Patent ID: 8258448

Claim:
An autofocus device comprising: an objective lens that converges light at a measurement surface of a measurement object; an observation optical system capable of observing an image of the measurement object based on the light from the objective lens; a driving mechanism that displaces the objective lens in an optical axis direction thereof on the basis of a contrast of the image of the measurement object obtained at the observation optical system; an illumination-optical-system optical path used to illuminate the measurement surface of the measurement object through the objective lens with the light; a pattern-projection-optical-system optical path, a light source; a light divider for dividing light from the light source into light that takes the illumination-optical-system optical path and light that takes the pattern-projection-optical-system optical path; and a light synthesizer for synthesizing the light from the pattern-projection-optical-system optical path with the light of the illumination-optical-system optical path wherein a pattern projection plate, having a predetermined pattern formed thereon, a projection lens, and a shutter are provided in the pattern-projection-optical-system optical path, wherein the shutter is movable between an opened position transmitting light and a closed position blocking transmission of light whereby, when a contrast value detected by the observation optical system is low, the shutter is moved from the closed position to the opened position so that the predetermined pattern of the pattern projection plate can be projected onto the measurement surface, thereby increasing the contrast value detected by the observation optical system and permitting focusing of the objective lens on the measurement surface, wherein the light divider includes a plurality of light dividing optical elements that divide the light from the light source into the light that takes the illumination-optical-system optical path and lights that take a plurality of pattern-projection-optical-system optical paths, and wherein the pattern projection plate includes a plurality of pattern projection plates and wherein the first shutter includes a plurality of shutters provided in the plurality of the pattern-projection-optical-system optical paths.