JPS5816692B

PURPOSE:To make it possible to detect a flaw in accordance with the influences at a mold jointing seam in accordance with the displacement in the generating instant of flaw signals from two light receivers, which are arranged cymmetrically with respect to an optical axis, by receiving the light from a test piece, which is torating at a right angle with respect to the optical axis, in the receivers. CONSTITUTION:If light receivers 6 and 7 are arranged symmetrically with respect to an optical axis 12, the light is refracted to the right and left by a flaw 13. This refraction is performed such that the light first comes to a point a of the flaw 13 and is refracted toward the receiver 6 and that the light then comes to the other point b and is refracted toward the receiver 7. As a result, there is a displacement in the generating instants of the flaw signals A and B resulting from the flaw 13. On the other hand, the optical changes of the light having passed therethrough due to the mold joining seam has the following characteristics. Namely, the light is refracted only in one direction toward the receiver 6 or 7, or the time displacement between the abnormal signals from the receivers 6 and 7 is equal to zero or a little, if any. Therefore, if an AND signal F is taken between suitably delayed timing pulses E and the binary signals D of the receiver 7, the signals of the seam and the flaw 13 can be discriminated.