Patent ID: 7868279

Claim:
A focus detecting apparatus for focusing on a transparent substrate having two surfaces, the focus detecting apparatus comprising: an objective lens; a point light source which emits illumination light for generating a focusing signal to the transparent substrate and irradiates the transparent substrate through the objective lens; mask means having a first shading part, the first shading part being configured to shade, out of luminous flux of the illumination light emanating from the point light source, luminous flux passing through one of two regions into which a space is divided by a first virtual plane, the first virtual plane being a plane in which an optical axis for the illumination light lies; and a photodetector, having two light receiving parts that are symmetrically arranged to sandwich a second virtual plane, for carrying out focusing of the objective lens on one of the two surfaces of the transparent substrate based on amounts of reflecting light from the transparent substrate that are detected through the two light receiving parts, respectively, the second virtual plane being a plane in which an optical axis for light reflecting from the transparent substrate lies; wherein the mask means further includes a second shading part, having a shape similar to one of the two light receiving parts of the photodetector, for intercepting, out of the luminous flux of the illumination light emanating from the point light source, a part of luminous flux passing through another of the two regions defined by division of the space by the first virtual plane so that, when a focus of the objective lens is positioned near one of the surfaces of the transparent substrate, reflected light from the one transparent substrate surface, which is positioned near the focus of the objective lens, enters the two light receiving parts, and reflected light from the other transparent substrate surface passes through an area off one of the light receiving parts arranged in one of two regions into which a space is divided by the second virtual plane.