The stress-optical coefficient functions of two optical-grade polycarbonates (PCs) have been obtained by simultaneous measurements of the relaxation modulus and strain-optical coefficient functions. Nonlinear behavior of the relaxation modulus and strain-optical coefficient was observed at small strains at room temperature. Comparison of these functions in the linear region with those of a commercial grade PC was made. These functions have been incorporated to linear viscoelastic and photoviscoelastic constitutive equations to calculate residual thermal birefringence in freely quenched PC plates. The numerical results have been compared with the measurements indicating a fair agreement between them.