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

For this model of the real crystal, the variable x is given by equation (6.4.8.6), with and g the refinable variables. Extinction factors are then calculated from equations (6.4.5.3), (6.4.5.4), and (6.4.5.5). For a reflection at a scattering angle of from a reasonably equiaxial crystal, the appropriate extinction factor is given by (6.4.7.1) as E(2θ) = .
It is a meaningful procedure to refine both primary and secondary extinction in this model. The reason for the high correlation between and g that is found when other theories are applied, for example that of Becker & Coppens (1974), lies in the structure of the quantity x. In the theory presented here, x is proportional to F^{2} for pure primary extinction and to for pure secondary extinction.
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
Becker, P. J. & Coppens, P. (1974). Extinction within the limit of validity of the Darwin transfer equations. I. General formalisms for primary and secondary extinction and their application to spherical crystals. Acta Cryst. A30, 129–147.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.