Optical surface inspection of steel mill products such as pipes, plates and slabs usually has the problem of overdetection,
which is caused by signals from harmless parts such as scale and surface texture. The authors propose a new inspection
technique based on the experience that most harmful defects on these products have a concave or convex shape, whereas
most harmless parts that might be overdetected have flat shapes. The proposed technique is called the ‘twin-illumination
and subtraction technique’.
In this technique, firstly, two images of the target area on a steel surface illuminated from the two sides are captured,
respectively. A subtraction image is then calculated from these images. Comparing the images illuminated from the
different sides, the images from concave or convex defects look different due to their different shadows, while images
from harmless flat parts look the same because illumination does not cause any shadow. As a result, two images with the
same appearances from a harmless part are canceled by subtraction, and two images with different appearances from a
concave or convex defect remain even after subtraction. Finally, it is possible to detect only concave or convex defects
without overdetecting flat patterns.
In this manuscript, first, we explain the proposed technique and confirmation experiments in the laboratory. We also
explain a new optical inspection system based on the concept described above and its application to moving hot pipes in
a steel manufacturing plant to prove the effectiveness of the technique. We concluded that the inspection system has
sufficient performance for use as a practical system.