Optical Inspection of Surface Roughness and Gloss

For a variety of products, presenting different industrial sectors, the quality of the product surface is quite often an important factor. The product surface may go through a sequence of finishing processes in order to improve the surface quality. In this chapter we mention a few examples where surface quality is a critical parameter. For instance, in the case of steel sheet production for the automotive industry a relatively thick steel strip is first hot-rolled and thereafter cold-rolled in a roller line. The rollers leave characteristic finishing marks on the surface of the steel strip. The surface profile of the roller surface is essential, in addition to the rolling load, as it concerns the desired surface roughness and surface texture of the steel sheet. Naturally there appears a wear on the envelope surface of the roller, and this wear causes the surface roughness of the metal strip to change as a function of the wear of the roller. A similar situation may appear also in another type of surface finishing, namely in the production of tablets in the pharmaceutical industry. There the surface finish of the punches of a tablet machine is crucial in order to prevent, e.g., sticking of the powder material onto the surface of the punch. Thus a very smooth punch surface is usually required for optimum tablet making. In the case of cellular phones high quality plastic covers are obtained using electro-discharge machining for injection mold surfaces. Plastic melt copies the mold surface and if wanted a relatively rough surface matte can be obtained. However, the tool is subject to wear and may cause local surface quality variation in the plastic product during a long duty cycle of the tool.

It is obvious that in some sectors of the industry there is a desire to get as smooth a product surface as possible, whereas in some other sectors a relatively high surface roughness is the measure of the quality of the product. A common factor for all such industrial branches is to get information of the surface roughness of the product, and also on the finishing marks and their orientations. The role of the manufacturing process and, for instance, the condition of the tool has great importance in finding the optimal process in order to gain optimal product surface condition [45]. Optical measurement techniques for ultra smooth surfaces is of great importance especially in the optimization of the surface quality of devices based on semiconductors and other high tech products. Optical measurement of ultra smooth surfaces have been described, e.g., in the book of Bennett and Matson [46].