Abstract: 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.