Abstract: This invention relates to marking the genuine CD in an easily detectable way while difficult to counterfeit. The improved CD has within its layers a second structured surface parallel to the flat surfaces of the CD. The second structured surface acts as a diffractive security device, that is to be read out from the same side of the CD, the verso side, as the data structure. In one example, the second structured surface is defined by a step of the refractive index or by a transparent reflective layer. Despite the diffractive rainbow colors observed on the verso side of the CD, the structure of the security device can be read out visually and/or with a CD-reader, even by a simple hand-held reader, and verified without the need of a bulky apparatus. A CD-reader prevents the playing of embodiments of the improved CDs.

Abstract: An optical data carrier is written along data tracks with diffraction patterns, each diffraction pattern representing a predetermined bit sequence with n bits. The bit sequences of a diffraction pattern can be determined with a number n of photodetectors which is smaller than the number m of the possible different diffraction patterns. It is particularly provided that the number m is a power of two of the number n: m=2.sup.n. The design of the diffraction pattern as a number of at most n juxtaposed diffraction elements which in the reading operation illuminate only a single photodetector permits simple production of an ROM and a WORM data carrier.

Abstract: An optical data carrier for use as a WORM memory contains memory cells where each memory cell is physically represented by a diffraction pattern. The content of each memory cell is a one bit information having a binary value "0" or "1". If a memory cell is altered by applying thermal energy it represents the complementary binary value. The unaltered memory cells are also used as security elements. This means that upon reading the content of an unaltered memory cell with a readout beam, the unaltered memory cell bends this reading beam into a plurality of different beams of predetermined outgoing directions. The intensities of these reflected beams are measured and compared with stored values. The data carrier is only then an authentic data carrier and not a fraudulent imitation, if the measured intensities of the reflected beams of such an unaltered memory cell lie within predetermined ranges.