1. Autocollimation light curtain comprising a light source (22) ; a light beam generating optical system (20, 23) ; a light deflecting device such as a mirror wheel (24) which receives the light beam, a reflector (19) at the end of a monitoring region (18) which is periodically scanned by the light beam and is subdivided in the scanning direction into a plurality of fields (12, 13) of two types which alternate with each other ; and also a light receiving arrangement (14) which receives a received light beam reflected out of the beam path in front of the light deflecting device and which has a photoreceiver (15) for receiving the light reflected back from the reflector (19) when the monitored region is free, characterised in that the two alternating types of fields (12, 13) of the reflector (19) have characteristics which influence the spectral range and/or the state of polarisation of the incident light and the light receiving arrangement (14) includes a second photoreceiver (16) which is so filtered that it receives only light reflected form the one type (12) of fields but not light from the other type (13) of fields, whereas the first photoreceiver responds to all light reflected back from the reflector (19) ; and in that both photoreceivers (15, 16) are connected to a logic circuit (17) which only signals freedom of the monitoring region when the second photoreceiver (16) transmits an alternating signal and the first photoreceiver (15) transmits a dc signal.