Abstract: The invention relates to optical memories making use of optical interaction between an incident ray and an information medium (83) capable of having data written thereon and of having data read therefrom, which data is organized in a adjacent track elements which are marked by prerecorded patterns. The invention provides an information medium (83) and a reader device therefor (84, 85, 86, 87, 88) including means (89, 91, 92, 93, 94) for sampled tracking of the track elements on the basis of patterns which are offset from the track scan line, said patterns forming discontinuous periodic wave sequences whose offset edges are symmetrically disposed about the middle of the inter-track gap. The invention is particularly applicable to optical memories using a preformated disk capable of recording, of being read, and where appropriate of being erased, with data in digital form.

Abstract: An information medium adapted to be written by thermo-optical means comprising a heat-sensitive structure adapted to store information to be read by optical means. This includes a thin metal layer, which, when incident writing radiation impinges on it, causes a transient localized temperature rise. Above a temperature threshold this causes a change in the optical characteristics of the heat-sensitive structure. The information medium is manufactured by a method involving depositing a continuous thin film of polycrystalline aggregate over an acceptor surface of the aforementioned structure by means of an incident flux of particles of a precious metal and a transition metal. In order to obtain a defined metallic composition, these metal particles are extracted from at least one target exposed to bombardment by other particles resulting from an electrical discharge in a rare gas at reduced pressure and confined by a magnetic field.