The dependence of the azimuth angle settings on the change in off-null intensity of a polarizer-.compensator-sample-analyzer ellipsometer owing to changes in sample properties is studied. First, a closed-form expression for the relationship between azimuth angles that fulfill the null condition is presented. An approximation for the off-null light intensity near null that is valid for small changes of the p- and s-reflection coefficients of an isotropic sample is then derived. This approximation shows that the intensity change near the null can be described by changes in the ellipsometric parameters tan q, and Delta only. Expressions for finding the azimuth angle that gives the maximum possible intensity change for a given change in the sample parameters are also derived. The importance of optimization of azimuth angle settings for different samples is investigated and found to depend on tan psi. Numerical and experimental results chosen from the investigation of gas sensors based on porous silicon are included to verify the approximations as well as the optimization. (C) 2003 Optical Society of America.