Following a short mathematical recapitulation of thin film theory, the stability of a coating is defined with respect to the optical thickness of each layer or with respect to wavelength. The first and second derivatives of the reflection factor are calculated, and these results are applied to practical examples.
Jacques Mouchart, "Thin film optical coatings. 1. Optical coating stabilities," Appl. Opt. 16, 2486-2490 (1977)