The light sources and their associated receivers are pref. activated individually in succession and esp. cyclically and the receivers detect whether an optical signal is received from an activated source or not and produce alarm signals when signals are not received. Light from a source reaches not only its receiver but at least one adjacent receiver when there is no obstruction and/or each receiver is illuminated not only by its source but by at least one other. Failure to receive from an activated source is checked by determining whether the receiver adjacent to the associated receiver receives light from the same source or not and/or whether the associated receiver receives light from an adjacent source. An alarm signal is produced if no signal is received. A defined number of reception results is checked to determine whether the presence of an obstruction is probable, in which case the alarm signal is maintained, or if it is not probable, a defect in a source and/or receiver is probable, in which case the alarm signal is turned off and a warning signal generated.
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