A note on the Darwin-Prins rocking curve for perfect crystals

For the purpose of calculating the reflectivity of perfect crystals in the Bragg case, taking into account both normal and Borrmann absorption, it is suggested that its original form is used expressed in terms of complex variables. Care is required regarding the phase angle for the algebraic expression of the square root. Two expressions which are directly usable for programming are presented. One of them is equivalent to the traditional one given by Zachariasen [Theory of X-ray Diffraction in Crystals (1945). New York: Wiley]. The other can be used without any problems of infinite values whatever values the Fourier coefficients of the polarizability takes. Some numerical examples of rocking curves are demonstrated for very special values of the Fourier coefficients.