1. An apparatus for determining a value indicative of retardance in a sample, comprising: a sample chamber for receiving the sample; an illuminator for providing an illumination light; optics for directing the illumination light toward the sample; a detector for measuring an intensity of light incident on the detector; optics for directing light that has interacted with the sample toward the detector; a first polarizer for selectively transmitting light that is substantially circularly polarized, the first polarizer being located one of between the illuminator and the sample chamber or between the sample chamber and the detector; a second polarizer for selectively transmitting light that has a selected elliptical polarization state, the second polarizer being located the other one of between the illuminator and the sample chamber or between the sample chamber and the detector; a controller for varying a selected elliptical polarization state of the second polarizer to correspond to a plural number of states χ i with a chosen Poincare latitude and longitude; and a processor connected to the detector for determining the value indicative of retardance in the sample from the measured incident light intensity obtained when the second polarizer is set to each of the states χ i , wherein none of the states χ i corresponds to circular polarization.