An Etymological Dictionary of Astronomy and Astrophysics

Four parameters which are needed to fully describe the → polarization state of → electromagnetic radiation. They involve the maximum and minimum intensity, the ellipticity, and the direction of polarization.

The four Stokes parameters are traditionally defined as follows:
I ≡ total intensity.
Q ≡ I₀ - I₉₀ = difference in intensities between → horizontal and → vertical  → linearly polarized components.
U ≡ I₊₄₅ - I_-45 = difference in intensities between linearly polarized components oriented at +45° and -45° (or 135°).
V ≡ I_rcp - I_lcp = difference in intensities between right and left → circularly polarized polarized components.

See also: → Stokes; → parameter.