Abstract: An apparatus adapted for thermally erasing or changing machine readable optical markings previously entered on a carrier of thermoplastic material which has two major sides, and wherein the optical markings cause a predetermined modification of any light rays impinging on the optical markings by either diffraction or refraction, include a erasing device adapted to be located on one major side of the carrier, for erasing or changing the markings, a reading device, operative during operation of the erasure device, and including at least two light sensors adapted to be located on the other major side of the carrier, so as to check the erasure or change of the markings effected by the erasing device. The light sensors provide output signals of respective opposite polarities following erasure or change of any one of the markings.

Abstract: Apparatus for storing and reading data in association with a card which has a timing track comprising at least three different kinds of optically-readable timing markings arranged in the timing track in a cyclic sequence, and a data track, the apparatus including three optical timing sensors respectively responsive to the three different kinds of timing markings, and a reading direction detector controlled by the timing sensors.