A natural framework for evaluating prism-coupler performance is provided by a recently developed geometrical construction in which refraction through a prism for an arbitrary angle of incidence is represented by an ellipse. The useful operating range of a prism coupler is represented by the portion of the representational ellipse that appears within a window determined by the mode propagation conditions of a thin-film-substrate system.
Albert Feldman, "Graphical representation of prism coupling into thin films," Appl. Opt. 22, 2380-2382 (1983)