Abstract:
A label ( 1 ) of this invention includes an optical function layer ( 13 ) configured to pass light of a certain wavelength, a light absorption layer ( 15 ) facing the optical function layer ( 13 ) and configured to absorb the light of the wavelength, and a light scattering layer ( 12 ) intervening between the optical function layer ( 13 ) and the light absorption layer ( 15 ) and including hollow bodies configured to scatter the light of the wavelength. The light scattering layer ( 12 ) is configured to raise a transmittance at the wavelength upon receiving external heat.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This is a Continuation Application of PCT application No. PCT/JP2012/068347, filed on 2012 Jul. 19, which was published under PCT Article 21(2) in Japanese. 
     This application is based upon and claims the benefit of priority from prior Japanese Patent Application No. 2011-213728, filed on 2011 Sep. 29, the entire contents of which are incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to, for example, a label usable for an anti-counterfeit purpose, and an adhesive label and a printed product including the same. 
     2. Description of the Related Art 
     In recent years, counterfeit articles circulating on the market have developed into a serious social issue. As a countermeasure, for example, a label that enables authenticity confirmation is put on an article. Examples of such labels or so-called anti-counterfeit labels are a label including a printed layer formed by a functional ink such as fluorescent ink or OVI (Optically Variable Ink), a label including a printed layer formed by special printing such as microprinting or intaglio printing, a label including a hologram or a diffraction grating, a label in which information is written by magnetic recording, and a label including an IC (Integrated Circuit) tag. 
     Many anti-counterfeit labels are themselves difficult to counterfeit. However, some anti-counterfeit labels can relatively easily be peeled off articles on which they are put. Such labels may be used dishonestly by peeling such labels off used articles and putting them on counterfeit articles. 
     Some anti-counterfeit labels employ measures to make such reuse impossible. 
     For example, an anti-counterfeit label has a notch. Such an anti-counterfeit label is designed to tear from the notch position when ripped off an article on which it is put. 
     Another anti-counterfeit label uses a base that causes brittle fracture by a relatively small force. Such a label is also designed to break when ripped off an article on which it is put. 
     Still another anti-counterfeit label includes a brittle layer that causes brittle fracture by a relatively small force. The adhesive strength between the brittle layer and a layer adjacent on the observer side changes depending on the position. When this label is ripped off an article on which it is put, the brittle layer breaks in a pattern corresponding to the adhesive strength distribution. As a result, for example, the brittle layer and the like partially remain on the article with a pattern corresponding to a character string “VOID”. A missing portion having a pattern corresponding to the character string “VOID” is formed in the brittle layer and the like of the anti-counterfeit label. 
     It is impossible or difficult to reuse such an anti-counterfeit label ripped off and released from an article. However, an organic solvent soaking into the adhesive layer or bonding layer may make it possible to release the anti-counterfeit label without damaging the label main body. 
     Some anti-counterfeit labels employ a technique of making this impossible or difficult. 
     For example, an anti-counterfeit label uses, as the material of an adhesive layer, a mixture of an adhesive and an additive insoluble in it (for example, see patent literature 1). When this anti-counterfeit label is released using an organic solvent, the surface of the adhesive layer becomes uneven due to the difference in solubility to the organic solvent between the adhesive and the additive. 
     Another anti-counterfeit label uses a printed layer containing a dye soluble in an organic solvent (for example, see patent literature 2). When this anti-counterfeit label is released using an organic solvent, the dye seeps from the printed layer. 
     Note that this label can be released without damage to the label main body or seepage of the dye when the surface is heated using a drier or the like. As an anti-counterfeit label taking a measure against the release by heating, there is, for example, an anti-counterfeit label using an adhesive layer containing foaming particles that foams when heated (for example, see patent literature 3). 
     It is impossible or difficult to reuse these anti-counterfeit labels released using an organic solvent or heat. However, after the expiration date of an article, the possibility that the anti-counterfeit label is removed together with the surface of the article on which it is put needs to be taken into consideration. The above-described measures cannot prevent reuse of an anti-counterfeit label removed in this way. 
     CITATION LIST 
     Patent Literatures 
     Patent Literature 1: Jpn. Pat. Appln. KOKAI Publication No. 2005-266147 
     Patent Literature 2: Jpn. Pat. Appln. KOKAI Publication No. 10-204363 
     Patent Literature 3: Jpn. Pat. Appln. KOKAI Publication No. 2000-293108 
     BRIEF SUMMARY OF THE INVENTION 
     It is an object of the present invention to make it possible to suppress reuse of a label put on an article after the expiration date of the article. 
     According to the first aspect of the present invention, there is provided a label comprising an optical function layer configured to pass light of a certain wavelength, a light absorption layer facing the optical function layer and configured to absorb the light of the wavelength, and a light scattering layer intervening between the optical function layer and the light absorption layer and including hollow bodies configured to scatter the light of the wavelength, wherein the light scattering layer is configured to raise a transmittance at the wavelength upon receiving external heat. 
     According to the second aspect of the present invention, there is provided a label wherein the light scattering layer raises the transmittance at the wavelength when the hollow bodies are broken. 
     According to the third aspect of the present invention, there is provided the label according to the first or second aspect, wherein the wavelength is in an infrared range, and the optical function layer comprises a black layer. 
     According to the fourth aspect of the present invention, there is provided the label according to the third aspect, wherein the wavelength is in a near infrared range, and a transmittance difference for a wavelength in one of a wavelength range of 700 to 800 nm of the near infrared range and a wavelength range of 800 to 1,500 nm of the near infrared range is not less than 10% in the optical function layer. 
     According to the fifth aspect of the present invention, there is provided the label according to any one of the first to fourth aspects, wherein the optical function layer comprises a colored pattern facing part of the light absorption layer while sandwiching the light scattering layer, and the light absorption layer comprises a colored layer having the same color as the optical function layer. 
     According to the sixth aspect of the present invention, there is provided the label according to any one of the first to fourth aspects, further comprising a light absorption pattern configured to absorb the light of the wavelength, the light absorption pattern intervening between the optical function layer and the light scattering layer or facing the light scattering layer while sandwiching the optical function layer. 
     According to the seventh aspect of the present invention, there is provided the label according to the sixth aspect, wherein the light absorption pattern and the optical function layer have the same color. 
     According to the eighth aspect of the present invention, there is provided an adhesive label comprising a label according to any one of the first to seventh aspects, and an adhesive layer facing a major surface of the label on a side of a light absorption layer. 
     According to the ninth aspect of the present invention, there is provided a printed product comprising a label according to any one of the first to seventh aspects; a print base facing a major surface of the label on a side of a light absorption layer, and an adhesive layer intervening between the label and the print base and configured so that the label can be put on the print base. 
     According to the present invention, it is possible to suppress reuse of a label put on an article after the expiration date of the article. 
     In the label according to the first aspect, when processing (to be referred to as “invalidation processing” hereinafter) of crushing the hollow bodies is performed for at least part of the light scattering layer, the transmittance of the light scattering layer at the wavelength (to be referred to as a “first wavelength” hereinafter) rises in that portion. As a result, a spectral characteristic of this label changes before and after the invalidation processing when illuminated with light of the first wavelength. 
     For this reason, for example, when the invalidation processing is performed after the expiration date of an article on which the label is put, the spectral characteristic when illuminated with light of the first wavelength can be changed. Hence, when the spectral characteristic when illuminated with light of the first wavelength is measured for the label put on an article whose authenticity is unknown, the authenticity of the article can be determined. 
     In the label according to the second aspect, the first wavelength is in an infrared range. That is, the first wavelength belongs to a range other than the visible light range. In this label, the optical function layer is a black layer. 
     For example, when the optical function layer covers the entire surface of the light scattering layer, and the near infrared range is a black layer, it is impossible or very difficult to grasp by the naked eye whether the label has undergone the invalidation processing. Hence, in this case, it is difficult to notice that the label has a special structure. It is therefore possible to suppress counterfeiting of the label itself. 
     In the label according to the third aspect, concerning the label according to the second aspect, the first wavelength is in a near infrared range, a transmittance of the optical function layer at the first wavelength is not less than 30%, and a transmittance difference for a wavelength in one of a wavelength range of 700 to 800 nm of the near infrared range and a wavelength range of 800 to 1,500 nm of the near infrared range is not less than 10% in the optical function layer. That is, the transmittance spectrum of the optical function layer in the near infrared range exhibits a high transmittance at the first wavelength. The transmittance difference for a wavelength in one of the wavelength range of 700 to 800 nm of the near infrared range and the wavelength range of 800 to 1,500 nm of the near infrared range is not more than 10%. It is therefore impossible or difficult for a person who is unaware of use of light of the first wavelength for authenticity determination to discriminate between the label before invalidation processing from that after invalidation processing. For this reason, a person who is committing dishonesty hardly notices that a measure against counterfeiting using the invalidation processing is taken. 
     In the label according to the fourth aspect, concerning the label according to any one of the first to third aspects, the optical function layer comprises a colored pattern, and the light absorption layer comprises a colored layer having the same color as the optical function layer. As described above, in this label, the light absorption layer is visualized in at least part of the label by the above-described invalidation processing. If the pattern formed by the optical function layer and the light absorption layer have the same color, it is impossible or very difficult to observe the pattern formed by the optical function layer after the processing. For example, when the optical function layer is formed into a one- or two-dimensional code pattern, observation of the code can be made impossible by the invalidation processing. As a result, the observer can clearly grasp by the naked eye that the label has undergone the invalidation processing. It is therefore possible to suppress an act of conducting counterfeiting by putting the label again. 
     The label according to the fifth aspect further comprises a light absorption pattern configured to absorb the light of the first wavelength. The light absorption pattern can intervene between the optical function layer and the light scattering layer or face the light scattering layer while sandwiching the optical function layer. 
     Employing this arrangement, when the label is observed at the first wavelength, an image corresponding to the light absorption pattern is observed. On the other hand, after the invalidation processing is performed, the image corresponding to the light absorption pattern cannot be observed due to absorption of light of the first wavelength by the light absorption layer. For this reason, when this arrangement is employed, authenticity determination can be performed relatively easily based on observation of the image at the first wavelength. 
     In the label according to the sixth aspect, concerning the label according to the fifth aspect, the light absorption pattern and the optical function layer have the same color. For this reason, when the label is observed by the naked eye, the existence of the light absorption pattern is hardly noticed. Hence, when this arrangement is employed, counterfeiting of the label itself can be suppressed. 
     An adhesive label according to the seventh aspect includes a label according to any one of the first to sixth aspects. The adhesive label is in a form usable when putting the label on an article. 
     A printed product according to the eighth aspect includes a label according to any one of the first to sixth aspects. The possibility that the label on the printed product is reused after its expiration date is low. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         FIG. 1  is a plan view schematically showing a label according to an embodiment of the present invention. 
         FIG. 2  is a sectional view of the label shown in  FIG. 1  taken along a line II-II. 
         FIG. 3  is a view schematically showing an example of an invalidation processing method of the label shown in  FIGS. 1 and 2 . 
         FIG. 4  is a plan view schematically showing an example of a label that has undergone invalidation processing. 
         FIG. 5  is a sectional view of the label shown in  FIG. 4  taken along a line V-V. 
         FIG. 6  is a plan view schematically showing a modification of the label shown in  FIGS. 1 and 2 . 
         FIG. 7  is a sectional view of the label shown in  FIG. 6  taken along a line VII-VII. 
         FIG. 8  is a sectional view schematically showing another modification of the label shown in  FIGS. 1 and 2 . 
         FIG. 9  is a sectional view schematically showing an example of an adhesive label. 
         FIG. 10  is a plan view schematically showing an example of a printed product. 
         FIG. 11  is a sectional view of the printed product shown in  FIG. 10  taken along a line XI-XI. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     An embodiment of the present invention will now be described in detail with reference to the accompanying drawings. Note that the same reference numerals denote constituent elements having the same or similar functions throughout the drawings, and a repeated description thereof will be omitted. “Near infrared range” here indicates a wavelength range of 700 to 1,500 nm. 
       FIG. 1  is a plan view schematically showing a label according to an embodiment of the present invention.  FIG. 2  is a sectional view of the label shown in  FIG. 1  taken along a line II-II. 
     A label  1  shown in  FIGS. 1 and 2  includes a base  11 , a light absorption layer  15 , a light scattering layer  12 , and an optical function layer  13 . The light absorption layer  15 , the light scattering layer  12 , and the optical function layer  13  are stacked on the base  11  in this order. The label  1  has a front surface on the side of the optical function layer  13  and a back surface on the side of the base  11 . 
     The optical function layer  13 , the light scattering layer  12 , and the light absorption layer  15  may be stacked on the base  11  in this order. In this case, the label  1  has a front surface on the side of the base  11  and a back surface on the side of the light absorption layer  15 . 
     The base  11  is, for example, a film made of a resin. As the resin, for example, plastics such as polyethylene terephthalate, polyethylene naphthalate, polypropylene, polycarbonate, and polyethylene are usable. The base is typically transparent but may be opaque like aluminum foil. However, when the label  1  has the front surface on the side of the base  11 , a material that passes light of a first wavelength or typically light of a first wavelength and a second wavelength different from each other is used as the base  11 . 
     The base  11  can have a single layer structure or a multilayer structure. The base  11  can be omitted. 
     The light absorption layer  15  is provided on one major surface of the base  11 . 
     The light absorption layer  15  absorbs light of the first wavelength. More specifically, the absorbance of the light absorption layer  15  at the first wavelength is higher than that of the light scattering layer  12  at the first wavelength and that of the optical function layer  13  at the first wavelength immediately after the manufacture of the label  1 . The absorbance of the light absorption layer  15  at the first wavelength is, for example, 70% or more and typically 90% or more. 
     When the first wavelength is in the near infrared range, the light absorption layer  15  contains, for example, a near infrared absorbent and a resin. As the near infrared absorbent, for example, carbon black used in a process india ink is usable. As the resin, for example, a material generally used in a process ink is usable. 
     The light absorption layer  15  is formed by, for example, a printing method. Examples of the printing method are offset printing, gravure printing, screen printing, and flexographic printing. The thickness of the light absorption layer  15  falls within the range of, for example, 0.5 to 10 μm, and typically falls within the range of 0.5 to 2 μm. 
     The light scattering layer  12  is provided on the light absorption layer  15 . The light scattering layer  12  contains hollow bodies that scatter light of the first wavelength. More specifically, the light scattering layer  12  scatters light of the first wavelength at least during a period from completion of the label  1  to application of invalidation processing. The light scattering layer  12  is configured to, when processing of breaking the hollow bodies is performed, increase the transmittance at the first wavelength at the position where the processing has been performed. 
     Immediately after the manufacture of the label  1 , a transmittance T 1  of the light scattering layer  12  with respect to light of the first wavelength falls within the range of, for example, 0% to 50%, and typically falls within the range of 20% to 40%. After invalidation processing, a transmittance T 2  of the light scattering layer  12  with respect to light of the first wavelength falls within the range of, for example, 60% to 100%, and typically falls within the range of 70% to 90%. The ratio of the transmittance T 2  to the transmittance T 1  is, for example, 1.2 or more, and typically falls within the range of 1.75 to 4.5. 
     An example of the hollow bodies contained in the light scattering layer  12  is an organic polymer having a hollow structure. A composition and manufacturing method of such an organic polymer are described in, for example, Jpn. Pat. Appln. KOKAI Publication No. 56-32513, 61-185505, 60-69103, 63-213509, 63-135409, 60-223873, 63-110208, 61-87734, or 62-127336. 
     Each of the hollow bodies contained in the light scattering layer  12  typically has a core component and a shell component surrounding it. The core component is formed using, for example, methacrylic acid, or methacrylic acid and another monomer. The shell component is formed using, for example, styrene. The particle diameter of the hollow bodies is, for example, 0.1 to 5 μm and, typically 0.3 to 1 μm. 
     A polymer that holds hollow bodies is typically an aqueous polymer having film forming properties. This polymer is typically synthesized by emulsion polymerization, solution polymerization, or bulk polymerization. A polymer that holds hollow bodies has such plasticity that does not impede breakage of the hollow bodies in invalidation processing (to be described later). The glass transition point of an aqueous polymer is, for example, 100° C. or less, and typically falls within the range of −80° C. to 25° C. 
     Examples of the aqueous polymer are water-dispersible polymers and water-soluble polymers. Water-dispersible polymers are dispersible in water. Water-soluble polymers are soluble in water. 
     Examples of monomers that form water-dispersible polymers are ethyl acrylate (EA), butyl acrylate (BA), 2-ethylhexyl acrylate (2EHA), and butadiene. Each of those monomers can form a homopolymer by itself, or form a copolymer together with one or more other monomers. A particularly preferable polymer is a polymer obtained by reaction between hexamethylene diisocyanate and polycarbonate polyol. 
     Examples of monomers that form water-soluble polymers are carboxylic acid derivatives of monomers exemplified for the water-dispersible polymers. Examples of the derivatives are acrylic acid (Aa), methacrylic acid, monomethyl itaconic acid (MMI), and 2-carboxyethyl acrylate. Polymers formed from those derivatives become soluble in water by changing at least some of the carboxy groups in monomers to a form of an alkali metal salt, an amine salt, or an ammonium salt. 
     The mass ratio of the hollow bodies in the light scattering layer  12  and the polymer that holds the hollow bodies falls within the range of, for example, 1:1 to 1:100. The light scattering layer  12  may further contain a plasticizer, a wetting agent, an antifoaming agent, a thickener, an emulsifying agent, and a wax such as carnauba wax, or paraffin wax. 
     The light scattering layer  12  has light scattering properties and normally takes on a white color. The light scattering layer  12  hides at least part of the light absorption layer  15  at least during a period from completion of the label  1  to application of invalidation processing. 
     The light scattering layer  12  is formed by, for example, a coating method. This coating can be performed using, for example, an air-knife coater, roll coater, spray coater, gravure coater, micro gravure coater, or bar coater. The film thickness of the light scattering layer  12  falls within the range of, for example, 5 to 20 μm, and typically falls within the range of 5 to 15 μm. 
     The optical function layer  13  is provided on the light scattering layer  12 . The optical function layer  13  passes light of the first wavelength. The transmittance of the optical function layer  13  with respect to light of the first wavelength is, for example, 30% or more, and typically falls within the range of 30% to 60%. 
     In the example shown in  FIGS. 1 and 2 , the optical function layer  13  is formed in a pattern.  FIGS. 1 and 2  illustrate an example in which the pattern of the optical function layer  13  forms a one-dimensional code. The pattern may form a two-dimensional code. Alternatively, the pattern may form another pattern such as a character, a symbol, a design, or a graphic. 
     The optical function layer  13  may be colored. For example, the optical function layer  13  may be a colored pattern. When the optical function layer  13  is a colored pattern, the optical function layer  13  and the light absorption layer  15  preferably have the same color. If the pattern formed by the optical function layer and the light absorption layer have the same color, it is impossible or very difficult to observe the pattern formed by the optical function layer after the invalidation processing. As a result, the observer can clearly grasp by the naked eye that the label has undergone the invalidation processing. It is therefore possible to psychologically suppress an act of conducting counterfeiting by putting the label again. 
     The optical function layer  13  is typically a black layer. For example, when the optical function layer  13  covers the entire surface of the light scattering layer  12 , and the optical function layer  13  is a black layer, it is impossible or very difficult to grasp by the naked eye whether the label has undergone the invalidation processing. Hence, in this case, it is difficult to notice that the label has a special structure. It is therefore possible to suppress counterfeiting of the label itself. 
     Note that “black” here indicates that the reflectance is 10% or less for all light components whose wavelengths fall within the range of 400 to 700 nm when the intensity of specular reflected light is measured. 
     When the first wavelength is within the near infrared range, a material whose transmittance at the first wavelength is 30% or more and in which the transmittance difference for a wavelength in one of the wavelength range of 700 to 800 nm of the near infrared range and the wavelength range of 800 to 1,500 nm of the near infrared range is 10% or more may be used as the optical function layer  13 . That is, as for the transmittance spectrum in the near infrared range, the optical function layer  13  may exhibit a high transmittance at the first wavelength and a low transmittance at other wavelengths. For example, the optical function layer  13  is assumed to have such an optical characteristic. In this case, the second wavelength is also assumed to be in the near infrared range, and the transmittance of the optical function layer  13  at the second wavelength is assumed to be lower than that of the optical function layer  13  at the first wavelength; for example, the difference from the transmittance of the optical function layer  13  at the first wavelength is 10% or more. 
     The optical function layer  13  having the above-described optical characteristic, that is, the optical characteristic of selectively passing light in a partial wavelength range of light in the near infrared range and absorbing the remaining light contains, for example, a predetermined near infrared absorbent and a resin. This near infrared absorbent absorbs, for example, light of the second wavelength. As the near infrared absorbent, for example, at least one material selected from the group consisting of phthalocyanine compounds, naphthalocyanine compounds, anthraquinone compounds, giimonium compounds, and cyanine compounds is usable. As the resin, for example, a material generally used in a process ink is usable. 
     The near infrared absorbent used here typically has an absorption spectrum in the near infrared range different from the near infrared absorbent used in the light absorption layer  15 . For example, the near infrared absorbent used here has a lower absorbance with respect to light of the first wavelength as compared to the near infrared absorbent used in the light absorption layer  15 . Alternatively, as the near infrared absorbent, a compound exemplified as the near infrared absorbent that the light absorption layer  15  can contain may be used. 
     The optical function layer  13  is formed by, for example, a printing method. Examples of the printing method are offset printing, gravure printing, screen printing, and flexographic printing. The thickness of the optical function layer  13  falls within the range of, for example, 0.5 to 10 μm, and typically falls within the range of 1 to 5 μm. 
     Authenticity determination of the label  1  described above is typically done by mechanical reading. For example, the authenticity determination can be performed using a sensor capable of detecting light in a specific wavelength range or a CCD (Charge Coupled Device) camera including a bandpass filter that passes light in a predetermined wavelength range. 
     When the label  1  shown in  FIGS. 1 and 2  is illuminated with light of the first wavelength, this light passes through the optical function layer  13  and is scattered by the light scattering layer  12 . Hence, in this case, when illuminated with light of the first wavelength, the label  1  exhibits a spectral characteristic unique to the optical function layer  13  based on the scattered light from the light scattering layer  12 . This spectral characteristic is a specific characteristic corresponding to the detailed structure of the label  1 . Hence, the authenticity of the label  1  can be determined by measuring the spectral characteristic. 
     In addition, invalidation processing to be described below can be performed for the label  1 . When this processing is performed, dishonest reuse of the label  1  can be suppressed. 
       FIG. 3  is a view schematically showing an example of the invalidation processing method of the label shown in  FIGS. 1 and 2 .  FIG. 4  is a plan view schematically showing an example of a label that has undergone invalidation processing.  FIG. 5  is a sectional view of the label shown in  FIG. 4  taken along a line V-V. 
     In the invalidation processing method shown in  FIG. 3 , a thermal head  41  is brought into contact with the label  1 , thereby heating at least part of the light scattering layer  12 . This breaks at least some of the hollow bodies contained in the heated portion of the light scattering layer  12 . 
     When this invalidation processing is performed, the transmittance of the light scattering layer  12  at the first wavelength rises at a position where the thermal head  41  comes into contact with the label  1 . As a result, as shown in  FIG. 5 , a first region  12   a  where the transmittance of the light scattering layer  12  at the first wavelength remains the same as that before the invalidation processing and a second region  12   b  where the transmittance of the light scattering layer  12  at the first wavelength is higher than that before the invalidation processing are formed in the light scattering layer  12 . 
     In a portion of the label  1  corresponding to the second region  12   b , light of the first wavelength passes through both the optical function layer  13  and the light scattering layer  12 . The light of the first wavelength is absorbed by the light absorption layer  15 . 
     Hence, in this case, when illuminated with light of the first wavelength, the portion of the label  1  corresponding to the second region  12   b  mainly exhibits a spectral characteristic resulting from absorption of the light absorption layer  15 . As a result, in this portion, it is impossible or very difficult to detect the spectral characteristic unique to the optical function layer  13  in this portion. That is, the spectral characteristic of this portion differs before and after the invalidation processing. Hence, it is possible to determine based on the difference in the spectral characteristic whether the label  1  has undergone the invalidation processing. 
     As described above, the hollow bodies in the light scattering layer  12  can be broken by, for example, applying heat/and or pressure to the label  1 . Alternatively, the hollow bodies may be broken physically using a microneedle or the like. 
     Breakage of the hollow bodies in the light scattering layer  12  is an irreversible change. Hence, once having undergone the invalidation processing, the label  1  cannot return to the state before the processing. For this reason, when the above processing is performed for the label  1 , dishonest reuse of the label  1  can reliably be suppressed. 
     Authenticity determination of the label  1  may be done using light of a plurality of wavelengths. For example, authenticity determination of the label  1  may be performed using light of the first wavelength and light of the second wavelength different from the first wavelength. Alternatively, authenticity determination of the label  1  may be performed using light of the first wavelength and light of two or more wavelengths different from the first wavelength. The number of wavelengths used for authenticity determination falls within the range of, for example, 1 to 5, and preferably falls within the range of 2 to 5. If the number of wavelengths used for authenticity determination is too large, the time required for authenticity determination of the label  1  may be excessively long. 
     The above-described label  1  can be variously modified. 
       FIG. 6  is a plan view schematically showing a modification of the label shown in  FIGS. 1 and 2 .  FIG. 7  is a sectional view of the label shown in  FIG. 6  taken along a line VII-VII. 
     The label  1  shown in  FIGS. 6 and 7  has the same structure as the label described with reference to  FIGS. 1 to 5  except that the optical function layer  13  covers the entire major surface of the light scattering layer  12 , and the label  1  further includes a light absorption pattern  14  facing the light scattering layer  12  while sandwiching the optical function layer  13  between them. 
     The light absorption pattern  14  absorbs light of the first wavelength. As the material of the light absorption pattern  14 , for example, the same materials explained above for the light absorption layer  15  are usable. 
     The light absorption pattern  14  preferably has the same color as the optical function layer  13  or a light color as long as it exhibits a sufficient absorbance with respect to light of the first wavelength. This makes it difficult to notice the existence of the light absorption pattern  14  when the label  1  is observed by the naked eye. 
     The light absorption pattern  14  is preferably distributed all over a region corresponding to the light scattering layer  12 . This can make it difficult to analyze the spectral characteristic of the optical function layer  13 . 
     The light absorption pattern  14  is formed by, for example, a printing method. Examples of the printing method are offset printing, gravure printing, screen printing, and flexographic printing. Alternatively, the light absorption pattern  14  may be formed using a thermal transfer ribbon, inkjet printing, or laser printing. The thickness of the light absorption pattern  14  falls within the range of, for example, 0.5 to 10 μm, and typically falls within the range of 0.5 to 2 μm. 
     The label  1  shown in  FIGS. 6 and 7  also exhibits a difference in spectral characteristic before and after the above-described invalidation processing when illuminated with light of the first wavelength. Hence, authenticity determination can be done by detecting the difference in the spectral characteristic. 
       FIG. 8  is a sectional view schematically showing another modification of the label shown in  FIGS. 1 and 2 . The label  1  shown in  FIG. 8  has the same structure as the label described with reference to  FIGS. 6 and 7  except that the light absorption pattern  14  intervenes between the optical function layer  13  and the light scattering layer  12 . 
     The label  1  shown in  FIG. 8  also exhibits a difference in spectral characteristic before and after the above-described invalidation processing when illuminated with light of the first wavelength. Hence, authenticity determination can be done by detecting the difference in the spectral characteristic. 
     Additionally, in the label  1  shown in  FIG. 8 , it is possible to make the existence of the light absorption pattern  14  unnoticeable by forming the optical function layer  13  as a colored layer, and in particular, forming the optical function layer  13  as a black layer. 
     An adhesive label and a printed product including the above-described label  1  will be described next. 
       FIG. 9  is a sectional view schematically showing an example of an adhesive label. 
     An adhesive label  10  shown in  FIG. 9  includes the label  1  described with reference to  FIGS. 1 and 2 , and an adhesive layer  2 . The adhesive layer  2  is provided on the back surface of the label  1 . 
     The adhesive label  10  is put on, for example, an article desired to be confirmed as authentic. Note that the adhesive label  10  may further include a release paper that releasably covers the surface of the adhesive layer  2 . 
       FIG. 10  is a plan view schematically showing an example of a printed product.  FIG. 11  is a sectional view of the printed product shown in  FIG. 10  taken along a line XI-XI. 
     A printed product  100  shown in  FIGS. 10 and 11  includes the label  1  described with reference to  FIGS. 1 and 2 , the adhesive layer  2 , and a printed product main body  3 . 
     The printed product main body  3  includes a print base  3   a  and a printed layer  3   b . The label  1  is put on the print base  3   a  via the adhesive layer  2 . 
     The print base  3   a  is made of, for example, paper, plastic, wood, glass, or resin. The print base  3   a  can have a single layer structure or a multilayer structure. The print base  3   a  can have a layer shape or another shape. 
     The printed layer  3   b  is provided on the print base  3   a . The printed layer  3   b  can wholly or only partially cover the print base  3   a.    
     Assume that the above-described invalidation processing is performed for the printed product  100  after its expiration date. This makes it possible to discriminate, for a printed product whose authenticity is unknown, whether the label  1  is reused. That is, it is possible to do authenticity determination for a printed product whose authenticity is unknown. It is therefore possible to discourage a person from committing dishonesty and thus suppress reuse of a label put on an article after the expiration date of the article. As a result, counterfeiting of the printed product  100  can be suppressed. 
     EXAMPLES 
     Examples of the present invention will be described below. 
     Example 1 
     The label  1  described with reference to  FIGS. 1 and 2  was manufactured by the following method. 
     First, a coated board was prepared as the base  11 . Next, india ink (Fine Star R92 Black: available from Toyo Ink) was applied to part of one major surface of the base  11  using a bar coater such that the dried film thickness became 2 μm. The light absorption layer  15  was formed in this way. 
     An ink A having a composition to be described below was applied onto the light absorption layer  15  using a bar coater such that the dried film thickness became 10 μm. The light scattering layer  12  was formed in this way. 
     An ink B having a composition to be described below was printed on the light scattering layer  12  to form a one-dimensional code pattern using an offset printing press. The pattered optical function layer  13  was formed in this way. The label is thus completed. 
     [Composition of Ink A] 
     Ropaque OP-84J (available from Dow Chemical Company) 
     25 parts by mass 
     Acrylic emulsion polymer 
     2.5 parts by mass 
     Water 
     30 parts by mass 
     [Composition of Ink B] 
     Near infrared absorbing dye YKR-3081 (available from Yamamoto Chemicals) 
     5 parts by mass 
     FD Karton ACE medium: available from Toyo Ink) 
     95 parts by mass 
     When the thus obtained label  1  was observed by a camera including a bandpass filter for passing a near infrared wavelength, the one-dimensional code as shown in  FIG. 1  could be read. 
     Invalidation processing was then performed for the label  1 . More specifically, a pressure of 1 kgf/cm 2  and heat of 160° C. were applied to the entire surface of the label  1 . When the label  1  was observed by the above camera after the processing, the one-dimensional code pattern as shown in  FIG. 1  could not be read. 
     Example 2 
     The label  1  described with reference to  FIGS. 6 and 7  was manufactured by the following method. 
     First, wood-free paper was prepared as the base  11 . Next, india ink (Fine Star R92 Black: available from Toyo Ink) was applied to part of one major surface of the base  11  using a bar coater such that the dried film thickness became 2 μm. The light absorption layer  15  was formed in this way. 
     The above-described ink A was applied onto the light absorption layer  15  using a bar coater such that the dried film thickness became 10 μm. The light scattering layer  12  was formed in this way. 
     An ink C having a composition to be described below was printed on the light scattering layer  12  using an offset printing press. The optical function layer  13  covering the entire surface of the light scattering layer  12  was formed in this way. 
     India ink (Fine Star R92 Black: available from Toyo Ink) was printed on the optical function layer  13  to form a one-dimensional code pattern using a gravure proof press. The light absorption pattern  14  was formed in this way. Note that the thickness of the light absorption pattern  14  was 1 μm. 
     [Composition of Ink C] 
     Fine Star R181 Red (available from Toyo Ink) 
     40 parts by mass 
     Fine Star R235 Yellow (available from Toyo Ink) 
     35 parts by mass 
     Fine Star R31 Indigo (available from Toyo Ink) 
     20 parts by mass 
     YKR-3081 (available from Yamamoto Chemicals) 
     5 parts by mass 
     The thus obtained label  1  was observed using camera 1 including a bandpass filter that passes a wavelength in a visible light range, camera 2 including a bandpass filter that passes the second wavelength (850 nm) belonging to the near infrared range, and camera 3 including a bandpass filter that passes the first wavelength (710 nm) belonging to the near infrared range. 
     Next, invalidation processing of the label  1  was performed under the same conditions as in Example 1. After that, observation was done using cameras 1 to 3 as in the above-described case. 
     Table 1 shows the results. In the columns of “camera 1”, “camera 2”, and “camera 3” of Table 1, “◯” indicates that the one-dimensional code was observable, and “x” indicates that the one-dimensional code was not observable. In the column of “authenticity determination”, “◯” indicates an authentic article, and “x” indicates a counterfeit that cannot be reused. 
     
       
         
               
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                 Invalidation 
                   
                   
                   
                 Authenticity 
               
               
                 processing 
                 Camera 1 
                 Camera 2 
                 Camera 3 
                 determination 
               
               
                   
               
             
             
               
                 Before 
                 x 
                 ∘ 
                 x 
                 ∘ 
               
               
                 After 
                 x 
                 x 
                 x 
                 x 
               
               
                   
               
             
          
         
       
     
     As shown in Table 1, before the invalidation processing, the one-dimensional code was observable by camera 2 but not observable by cameras 1 and 3. On the other hand, after the invalidation processing, the one-dimensional code was not observable by any of cameras 1 to 3. As described above, the authenticity of the label  1  could be judged by detecting the difference in the spectral characteristic of the label  1  before and after the invalidation processing.