| 摘要 |
PURPOSE:To obtain focusing with high accuracy, by branching an optical path passing an objective system, observing an object to be examined with the one thereof, branching further the other optical system and providing a light projecting means in the one t hereof and a photodetecting means in the other. CONSTITUTION:When a semiconductor laser 17 is oscillated, laser light is converged with a lens 18 and the converged light illuminates a mask 19. The luminous flux past the area on the outside of the mask 19 passes a semitransparent mirror 20 and is reflected by an optical branching filter 13, from which the luminous flux is made incident through an objective lens 12 to an object 11 to be examined. This flux is reflected by said object, is passed through the lens 12 and is further reflected by optical branching filters 13, 20. The image thereof is formed with a lens 21, and is made incident via a filter 22 to a photodetector 23. If the object surface 11 and the photodetector are conjugate with each other, the light distribution on the photodetector 23 is such that the position of the optical axis peaks. However, in the case of a front focus or rear focus, the one side of the projected light is vignetted by the mask 19; therefore, the photodetector 23 is driven to scan the light distribution and the scanning is continued until the peak of the light distribution coincides with the boundary of the regions 23, 24. |
