Measurement of reflectance at normal incidence R and its fractional change ΔR/R caused by a change of the angle of incidence from 0 to a small angle ϕ (ϕ≲ 20°) permits explicit determination of both the refractive index n and extinction coefficient k of an isotropic absorbing medium. The medium of incidence (ambient) is assumed to have a known refractive index (e.g., =1 for vacuum or air), and the incident light is either p or s linearly polarized.
Journal of the Optical Society of America (1917-1983)
R. M. A. Azzam, "Explicit determination of the complex refractive index of an absorbing medium from reflectance measurements at and near normal incidence," J. Opt. Soc. Am. 72, 1439-1440 (1982)