Polarization imaging apparatus with auto-calibration

Abstract

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, I, I, Iand Iof 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, S, S, Sand Swere calculated using the four intensity images.