An Application of Laser Speckle-Based Coating Thickness Measurement

A non-contacting method based on laser speckles for determining thin liquid layers on optically rough surfaces is presented. The optical system allows large stand-off distances between the specimen and the sensor so the system can also be used in harsh environments and industrial applications. The thickness of organic residues or polar liquids to be measured can be in the typical range of about the same as the roughness of the surface. The distribution of the oil on the rough surface is dependent on the adhesion and cohesion forces, the temperature and mainly of the surface topography itself. The speckle generating effect is thus dependent on the surface roughness as well as the film thickness (and also its optical properties).These two effects change the structure of the so-called laser speckles due to laser light illumination on any dielectric coating on metallic surfaces. The speckle patterns viewed at by a video camera are evaluated and their rather strong dependence on liquid film properties are presented in this paper.