The relative dispersion of birefringence, D-b, is determined for isotactic polypropylene and Kapton HA PMDA-ODA polyimide films. D-b is then used to identify the dependence of dispersion on the orientation of polymers and is interpreted in terms of fringe jumping. Optical relationships between the in-plane birefringence and wavelength are formulated to predict the in-plane birefringence of oriented Kapton HA PMDA-ODA polyimide films at any wavelength. These relations can be used for the direct comparison of the in-plane birefringence of Kapton PMDA-ODA polyimide films obtained from different optical techniques (i.e., polarized microscopy, polarized refractometry, wave guide coupling, etc.).