In the vacuum-ultraviolet region of the spectrum, the optical constants of vacuum-deposited absorbing films can be determined by measuring their specular reflectances at a number of angles of incidence. These reflectance values are then used to solve the generalized Fresnel reflection equations to obtain n and k. If the film is thin enough so that interference occurs between the wave fronts reflected from the film-vacuum and film-substrate interfaces, the errors in determining n and k may be large. Previously an opaque film, one that would transmit only 0.1% of the incident radiant flux at normal incidence if it were free-standing, was considered necessary if the n, k values of the film were to be determined with an accuracy of 1%. This paper presents the results of calculations demonstrating that the film thickness necessary to reduce the errors in n and k to 1% is dependent on n and k and is generally less than the opaque thickness.
W. R. Hunter and G. Hass, "Thickness of absorbing films necessary to measure their optical constants using the reflectance-vs-angle-of-incidence method," J. Opt. Soc. Am. 64, 429-433 (1974)