Some instrument considerations for detecting very low levels of light polarization -- below 1 ppm --are discussed. Recent linear and circular polarimetry of the whole disk of the Sun, at levels down to 3 x 10-7, is reviewed as an outstanding example. Further developments and applications are indicated.
James C. Kemp,
"Detecting Polarized Light At Levels Below 1 ppm.", Proc. SPIE 0891, Polarization Considerations for Optical Systems, (29 June 1988); doi: 10.1117/12.944295; https://doi.org/10.1117/12.944295