Excerpt from the patent specification:
FIG. 48 depicts a process whereby one of three additional antipiracy techniques can be used. Under one technique, a random pattern is generated and printed on a card with ink (UV or IR) that is only visible with ultraviolet or infrared light 4804; an example of such a pattern appears in FIG. 49A. Under another technique, slightly different shades of ink are used during different printing runs and correlated with cards 4806; cards that appear to be identical but do not use the same shade of ink are counterfeit, as per the comparison depicted in FIG. 49C. Under another technique, impurities are added 4805 to the paper upon which cards are printed; these impurities are mapped using penetrating light so as to produce a unique fingerprint for each card 4807. This fingerprint may also be recorded in a barcode printed on the card itself and can be used to authenticate cards thereafter. An example of such a fingerprint is depicted in FIG. 49B.
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