Surface defect inspecting device and method for steel sheets

A surface defect inspecting device for steel sheets, comprising:

• an illuminating unit having a light source configured to illuminate an imaging target portion, with light of the light source, on a surface of a steel sheet;
• a first diffuse reflection light imaging unit having a first light receiver arranged, at a first angle with respect to a specular reflection direction of illuminated light, in such a manner that the first diffuse reflection light imaging unit images reflection light, received by the first light receiver, reflected in a direction of the first angle from the imaging target portion so as not to image specular reflection light but to image first diffuse reflection light;
• a second diffuse reflection light imaging unit having a second light receiver arranged, at a second angle larger than the first angle with respect to a specular reflection direction of illuminated light, in such a manner that the second diffuse reflection light imaging unit images reflection light, received by the second light receiver, reflected in a direction of the second angle from the imaging target portion so as not to image specular reflection light but to image second diffuse reflection light;
• and
• an image signal processor configured to process a first diffuse reflection image signal acquired by the first diffuse reflection light imaging unit as a result of performing imaging and a second diffuse reflection image signal acquired by the second diffuse reflection light imaging unit as a result of performing imaging, wherein
• the first diffuse reflection light imaging unit and the second diffuse reflection light imaging unit simultaneously image reflection light reflected from the imaging target portion, and wherein
• the image signal processor detects, as a surface defect portion, a portion for which brightness level is lower than a first predetermined threshold in the first diffuse reflection image signal acquired by the first diffuse reflection light imaging unit as a result of performing imaging as well as higher than a second predetermined threshold in the second diffuse reflection image signal acquired by the second diffuse reflection light imaging unit as a result of performing imaging.