In industry coating of materials is a standard process. Due to cost reduction and better production processes the thickness of the coatings becomes thinner and thinner under a comparable performance. For quality control, the thickness of the coating layer ideally needs to be measured already in the production process (in-line). Further the measurement principle needs to allow for a contactless measurement and should further provide a large stand-off distance to reduce the impact on the production process. For thickness measurements of optically transparent non-polar liquids or organic residues in the µm range laser-speckle based methods are very suitable. The speckle pattern results from the constructive and destructive interferences of diffracted laser light off of a rough surface. The statistical properties of the speckle pattern depend on several parameters like the roughness of the surface and the coating layer, the wavelength and diameter of the laser beam, and the refractive index of the coating. The thickness distribution of the coating is dependent on adhesive and cohesive forces, the temperature and mainly on the surface topography. The thickness of transparent coatings can be evaluated by considering the parameters of wavelength and roughness by using statistical data processing.
In this work several different noncontacting laser-speckle-based measurement systems are presented to determine the thickness of the µm-thin liquid layers on rough metallic surfaces. The measurement principles and the data processing, as well as the measurement results are presented. The advantages and disadvantages of different principles and the preliminary analyses of the speckle patterns are discussed. For a better understanding the theory of laser speckle and the theory of the distribution of liquids on rough surfaces will be detailed. At the beginning there is a discussion on laser speckles on different application areas to get an overview about the topic.
Keywords: laser-speckles, non-contacting, thin coatings, surface roughness