| 摘要 |
PURPOSE:To detect an exact focus shift signal even if a measuring surface is inclined against an incident luminous flux by dividing a reflected luminous flux into two, and leading both of them to a critical prism through Selfoc lenses of an erect unmagnification and an inverted unmagnification. CONSTITUTION:A reflected luminous flux is divided into two by a half mirror 10, a transmitting light is made incident on a Selfoc lens 12 of an erect unmagnification, a reflected light of the mirror 10 is made incident on a Selfoc lens 13 of an inverted unmagnification, and both of them are reflected several times in a critical prism 7 and made incident on photodetectors 8, 9. In case when a measuring surface 6 is inclined against an incident light, a moving direction of a reflected luminous flux by an incident end of the Selfoc lenses 12, 13 is the same, but it moves in the opposite direction in the emitting end, and a movement of a spot of the photodetectors 8, 9 becomes the opposite direction. Therefore, by taking the sum of error signals of the photodetectors 8, 9, a variation of a signal by a parallel movement is offset, and an exact focus shift signal can be obtained. |
