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.
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.Google Scholar