Where Γ is the quantitative retardation of the material, t could be the thickness of the birefringent crystal (or product) and B is birefringence as outlined previously mentioned. Things contributing to the value of retardation tend to be the magnitude of the difference in refractive indices for the environments https://travisuwrpf.fitnell.com/42133282/what-does-birefringent-effects-mean