Abstract: A protective zone apparatus on a vehicle capable of travelling along a path has a light transmitter (16) and a light receiver (18). The transmitted light beam (11) is obliquely directed onto the path of travel (12) at a first fixed angle (.alpha.) and executes a scanning movement about an axis (13) perpendicular to the path of travel (12). The received light beam extends at a second fixed angle (.beta.) from the point of impingement (15) of the transmitted light beam (11) on the path of travel (12) to the perpendicular axis (13) and executes a scanning movement synchronous with that of the transmitted light beam (11). An electronic processing circuit transmits a control signal which stops the vehicle if an obstacle arises in the protective zone (FIG. 1).

Abstract: A retroreflector consists of a support surface 12 coated with a layer 11 of small retroreflecting particles such as glass spheres. The support surface comprises a plurality of regularly repeating, alike, structural elements 13 which are inclined to the overall plane of the retroreflector. The surface elements 13 can be of various shapes and serve to increase the angular range over which retroreflection can occur. The optical characteristics of the proposed retroreflectors can be tailored so that they are eminently suitable for use in certain beam scanning applications such as continuous monitoring of doorways and windows by an optical beam scanning device located at one corner thereof in order to detect, for example, intruders.