The arbitrary choice between phase conventions places restrictions upon the interference equation for reflected light, the discontinuous phase shift upon reflection, and the complex refractive index. An attempt has been made to formulate a set of rules within which there is consistency among the existent mathematical framework for treatment of intereference, some experimental observations, and the requirement that the amplitude of a wave decrease with increasing penetration in an absorbing medium.
W. P. ELLIS, "Phase Conventions and the Interference Equation," J. Opt. Soc. Am. 56, 746-748 (1966)