Abstract: In the interferometry measurement of lubricating film thickness in a slider-on-glass disc contact, inclination angle of the slider with respect to the disc has to be determined. Usually, the number of fringes is counted artificially to deduce the inclination angles, which is inefficient and leads to large errors. Fringes generated by the slider-on-glass disc contact are of equal distance and the interference intensity varies periodically with film thickness. Therefore, a Fouriertransform-based approach has been proposed to accurately calculate the fringe number, in which a frequency interpolation is used to eliminate frequency leaking by combining the Rife-Jane method and overlapping FFT phase difference method. Experiments show that the relative error of the proposed approach is less than 1% when the fringe number larger than 5. Strong reliability is demonstrated. The film thickness at different inclination angles was measured, and dimensionless capacity vs. convergence ratio was obtained. The results revealed that when the film thickness is high, the maximum load-carrying capacity appears around a convergence ratio of 1.2, which is correlated well to the classical theory. However, when the film thickness gets low, the maximum load-carrying capacity reaches its maximum when the convergence ratio is larger than 2, showing obvious deviation from existing theory.