Patent Application
20200215843
The present disclosure relates to a forgery prevention structure for preventing forgery, a forgery prevention medium having the forgery prevention structure, and a method for inspecting the forgery prevention structure.
Conventionally, forgery prevention structures for preventing forgery have been provided in sheet-like valuable mediums such as banknotes, stock certificates, bonds, checks, and coupons. For example, Patent Literature 1 discloses a technique in which a conductive layer having a split ring resonator (hereinafter, abbreviated as “SRR”) is used as a forgery prevention structure. A meta-material formed by a micro SRR which has an outer diameter of about several hundred microns and acts on a terahertz electromagnetic wave, is used for forgery prevention.
Specifically, a conductive layer in which the SRRs having predetermined shapes are arranged at regular intervals into a matrix, is formed such that a transmissivity indicates a predetermined value when a terahertz electromagnetic wave having a specific frequency is irradiated. The conductive layer is provided inside or on a medium as the forgery prevention structure. Authentication of the medium can be performed based on a value of the transmissivity obtained by the terahertz electromagnetic wave being irradiated to the forgery prevention structure.
The transmissivity of the terahertz electromagnetic wave transmitted through the conductive layer changes depending on a relationship between a polarization direction of the terahertz electromagnetic wave and a direction of an open part of the SRR. When the conductive layer is divided into a plurality of regions, and the directions of the open parts of the SRRs in the respective regions are made different, a forgery prevention structure in which the transmissivity is different for each region can be achieved. When the transmissivity is measured while each region in the forgery prevention structure is scanned using the terahertz electromagnetic wave, authentication of the medium can be performed by determining whether or not the transmissivity changes according to a transmissivity in each region and a scanning width.
[PTL 1] Japanese Laid-Open Patent Publication No. 2016-498
In order to perform high-accuracy authentication of a forgery prevention medium having a forgery prevention structure, a forgery prevention structure provided in or on a medium for performing authentication of the medium is disclosed. Anisotropic and isotropic resonators are used. The anisotropic resonator resonates in response to being irradiated with a first terahertz electromagnetic wave at a first frequency, a transmissivity of the first terahertz electromagnetic wave changes with a polarization direction of the first terahertz electromagnetic wave. The isotropic resonator resonates in response to be irradiated with a second terahertz electromagnetic wave at a second frequency, a transmissivity of the second terahertz electromagnetic wave does not change with a polarization direction of the second terahertz electromagnetic wave. As such, a forgery prevention structure, forgery prevention medium, and method for inspecting the forgery prevention structure are disclosed.
As recognized by the present inventors, in the above-described conventional art, high-accuracy authentication of a medium having the forgery prevention structure cannot be performed in some cases. For example, in order to measure a transmissivity of the terahertz electromagnetic wave, positions of a transmitter and a receiver for the terahertz electromagnetic wave are fixed, and the medium is transported such that the forgery prevention structure passes between the transmitter and the receiver. When the medium is transported and the forgery prevention structure formed by the SRRs passes so as to block the terahertz electromagnetic wave that is transmitted by the transmitter and received by the receiver, the transmissivity changes according to the direction of the open part of the SRR. When the medium being transported is tilted (skewed) and an angle between the open part and the polarization direction of the terahertz electromagnetic wave changes, the transmissivity also changes. For example, when a medium is skewed and tilted by an angle of −15 to 15 degrees in a forgery prevention structure designed such that an angle between the open part and the polarization direction of the terahertz electromagnetic wave is 60 degrees, the transmissivity has a value varying between 30% and 60%. The authentication is performed by comparing the value of the transmissivity with a threshold value. However, when the threshold value is set so as to allow such a great variation in transmissivity, high-accuracy authentication cannot be performed.
The variation range of the transmissivity in a tilted medium depends on the direction of the open part of the SRR. In the above-described conventional art, the forgery prevention structure is divided into a plurality of regions, and the direction of the open part is set to be different for each region. In this case, when the medium is skewed and tilted, the transmissivity in each region varies in a different variation range depending on the direction of the open part. Therefore, the change of the transmissivity observed by scanning the forgery prevention structure may be different from a change to be observed, and high-accuracy authentication may not be performed. That is, similarly to the SRR, in a resonator structure in which a transmissivity changes according to change of an angle between the polarization direction of the terahertz electromagnetic wave and the resonator structure, high-accuracy authentication may not be performed when a medium is tilted.
The structures and methods presented in the present disclosure overcome these and other problems of the aforementioned conventional art by providing a forgery prevention structure, a forgery prevention medium, and a method for inspecting the forgery prevention structure, which are capable of high-accuracy authentication.
In order to overcome the aforementioned and other problems, one aspect of the present disclosure is directed to a forgery prevention structure provided in a medium for performing authentication of the medium. The forgery prevention structure includes an anisotropic resonator that resonates in response to being irradiated with a first terahertz electromagnetic wave at a first frequency, a transmissivity of the first terahertz electromagnetic wave changes with a polarization direction of the first terahertz electromagnetic wave, and an isotropic resonator that resonates in response to be irradiated with a second terahertz electromagnetic wave at a second frequency, a transmissivity of the second terahertz electromagnetic wave does not change with a polarization direction of the second terahertz electromagnetic wave.
According to the present disclosure, the first frequency and the second frequency being a same frequency in the above-described disclosure.
According to the present disclosure, the anisotropic resonator and the isotropic resonator are mixed a region with at least one of another anisotropic resonator or another isotropic resonator is provided in the above-described disclosure.
According to the present disclosure, a region in which the anisotropic resonator being one of a plurality of kinds of anisotropic resonators mixed in a fixed ratio in a region, the plurality of kinds of anisotropic resonators respectively resonate with terahertz electromagnetic waves having different polarization directions is provided in the above-described disclosure.
According to the present disclosure, the anisotropic resonator and the isotropic resonator are arranged in a basic pattern, the basic pattern being repeated with other anisotropic resonators and other isotropic resonators in a region is provided in the above-described disclosure.
According to the present disclosure, the anisotropic resonator and the isotropic resonator being arranged in a first region, the first region being one of a plurality of regions that impart different transmissivities on a predetermined terahertz electromagnetic wave in response to being irradiated with the predetermined terahertz electromagnetic wave are provided in the above-described disclosure.
According to the present disclosure, the anisotropic resonator also includes another anisotropic resonator, the another anisotropic radiator resonates with a terahertz electromagnetic wave having a polarization direction different by 90 degrees than the first terahertz electromagnetic wave.
According to the present disclosure, a hologram layer having a predetermined pattern observed under visible light is further provided in the above-described disclosure.
According to the present disclosure, the forgery prevention structure according to the above-described disclosure is formed on a banknote.
Furthermore, the present disclosure is directed to a forgery prevention medium that has the forgery prevention structure according to the above-described disclosure.
Furthermore, the present disclosure is directed to a method for inspecting the forgery prevention structure according to the above-described disclosure, and the method includes: irradiating the terahertz electromagnetic wave to a forgery prevention medium; detecting the terahertz electromagnetic wave having been reflected or transmitted; and comparing a detected intensity, transmissivity, or reflectivity with previously stored reference data to determine whether an article to which the forgery prevention medium is disposed is a forgery.
According to the present disclosure, as compared with a forgery prevention structure in which split ring resonators in the same region have respective open parts in the same direction, a transmissivity is inhibited from changing when the forgery prevention structure is tilted and high-accuracy authentication can be performed.
A forgery prevention structure, a forgery prevention medium, and a method for inspecting the forgery prevention structure according to the present disclosure will be described below in detail with reference to the accompanying drawings. The present disclosure has a feature in which an anisotropic resonator that is an anisotropic resonator structure such as a split ring resonator (hereinafter, abbreviated as “SRR”), and an isotropic resonator that is an isotropic resonator structure such as a closed ring resonator (hereinafter, abbreviated as “CRR”) structure are used to form a forgery prevention structure such that a transmissivity of a terahertz electromagnetic wave transmitted through the forgery prevention structure indicates a predetermined value.
When a polarized terahertz electromagnetic wave is irradiated to the resonator structure, and a transmissivity is obtained by measurement of an intensity of the transmitted terahertz electromagnetic wave, the transmissivity changes according to a frequency of the terahertz electromagnetic wave. This is because the resonator structure resonates with a terahertz electromagnetic wave in a specific frequency domain. The change of the transmissivity is different depending on a method of forming the resonator structure, and a transmissivity at a frequency at which the resonator structure resonates indicates a lower value in some cases and a higher value in other cases, as compared with a transmissivity at a frequency at which the resonator structure does not resonate. Hereinafter, a frequency domain in which the resonator structure resonates to change a transmissivity is referred to as a resonance frequency.
When a polarized terahertz electromagnetic wave having a resonance frequency is irradiated, a transmissivity changes in some of the resonator structures and does not change in the other of the resonator structures, depending on a polarization direction of the terahertz electromagnetic wave. Hereinafter, a resonator structure in which the transmissivity changes depending on the polarization direction of the terahertz electromagnetic wave is referred to as an anisotropic resonator, and a resonator structure in which the transmissivity does not change depending on the polarization direction of the terahertz electromagnetic wave is referred to as an isotropic resonator.
When the shape of the resonator structure is changed, a resonance frequency also changes. The width of the resonance frequency is also different depending on the resonator structure, and may be narrow in some cases and wide in the other cases. Hereinafter, in an exemplary case where the width of the resonance frequency is relatively narrow, a frequency at which the transmissivity indicates a (maximum or minimum) peak is described as a resonance frequency. However, the resonance frequency includes not only a single frequency but also the neighboring frequencies. The width of the resonance frequency can be represented based on a peak value. For example, when a relationship between a frequency and a transmissivity is represented as a graph, a width (half value width) at which the transmissivity is 50% of the peak value may be the width of the resonance frequency. When the width of the resonance frequency is wide, the width at which the transmissivity is 90% or 80% of the peak value may be set as the width of the resonance frequency.
The SRR is a kind of the anisotropic resonator, and has a ring shape having an open part (split). The ring shape is a ring-like shape, having an open part, such as an almost C-like shape that is an annular shape with an open part, or a quadrangular-ring-like shape with an open part. A conductive material is formed, into the shape of the resonator structure such as the ring shape of the SRR, on a sheet of an insulating material, thereby forming the resonator structure. When a polarized terahertz electromagnetic wave is irradiated to the anisotropic resonator such as the SRR which is formed in this manner, a transmissivity of the terahertz electromagnetic wave changes according to a frequency and a polarization direction of the terahertz electromagnetic wave. A transmissivity of a terahertz electromagnetic wave with which the resonator structure resonates indicates a value less than a transmissivity of a terahertz electromagnetic wave with which the resonator structure does not resonate.
For example, a sheet of a conductive material is cut out to form a through groove having the shape of the resonator structure such as a ring shape having an open part, thereby forming the resonator structure such as the SRR. The SRR formed by cutting out the conductive material is particularly called a complementary split ring resonator (CSRR). Also, in a case where a sheet of a conductive material is cut out to form the resonator structure, when a terahertz electromagnetic wave is irradiated to the resonator structure, a transmissivity of the terahertz electromagnetic wave transmitted through the sheet changes according to a frequency and a polarization direction of the terahertz electromagnetic wave. A transmissivity of the terahertz electromagnetic wave with which the resonator structure resonates indicates a value higher than a transmissivity of a terahertz electromagnetic wave with which the resonator structure does not resonate.
The CRR is a kind of an isotropic resonator and has a ring shape formed by removing an open part from the SRR. The CRR can be formed similarly to the SRR such that a conductive material is formed, on a sheet of an insulating material, into a shape of the resonator structure such as an annular shape of the CRR having no open part or a sheet of a conductive material is cut out to form the shape. When a polarized terahertz electromagnetic wave is irradiated to the isotropic resonator such as a CRR, the transmissivity of the terahertz electromagnetic wave changes depending on a frequency of the terahertz electromagnetic wave, whereas the transmissivity of the terahertz electromagnetic wave does not change according to change of the polarization direction of the terahertz electromagnetic wave with which the isotropic resonator resonates. When a conductive material is formed into the shape of the resonator structure, the transmissivity of the terahertz electromagnetic wave with which the isotropic resonator resonates indicates a value less than the transmissivity of the terahertz electromagnetic wave with which the isotropic resonator does not resonate, similarly to the SRR. When a sheet of a conductive material is cut out to form the shape of the resonator structure, the transmissivity of the terahertz electromagnetic wave with which the isotropic resonator resonates indicates a value higher than the transmissivity of the terahertz electromagnetic wave with which the isotropic resonator does not resonate.
When the SRR and the CRR are designed to have almost the same shape except for presence or absence of the open part, a resonance frequency, for the SRR, of the terahertz electromagnetic wave having the polarization direction perpendicular to the direction of the open part is almost the same as a resonance frequency for the CRR. Therefore, the SRR and the CRR can simultaneously resonate with the terahertz electromagnetic wave having the resonance frequency (and the neighboring frequencies). Furthermore, by adjusting the shapes of the SRR and the CRR and an interval of arrangement of the SRR and the CRR, a frequency at which the transmissivity has its peak can be made the same between the SRR and the CRR.
By forming the forgery prevention structure including the resonator structures such that the anisotropic resonator such as the SRR and the isotropic resonator such as the CRR are mixed, a transmissivity is inhibited from changing when the forgery prevention structure is tilted relative to the polarized terahertz electromagnetic wave, and authentication can be determined with high accuracy based on presence or absence of the forgery prevention structure.
As described above, a transmissivity changes in the anisotropic resonator such as the SRR when the polarization direction changes, whereas a transmissivity does not change in the isotropic resonator such as the CRR even when the polarization direction changes. Therefore, the transmissivity can be inhibited from changing when the forgery prevention structure is tilted, according to a proportion of the isotropic resonators such as the CRRs in the forgery prevention structure.
The characteristics of the detected transmissivity can be complicated due to the polarization direction of the irradiated terahertz electromagnetic wave, as compared with a case where the forgery prevention structure is formed by the SRRs in which the directions of the open parts are the same as in Patent Literature 1. Therefore, the forgery prevention structure that can make forgery difficult and allow high-accuracy authentication, can be obtained.
By forming a region including multiple resonator structures, a transmissivity of a terahertz electromagnetic wave having a specific frequency in the region can be controlled. The resonator structures may be arranged into a matrix-like pattern in which the multiple resonator structures are aligned at regular intervals in the longitudinal and transverse directions, a checkered pattern, or a honeycomb-shaped pattern.
As a method of forming a region in which a predetermined transmissivity is indicated, a method of forming, on a sheet of an insulating material, the resonator structure having a predetermined shape by using a conductive material, and a method of cutting out a sheet of a conductive material into a predetermined shape to form the resonator structure, can be used. In either method, a region in which the transmissivity of the terahertz electromagnetic wave indicates a predetermined value can be formed. In the present embodiment, an exemplary case where a conductive material is cut out to form the resonator structure will be described.
The forgery prevention structure of the present embodiment includes a conductive layer in which a transmissivity indicates a predetermined value when measured by irradiating a terahertz electromagnetic wave which has a predetermined frequency and has a polarization direction in a predetermined direction. At least two kinds of resonator structures are arranged in the conductive layer. The conductive layer includes anisotropic resonators such as the SRRs that resonate with terahertz electromagnetic waves of which polarization directions are different in units of 90 degrees when irradiated with the terahertz electromagnetic waves having a predetermined frequency, and/or isotropic resonators such as the CRRs that resonate with the terahertz electromagnetic waves regardless of the polarization directions. The predetermined direction in the present embodiment represents a direction selected as the polarization direction of a terahertz electromagnetic wave to be irradiated for measuring a transmissivity. The predetermined frequency in the present embodiment represents a frequency (resonance frequency) at which the resonator structure resonates with the terahertz electromagnetic wave and is a frequency selected as a frequency of a terahertz electromagnetic wave to be irradiated for measuring a transmissivity. In order to measure difference in transmissivity due to the resonator structure, the predetermined frequency is preferably a resonance frequency at which change of a transmissivity is great when a direction of the anisotropic resonator changes with respect to the predetermined direction (polarization direction). Specifically, the predetermined frequency is preferably in a frequency band including frequencies higher and lower than the frequency at which the transmissivity has its peak. However, when a frequency at which the transmissivity has its peak is stable for each forgery prevention structure, the predetermined frequency may be a single frequency. When variation in detected transmissivity can be allowed, the predetermined frequency may be a frequency other than the frequency at which the transmissivity has its peak.
A specific example of the forgery prevention structure formed by the resonator structure will be described using the SRR as an example.
The forgery prevention structure 10 has a conductive layer 16 in which a plurality of kinds of resonator structures selected from the SRRs 20 to 23, the CRRs 24, and the like are formed at regular intervals into a matrix. The SRRs 20 to 23 each have an almost C-shape obtained by cutting a part of the ring to form each of open parts 20a to 23a. As shown in
As shown in the partially enlarged view in the upper-right portion in
The conductive layer 16 made of a conductive material is cut out to form the SRRs 20 to 23 and the CRR 24. The four kinds of the SRRs 20 to 23 have the same structure except that the open parts 20a to 23a are formed in different directions (positions on the ring). The SRRs 21 to 23 can be realized by rotating the SRR 20. The CRR 24 can be formed so as to have no open part. Therefore, the specific structure will be described by using the SRR 20 as an example.
The SRR 20 is formed by removing the almost C-shaped region from the conductive layer 16 formed on the base member 17. Specifically, the conductive layer 16 is cut out to leave the open part 20a such that a ring-like shape having a predetermined width in the radial direction is formed, thereby forming the SRR 20. As a result, a region of the almost C-shaped ring portion is formed as a groove, and the surface of the base member 17 is exposed on the bottom surface of the groove. Meanwhile, in a region, other than the ring portion, including the open part 20a, the surface of the base member 17 is left covered with the conductive layer 16. The SRRs 21 to 23 can be formed by changing regions to be left as the open parts 21a to 23a when forming the almost C-shaped groove. A method for forming the SRR on the conductive layer, the function of the SRR, and the like are disclosed in Japanese Laid-Open Patent Publication No. 2016-498 and therefore, a detailed description is omitted.
For example, the sheet-like forgery prevention structure 10 has an about 20 mm×20 mm square shape. An inner diameter d of the SRR 20 shown on the upper side in
The thin-film-like forgery prevention structure 10 can be embedded in a medium such as a coupon to be subjected to the forgery prevention, or may be adhered on the medium. The forgery prevention structure 10 may be formed such that both the conductive layer 16 and the base member 17 are newly provided, or a medium such as a coupon is used as the base member 17 and the conductive layer 16 is formed directly on the medium.
Next, the frequency of a terahertz electromagnetic wave used for detecting transmission characteristics will be described.
When the polarization direction of the terahertz electromagnetic wave irradiated to the area and the direction of the open parts of the SRRs formed in the irradiated area are the same, that is, parallel to each other, the frequency characteristics indicated by a solid line in
When the direction of the open parts of the SRRs and the polarization direction of the terahertz electromagnetic wave are the same, two clear peaks P1, P2 are observed as indicated by the solid line in
When terahertz electromagnetic waves having the same polarization directions as the terahertz electromagnetic waves having the frequency characteristics shown in
When the inner diameter, and the width in the radial direction (d and W in
As described above, the frequency (predetermined frequency) of the terahertz electromagnetic wave is preferably a resonance frequency at which change of a transmissivity is great when the direction of the open parts of the SRRs is changed with respect to the polarization direction (predetermined direction) of the terahertz electromagnetic wave. When comparing a ratio between a transmissivity (solid line) for the X polarized light and a transmissivity (broken line) for the Y polarized light for the peaks P1, V1 and P2, the peaks at which the ratio is great are P1 and V1. When the CRR is used, a frequency at which both the SRR and the CRR can resonate is preferable. The peak V1 representing the resonance frequency for the SRR and the peak V2 representing the resonance frequency for the CRR are almost the same frequency, and both the SRR and the CRR can resonate at the frequencies. Therefore, in order to detect a transmissivity in a region in which the SRR and the CRR are mixed, a terahertz electromagnetic wave having a resonance frequency common to the SRR and the CRR such as the peak V1, the peak V2, or a mid-value between both the frequencies is used. As described above, the resonance frequency may be in a frequency band including a peak frequency and the neighboring frequencies.
Next, an example of the resonator structure other than the SRR and the CRR will be described. The SRR is an anisotropic resonator in which a transmissivity of a terahertz electromagnetic wave changes depending on whether the polarization direction of the terahertz electromagnetic wave is the X-axis direction or the Y-axis direction.
In the LC resonators 221, 222, similarly to the resonance frequencies P1, V1 of the SRR, an observed resonance frequency is different for each polarization direction of the irradiated terahertz electromagnetic wave. Specifically, a resonance frequency corresponding to the resonance frequency P1 for the SRR is observed when the polarization direction is parallel to the counter electrode of the capacitor portion 221b, 222b, and a resonance frequency corresponding to the resonance frequency V1 for the SRR is observed when the polarization direction is perpendicular to the counter electrode. Meanwhile, in the slit resonators 223, 224, unlike in the SRR, when the irradiated terahertz electromagnetic wave has the polarization direction perpendicular to the directions of a plurality of slits, a resonance frequency in a wide range including frequencies corresponding to the resonance frequencies P1, V1 for the SRR is observed.
In the example described below, the frequency (predetermined frequency) of the terahertz electromagnetic wave corresponds to the resonance frequency V1 for the SRR, and the SRRs 20 to 23, the LC resonators 221, 222, the slit resonators 223, 224, the CRR 24, and a hole-shaped resonator 241, a disk-shaped resonator 241, and a cross-shaped resonator 242 which are described below are designed to resonate with the terahertz electromagnetic wave having the predetermined frequency.
These resonator structures may be each formed by a sheet of a conductive material being cut out to form the shape of the resonator structure or by a conductive material being formed on a sheet of an insulating material into the shape of the resonator structure, similarly to the SRR. Similarly to the SRR, when a terahertz electromagnetic wave having the resonance frequency is irradiated to the resonator structure, the transmissivity indicates a higher value than that at a frequency at which the resonator structure does not resonate in the former case, and the transmissivity indicates a lower value than that at a frequency at which the resonator structure does not resonate in the latter case. A plurality of kinds of the resonator structures that form a region of the forgery prevention structure are all formed in the same manner.
The LC resonator 221 on the left side in
The slit resonator 223 on the left side in
The CRR is an isotropic resonator in which the transmissivity of the terahertz electromagnetic wave indicates the same value regardless of whether the polarization direction of the terahertz electromagnetic wave is the X-axis direction or the Y-axis direction.
Each of the hole-shaped resonator 241 and the cross-shaped resonator 242 shown in
A first pattern 31 shown in
A second pattern 32 shown in
When the terahertz electromagnetic wave which has the predetermined frequency (V1 in
In a case where a terahertz electromagnetic wave is irradiated to the forgery prevention structure 10 that includes a plurality of kinds of the SRRs having the open parts in different directions, the transmissivity indicates a value between a transmissivity Tx in a case where all the SRRs have open parts in the direction parallel to the polarization direction of the terahertz electromagnetic wave, and a transmissivity Ty in a case where all the SRRs have open parts in the direction perpendicular to the polarization direction of the terahertz electromagnetic wave.
A third pattern 33 shown in
A fourth pattern 34 shown in
The fifth pattern 35 shown in
The basic pattern is thus formed by the resonator structures being selected from the four kinds of the SRRs 20 to 23 having the open parts 20a to 23a in the direction parallel or perpendicular to the X-axis direction, and the CRR 24 having no open parts. The transmissivity of the terahertz electromagnetic wave has a different value by changing, for example, the kinds and the number of the resonator structures to be selected. The first pattern 31 to the fifth pattern 35 are set so as to indicate different transmissivities, respectively. When the forgery prevention structure 10 is formed by arranging the basic pattern such as the second pattern 32 to the fifth pattern 35 which are formed of the resonator structures selected from the SRRs 20 to 23 and the CRR 24 continuously in a matrix, the transmissivity can be inhibited from changing due to the forgery prevention structure 10 being tilted.
The forgery prevention structure 10 of the second pattern 32 can reduce the variation range of the transmissivity since a plurality of kinds of the SRRs 20 to 23 having the open parts in different directions in units of 90 degrees are mixed. Specifically, in a case where the terahertz electromagnetic wave is irradiated, the transmissivity is reduced when the SRR in which the direction of the open part is perpendicular to the polarization direction of the terahertz electromagnetic wave is tilted, whereas the transmissivity increases when the SRR in which the direction of the open part is parallel to the polarization direction is tilted. Therefore, reduction and increase of transmissivity are compensated with each other, thereby reducing the variation range of the transmissivity.
In a case where the SRRs which increase the transmissivity when the forgery prevention structure 10 is tilted relative to the polarization direction of the terahertz electromagnetic wave and the other SRRs which reduce the transmissivity at that time, are mixed, an effect of reducing the variation range of the transmissivity with respect to the tilting can be exerted. Therefore, the kinds of the SRRs of the forgery prevention structure 10 are not limited to the SRRs in which the directions of the open parts are different by 90 degrees. However, by mixing the SRRs in which the directions of the open parts are different by 90 degrees, when the forgery prevention structure 10 irradiated with the terahertz electromagnetic wave is tilted, there are SRRs which increase the transmissivity and other SRRs which reduce the transmissivity, regardless of the polarization direction of the terahertz electromagnetic wave. Therefore, an effect of reducing the variation range of the transmissivity with respect to tilting of the forgery prevention structure 10 regardless of the polarization direction of the terahertz electromagnetic wave can be exerted.
The forgery prevention structures 10 of the third pattern 33 to the fifth pattern 35 can reduce the variation range of the transmissivity because the CRR 24 that is an isotropic resonator is included therein. Specifically, when the forgery prevention structure 10 is tilted, the transmissivity in the anisotropic resonator changes whereas the transmissivity in the isotropic resonator does not change. Therefore, change of the transmissivity can be reduced.
Hereinafter, an authentication apparatus for determining authentication of a sheet-like medium having the forgery prevention structure, based on the transmissivity of a terahertz electromagnetic wave irradiated to the forgery prevention structure, will be described. Thereafter, an example of the forgery prevention structure having a combination of a plurality of kinds of patterns will be described.
The controller 64 controls, for example, transport of the medium 100 by the transport unit 63, and transmission and reception of the terahertz electromagnetic wave by the terahertz electromagnetic wave transmitter 61 and the terahertz electromagnetic wave receiver 62. The controller 64 obtains, for example, the value of the transmissivity of the terahertz electromagnetic wave transmitted through the forgery prevention structure 10, and/or a waveform of the transmissivity. The controller 64 compares at least one of the value of the transmissivity, the waveform of the transmissivity, the characteristics of the waveform, and the like, with the reference data that is prepared in advance in the memory 65, to perform authentication of the medium 100. The controller 64 outputs the result of the authentication to a not-illustrated external apparatus. For example, the result of the authentication is outputted to and displayed on the display unit to notify the result.
For example, the authentication apparatus 1 may be used for inspection of the forgery prevention structure 10 produced on the medium 100 in addition to authentication of the medium 100 having the forgery prevention structure 10. The authentication apparatus 1 may output an intensity, a transmissivity, or a reflectivity of the terahertz electromagnetic wave that has been transmitted through or reflected by the forgery prevention structure 10, in addition to outputting the result of the authentication. The authentication apparatus 1 uses the intensity, transmissivity, or reflectivity to inspect the forgery prevention structure 10. Specifically, the intensity, transmissivity, or reflectivity of the terahertz electromagnetic wave to be detected at the inspection is prepared in the memory 65, by using the forgery prevention structure 10 which has been properly produced. In producing the forgery prevention structure 10, a terahertz electromagnetic wave is transmitted by the terahertz electromagnetic wave transmitter 61 and irradiated to the forgery prevention structure 10 to be inspected, and the terahertz electromagnetic wave receiver 62 receives the terahertz electromagnetic wave having been transmitted through or reflected by the forgery prevention structure 10. The intensity, transmissivity, or reflectivity of the terahertz electromagnetic wave which has been thus detected from the forgery prevention structure 10 to be inspected is compared with the reference data, and whether or not the intensity, transmissivity, or reflectivity matches the reference data is determined, that is, whether or not the forgery prevention structure 10 having been produced passes the inspection is determined. Thus, the authentication apparatus 1 may compare the data detected from the forgery prevention structure 10 with the reference data, for pass/fail determination as well as authentication.
Next, an example of a forgery prevention structure having a combination of a plurality of kinds of patterns will be described. Examples of the forgery prevention structure 10, and the basic patterns 30, 33, 34 that include both the anisotropic resonators and the isotropic resonators have been described with reference to
The forgery prevention structure 110 has an about 20 mm×20 mm square sheet-like shape, and has four divisional elongated regions that are arranged at regular intervals in the Y-axis direction. Black regions and white regions of the forgery prevention structure 110 are each formed of the resonator structures. Specifically, as shown in partially enlarged views in
The black region and the white region include different kinds of resonator structures, and indicate different transmissivities depending on the polarization direction of the terahertz electromagnetic wave. The SRR 23 in the first pattern 31 of the black region is an anisotropic resonator. Therefore, in the black region, when the terahertz electromagnetic wave having the polarization direction in the X-axis direction is irradiated, the transmissivity becomes high, and, when the terahertz electromagnetic wave having the polarization direction in the Y-axis direction is irradiated, the transmissivity becomes substantially 0 (zero). The CRR 24 in the fifth pattern 35 of the white region is an isotropic resonator. Therefore, the transmissivity obtained by irradiating the terahertz electromagnetic wave having the polarization direction in the X-axis direction, and the transmissivity obtained by irradiating the terahertz electromagnetic wave having the polarization direction in the Y-axis direction, indicate almost the same value. The transmissivity in the white region shown in
A transmissivity waveform 141a obtained when the forgery prevention structure 110 is scanned in the Y-axis direction by the terahertz electromagnetic wave having the polarization direction in the X-axis direction is indicated above the forgery prevention structure 110 in
A transmissivity waveform 141b obtained when the forgery prevention structure 110 is scanned in the Y-axis direction by the terahertz electromagnetic wave having the polarization direction in the Y-axis direction is indicated below the forgery prevention structure 110 in
Thus, the transmissivity waveform obtained when the terahertz electromagnetic wave having the polarization direction in the X-axis direction is used, is different from the transmissivity waveform obtained when the terahertz electromagnetic wave having the polarization direction in the Y-axis direction is used. By utilizing this feature, authentication of the forgery prevention structure 110, that is, authentication of the medium 100 having the forgery prevention structure 110 can be determined.
The second pattern 32 in the black region shown in
As a result, as shown on the upper side in
The third pattern 33 in the black region shown in
As a result, as shown on the upper side in
The fourth pattern 34 in the black region shown in
As a result, as shown on the upper side in
Thus, when a plurality of kinds of regions, in which the basic patterns 31 to 35 shown in
As shown in
The forgery prevention structure 160 shown in
The proportion of the number of the SRRs 20 included in each pattern is reduced in the order from the pattern A to the pattern D. That is, the proportion of the number of the resonator structures that resonate with the terahertz electromagnetic wave having the polarization direction in the Y-axis direction is reduced. The proportion of the number of the SRRs 20 included in each column is reduced in the order from the left end column to the right end column in the forgery prevention structure 160. That is, the proportion of the number of the resonator structures that resonate with the terahertz electromagnetic wave having the polarization direction in the Y-axis direction is reduced. Therefore, when the forgery prevention structure 160 is scanned in the Y-axis direction from the left end column to the right end column by the terahertz electromagnetic wave having the polarization direction in the Y-axis direction, the waveform of the transmissivity is obtained such that the transmissivity is gradually reduced. Meanwhile, when the forgery prevention structure 160 is similarly scanned by the terahertz electromagnetic wave having the polarization direction in the X-axis direction, the waveform of the transmissivity is obtained such that the transmissivity gradually increases. Thus, when the forgery prevention structure 160, in which the characteristic transmissivity waveform representing continuously changing transmissivity value is obtained, is provided on a medium, authentication of the medium can be determined.
In the present embodiment, the example where the first pattern 31 to the fifth pattern 35 shown in
In the present embodiment, the example where the polarization direction of the terahertz electromagnetic wave used for authentication is mainly the X-axis direction, has been described. However, the terahertz electromagnetic wave in which the polarization direction is the Y-axis direction may be used. When the polarization direction of the terahertz electromagnetic wave changes, the transmissivity in each basic pattern also changes. However, the authentication can be determined as described above by preparing reference data of the transmissivity corresponding to the polarization direction.
In the present embodiment, the example where a transmissivity of a terahertz electromagnetic wave is used for authentication of the forgery prevention structure, has been described. However, a reflectivity of a terahertz electromagnetic wave may be used. A transmissivity and a reflectivity of a terahertz electromagnetic wave have such a relationship that, when one of the transmissivity and the reflectivity increases, the other thereof is reduced. For example, the structure is changed such that the terahertz electromagnetic wave transmitter 61 and the terahertz electromagnetic wave receiver 62 that are disposed so as to face each other across the transported medium 100 in
As described above, when the authentication apparatus according to the present embodiment is used, a terahertz electromagnetic wave is irradiated to a forgery prevention medium such as a banknote and a coupon having the forgery prevention structure, and authentication of the forgery prevention medium can be determined based on the transmission characteristics such as the frequency and the transmissivity of the transmitted terahertz electromagnetic wave.
Each of a plurality of kinds of the resonator structures in the forgery prevention structure resonates in some cases and does not resonate in the other cases depending on the frequency and the polarization direction of the terahertz electromagnetic wave to be irradiated to the forgery prevention medium for authentication. By adjusting a ratio between the number of the resonator structures that resonate and the number of the resonator structures that do not resonate, a region through which the terahertz electromagnetic wave having a predetermined frequency is transmitted at a predetermined transmissivity can be obtained. A plurality of regions in which transmissivities are different can be combined to form the forgery prevention structure. Use of a plurality of kinds of the resonator structures in which the polarization directions of the terahertz electromagnetic waves with which the resonator structures resonate are different inhibits the transmissivity from changing when the forgery prevention structure is tilted relative to the polarization direction of the terahertz electromagnetic wave. Thus, high-accuracy authentication with the forgery prevention structure can be performed.
As described above, the forgery prevention structure, the forgery prevention medium, and the method for inspecting the forgery prevention structure according to the present disclosure are useful for high-accuracy authentication of a forgery prevention medium having a forgery prevention structure.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2017-181986 | Sep 2017 | JP | national |
The present application is a bypass continuation of PCT Application No. PCT/JP2018/034607 filed on Sep. 19, 2018, and contains subject matter related to Japanese priority document 2017-181986, filed in the Japanese Patent Office on Sep. 22, 2017, the entire contents of each of which being incorporated herein by reference in its entirety.
| Number | Date | Country | |
|---|---|---|---|
| Parent | PCT/JP2018/034607 | Sep 2018 | US |
| Child | 16823359 | US |
