Scattered-light photoelasticity provides a nondestructive approach for analyzing the secondary principal stresses in a 3D photoelastic model. In this paper, a method is proposed to determine the whole-field secondary photoelastic parameters (isoclinics and isochromatics) using polarized incident light in the scatted-light photoelasticity. Relevant theory is derived and explained. Two polarization-stepped loaded model images and the associated unloaded model images are used to determine the isoclinic angle and the fractional retardation or fringe order. Tests of this method on a circular disk under diametrical compression are demonstrated. The digitally determined secondary principal stress direction agrees well with the theory.