Abstract
This paper introduces a technique for precisely measuring the vertical birefringence of an optical polymer film. Analytical equations are derived to relate the phase retardation to the optical film birefringence. In the measurement, we first measure the in-plane birefringence of the film and locate the principal axes of refractive index ellipsoid. The film is then rotated with its normal at an angle to the incident light wave vector. The vertical birefringence is calculated from the retardations measured at several angles. Using the procedure, the vertical birefringence can be measured to a precision of 0.2 ×10-3.