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Expressions are developed for reflecting range from various models, and the practical application of these in terms of reflection prediction and calculation of partiality is discussed here for conventional sources. Synchrotron sources utilizing focusing monochromator systems, where the wavelength of the radiation incident at the sample is correlated with the angular direction of that radiation, are dealt with elsewhere. Even for conventional sources spectral dispersion is shown to be an important factor, particularly in the case of high-resolution data. The various published methods are discussed and expressions are derived from first principles showing that all are inherently equivalent, differing only when approximations are used, and revealing a missing factor of two in the treatments of Rossmann [J. Appl. Cryst. (1979), 12, 225-238] and Rossmann, Leslie, Abdel-Meguid & Tsukihara [J. Appl. Cryst. (1979), 12, 570-581]. Particular emphasis is placed on the method which uses a spherical reciprocal-lattice volume element whose dimensions are designed to reproduce the expected or observed reflecting ranges, showing that for all practical purposes the effects of beam cross-fire, mosaic spread and spectral dispersion can be adequately simulated by such a volume. The equations for reflecting range are of particular interest in the electronic stationary-picture method and in the use of electronic area detectors.
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