- A new technique for birefringence measurement and extracting the coefficients of the dispersion relation such as Cauchy or Sellmeier equations is proposed. The main principle of the technique is based on finding accurately the wavelengths that the birefringent plate operates as a quarter-wave plate (QWP) and measuring the birefringence at these points. As the projections of the ordinary and extraordinary beams on the analyzer axis interfere, the setup is a form of common path interferometer and these QWP points are the quadrature points at which the sensitivity of the interferometer is optimum. An algorithm is developed to find these crossing points precisely. Implementation of this technique has been done on two different kinds of nematic liquid crystal wave plates made of Merck E44 and BL036.