Hydrophobicity estimation of HV polymeric insulating materials. Development of a digital image processing method

Image analysis of water drop patterns on an inclined flat polymeric insulator surface has been performed in order to find a simple mathematical function that indicates the level of hydrophobicity of the insulator surface. A simple function, given the acronym average of normalized entropies (ANE), seems to correlate well with hydrophobicity as defined by the Swedish Transmission Research Institute (STRI) hydrophobicity classification. It is a composition of three other functions, viz. the standard deviation, the Shannon information entropy and the ´fraction of small differences´. All these are in turn based on the histogram of horizontal nearest-neighbor pixel differences for a given digital greyscale image of a water drop pattern. ANE is fairly independent of illumination intensity (exposure) as well as total gain and offset in a camera system (linear sensor). The experimental results also indicate that ANE is fairly independent of limited changes in the surface inclination, although this needs further investigation. Some of the various pitfalls associated with the photography of water drop patterns and processing of images are identified, together with possible solutions for avoiding them