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Even small collection angles in the receiving optics can appreciably broaden, distort, and shift Brillouin lines observed in a light-scattering experiment, thereby causing errors in both sound absorption and sound velocity. A general expression for the observed lineshape is derived for a Lorentzian scattering line in terms of the distribution of thermal phonons and the shape of the collection aperture. Relaxational effects are also considered. For a square aperture, a closed-form solution is obtained and discussed. Applicable correction formulas for the true linewidth and lineshift are derived. To first approximation, Brillouin line is the sum of two arctan functions.
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H. G. Danielmeyer (1970) studied this question.
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