The radiant absorptance of an arbitrary multilayer stack depends upon the amplitude reflectance of the medium on its emergent side. Contours of constant absorptance are drawn on the reflectance plane which enable the designer to determine the maximum and minimum absorptance. This is applied to optimize the absorptance of an interference photocathode.
R. Gelber and P. Baumeister, "The Control of Absorptance of a Multilayer," Appl. Opt. 9, 863-865 (1970)