Photoelastic-modulator polarimeters in astronomy

Instruments used to detect polarized light from astronomical objects are briefly compared. Photoelastic-modulator polarimeters, introduced in 1970, have advantages, particularly for extreme-sensitivity measurements of linear and circular polarization, down to the levels of 0.01% and below. They have made possible several discoveries. Versions of such instruments for the UV-visible and for the infrared (1-13 microns) are described.