“…The simplest cases are the onebeam cases, such as forward and off-Bragg scattering, and the two-beam cases, the Bragg-Laue scattering (Hannon & Trammell, 1969;Sturhahn & Gerdau, 1994) and the grazing incidence scattering (Röhlsberger et al, 2003;Hannon & Trammell, 1969;Hannon et al, 1985b;Irkaev et al, 1993;Deák et al, 1996). In the grazing incidence limit, an optical model was derived from the dynamical theory (Hannon & Trammell, 1969;Hannon et al, 1985b), which has been implemented in numerical calculations (Röhlsberger et al, 2003). The reflectivity formulae given by Deák et al (1996) and Deák et al (2001) are suitable for fast numerical calculations in order to actually fit the experimental data (Spiering et al, 2000) and, as has been shown , this optical method is equivalent to that of the other approaches in the literature (Röhlsberger et al, 2003;Hannon et al, 1985b).…”