A light barrier or light grid which has at least one light emitter arranged in a first housing and at least one associated light receiver arranged in a second housing. At least one emitter of visible alignment light installed in the first or the second housing generates one or more alignment light beams which is used to geometrically orient the light emitter relative to the opposite light receiver. For optically aligning the two housings, at least one alignment mechanism is present in the housing which receives the alignment beam. The alignment mechanism has at least one surface which optically deflects the alignment beam towards a target plane.