| 摘要 |
PURPOSE:To enable the detection of the abnormal state and the position of the surface of an object to be detected in a short time, by optically scanning the surface of the object being detected to detect the 0-order reflected light at least with a one-dimensional photoelectric detection element. CONSTITUTION:A scanning is done over a reticle 7 by a beam from a laser 1. The 0-order reflected light from the reticle surface reaches a position sensor 10 set on the focus of a condenser lens 6. Without any reticle pattern, dirt or the like on the reticle surface, the 0-order light reflected from the reticle surface is made incident to the center of a position sensor 10, where the outputs ix and ix' will be equal. With any dirt existing on the reticle, light beam will be incident into the point other than the center of the position sensor 10 as the laser beam passes over the dirt and the outputs Vx and Vx' of preamplifiers 11 and 12 will give uneven waveforms to generate a dirt signal. Thus, the position of the dirt is determined from the scanning position of the beam and the output of an encoder 15 at the time when the dirt signal is generated. |
