LATENT IMAGE CARRIER, PRODUCTION METHOD THEREOF, LATENT IMAGE READER, AND ITS READING SYSTEM

Abstract: There is provided a latent image carrier from which data can be reliably read for a long time. It includes a data storage layer (2) having a latent image pattern formed of fine phosphor paticles dispersed and held on a substrate (4a). When the layer (2) is irradiated with light having a predetermined wavelength, the luminescence from the fine phosphor particles is read as optical information. When irradiated with such light, the surface of the substrate (4a) has a reflectivity of at least 20%. The data storage layer (2) base thickness T of 0.5 to 20 µm above the upper surface of the substrate (4a), and a protective sheet (6a) is laminated on the substrate (4a) to cover the data storage layer (2) as a whole.

Description: The present invention relates to a latent image forming member, such as an ID card, and more particularly to a latent image forming member having a latent image pattern, such as a bar code having fluorescent grains dispersed and held therein, and a method of manufacturing the same. In recent years, control of articles by means of bar codes has been performed widely in various industrial fields while centering the PDS (Point Of Sales) in the distribution field. Along with this, a suggestion has been made that a non-visible bar code, that is, a latent image bar code is put on in terms of the improving design and security (see, for example, Japanese Patent Application Laid-Open No. 52-80906). As for manufacturing of the latent image bar code, a method has been suggested (see, for example, Japanese Patent Application Laid-Open No. 53-9607) in which a ribbon containing fluorescent gains is made to be transferred to paper by impact so that the latent image bar code is printed on to the paper. At present, if the latent image bar code of the foregoing type is formed on an ID card for use as a credit card or the like, there arise a problem of unsatisfactory durability, such as the wear resistance and chemical resistance against, for example, alcohol. It will come to be difficult to read, the latent image bar code as the card is repeatedly used. Therefore, the code of the foregoing type has a disadvantage that the card cannot be used for a long time. The latent image can be obtained by printing, on, for example, white paper or plastic, with ink which is obtained by mixing 200g of sodium yttrium fluoride (NaYF 4 ) fluorescent substance activated by, for example, ytterbium and erbium with 160g of vinyl chloride type resin and 40g of plasticizer, followed by dispersing the mixture in a 400g of solvent. When the latent image of the foregoing type is irradiated with infrared rays by, for example, a gallium arsenide (GaAs) infrared ray emitting diode, the ink emits light, the excited light having a wavelength different from that of the infrared ray. The reading apparatus for reading the latent image receives the excited light from the ink, and treat or process a signal of the light to read out the data recorded in the latent image. Since the excited light obtainable from the latent image is very weak, if infrared rays from the infrared ray source is not efficiently applied to the latent image or if the excited light from the latent image is not efficiently incident upon a light detection device disposed in the reading apparatus, a high level of reading signal cannot be obtained and a reading error becomes liable to occur. JP-A-2-225,091 discloses an identity card with a fluorescent mark printed on the back of a transparent sheet which is then laminated with an opaque substrate. An aim of the present invention is to overcome at least partially the foregoing problems experienced with the conventional technology. More specifically, a first object of the present invention is to provide a latent image forming member which is capable of surely reading information to have high reliability and which can be used for a long time. According to the present invention, there is provided a latent image forming member comprising a base, and an information indication layer formed on the base and having a latent image pattern, in which fluorescent grains are dispersed and held, so that when said information indication layer is irradiated with light having a predetermined wavelength to excite said fluorescent grains, information of light emitted from said fluorescent grains may be optically read, characterised in that: reflectance of a surface of said base with respect to light having said predetermined wavelength is 20% or higher, a projection amount of said information indication layer from a top surface of said base is restricted to a range of from 0.5 to 20 µm, and a protection sheet is disposed which is laminated onto said base to cover all over said information indication layer and which permits light having said predetermined wavelength to pass therethrough. The term "information" in this specification includes, in its wide sense, information representing truth/falseness by presence/absence of the latent image pattern (for example, presence of a certain latent image pattern represents "true" and absence of the same represents "false"). That is, the latent image pattern does not need to be coded. However, the following description is made taking a case of narrow sense of information in which the latent image pattern is coded in order to simplify the description. In the present invention, as the light reflectance of the base adjacent to the information indication layer at a side thereof opposite a side on which excited light is incident is made to be 20% or higher, a part, of excitation light applied toward the information indication layer, directly impinges the fluorescent grains in the information indication layer to excite the fluorescent grains. Further, a part, of the excitation light passes through the information display layer to be reflected by the surface of the base, the reflected light being allowed to impinge the fluorescent grains to excite the fluorescent grains. Therefore, the excitation of the fluorescent material can be performed efficiently. Further, the arrangement that the projection amount T of the information indication layer is restricted to a range of 0.5 to 20 mm enables the light emission power (luminescent power) from the information indication layer to be obtained sufficiently. Further, the deformation (distortion due to sagging) of the latent image, such as the bar code, in the information indication layer can be prevented at the time of laminating the protection sheet. Therefore, information of the information indication layer can accurately be read. Since the overall body of the information display layer is laminated by the protection sheet, various advantages such that excellent durability and chemical resistance can be attained and it can be used for a long time. Therefore, a highly reliable latent image forming member can be provided. The foregoing and other aims, features and advantages of the invention will be made clearer from the following description of non-limiting embodiments with reference to the drawings, in which: Fig. 1 is a plan view which illustrates an ID card according to a first embodiment of the present invention; Fig. 2 is an enlarged cross sectional view of the ID card; Fig. 3 is an enlarged sectional view of an information indication layer of the ID card; Fig. 4 is a view which illustrates a projection amount of the information indication layer; Fig. 5 is a view which illustrates a process for forming the information indication layer on a base of the ID card; Fig. 6 is a view which illustrates a process for bonding the base, on which the information indication layer has been formed, and a protection sheet to each other; Fig. 7 is an enlarged sectional view of an ID card according to a second embodiment of the present invention; Fig. 8 is a view which illustrates the projection amount of the information indication layer of the ID card; Fig. 9 is an enlarged sectional view of an ID card according to a third embodiment of the present invention; Fig. 10 is a sectional view which illustrates an apparatus for reading the ID card; and Fig. 11 is a characteristic graph which illustrates the relationship between the information indication layer and output voltage. Preferred embodiments of the present invention will now be described. Fig. 1 is a plan view of an ID card serving as a latent image forming member according to a first embodiment, Fig. 2 is an enlarged sectional view of the ID card, Fig. 3 is an enlarged sectional view of an information indication layer of the ID card, and Fig. 4 is a view which illustrates an amount of projection of the information indication layer. The ID card 1 according to this embodiment has an information indication layer 2 at a predetermined position as shown in Fig. 1, the information indication layer 2 having a bar code 3 serving as a non-visible latent image pattern. An internal structure of the ID card 1, as shown in Fig. 2, comprises, bases 4a and 4b, the information indication layer 2 formed on the same by printing, non-solvent type adhesive agent layers 5a and 5b made of materials such as hot-melt type adhesive agent and protection sheets 6a and 6b, in the form of an integrally stacked structure. In the case of this embodiment, the information indication layer 2 is, by printing, formed on the opaque base 4a having a flat surface. The opaque base 4a is made of a vinyl chloride type resin sheet in which white pigment such as titanium oxide is dispersed and held. The base 4b may be opaque or transparent. It is preferable that the thickness of each of the bases 4a and 4b is about 300 µm. By using a member formed by bonding the two relatively thick bases 4a and 4b, printing of the information indication layer 2 can be performed satisfactorily. If the base is relatively thick, quality of the printed information indication layer 2 is unsatisfactory. The information indication layer 2, as shown in Fig. 3, comprises fluorescent grains 7 which are excited when irradiated with, for example, infrared rays 6, and transparent binder 8 for dispersing and holding the fluorescent grains 7. As the fluorescent grains 7, an organic compound such as dye named rhodamine 6G , thioflavine or eosin, or an inorganic compound such as NdP 5 O 14 , LiNdP 4 O 12 or Al 3 Nd (BO 3 ) 4 may be employed. It is preferable that the average grain size of the fluorescent grains 7 is about 0.5 to 2 µm. As the binder 8, a mixture of, for example, wax, vinyl chloride-vinyl acetate copolymer, ethylene-vinyl acetate copolymer, polyester, polyurethane and carbonate is used. If necessary, additives such as a plasticizer and a surface active agent are included in adequate quantities. It is appropriate to make a content of the fluorescent grains 7 in the information indication layer 2 to be ranged from 50 to 80 wt%. It is 75 wt% in this embodiment. If the content of the fluorescent grains 7 is less than 50 wt%, the output from the information indication layer 2 is too weak. If the content exceeds 80 wt%, there arises a risk of defective printing at the time of printing the information indication layer 2. A softening point of the information indication layer 2 is adjusted to range from a temperature lower, by about 40°C, than a temperature at which the base 4a and the protection sheet 6a are laminated to each other to a temperature higher, by about 10°C, than the same. Specifically, the softening point of the information indication layer 2 is 80°C or higher, preferably 80°C to 105°C. If the temperature is lower than 80°C, the shape of the bar code 3 is likely to be distorted (deformed) due to the temperature at which the base 4a and the protection sheet 6a are laminated to each other. If the softening point of the information indication layer 2 exceeds 105°C, the thermal head consumes excessively large electric power at the time of printing the bar code 3. What is worse, a life of the thermal head will be shortened or there arise a risk that the bar code 3 cannot be adequately printed. In this embodiment, an ink ribbon was used which was manufactured by forming a thermal transfer recording layer (softening point 89°C) of a thickness of 2.5 µm formed by a mixture of 76 parts by weight of NdP 5 O 14 serving as the fluorescent member with 12 parts by weight of tepoly urethane (UR-8200 manufactured by Toyobo Co., Ltd.) and 12 parts by weight of polycarbonate (S-8200 manufactured by Asahi Glass Co., Ltd.) serving as the binder on a base film made of a 4.5 µm-thick polyethylene terephthalate film. The ink ribbon is fastened to the thermal head to print individual bar bodes 3 on the base 4 (the base 4a and the base 4b are integrated) by a thermal transfer process. When the infrared rays 10 having a center wavelength adaptable to the excitation wavelength of the employed fluorescent member are applied to the information indication layer 2 as shown in Fig. 3, the fluorescent grains 7 receive the infrared rays 10 and are excited to emit light. In order to efficiently excite the fluorescent grains 7, the light reflectance of the base 4a is made to be 20% or more. That is, by the base 4a of the high reflectance, the infrared rays 10 are reflected on the surface of the base 4a as shown in Fig. 3 to enhance excitation of the fluorescent grains 7. By adding the white pigment to the base 4a as described above, the reflectance of the base 4a can be raised. In this embodiment, the base 4a having a reflectance of 80% is used. If the reflectance is lower than 20%, the excitation of the fluorescent grains 7 cannot be efficiently performed. Therefore, the light emission power from the information indication layer 2 is unsatisfactory with respect to a supplied energy of the infrared rays 10. Therefore, the light reflectance of the base 4a adjacent to the information display layer 2 and opposite the excited light-incident side should be 20% or higher. Next Table 1 shows results of measurements performed in such a manner that samples each of which was manufactured by forming the information indication layer having a thickness of 4 µm on the 5.5 cm (length) x 8.5 cm (width) base of a thickness of 188 µm containing various proportion (contents) of white pigment to provide various the reflectances of the base were used to measure an output voltage and an S/N (ratio of a signal output (S) to a noise output (N)) after the light emission power from the information indication layer 2 has been converted photo-voltacically. As the measuring apparatus, an infrared ray emitting diode TLN201 (a GaAlAs infrared ray emitting diode having a center wavelength of 880 nm) manufactured by Toshiba Corp. was used in this case. The infrared rays from the light emitting diode were irradiated normally on the information indication layer through an optical fiber (having a diameter of 1 mm). Light from the information indication layer was passed through a film made of InP having a thickness of 0.3 mm by way of an optical fiber (having a diameter of 1.2 mm) to be received by photo-diode TPS703 (a light receiving device made of silicon) manufactured by Toshiba Corp. Relationship Between Reflectance of Base and Output Reflectance of Base (%) Output Voltage (mV) S/N 2 23 1.10 5 36 1.25 20 61 1.61 30 77 1.83 50 120 2.51 60 130 2.93 80 580 3.60 As apparent from Table 1, if the reflectance of the base is less than 20%, the S/N is low and the output voltage cannot be satisfactorily high. Therefore, information cannot be accurately read. If the reflectance of the base is 20% or higher, preferably 50% or higher, a satisfactory S/N and output voltage can be obtained to read information correctly. As shown in Fig. 4, a projection amount T (which corresponds to the thickness of the information indication layer 2 in this embodiment) of the information indication layer 2 from a top surface 9 of the base 4a in a region where the information indication layer 2 is not formed is restricted to range from 0.5 to 20 µm, preferably from 1 to 10 µm. If the projection amount T is less than 0.5 µm, a quantity of the fluorescent substance is insufficient and the light emission power from the information display layer 2 is too weak to correctly read information of the bar code 3. If the projection amount T exceeds 20 µm, deformation (shape-distortion occurring due to sagging) of the bar code 3 upon laminating the protection sheet 6a becomes significant. Therefore, information of the bar code 3 cannot be accurately read. Because of the foregoing reasons, the projection amount T should be restricted to range from 0.5 to 20 µm. Fig. 11 is a characteristic graph for illustrating technical basis of the lower limit of the projection amount T (the thickness of the information indication layer 2) of the information indication layer 2, in which the abscissa stands for the thickness of the information indication layer 2 and the ordinate stands for the output voltage after photo-voltaic conversion of the light emission power from the information display layer 2. Curve A shown in Fig. 11 is a characteristic curve showing the relationship between the thickness of the information indication layer 2 and the output voltage when the information indication layer 2 is formed on the surface of the base containing the white pigment as described above. Line B is a straight line showing the output level sy which the output from the information indication layer 2 is 1.6 times the output voltage from only the base (average light reflectance in a visible ray region is 80%) containing the white pigment. As a result of various experiments carried out by the inventors of the present invention, if the output voltage from the information indication layer 2 is lower than 1.6 times of the output voltage from only the base, the bar code formed by the information indication layer 2 cannot be read fully. Therefore, the output voltage from the information indication layer 2 should be 1.6 times or more that from only the base. The minimum thickness of the information indication layer 2 at this value is 0.5 µm as can be apparent from the graph. Although the output voltage is increased according to an increase in the thickness of the information indication layer 2, it becomes substantially constant when the thickness of the information indication layer 2 reaches about 50 µm. As the adhesive agent layers 5a and 5b, a hot-melt type adhesive agent such as an ethylene-vinyl acetate copolymer, polyethylene, polyamide or polyester is used. In particular, it is preferable to use a hot-melt type adhesive agent having slight adhesiveness. If the adhesive agent layer 5a is a solvent type agent, there is a risk that residual solvent is transferred to the information indication layer 2 to cause the deformation of the bar code 3 or the like due to swelling or dissolution. Therefore, at least the adhesive agent layer 5a in contact with the information indication layer 2 should be a non-solvent type agent. The protection sheets 6a and 6b are made of transparent synthetic resin sheets each having a thickness of about 50 to 150 µm. Although 80 µm vinyl chloride resin sheets are employed in this embodiment, the sheets may be made of polyethylene terephthalate type resin, polypropylene, polycarbonate or acrylic resin. The thickness of the protection sheet 6a is not particularly limited so far as the thickness is large enough to protect the information indication layer 2. Fig. 5 is a view for illustrating a process for forming the information indication layer 2 on the base 4. A bonded member 4 formed by bonding the base 4a and the base 4b is inserted into between an ink ribbon 10 made of the base film having thereon the thermal transfer recording layer containing therein the fluorescent substance and a platen roller 14. Disposition is so made that the thermal transfer recording layer of the ink ribbon 10 faces the bonded member 4, so that a peculiar bar code is thermally transferred to a predetermined position of the base 4a by the thermal head 12. It is preferable that the thickness of the thermal transfer recording layer is 1 to 4 µm, more preferably a range of 1.5 to 2.5 µm If the thickness of the thermal transfer recording layer is less than 1 µm, the power from the formed information indication layer 2 is too weak to adequately read the information. If the thickness of the thermal transfer recording layer exceeds 4 µm, the bar code cannot be satisfactorily printed. A coloring material such as pigment may be added to the thermal transfer recording layer. Fig. 6 is a view for illustrating a process for bonding the bases 4a and 4b having the information indication layer 2 formed thereon with the protection sheets 6a and 6b to one another. Web-like protection sheets 6a and 6b having the adhesive agent layers 5a and 5b are respectively stacked on upper and lower surfaces of a web-like member made by forming the information indication layers 2 with intervals on predetermined positions of the bonded member 4 of the base 4a and the base 4b as having been described with reference to Fig. 5. They are pressed by rollers 13a and 13b to be temporarily integral followed by being subjected to to a press machine to be heated and pressed at 120°C for 20 minutes. As a result, the bases 4a, 4b, the protection sheets 6a and 6b are fused to each other. Then, the thus bonded member is cooled, and then it is sequentially cut along a line X so that ID cards each having a predetermined size and shape are obtained. Figs. 7 and 8 are views which illustrate a second embodiment of the present invention. The second embodiment differs from the first embodiment in that a recessed portion 15, which is a little wider than the information of the base 4a layer 2, is formed beforehand by a work such as an embossing work in a position of the base 4a at which the information indication layer 2 is to be provided. The information indication layer 2 is printed into the recessed portion 15 by thermal transfer. Thus, the distance from the top surface 9 of the base 4a that has not the recessed portion 15 to the top surface of the information indication layer 2 is the projection amount T in the case of this embodiment, the projection amount T being restricted to range from 0.5 to 20 µm, preferable from 1 to 10 µm. If the recessed portion 15 is formed in the base 4a and the information indication layer 2 is formed in the recessed portion 15 as is done in this embodiment, the substantial thickness of the information indication layer 2 can be thickened while preventing the excessive increase in the projection amount T of the information indication layer 2 so that the output (power) can be increased. Fig. 9 is a view which illustrates a third embodiment of the present invention. In the case of this embodiment, the base 4a is also made of a transparent sheet similarly to the protection sheet 6a. The information indication layer 2 is, by thermal transfer, printed on to the lower surface of the base 4a. The base 4b and the base 4a each having the light reflectance of 20% or higher are bonded to each other with or without an adhesive agent layer therebetween. Also in this embodiment, the projection amount T from the lower surface of the base 4a to the lower surface of the information indication layer 2 is restricted to range from 0.5 to 20 µm, preferable from 1 to 10 µm. In this embodiment, a printed layer 16 for hiding the information indication layer 2 (the bar code 3) is formed on the top surface of the base 4a. Although the printed layer 16 according to this embodiment is formed by a paint containing a soluble-type ink solvent and a coloring pigment, ultraviolet-curing ink may be used in place of the soluble-type solvent ink. Fig. 10 is a sectional view which illustrates a reading apparatus for optically reading the bar code (a latent image pattern) of the information indication layer of the ID card according to each of the foregoing embodiments. Reference numeral 1 shown in Fig. 10 represents the ID card described in each of the foregoing embodiments, 50 represents an infrared ray emission source incorporating therein an infrared ray emitting diode, 51 represents a light detection device made of a photo-diode, 52 represents a filter that permits only light from the bar code (the latent image pattern) to pass therethrough, and 53 represents a holder for holding elements such as the infrared ray emission source 50, the light detection device 51 and the filter 52. Reference numeral 54 represents a slit plate fastened to an opening portion 55 of the holder 53, and 56 represents a card conveyance passage. When the ID card 1 is inserted into the card conveyance passage 56 in a state where a side of the card 1 having the information indication layer faces upwards, infrared ray emitted from the infrared ray emission source 50 is applied to the foregoing information indication layer. As a result, the fluorescent grains dispersed and held in the information indication layer are excited, to emit light, so that the emitted light is received by the light detection device 51 by way of the slit plate 54 and the filter 52. Therefore, information of the information indication layer is optically read by the light detection device 51. Although each of the foregoing embodiments has the arrangement that the light reflectance of the base is made to be 20% or higher by adding the white pigment or the like, the present invention is not limited to this. For example, a metal film made of aluminum or the like may be deposited on the surface of the base adjacent to the information indication layer 2 by vacuum evaporation or sputtering to make the light reflectance of the base to be 20% or higher. Although the foregoing embodiments comprise the bonded member of the two bases, one relatively-thick base may be used. Although the foregoing embodiments has the arrangement that the bar code 3 is formed in the information indication layer 2, the present invention is not limited to this. For example, an information indication layer may include symbols, such as characters, or dots. Although the foregoing embodiments have been described about the card-shaped latent image forming member, the present invention is not limited to this. For example, the latent image forming member may be formed into another shape, such as a coin-like shape, a seal-like shape or a bar-like shape. As described above, the present invention has the arrangement that the light reflectance of the base adjacent to the information indication layer at the side thereof opposite to the side on which excited light is incident is made to be 20% or higher, a part of excitation light applied to the information indication layer impinges directly on the fluorescent grains in the information indication layer to excite the fluorescent grains, while a part of the excitation light transmits through the information indication layer, and is then reflected by the surface of the base. Reflected light from the surface impinges on the fluorescent grains to excite the fluorescent grains. Therefore, the excitation of the fluorescent member can efficiently be performed. Since the projection amount T of the information indication layer is restricted to range from 0.5 to 20 µm, a sufficiently high light emission power can be obtained from the information indication layer. Further, there is not a possibility that the latent image, such as the bar code of the information indication layer, is distorted (deformed due to sagging) at the time of laminating the protection sheet. As a result, information of the information indication layer can accurately be read. Since the overall body of the information indication layer is laminated over by the protection sheet, various advantages such as excellent durability chemical resistance and capability of being used for a long time are attained. Therefore, a reliable latent image forming member can be provided. By forming on the base the information indication layer having the latent image pattern by printing thereon the fluorescent paint containing the fluorescent grains and by applying the protection sheet having the hot-melt adhesive agent layer on one side thereof to the base to cover the overall body of the information indication layer to fuse the base and the protection sheet to each other with the hot-melt adhesive agent layer therebetween, the deformation of the latent image pattern due to swelling and dissolution can be prevented while obtaining high manufacturing efficiency.

Claims: A latent image forming member (1) comprising a base (4a, 4b), and an information indication layer (2) formed on the base and having a latent image pattern, in which fluorescent grains are dispersed and held, so that when said information indication layer is irradiated with light having a predetermined wavelength to excite said fluorescent grains, information of light emitted from said fluorescent grains may be optically read, characterised in that: reflectance of a surface of said base with respect to light having said predetermined wavelength is 20% or higher, a projection amount (T) of said information indication layer from a top surface of said base is restricted to a range of from 0.5 to 20 µm, and a protection sheet (6a, 6b) is disposed which is laminated onto said base to cover all over said information indication layer and which permits light having said predetermined wavelength to pass therethrough. A latent image forming member according to claim 1, wherein a non-solvent type adhesive agent layer (5a, 5b) is formed on a surface, of said protection sheet (6a, 6b), adjacent to said information indication layer (2). A latent image forming member according to claim 1 or 2, wherein a softening point of said information indication layer (2) is 80°C or higher. A latent image forming member according to claim 1, 2 or 3, wherein said base (4a, 4b) is made of white opaque vinyl chloride type resin. A latent image forming member according to claim 1, 2, 3 or 4, having a shape in the form of a card. A method of producing a latent image forming member according to any one of the preceding claims comprising the steps of: forming said information indication layer having said latent image pattern by printing, onto said base (4a, 4b), fluorescent paint (2) containing fluorescent grains; and applying said protection sheet (6a, 6b) having, on one side thereof, a hot-melt type adhesive agent layer (5a, 5b) to said base so as to cover all over said information indication layer so that said base and said protection sheet are fused to each other with said hot-melt type adhesive agent layer therebetween.

Inventors: ITOH AKIHIKO, MATSUMOTO TAIJI, NISHIDA MASATO, OHIWA TSUNEMI, OSHIMA TOSHIO, TAKEUCHI YOJI, ITOH, AKIHIKO, MATSUMOTO, TAIJI, NISHIDA, MASATO, OHIWA, TSUNEMI, OSHIMA, TOSHIO, TAKEUCHI, YOJI
Applicants: HITACHI MAXELL, HITACHI MAXELL, LTD.
Application year: 1993
Main classification: B42D15
IPC codes: B42D15, G06K7, G06K19