Opticaly, the point measurement method of two-dimensional photoelasticity is as good as the means to determine the fast and slow axis' directions of an ideal linear retarder and their relative retardation. The measurement equation is formulated in ellipsometry is a complex equation which can be changed into two real equations. Therefore, no matter which one kind of ellipsometers is used, such as null ellipsomer, rotating-analyzer ellipsometer or polarization modulated ellipsometre, the difference and directions of the principal stresses can be measured at once by any of them.In this paper the theorem of contemporaneous determination of these two magnitudes by these three ellipsometers is described and the formula for calculation are also derived.Sinece the mini-computers have been used widely the complexity of calculation is not a problem at all.