1. A polarization imaging apparatus for measuring polarization images of a sample comprising: an optical assembly; a first variable phase retarder having a first controllable phase retardation; a second variable phase retarder optically coupled to the first variable phase retarder, the second variable phase retarder having a second controllable phase retardation; a linear polarizer optically coupled to the second variable phase retarder; an imaging sensor optically coupled to the polarizer for sensing the intensity images of the sample through the linear polarizer; a controller for varying the first controllable phase retardation of the first variable phase retarder and for varying the second controllable phase retardation of the second variable phase retarders; and a computer for controlling the controller, recording the sequential intensity images, calibrating the alignment errors of the first and second variable phase retarders, and converting four sequential intensity images to four components of a Stokes image.