JPH057750B

PURPOSE:To perform efficient and inexpensive detection of surface conditions on an object by irradiating parallel light at a prescribed angle from an oblique direction with respect to the object surface, and equalizing a light irradiation angle at least on the surface to be detected. CONSTITUTION:A device turns light into parallel light by an optical system 13 consisting of an extinction filter 11 and a parallel light lens 12 from a light source 2, irradiates the parallel light at the prescribed angle from an oblique direction on a flap side with respect to the reverse surface of a mail P, and projects the reflected light on the light receiving surface of a line sensor 4 by a projecting lens 3 and then performs photoelectric conversion. By irradiating the parallel light at the prescribed angle from said direction on the flap side with respect to said P, the irradiating angle will become equal (theta) at any position on the P, by which a shade S produced by a difference in level of the flap 1 is generated regardless of the shape and location of the flap, thereby enabling secure flap detection over the upper/lower directional wide range of the P.