polarimetry

Polarimetry is the measurement and interpretation of the polarization state of light. Polarized light has a preferred orientation of its electric field; common states are linear, circular, and elliptical. Polarimetry quantifies the polarization and its changes after interaction with matter or propagation. The standard formalism uses the Stokes parameters I, Q, U, and V, where I is total intensity; Q and U describe linear polarization; V describes circular polarization. The degree of polarization is p = sqrt(Q^2 + U^2 + V^2) / I and the polarization angle is chi = 1/2 arctan(U/Q).

Measurements require calibration to separate intrinsic polarization from instrumental or environmental effects. Instruments include polarimeters, spectropolarimeters, and imaging polarimeters; ellipsometers assess thin films. A common framework is Mueller calculus: an optical element is described by a Mueller matrix that acts on a Stokes vector. Polarization signals are often modulated to improve accuracy and to retrieve Q, U, and V as a function of wavelength or image position.

Applications are diverse. In astronomy, polarimetry probes magnetic fields, scattering geometries, and dust properties; measurements of cosmic microwave background polarization constrain early-universe physics. In atmospheric science, polarimetry aids remote sensing of aerosols and clouds. In materials science, ellipsometry and polarimetric imaging measure optical constants, film thickness, and stress through photoelasticity. The field has historical roots in 19th-century experiments by Malus and Brewster and was later extended by Faraday to magneto-optic effects.