Abstract: A photoelectronic apparatus, in which a photoelectric convertor (the operational convertor) receives a beam of light and produces an output signal proportional to the intensity of the light received, is protected against the effects of unwanted light by providing an auxiliary photoelectric convertor arranged in the vicinity of the operational convertor. The auxiliary convertor detects the presence of unwanted light and its output is subtracted from that of the operational convertor and the resultant signal is then examined to distinguish between events occuring in the light beam and events simulated by the unwanted light.