METHOD FOR DETECTING FLAW ON SURFACE OF METALLIC OBJECT

摘要

PURPOSE:To enable flaw detection at high S/N by setting the angle between the incident direction of the irradiated light having a prescribed plane of polarization and the normal on the surface of a specimen at 15 deg.-55 deg. and detecting the scattered light of <=20 deg. angle to the incident direction and the regularly reflected light. CONSTITUTION:A laser light source 22 disposed in the diagonal direction intersecting orthogonally with the traveling direction of a continuous casting slab 20 on the lateral side of the traveling line of said slab irradiates the external light having a linear polarization characteristic with a rotor 24 for the plane of polarization and a cylindrical lens 26 in such a way that the angle theta1 between the normal on the surface of the slab 20 and the incident direction of the irradiated light attains 15 deg.-55 deg.. On the other hand, a photodetection camera 30 which is disposed on the same lateral side as the light source 22 and has a polarization filter 28 detects the component of the light having the same plane of polarization as the plane of polarization of the irradiated light in the scattered and reflected light of <=20 deg. angle theta2 to the incident direction. The outputs of the camera 30 and a photodetection camera 34 for regularly reflected light is compared in a comparator 40 via signal processing circuits 36, 38 and only the defect signal removed of the spurious defect signal by a scale or the like is outputted.