A Dichromatic Interference Intensity Modulation Approach to Measurement of Lubricating Film Thickness

The study aims to extend the measurement range of dichromatic interferometry in lubricating film thickness measurement. Subtraction of two sets of interference fringes, which are produced by dichromatic (red and green) lights, gives a modulated signal of intensity against film thickness. The measuring range without wavelength ambiguity is limited to the first half-cycle of the beat wave in the modulated signal. Theoretical and experimental analyses of the modulated interference intensity variation against film thickness were conducted. The differences in the pattern of the modulated interference intensity were obtained in the first three half-cycles of the beat wave. Based on that, a robust dichromatic interference intensity modulation approach for lubricating film measurement was derived and its measuring range without wavelength ambiguity was successfully extended to the magnitude equivalent to three half-cycles of the beat wave. To test the feasibility and credibility of this approach, the film thickness in elastohydrodynamic and hydrodynamic lubricated contacts was measured. The results were well correlated with the classical theory. Using red and green lasers with wavelength of 653 and 532 nm, respectively, the measurement range can be extended to 4 μm.