Recording medium and information recording and reproducing method using the same

Abstract

A recording medium comprises a substrate and a recording layer overlaid on the substrate. The recording layer comprises a material, which has properties such that, when recording light having a predetermined wavelength λ 1 is irradiated to the material, the material is capable of being caused to change into a fluorescent material and such that, when excitation light having a wavelength λ 2 is then irradiated to the thus formed fluorescent material, the fluorescent material is capable of being caused to produce fluorescence. The wavelength λ 1 of the recording light and the wavelength λ 2 of the excitation light may be identical or different. The substrate may be constituted of a material having properties such that, when the excitation light is irradiated to the material, the material does not produce fluorescence having a wavelength identical with the wavelength of the fluorescence produced by the fluorescent material.

1 - 66 . (canceled) 
     
     
         67 . A recording medium, comprising:
 i) a substrate, and   ii) a recording layer overlaid on the substrate,   wherein the recording layer comprises multiple kinds of materials uniformly mixed together, each of which has properties such that, when recording light having a predetermined wavelength λ 1  is irradiated to the material, the material is capable of being caused to change into a fluorescent material,   the multiple kinds of the materials are capable of being caused by the recording light having different wavelengths λ 1  to change into fluorescent materials,   the fluorescent materials, which have been formed by the multiple kinds of the materials, are capable of being caused by excitation light having different wavelengths λ 2  to produce fluorescence, and   the fluorescent materials, which have been formed by the multiple kinds of the materials, produce the fluorescence having different wavelengths λ 3 .   
     
     
         68 . A recording medium, comprising:
 i) a substrate, and   ii) a recording layer overlaid on the substrate,   wherein the recording layer comprises multiple kinds of materials uniformly mixed together, each of which has properties such that, when recording light having a predetermined wavelength λ 1  is irradiated to the material, the material is capable of being caused to change into a fluorescent material,   the multiple kinds of the materials are capable of being caused by the recording light having different wavelengths λ 1  to change into fluorescent materials,   the fluorescent materials, which have been formed by the multiple kinds of the materials, are capable of being caused by excitation light having an identical wavelength λ 2  to produce fluorescence, and   the fluorescent materials, which have been formed by the multiple kinds of the materials, produce the fluorescence having different wavelengths λ 3 .   
     
     
         69 . A recording medium, comprising:
 i) a substrate, and   ii) a recording layer overlaid on the substrate,   wherein the recording layer comprises multiple kinds of materials uniformly mixed together, each of which has properties such that, when recording light having a predetermined wavelength λ 1  is irradiated to the material, the material is capable of being caused to change into a fluorescent material,   the multiple kinds of the materials are capable of being caused by the recording light having different wavelengths λ 1  to change into fluorescent materials,   the fluorescent materials, which have been formed by the multiple kinds of the materials, are capable of being caused by excitation light having different wavelengths λ 2  to produce fluorescence, and   the fluorescent materials, which have been formed by the multiple kinds of the materials, produce the fluorescence having an identical wavelength λ 3 .   
     
     
         70 . A recording medium as defined in  claim 67 ,  68 , or  69  wherein the recording layer is constituted of a thin film, which contains the multiple kinds of the materials mixed together. 
     
     
         71 . A recording medium as defined in  claim 67 ,  68 , or  69  wherein the substrate comprises a dielectric material, a metal film is overlaid on one surface of the dielectric material, and the recording layer is overlaid on the metal film. 
     
     
         72 . A recording medium as defined in  claim 71  wherein the metal film comprises a metal selected from the group consisting of gold and silver. 
     
     
         73 . A recording medium as defined in  claim 67 ,  68 , or  69  wherein the substrate comprises a material permeable to the excitation light having the wavelengths λ 2 . 
     
     
         74 . A recording medium as defined in  claim 67 ,  68 , or  69  wherein at least one material among the multiple kinds of the materials, each of which has the properties such that, when the recording light having the predetermined wavelength λ 1  is irradiated to the material, the material is capable of being caused to change into the fluorescent material, is a compound, in which a certain functional group of a compound [FL] has been protected with a protecting group [PR]−, and in which the production of the fluorescence is thereby restricted, the compound being represented by Formula (I):
   [FL]−[PR]  (I)   wherein [FL] represents the compound residue capable of producing the fluorescence, and [PR] represents the group capable of being separated from [FL] when light is irradiated to [FL].   
     
     
         75 . A recording medium as defined in  claim 67 ,  68 , or  69  wherein at least one material among the multiple kinds of the materials, each of which has the properties such that, when the recording light having the predetermined wavelength λ 1  is irradiated to the material, the material is capable of being caused to change into the fluorescent material, is a material, which comprises a combination of a chemical species [FL] capable of producing the fluorescence and a chemical species [Q] capable of quenching the fluorescence, the material being represented by Formula (II):
   [FL]+[Q]  (II)   wherein [FL] represents the chemical species capable of producing the fluorescence, and [Q] represents the chemical species capable of quenching the fluorescence.   
     
     
         76 . A recording medium as defined in  claim 75  wherein the chemical species [FL] is a compound represented by Formula (II-1): 
       
         
           
           
               
               
           
         
         wherein Z 1  and Z 2  each represent an atom group necessary for forming a five-membered or six-membered, nitrogen-containing heterocyclic ring; R 30  and R 31  each independently represent an alkyl group or an aryl group; L 3 , L 4 , L 5 , L 6 , and L 7  each independently represent a substituted or unsubstituted methine group, provided that, in cases where L 3  to L 7  are substituted by substituents, the substituents may optionally be connected with one another to form a ring; p and q each independently represent 0 or 1; n1 and n2 each independently represent 0, 1, or 2; M1 represents a charge balancing counter ion; and m1 represents a number necessary for keeping charge balance; and 
         the chemical species [Q] is a compound represented by Formula (II-2): 
       
       
         
           
           
               
               
           
         
       
       wherein m and n each independently represent an integer of 0 to 2; X 1  and X 2  each represent ═NR 1  or ═CR 2 R 3 , in which R 1 , R 2 , and R 3  each represent a substituent; and L 1  and L 2  each independently represent a bivalent linking group. 
     
     
         77 . A recording medium as defined in  claim 75  wherein the chemical species [FL] is an anion represented by Formula (II-3): 
       
         
           
           
               
               
           
         
       
       wherein Za and Zb each independently represent an atom group necessary for forming a five-membered or six-membered, nitrogen-containing heterocyclic ring; R 1  and R 2  each independently represent an alkyl group or an aryl group; L 1 , L 2 , L 3 , L 4 , and L 5  each independently represent a substituted or unsubstituted methine group, provided that, in cases where L 1  to L 5  are substituted by substituents, the substituents may optionally be connected with one another to form a ring; n represents an integer of at least 1; j represents 0, 1, or 2; and k represents 0 or 1; and the chemical species [Q] is a cation represented by Formula (II-4): 
       
         
           
           
               
               
           
         
       
       wherein R 5  and R 6  each independently represent a substituent group; R 7  and R 8  each independently represent an alkyl group, an alkenyl group, an alkynyl group, an aralkyl group, an aryl group or a heterocyclic group, provided that R 5  and R 6 , R 5  and R 7 , R 6  and R 8 , or R 7  and R 8  may be connected with each other to form a ring; and r and s each independently represent an integer of 0 to 4, provided that, in cases where r and s each represent an integer of at least 2, the at least two substituents R 5  may be identical or different, and the at least two substituents R 6  may be identical or different. 
     
     
         78 . A recording medium as defined in  claim 75  wherein the chemical species [FL] is an anion represented by either one of Formula (II-5) and Formula (II-6): 
       
         
           
           
               
               
           
         
       
       wherein A 1 , A 2 , B 1 , and B 2  each independently represent a substituent; L 1 , L 2 , L 3 , L 4 , and L 5  each represent a methine group; X 1  represents ═O, ═NR, or ═C(CN) 2 , in which R represents a substituent; X 2  represents —O, —NR, or —C(CN) 2 , in which R represents a substituent; m and n each represent an integer of 0 to 2; Y 1  and E each represent an atom or an atom group necessary for forming a carbocyclic ring or a heterocyclic ring; Z 1  and G each represent an atom or an atom group necessary for forming a carbocyclic ring or a heterocyclic ring; x and y each independently represent 0 or 1; M k+  represents an onium ion; and k represents the number of charges; and
 the chemical species [Q] is a cation represented by Formula (II-4): 
 
       
         
           
           
               
               
           
         
       
       wherein R 5  and R 6  each independently represent a substituent group; R 7  and R 8  each independently represent an alkyl group, an alkenyl group, an alkynyl group, an aralkyl group, an aryl group or a heterocyclic group, provided that R 5  and R 6 , R 5  and R 7 , R 6  and R 8 , or R 7  and R 8  may be connected with each other to form a ring; and r and s each independently represent an integer of 0 to 4, provided that, in cases where r and s each represent an integer of at least 2, the at least two substituents R 5  may be identical or different, and the at least two substituents R 6  may be identical or different. 
     
     
         79 . An information recording method, in which information is recorded on a recording medium as defined in  claim 67 ,  68 , or  69 , the method comprising the step of:
 irradiating beams of the recording light carrying recording information and having a plurality of different wavelengths, each of which wavelengths is capable of causing one of the multiple kinds of the materials to change into the fluorescent material, to the recording layer of the recording medium, such that the beams of the recording light are capable of impinging upon an identical site on the recording layer, the multiple kinds of the materials, which are located at the site having been exposed to the recording light, being thereby caused to change into the fluorescent materials.   
     
     
         80 . (canceled) 
     
     
         81 . An information recording apparatus, in which information is recorded on a recording medium as defined in  claim 67 ,  68 , or  69 , the apparatus comprising:
 recording means for irradiating beams of the recording light carrying recording information and having a plurality of different wavelengths, each of which wavelengths is capable of causing one of the multiple kinds of the materials to change into the fluorescent material, to the recording layer of the recording medium, such that the beams of the recording light are capable of impinging upon an identical site on the recording layer, in order to cause the multiple kinds of the materials, which are located it at the site having been exposed to the recording light, to change into the fluorescent materials.   
     
     
         82 .- 83 . (canceled) 
     
     
         84 . An information reproducing method for reproducing recording information from a recording medium as defined in  claim 67 , on which the recording information has been recorded, wherein the recording information has been recorded on the recording medium by irradiating beams of the recording light carrying recording information and having a plurality of different wavelengths, each of which wavelengths is capable of causing one of the multiple kinds of the materials to change into the fluorescent material, to the recording layer of the recording medium, such that the beams of the recording light are capable of impinging upon an identical site on the recording layer, the multiple kinds of the materials, which are located at the site having been exposed to the recording light, being thereby caused to change into the fluorescent materials, the method comprising the steps of:
 i) irradiating the excitation light having a plurality of different wavelengths, each of which wavelengths falls within an excitation wavelength region for one of the fluorescent materials, to the recording layer, the fluorescent materials being thereby caused to produce the fluorescence having different wavelengths, and   ii) detecting the fluorescence through wavelength discrimination.   
     
     
         85 . An information reproducing method for reproducing recording information from a recording medium as defined in  claim 68 , on which the recording information has been recorded, wherein the recording information has been recorded on the recording medium by irradiating beams of the recording light carrying recording information and having a plurality of different wavelengths, each of which wavelengths is capable of causing one of the multiple kinds of the materials to change into the fluorescent material, to the recording layer of the recording medium, such that the beams of the recording light are capable of impinging upon an identical site on the recording layer, the multiple kinds of the materials, which are located at the site having been exposed to the recording light, being thereby caused to change into the fluorescent materials, the method comprising the steps of:
 i) irradiating one kind of the excitation light having a wavelength, which falls within an excitation wavelength region common to the fluorescent materials, to the recording layer, the fluorescent materials being thereby caused to produce the fluorescence having different wavelengths, and   ii) detecting the fluorescence through wavelength discrimination.   
     
     
         86 . An information reproducing method for reproducing recording information from a recording medium as defined in  claim 69 , on which the recording information has been recorded, wherein the recording information has been recorded on the recording medium by irradiating beams of the recording light carrying recording information and having a plurality of different wavelengths, each of which wavelengths is capable of causing one of the multiple kinds of the materials to change into the fluorescent material, to the recording layer of the recording medium, such that the beams of the recording light are capable of impinging upon an identical site on the recording layer, the multiple kinds of the materials, which are located at the site having been exposed to the recording light, being thereby caused to change into the fluorescent materials, the method comprising the steps of:
 i) irradiating the excitation light having a plurality of different wavelengths, each of which wavelengths falls within an excitation wavelength region for one of the fluorescent materials, with different timings to the recording layer, the fluorescent materials being thereby caused to produce the fluorescence having the identical wavelength, and   ii) detecting the fluorescence.   
     
     
         87 . (canceled) 
     
     
         88 . An information reproducing method as defined in  claim 84 ,  85 , or  86  wherein light having the wavelengths different from the wavelengths of the recording light is employed as the excitation light. 
     
     
         89 . An information reproducing method as defined in  claim 84 ,  85 , or  86  wherein light irradiated to a fine range is employed as the excitation light,
 the recording layer is scanned with the excitation light, and   the fluorescence produced from the recording layer during the scanning with the excitation light is detected with respect to each of positions which are being scanned.   
     
     
         90 - 91 . (canceled) 
     
     
         92 . An information reproducing method as defined in  claim 84 ,  85 , or  86  wherein the recording medium is constituted such that the substrate comprises a dielectric material, a metal film is overlaid on one surface of the dielectric material, and
 the recording layer is overlaid on the metal film, and the excitation light is irradiated from the substrate side to the recording medium such that the excitation light impinges at a specific angle of incidence upon the metal film.   
     
     
         93 . An information reproducing apparatus for reproducing recording information from a recording medium as defined in  claim 67 , on which the recording information has been recorded, wherein the recording information has been recorded on the recording medium by irradiating beams of the recording light carrying recording information and having a plurality of different wavelengths, each of which wavelengths is capable of causing one of the multiple kinds of the materials to change into the fluorescent material, to the recording layer of the recording medium, such that the beams of the recording light are capable of impinging upon an identical site on the recording layer, the multiple kinds of the materials, which are located at the site having been exposed to the recording light, being thereby caused to change into the fluorescent materials, the apparatus comprising:
 i) excitation means for irradiating the excitation light having a plurality of different wavelengths, each of which wavelengths falls within an excitation wavelength region for one of the fluorescent materials, to the recording layer in order to cause the fluorescent materials to produce the fluorescence having different wavelengths, and   ii) read-out means for detecting the fluorescence through wavelength discrimination.   
     
     
         94 . An information reproducing apparatus for reproducing recording information from a recording medium as defined in  claim 68 , on which the recording information has been recorded, wherein the recording information has been recorded on the recording medium by irradiating beams of the recording light carrying recording information and having a plurality of different wavelengths, each of which wavelengths is capable of causing one of the multiple kinds of the materials to change into the fluorescent material, to the recording layer of the recording medium, such that the beams of the recording light are capable of impinging upon an identical site on the recording layer, the multiple kinds of the materials, which are located at the site having been exposed to the recording light, being thereby caused to change into the fluorescent materials, the apparatus comprising:
 i) excitation means for irradiating one kind of the excitation light having a wavelength, which falls within an excitation wavelength region common to the fluorescent materials, to the recording layer in order to cause the fluorescent materials to produce the fluorescence having different wavelengths, and   ii) read-out means for detecting the fluorescence through wavelength discrimination.   
     
     
         95 . An information reproducing apparatus for reproducing recording information from a recording medium as defined in  claim 69 , on which the recording information has been recorded, wherein the recording information has been recorded on the recording medium by irradiating beams of the recording light carrying recording information and having a plurality of different wavelengths, each of which wavelengths is capable of causing one of the multiple kinds of the materials to change into the fluorescent material, to the recording layer of the recording medium, such that the beams of the recording light are capable of impinging upon an identical site on the recording layer, the multiple kinds of the materials, which are located at the site having been exposed to the recording light, being thereby caused to change into the fluorescent materials, the apparatus comprising:
 i) excitation means for irradiating the excitation light having a plurality of different wavelengths, each of which wavelengths falls within an excitation wavelength region for one of the fluorescent materials, with different timings to the recording layer in order to cause the fluorescent materials to produce the fluorescence having the identical wavelength, and   ii) read-out means for detecting the fluorescence.   
     
     
         96 . An information reproducing apparatus as defined in  claim 93 ,  94 , or  95  wherein the excitation means scans the recording layer with converged beam-like excitation light, and
 the read-out means detects the fluorescence with respect to each of positions which are being scanned with the excitation light.   
     
     
         97 . An information reproducing apparatus as defined in  claim 96  wherein the recording medium is constituted such that the substrate comprises a dielectric material, a metal film is overlaid on one surface of the dielectric material, and the recording layer is overlaid on the metal film, and
 the excitation means irradiates the excitation light to the recording medium such that the excitation light impinges upon the metal film from the substrate side.   
     
     
         98 - 100 . (canceled) 
     
     
         101 . An information reproducing method for reproducing recording information from a recording medium, on which the recording information has been recorded, wherein the recording information has been recorded on the recording medium by irradiating the recording light having the predetermined wavelength λ 1 , which recording light carries the recording information, to the recording layer of the recording medium, the material, which is located at a site having been exposed to the recording light, being thereby caused to change into the fluorescent material, the method comprising the steps of:
 i) providing the recording medium, the recording medium comprising:
 a substrate, and 
 a recording layer overlaid on the substrate, 
 wherein the recording layer comprises a material, which has properties such that, when recording light having a predetermined wavelength λ 1  is irradiated to the material, the material is capable of being caused to change into a fluorescent material and such that, when excitation light having a wavelength λ 2  is then irradiated to the thus formed fluorescent material, the fluorescent material is capable of being caused to produce fluorescence; 
   ii) irradiating the excitation light having the wavelength λ 2 , which falls within an excitation wavelength region for the fluorescent material, to the recording layer, the fluorescent material being thereby caused to produce the fluorescence; and   iii) detecting the fluorescence,   wherein light having the wavelength λ 2  different from the wavelength λ 1  of the recording light is employed as the excitation light.   
     
     
         102 . An information reproducing method for reproducing recording information from a recording medium, on which the recording information has been recorded, wherein the recording information has been recorded on the recording medium by irradiating the recording light having the predetermined wavelength λ 1 , which recording light carries the recording information, to the recording layer of the recording medium, the material, which is located at a site having been exposed to the recording light, being thereby caused to change into the fluorescent material, the method comprising the steps of:
 i) providing the recording medium, the recording medium comprising:
 a substrate, and 
 a recording layer overlaid on the substrate, 
 wherein the recording layer comprises a material, which has properties such that, when recording light having a predetermined wavelength λ 1  is irradiated to the material, the material is capable of being caused to change into a fluorescent material and such that, when excitation light having a wavelength λ 2  is then irradiated to the thus formed fluorescent material, the fluorescent material is capable of being caused to produce fluorescence; 
   ii) irradiating the excitation light having the wavelength λ 2 , which falls within an excitation wavelength region for the fluorescent material, to the recording layer, the fluorescent material being thereby caused to produce the fluorescence; and   iii) detecting the fluorescence,   wherein the recording medium is constituted such that the substrate comprises a dielectric material, a metal film is overlaid on one surface of the dielectric material, and the recording layer is overlaid on the metal film, and   wherein the excitation light is irradiated from the substrate side to the recording medium such that the excitation light impinges at a specific angle of incidence upon the metal film.   
     
     
         103 . The information reproducing method of  claim 102 , wherein the wavelength λ 1  of the recording light and the wavelength λ 2  of the excitation light are different from each other. 
     
     
         104 . An information reproducing apparatus for reproducing recording information from a recording medium, on which the recording information has been recorded, wherein the recording information has been recorded on the recording medium by irradiating the recording light having the predetermined wavelength λ 1 , which recording light carries the recording information, to the recording layer of the recording medium, the material, which is located at a site having been exposed to the recording light, being thereby caused to change into the fluorescent material, the apparatus comprising:
 i) excitation means for irradiating the excitation light having the wavelength λ 2 , which falls within an excitation wavelength region for the fluorescent material, to the recording layer in order to cause the fluorescent material to produce the fluorescence, and   ii) read-out means for detecting the fluorescence,   wherein the recording medium comprises:
 a substrate, and 
 a recording layer overlaid on the substrate, 
   wherein the recording layer comprises a material, which has properties such that, when recording light having a predetermined wavelength λ 1  is irradiated to the material, the material is capable of being caused to change into a fluorescent material and such that, when excitation light having a wavelength λ 2  is then irradiated to the thus formed fluorescent material, the fluorescent material is capable of being caused to produce fluorescence, and   wherein the excitation means irradiates light, which has the wavelength λ 2  different from the wavelength λ 1  of the recording light, as the excitation light to the recording layer.   
     
     
         105 . An information reproducing apparatus for reproducing recording information from a recording medium, on which the recording information has been recorded, wherein the recording information has been recorded on the recording medium by irradiating the recording light having the predetermined wavelength λ 1 , which recording light carries the recording information, to the recording layer of the recording medium, the material, which is located at a site having been exposed to the recording light, being thereby caused to change into the fluorescent material, the apparatus comprising:
 i) excitation means for irradiating the excitation light having the wavelength λ 2 , which falls within an excitation wavelength region for the fluorescent material, to the recording layer in order to cause the fluorescent material to produce the fluorescence, and   ii) read-out means for detecting the fluorescence,   wherein the recording medium comprises:
 a substrate, and 
 a recording layer overlaid on the substrate, 
   wherein the recording layer comprises a material, which has properties such that, when recording light having a predetermined wavelength λ 1  is irradiated to the material, the material is capable of being caused to change into a fluorescent material and such that, when excitation light having a wavelength λ 2  is then irradiated to the thus formed fluorescent material, the fluorescent material is capable of being caused to produce fluorescence,   wherein the excitation means scans the recording layer with converged beam-like excitation light,   wherein the read-out means detects the fluorescence with respect to each of positions which are being scanned with the excitation light,   wherein the recording medium is constituted such that the substrate comprises a dielectric material, a metal film is overlaid on one surface of the dielectric material, and the recording layer is overlaid on the metal film, and   wherein the excitation means irradiates the excitation light to the recording medium such that the excitation light impinges upon the metal film from the substrate side.   
     
     
         106 . The information reproducing apparatus of  claim 105 , wherein the wavelength λ 1  of the recording light and the wavelength λ 2  of the excitation light are different from each other. 
     
     
         107 . A recording medium, comprising:
 i) a substrate, and   ii) a recording layer overlaid on the substrate,   wherein the recording layer comprises a material, which has properties such that, when recording light having a predetermined wavelength λ 1  is irradiated to the material, the material is capable of being caused to change into a fluorescent material and such that, when excitation light having a wavelength λ 2  is then irradiated to the thus formed fluorescent material, the fluorescent material is capable of being caused to produce fluorescence having an intensity in accordance with the intensity of the recording light, and   wherein the wavelength λ 1  of the recording light and the wavelength λ 2  of the excitation light are different from each other.   
     
     
         108 . A recording medium, comprising:
 i) a substrate, and   ii) a recording layer overlaid on the substrate,   wherein the recording layer comprises a material, which has properties such that, when recording light having a predetermined wavelength λ 1  is irradiated to the material, the material is capable of being caused to change into a fluorescent material and such that, when excitation light having a wavelength λ 2  is then irradiated to the thus formed fluorescent material, the fluorescent material is capable of being caused to produce fluorescence having an intensity in accordance with the intensity of the recording light, and   wherein the substrate comprises a dielectric material, a metal film is overlaid on one surface of the dielectric material, and the recording layer is overlaid on the metal film.   
     
     
         109 . The recording medium of  claim 108 , wherein the wavelength λ 1  of the recording light and the wavelength λ 2  of the excitation light are different from each other. 
     
     
         110 . The recording medium of  claim 108 , wherein the metal film comprises a metal selected from the group consisting of gold and silver. 
     
     
         111 . The recording medium of  claim 109 , wherein the metal film comprises a metal selected from the group consisting of gold and silver. 
     
     
         112 . A recording medium, comprising:
 i) a substrate, and   ii) a recording layer overlaid on the substrate,   wherein the recording layer comprises a material, which has properties such that, when recording light having a predetermined wavelength λ 1  is irradiated to the material, the material is capable of being caused to change into a fluorescent material and such that, when excitation light having a wavelength λ 2  is then irradiated to the thus formed fluorescent material, the fluorescent material is capable of being caused to produce fluorescence having an intensity in accordance with the intensity of the recording light, and   wherein the substrate comprises a material permeable to the excitation light having the wavelength λ 2 .   
     
     
         113 . The recording medium of  claim 112 , wherein the wavelength λ 1  of the recording light and the wavelength λ 2  of the excitation light are different from each other. 
     
     
         114 . A multi-valued information reproducing method for reproducing multi-valued information from a recording medium, on which the multi-valued information has been recorded, wherein the multi-valued information has been recorded on the recording medium by irradiating the recording light having the predetermined wavelength λ 1 , which recording light has an intensity in accordance with the multi-valued information, to the recording layer of the recording medium, the material, which is located at a site having been exposed to the recording light, being thereby caused to change into the fluorescent material, the method comprising the steps of:
 i) providing the recording medium, the recording medium comprising:
 a substrate, and 
 a recording layer overlaid on the substrate, 
 wherein the recording layer comprises a material, which has properties such that, when recording light having a predetermined wavelength λ 1  is irradiated to the material, the material is capable of being caused to change into a fluorescent material and such that, when excitation light having a wavelength λ 2  is then irradiated to the thus formed fluorescent material, the fluorescent material is capable of being caused to produce fluorescence having an intensity in accordance with the intensity of the recording light; 
   ii) irradiating the excitation light having the wavelength λ 2 , which falls within an excitation wavelength region for the fluorescent material, to the recording layer, the fluorescent material being thereby caused to produce the fluorescence having an intensity in accordance with the intensity of the recording light; and   iii) detecting the fluorescence,   wherein light having the wavelength λ 2  different from the wavelength λ 1  of the recording light is employed as the excitation light.   
     
     
         115 . A multi-valued information reproducing method for reproducing multi-valued information from a recording medium, on which the multi-valued information has been recorded, wherein the multi-valued information has been recorded on the recording medium by irradiating the recording light having the predetermined wavelength λ 1 , which recording light has an intensity in accordance with the multi-valued information, to the recording layer of the recording medium, the material, which is located at a site having been exposed to the recording light, being thereby caused to change into the fluorescent material, the method comprising the steps of:
 i) providing the recording medium, the recording medium comprising:
 a substrate, and 
 a recording layer overlaid on the substrate, 
 wherein the recording layer comprises a material, which has properties such that, when recording light having a predetermined wavelength λ 1  is irradiated to the material, the material is capable of being caused to change into a fluorescent material and such that, when excitation light having a wavelength λ 2  is then irradiated to the thus formed fluorescent material, the fluorescent material is capable of being caused to produce fluorescence having an intensity in accordance with the intensity of the recording light; 
   ii) irradiating the excitation light having the wavelength λ 2 , which falls within an excitation wavelength region for the fluorescent material, to the recording layer, the fluorescent material being thereby caused to produce the fluorescence having an intensity in accordance with the intensity of the recording light; and   iii) detecting the fluorescence,   wherein the recording medium is constituted such that the substrate comprises a dielectric material, a metal film is overlaid on one surface of the dielectric material, and the recording layer is overlaid on the metal film, and   wherein the excitation light is irradiated from the substrate side to the recording medium such that the excitation light impinges at a specific angle of incidence upon the metal film.   
     
     
         116 . The multi-valued information reproducing method of  claim 115 , wherein the wavelength λ 1  of the recording light and the wavelength λ 2  of the excitation light are different from each other. 
     
     
         117 . A multi-valued information reproducing apparatus for reproducing multi-valued information from a recording medium, on which the multi-valued information has been recorded, wherein the multi-valued information has been recorded on the recording medium by irradiating the recording light having the predetermined wavelength λ 1 , which recording light has an intensity in accordance with the multi-valued information, to the recording layer of the recording medium, the material, which is located at a site having been exposed to the recording light, being thereby caused to change into the fluorescent material, the apparatus comprising:
 i) excitation means for irradiating the excitation light having the wavelength λ 2 , which falls within an excitation wavelength region for the fluorescent material, to the recording layer in order to cause the fluorescent material to produce the fluorescence having an intensity in accordance with the intensity of the recording light; and   ii) read-out means for detecting the fluorescence,   wherein the recording medium comprises:
 a substrate, and 
 a recording layer overlaid on the substrate, 
   wherein the recording layer comprises a material, which has properties such that, when recording light having a predetermined wavelength λ 1  is irradiated to the material, the material is capable of being caused to change into a fluorescent material and such that, when excitation light having a wavelength λ 2  is then irradiated to the thus formed fluorescent material, the fluorescent material is capable of being caused to produce fluorescence having an intensity in accordance with the intensity of the recording light, and   wherein the excitation means irradiates light, which has the wavelength λ 2  different from the wavelength λ 1  of the recording light, as the excitation light to the recording layer.   
     
     
         118 . A multi-valued information reproducing apparatus for reproducing multi-valued information from a recording medium, on which the multi-valued information has been recorded, wherein the multi-valued information has been recorded on the recording medium by irradiating the recording light having the predetermined wavelength λ 1 , which recording light has an intensity in accordance with the multi-valued information, to the recording layer of the recording medium, the material, which is located at a site having been exposed to the recording light, being thereby caused to change into the fluorescent material, the apparatus comprising:
 i) excitation means for irradiating the excitation light having the wavelength λ 2 , which falls within an excitation wavelength region for the fluorescent material, to the recording layer in order to cause the fluorescent material to produce the fluorescence having an intensity in accordance with the intensity of the recording light; and   ii) read-out means for detecting the fluorescence,   wherein the recording medium comprises:
 a substrate, and 
 a recording layer overlaid on the substrate, 
   wherein the recording layer comprises a material, which has properties such that, when recording light having a predetermined wavelength λ 1  is irradiated to the material, the material is capable of being caused to change into a fluorescent material and such that, when excitation light having a wavelength λ 2  is then irradiated to the thus formed fluorescent material, the fluorescent material is capable of being caused to produce fluorescence having an intensity in accordance with the intensity of the recording light,   wherein the excitation means scans the recording layer with converged beam-like excitation light,   wherein the read-out means detects the fluorescence with respect to each of positions which are being scanned with the excitation light,   wherein the recording medium is constituted such that the substrate comprises a dielectric material, a metal film is overlaid on one surface of the dielectric material, and the recording layer is overlaid on the metal film, and   wherein the excitation means irradiates the excitation light to the recording medium such that the excitation light impinges upon the metal film from the substrate side.   
     
     
         119 . The multi-valued information reproducing apparatus of  claim 118 , wherein the wavelength λ 1  of the recording light and the wavelength λ 2  of the excitation light are different from each other.