Discussion on the Technique of Relative Optical Interference Intensity for the Measurement of Lubricant Film Thickness

Abstract

The technique of relative optical interference intensity (ROII) has been developed for measuring lubricant film thickness. Here, we describe in detail the principle and the characteristics of the film thickness measurement system based on the existing ROII method. Some researchers have reported a large calculation error in film thickness with the use of ROII method. The description and analysis of the resolution, accuracy and important factors influencing the measurement results of this measurement technique presented here illustrate that the ROII method should be used in a specific optical system with negligible multiple beam reflections, but not in an ordinary one. The reported calculation error in film thickness using the ROII method comes from using an inappropriate optical system and inappropriate coating parameters. In fact, the ROII method, which is a very accurate method for measuring the thickness of a thin lubricant film, with a high resolution in film thickness of 0.5 nm and a horizontal resolution of 1.4 μm, can only be used in a two-beam interference system.