The present invention generally pertains to paper products and is particularly directed to providing a paper product that is secure both from xerographic copying and from removal from secure premises.
A xerographic copier includes a semiconductor layer that conducts electricity upon exposure to light but behaves as an insulator in the dark. In accordance with the xerographic copying process, the semiconductor layer senses the image to be copied when a mirror image of the image to be copied is reflected onto the semiconductor layer by a high energy light within a predetermined portion of the light spectrum. Such portion includes ultraviolet light. Light sources commonly used in xerographic copiers include quartz (tungsten) halogen lamps having an operational range between 400 and 900 nm and xenon lamps having an operational range between 380 and 1900 nm.
In the copying process, first, the semiconductor layer is electrostatically charged. Then, a mirror image of the image to be copied is projected onto the semiconductor layer by reflecting high energy light off of an original paper containing a printed image to be copied. In the areas of the semiconductor layer that sense the reflected light, the electric charge is dissipated. However, the residual charge is retained in the areas of the semiconductor layer that do not sense the reflected light, as a result of the high energy light being absorbed by the print on the original paper that defines the image to be copied. The semiconductor layer is then dusted with an oppositely charged toner powder which adheres to the residually charged areas to form the mirror image on the semiconductor layer. The image is transferred as a reproduced true image onto a copy paper that is brought into contact with the semiconductor layer and electrostatically charged from the rear to attract the toner powder onto the copy paper. The toner powder is then fused to the copy paper by heat to provide a permanent copy of the reproduced image on the copy paper.
Paper products that are more or less secure from copying by xerographic photocopiers are known. One such paper product is distributed by the Fine Paper Company of Canada under the trademark "NOCOPI". This paper product is a standard paper characterized by a heavily dyed coating that is so dark that images printed on the paper can be seen with only great difficulty. When an attempt is made to copy the image by xerographic copying, the copy paper is turned totally dark. Another such paper product is made by the Xerox Research Center in Canada. This paper product contains a light sensitive matrix that is combined with the cellulose of the paper to cause the word void to appear on the copy paper when an attempt is made to copy an image from the paper product by xerographic copying. One drawback to this paper product is that after it has been imaged a few times with a halogen lamp (such as contained in many xerographic copiers) the paper product changes color so that the images on the paper product become unreadable to the naked eye.
There are systems for preventing the removal of a paper product from secure premises. One such system is described in U.S. Pat No. 3,665,449 to Elder et al. Such a system has been used to prevent unauthorized removal of documents from a secure area. A marker that produces a unique signal in response to an interrogation signal when transported through an interrogation zone of an electronic article surveillance (EAS) system is affixed to the document. The unique signal consists of harmonics of the interrogation signal that are uniquely characteristic of the marker material so that they may be distinguished from harmonics produced by other materials in response to the interrogation signal.