Results are presented from a rheo-optical study of a poly(dimethyl siloxane) (PDMS) fluid in a converging wedge flow cell at room temperature for estimation of the linear stress-optical coefficient. The linearity of the stress and polarizability or refractive index tensors were checked by flow birefringence measurements in pure extensional flow along the centerline of the converging flow cell that extended well into the non-Newtonian region. The stresses were evaluated using (a) shear and extension rates from laser Doppler anemometry (LDA) velocity measurements and (b) information obtained from orientation angle as a test of constitutive equations. All measurements were optical (i.e., non-intrusive), and a linear stress-optical coefficient of 1.475 x 10(-10) Pa-1 was obtained using a two-term Goddard-Miller model for room temperature flow of a PDMS fluid in combined shear and extension in a converging wedge flow cell.