Optical data carriers and reading devices therefor

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. If relief structures with an asymmetrical profile are also used as diffraction elements and if the reading device subjects the signals supplied by the photodetectors to an analysis operation and only then determines the bit sequence to be associated with a diffraction pattern, the number m of different diffraction patterns which can be read with a predetermined number n of photodetectors can be further increased. If the data carrier serves as a WORM memory with a high level of security in relation to forgeries and unauthorized manipulation, then the unaltered diffraction pattern 2 in the form of a security feature represents on the one hand the bit sequence and on the other hand a memory cell for a single bit.