The target contact zone was visualized using LED (light-emitting diode) polarized interferometry. Under the condition of non-lubricated static Hertzian contact between a sapphire plate and a piano-convex or a lenticular lens made of PMMA (polymethyl methacrylate), a statistical measurement of the real contact area was carried out by using image intensity histograms. The results are as follows: (1) The Gaussian distribution characteristics at the lowest region of the intensity histogram distribution were obtained, and the size of the domain as extracted through Gaussian distribution fitting was in good agreement with the theoretically obtained Hertzian contact area. By using the right and left symmetricalness of Gaussian distributions, it was possible to statistically measure the size of the domain, that is, the real contact area, in a straightforward manner. (2) It was suggested to measure the real contact area statistically by using the proposed method, which is capable of detecting gaps of the order of nanometers without requiring a threshold value for binarization. (3) In the case of a rough model surface when using a lenticular lens, the results agreed with the theoretical results for the Hertzian contact area as obtained by performing background correction. Thus, the effectiveness of measuring the real contact area by using Gaussian distribution fitting was demonstrated.
Statistical measurement of real contact area was applied to a paper-based wet friction material by using light interferometry images. It was demonstrated that extraction of the real contact area was possible in spite of the difficulty in determining the threshold value for binarization, where a valley did not appear in the lower region of the intensity histogram. The following points were clarified: (1) The intensity histogram, which was obtained through shading correction for non-uniform light illumination, was divided into four domains corresponding to the individual distribution of clearances between a glass plate and a paper-based friction material. (2) The real contact area was estimated by utilizing the "Gauss 1"domain, which was extracted by performing Gaussian distribution fitting to the intensity histogram. The extraction precision was estimated to be 20 nm for a gap conversion range of 3σ（standard deviation). (3) By dividing the intensity histogram into real contact area ("Gauss 1") and vicinities of micro-gaps ("Gauss 3", "Area 2", and "Area 4"), it was possible to estimate the statistical ratio of the micro-gap areas, and as a result the dependence of the changes in those areas on the contact pressure was demonstrated. (4) Since the abovementioned knowledge is applicable to general surfaces with rough texture and/or low reflectance as well as to paper-based friction materials, the proposed method is effective as a simple tool for measurement and analysis of the real contact area.