## Rayleigh ratio

https://doi.org/10.1351/goldbook.R05159
The quantity used to characterize the scattered intensity at the scattering angle θ, defined as R(θ) = iθr2/IfV, where I is the intensity of the incident radiation, iθ is the total intensity of scattered radiation observed at an angle θ and a distance r from the point of scattering and V is the scattering volume. The factor f takes account of polarization phenomena. It depends on the type of radiation employed.
1. For light scattering, dependent on the polarization of the incident beam, f = 1 for vertically polarized light, f = 1 - cos^2 θ for horizontally polarized light and f = (1 + cos^2 θ)/2 for unpolarized light.
2. For small-angle neutron scattering, f = 1.
3. For small-angle X-ray scattering, f ≈ 1, if θ < ca. 5°.
Notes:
1. The dimension of R(θ) is (length-1).
2. In small-@A00346@ @N04116@ @S05487@ the term cross-section is often used instead of R(θ); the two quantities are identical.
3. An alternative recommended symbol is R(θ).
Source:
Purple Book, 1st ed., p. 65 (http://old.iupac.org/publications/books/author/metanomski.html)