Exact expressions for phase difference in birefringent wave plate

Approximation assumptions are involved in the common used traditional expressions for phase difference. In order to describe the property of birefringent wave plate more accurately, a concept which eliminates the approximation assumptions is proposed. According to the features of crystal birefringence and combining with the structural characteristics of birefringent wave plate, a more exact expression for phase difference between normal of e ray and normal of o ray can be derived through representing refracting angle and refractive index of normal of e ray in wave plate with the incident angle and crystal parameters. In order to study the interference between polarization lights, an exact expression for phase difference can be derived for exactly describing light path in crystal plate and the light path outside crystal plate. More exact expression for phase difference provides a convenience for more accurately analyzing the characteristics of birefringent wave plate.