The general theory for calibrating two arbitrarily oriented phase plates

The general expressions for the retardances of two phase plates arbitrarily oriented between two polarizing prisms are presented. It is shown that more accurate results are obtained if measurements are performed at particular orientations of the plates. Compact formulas for the retardances at these orientations are directly derived as special cases from the general expressions. Also, some aspects related to the calibration model like the sensitivity of measurements to the relative orientations of the plates can only be treated using the general expressions.