A general equivalence theorem for all unitary optical system is presented. In particular the case of the twisted nematic liquid crystal (LC) cell is shown to have very interesting equivalence properties. Specific formulas for the equivalence are derived. Applications of this optical equivalence to LCD measurements and modeling are discussed.
S. T. Tang,
"Optical equivalence theorem for liquid crystal layers: principles and applications", Proc. SPIE 4463, Liquid Crystals V, (11 December 2001); doi: 10.1117/12.449939; https://doi.org/10.1117/12.449939