Problems and precautions in high-precision two-beam microinterferometry

摘要

Stray light (glare) in Jamin-Lebedeff interference microscopes, partly due to bubbles in the calcite plates of the objectives, can be reduced by limiting the area of field illuminated. Glare degrades visual image contrast and can cause measuring errors in systems which depend on the measurement or comparison of light intensities, but has no significant effect on measurements in a microinterfereometer which uses phase-modulated light. The apparent OPD of a very thin object mounted in a given medium can be significantly affected by phase changes due to reflections at the interfaces with the preceding and successive media. The error can be reduced or eliminated if the refractive indices of the specimen and the supporting slide are similar, and/or if the reference medium is air and no coverglass is used. In principle, the geometrical thickness and refractive index of a plane object such as a plastic microtome section in a given reference medium can be obtained from microinterferometric measurements using different obliquities of transmitted light. Currently available microinterferometers are not, however, precise and accurate enough for the method to be practical with objects only about a wavelength thick.