High sensitivity polarimetry : techniques and applications

High sensitivity polarimetry will refer to fractional polarizations of below, and often significantly below, 103. Measuring low fractional polarization requires a large number of photons and also special techniques are needed to remove systematic effects. The highest sensitivities have been achieved using very fast modulation with single-element detectors, giving sensitivities of 106, or better, in fractional polarization. However, it is often important to have good spatial and/or spectral information and to provide this efficiently requires the use of area detectors which bring their own problems, particularly for the highest sensitivities. Applications of highly sensitive polarimetry described here, include observing the reflected light from exoplanets, the properties of dust in the Earth’s atmos-phere, and the remote sensing of biomarkers.