Absorption and secondary extinction

A comparison is made of the treatments of absorption in the theories of secondary extinction by Werner [J. Appl. Phys. (1974), 45, 3246-3254] and by Becker & Coppens [Acta Cryst. (1974), A30, 129-147]. The Werner approach treats absorption and extinction together, whereas Becker & Coppens assume a prior correction for absorption, and make an approximate allowance for the effect of absorption on the extinction by modifying the parameter representing the effective crystal dimension. By consideration of the form of the corrections for a specially simple crystal shape, it is shown that the Becker & Coppens method predicts slightly greater extinction than does the unified treatment of Werner. However, the difference is small, and likely to be unimportant in practice The Werner solution of the Hamilton-Zachariasen transfer equations is exact, but not easy to use. The Becker & Coppens results are approximate, but they are presented in a form convenient for computation.