Abstract:
The object to be identified is affixed with an identification medium comprising a reflective layer consisting of a reflective hologram or diffraction grating. 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.

Description:
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 λ which is given by λ=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 λ 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 λ 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 λ 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 properties, the small regions being sized and distributed so that a plurality of regions including those of different diffractive properties may be simultaneously accessed by each of the light receiving units. This also enhances the security of the system. 
     The high polymer cholesteric liquid crystal material used for the present invention should be capable of retaining a shape as a solid substance, and should have a molecular weight which is required to be used as a reflective layer in a hologram or diffraction grating. The high polymer cholesteric liquid crystal material has an optical property which is highly stable when exposed to a magnetic field, electric field and temperature, as opposed to the low polymer cholesteric liquid crystal material. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Now the present invention is described in the following with reference to the appended drawings, in which: 
     FIG. 1 is a perspective view of an optical identification system embodying the present invention; 
     FIG. 2 is a sectional view of the identification medium for the optical identification system according to the present invention; 
     FIG. 3 is a simplified longitudinal view of a second embodiment of the present invention; 
     FIG. 4 is a bottom view of the identification device shown in FIG. 3; 
     FIG. 5 is a graph showing the output signals from the identification device of FIG. 4 according to the first pattern; 
     FIG. 6 is a graph showing the output signals from the identification device of FIG. 4 according to the second pattern; 
     FIG. 7 is a simplified longitudinal view of a second embodiment of the present invention; 
     FIG. 8 is a simplified longitudinal view of a third embodiment of the present invention; 
     FIG. 9 is a graph showing the output signals from the identification device of FIG. 8 according to the first pattern; 
     FIG. 10 is a graph showing the output signals from the identification device of FIG. 8 according to the second pattern; 
     FIG. 11 is a simplified longitudinal view of a fourth embodiment of the present invention; 
     FIG. 12 is a bottom view of the identification device shown in FIG. 11; 
     FIG. 13 is a simplified longitudinal view of a fifth embodiment of the present invention; 
     FIG. 14 is a graph showing the output signals from the identification device of FIG. 13 according to the second pattern; 
     FIG. 15 is a simplified front view of a sixth embodiment of the present invention; 
     FIG. 16 is a bottom view of the identification device shown in FIG. 15; 
     FIG. 17 is a plan view of an identification medium for a seventh embodiment of the present invention; 
     FIG. 18 is a bottom view of an identification device for the identification medium of FIG. 17; 
     FIG. 19 is a plan view of an identification medium for an eighth embodiment of the present invention; 
     FIG. 20 is a simplified longitudinal view of the identification system for the identification medium shown in FIG. 19; 
     FIG. 21 is a bottom view of the identification device of the identification system shown in FIG. 20; 
     FIG. 22 is a simplified longitudinal view of a ninth embodiment of the present invention; 
     FIG. 23 is a bottom view of the identification device for the identification system shown in FIG. 22; and 
     FIG. 24 is a plan view of an identification medium for the ninth embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 is a perspective view of an identification system embodying the present invention which comprises a hologram foil  1  serving as an identification medium  1  and an identification device  10 . The hologram foil  1  may be affixed to a selected location, two or more locations or the entire surface of an object X such as a card, passport, security note or gift certificate, by a hot stamping process. The hot stamping process consists of transferring an ornamental film onto the surface of an article with an instantaneous application of heat and pressure. 
     As shown in FIG. 2, the hologram foil  1  is formed by laminating a bonding layer  2  for the surface of an object X, a high-polymer cholesteric liquid crystal layer  3  serving as a reflective layer, a hologram forming layer  4  and a protective layer  5 , and is adapted to be transferred onto the object X by the protective layer  5  serving as a peel layer separating it from a base film not shown in the drawing by heat and pressure during the hot stamping process. 
     Referring also to FIGS. 3 and 4, the identification device  10  is provided with a centrally located light source  11  for emitting a random light beam, and a pair of light receiving units  12   a  and  12   b  located at symmetric positions with respect to the light source  11 . A circularly polarizing filter  13  consisting of a polarizing filter  14  and a quarter-wave plate  15  is placed between the hologram foil  1  and one of the light receiving units  12   a  at the time of identification or only in front of one of the light receiving units  12   a . Additionally, a pair of band pass filters  17  are  18  for passing only light of a prescribed wavelength, for instance only green light, are placed in front of the circularly polarizing filter  13  of one of the light receiving units  12   a  and in front of the other light receiving unit  12   b . These band pass filters  17  and  18  are optional, and improve the identification performance particularly in environments where a light source of a special wavelength is employed or where noises of prescribed wavelengths are required to be eliminated. The two light receiving units  12   a  and  12   b  are adapted to individually detect the received light, and the output signals thereof are forwarded to a control unit  16  for processing these output signals (see FIG.  1 ). 
     In this embodiment, the identification device  10  is stationary white the object X is transported in the direction indicated by the arrow in FIG. 1 so that the identification device  10  may scan the hologram foil  1 . The hologram foil  1  may also consist of diffraction grating, and the term “diffraction grating” as used in the claims of this application should be understood as covering hologram, diffraction grating and any other optical elements which demonstrate a selective reflective directivity produced by making use of optical diffraction. 
     In this hologram foil  1 , the grid lines extend perpendicularly to the direction of transportation or scanning so that the incident light directed perpendicularly to the foil surface is diffracted obliquely in the fore-and-aft direction of the scanning direction, and symmetrically with respect to the optical center line of the incident light beam. The light receiving units  12   a  and  12   b  are arranged along the scanning direction so as to capture the diffracted light beams. 
     For practical purposes, the direction of the grid lines of the hologram foil  1  as well as the positions of the light receiving units  12   a  and  12   b  can be arbitrarily selected as long as the light receiving units  12   a  and  12   b  are arranged so as to be symmetric to each other with respect to the light source  11  and to capture the diffracted light beams. However, if the arrangement is such that the incident light beam impinging upon the hologram foil  1  from a perpendicular direction is diffracted laterally with respect to the scanning direction, it may be relatively easy to simulate the diffraction with other optical elements such as mirrors and prisms. On the other hand, it is extremely difficult to build an arrangement which can simulate a diffraction grating which diffracts the incident light beam in the fore-and-aft directions with respect to the scanning direction substantially over the entire scanning length, and to arrange the light receiving units accordingly. Therefore, the latter arrangement is more preferable. 
     In the various embodiments described in this application, two different patterns are placed under consideration. According to the first pattern, the high polymer cholesteric liquid crystal layer  3  of the hologram foil  1  reflects only right-handed circularly polarized light, and the circularly polarizing filter  13  transmits only right-handed circularly polarized light while shutting off left-handed circularly polarized light. According to the second pattern, the high polymer cholesteric liquid crystal layer  3  of the hologram foil  1  likewise reflects only right-handed circularly polarized light, but the circularly polarizing filter  13  transmits only left-handed circularly polarized light while shutting off right-handed circularly polarized light. The mode of operation of these patterns is described in the following. The right and left senses in the circular polarization of the first and second patterns are mutually interchangeable, and description of such obvious variations is omitted. However, various other combinations of the right and left senses in the circular polarization of the first and second patterns should be understood as being included in the overall all concept and spirit of the present invention. 
     According to the first pattern where the high polymer cholesteric liquid crystal layer  3  of the hologram foil  1  reflects only right-handed circularly polarized light, and the circularly polarizing filter  13  transmits only right-handed circularly polarized light while shutting off left-handed circularly polarized light, as the object X is transported and the hologram foil  1  is scanned, because the light diffracted onto the light receiving unit  12   a  is not prevented, the intensity A of the light received by the light receiving unit  12   a  is substantially equal to the intensity B of the light received by the other light receiving unit  12   b  (A/B=1). When the hologram or diffraction grating is not provided with a high polymer cholesteric liquid crystal layer, because the left-handed circularly polarized component of the light diffracted to the light receiving unit  12   a  is shut off, the intensity A of the light received by the light receiving unit  12   a  is only about one half the intensity B of the light received by the other light receiving unit  12   b  (A/B≈{fraction (1/2+L )}). Therefore, the authenticity of the object can be readily determined. The intensities of the light received by the light receiving units  12   a  and  12   b  in this case are indicated in Fig.  5 . Obviously, when the light is reflected by normal optical elements, the reflected light would not be normally received by the light receiving units  12   a  and  12   b . Even if the light is directed to the light receiving units  12   a  and  12   b  by using mirrors and prisms and properly adjusting their angles, as was the case with the hologram or diffraction grating having no high polymer cholesteric liquid crystal layer, the left-handed circularly polarized component of the light reflected to the light receiving unit  12   a  is shut off so that the intensity A of the light received by the light receiving unit  12   a  is only about one half the intensity B of the light received by the other light receiving unit  12   b  (A/B={fraction (1/2+L )}). The intensities of the light received by the light receiving units  12   a  and  12   b  in this case are also indicated in FIG.  5 . As one can readily appreciate, this equally applies to other embodiments described below, and, therefore, any further discussion on any attempts to simulate the present invention with optical elements, instead of a hologram foil  1  affixed to the object X, is omitted. 
     According to the second pattern where the high polymer cholesteric liquid crystal layer  3  of the hologram foil  1  likewise reflects only right-handed circularly polarized light, but the circularly polarizing filter  13  transmits only left-handed circularly polarized light while shutting off right-handed circularly polarized light, as the object X is transported and the hologram foil  1  is scanned, because the light diffracted onto the light receiving unit  12   a  is shut off, the intensity A of the light received by the light receiving unit  12   a  is significantly smaller than the intensity B of the light received by the other light receiving unit  12   b  (AB=0) as shown in the graph of FIG.  6 . When the hologram or diffraction grating is not provided with a high polymer cholesteric liquid crystal layer, because the right-handed circularly polarized component of the light diffracted to the light receiving unit  12   a  is shut off, the intensity A of the light received by the light receiving unit  12   a  is only about one half the intensity B of the light received by the other light receiving unit  12   b  (A/B≈{fraction (1/2+L )}). The intensities of the light received by the light receiving units  12   a  and  12   b  in this case are also indicated in FIG.  6 . Therefore, the authenticity of the object can be readily determined. 
     The circularly polarizing filter was provided on only one of each pair of light receiving units in the above described embodiments but it is also possible to provide circularly polarizing filters of opposite senses on corresponding ones of each pair of light receiving units and effect a similar identification process. Referring to FIG. 7 showing a second embodiment of the present invention, the light receiving unit  12   a  is provided with a first circularly polarizing filter  13 , and the other light receiving unit  12   b  is provided with a second circularly polarizing filter  16  of an opposite sense. In other words, the circularly polarizing filter  13  shuts of the left-handed circularly polarized light (first pattern) while the second circularly polarizing filter  16  placed in front of the light receiving unit  12   b  shuts of the right-handed circularly polarized light (second pattern). In this case, the intensity A of the light received by the light receiving unit  12   a  is significantly smaller than the intensity B of the light received by the other light receiving unit  12   b  (A/B≈0). When a hologram or diffraction grating not provided with a high polymer cholesteric liquid crystal layer is used, because the right-handed circularly polarized light is shut off from the light receiving unit  12   a , and the left-handed circularly polarized light is shut off from the light receiving units  12   b  so that the intensity A of the light received by the light receiving units  12   a  is substantially equal to the intensity B of the light received by the light receiving unit  12   b  (A/B=1). This equally applies to other embodiments. 
     Referring to FIG. 8 showing a third embodiment of the present invention, as a possible modification of the above described embodiment, in the case of the first pattern where the circularly polarizing filter  13  transmits only the right-handed circularly polarized light while shutting off the left-handed circularly polarized light, the light source  11  may be turned into the right-handed circularly polarized light by placing a circularly polarizing filter  19 , which transmits only the right-handed circularly polarized light, and shuts off the left-handed circularly polarized light, between the hologram foil  1  and the light source  11 . In this case, as the object X is transported and the hologram foil  1  is scanned, because the diffracted light directed to the light receiving unit  12   a  is not shut off, the intensity A of the light received by the light receiving unit  12   a  is substantially equal to the intensity B of the light received by the other light receiving unit  12   b  (A/B=1). In this embodiment, the circularly polarizing filter  19  consists of an integral extension of the circularly polarizing filter  13  for the first light receiving unit  12   a . When the hologram or diffraction grating is not provided with a high polymer cholesteric liquid crystal layer, because the diffraction grating converts the incident right-handed circularly polarized light into left-handed circularly polarized light, and the diffracted light directed to the light receiving unit  12   a  is therefore totally shut off, the intensity A of the light received by the light receiving unit  12   a  is significantly smaller than the intensity B of the light received by the other light receiving unit  12   b  (A/B≈0). Therefore, the authenticity of the object can be readily determined. The intensities of the light received by the light receiving units  12   a  and  12   b  in this case are indicated in FIG.  9 . 
     In the case of the second pattern where the circularly polarizing filter  13  transmits only the left-handed circularly polarized light while shutting off the right-handed circularly polarized light, the light source may be likewise turned into the right-handed circularly polarized light. Therefore, the circularly polarizing filter  16  for the light source must be provided separately from the circularly polarizing filter  13  for the first light receiving unit  12   a . In this case, as the object X is transported and the hologram foil  1  is scanned, because the diffracted light directed to the light receiving unit  12   a  is totally shut off, the intensity A of the light received by the light receiving unit  12   a  is significantly smaller than the intensity B of the light received by the other light receiving unit  12   b  (A/B≈0). When the hologram or diffraction grating is not provided with a high polymer cholesteric liquid crystal layer, because the diffracted light is turned into left-handed circularly polarized light, and the diffracted light directed to the light receiving unit  12   a  is therefore not shut off, the intensity A of the light received by the light receiving unit  12   a  is substantially equal to the intensity B of the light received by the other light receiving unit  12   b  (A/B=1). Therefore, the authenticity of the object can be readily determined. The intensities of the light received by the light receiving units  12   a  and  12   b  in this case are indicated in FIG.  10 . 
     If the light source consists of left-handed circularly polarized light and the high polymer cholesteric liquid crystal layer  3  reflects only left-handed circularly polarized light, the output results of the first pattern and the second pattern are simply reversed. 
     Thus, when the light source consists of circularly polarized light, instead of random light, the difference between the outputs of the two light receiving units  12   a  and  12   b  is amplified, and the S/N ratio of the output is increased. Therefore, a more reliable detection is made possible without being interfered by contamination. The same is true with the following embodiments although exhaustive description of such combinations are omitted from description. 
     FIGS. 11 and 12 show a fourth embodiment of the present invention. In this embodiment, an identification device  20  based on the first pattern where only the right-handed circularly polarized light is transmitted while the left-handed circularly polarized light is shut off, and another identification device  21  based on the second pattern where only the left-handed circularly polarized light is transmitted while the right-handed circularly polarized light is shut off are arranged in series along the scanning direction so that the identification action may occur sequentially. The two identification devices  20  and  21  in this case are provided with separate light sources  20   a  and  21   a , respectively. The high polymer cholesteric liquid crystal layer  3  reflects only the right-handed circularly polarized light. In this case, as an identification process is executed by the identification device  20  while the object X is transported and the hologram foil  1  is thereby scanned, the diffracted light directed to the light receiving unit  22   a  is not shut off by the circularly polarizing filter  24  so that the intensity A of the light received by the light receiving unit  22   a  is substantially equal to the intensity B of the light received by the other light receiving unit  22   b  (A/B=1). As an identification process is thereafter executed by the other identification device  21 , the diffracted light directed to the light receiving unit  23   a  is totally shut off by the circularly polarizing filter  25  so that the intensity C of the light received by the light receiving unit  23   a  is significantly smaller than the intensity D of the light received by the other light receiving unit  23   b  (C/D≈0). 
     When the hologram or diffraction grating is not provided with a high polymer cholesteric liquid crystal layer, because the right-handed circularly polarized component of the diffracted light to the light receiving unit  22   a  and the left-handed circularly polarized component of the diffracted light to the light receiving unit  23   a  are both shut off, the intensity A of the light received by the light receiving unit  22   a  is only about half the intensity B of the light received by the other light receiving unit  22   b  (A/B≈{fraction (1/2+L )}) while the intensity C of the light received by the light receiving unit  23   a  is only about half the intensity D of the light received by the other light receiving unit  23   b  (C/D≈{fraction (1/2+L )}). The authenticity of the object can be determined from these results. 
     It is also possible to find (A−C):(A+C) from the intensities A and C of light received by the light receiving units  22   a  and  23   a , respectively, and use this ratio for the determination of the authenticity. Such a process can be executed in the control unit  6  illustrated in FIG.  1 . In case of a hologram or diffraction grating provided with a high polymer cholesteric liquid crystal layer (A−C):(A+C)≈1:1 because A=1 and C≈0. On the other hand, in case of a hologram or diffraction grating lacking a high polymer cholesteric liquid crystal layer (A−C):(A+C)≈1: X (X is a certain small number substantially smaller than {fraction (1/2+L )} which depends on the measured value) because A≈C with the right-handed circularly polarized component in the diffracted light directed to the light receiving unit  22   a  and the left-handed circularly polarized component in the diffracted light directed to the light receiving unit  23   a  both shut off. The authenticity of the object can be particularly distinctly determined from these results. 
     It is also conceivable, with an intent to simulate an authentic hologram foil  1 , to use a hologram or diffraction grating in which each of the grids of the diffraction grating is formed in the shape of a saw tooth so that one side of the saw tooth is parallel to the incident light and the other side is inclined, and the diffracted light may be substantially entirely directed to the light receiving units  22   b  and  23   b . In this case, even though the intensity C of the light received by the light receiving unit  23   a  is significantly smaller than the intensity D of the light received by the other light receiving unit  23   b  (C/D≈0), the intensity A of the light received by the light receiving unit  22   a  is significantly smaller than the intensity B of the light received by the other light receiving unit  22   b  (A/B≈0) so that the determination of the authenticity of the hologram foil  1  can be readily accomplished. Also when the hologram or diffraction grating is constructed in such a manner that the diffracted light may be substantially entirely directed to the light receiving unit  22   a  and  23   a , even though the intensity A of the light received by the light receiving unit  22   a  is substantially equal to the intensity B of the light received by the other light receiving unit  22   b  (A/B=1), because the intensity C of the light received by the light receiving unit  23   a  is also substantially equal to the intensity B of the light received by the other light receiving unit  23   b  (C/D=1), the authenticity of the hologram foil  1  can be again readily determined. Such an arrangement for adjusting the intensity of light for each direction of diffraction requires a special technology and a high cost in forming the saw-teeth shaped grids for the diffraction grating. However, any one of the embodiments of the present invention can readily detect such an arrangement because of the use of the polarized light for identification. 
     FIG. 13 shows a fifth embodiment of the present invention. A circularly polarizing filter  28  which transmits only the right-handed circularly polarized light while shutting off the left-handed circularly polarized light is interposed between the hologram foil  1  and the light source  20   a  of a first identification device  20  based on the first pattern where the circularly polarizing filter  24  transmits only the right-handed circularly polarized light, and shuts off the left-handed circularly polarized light. Also, a circularly polarizing filter  29  which transmits only the left-handed circularly polarized light while shutting off the right-handed circularly polarized light is interposed between the hologram foil  1  and the light source  21   a  of a second identification device  21  based on the second pattern where the circularly polarizing filter  25  transmits only the left-handed circularly polarized light, and shuts off the right-handed circularly polarized light. The high polymer cholesteric liquid crystal layer  3  of the hologram foil  1  reflects only the right-handed circularly polarized light. In this case, as the object X is transported and the hologram foil  1  is scanned, because the light from the light source  20   a  consists of right-handed circularly polarized light, the diffracted light directed to the light receiving unit  22   a  is not shut off by the circularly polarizing filter  24 , and the intensity A of the light received by the light receiving unit  22   a  is substantially equal to the intensity B of the light received by the other light receiving unit  22   b  (A/B=1). As the other identification device  21  executes an identification process, because the light from the light source  21   a  consists only of the left-handed circularly polarized light, no diffracted light is emitted from the hologram foil  1 , and the intensity C of the light received by the light receiving unit  23   a  and the intensity D of the light received by the other light receiving unit  23   b  are both substantially zero (C≈0, D≈0). The authenticity of the object can be readily determined from these results. 
     When the hologram or diffraction grating is not provided with a high polymer cholesteric liquid crystal layer, during the identification process at the identification device  20 , because the right-handed circularly polarized light emitted from the light source  20   a  turns into left-handed circularly polarized light as it is diffracted, the circularly polarizing filter  24  substantially shuts off the diffracted light directed to the light receiving unit  22   a  so that the intensity A of the light received by the light receiving unit  22   a  is significantly smaller than the intensity B of the light received by the other light receiving unit  22   b  (A/B=0). During the subsequent identification process at the identification device  21 , because the left-handed circularly polarized light emitted from the light source  21   a  turns into right-handed circularly polarized light as it is diffracted, the diffracted light directed to the light receiving unit  23   a  is substantially shut off by the circularly polarizing filter  25  so that the intensity C of the light received by the light receiving unit  23   a  is significantly smaller than the intensity D of the light received by the other light receiving unit  23   b  (C/D≈0). The authenticity of the object can be readily determined from these results. 
     Even when the identification process is executed by using only one identification device having only one light source as was the case with the second embodiment, the S/N ratio of the output signal can be raised as mentioned earlier by using a light source for producing right-handed circularly polarized light. When a transparent tape is affixed to the surface of the hologram or diffraction grating not provided with a high polymer cholesteric liquid crystal layer so as to produce a half-wave phase difference, because the diffracted light consists of right-handed circularly polarized light in the same way as the hologram or the diffraction grating is provided with a high polymer liquid crystal layer so that the output is not different from that would be obtained from the hologram foil  1  of the present invention. However, when a plurality of identification devices having light sources of circularly polarized light of opposite senses are used, it is possible to detect a use of a plastic tape which causes a half-wave phase difference while maintaining a high S/N ratio. 
     More specific explanation is given in the following in connection with the arrangement illustrated in FIG.  13 . Suppose that a transparent plastic tape for causing a half-wave phase difference is placed over the surface of the hologram or diffraction grating lacking a high polymer cholesteric liquid crystal layer. During the identification process by the identification device  20 , because the right-handed circularly polarized light from the light source  20   a  is diffracted without any rotation, the intensity A of the light received by the light receiving unit  22   a  is substantially equal to the intensity B of the light received by the other light receiving unit  22   b  (A/B≈1). During the subsequent identification process by the identification device  21 , because the left-handed circularly polarized light from the light source  21   a  is diffracted without any rotation, the intensity C of the light received by the light receiving unit  23   a  is substantially equal to the intensity D of the light received by the other light receiving unit  23   b  (C/D≈1). These results are different from those obtained when the hologram foil  1  of the present invention is used so that the authenticity of the object can be readily determined. The intensities of the light received by the light receiving units  22   a ,  22   b ,  23   a  and  23   b  in this case are indicated in FIG.  14 . 
     A pair of sensors having an identical structure are used in the above described embodiment, and this is more economical than using a pair of sensors having different strictures. 
     The improvement of the S/N ratio by using the circularly polarized light for the light source can be also effected in the embodiments described hereinafter. Such arrangements are all within the purview of the present invention although description of the specific arrangements and the resulting advantages are omitted in the following description. 
     According to the above embodiments using two identification devices, the identification process was sequentially executed by the first identification device  20  and the second identification device  21  with a certain time lag. Therefore, it is conceivable that an attempt may be made to deceive the identification system by changing the identification medium during the process. It can be avoided by a sixth embodiment illustrated in FIGS. 15 and 16 in which a pair of identification devices  20  and  21  are arranged in slanted orientations so that the incident light beam from each of the light sources  20   a  and  21   a  may be directed to the other light receiving unit, and the diffracted light beams may be simultaneously received by the light receiving units  22   a ,  22   b ,  23   a  and  23   b  of the other identification devices  20  and  21 . This modification can be applied also to the embodiments described in the following. 
     FIG. 17 is a plan view schematically illustrating the structure of the hologram foil  31  serving as the identification medium for a seventh embodiment of the present invention, and FIG. 18 is a bottom view of an identification device  40  for this hologram foil  31 . In this arrangement, a large number of rectangular small regions  31   a  and  31   b  of two different diffractive directions but of a same diffractive angle are arranged in a checkerboard pattern in an alternate fashion. These regions  31   a  and  31   b  are small enough for a spot of the light beam from a light source  41  to simultaneously cover a large number of these regions. The grid lines of the diffraction grating of the regions  31   a  extend perpendicularly to the scanning direction so that the incident light beam impinging upon the hologram foil in a perpendicular direction diffracts obliquely upward in the fore-and-aft direction with respect to the scanning direction, and symmetrically with respect to the incident light beam. The grid lines of the diffraction grating of the other regions  31   b  extend in parallel with the scanning direction so that the incident light beam impinging upon the hologram foil in a perpendicular direction diffracts laterally and obliquely with respect to the scanning direction, and symmetrically with respect to the incident light beam. The high polymer cholesteric liquid crystal layer serving as the reflective layer of the hologram foil  31  likewise only reflects right-handed circularly polarized light. 
     The identification device  40  is provided with two pairs of light receiving units  42   a ,  42   b ,  43   a  and  43   b  arranged in an orthogonal cross pattern centered around a light source  41 . The light receiving units  42   a  and  42   b  are arranged along the scanning direction so as to receive the light diffracted from the regions  31   a . The receiving units  43   a  and  43   b  are arranged perpendicular to the scanning direction so as to receive the light diffracted from the regions  31   b . One of the paired light receiving units  42   a  is provided with a circularly polarizing filter  45  in front thereof so as to transmit only the right-handed circularly polarized light while shutting off the left-handed circularly polarized light (the first pattern). One of the other paired light receiving units  43   a  is provided with a circularly polarizing filter  46  in front thereof so as to transmit only the left-handed circularly polarized light while shutting off the right-handed circularly polarized light (the second pattern). 
     As the object X is transported and the hologram foil  31  is scanned, the first paired light receiving units  42   a  and  42   b  receive right-handed circularly polarized light from the regions  31   a . This light is not shut off by the circularly polarizing filter  45  so that the intensity A of the light received by the light receiving unit  42   a  is substantially equal to the intensity B of the light received by the other light receiving unit  42   b  (A/B=1). Likewise, the second paired light receiving units  43   a  and  43   b  receive right-handed circularly polarized light from the regions  31   b . This light is however totally shut off by the circularly polarizing filter  46  so that the intensity C of the light received by the light receiving unit  43   a  is significantly smaller than the intensity D of the light received by the other light receiving unit  43   b  (C/D=0). The authenticity can be thus readily determined from these results. 
     When the hologram or diffraction grating is not provided with a high polymer cholesteric liquid crystal layer, because the left-handed circularly polarized component of the light diffracted to the light receiving unit  42   a  and the right-handed circularly polarized component of the light diffracted to the light receiving unit  43   a  are both shut off, the intensities A and C of the light received by the light receiving units  42   a  and  43   a  are only about one half the intensities B and D of the light received by the other light receiving units  42   b  and  43   b , respectively, (A/B≈C/D≈{fraction (1/2+L )}). 
     When the hologram or diffraction grating is not provided with a high polymer cholesteric liquid crystal layer, and the shape of the diffraction grating is selected such that the diffracted light is directed only to one of the paired light receiving units, either A/B or C/D is approximately {fraction (1/2+L )}, instead of the relationship of A/B≈1 between the intensities A and B of the light received by the light receiving units  42   a  and  42   b , and C/D=0 between the intensities C and D of the light received by the light receiving units  43   a  and  43   b , respectively. The authenticity of the hologram foil  31  can be thus readily determined from these results. 
     The directions of diffraction in the above described embodiment were directed in the scanning direction and the direction perpendicular thereto, but it is obvious that arbitrary directions can be used. Also, three or more directions of diffraction may be selected while the paired light receiving units are arranged accordingly. 
     FIG. 19 is a plan view schematically illustrating the structure of the hologram foil  51  serving as the identification medium for an eighth embodiment of the present invention, and FIGS. 20 and 21 illustrate the identification device  60  for this hologram foil  51 . In this arrangement, a large number of rectangular small regions  51   a  and  51   b  of a same diffractive direction but of different diffractive angles or of different grid pitches are arranged in a checkerboard pattern in an alternate fashion. These regions  51   a  and  51   b  are small enough for a spot of the light beam form a light source  61  to simultaneously cover a large number of these regions. The grid lines of the diffraction grating of the regions  51   a  extend perpendicularly to the scanning direction, similarly as the regions  31   a  of the previous embodiment, so that the incident light beam impinging upon the hologram foil  51  in a perpendicular direction diffracts obliquely upward in the fore-and-aft direction with respect to the scanning direction, and symmetrically with respect to the incident light beam. The grid lines of the diffraction grating of the other regions  51   b  give rise to a same diffraction direction as the regions  51   a , but are more widely spaced than the regions  51   a  so that the diffracted light is angularly directed further away from the incident light beam. The high polymer cholesteric liquid crystal layer serving as the reflective layer of the hologram foil  51  in this case likewise only reflects right-handed circularly polarized light. 
     The identification device  60  is provided with a first pair of light receiving units  62   a  and  62   b  which are arranged along the scanning direction at mutually symmetric positions with respect to the light source  61  to receive the light diffracted from the regions  51   a . A second pair of light receiving units  63   a  and  63   b  are arranged along the scanning direction at mutually symmetric positions with respect to the light source  61  on the outside of the aforementioned light receiving units  62   a  and  62   b  to receive the light diffracted from the regions  51   b . One of the first paired light receiving units  62   a  is provided with a circularly polarizing filter  65  in front thereof so as to transmit only the right-handed circularly polarized light while shutting off the left-handed circularly polarized light (the first pattern). One of the second paired light receiving units  63   a  is provided with a circularly polarizing filter  66  in front thereof so as to transmit only the left-handed circularly polarized light while shutting off the right-handed circularly polarized light (the second pattern). 
     As the object X is transported and the hologram foil  51  is scanned, the first paired light receiving units  62   a  and  62   b  receive right-handed circularly polarized light from the regions  51   a . This light is not shut off by the circularly polarizing filter  65  so that the intensity A of the light received by the light receiving unit  62   a  is substantially equal to the intensity B of the light received by the other light receiving unit  62   b  (A/B≈1). Likewise, the second paired light receiving units  63   a  and  63   b  receive right-handed circularly polarized light from the regions  31   b . This light is totally shut off by the circularly polarizing filter  66  so that the intensity C of the light received by the light receiving unit  63   a  is significantly smaller than the intensity D of the light received by the other light receiving unit  63   b  (C/D≈0). The authenticity can be thus readily determined from these results. 
     When the hologram or diffraction grating is not provided with a high polymer cholesteric liquid crystal layer, because the left-handed circularly polarized component of the light diffracted to the light receiving unit  62   a  and the right-handed circularly polarized component of the light diffracted to the light receiving unit  63   a  are both shut off, the intensities A and C of the light received by the light receiving units  62   a  and  63   a  are only about one half the intensities B and D of the light received by the other light receiving units  62   b  and  63   b , respectively, (A/B≈C/D≈{fraction (1/2+L )}). 
     When the hologram or diffraction grating is not provided with a high polymer cholesteric liquid crystal layer, and the shape of the diffraction grating is selected such that the diffracted light is directed only to one of the paired light receiving units, either A/B or C/D is approximately {fraction (1/2+L )}, instead of the relationships of A/B≈1 between the intensities A and B of the light received by the light receiving units  62   a  and  62   b , and C/D=1 between the intensities C and D of the light received by the light receiving units  63   a  and  63   b , respectively. The authenticity of the hologram foil  51  can be thus readily determined from these results. 
     Two different angles of diffraction were used in the above described embodiment, but it is obvious that three or more angles of diffraction may be selected while the paired light receiving units are arranged accordingly. Also, additionally combining the variations in the directions of diffraction to the variations in the angle of diffraction further increases the number of possible variations. 
     FIGS. 22 and 23 illustrate an identification device  70  for a ninth embodiment of the present invention. Referring to FIG. 23, the identification device  70  of this embodiment comprises a centrally placed light source  71  for emitting random light, and a disk-shaped light receiving element  72  surrounding the light source  71 . This disk-shaped light receiving element  72  is divided into eight identical sector-shaped light receiving units  72   a ,  72   b ,  72   c ,  72   d ,  72   c ,  72   f ,  72   g  and  72   h . These sector-shaped light receiving units  72   a ,  72   b ,  72   c ,  72   d ,  72   e ,  72   f ,  72   g  and  72   h  are arranged symmetrically with respect to the light source  71  and form four pairs, light receiving units  72   a  and  72   e , light receiving units  72   b  and  72   f , light receiving units  72   c  and  72   g  and light receiving units  72   d  and  72   h , each pair being adapted to simultaneously receive the diffracted light from the hologram foils  81 . 
     As also shown in FIG. 22, one of each pair of the light receiving units  72   a ,  72   b ,  72   c  and  72   d  is provided with a circularly polarizing filter  73  of the first pattern in front thereof, the circularly polarizing filter  73  transmitting only the right-handed circularly polarized light while shutting off the left-handed circularly polarized light. Additionally, a band pass filter  77  covering the entire surface of the light receiving element  72  is placed between the circularly polarizing filter  73  and the hologram foil  1 . 
     Referring to FIG. 24, the object X is provided with four hologram foils  81   a ,  81   b ,  81   c  and  81   d  as identification media. These hologram foils  81   a ,  81   b ,  81   c  and  81   d  are adapted to diffract light in four different directions corresponding to the four pairs of light receiving units. The high polymer cholesteric liquid crystal layer serving as the reflective layer for these hologram foils  81   a ,  81   b ,  81   c  and  81   d  reflects only the right circularly polarized light. 
     As the object X is transported, the hologram foils  81   a ,  81   b ,  81   c  and  81   d  are sequentially scanned. Then, the right-handed circularly polarized light is diffracted from the hologram foil  81   a  to the paired light receiving units  72   a  and  72   e , for instance. Because this light is not shut off by the circularly polarizing filter  73 , the intensity A of the light received by the light receiving unit  72   a  is substantially equal to the intensity E of the light received by the other light receiving unit  72   e  (A/E=1). Then, the right-handed circularly polarized light is diffracted from the hologram foil  81   b  to the paired light receiving units  72   b  and  72   f , for instance. Because this light is not shut off by the circularly polarizing filter  73 , the intensity B of the light received by the light receiving unit  72   b  is substantially equal to the intensity E of the light received by the other light receiving unit  72   f  (B/F≈1) Likewise, the intensity C of the light received by the light receiving unit  72   c  is substantially equal to the intensity G of the light received by the other light receiving unit  72   g  (C/G≈1), and the intensity D of the light received by the light receiving unit  72   d  is substantially equal to the intensity H of the light received by the other light receiving unit  72   h  (D/H≈1). 
     When a hologram or diffraction grating not provided with a high polymer cholesteric liquid crystal layer is used with the intent to emulate each of the hologram foils  81   a ,  81   b ,  81   c  and  81   d , because the left-handed circularly polarized light is shut off from the paired light receiving units  72   a  and  72   e , for instance, the intensity A of the light received by the light receiving units  72   a  is only about one half the intensity E of the light received by the other light receiving unit  72   e  (A/E={fraction (1/2+L )}). The output of one of the remaining pairs of light receiving units is also about one half of the output of the other (B/F≈{fraction (1/2+L )}, C/G={fraction (1/2+L )} and D/H={fraction (1/2+L )}). 
     In this embodiment, the hologram foil  81   a  is associated with the paired light receiving units  72   a  and  72   e , the hologram foil  81   b  is associated with the paired light receiving units  72   b  and  72   f , the hologram foil  81   c  is associated with the paired light receiving units  72   c  and  72   g , and the hologram foil  81   d  is associated with the paired light receiving units  72   d  and  72   h , but other combinations are also possible. The object X may also be provided with a larger number of hologram foils, and the light receiving element  72  may be divided into a larger number of light receiving units. The hologram foils may serve the purpose of determining ID and carrying various forms of data by using various combinations of these hologram foils. The above arrangement used only one identification device, but an even more reliable identification is possible if the light from the light source consists of circularly polarized light of two opposite senses and two identification devices are arranged along the direction of transportation as in the fourth embodiment. 
     The light source consisted of random light in the above described embodiments, but may also consist of laser light. 
     As can be appreciated from the above description, according to the system for identifying an object of the present invention, the object is affixed with an identification medium comprising a reflective layer consisting of a reflective hologram or diffraction grating which diffracts incident light from a light source onto a pair of or a plurality of pairs of symmetric positions with respect to the optical center of the incident light, the reflective layer being formed by a high polymer cholesteric liquid crystal layer, and a light beam is impinged thereon. One of a pair of diffracted light beams is received via a circularly polarizing filter consisting of a polarizing filter and a wave plate, and the other of the pair of the diffracted light beams is directly received. Alternatively, one of a pair of diffracted light beams is received via a first circularly polarizing filter having a first circularly polarizing property while the other of the pair of diffracted light beams is received via a second circularly polarizing filter having a second circularly polarizing property which is an opposite of the first circularly polarizing property. The authenticity of the object can be determined by comparing the results of receiving the light beams. Thus, the polarizing property of the cholesteric liquid crystal layer and the diffractive property of hologram are symbiotically combined so as to improve the performance of identification. Because the circularly polarized light is detected, instead of normal reflected light, undesirable influences from irregular reflections can be minimized. Also, the light beam is in effect split by the identification medium through diffraction, the identification device is not required to be provided with expensive beam splitters. Therefore, the necessary number of component parts can be minimized, compact and light-weight design of the system is made possible, and the cost is minimized. 
     Although the present invention has been described in terms of preferred embodiments thereof, it is obvious to a person skilled in the art that various alterations and modifications are possible without departing from the scope of the present invention which is set forth in the appended claims.