International
Tables for Crystallography Volume C Mathematical, physical and chemical tables Edited by E. Prince © International Union of Crystallography 2006 
International Tables for Crystallography (2006). Vol. C, ch. 6.4, pp. 611612

The expressions for the extinction factor have been given, by default, for the σpolarization state, in which the electric field vector of the incident radiation is perpendicular to the plane defined by the incident and diffracted beams. For this state, the polarization factor is unity. For the πpolarization state, in which the electric vector lies in the diffraction plane, the factor is . The appropriate values for the extinction factors for this state are given by multiplying F by wherever F occurs.
For neutrons, which are matter waves, the polarization factor is always unity.
For an unpolarized beam from an Xray tube, the observed integrated intensity is given by . In the kinematic limit, , and the power to which is raised (the polarization index n) is 2. In the pure primaryextinction limit, , while . Hence, n = 1. In the pure secondaryextinction limit, where g is the mosaicspread parameter, while Hence, n = 0. In all real cases, n will lie between 0 and 2, and its value will reflect departures from kinematic behaviour.