Optical: systems and elements – Diffraction – From grating
Reexamination Certificate
1999-11-16
2001-10-09
Spyrou, Cassandra (Department: 2872)
Optical: systems and elements
Diffraction
From grating
C359S002000, C359S572000, C359S483010, C349S194000, C356S071000, C250S556000
Reexamination Certificate
active
06301047
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a system for preventing forgery of two-dimensional and three-dimensional objects such as passports, cards, security notes, gift certificates, pictures, public transportation tickets, and betting tickets, and in particular to a system for verifying the authenticity of an object by optically and mechanically recognizing a security medium affixed to the object.
BACKGROUND OF THE INVENTION
Conventionally, the use of cholesteric liquid crystal layers for identifying cards and security notes has been known. Cholesteric liquid crystals normally have a layered structure, and the axial directions of the molecules in each layer are parallel to each other as well to the plane of each layer. Each layer is slightly twisted relative to the adjacent layer so that a three-dimensional spiral structure is produced. This structure demonstrates the property to selectively reflect a circularly polarized light having a wavelength of &lgr; which is given by &lgr;=n−p where p is the depth of the layers for this axial direction to turn 360 degrees or the pitch, and n is the average index of refraction of each layer. Therefore, if the direction of the liquid crystals in each layer turns counter-clockwise with respect to the incident light, the left-handed circularly polarized component of the incident light having the wavelength of &lgr; is reflected while the right-handed circularly polarized component passes through. The light having any other wavelength passes through. For instance, when a cholesteric liquid crystal material having a property to reflect red light having the wavelength of &lgr;
R
is placed on a material which absorbs light in the visible range, and a random light such as sunlight is radiated thereon, the transmitted light is all absorbed, and only a left-handed circularly polarized light having the wavelength of &lgr;
R
is reflected.
For instance, Japanese patent laid-open publication (kokai) No. 4-144796 discloses a system in which random light is radiated upon a cholesteric liquid crystal layer, and the reflected circularly polarized light is passed through a band pass filter and a quarter-wave plate to convert the incident light into a linearly polarized light. The linearly polarized light is divided by a beam splitter, and a right-handed circularly polarized light or a left-handed circularly polarized light is detected by using a suitable polarizing plate.
However, when reflected light is used for identification purpose, the surface contamination and/or irregular reflection from the background may cause noises which are significant enough to impair the reliability of the system. Also, the reliance on the simple use of a liquid crystal layer may not be effective enough because duplication or forgery is relatively easy. To individually detect a right-handed or left-handed circularly polarized light, an expensive beam splitter is required. This leads to an increase in the number of necessary components, in the size of the system, and in the overall cost.
It has also been proposed to affix a hologram on the surface of an object and to identify the authenticity of the object by visually identifying it. It has also been proposed, to eliminate the possible uncertainty associated with visual identification, to use a hologram or diffraction grating having a specific diffractive property, impinge a light beam having a prescribed wavelength upon the hologram, and determine the authenticity of the object by comparing the intensity of the light diffracted onto a prescribed position with the intensity of the light obtained at a different position,
However, due to the recent popularization of the technology of preparing hologram, the hologram technology has become so readily available that illicit duplication of hologram which is hardly distinguishable from an authentic hologram can now be made without any significant difficulty. In other words, the hologram has become less effective in discouraging illicit duplication. A light beam diffracted by a hologram or diffraction grating is typically detected by comparing its intensity with the intensity of a light beam obtained elsewhere and determining if the difference is greater than a prescribed threshold level or not. However, because of the need for an additional light receiving unit to be placed at a position other than that for the diffracted light beam, an increase in both size and cost was unavoidable. Also, any irregular reflection and/or insufficient reflection due to surface contamination could cause detection errors.
Other technologies for preventing forgery are known, but are so costly that they are not suitable for use on common commercial goods. Thus, there is a need for a novel technology for preventing forgery.
BRIEF SUMMARY OF THE INVENTION
In view of such problems of the prior art, a primary object of the present invention is to provide an optical identification system which is highly difficult to illicitly duplicate.
A second object of the present invention is to provide an optical identification system which is capable of producing highly distinct results, and hence is highly reliable in use.
A third object of the present invention is to provide an optical identification system which is economical enough to be affixed to inexpensive commercial goods.
A fourth object of the present invention is to provide an optical identification system which uses durable identification media highly resistant to contamination.
According to the present invention such objects can be accomplished by providing a system for optically identifying the authenticity of an object, comprising: a diffraction grating affixed to an object, the diffraction grating comprising a high polymer cholesteric liquid crystal layer for a reflective layer of the diffraction grating; a light source for impinging an incident light beam onto the diffraction grating; a first light receiving unit placed at a position for receiving a diffracted light beam from the diffraction grating and providing an output signal; a circularly polarizing filter placed between the first light receiving unit and the diffraction grating; and a second light receiving unit placed at a different position for receiving a diffracted light beam from the diffraction grating and providing a reference signal therefrom.
The high polymer cholesteric liquid crystal provides an inexpensive identification medium, and the use of the second light receiving unit for providing a reference signal provides a high S/N output signal which is highly distinct and resistant to the contamination of the identification medium. Also, the elimination of the need for expensive optical elements such as beam splitters also contributes to the reduction of cost.
When a second circularly polarizing filter of an opposite sense is placed between the second light receiving unit and the diffraction grating, an even more distinct output signals can be obtained, and the security of the system can be enhanced. Also, distinct output signals can be obtained also by the use of a second pair of light receiving units placed at positions for receiving diffracted light beams from the diffraction grating, and a circularly polarizing filter of an opposite sense placed between only one of the second pair of light receiving units and the diffraction grating, or a pair of circularly polarizing filter of mutually opposite senses each placed between a corresponding one of the second pair of light receiving units and the diffraction grating, as the case may be. The use of band pass filters also contributes to producing distinct outputs.
To enhance the effectiveness in the security of the system, the two pairs of light receiving units may be adapted to simultaneously receive diffracted light from a common spot in the diffraction grating. The security of the system can be also enhanced by placing an additional circularly polarizing filter between the light source and the diffraction grating.
The diffraction grating may comprises small regions having at least two different diffractive proper
Hoshino Hidekazu
Shibuya Seiya
Takeuchi Itsuo
Assaf Fayez
Marger & Johnson & McCollom, P.C.
NHK Spring Co. Ltd.
Spyrou Cassandra
LandOfFree
System for optically identifying an object does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with System for optically identifying an object, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and System for optically identifying an object will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2616822