The technique for using conoscopy to determine the optical orientation and birefringences of stretched polymer films is presented here in detail. The method is nondestructive and has the capacity for implementation both off-line and online during processing. Analysis was performed on films of polyethylene naphthalate (PEN) and poly(ethylene terephthalate) (PET), with birefringence results compared to measurements using a Berek compensator. The technique is highly effective at measuring the orientation and in-plane principal birefringence, though an accurate measure for a film's out-plane birefringence requires knowledge of at least an approximate value for the material's average refractive index.