Simple equations are given for determining both refractive index and film thickness from a measurement of interference fringe separation where the question of phase change and the order of the fringes can be disregarded. The equations are quite general, since they apply to fringe maxima or minima for either transmission or reflection and can be used for free-standing films or films on substarates of higher refractive index. Advantages of recording fringes via reflection rather than transmission are discussed. A unique attachment for commercial spectrophotometers for making measurements over a wide range of angles of incidence is described.
N. J. Harrick, "Determination of Refractive Index and Film Thickness from Interference Fringes," Appl. Opt. 10, 2344-2349 (1971)