Abstract: Subtle streaklike defects are detected in running webs and sheets by transversely scanning distinct lateral sections of the webs or sheets by means of lines of radiant energy extending parallel with the direction of movement of the webs or sheets. Transmitted or reflected radiation is received on corresponding photocells that move synchronously with the lines of radiant energy. During each scan of each section of the webs or sheets, a large number of comparisons occur between the output signals of each photocell at a given first lateral position and at a second lateral position that is separated from the first one by a distance that is of the order of magnitude of the width and preferably half the width, of a streaklike defect to be identified.