Abstract: Requirement of surface quality for high-quality metal plates and strips is very strict, and tiny defect is not allowed on the surface. Existing surface inspection systems based on machine vision are difficult to detect tiny defects. Photometric stereo is applied to detect tiny surface defects with surface normal calculated from two gray images under different illumination angles. One color camera with triple line-scan charge-coupled device(CCD) is mounted perpendicular to the surface of the metal. One red lighting and one blue lighting are mounted symmetrically on both sides of the camera. Three channels of gray image including red, green, blue are separated from the color image captured by the camera, and red, blue channel gray images represent reflected lights of the red lighting and the blue light. The distribution map of surface slope angles is calculated from red and blue channel gray images with the developed algorithm. Tiny defects can be detected from the distribution map of surface slope angles. Experiments showe that pinholes with the diameter less than 0.1 mm can be detected. The scheme of detection normal defects and tiny defects with one apparatus is discussed. One green lighting is added, and mounted to the camera at the position between bight-field illumination and dark-field illumination. Normal defects can be detected from the green channel gray image of the camera which represents reflected lights of the green lighting.

Key words: Metal plate and strip, Photometric stereo, Surface defect, Surface inspection