Previous research has established that the laser beam diffraction method can be used for measurement of film thicknesses between two adjacent flat surfaces. This paper describes further experimentation on this method, where oil film thicknesses between rollers is measured. Calibration curves are obtained between the experimental (set) gap and the gap dimensions which are deduced from the diffraction patterns when the laser beam is passed through the film. These set versus calculated film thickness curves are extremely linear for films which are less than 1 mm depth, the coefficients of correlation with straight lines being close to unity. Data based upon several oils used with 76.2 mm dia rollers are presented and empirical equations established. These equations allow the researcher to obtain the film thickness between the rollers in terms of oil properties (density, viscosity and refractive index) and the geometry of the light beam (wavelength, distance from oil film to diffraction screen, and diffraction pattern bandwidth). The empirical constants in the equations for film thickness appear to apply to specific oil types, and remain the same for each family or group of oil used. The groups used here were SAE 10 through 40, low viscosity index and high viscosity index groups. Oil films from 7.62 × 10−3 mm up to 76.2 × 10−3 mm thickness were measured using this method.

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