| 摘要 |
PURPOSE:To detect the period until an image pickup element is normally activated and to shut light to the element with a shut mechanism during this period, by comparing an optical signal voltage with a prescribed reference signal. CONSTITUTION:A picture output of a pickup tube 14 is clamped via a preamplifier 15 and this signal is compared with a reference voltage to discriminate a beam current of the tube 14 at a comparator 17. A power supply 32 is inputted to a CLR terminal of an FF20 via a time constant circuit and outputs an output signal at a terminal Q. When a switch 29 is set on, an iris is opened with a delay t1 at a time constant circuit. In this case, the output of a clamp circuit 16 is 0V and an output Q of an FF20 and the comparator 17 is ''L''. When a beam flows to the tube 14, a signal current is outputted to the picture output due to bias light, and since the input to the comparator 17 exceeds the reference voltage, an output is given. Thus, the FF20 reads an input D, a high level is obtained at the output Q, and the closing state of the iris is released. The power to the iris control section 12 is applied to a control circuit 12 for several seconds at the time constant circuit even if the switch 29 is set off. |
