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In this paper we consider the inspection of surface imperfections in ceramics, in particular porcelain products. An optical system is presented which addresses three critical aspects of the detection: the characteristics of the surface; the right choice of illumination system; and the collection of the diffused light which comes from the porcelain. Furthermore, a computer- based approach, with its fundamentals in the binary morphological image analysis, is used to support the processing and quality control. The motivation for working the morphology is the fact that we can acquire a low gradient image which is ideal for thresholding purposes, resulting in an effective binary image. The porcelain that we have analyzed has a glazed surface. As a matter of fact, it exhibits a significant coefficient of specular reflection. An extended white light source is used to illuminate a quarter of the surface of the plate. Along the axis of the beam a variable-aperture diaphragm is mounted for controlling the amount of light which reaches the surface. Other devices in the path are two linear dichroic polarizers for creating the shading field. This kind of illumination isolates an appropriate portion of the plate being inspected with low gradient contrast over the part being observed. By adjusting the relative orientations of the polarizers we obtain a contrast optimized image when the axis of the polarizers are nearly perpendicular. The diffused beam enters the CCD camera objective after passing through a large diameter positive lens. The method, as described above, improves the performance of the inspection process. It is found that the inspection time per plate is substantially lower, 2 seconds instead of 15 seconds, and that the automation eliminates the possibility of the subjective element introduced by human visual perception. Regarding the effectiveness of the method, we are making successive improvements in order to attenuate considerably the specular component of reflection. To this end, we are considering the use of a different involving media whose refractive index is close to that of the glaze used on the porcelain.
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