Rayleigh ratio

The quantity used to characterize the scattered intensity at the scattering angle $θ$ , defined as $R θ = i θ r 2 I f V$ , where $I$ is the intensity of the incident radiation, $i θ$ 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.

For light scattering, dependent on the polarization of the incident beam, $f = 1$ for vertically polarized light, $f = cos 2 ⁡ θ$ for horizontally polarized light and $f = 1 + cos 2 ⁡ θ 2$ for unpolarized light.
For small-angle neutron scattering, $f = 1$ .
For small-angle X-ray scattering, $f ≈ 1$ , if $θ < ca. 5 °$ .

Notes:

The dimension of $R θ$ is $length −1$ .
In small-angle neutron scattering the term cross-section is often used instead of $R θ$ ; the two quantities are identical.
An alternative recommended symbol is $R θ$ .

Source:

Purple Book, p. 65