| 摘要 |
PURPOSE:To correctly compare a front focus state and a rear focus state by inputting outputs of a first and a second photoelectric converters for photodetecting a transmission light of a pattern mask for a front focus and for a rear focus, and positioning a sample in a focused position of an objective lens by an output signal from a difference device for deriving its difference. CONSTITUTION:An optical path of an image forming lens 13 is used for a front focus and a front focus use pattern mask 14 and a photoelectric converter 15 are provided against it, and an optical path of an image forming lens 16 is used as a rear pin and a rear focus use pattern mask 17 and a photoelectric converter 18 are provided against it. In this state, a difference of outputs of the photoelectric converter 15 and 18 is calculated by a difference device 19. In such a case, when the front focus use pattern mask 14 and the rear focus use pattern mask 17 are placed so that a photoelectric output (a) and (b) from the photoelectric converters 15, 18 intersect with each other, an output signal from the difference device 19 becomes '0', and this point becomes a focused position of a sample 1 an objective lens 2. In such a way, the front focus state and the rear focus state can be compared correctly. |
