| 摘要 |
PURPOSE:To measure the focal distance of an optical lens with an error almost as large as a photodetector by providing the photodetector which outputs different logical values in accordance with the light quantity of a surface and a logical operation circiut which processes the logical values. CONSTITUTION:For example, the concentric photodetectors 1a-1d output the logical value '1'(0) to signal lines 2a-2b respectively when light through the optical lens is cast(is not cast) to the photodetectors. When the logical values of two input signals are same(different), exclusive OR circuits 3a-3d output the logical value '0'(1). An adder 5 of four bits subtracts the value of a latch circiut 4b of four bits from the value of a latch circuit 4a of four bits and outputs a carry signal 6. When a controller receives the signal 6, it moves a semiconductor element 10 in a direction where the received light quantity is reduced. Finally, the movement of the element 10 is stopped when the logical value of the signal line 7(the most significant bit of the circuit 4a)is '1', that is, when the light is not cast to the photodetectors 1a-1d. A distance of the element 10 from the lens at that time is the focal distance of the lens. |
