Influence of the Fourier transform of the incident beam on the measured dif
fracted intensity in X-ray diffraction is analysed. The theory presented is
a more rigorous physical picture of X-ray diffraction in the case of a nar
row incident beam than the usually assumed spherical-wave theory. The shape
of diffracted beam amplitude depends on the FT of the incident beam which
is fully determined by experimental conditions. The proposed methods may be
used for direct calculations of the correlation function for electromagnet
ic fields and studies of the coherence degree of X-ray radiation. (C) 2001
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