When is retardance autocalibration of microgrid-based full Stokes imagers possible and useful?

Full Stokes polarimetric images can be obtained from two acquisitions with a microgrid polarization camera equipped with a retarder. When the retardance is imperfectly known, it can be calibrated from the measurements, but this requires three image acquisitions and may cause divergence of estimation variance at a low signal-to-noise ratio. We determine closed-form equations allowing one to decide in which experimental conditions autocalibration is possible and useful, and to quantify the performance gain obtained in practice. These results are validated by real-world experiments.