Optimum angle of incidence for observing thin-film interference colors.

A colorimetric analysis of white-light interference colors for transparent films on absorbing substrates has shown that specific angles of incidence exist for which the purity of interference colors is predicted to be greatest for s- or p-polarized light. These optimum angles of incidence depend primarily on the optical constants of the substrate, and they decrease from the first to the third order of interference. They provide the best conditions for the determination of film thicknesses from interference colors, and their use is essential for making interference colors visible with highly reflective substrates. As an alternate measure of the observability of interference colors, a polychromatic fringe visibility has been introduced. Its use provides similar predictions of optimum angles. The computations are in good agreement with experimental observations.