Patent Publication Number: US-2005134580-A1

Title: Display device

Description:
BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The present invention relates to a display device, and more particularly to a display device making use of the movement of a penetrant under the influence of an electric field.  
      2. Description of the Related Art  
      There is known a display device that performs display by changing the reflection, refraction, and scattering of light incident on a porous body, utilizing the movement of a penetrant into the porous body across which a difference of potential is applied. For example, as shown in  FIGS. 11A and 11B , such a display device is made up of a transparent porous panel  82  with a great number of grooves  81  formed in the screen; top electrodes  83   a  (which are also used as light-intercepting plates) disposed on the top surface of the porous panel  82 ; and transparent bottom electrodes  83   b  disposed on the bottom surface of the porous panel  82  (see Japanese Unexamined Patent Publication No. 56(1981)-88177). This display device forms an electric field in the direction of the thickness of the porous panel  82  by applying a potential difference between the top and bottom electrodes. This causes the display device to change the reflection, refraction, and scattering of light L e  incident on the side of the top electrodes  83   a  or on the side of the bottom electrodes  83   b , by switching between a supply state in which a penetrant L 5  is moved toward the top electrodes  83   a  and penetrates into the spaces  84  surrounded by the grooves  81  (see  FIG. 11A ), and a discharge state in which the penetrant L 5  is discharged from the spaces  84  and moved toward the bottom electrodes  83   b  (see  FIG. 11B ). In this manner, display is performed.  
      However, because there is no possibility that light incident on the surface, on which the top electrodes  83  are disposed and in which the grooves  81  are not formed, will be utilized for display, it is difficult for the above-described display device to have sufficient contrast. In addition, in the display device, no electric field is formed in the spaces  84  where the porous body of the porous panel  82  is not present, so the control of the penetrant L 5  in the grooved surface portions is reduced. Because of this, there are cases where the spaces  84  are not sufficiently filled with the penetrant L 5 , or discharge of the penetrant L 5  from the spaces  84  is not sufficient. As a result, the reflection, refraction, and scattering of light L e  incident on the spaces  84  are reduced, and contrast is decreased.  
     SUMMARY OF THE INVENTION  
      The present invention has been developed in view of the circumstances described above. Accordingly, it is the primary object of the present invention to provide a display device that is capable of enhancing contrast.  
      To achieve this end, there is provided a display device comprising (1) a transparent porous layer, formed from a transparent material, which has pores into which a penetrant is penetrable; (2) a colored layer stacked on one surface of the transparent porous layer; and (3) penetrant-moving means for performing penetration of the penetrant into the transparent porous layer or discharge of the penetrant from the transparent porous layer. After the penetration of the penetrant into the transparent porous layer is performed by the penetrant-moving means, the transparent porous layer becomes transparent and therefore the colored layer is displayed through the transparent porous layer. After the discharge of the penetrant from the transparent porous layer performed by the penetrant-moving means, the transparent porous layer becomes opaque and therefore the transparent porous layer is displayed.  
      In the display device according to the present invention, the aforementioned colored layer may be a colored porous layer with pores into which the penetrant is penetrable. When the penetrant penetrates into the transparent porous layer, the colored porous layer supplies the penetrant from the colored porous layer to the transparent porous layer. When the penetrant is discharged from the transparent porous layer, the colored porous layer absorbs the discharged penetrant.  
      In the display device according to the present invention, the aforementioned penetrant-moving means may be made up of electrodes, which are respectively disposed on the top and bottom surfaces of a stacked structure comprising the transparent porous layer and colored porous layer; and a potential difference application part for applying a potential difference between the electrodes disposed on the top and bottom surfaces of the stacked structure. The aforementioned penetrant-moving means may also be made up of electrodes, which are respectively disposed on the top and bottom surfaces of the colored porous layer; and a potential difference application part for applying a potential difference between the electrodes disposed on the top and bottom surfaces of the colored porous layer. Furthermore, the aforementioned penetrant-moving means may be made up of electrodes, which are respectively disposed on the top and bottom surfaces of the transparent porous layer; and a potential difference application part for applying a potential difference between the electrodes disposed on the top and bottom surfaces of the transparent porous layer.  
      In the display device according to the present invention, capillary attraction, which is produced in the colored porous layer when the penetrant penetrates into the colored porous layer, may be made greater than that of the transparent porous layer.  
      The display device of the present invention may further include a transparent penetrant storage layer, interposed between the transparent porous layer and the colored layer, for storing the penetrant. When the penetrant penetrates into the transparent porous layer, the penetrant storage layer supplies the penetrant from the penetrant storage layer to the transparent porous layer. When the penetrant is discharged from the transparent porous layer, the penetrant storage layer stores the discharged penetrant. Moreover, the aforementioned penetrant-moving means may be made up of eletrodes, which are respectively disposed on the top and bottom surfaces of the transparent porous layer; and a potential difference application part for applying a potential difference between the electrodes disposed on the top and bottom surfaces of the transparent porous layer.  
      The aforementioned penetrant storage layer may be of any type, so long as it is in the form of a layer capable of storing a penetrant. It may also be in the form of a container to store a penetrant. Moreover, it may be formed from a foaming resin material.  
      In the display device according to the present invention, a pore diameter in the transparent porous layer can be made greater the further it is away from the colored layer in the direction of the thickness of the two layers. Note that if the pore diameter becomes greater the further it is away from the colored layer in the thickness direction, it means that capillary attraction becomes smaller the further it is away from the colored layer in the thickness direction.  
      It is preferable that a pore diameter in the transparent porous layer be between 0.1 μm and 10 μm. When the colored layer is the colored porous layer, it is preferable that a pore diameter in the colored porous layer be between 0.1 μm and 10 μm. Note that the expression “a pore diameter is between 0.1 μm and 10 μm” as used herein is not limited to the case where the pore diameter of all pores in the transparent porous layer and colored porous layer is between 0.1 μm and 10 μm. However, it is necessary that the pore diameter of 90% or more of all pores in the transparent porous layer and colored porous layer be between 0.1 μm and 10 μm.  
      In the display device according to the present invention, the aforementioned colored layer may be colored black.  
      The aforementioned penetrant-moving means may exert the Coulomb forces of different magnitudes on the penetrant in the thickness direction at different positions on a plane perpendicular to the thickness direction. Also, the aforementioned colored layer may be colored in different hues at different positions on a plane perpendicular to the thickness direction. The different hues maybe yellow, magenta, cyan, and black.  
      In the display device of the present invention, a main material constituting the aforementioned transparent porous layer may be cellulose nitrate, acetyl cellulose, cellulose acetate, vinyl chloride, polypropylene, polyamide, polytetrafluorethylene, polyolefin, polysulfone, glass fiber, or alumina. Also, a main material constituting the colored porous layer may be cellulose nitrate, acetyl cellulose, cellulose acetate, vinyl chloride, polypropylene, polyamide, polytetrafluorethylene, polyolefin, polysulfone, glass fiber, or alumina.  
      The aforementioned transparent porous layer may be formed from a mixture of any two or more of resin, ceramic, and glass materials. Also, the aforementioned colored porous layer may be formed from a mixture of any two or more of resin, ceramic, and glass materials.  
      The expression “the colored layer is displayed” as used herein is intended to mean that the colored layer is visible to the naked eye through the transparent porous layer.  
      The aforementioned colored layer is not limited to the case where it is colored in various hues. That is, it may be colored black or gray. Furthermore, the colored layer is not limited to the case where it is formed by being colored. For instance, a material that constitutes the colored layer may assume the aforementioned color.  
      The aforementioned transparent porous layer is not always colorless and transparent, but may assume any color if it is transparent.  
      Note that the electrodes disposed on the transparent porous layer are constructed so that when the transparent porous layer is made transparent and the colored layer is displayed, the colored layer is visible to the naked eye through the transparent porous layer. For instance, these electrodes may be electrodes arranged in the form of a mesh, or transparent electrodes.  
      Also, the electrodes between the aforementioned transparent porous layer and colored porous layer, or between the transparent porous layer and penetrant storage layer are disposed so the penetrant can pass through them.  
      As set forth above, the display device of the present invention comprises the transparent porous layer, formed from a transparent material, which has pores into which a penetrant is penetrable; the colored layer stacked on one surface of the transparent porous layer; and the penetrant-moving means for performing penetration of the penetrant into the transparent porous layer or discharge of the penetrant from the transparent porous layer. After the penetration of the penetrant into the transparent porous layer performed by the penetrant-moving means, the transparent porous layer becomes transparent and the colored layer is displayed through the transparent porous layer. And after the discharge of the penetrant from the transparent porous layer performed by the penetrant-moving means, the transparent porous layer becomes opaque and the transparent porous layer is displayed. Therefore, by making the entire surface of the transparent porous layer opaque or transparent, the colored layer can be displayed. Thus, the rate of an area where the reflection, refraction, and scattering of light occur can be increased compared to the prior art, so contrast can be enhanced. In addition, the Coulomb force can be exerted on a penetrant in the entire surface of the aforementioned transparent porous layer, so control in moving the penetrant can also be enhanced.  
      According to the present invention, the colored layer is a colored porous layer with pores into which the penetrant is penetrable. When the penetrant penetrates into the transparent porous layer, the colored porous layer supplies the penetrant from the colored porous layer to the transparent porous layer. When the penetrant is discharged from the transparent porous layer, the colored porous layer absorbs the discharged penetrant. Therefore, the supply and discharge of the penetrant with respect to the transparent porous layer can be easily performed.  
      According to the present invention, the penetrant-moving means is made up of electrodes, which are respectively disposed on the top and bottom surfaces of a stacked structure comprising the transparent porous layer and colored porous layer; and a potential difference application part for applying a potential difference between the electrodes disposed on the top and bottom surfaces of the stacked structure. Or, the penetrant-moving means is made up of electrodes, which are respectively disposed on the top and bottom surfaces of the colored porous layer; and a potential difference application part for applying a potential difference between the electrodes disposed on the top and bottom surfaces of the colored porous layer. Therefore, the Coulomb force can be reliably exerted on the penetrant.  
      If the capillary attraction, which is produced in the colored porous layer when the penetrant penetrates into the colored porous layer, is greater than that of the transparent porous layer, the penetrant is discharged from the transparent porous layer when no Coulomb force is exerted on the penetrant by the penetrant-moving means. This state, in which the transparent porous layer becomes opaque after the movement of the penetrant into the colored porous layer, can be employed as a default state.  
      According to the present invention, the display device is further equipped with a transparent penetrant storage layer, interposed between the transparent porous layer and the colored layer, for storing the penetrant. When the penetrant penetrates into the transparent porous layer, the penetrant storage layer supplies the penetrant from the penetrant storage layer to the transparent porous layer. When the penetrant is discharged from the transparent porous layer, the penetrant storage layer stores the discharged penetrant. Therefore, the supply and discharge of the penetrant with respect to the transparent porous layer can be readily performed. In addition, according to the present invention, the penetrant-moving means is made up of electrodes, which are respectively disposed on the top and bottom surfaces of the transparent porous layer; and a potential difference application part for applying a potential difference between the electrodes disposed on the top and bottom surfaces of the transparent porous layer. This makes it possible to exert the Coulomb force on the penetrant more reliably.  
      According to the present invention, a pore diameter in the transparent porous layer is made greater the further it is away from the colored layer in the direction of the thickness of the two layers. For example, when the transparent porous layer is displayed by discharging the penetrant from the transparent porous layer, and when the colored porous layer is displayed by penetration of the penetrant into the transparent porous layer, a change in display can be made appropriate. Therefore, when a specified brightness is shifted to a brightness different from that brightness, the afterimage of the display can be optimized. Also, when the colored layer is a colored porous layer with pores into which a penetrant is penetrable, a region of the transparent porous layer in contact with the colored layer plays a role of preventing the moving time of the penetrant from being too short when the penetrant in the colored layer penetrates into the transparent porous layer during application of the Coulomb force, if a pore diameter in that contact region is made smallest and the thickness of a region whose pore diameter is smaller than that of the colored layer is made sufficiently thinner than that of the colored layer. In addition, when no Coulomb force is exerted, the penetrant in the transparent porous layer plays a role of resistance in penetrating into the colored layer and can prolong the moving time of the penetrant. Thus, display is able to have an afterimage (memory effect).  
      According to the present invention, a pore diameter in the transparent porous layer is between 0.1 μm and 10 μm. When the colored layer is the colored porous layer, a pore diameter in the colored porous layer is between 0.1 μm and 10 μm. Therefore, penetration and discharge of the penetrant with respect to the transparent porous layer and colored porous layer can be more easily performed.  
      According to the present invention, the penetrant-moving means is able to exert the Coulomb forces of different magnitudes on the penetrant in the thickness direction at different positions on a plane perpendicular to the thickness direction. This makes it possible to change brightness at different positions. For example, images can be displayed. In addition, the colored layer can be colored in different hues at different positions on a plane perpendicular to the thickness direction. This renders it possible to display color images. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The present invention will be described in further detail with reference to the accompanying drawings wherein:  
       FIG. 1  is a perspective view showing a display device constructed in accordance with a first embodiment of the present invention;  
       FIG. 2  is an exploded perspective view showing a construction for exerting the Coulomb force on a penetrant;  
       FIG. 3A  is a perspective view showing another construction for exerting the Coulomb force on the penetrant;  
       FIG. 3B  is a perspective view showing the construction of  FIG. 3A  in further detail;  
       FIG. 3C  is a diagram showing a circuit for exerting the Coulomb force on the penetrant;  
       FIG. 4A  is a perspective view showing the state in which white is displayed by the display device shown in  FIG. 1 ;  
       FIG. 4B  is a perspective view showing the state in which black is displayed by the display device shown in  FIG. 1 ;  
       FIG. 4C  is a perspective view showing the state in which gray is displayed by the display device shown in  FIG. 1 ;  
       FIGS. 5A and 5B  are side sectional views showing a first modification of the display device shown in  FIG. 1 ;  
       FIGS. 6A and 6B  are side sectional views showing a second modification of the display device shown in  FIG. 1 ;  
       FIGS. 7A and 7B  are side sectional views showing a third modification of the display device shown in  FIG. 1 ;  
       FIG. 8  is a side sectional view showing a modification of the display device of  FIG. 7  provided with partition walls;  
       FIG. 9A  is a side sectional view showing the initial state of a display device constructed in accordance with a second embodiment of the present invention;  
       FIG. 9B  is a side sectional view showing the state in which black is displayed by the display device shown in  FIG. 9A ;  
       FIG. 9C  is a side sectional view showing the state in which white is displayed by the display device shown in  FIG. 9A ;  
       FIG. 9D  is a side sectional view showing the state in which gray is displayed by the display device shown in  FIG. 9A ;  
       FIG. 10A  is a side sectional view showing the initial state of a display device constructed in accordance with a third embodiment of the present invention;  
       FIG. 10B  is a side sectional view showing the state in which gray is displayed by the display device shown in  FIG. 10A ;  
       FIG. 10C  is a side sectional view showing the state in which black is displayed by the display device shown in  FIG. 10A ; and  
       FIGS. 11A and 11B  are side sectional views showing a conventional display device. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
      Referring to  FIGS. 1 through 4 , there is shown a display device  101  constructed in accordance with a first embodiment of the present invention.  
      The display device  101  is formed from a transparent material, and includes a transparent porous layer  10  with pores into which a penetrant L 1  is penetrable; a colored layer  20  stacked on one surface of the transparent porous layer  10 ; and penetrant-moving means  30  for exerting the Coulomb force on the penetrant L 1  to perform penetration of the penetrant L 1  into the transparent porous layer  10  or discharge of the penetrant L 1  from the transparent porous layer  10 .  
      In the display device  101 , when the penetrant L 1  penetrates into the transparent porous layer  10 , the transparent porous layer  10  becomes transparent and therefore the colored porous layer  20  is displayed through the transparent porous layer  10 . On the other hand, when the penetrant L 1  is discharged from the transparent porous layer  10 , the transparent porous layer  10  becomes opaque and therefore the transparent porous layer  10  is displayed. The above-described display is performed by illuminating the transparent porous layer  10  of the display device  101  with light L a . The surface of the display device  101  illuminated with light L a  will hereinafter be referred to as a screen.  
      The colored porous layer  20  is formed from a porous body into which the penetrant L 1  is penetrable. When the penetrant L 1  penetrates into the transparent porous layer  10  by the Coulomb force exerted on the penetrant L 1  by the penetrant-moving means  30 , the colored porous layer  20  supplies the penetrant L 1  in the colored porous layer  20  to the transparent porous layer  10 . When the penetrant L 1  is discharged from the transparent porous layer  10 , the colored porous layer  20  absorbs the discharged penetrant L 1 . The colored porous layer  20  can employ cellulose or acetyl cellulose. The colored porous layer  20  is colored black. For example, it can be colored by employing color ink, carbon black, etc.  
      The penetrant-moving means  30  consists of top electrodes  31   a , bottom electrodes  31   b , and a power supply  35 . The top electrodes  31   a  are disposed on the top surface of a stacked structure  15  consisting of the transparent porous layer  10  and colored porous layer  20 . The bottom electrodes  31   b  are disposed on the bottom surface of the stacked structure  15 . The power supply  35  is a potential difference application part for applying a potential difference between the top electrodes  31   a  and the bottom electrodes  31   b . The penetrant-moving means  30  exerts the Coulomb force on the penetrant L 1  by forming an electric field in the direction of the thickness of the stacked structure  15 . Note that the top electrodes  31   a  are arranged on the side of the transparent porous layer  10 . Also, the bottom electrodes  31   b  are arranged on the side of the colored porous layer  20 .  
      The top stripe electrodes  31   a  disposed on one surface (screen side) of the stacked structure  15 , and the bottom stripe electrodes  31   b  disposed on the other surface of the stacked structure  15 , are disposed to cross one another across the stacked structure  15 . More specifically, the top stripe electrodes  31   a  and bottom stripe electrodes  31   b  are disposed on a top electrode-mounting panel  30   a  and bottom electrode-mounting panel  30   b , which are in turn disposed on the top and bottom surfaces of the stacked structure  15  (see  FIG. 2 ).  
      Each of the above-described stripe electrodes may use an electrode formed from a transparent material such as In 2 O 3 -SnO 2  (ITO) and ZnO. Also, the bottom stripe electrodes  31   b  may use electrodes formed from an opaque material such as Ni, Al, Pt, Ag, black lead, etc. Furthermore, the bottom stripe electrodes  31   b  may use transparent electrodes that are colored. The above-described electrodes may be formed, for instance, by a thin-film formation method such as sputtering. The thin-film formation method is capable of forming electrodes on the surfaces of the transparent porous layer  10  and colored porous layer  20  so the penetrant can pass through, without closing the pores in the two layers.  
      The penetrant-moving means  30  is capable of exerting the Coulomb forces of different magnitudes in the thickness direction (indicated by arrow Z) on the penetrant L 1  present at positions different from one another in the layer direction (indicated by arrows X and Y). The penetrant-moving means  30  adopts a passive drive method of exerting the Coulomb forces of different magnitudes on the penetrant L 1  present at different positions, through the top and bottom stripe electrodes  31   a  and  31   b  arranged in the form of a lattice with respect to the stacked structure  15 . The different positions on the plane perpendicular to the thickness direction are positions at which the top stripe electrodes  31   a  and bottom stripe electrodes  31   b  cross one another in the form of a lattice. For example, these positions are indicated by position ( 1 ,  1 ), position ( 2 ,  1 ), position ( 3 ,  1 ), position ( 1 ,  2 ), position ( 2 ,  2 ), position ( 3 ,  2 ), etc.  
      Note that the penetrant-moving means  30  may adopt a different method than the above-described drive method. For instance, the penetrant-moving means  30  may adopt an active drive method, as shown in  FIGS. 3A  to  3 C. In the active drive method, an electrode panel  33  is mounted on one surface side (e.g., screen side) of the above-described stacked structure  15  and has a great number of electrodes  35 , which are disposed in the form of a matrix and connected to thin-film transistors  34 . The other surface side of the stacked structure  15  is grounded. The Coulomb forces different from one another are exerted on the penetrant L 1  present at different positions, through the electrodes  35 , by driving each thin-film transistor  34 .  
      With adoption of the above-described penetrant-moving means, the colored porous layer  20  or transparent porous layer  10  can be displayed at an arbitrary position on the screen of the display device. This device is able to display a still picture image such as characters, figures, etc., and a motion picture image.  
      The penetrant L 1  may be, for example, dimethyltriphenyltrimethoxysiloxane having a refractive index of 1. 51 (see Japanese Unexamined Patent Publication Nos. 60(1985)-6928 and 5(1993)-246021).  
      It is preferable that the transparent porous layer  10  employ a microporous membrane filter formed from a transparent resin material. A main material constituting the transparent resin material may be vinyl chloride, polypropylene, polyamide, polytetrafluorethylene, polyolefin, polysulfone, etc. It may also be a cellulosic material (such as cellulose nitrate, acetyl cellulose, and cellulose acetate), a ceramic material (such as alumina), glass fabric, etc. Furthermore, it may be a mixture of any two or more of resin, cellulosic, glass, and ceramic materials. Similarly, the colored porous layer  20  maybe formed by the same materials as the aforementioned materials.  
      In this embodiment, the transparent porous layer  10  is colorless and transparent. However, it maybe a colored layer if it is transparent.  
      It is preferable that the refractive index of the transparent porous layer  10  be close to that of the penetrant L 1 . That is, if both refractive indices are close to each other, the scattering, etc., of light passing through the transparent porous layer  10  can be reduced when the penetrant L 1  has penetrated into the transparent porous layer  10 , so the transparency of the transparent porous layer  10  into which the penetrant L 1  has penetrated can be enhanced.  
      The capillary attraction of the colored porous layer  20  is greater than that of the transparent porous layer  10 . When there is no potential difference between the top electrodes  31   a  and the bottom electrodes  31   b , the penetrant L 1  penetrates into the colored porous layer  20  whose capillary attraction is greater, and becomes stable. The capillary attraction can be expressed by the following Equation:
 
 P =(2σcos θ)/ r 
 
 in which 
 
      P: capillary attraction (Pa),  
      σ: surface tension (N/m),  
      θ: angle of contact between the liquid and the capillary interior wall (°),  
      r: radius of the capillary (mm).  
      In the above-described Equation, the capillary corresponds to a pore in the above-described porous layer. For example, when the above-described surface tension a and the angle of contact θ are the same between the transparent porous layer  10  and the colored porous layer  20 , and the transparent porous layer  10  and the colored porous layer  20  are uniform in pore diameter, the capillary attraction of the colored porous layer  20  can be made greater than that of the transparent porous layer  10 , if the pore diameter of the colored porous layer  20  is made smaller than that of the transparent porous layer  10 .  
      It is preferable that the pore diameters of the transparent porous layer  10  and colored porous layer  20  be between 0.1 μm and 10 μm and further preferable that they be between 0.1 μm and 5 μm. If the pore diameter becomes small, the capillary attraction becomes great. As a result, since the resistance of the pore is increased when the penetrant is moved, the power supply  35  must consume more power when applying a potential difference between the above-described electrodes. On the other hand, if the pore diameter becomes great, light scattering at the transparent porous layer  10  is reduced and brightness is decreased, when the penetrant L 1  is discharged from the transparent porous layer  10  to make the transparent porous layer  10  opaque.  
      It is also preferable that the thickness of the transparent porous layer  10  be between 10 μm and 200 μm. If the thickness is decreased, light passes through the transparent porous layer  10  even when the penetrant L 1  is discharged from the transparent porous layer  10 , and it becomes difficult to display the transparent porous layer  10  accurately. On the other hand, if the thickness is greater than 200 μm, the amount of the penetrant L 1  to be moved in and out of the transparent porous layer  10  is increased and therefore the time required for moving the penetrant L 1  becomes long.  
      Now, a description will be given of the operation of the display device constructed in accordance with the first embodiment of the present invention.  
      In the initial state where there is no potential difference across the stacked structure  15  consisting of the transparent porous layer  10  and colored porous layer  20 , the penetrant L 1  is discharged from the transparent porous layer  10  and penetrates into the colored porous layer  20  whose capillary attraction is great, as shown in  FIG. 4A . In this state, if the transparent porous layer  10  is illuminated with light L a , the light L a  is scattered at the transparent porous layer  10  and therefore the transparent porous layer  10  is displayed as opaque white.  
      When displaying black, the power supply  35  applies a potential difference between the top electrodes  31   a  and the bottom electrodes  31   b , as shown in  FIG. 4B . For example, when the potential difference is applied at positions ( 1 ,  1 ) and ( 3 ,  1 ), electric fields are formed in the corresponding portions of the stacked structure  15  in the thickness direction. As a result, the penetrant L 1  in the colored porous layer  20  is moved into the transparent porous layer  10  by the Coulomb force, so the transparent porous layer  10  becomes transparent. At the positions ( 1 ,  1 ) and ( 3 ,  1 ), the colored porous layer  20  is displayed through the transparent porous layer  10  that became transparent by the illumination of light L a . That is, black, which is the color of the colored porous layer  20 , is displayed at the positions ( 1 ,  1 ) and ( 3 ,  1 ). On the other hand, at positions ( 2 ,  1 ), ( 1 ,  2 ), ( 2 ,  2 ), and ( 3 ,  2 ) where there is no potential difference between the top electrodes  31   a  and the bottom electrodes  31   b , the penetrant L 1  remains stayed in the colored porous layer  20  and therefore the transparent porous layer  10  is displayed.  
      When again displaying the entire surface of the screen in white, the application of the potential difference between the top electrodes  31   a  and the bottom electrodes  31   b  by the power supply  35  is stopped (potential difference is made zero). As a result, the penetrant L 1  is moved from the transparent porous layer  10  into the colored porous layer  20  by capillary attraction, and in the same manner as described above, the transparent porous layer  10  is displayed in white (see  FIG. 4A ).  
      When displaying gray, a potential difference, smaller than the potential difference applied in displaying black, is applied between the top electrodes  31   a  and the bottom electrodes  31   b  by the power supply  35 . For example, when the smaller potential difference is applied at positions ( 1 ,  1 ) and ( 3 ,  1 ) in  FIG. 4C , electric fields are formed in the direction of the thickness of the stacked structure  15 , and the penetrant L 1  in the colored porous layer  20  is moved into the transparent porous layer  10  by the Coulomb forces. However, if the Coulomb forces of moving the penetrant L 1  into the transparent porous layer  10  become equal to the capillary attraction of attracting the penetrant L 1  into the colored porous layer  20 , the movement of the penetrant L 1  from colored porous layer  20  into the transparent porous layer  10  is stopped and the transparent porous layer  10  becomes semitransparent at the positions ( 1 ,  1 ) and ( 3 ,  1 ). Therefore, the black of the colored porous layer  20  is displayed through the transparent porous layer  10  that became semitransparent at the positions ( 1 ,  1 ) and ( 3 ,  1 ). In this way, gray is displayed.  
      Thus, the display of the display device  101  can be variously changed by applying the above-described potential difference with the power supply  35 .  
      Referring to  FIGS. 5A and 5B , there is shown a first modification of the display device  101  constructed in accordance with the first embodiment of the present invention.  
      In the first modification, the transparent porous layer  10  of the display device  101  of FIGS.  1  to  4  is improved so that the diameter of each pore in the transparent porous layer  10  becomes larger the further it is away from the colored porous layer  20  in the thickness direction. More particularly, as shown in  FIGS. 5A and 5B , a transparent porous layer  10 A in the first modification consists of a first transparent porous layer  10   a  with a great pore diameter disposed on the side of light L a , and a second transparent porous layer  10   b  with a small pore diameter disposed on the side of the colored porous layer  20 . The first transparent porous layer  10   a  is stacked on the second transparent porous layer  10   b . The remaining structure is the same as the display device  101  of the first embodiment of FIGS.  1  to  4 . In the following description, the same reference numerals will be applied to the same parts as the first embodiment. Therefore, detailed descriptions there of will be omitted unless particularly necessary. Note that because the pore diameter of the colored porous layer  20  is smaller than that of the transparent porous layer  10   b , the pore diameter of the colored porous layer  20  is smaller than that of the transparent porous layer  10 A.  
      Now, a description will be given of the operation of the display device of the first modification shown in  FIGS. 5A and 5B .  
      In the initial state where there is no potential difference between the top electrodes  31   a  and the bottom electrodes  31   b , the penetrant L 1  is discharged from the transparent porous layer  10 A and penetrates into the colored porous layer  20 , as shown in  FIG. 5A . In this state, if the transparent porous layer  10 A is illuminated with light L a , the transparent porous layer  10 A is displayed in white, as with the first embodiment of FIGS.  1  to  4 .  
      When displaying black, the power supply  35  applies a potential difference between the top electrodes  31   a  and the bottom electrodes  31   b , as shown in  FIG. 5B . With the application of the potential difference, an electric field is formed in the thickness direction of the stacked structure  15 . As a result, the penetrant L 1  in the colored porous layer  20  is moved into the transparent porous layer  10 A by the Coulomb force, so the transparent porous layer  10  is filled with the penetrant L 1  and becomes transparent. Because the colored porous layer  20  is displayed through the transparent porous layer  10 A, black is displayed.  
      When displaying white again, the potential difference between the top electrodes  31   a  and the bottom electrodes  31   b  is made 0 V by the power supply  35 . As a result, the penetrant L 1  is moved from the transparent porous layer  10 A into the colored porous layer  20  by capillary attraction, and in the same manner as described above, the transparent porous layer  10 A is displayed in white (see  FIG. 5A ). When the penetrant L 1  penetrates into the transparent porous layer  10   b  whose pore diameter is small, resistance to the small pore is great. On the other hand, when the penetrant L 1  penetrates into the transparent porous layer  10   a  whose pore diameter is large, resistance to the large pore is small. Therefore, during the period from the movement of the penetrant L 1  into the colored porous layer  20  to the display (white) of the transparent porous layer  10 A, display is properly changed. Also, during the period from the movement of the penetrant L 1  into the transparent porous layer  10 A to the display of the colored porous layer  20 , display is properly changed. Therefore, when a specified brightness is shifted to brightness different from that brightness, the afterimage of the display can be optimized. That is, when the penetrant L 1  in the colored porous layer  20  penetrates into the transparent porous layer  10 A by the application of a potential difference between the top electrodes  31   a  and the bottom electrodes  31   b , the second transparent porous layer  10   b  plays a role of preventing the moving time of the penetrant L 1  from becoming too short. Also, when the penetrant L 1  in the transparent porous layer  10 A is moved into the colored porous layer  20  by making the above-described potential difference 0 V, the transparent porous layer  10   b  fulfills its role as resistance and prolongs the moving time of the penetrant L 1 , so display can have an afterimage (memory effect). The transparent porous layer  10   b  is sufficiently thinner in thickness than the other layers and is smallest in pore diameter.  
      The structure of making the pore diameter of each pore in the transparent porous layer  10 A larger the further it is away from the colored porous layer  20  in the thickness direction is not limited to the above-described structure consisting of two layers which are different from each other in pore diameter, but may be a structure consisting of three or more layers, or a structure whose pore diameter changes continuously in the thickness direction.  
      Referring to  FIGS. 6A and 6B , there is shown a second modification of the display device  101  constructed in accordance with the first embodiment of the present invention.  
      In the second modification, the power supply  35  of the display device  101  of FIGS.  1  to  4  is improved to apply a potential difference between the electrodes disposed on the top and bottom surfaces of the colored porous layer  20 . More particularly, as shown in  FIGS. 6A and 6B , penetrant-moving means has top electrodes  31   c  and bottom electrodes  31   b  disposed on the top and bottom surfaces of the colored porous layer  20 . The power supply  35  applies a potential difference between the top electrodes  31   c  and the bottom electrodes  31   b  to exert the Coulomb forces on a penetrant L 1 . The remaining structure is the same as the display device  101  of the first embodiment of FIGS.  1  to  4 . In the following description, the same reference numerals will be applied to the same parts as the first embodiment. Therefore, detailed descriptions thereof will be omitted unless particularly necessary. Note that the top electrodes  31   c  are arranged on the top surface of the colored porous layer  20  that is on the side of the transparent porous layer  10 . Also, the bottom electrodes  31   b  are arranged on the bottom surface of the colored porous layer  20  remote from the transparent porous layer  10 . The top electrodes  31   c  are transparent electrodes through which the penetrant L 1  can pass.  
      Now, a description will be given of the operation of the display device of the second modification shown in  FIGS. 6A and 6B .  
      In the initial state where there is no potential difference between the top electrodes  31   c  and the bottom electrodes  31   b , the penetrant L 1  is discharged from the transparent porous layer  10  and penetrates into the colored porous layer  20  (where pore diameter is small and capillary attraction is great, compared to the transparent porous layer  10 ), as shown in  FIG. 6A . In this state, if the transparent porous layer  10  is illuminated with light L a , the transparent porous layer  10  is displayed in white, as with the first embodiment of FIGS.  1  to  4 .  
      When displaying black, the power supply  35  applies a potential difference between the top electrodes  31   c  and the bottom electrodes  31   b , as shown in  FIG. 6B . With the potential difference, an electric field is formed in the thickness direction of the colored porous layer  20 . As a result, the penetrant L 1  in the colored porous layer  20  is moved into the transparent porous layer  10  by the Coulomb force, so the transparent porous layer  10  is filled with the penetrant L 1  and becomes transparent. Because the colored porous layer  20  is displayed through the transparent porous layer  10 , black is displayed.  
      When displaying white again, the potential difference between the top electrodes  31   c  and the bottom electrodes  31   b  is made 0 V by the power supply  35 . As a result, the penetrant L 1  is moved from the transparent porous layer  10  into the colored porous layer  20  by capillary attraction, and in the same manner as described above, the transparent porous layer  10  is displayed in white (see  FIG. 6A ).  
      Referring to  FIGS. 7A and 7B , there is shown a third modification of the display device  101  constructed in accordance with the first embodiment of the present invention.  
      In the third modification, the display device  101  of FIGS.  1  to  4  is improved so it can display colors. As shown in  FIGS. 7A and 7B , a colored porous layer  20  is colored yellow (Y), magenta (M), cyan (C), and black (K) at positions ( 1 ), ( 2 ), ( 3 ), and ( 4 ), which are different from one another, on the plane perpendicular to the thickness direction. At the different positions ( 1 ), ( 2 ), ( 3 ), and ( 4 ), the power supply  35  is able to exert the Coulomb forces different from one another, on the penetrant L 1 . The remaining structure is the same as the display device  101  of the first embodiment of FIGS.  1  to  4 . In the following description, the same reference numerals will be applied to the same parts as the first embodiment. Therefore, detailed descriptions thereof will be omitted unless particularly necessary.  
      Now, a description will be given of the operation of the display device of the third modification shown in  FIGS. 7A and 7B .  
      In the initial state where there is no potential difference between the top electrodes  31   a  and bottom electrodes  31   b  disposed on the top and bottom surfaces of a stacked structure  15 C consisting of a transparent porous layer  10  and a colored porous layer  20 C, the penetrant L 1  is discharged from the transparent porous layer  10  and penetrates into the colored porous layer  20 , as shown in  FIG. 7A . In this state, if the transparent porous layer  10  is illuminated with light L a , the transparent porous layer  10  is displayed in white.  
      For instance, when displaying yellow (Y) and cyan (C), the power supply  35  applies a potential difference between the top electrodes  31   a  and the bottom electrodes  31   b  at the positions ( 1 ) and ( 3 ) corresponding to the yellow (Y) and cyan (C) in the stacked structure  15 C. The application of the potential difference causes electric fields to be generated at the positions ( 1 ) and ( 3 ) in the thickness direction of the stacked structure  15 C. As a result, at the positions ( 1 ) and ( 3 ), the penetrant L 1  in the colored porous layer  20 C is moved into the transparent porous layer  10 , and the transparent porous layer  10  becomes transparent. Therefore, at the positions ( 1 ) and ( 3 ), the colored porous layer  20 C colored yellow (Y) and cyan (C) is displayed through the transparent porous layer  10 . On the other hand, at the positions ( 2 ) and ( 4 ) corresponding to magenta (M) and black (K), the display device remains displayed in white because there is no potential difference between the top electrodes  31   a  and the bottom electrodes  31   b.    
      When displaying white at the positions ( 1 ) and ( 3 ) again, the potential difference between the electrodes corresponding to those positions is made 0 V. As a result, the penetrant L 1  in the transparent porous layer  10  is moved into the colored porous layer  20 C by capillary attraction, and white is displayed at the positions ( 1 ) and ( 3 ) (see  FIG. 7A ).  
      Note that the third modification maybe further improved. As shown in  FIG. 8 , the stacked structure  15 C may be provided with partition walls  41 , which extend in the thickness direction to individually separate the positions ( 1 ), ( 2 ), ( 3 ), and ( 4 ). These partition walls  41  prevent the penetrant L 1  from passing through them. Therefore, at the positions ( 1 ), ( 2 ), ( 3 ), and ( 4 ), the penetration of the penetrant L 1  into the transparent porous layer  10  and discharge of the penetrant L 1  from the transparent porous layer  10  can be distinctly performed. Thus, displaying can be performed with the contour of each area emphasized.  
      Referring to  FIG. 9 , there is shown a display device  102  constructed in accordance with a second embodiment of the present invention.  
      The display device  102  is formed from a transparent material, and includes a transparent porous layer  50  with pores into which a penetrant L 1  is penetrable; a colored layer  60  stacked on one surface of the transparent porous layer  50 ; penetrant-moving means  70  for exerting the Coulomb force on the penetrant L 1  to perform penetration of the penetrant L 1  into the transparent porous layer  50  or discharge of the penetrant L 1  from the transparent porous layer  50 ; and a penetrant storage layer  80  for storing the penetrant L 1  between the transparent porous layer  50  and the colored layer  60 .  
      In the display device  102 , when the penetrant L 1  penetrates into the transparent porous layer  50 , the transparent porous layer  50  becomes transparent and therefore the colored porous layer  60  is displayed through the transparent porous layer  50 . On the other hand, when the penetrant L 1  is discharged from the transparent porous layer  50 , the transparent porous layer  50  becomes opaque and therefore the transparent porous layer  50  is displayed. The above-described display is performed by illuminating the transparent porous layer  50  of the display device  102  with light L a . The surface of the display device  102  illuminated with light L a  will hereinafter be referred to as the screen.  
      Note that the colored layer  60  is constructed of a black light-absorbing layer of resin.  
      When the penetrant L 1  is moved into the transparent porous layer  50  by the Coulomb force exerted on the penetrant L 1  by the penetrant-moving means  70 , the penetrant storage layer  80  supplies the penetrant L 1  to the transparent porous layer  50 . Also, when the penetrant L 1  is discharged from the transparent porous layer  50 , the penetrant storage layer  80  stores the discharged penetrant L 1 .  
      The penetrant-moving means  70  consists of top electrodes  31   a , bottom electrodes  31   b , and a power supply  75 . The top electrodes  31   a  and bottom electrodes  31   b  are disposed on the top and bottom surfaces of the transparent porous layer  50 , respectively. The power supply  75  is a potential difference application part for applying a potential difference between the top electrodes  31   a  and the bottom electrodes  31   b , forming an electric field, and exerting the Coulomb force on the penetrant L 1 . Note that the top electrodes  31   a  are arranged on the top surface of the transparent porous layer  50 . Also, the bottom electrodes  31   b  are arranged on the bottom surface of the transparent porous layer  50  that is on the side of the penetrant storage layer  80 .  
      In the transparent porous layer  50 , the pore diameter becomes larger the further it is away from the colored layer  60 . The average value of pore diameters in the vicinity of the top electrodes  32   a  disposed on the screen side of the transparent porous layer  50  is 2 μm, while the average value of pore diameters near the bottom electrodes  32   b  disposed on the bottom surface of the transparent porous layer  50  is 0.3 μm. Also, the thickness of the transparent porous layer  50  is 80 μm.  
      The remaining structure is the same as the first embodiment of FIGS.  1  to  4 .  
      Next, a description will be given of the operation of the display device  102  of the second embodiment constructed as described above.  
      In the initial state where there is no potential difference across the transparent porous layer  50 , the penetrant L 1  is stored in the penetrant storage layer  80 , as shown in  FIG. 9A . In the transparent porous layer  50 , the penetrant L 1  has penetrated on a side of the transparent porous layer  50  near the penetrant storage layer  80  in which the pore diameter is small and capillary attraction is great (this side, indicated by arrow G 1 , will hereinafter be referred to as the small-diameter side). Also, on the screen side of the transparent porous layer  50  where the pore diameter is large and capillary attraction is small (this side indicated by arrow G 2  will hereinafter be referred to as the large-diameter side), the penetrant L 1  has been discharged. In this state, if the transparent porous layer  50  is illuminated with light L a , the light L a  incident on the transparent porous layer  50  is scattered at the region on the large-diameter side of the transparent porous layer  50  and therefore the transparent porous layer  50  becomes opaque and is displayed in white.  
      When displaying black, the power supply  75  applies a potential difference of 100 V between the top electrodes  32   a  and the bottom electrodes  32   b , as shown in  FIG. 9B . The application of the potential difference causes an electric field to be generated across the transparent porous layer  50 . The penetrant L 1  in the penetrant storage layer  80  is moved into the transparent porous layer  50  by the Coulomb force exerted on the transparent porous layer  50 , so the transparent porous layer  50  is filled with the penetrant L 1  and becomes transparent. Thus, the colored layer  60  is displayed and black is displayed.  
      When displaying white, the potential difference between the top electrodes  32   a  and the bottom electrodes  32   b  that is applied by the power supply  35  is made opposite in polarity to the case of displaying black, as shown in  FIG. 9C . The application of the opposite potential difference causes the Coulomb force to be exerted on the penetrant L 1  in the transparent porous layer  50 , so the penetrant L 1  is moved into the penetrant storage layer  80 . Because the penetrant L 1  is discharged from the transparent porous layer  50 , the transparent porous layer  50  is displayed in white, as described above. Note that if the potential difference across the transparent porous layer  50  is made 0 V after the discharge of the penetrant L 1  from the transparent porous layer  50 , the penetrant L 1  is moved into the small-diameter side of the transparent porous layer  50  by capillary attraction and is caused to be in the same state as the initial state.  
      When displaying gray from the above-described state in which white is displayed, the power supply  75  applies a potential difference, which is the same polarity as when displaying black but smaller than when displaying black, between the top electrodes  32   a  and the bottom electrodes  32   b , as shown in  FIG. 9D . With application of the potential difference, an electric field is generated in the transparent porous layer  50  and the Coulomb force is exerted on the penetrant L 1 , so the penetrant L 1  is moved toward the large-diameter side of the transparent porous layer  50 . However, since the penetration of the penetrant L 1  into the transparent porous layer  50  is stopped without being completely filled with the penetrant L 1 , scattering of light L a  occurs more or less on the large-diameter side of the transparent porous layer  50 . Therefore, the transparent porous layer  50  becomes semitransparent. Thus, since the colored layer  60  which is black is displayed through the transparent porous layer  50  made semitransparent, gray is displayed.  
      Note that the transparent porous layer  50  is not limited to the case where the pore diameter in the transparent porous layer  50  is larger the further it is away from the colored layer  60  in the thickness direction. For instance, even when the pore diameter in the transparent porous layer  50  is uniform regardless of the pore position, displaying can be performed in approximately the same manner in the case other than the initial state. That is, in the initial state where the potential difference between the top electrodes  32   a  and the bottom electrodes  32   b  is 0 V, the penetrant L 1  penetrates uniformly into the transparent porous layer  50  by capillary attraction. Therefore, since scattering of light L a  on the large-diameter side of the transparent porous layer  50  is reduced and the transparent porous layer  50  becomes semitransparent, the colored layer  60  that is black is also displayed through the transparent porous layer  50  made semitransparent. Therefore, in this case, brightness is reduced, compared to the white of the initial state in the case where pore diameter changes in the thickness direction.  
      Note that the penetrant-moving means may be made up of electrodes, which are respectively disposed on the top and bottom surfaces of a stacked structure consisting of the transparent porous layer and penetrant storage layer, and a power source for applying a potential difference between the electrodes.  
      Referring to  FIGS. 10A through 10D , there is shown a display device  103  constructed in accordance with a third embodiment of the present invention.  
      The display device  103  of the third embodiment is formed from a transparent material, and includes a transparent porous layer  110  with pores into which a penetrant L 1  is penetrable; a colored porous layer  120  stacked on one surface of the transparent porous layer  110 ; and penetrant-moving means  30  for exerting the Coulomb force on the penetrant L 1  to perform penetration of the penetrant L 1  into the transparent porous layer  110  or discharge of the penetrant L 1  from the transparent porous layer  110 .  
      In the display device  103 , when the penetrant L 1  penetrates into the transparent porous layer  110 , the transparent porous layer  110  becomes transparent and therefore the colored porous layer  120  is displayed through the transparent porous layer  110 . On the other hand, when the penetrant L 1  is discharged from the transparent porous layer  110 , the transparent porous layer  110  becomes opaque and therefore the transparent porous layer  110  is displayed. These displays are performed by illuminating the transparent porous layer  110  with light L a . The surface of the display device  103  illuminated with light L a  will be referred to as the screen.  
      The stacked structure  115 , consisting of the transparent porous layer  110  and colored porous layer  120 , is formed in the following manner, with a homogeneous porous panel  116  as a base.  
      That is, the porous panel  116  employs a microporous nitrocellulose material with a thickness of 80 μm, an average pore diameter of 5 μm, and an refractive index of 1.51. Initially, top stripe electrodes  131   a  and bottom stripe electrodes  131   b  are respectively disposed on the top and bottom surfaces of the porous panel  116  so that they cross one another across the porous panel  116 . The top stripe electrodes  131   a  and bottom stripe electrodes  131   b  are formed as transparent electrodes of thickness 200 nm consisting of In 2 O 3 -SnO 2  (ITO) by sputtering. Note that the top stripe electrodes  131   a  and bottom stripe electrodes  131   b , formed to cross one another across the porous panel  116 , may be the same electrodes as the first embodiment.  
      Next, the transparent porous layer  110  is formed in the porous panel  116 . The side of the top electrodes  131   a  of the porous panel  116  is coated with a water-repellent and oil-repellent coating (SAITOP made by Asahi Glass) with a surface tension of 19 N/m so that the coating penetrates to a depth of 40 μm. In this way, the transparent porous layer  110  is formed in the region from the top surface of the porous panel  116  to depth 40 μm.  
      Subsequently, the colored porous layer  120  is formed in the porous panel  116 . The side of the bottom electrodes  131   b  of the porous panel  116  is coated with ink containing carbon black as a pigment (GA Black  1 ), the ink penetrates to a depth of 40 μm, and the porous panel  116  is colored black. In this way, the colored porous layer  120  is formed in the region from the bottom surface of the porous panel  116  to depth 40 μm.  
      With formation of the stacked structure  115 , the electrodes disposed on the side of the transparent porous layer  110  are formed as the top electrodes  131   a . The electrodes arranged on the side of the colored porous layer  120  are formed as the bottom electrodes  131   b . The side of the top electrodes  131   a  is formed as the screen.  
      The penetrant-moving means  130  has the top electrodes  131   a  and  131   b  respectively disposed on the transparent porous layer  110  and colored porous layer  120 , and a power supply  135 . The power supply  135  is a potential difference application part to exert the Coulomb force on the penetrant L 2  by applying a potential difference between the top and bottom electrodes  131   a  and  131   b.    
      Note that the penetrant L 2  is composed of dimethyltriphenyltrimethoxysiloxane, having a refractive index of 1. 51.  
      A description will hereinafter be given of the operation of the display device  103  constructed as described above.  
      When the power supply  135  applies no potential difference between the top and bottom electrodes  131   a  and  131   b , the penetrant L 2  is more stably present in the black-colored porous layer  120  than in the transparent porous layer  110 , as shown in  FIG. 10A . In this state, if the transparent porous layer  110  is illuminated with light L a , the light L a  is scattered within the transparent porous layer  110  and therefore the transparent porous layer  110  is displayed as opaque white.  
      When the power supply  135  applies 0 V to the bottom electrodes  131   b  and −50 V to the top electrodes  131   a , the penetrant L 2  is attracted toward the top electrodes  131   a  and penetrates into a portion of the transparent porous layer  110 . As a result, scattering of the light L a  in the transparent porous layer  110  is reduced and the transparent porous layer  110  becomes semitransparent. Because the black-colored porous layer  120  is displayed through the transparent porous layer  110 , which is made semitransparent, gray is displayed.  
      When the power supply  135  applies 0 V to the bottom electrodes  131   b  and −100 V to the top electrodes  131   a , the penetrant L 2  is further attracted toward the top electrodes  131   a  and penetrates into approximately the entire surface of the transparent porous layer  110 . As a result, there is no scattering of the light L a  in the transparent porous layer  110  and the transparent porous layer  110  becomes transparent. Since the black-colored porous layer  120  is displayed through the transparent porous layer  110 , which is made transparent, black is displayed.  
      Thus, the display device  103  is capable of displaying white, gray, and black by changing a potential difference between the top and bottom electrodes  131   a  and  131   b  by the penetrant-moving means  130 .  
      While the present invention has been described with reference to the preferred embodiments thereof, the invention is not to be limited to the details given herein, but may be modified within the scope of the invention hereinafter claimed. For example, although the supply of the penetrant to the transparent porous layer and the absorption of the penetrant discharged from the transparent porous layer are performed by the penetrant storage layer or colored porous layer, the structure of performing the supply and absorption of the penetrant may be any structure, so long as it is able to supply and absorb the penetrant.