An explanation is presented for the isoclinic band‐spreading patterns associated with the measurement of flow birefringence in a two‐dimensional slit‐flow geometry. Band spreading is attributed to the positional dependence of the shear stress in the vicinity of the side wall viewing windows leading to a range of positions along the channel depth at which a given stress level may exist. Analyses are presented for the stress fields for Newtonian and power law flow which enable comparison to experimental data for the band spread. Results show that for a given rotation of the polars, the inside line of the resulting band represents the stress characteristic of the one‐dimensional centerline flow to be used for the evaluation of the isoclinic angle. Data are also presented for the stress‐optic coefficient of poly(vinylidene fluoride).