| 摘要 |
PURPOSE:To perform focusing of high accuracy with simple constitution by detecting the quantity of reflected light from an object to be focused, and controlling the output voltage to be inputted to a control circuit in such a way as to be kept within a prescribed range. CONSTITUTION:The beam from a semiconductor laser 8 is projected to the surface of a mask 2 via an objective lens 6, and the reflected beam is formed image at an image forming point A via a beam splitter 9 and an image forming lens 10. The outputs of pin photodiodes 11, 12 are inputted via amplifiers 16, 17 to a comparator 18, and a pulse motor 13 is driven with a control part 19 to move a mask 2 vertically until both the inputs thereof are made equal whereby focusing is accomplished. On the other hand, the outputs of the diodes 11, 12 are inputted to a control means 20, and the output of a semiconductor laser 8 is controlled so that the quantity of reflected beam is kept always within a prescribed range. Thereby, the focusing of high accuracy is accomplished with the simple constitution. |
