This invention is directed to methods of unambiguously measuring the absolute retardance, δA of an optical sample. A method for measuring absolute retardance of an optical sample includes directing light comprising a plurality of wavelengths through a polarization state generator source, the optical sample, and a polarization state analyzer, detecting, at an imaging device, retardance measurement light emanating from the optical sample after also passing through the polarization state analyzer at the plurality of wavelengths, determining a measurement retardance associated with the detected retardance measurement light at each of the wavelengths, and determining an absolute retardance associated with the optical sample based on the measurement retardances determined at each of the wavelengths.