Optical microscopy and optical interferometry

Introduction

The availability of powerful tools for viewing surfaces at very high magnifications through the medium of highly resolved images has often caused the optical microscope to be overlooked. As we shall see, this may be related to the fact that the depth of field of an optical micrograph varies inversely with increased magnification, whereas with electron microscopy, the depth of field is almost infinite. Nevertheless, in tribology, many of the processes occur on a scale for which the optical microscope is probably the best and easiest instrument to use, especially when used in the reflection mode. Furthermore, when the optical microscope is combined with an interferometer device, we have a surface examination tool which has a vertical resolution equal to the resolution of many electron microscopes (and at a fraction of the capital outlay). We shall see how optical interferometry has indeed been used to give information about lubricant film thickness contours within the contact between a ball and a flat. These thicknesses are of the order of micrometres. Interference techniques have been used (Tolansky, 1970) to deduce the height of 40 Å steps in cleared mica. The present author has used a home-made interferometer to deduce the thickness of vacuum-evaporated films of iron used by Halliday (1960) in his work on using the contrast of electron diffraction patterns to determine thickness.