| 摘要 |
PURPOSE:To utilize effectively incident light, and also to execute a detection of a good S/N extending over a wide range by controlling a photocurrent quantity to an optimum value in accordance with an incident light level. CONSTITUTION:Incident light is diffused by a diffusing plate 6, made incident uniformly on the whole photodetecting surface, and converted to a photocurrent corresponding to an incident quantity level by each photodiode 7. As the incident light quantity increases, an output of an amplifier 8 rises, and an output of a voltage comparator 9 becomes a high level. If a switch S is opened so that the photocurrent decreases in a switch changeover driving circuit 12, and for instance, comparators 9, 10 are set to VH>10VL, the photocurrent is set to 1/10 and the amplification degree is switched to one time. On the contrary, when the incident light quantity decreases, the output of the amplifier 8 falls, and an output of the comparator 10 becomes a high level. In this case, the photocurrent quantity is set to 10 times by the circuit 12 by connecting the switch S, and the amplification degree is switched to 10 times. In this way, a photodetection of a good S/N can be executed extending over a wide range. |
