Correction for pressure dependence of the refractive index in measurementsof lubricant film thickness with image analysis

The lubricant film thickness between contacting bodies may be determined fr om interferograms by image analysis. Look-up tables employed in the analysi s are obtained from calibration interferograms of contacts of bodies of kno wn shape. Calibration and measurement interferograms are, however, obtained under different pressure conditions and, since the refractive index of lub ricants is pressure dependent, the use of look-up tables can introduce sign ificant errors (in the range of 10 per cent). In this paper it is shown that a straightforward correction for this error is available. In an iterative procedure the uncorrected film thickness is f irst determined through looking up tables. This allows the pressure distrib ution over the contact to be calculated and a corrected refractive index ma y then be obtained. The corrected refractive index in turn allows a correct ed film thickness to be calculated. This procedure can be followed iterativ ely. After two iterations the corrections have been found to be of the same order as the resolution yielded by the method, as has been implemented.