Concealed identification code for credit cards etc. - is pressed in stress field and heated to reveal surface markings

摘要

A credit card or similar document is provided with a means of identifying its authenticity by a plastic strip embedded in the card surface. The strip (2) is initially produced with a series of transverse ridges of rectangular cross-section (6). The identification code is concealed in the pattenr of the ridges. The strip (2) is then subjected to a pressing or rolling processing which forces the ridges (6) into the body of the strip to produce a flat surface (7). At the same time this deformation of the strip (2) produces zones of stress (8). In order to read the code, the strip (2) is heated to temp. above the transition temp. of the plastic so that the strip resumes its initial shape.