| 摘要 |
PURPOSE:To electrically detect defocusing of an objective lens to a minute target by providing two photodetectors in positions equally distant from the position conjugate to the reflected optical image from the target and obtaining the difference between their detection outputs. CONSTITUTION:A laser light 19 is condensed on a minute alignment target 11 through a transmission mirror 10 and an objective lens 12. The reflected light from the target 11 is condensed by an eyepiece 13; and when first and second photodetectors 14 and 16 are arranged before and after the position conjugate to the reflected optical image in positions equally distant from this position, the difference between their detection outputs indicates the extent of deviation between the focus of the objective lens 12 and the target 11. Consequently, detection outputs are inputted to a differential amplifier 18 and the objective lens 12 is so adjusted that the difference is zero, thereby objectively and quickly detecting the defocusing to the minute target 11 as an electric signal. |
