Abstract: A concealed code of a card or badge arranged along a line parallel to one edge is read by advancing the card or badge along a slot defined between two internal chambers of a sensor housing that accomodate an infra-red light-emitting diode and a phototransistor respectively. The two chambers are provided with holes disposed on opposite sides of the slot to facilitate a beam of infra-red radiation passing through the holes and across the slot to read the code embodied in the card or badge. The surface at the bottom of the slot slopes upwardly from the peripery of the housing to a point of maximum height midway through the slot that is immediately below the point at which the infra-red beam passes across the slot so that the beam reads the code even when the edge of the card 1 is slightly misaligned.

Abstract: An instruction or identification card or badge for use in a time recording system, to authorize admission to restricted locations or for use in credit transactions comprises layers of material that is opaque to visible light but transparent to infra-red radiation. A code that is readable by an optoelectronic reader but is invisible in ordinary light is provided by transparent windows alternating with infra-red opaque regions arranged along a line parallel to one edge of the card, one binary digit being represented by wide rectangular windows and the other binary digit being represented by narrower windows that when read in a badge reader give different peak heights. The code is self-clocking and need occupy only a small portion of the card area.