Patent ID: 7495779

Claim:
A level detection apparatus comprising: an illumination slit in which a rectangular first opening which causes illumination light to pass is formed; an optical system configured to illuminate a target object surface by illumination light passing through the illumination slit and focuses reflected light from the target object surface; first and second detection slits which are arranged in front of and in back of a focal point and in each of which a second opening is formed such that a short side of a rectangle is shorter than a short side of a illumination slit image formed by the illumination slit and a long side of the rectangle is larger than a long side of the illumination slit image; first and second light amount sensors configured to detect amounts of light of the reflected lights passing through the first and second detection slits; and a calculating unit configured to calculate a level of the target object surface based on outputs from the first and second light amount sensors.