SURFACE INSPECTION APPARATUS AND MANUFACTURING METHOD FOR STEEL PLATE WITHOUT VERY SMALL IRREGULARITY

PROBLEM TO BE SOLVED: To provide an apparatus and a method wherein a very small irregularity flaw whose irregularity part is about several μm can be detected surely even in an object, to be inspected, whose surface roughness is coarse. SOLUTION: A light source 4 is installed in a detection head 3. The surface of a steel plate 1 is irradiated with convergent light at a wavelength in a visible range at a large angle of incidence θof near 90 deg.C. Light which is reflected by the surface of the steel plate 1 forms an image on a semitransparent screen 6. The image is imaged by a two-dimensional camera 7 from the rear of the screen 6, the image is processed by a signal processor 8, and the convexo- concave flaw is detected. The wavelength of light form the light source is designated as λ, and the angle of incidence of irradiation light on the steel plate 1 is designated as θ. When con θ/λ is set at a prescribed value or less, reflected light from the surface of the steel plate becomes specularly reflected light. When the irregularity flaw exists. its part is projected on the screen 6 as a black part, and the existence of the flaw can be detected.