apparatus for detecting marks or other objects by comparing a decrease in reflected light while the mark or object is moving across optics , to the light reflected before the mark or object was encountered by the optics . a threshold reference voltage level is derived directly from the conductance level of a phototransistor prior to the introduction of the mark or object to be detected . this reference level is developed in such a manner as to cause it to be a dynamic voltage , varying percentage - wise as the conductance of the photo - transistor . the dynamic reference level so determined is thus always directly a portion of the level present before the mark or object to be sensed was introduced .