Simple physical models are taken to calculate thermal and electronic contributions to absorption coefficients in Al. N-beam systematic calculations are interpreted by means of 2-beam theory, making possible a comparison with the experimental values of Watanabe. The mean absorption coefficient agrees to within 25 per cent. Spherical aberration and thermal diffuse scattering within the objective aperture are shown to explain the poorer agreement for the anomalous absorption coefficient. It is concluded that weak beams are significant in the measurement of absorption coefficients. Weak systematic beams are shown to have a significant effect on the positions of the thickness fringe maxima. A brief comparison of 2- and N-beam rocking curves is given.
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P. A. Doyle (1970) studied this question.