A polarization imaging apparatus measures the Stokes image of a sample. The apparatus consists of an optical lens set, a first variable phase retarder (VPR) with its optical axis aligned 22.5°, a second variable phase retarder with its optical axis aligned 45°, a linear polarizer, a imaging sensor for sensing the intensity images of the sample, a controller and a computer. Two variable phase retarders were controlled independently by a computer through a controller unit which generates a sequential of voltages to control the phase retardations of the first and second variable phase retarders. A auto-calibration procedure was incorporated into the polarization imaging apparatus to correct the misalignment of first and second VPRs, as well as the half-wave voltage of the VPRs. A set of four intensity images, I0, I1, I2 and I3 of the sample were captured by imaging sensor when the phase retardations of VPRs were set at (0,0), (π,0), (π,π) and (π/2,π), respectively. Then four Stokes components of a Stokes image, S0, S1, S2 and S3 were calculated using the four intensity images.