The ratio of two Rayleigh ratios for different angles of observation, i.e. \[z(\theta _{1},\, \theta _{2}) = \frac{R(\theta _{1})}{R(\theta _{2})}\] \[\theta _{1} < \theta _{2}\] The angles must be specified; in light scattering it is customary to let theta = 1 - theta1 and, most frequently, theta1 = 45° and theta2 = 135°.
Source:
Purple Book, 1st ed., p. 67 (http://old.iupac.org/publications/books/author/metanomski.html)
See also:
PAC, 1983, 55, 931. 'Definitions, terminology and symbols in colloid and surface chemistry. Part 1.14: Light scattering (Provisional)' on page 932 (https://doi.org/10.1351/pac198355060931)