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The transmission of an incident X-ray beam in a particular direction depends on the absorption properties of the crystal in this direction. A measurement of the relative transmission of the incident beam in all possible directions allows, therefore, in principle, the determination of the three-dimensional absorption surface of the crystal. The relative transmission factor of the incident beam for all directions of interest can easily be determined on a precession or rotation camera if the beam cross section is smaller than the crystal size. The absorption surface determined in this way may be used to correct X-ray intensity films obtained under similar geometrical conditions, i.e. screenless precession or rotation photographs. The method was tested by its effect on the symmetry averaging reliability index of a large number of reflexions from various films of various crystals. This value decreased typically from 8.2 to 6.1% after application of the absorption correction, indicating that the correction accounts for a large part of the absorption effect.
