This invention discloses an optical and computation system that enables the magnitude of the retardation, or the birefringence, in a birefringent material to be measured. This is achieved by consideration of the spectral interference pattern generated by combining quadrature axes of polarized light that have passed through the material, however, unlike other approaches, this invention removes the spectral intensity variations of the light source and the spectral attenuation variations of the optical system before analyzing the resultant spectral interference pattern. Since the spectral interference pattern is unique for each retardation or birefringence value, this invention provides an absolute measure of these quantities. Additionally this invention permits the full range of retardations or equivalent birefringence values to be measured, from zero retardation to any (large) value that does not create interference modulations, the frequency of which exceed the Shannon-Kotelnikov criteria for the wavelength or spectral sampling implemented. Further, in the second main embodiment of this invention, the dependence on stored light source spectral intensities and stored optical light path attenuations is removed, with the system being independent of any time dependent variations in intensity and/or attenuations and additionally, being independent of any axial alignment or setup requirements.