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, p. 611

When this model is used, two values of x are required. These are designated for primary extinction and for secondary extinction. Equation (6.4.8.1) is used to obtain a value for . The primaryextinction factors are then calculated from (6.4.5.3), (6.4.5.4) and (6.4.5.5), and is given by equation (6.4.7.1). In the second step, is obtained from equation (6.4.9.1), and the secondaryextinction factors are calculated from either (6.4.9.2) and (6.4.9.3) or (6.4.9.4) and (6.4.9.5). The result of these calculations is then used in equation (6.4.7.1) to give . It is emphasised that includes the primaryextinction factor. Finally, .
Application of both models to the analysis of neutron diffraction data has been carried out by Kampermann, Sabine, Craven & McMullan (1995).
References
Kampermann, S. P., Sabine, T. M., Craven, B. M. & McMullan, R. K. (1995). Hexamethylenetetramine: extinction and thermal vibrations from neutron diffraction at six temperatures. Acta Cryst. A51, 489–497.