The mathematical theory of the operation of the Sénarmont compensator for measuring flow birefringence is developed. Equations are obtained for the motion and intensity of the cross of isocline. It is shown that the equation used by Rich is good for small values of the retardation but does not describe accurately the behavior of the cross of isocline for large retardations. In particular it is found that the collapse of the cross occurs when the analyzer is at an angle less than ½δ. The angle ½δ is the position of the analyzer at which the darkness of the already collapsed cross is a maximum.
C. A. HOLLINGSWORTH and W. T. GRANQUIST, "Theory of Measurement of Flow Birefringence by Use of the Sénarmont Compensator," J. Opt. Soc. Am. 52, 562-563 (1962)