Optical element for forgery proof
There is provided a versatile optical element applicable both to an electrode layer required in a bank bill field and to an optical element required in an ID field. In an optical element ( 1 ) according to one embodiment of the present invention, a first layer ( 2 ) is arranged on a second layer ( 3 ) having a relief structure on a surface thereof, and a first region ( 4 ) and a second region ( 5 ) are provided. Electromagnetic waves incident at a preset specific angle from a side of the first layer ( 2 ) are totally reflected due to at least one of the relief structure in the first region ( 4 ) and a ratio of a refractive index of the second layer ( 3 ) with respect to a refractive index of the first layer ( 2 ), the electromagnetic waves incident at the specific angle from the side of the first layer ( 2 ) are not totally reflected but transmitted or refracted due to at least one of the relief structure in the second region ( 5 ) and the ratio of the refractive index of the second layer ( 3 ) with respect to the refractive index of the first layer ( 2 ), and only in case of observation performed from the specific angle on the first layer ( 2 ) side, the second region ( 5 ) has higher transparency than the first region ( 4 ), and a preset image is expressed by a transparency contrast therebetween.
1. An optical element for forgery proof comprising a first layer and a second layer, the optical element being divided into at least a first region and a second region, wherein,
the first layer is arranged on the second layer having a relief structure on a surface thereof, and the first layer and the second layer have mutually different refractive indexes,
electromagnetic waves incident at a preset specific angle from a side of the first layer are totally reflected due to at least one of the relief structure in the first region and a ratio of the refractive index of the second layer with respect to the refractive index of the first layer,
the electromagnetic waves incident at the specific angle from the side of the first layer are not totally reflected but transmitted or refracted due to at least one of the relief structure in the second region and the ratio of the refractive index of the second layer with respect to the refractive index of the first layer,
wherein when observed from a direction perpendicular to a plane of the optical element, at least one of the first region and the second region generates a parallax image by a convergence angle defined by positions of both eyes and the optical element, and
wherein
the second layer has the lower refractive index than the first layer, and the relief structure has an oblique surface tilted with respect to the plane of the optical element, and
upon observation performed from a given observation point with a light source placed on an observation point side, Formulae 1 and 2 are satisfied:
θ f1 ≥arcsin( N 2 /N 1 ) (Formula 1)
arcsin( N 4 /N 3 )>θ f2 (Formula 2)
where
θ f1 denotes an incidence angle on the oblique surface of the relief structure in the first region,
θ f2 denotes an incidence angle on the oblique surface of the relief structure in the second region,
N 1 denotes the refractive index of the first layer in the first region,
N 2 denotes the refractive index of the second layer in the first region,
N 3 denotes the refractive index of the first layer in the second region, and
N 4 denotes the refractive index of the second layer in the second region.
2. The optical element for forgery proof according to claim 1 , further comprises a third layer arranged in contact with the second layer, wherein,
the third layer is a printing layer on which at least one of a symbol and an image pattern is written.
3. The optical element for forgery proof according to claim 1 , further comprises a fourth layer arranged in contact with the second layer, wherein
the fourth layer is a colored layer.
4. The optical element according forgery proof to claim 3 , further comprises a fifth layer arranged in contact with a surface of the fourth layer opposite to the second layer, wherein
the fifth layer is an electromagnetic wave absorption layer.
5. The optical element for forgery proof according to claim 1 , further comprises a sixth layer arranged in contact with the first layer and a seventh layer arranged in contact with the second layer, wherein
the sixth layer is a colored layer and the seventh layer is an electromagnetic wave absorption layer.
6. An optical element for forgery proof configured to include two or more optical elements for forgery proof according to claim 1 superposed on each other.
7. The optical element for forgery proof according to claim 6 , wherein
the at least two superposed layers of the optical elements have cyclic relief structures, and
the cyclic relief structures cause moire.
8. The optical element for forgery proof according to claim 1 , wherein
at least part of the relief structure has a cyclic structure, and
the optical element further comprises an eighth layer configured to have the cyclic structure and to be capable of causing moire.
9. An optical element for forgery proof configured to include two or more optical elements for forgery proof according to claim 2 superposed on each other.
10. An optical element for forgery proof configured to include two or more optical elements for forgery proof according to claim 3 superposed on each other.
11. An optical element for forgery proof configured to include two or more optical elements for forgery proof according to claim 4 superposed on each other.
12. An optical element for forgery proof configured to include two or more optical elements for forgery proof according to claim 5 superposed on each other.
13. The optical element for forgery proof according to claim 2 , wherein
at least part of the relief structure has a cyclic structure, and
the optical element further comprises an eighth layer configured to have the cyclic structure and to be capable of causing moire.
14. The optical element for forgery proof according to claim 3 , wherein
at least part of the relief structure has a cyclic structure, and
the optical element further comprises an eighth layer configured to have the cyclic structure and to be capable of causing moire.