Abstract

Observation of path differences by means of an interferometric arrangement making use of birefringent crystals has been described by M. Françon.1 Important advantages of the apparatus are concentration of the interferometric part within the eyepiece, and ruggedness because the interference takes place within the crystal plates. This method can therefore be adapted for purposes other than microscopic observation. Only those objects which lie within the depth limits of the field give an observable effect. If a large aperture object lens is used, this depth can be kept within narrow limits. Thus, phase objects outside the zone of sharp focus have little or no effect. This means that the object to be studied can be placed between glass plates of poor quality.

We have found it possible to use this technique to measure the thickness of coatings in a vacuum, that is, during the process of evaporation. The light enters and leaves the vacuum vessel through windows of thick plate glass. The light source and the interference eyepiece are mounted outside of the vessel. The thickness of the coating is measured by means of a plane-parallel glass plate, placed in a nonparallel part of the light beam. When white light is used, the field is divided into sections of different colors. By comparing these colors to a screened part of the same surface, the thickness of the film can be measured. More sensitive and accurate comparisons can be made with monochromatic light. It has proved possible to measure simultaneously the thickness and the absorption of a film. The precision of the measurement of path difference appeared to be at least λ/200.

© 1957 Optical Society of America

Objects to Test the Sensitivity of Phase Contrast and Interference Microscopes*†

E. Menzel
J. Opt. Soc. Am. 47(6) 563-563 (1957)

Modern Trends in the Evaluation of Optical Images*

G. Toraldo di Francia
J. Opt. Soc. Am. 47(6) 507-507 (1957)

Optical Properties of Thin Absorbing Films*

F. Abelès
J. Opt. Soc. Am. 47(6) 473-482 (1957)

Coherence and the Quantum*

R. H. Dicke
J. Opt. Soc. Am. 47(6) 527-527 (1957)

Brewster Angle Apparatus for Thin Film Index Measurements

Alan C. Traub and Harold Osterberg
J. Opt. Soc. Am. 47(1) 62-64 (1957)