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

A separate treatment of secondary extinction is required only in the uncorrelated block model, and the method given by Hamilton (1957) is used in this work. The coupling constant in the H–D equations is given by where for equatorial reflections in the neutron case, is the correction for primary extinction evaluated at the angle θ, and W(Δθ) is the distribution function for the tilts between mosaic blocks. The choice of this function has a significant influence on the final result (Sabine, 1985), and a rectangular or triangular form is suggested.
In the following equations for the secondaryextinction factor, and A and B are given by equations (6.4.5.6) and (6.4.5.7). The average path length through the crystal for the reflection under consideration is D and G is the integral breadth of the angular distribution of mosaic blocks. It is important to note that A should be set equal to one if the data have been corrected for absorption, and B should be set equal to one if absorptionweighted values of D are used. If D for each reflection is not known, the average dimension of the crystal may be used for all reflections.
For a rectangular function, W(Δθ) = G for otherwise, and the secondaryextinction factor becomes For a triangular function, W(Δθ) = for otherwise, and the secondaryextinction factor becomes
References
Hamilton, W. C. (1957). The effect of crystal shape and setting on secondary extinction. Acta Cryst. 10, 629–634.Sabine, T. M. (1985). Extinction in polycrystalline materials. Aust. J. Phys. 38, 507–518.