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 applied. 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 applied 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, the forgery prevention structure provided in or on the medium for performing authentication of the medium is formed so as to include a mixed area with a plurality of kinds of resonator structures each of which resonates with a terahertz electromagnetic wave having a different polarization direction. 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 the medium and a plurality of kinds of resonator structures disposed in or on a mixed area of the medium, wherein a first kind of resonator structure resonates in response to being exposed to a first terahertz electromagnetic wave having a first polarization direction, and a second kind of resonator structure resonates in response to being exposed to a second terahertz electromagnetic wave having a second polarization direction, the first polarization direction being different than the second polarization direction.
According to the present disclosure, the first terahertz electromagnetic wave and the second terahertz electromagnetic wave having a same frequency.
According to the present disclosure, an arrangement of the plurality of kinds of resonator structures in the mixed area includes multiple instances of a basic pattern, the basic pattern being a combination of the plurality of kinds of resonator structures in a predetermined arrangement.
According to the present disclosure, a transmissivity of an inspection terahertz electromagnetic wave having a predetermined frequency and a predetermined polarization direction in the mixed area indicates a constant value based on a ratio of each of the plurality of kinds of resonator structures, in the above-described disclosure.
According to the present disclosure, the first polarization direction being different than the second polarization direction by substantially 90 degrees, in the above-described disclosure.
According to the present disclosure, the medium includes a plurality of regions that exhibit different transmissivities when irradiated with an inspection terahertz electromagnetic wave having a predetermined frequency and a predetermined polarization direction, and at least one of the plurality of regions including the mixed area, in the above-described disclosure.
According to the present disclosure, the plurality of regions include at least one other mixed area, and resonator structures in the at least one other mixed area include the plurality of kinds of resonator structures mixed in a different ratio than for the mixed area, and cause a different transmissivity than that caused by the mixed area on the inspection terahertz electromagnetic wave having the predetermined frequency and the predetermined polarization direction, in the above-described disclosure.
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 of the inspection terahertz electromagnetic wave with previously stored reference data to determine whether an article (such as a banknote) 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 a resonator structure such as a split ring resonator (hereinafter, abbreviated as “SRR”) is used 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 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 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 resonator structure 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.
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 polarization directions of the terahertz electromagnetic waves with which these resonator structures resonate are different in units of 90 degrees. 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 detect a 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 in which the resonator structure resonates 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 the SRRs 20 to 23 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 an almost C-shape, thereby forming the SRRs 20 to 23. 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 and 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 20×20 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 order of the resonance frequency 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
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. In order to compare transmissivities when a terahertz electromagnetic wave having a predetermined polarization direction is irradiated to the SRRs, the peak P1 and the peak V1 are preferably used. Therefore, in the present embodiment, examples, in which the frequency at the peak P1 is a primary resonance frequency and the frequency at the peak V1 is a secondary resonance frequency, are described below. The primary resonance frequency may be in a frequency band that includes the frequency at the peak P1 and the neighboring frequencies, and the secondary resonance frequency may be in a frequency band that includes the frequency at the peak V1 and the neighboring frequencies, as described above.
Next, an example of the resonator structure other than the SRR will be described.
The LC resonator 221 on the left side in
The slit resonator 223 on the left side in
The SRR 20, the SRR 22, the LC resonator 221, and the slit resonator 223 shown on the upper side in
The SRR 21, the SRR 23, the LC resonator 222, and the slit resonator 223 shown on the upper side in
A plurality of kinds of the resonator structures are selected from these resonator structures according to the frequency of the terahertz electromagnetic wave to form a basic pattern, and a plurality of the basic patterns are aligned to form the forgery prevention structure. In this case, the resonance frequency is used for the frequency of the terahertz electromagnetic wave for irradiating the resonator structures, and, in particular, the primary resonance frequency and the secondary resonance frequency are preferably used.
As shown in
A first pattern 31 shown in
When the terahertz electromagnetic wave which has the predetermined frequency (primary resonance frequency) and has the polarization direction in the X-axis direction is irradiated, the transmissivity in the SRRs 20, 22 having the open parts 20a, 22a in the X-axis direction that is the polarization direction becomes maximum. Therefore, the transmissivity of the terahertz electromagnetic wave irradiated to the forgery prevention structure 10 of the first pattern 31 indicates the maximum value when the polarization direction is the X-axis direction.
A second pattern 32 shown in
When the terahertz electromagnetic wave which has the predetermined frequency (primary resonance frequency) and has the polarization direction in the X-axis direction is irradiated, the transmissivity in the SRRs 20, 22 having the open parts 20a, 22a in the direction parallel to the polarization direction (X-axis direction) becomes maximum. Meanwhile, the transmissivity in the SRRs 21, 23 having the open parts 21a, 23a in the direction perpendicular to the polarization direction (X-axis direction) becomes minimum.
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.
In the forgery prevention structure 10 of the second pattern 32, a ratio of the number of the SRRs in which the directions of the open parts are parallel to the polarization direction (X-axis direction), to the number of the SRRs in which the directions of the open parts are perpendicular to the polarization direction (X-axis direction) is 3:1. Therefore, when the terahertz electromagnetic wave which has the predetermined frequency (primary resonance frequency) and has the polarization direction in the X-axis direction is irradiated to the forgery prevention structure 10 of the second pattern 32, the value of the transmissivity is approximate to (3×Tx+Ty)/4. The size of the area to which the above-described terahertz electromagnetic wave is irradiated is determined such that the wave is irradiated to an area larger than an area including at least the SRRs in two columns and two rows.
A third pattern 33 shown in
In a case where a terahertz electromagnetic wave which has the predetermined frequency (primary resonance frequency) and has the polarization direction in the X-axis direction is irradiated to the forgery prevention structure 10 of the third pattern 33, the transmissivity indicates a value between a transmissivity Tx in a case where all the SRRs have the open parts in the direction parallel to the polarization direction (X-axis direction), and a transmissivity Ty in a case where all the SRRs have the open parts in the direction perpendicular to the polarization direction (X-axis direction). In the third pattern 33, a ratio of the number of the SRRs in which the directions of the open parts are parallel to the polarization direction (X-axis direction) to the number of the SRRs in which the directions of the open parts are perpendicular to the polarization direction (X-axis direction) is 1:1. Therefore, the value of the transmissivity is approximate to (Tx+Ty)/2. The size of the area to which the above-described terahertz electromagnetic wave is irradiated is determined such that the wave is irradiated to an area larger than an area including at least the SRRs in two columns and two rows.
The pattern formed by the four kinds of the SRRs 20 to 23 is not limited to the pattern formed by the SRRs in two columns and two rows.
The fourth pattern 34 shown in
In a case where the terahertz electromagnetic wave which has the predetermined frequency (primary resonance frequency) and has the polarization direction in the X-axis direction is irradiated to the forgery prevention structure 10 of the fourth pattern 34, the transmissivity indicates a value between a transmissivity Tx in a case where all the SRRs have the open parts in the direction parallel to the polarization direction (X-axis direction) and a transmissivity Ty in a case where all the SRRs have the open parts in the direction perpendicular to the polarization direction (X-axis direction). In the fourth pattern 34, a ratio of the number of the SRRs in which the directions of the open parts are parallel to the polarization direction (X-axis direction), to the number of the SRRs in which the directions of the open parts are perpendicular to the polarization direction (X-axis direction) is 5:4. Therefore, the value of the transmissivity is approximate to (5×Tx+4×Ty)/9. The size of the area to which the above-described terahertz electromagnetic wave is irradiated is determined such that the wave is irradiated to an area larger than an area including at least the SRRs in three columns and three rows.
The basic pattern is thus formed by the SRRs 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. The transmissivity of the terahertz electromagnetic wave has a different value by changing, for example, the kinds and the number of the SRRs to be selected. The first pattern 31 to the fourth pattern 34 are set so as to indicate different transmissivities, respectively. The basic patterns set by using the SRRs 20 to 23 are continuously arranged into a matrix, to form the forgery prevention structure 10, whereby the transmissivity is inhibited from changing due to the forgery prevention structure 10 being tilted.
Specifically, when the terahertz electromagnetic wave which has the primary resonance frequency f1 (THz) and has the polarization direction in the X-axis direction is irradiated to the forgery prevention structure 10 in a state where the forgery prevention structure 10 is not tilted (α=0 degrees), the transmissivity of the forgery prevention structure 10 of the first pattern 31 is about 40%. The transmissivity of the forgery prevention structure 10 of the second pattern 32 is about 35%. The transmissivity of the forgery prevention structure 10 of the third pattern 33 is about 30%. The transmissivity of the forgery prevention structure 10 of the fourth pattern 34 is about 30%. In a conventional forgery prevention structure in which an angle between the polarization direction of the terahertz electromagnetic wave and the open part of each SRR is 60 degrees, when the terahertz electromagnetic wave is similarly irradiated, the transmissivity is about 30%.
In a case where the forgery prevention structure 10 is tilted at an angle ranging from −15 to 15 degrees (−15 degrees≤α≤15 degrees), when the terahertz electromagnetic wave is similarly irradiated, the transmissivity varies between about 40% and about 38% in the forgery prevention structure 10 of the first pattern 31, and the variation range of the transmissivity is about 2%. Similarly, the variation range of the transmissivity in the forgery prevention structure 10 of the second pattern 32 is about 1%, the variation range of the transmissivity in the forgery prevention structure 10 of the third pattern 33 is about 0%, and the variation range of the transmissivity in the forgery prevention structure 10 of the fourth pattern 34 is about 0.3%. When the above-describe conventional forgery prevention structure in which the angle of the open parts is 60 degrees is tilted at an angle ranging from −15 to 15 degrees, the angle varies between 45 degrees and 75 degrees, and the variation range of the transmissivity is about 20%.
When the forgery prevention structure 10 is not tiled, the forgery prevention structure 10 of the third pattern 33, the forgery prevention structure 10 of the fourth pattern 34, and the conventional forgery prevention structure have almost the same transmissivity of about 30%. Meanwhile, when the forgery prevention structure is tilted at an angle ranging from −15 to 15 degrees, the variation range of the transmissivity in the conventional forgery prevention structure is about 20% whereas the variation range of the transmissivity in each of the forgery prevention structure 10 of the third pattern 33 and the forgery prevention structure 10 of the fourth pattern 34 is less than 1%. That is, the forgery prevention structure 10 according to the present embodiment can reduce the variation range of the transmissivity when the forgery prevention structure 10 is tilted as compared with a conventional art.
The forgery prevention structure 10 of the first pattern 31 can reduce the variation range of the transmissivity since the variation range with respect to tilting is reduced when the direction of the open part of the SRR is parallel to the polarization direction of the terahertz electromagnetic wave.
The forgery prevention structures 10 of the second pattern 32 to the fourth pattern 34 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, for example, in a case where the terahertz electromagnetic wave having the primary resonance frequency is irradiated, the transmissivity is reduced when the SRR in which the direction of the open part is parallel 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 perpendicular 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.
As shown in the partially enlarged view on the right side in
The second region 12 is formed by a fifth pattern 35 obtained by rotating the first pattern 31 counterclockwise by 90 degrees.
When the medium 100, that is, the forgery prevention structure 50 is not tilted, almost the center portion, in the X-axis direction, of the forgery prevention structure 50 is scanned in the direction indicated by the arrow 200 by the terahertz electromagnetic wave which has the predetermined frequency and has the polarization direction in the X-axis direction. The transmissivity in the first region 11 of the first pattern 31, the transmissivity in the second region 12 of the fifth pattern 35, and the transmissivity in the third region 13 of the third pattern 33 indicate different values according to the patterns, respectively.
For example, at the primary resonance frequency, the transmissivity in the first region 11 indicates a high value (about 40%), and the transmissivity in the second region 12 indicates a very small value (about 2%). The transmissivity in the third region 13 indicates a value (about 20%) between the transmissivity in the first region 11 and the transmissivity in the second region 12. Therefore, as indicated in the lower portion in
Even when the forgery prevention structure 50 is tilted by 15 degrees, variation range of the transmissivity in the first region 11 of the first pattern 31 and the variation range of the transmissivity in the second region 12 are about 2%, and the transmissivity in the third region 13 of the third pattern 33 does not substantially change. Therefore, even when the forgery prevention structure 50 is tilted, a stepwise waveform in which the transmissivity is reduced from the waveform 71 to the waveform 72 and the transmissivity increases from the waveform 72 to the waveform 73, is obtained as shown in the lower portion in
The exemplary case in which the terahertz electromagnetic wave which has the primary resonance frequency (P1 in
When the medium 100, that is, the forgery prevention structure 50 shown in
Different transmissivities are obtained among the first region 11 to the third region 13 also at the secondary resonance frequency. The transmissivity indicates a very small value of several % in the first region 11, and the transmissivity indicates a high value in the second region. In the third region 13, the transmissivity indicates a value between the transmissivity in the first region 11 and the transmissivity in the second region 12. Therefore, as shown in the lower portion in
When the forgery prevention structure 50 is tilted by 15 degrees, the variation range of the transmissivity in the first region 11, the variation range of the transmissivity in the second region 12, and the variation range of the transmissivity in the third region 13 are small, similarly to the primary resonance frequency, and are each about 4%. Therefore, even when the forgery prevention structure 50 is tilted, a stepwise waveform in which the transmissivity increases from the waveform 81 to the waveform 82 is obtained as shown in the lower portion in
In
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 50, 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.
The example where the direction of the open part of the SRR is parallel or perpendicular to the polarization direction of the terahertz electromagnetic wave has been described. However, the direction of the open part is not limited thereto.
The structure in which the transmissivity changes for each region when the forgery prevention structure 50 is scanned while the medium 100 is transported may be different from the structure shown in
As shown in
The forgery prevention structure 160 shown in
The ratio of the number of the X-direction resonator structures 231 included in each pattern is reduced in the order from the pattern A to the pattern E. The ratio of the number of the X-direction resonator structures 231 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. 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 which has the primary resonance frequency and has the polarization direction in the X-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 which has the primary resonance frequency and has the polarization direction in the Y-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 regions of the forgery prevention structure 50 shown in
In the present embodiment, the example where the first pattern 31 to the fifth pattern 35 are used as the basic pattern, has been described. However, the basic pattern is not limited thereto. As long as, when an arbitrary region having the same size as the basic pattern is selected from a region in which the basic pattern is repeatedly arranged to form a matrix, a ratio between the number of the resonator structures, in the selected region, which resonate with a terahertz electromagnetic wave having the polarization direction in the X-axis direction, and the number of the resonator structures, in the selected region, which resonate with a terahertz electromagnetic wave having the polarization direction in the Y-axis direction is equal to the ratio in the basic pattern, the shape of the basic pattern, and the kinds, the number, arrangement positions, and the like of the resonator structures that form the basic pattern are not particularly limited. Specifically, for example, the resonator structures in the basic pattern may be arranged into not only a matrix in which the resonator structure is repeatedly arranged longitudinally and transversely, but also a checkered pattern or a honeycomb pattern. Also, in the method of arranging the basic pattern in each region, the basic pattern may be repeated in any manner. For example, the basic patterns may be arranged into a block pattern or a honeycomb pattern as well as a matrix. The shape of the SRR is not particularly limited as long as the transmissivity can be obtained as desired when a terahertz electromagnetic wave having a predetermined frequency is irradiated. For example, a rectangular ring-like shape may be formed. Another resonator structure may replace the illustrated resonator structure when each of the frequency and the polarization direction of the terahertz electromagnetic wave with which the resonator structure resonates is the same.
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 |
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2017-181979 | Sep 2017 | JP | national |
The present application is a bypass continuation of PCT Application No. PCT/JP2018/034601, filed on Sep. 19, 2018, and contains subject matter related to Japanese priority document 2017-181979, 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 | |
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Parent | PCT/JP2018/034601 | Sep 2018 | US |
Child | 16824703 | US |