https://doi.org/10.1351/goldbook.08677
Relation between the propagation directions, \(k_{\rm{i}}\), \(k_{\rm{s}}\), and the polarization directions, \(p_{\rm{i}}\), \(p_{\rm{s}}\), of the incident and scattered radiation, usually expressed in terms of laboratory-fixed or crystal-fixed Cartesian axes.
Note: A common notation, due to Porto, is \(k_{\rm{i}}(p_{\rm{i}}\,p_{\rm{s}})k_{\rm{s}}\).