
An apparatus for detecting the presence of a foreign substance adhering to a planar substrate comprises irradiating means capable of emitting a light beam incident on one surface of the substrate obliquely relative to the surface, means for displacing the irradiating means and the substrate relative to each other so that the position of incidence of the light beam onto the substrate is scanned on said one surface, metering means capable of receiving a plurality of irregular reflected light beams...











