A curved cylindrical segment platen (18) has a concave touching surface (18b). An interrogating slit light beam is scanned around the platen to scan a finger held against the touching surface. An optical imaging system images (20) the zone illuminated onto an array of photo-transducers (22) to provide a signal representing the fingerprint. The optical system is rotated (40) around the platen. The axis (17) of the interrogating light beam (16) is substantially normal to the touching surface (18b). The axis (28a) of the imaging system is off normal by about 56 degrees, an amount which is greater than a critical angle equal to the maximum angle of refraction (MAR) in the platen. This critical angle is approximately 41 degrees. The axis (28) of the imaging system is also off normal to the non-touching surface (18a) by an angle that is less than this critical angle of 4l degrees and thus less than the angle of total internal reflection (TIR). As a consequence, the image at the photo-transducers provides a completely dark valley zone and a sharply contrasting light image ridge ,zone.