Flow birefringence is used to study stress relaxation following step strain deformations of a well entangled polyisoprene melt. The optical method employs multiple light paths to fully sample the three-dimensional stress tensor, and hence provides measurements of all three independent shear material functions (shear stress and both first and second normal stress differences). Experiments are complicated by multiple orders in retardation. However, data show that the ratio of the second to the first normal stress difference is a strain thinning function, with magnitude intermediate between the predictions of the Doi-Edwards model with and without independent alignment.