Stokesparametrit

Stokesparametrit, or Stokes parameters, are a compact description of the polarization state of electromagnetic radiation. They consist of four values, I, Q, U, and V, often written as the Stokes vector S = (I, Q, U, V)ᵀ. I represents the total intensity, while Q and U describe linear polarization along two orthogonal axes and together determine the magnitude and orientation of linear polarization. V describes circular polarization.

The parameters can be defined from the electric field components Ex and Ey (typically treated as complex quantities) through the coherency matrix. A common form is I = ⟨|Ex|²⟩ + ⟨|Ey|²⟩, Q = ⟨|Ex|²⟩ − ⟨|Ey|²⟩, U = ⟨Ex Ey*⟩ + ⟨Ex* Ey⟩ = 2 Re⟨Ex Ey*⟩, and V = i(⟨Ey Ex*⟩ − ⟨Ex Ey*⟩) = 2 Im⟨Ex Ey*⟩. The Stokes vector provides a complete description of the polarization state for any partially polarized light and relates to the density (coherency) matrix formalism used in optics.

Measurement and interpretation: Stokes parameters are determined by sequences of polarization optics, such as linear polarizers at different angles and quarter-wave plates, combined with detectors. From the measured parameters one defines the degree of polarization p = sqrt(Q² + U² + V²) / I, the linear polarization degree p_lin = sqrt(Q² + U²)/I, and the polarization angle χ = 1/2 arctan(U/Q). The circular fraction is V/I.

Applications: Stokes parameters are widely used in astronomy, remote sensing, atmospheric science, optical communications, and imaging polarimetry to analyze and quantify polarization states. Transformations: rotating the reference frame changes Q and U as Q' = Q cos 2θ + U sin 2θ and U' = −Q sin 2θ + U cos 2θ, while V remains unchanged.

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