Abstract

An apparently conventional playing card is invisibly coded so that it can only be read face down, by an electrooptic reading means. The card may be of non-laminated conventional card stock which has a substantially white surface conventionally printed with the identification of the suit and value of the card with inks chosen because they are visible but substantially transparent to wavelengths outside the visible range. The face of the card is coded with indicia inklessly marked across its surface with a compound which absorbs wavelengths (outside the visible range) which wavelengths are used by the reading means to read the indicia. The indicia, invisible to the human eye, correspond to a code which uniquely identifies the card. The card may be laminated from top and base sheets and the code concealed behind the front printed face of the top sheet. The upper surface of the top sheet is imprinted with the face value of the card with the inks described. The base sheet serves as a support layer, either for the indicia per se, or for an intermediate layer on which the indicia may be printed. The code is read because there is sufficient contrast between the transmitted and absorbed light in the wavelength used by the reading means. A coating or auxiliary layer may be provided to enhance the contrast.