| 摘要 |
A method and apparatus for document verification exploits a temporally variable physical process to generate a reproducible effect that cannot be copied. A document such as a credit card is provided with a spot or stripe that incorporates at least one, and preferably a large plurality of photorefractive crystals arrayed in a random manner. The document verifying apparatus includes a coherent light source such as a diode laser to illuminate the photorefractive crystals, and a photosensor to receive light scattered from the photorefractive crystals. The random distribution and orientation of photorefractive crystals comprises a unique characteristic for each card or document, and this characteristic is not based on any assigned number or code. The response of photorefractive crystals to the coherent illumination comprises a time-varying characteristic that is dependent upon the intensity and temporal nature of the illumination itself. Input to the laser illuminator may be varied to elicit differing responses from the photorefractive crystals, and this factor may be very difficult for a counterfeiter to ascertain. Also, for any given illumination intensity or temporal pattern, the image received by the photosensor varies with time. The time at which the photosensor signal is sampled to obtain an identifying image may also be varied, thereby further compounding the difficulty for a counterfeiter to overcome. A large number of "snapshots" of the time-varying image of the document is electronically captured, digitized, and stored in an electronic media. The photosensor signal is compared to the stored data; a match indicates a valid document, and no match indicates an invalid or unauthorized document. The image recognition process can be enhanced by comparing the rate of change in a sequence of images elicited by the laser illuminator.
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