxazolyl) stearylidene] -3-isopropylidene succinic anhydride ". The maximum absorption wavelength of these compounds in a color-developed state is about 430 nm to 460 nm.
[0060]
Further, as the magenta coloring compound, for example,
"2- [1- (2,5-dimethyl-1-phenylpyrazolyl) ethylidene] -3-isopropylidene succinic anhydride", "2- [1- (3-methoxy-5-methyl-1-phenyl- 4-pyrazolyl) ethylidene] -3-isopropylidene succinic anhydride, "2- [1- (2-methyl-5-styryl-3-thienyl) ethylidene] -3-isopropylidene succinic anhydride" and the like. No. The maximum absorption wavelength of these compounds in a colored state is about 550 nm to 560 nm.
[0061]
Further, as the cyan coloring compound, for example,
"2- [1- (1,2,5-trimethyl-3-pyrrolyl) ethylidene] -3-isopropylidene succinic anhydride", "2- [2,6-dimethyl-3,5-bis (p- Dimethylaminostyryl) benzylidene] -3-isopropylidenesuccinic anhydride. The maximum absorption wavelength of these compounds in a colored state is about 600 nm to 650 nm.
[0062]
The photochromic compound in the photosensitive layer may be dispersed in a resin such as an acrylic resin, a vinyl chloride resin, a polyester resin, a polyamide resin, a polyolefin resin or a urethane resin, or may be encapsulated in microcapsules. Is also good.
[0063]
As a substance included in the light shielding provided between the photosensitive layers, for example, 2- (5-methyl-2-hydroxyphenyl) benzotriazole, 2- (3,5-di-t-butyl-2-hydroxyphenyl) benzotriazole, indole Dyes and the like.
[0064]
Examples of the ultraviolet polarizing filter include a material obtained by uniaxially stretching polyvinyl alcohol (PVA) dyed with an azo or anthraquinone dichroic dye.
[0065]
Further, needless to say, an image display medium in which a polyvinyl alcohol thin film or the like is formed on the surface of the photosensitive layer as a protective film for preventing deterioration of the photosensitive layer is also included in the present invention.
[0066]
【Example】
Next, the present invention will be described in more detail with reference to examples. However, the present invention is not limited by the following examples.
As the photochromic compound used in the examples, a photochromic compound (1) exhibiting cyan coloring in a coloring state, a photochromic compound (2) exhibiting magenta coloring in a coloring state, and a photochromic compound (3) exhibiting yellow coloring in a coloring state are used. Was.
[0067]
Specifically, as the photochromic compound (1), 2- [1- (1,2,5-trimethyl-3-pyrrolyl) ethylidene] -3-isopropylidene succinic anhydride (hereinafter referred to as PC1), the photochromic compound (2) ) As 2- [1- (5-methyl-2-p-dimethylaminophenyl-4-oxazolyl) ethylidene] -3-isopropylidene succinic anhydride (hereinafter referred to as PC2); [1- (5-Methyl-2-phenyl-4-oxazolyl) ethylidene] -3-isopropylidene succinic anhydride (hereinafter referred to as PC3) was used.
[0068]
The coloring threshold wavelength of the photochromic compound (1) was 400 nm, the coloring threshold wavelength of the photochromic compound (2) was 380 nm, and the coloring threshold wavelength of the photochromic compound (3) was 360 nm.
[0069]
For the purpose of reversibly changing the coloring threshold of the photochromic compound (2), the photochromic compound (2) and tetradecylphosphonic acid were mixed at a molar ratio of 1:10. It was examined whether or not the coloration threshold of the mixed substance (hereinafter referred to as PS1) changes depending on the heat treatment temperature. As a result, it was found that the color development threshold shifted to 340 nm in a state where the color was heated to 110 ° C. and then cooled to room temperature. Further, thereafter, in a state of heating to 70 ° C. and then cooling to room temperature, the coloring threshold returned to 380 nm, and it was confirmed that the coloring threshold could be reversibly changed depending on the applied temperature.
[0070]
For the purpose of reversibly changing the photochromic compound (3), the photochromic compound (3) and tetradecylphosphonic acid were mixed at a molar ratio of 1:10. It was examined whether or not the coloration threshold of the mixed substance (hereinafter referred to as PS2) changes depending on the heat treatment temperature. As a result, it was found that the color development threshold shifted to 320 nm when heated to 110 ° C. and then cooled to room temperature. After that, the color development threshold returned to 360 nm when heated to 70 ° C. and then cooled to room temperature, and it was confirmed that the color development threshold could be reversibly changed depending on the applied temperature.
[0071]
Example 1
As Example 1, the structure shown in FIG. 3 was formed by the following procedure.
2- (5-methyl-2-hydroxyphenyl) benzotriazole (hereinafter abbreviated as S1) was used as a light-shielding substance contained in the light-shielding layer. S1 has a property of not transmitting ultraviolet light having a wavelength of 350 nm. As the ultraviolet polarizing filter, a film (hereinafter referred to as P1) formed by dissolving an azobenzene-based dichroic dye having an absorption peak in the ultraviolet region (Direct Yellow 8) in PVA and stretching the film uniaxially was used.
[0072]
First, PC1 was dispersed at 10 wt% in polystyrene, and spin-coated to a thickness of 2 μm on a white polyethylene terephthalate substrate 1 (thickness: 0.5 mm). P1 was adhered thereon using a photo-curing resin, and PS1, S1, and PC3 were each further spin-coated with a material in which 10 wt% was dispersed in polystyrene in a thickness of 2 μm in this order. Finally, a thin film (2 μm) of polyvinyl alcohol was applied as a protective film on the surface of the photosensitive layer to produce a reversible image display medium. In this state, the mixture was heated to 70 ° C. and returned to room temperature.
[0073]
Example 2
With respect to the decolored state of the reversible image display medium manufactured in Example 1, light having a peak wavelength of 370 nm and a half value width of 5 nm (1 mW / cm) extracted from a xenon lamp and an interference filter was used.²) Was transmitted through a polarizing filter and irradiated with light having a polarization orthogonal to the polarization plane of the ultraviolet polarizing filter layer for 100 seconds, so that the color became magenta. This is presumably because PS1 selectively developed color.
[0074]
Example 3
With respect to the decolored state of the reversible image display medium manufactured in Example 1, light having a peak wavelength of 350 nm and a half-width of 5 nm (1 mW / cm) extracted from a xenon lamp and an interference filter was used.²) Was irradiated for 100 seconds and turned yellow. This is probably because PC3 selectively developed color.
[0075]
Example 4
The decolored state of the reversible image display medium manufactured in Example 1 was heated to 110 ° C. and then returned to room temperature. On the other hand, a light having a peak wavelength of 370 nm and a half value width of 5 nm (1 mW / cm) extracted from the xenon lamp and the interference filter.²) Was transmitted through a polarizing filter, and then irradiated with light having a polarization parallel to the polarization plane of the ultraviolet polarizing filter layer for 100 seconds, and turned cyan. This is probably because PC1 selectively developed color.
[0076]
Example 5
As Example 5, the structure of FIG. 5 was formed by the following procedure.
PC1, S1, and PC2 were each dispersed in polystyrene at 10 wt%, and spin-coated in this order on a white polyethylene terephthalate substrate (thickness: 0.5 mm) by 2 μm. P1 was adhered thereon using a photocurable resin, and a material in which PS2 was dispersed at 10 wt% in polystyrene was spin-coated thereon. Finally, a thin film (2 μm) of polyvinyl alcohol was applied as a protective film on the surface of the photosensitive layer to produce a reversible image display medium. In this state, the mixture was heated to 70 ° C. and returned to room temperature.
[0077]
Example 6
With respect to the decolored state of the reversible image display medium manufactured in Example 5, light having a peak wavelength of 350 nm and a half value width of 5 nm (1 mW / cm) extracted from a xenon lamp and an interference filter was used.²) Was transmitted through a polarizing filter, and then irradiated with light having a polarization orthogonal to the polarizing plane of the ultraviolet polarizing filter layer for 100 seconds, and turned yellow. This is probably because PS2 selectively developed color.
[0078]
Example 7
With respect to the decolored state of the reversible image display medium manufactured in Example 5, light having a peak wavelength of 390 nm and a half value width of 5 nm (1 mW / cm) extracted from a xenon lamp and an interference filter was used.²) Was irradiated for 100 seconds and turned cyan. This is probably because PC1 selectively developed color.
[0079]
Example 8
The decolorized state of the reversible image display medium produced in Example 5 was heated to 110 ° C. and then returned to room temperature. On the other hand, light having a peak wavelength of 350 nm and a half width of 5 nm (1 mW / cm) extracted from a xenon lamp and an interference filter.²) Was transmitted through a polarizing filter, and then irradiated with light having a polarization parallel to the polarization plane of the ultraviolet polarizing filter layer for 100 seconds, to obtain a magenta color. This is probably because PC2 selectively developed color.
[0080]
From the above examples, it was confirmed that a photosensitive layer containing three types of photochromic compounds having different color hues can be independently colored using one type of light-shielding layer and two types of light sources.
[0081]
【The invention's effect】
As described above, according to claim 1, in the reversible image display medium for performing multicolor display by laminating photosensitive layers individually containing a plurality of types of photochromic compounds having different color hues, the plurality of types of photosensitive layers Of the at least one photosensitive layer, the color development threshold wavelength of irradiation light to the photochromic compound that transitions from the decolored state to the colored state by light absorption, reversibly by external stimulation to the one type of photosensitive layer A reversible image display medium characterized by comprising a photosensitive layer material that can be changed into two states can be provided with two states, that is, a state in which color is generated for light of one wavelength and a state in which no color is generated. Even when the photosensitive layers are independently colored, the types of light sources used for coloring can be reduced.
[0082]
According to claim 2, in a reversible image display medium for performing multicolor display by laminating photosensitive layers individually containing a plurality of types of photochromic compounds having different color hues, the laminated lowermost photosensitive layer and A polarizing filter layer having a function of transmitting only a specific polarization plane of ultraviolet light is provided between the photosensitive layer positioned or between the uppermost photosensitive layer and the photosensitive layer located thereunder. With the reversible image display medium, the transmission of light can be controlled by rotating the angle of the plane of polarization of incident light, so that only one type of light-shielding layer is used even when three types of photosensitive layers are independently colored, It is possible to increase the degree of freedom for coloring a specific photosensitive layer.
[0083]
According to claim 3, in claim 2, at least one of the plurality of types of photosensitive layers is reversibly changed in color erasing threshold wavelength by an external stimulus in a decolored state. The color development according to claim 1, wherein at least one of the three types of photosensitive layers is in a decolored state by a reversible image display medium characterized in that the photosensitive layer is made of a photosensitive layer material that can be formed. The feature that the threshold wavelength is reversibly changed by an external stimulus makes it possible to have two states, that is, a state in which one wavelength of light is colored and a state in which it is not colored, and is incident. Since the transmission of light can be controlled by the difference in the polarization plane of light, even when three types of photosensitive layers are independently colored, the types of light sources used for coloring can be reduced, and the light shielding used can be reduced. There it is possible to requires only one type.
[0084]
According to the fourth aspect, in the reversible image display medium, the color development threshold wavelength λ of the irradiation light to the lowermost photosensitive layer is set.₁Is the color development threshold wavelength λ of the photosensitive layer on the top₃Wavelength between the uppermost photosensitive layer and the middle photosensitive layer.₁And λ₃At least some wavelengths in the intermediate wavelength range of₃A light-shielding layer having a property of shielding light of at least a part of the wavelength in a shorter wavelength region is provided, and only a specific polarization plane of ultraviolet light is provided between the lowermost photosensitive layer and the photosensitive layer located in the middle. A polarizing filter layer having a function of transmitting light is provided, and the photosensitive layer located in the middle between the uppermost and lowermost photosensitive layers has a wavelength of λ due to external stimulation.₃Longer, wavelength λ₁Shorter wavelength X₁For the wavelength X₁Longer color development threshold wavelength and wavelength X₁4. The light-reversible image display medium according to claim 1, wherein the light-sensitive layer is a photosensitive layer that can be reversibly changed between a state of a shorter coloring threshold wavelength and a light-shielding layer. In this configuration, only three types of photosensitive layers can be independently colored, and the number of light sources used for coloring can be reduced.
[0085]
According to claim 5, in claim 4, the color development threshold wavelength of the photosensitive layer located in the intermediate portion is set to the wavelength X.₁Longer than the wavelength X having a polarization plane that does not transmit through the lowermost photosensitive layer and the polarizing filter layer located in the middle.₁A first light irradiation step of irradiating only the photosensitive layer located in the intermediate portion by irradiating the ultraviolet ray, the color development threshold wavelength of the photosensitive layer located in the intermediate portion is set to the X₁Shorter than the wavelength X having a polarization plane transmitting through the ultraviolet polarization filter layer.₁A second light irradiation step of irradiating the ultraviolet light to only develop a color in the photosensitive layer located at the lowermost part;₃Light X in a shorter wavelength range than₂Irradiating the photosensitive layer selectively containing three types of photochromic compounds by three light irradiating steps of a third light irradiating step in which only the uppermost photosensitive layer is colored by irradiating the photosensitive layer. According to the image forming method, the three types of photosensitive layers can be independently colored.
[0086]
According to the sixth aspect, in the reversible image display medium according to the first to third aspects, the coloring threshold wavelength λ of the lowermost photosensitive layer is set.₁Is the coloration threshold wavelength λ of the photosensitive layer located in the middle₂Longer than the wavelength, between the uppermost photosensitive layer and the photosensitive layer located in the middle there is provided an ultraviolet polarizing filter layer having a function of transmitting only a specific polarization plane of ultraviolet light, the lowermost photosensitive layer and Λ between the photosensitive layers located in the middle₁Shorter than λ₂Transmits light of at least some wavelengths within a longer wavelength range than λ₂A light-blocking layer having a property of blocking light of at least a part of the wavelength in a shorter wavelength region is provided, and the uppermost photosensitive layer has a wavelength of λ.₂Wavelength X shorter than₃The color development threshold wavelength is X₃Longer state and X₃The reversible image display medium, which changes reversibly during a shorter state due to an external stimulus, enables the three types of photosensitive layers to be independently colored even in a configuration having only one light-shielding layer. In addition, it is possible to reduce the number of light sources used for coloring.
[0087]
According to claim 7, in claim 6, the color development threshold wavelength of the uppermost photosensitive layer is set to the X threshold.₃Shorter than the wavelength X having a polarization plane transmitting the polarization filter layer.₃The first light irradiation step of irradiating only the photosensitive layer located in the intermediate portion by irradiating the light, the color development threshold wavelength of the photosensitive layer located in the uppermost portion is X₃Longer than the wavelength X having a polarization plane that does not transmit through the ultraviolet polarization filter layer.₃A second light irradiation step of irradiating shorter ultraviolet rays to develop a color only in the uppermost photosensitive layer;₁Shorter than λ₂Light X in a longer wavelength range than₄Irradiating the photosensitive layer selectively containing three types of photochromic compounds by three light irradiating steps of a third light irradiating step in which only the lowermost photosensitive layer is colored by irradiating the photosensitive layer. According to the image forming method, the three types of photosensitive layers can be independently colored in the structure of the sixth aspect.
[0088]
According to the eighth aspect, the photosensitive layer containing the photochromic compound having the characteristic that the color development threshold wavelength used in the first to seventh aspects is reversibly changed is chemically interacted with the photochromic compound by an external stimulus. The use of a reversible image display medium characterized in that it contains a compound that causes the following problems, without having a complicated structure, the type of light source used for coloring even when three types of photosensitive layers are independently colored. It is possible to reduce the number and to use only one kind of light-shielding layer.
[0089]
According to claim 9, according to claim 8, the compound causing a chemical interaction with the photochromic compound is an electron-accepting compound or an electron-donating compound. A change occurs, and it is possible to easily create two states, ie, a state in which no color is generated even when light of one wavelength is used and a state in which color is generated.
[0090]
According to the tenth aspect, in the eighth aspect, the external stimulus is a thermal stimulus, and the external stimulus is a thermal stimulus. It is possible to provide external stimuli with simple components.
[Brief description of the drawings]
FIG. 1 is a cross-sectional view showing a configuration of an image display medium shown in a conventional example.
FIG. 2 is an explanatory diagram for explaining a color forming process of each layer of a conventional image display medium.
FIG. 3 is a cross-sectional view illustrating a configuration of a reversible image display medium of the present invention.
FIG. 4 is an explanatory diagram illustrating a color forming process of each layer of the reversible image display medium of the present invention.
FIG. 5 is a sectional view showing another example of the configuration of the reversible image display medium of the present invention.
FIG. 6 is an explanatory diagram of another example of the color forming process of each layer of the reversible image display medium of the present invention.
[Explanation of symbols]
1 substrate
2 Photosensitive layer containing photochromic compound (1)
3 Shading layer
4 Photosensitive layer containing photochromic compound (2)
5 Shading layer
6. Photosensitive layer containing photochromic compound (3)
7 Polarizing filter layer
8 Photosensitive layer containing photochromic compound (2) having reversible coloration threshold wavelength
9 Photosensitive layer containing photochromic compound (3) having reversible color development threshold wavelength
11 Photochromic compound (1)
12 Photochromic compound (2)
13 Photochromic compound (3)
14 Step 14
15 Step 15
16 Step 16
17 Photochromic compound having a reversible coloration threshold wavelength (2) Photosensitive layer
18 Photochromic compound having a reversible coloration threshold wavelength (3) Photosensitive layer
19 Shading layer
20 Shading layer

Claims (10)

In a reversible image display medium that performs multicolor display by laminating photosensitive layers individually containing a plurality of types of photochromic compounds having different color hues, at least one of the plurality of types of photosensitive layers has light absorption. The color development threshold wavelength of the irradiation light to the photochromic compound that transitions from the decolored state to the color development state by, comprising a photosensitive layer material that can be reversibly changed by an external stimulus to the one type of photosensitive layer. Reversible image display medium. In a reversible image display medium that performs multicolor display by laminating photosensitive layers individually containing a plurality of types of photochromic compounds having different color hues, between a laminated lowermost photosensitive layer and a photosensitive layer located thereon. Alternatively, a reversible image display medium characterized in that a polarizing filter layer having a function of transmitting only a specific polarization plane of ultraviolet light is provided between the uppermost photosensitive layer and the photosensitive layer located thereunder. In claim 2, at least one of the plurality of types of photosensitive layers is formed of a photosensitive layer material capable of reversibly changing a coloration threshold wavelength according to claim 1 in an erased state by an external stimulus. A reversible image display medium, which is a photosensitive layer. In the reversible image display medium, color threshold wavelength lambda ₁ of the light irradiated to the bottom of the photosensitive layer has a longer wavelength than the color threshold wavelength lambda ₃ of the light-sensitive layer to the top, the photosensitive layer and the intermediate portion of the top Between the photosensitive layers located, light of at least some wavelengths in a wavelength region between wavelengths λ ₁ and λ ₃ is transmitted, and light of at least some wavelengths in a wavelength region shorter than wavelength λ ₃ is blocked. A light-shielding layer having the property of providing a characteristic is provided, and a polarizing filter layer having a function of transmitting only a specific polarization plane of ultraviolet light is provided between a lowermost photosensitive layer and a photosensitive layer located at an intermediate portion. a photosensitive layer located in the middle portion of the bottom of the photosensitive layer, by external stimulation, longer than the wavelength lambda _3, with respect to short wavelength X ₁ than the wavelength lambda _1, the long color threshold wavelength than the wavelength X ₁ state reversible between a state of short color threshold wavelength than X ₁ and Reversible image display medium according to any one of claims 1 3, characterized in that it is possible to change a photosensitive layer possible. In claim 4, the color threshold wavelength of the photosensitive layer located in the middle part had been a long state than the wavelength X _1, the plane of polarization is not transmitted through the polarization filter layer located on the photosensitive layer and the intermediate portion of the bottom the first light irradiation step of coloring only the photosensitive layer located in the middle part, shorter state than the color threshold wavelength wherein X ₁ of the photosensitive layer located on the intermediate portion by irradiation of ultraviolet light of wavelength X ₁ having a second light irradiation step of coloring only the photosensitive layer located on the bottom by irradiation of ultraviolet light of wavelength X ₁ having a polarization plane which transmits the ultraviolet polarizing filter layer leave this, than the lambda ₃ the three light irradiation step of the third light irradiation step of coloring only the photosensitive layer located on the top by irradiating light X ₂ short wavelength region, the individual three photochromic compounds Image forming method characterized by selectively coloring laden photosensitive layer. In the reversible image display medium according to claims 1 to 3, coloration threshold wavelength lambda ₁ of the bottom of the photosensitive layer wavelength than color threshold wavelength lambda ₂ of the light-sensitive layer positioned intermediate portion is long, the top of the photosensitive layer An ultraviolet polarizing filter layer having a function of transmitting only a specific polarization plane of ultraviolet light is provided between the photosensitive layer located in the middle part and the photosensitive layer located in the middle part between the bottom photosensitive layer and the photosensitive layer located in the middle part. shielding layer is provided having a characteristic of shielding light in at least part of the wavelength of at least part of a wavelength region shorter than the transmittance and lambda ₂ light of wavelengths longer wavelength region than the shorter lambda ₂ than lambda ₁ and reversibly changed by external stimuli between color threshold wavelength of the photosensitive layer is λ wavelength X ₃ wavelength is shorter than ₂ positioned at the top of a short state longer state and X ₃ than X ₃ A reversible image display medium. According to claim 6, by irradiating leave the color threshold wavelength of the photosensitive layer located on top short state than the X _3, light having a wavelength X ₃ having a plane of polarization that passes through the polarizing filter layer first light irradiation step of coloring only the photosensitive layer located on the intermediate portion, said color threshold wavelength of the photosensitive layer at the top are kept in the longer state than said X _3, the UV polarization filter layer a second light irradiation step of coloring only the photosensitive layer located on the top by irradiating a shorter than the wavelength X ₃ having a polarization plane which does not transmit ultraviolet rays, wavelength region longer than shorter also lambda ₂ than the lambda ₁ feeling of the three light irradiation step of the third light irradiation step of coloring only the photosensitive layer located on the bottom by irradiating light X _4, including three photochromic compounds individually Image forming method characterized by selectively coloring the layer. 8. A photosensitive layer containing a photochromic compound having a feature that the color development threshold wavelength reversibly changes as used in claim 1 to 7, contains a compound that causes a chemical interaction with the photochromic compound by an external stimulus. Reversible image display medium. 9. The reversible image display medium according to claim 8, wherein the compound causing a chemical interaction with the photochromic compound is an electron accepting compound or an electron donating compound. 9. The image forming method according to claim 8, wherein the external stimulus is a thermal stimulus.

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