Photoelectronic apparatus with protection from effects of undesired light

Claim 1

Photoelectronic light barrier apparatus comprising light source means for producing an operational light beam, light receiving means including an operational photoelectric converter disposed in fixed disposition with respect to the light source means to receive the operational light beam, an auxiliary photoelectric converter arranged in the immediate vicinity of the operational photoelectric converter and positioned outside the light beam so as to receive substantially no light from the light source means but so as to receive unwanted light incident on the light receiving means, an electronic processing circuit, circuit means for forming a weighted difference signal from the outputs of the operational and auxiliary photoelectric converters and for passing the weighted difference signal to the electronic processing circuit, the electronic pro-cessing circuit including means defining a threshold and being adapted to produce a first signal when the weighted difference signal exceeds the threshold and a second signal when the weighted difference signal is less than the threshold, one of the first and second signals indicating the presence of an obstruction in the light beam and the other being indictive of the absence of an obstruction in the light beam, and wherein the circuit means applies a level of weighting in forming the weighted difference signal which defines a threshold for stray light below which the photo-electric light barrier apparatus is compensated for the effects of stray light to reliably indicate the presence of an obstruction and above which the light barrier apparatus will always indicate the presence of an obstruction, irrespective of whether or not an obstruction is actually present.