PURPOSE: To accurately decide whether a mark is present or not by comparing the threshold level of a mark on a film with that of the output of a photoelectric transducing means.
CONSTITUTION: When rays of light are transmitted to a sensor 21 from an optical fiber 20 through the mark 16, the sensor 21 generates a voltage. Since said voltage changes on the mark 16 and other places, a maximum detecting circuit 23 and a minimum detecting circuit 24 obtain maximum and minimum values. A threshold level is set by a threshold level setting circuit 26 and is stored by a threshold level memory circuit 27 to which a backup power source 28 is connected. Even if a main power source is disconnected, the circuit 27 continues storing the threshold, and moreover it can detect a mark at the same level even if a power source is replaced in the middle of charging.
MIZUSHIMA TOSHIAKI
KASHIWAGI KAZUO
