Interferometry-based measurements of oil-film thickness

Measurement of the thickness of thin lubricant films separating rotating su rfaces in elastohydrodynamic experiments presents some challenging problems . The nature of the experimental apparatus inhibits the use of most commonl y applied interferometric phase measurement methods. Also the absolute thic kness of the separating film must be determined, as opposed to relative dis tances that would be sufficient in most other measurement scenarios where i nterferometry methods are used. In this paper, computer-based analysis of i nterferograms recorded using an elastohydrodynamic lubrication Fitzeu inter ferometer (a so-called ball-and-disc apparatus) is discussed, the main obje ctive being to extract the absolute oil-film thickness. Intensity based met hods {most importantly, calibration look-up procedures where colour paramet ers from recorded dynamic interferograms are compared with table values cor responding to known film thicknesses, but also a phase measurement approach based on mufti-channel interferometry using trichromatic light) are descri bed. A discussion regarding compensation for measurement errors due to the pressure dependence of the refractive index of the lubricant is also includ ed.