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The '''angular spectrum method''' is a technique for modeling the propagation of a [[wave field]]. This technique involves expanding a complex wave field into a summation of infinite number of [[plane wave]]s of the same frequency and different directions. Its mathematical origins lie in the field of [[Fourier optics]]<ref>''Digital Picture Processing'', 2nd edition 1982, Azriel Rosenfeld, Avinash C. Kak, {{ISBN|0-12-597302-0}}, Academic Press, Inc.</ref><ref>''Linear Systems, Fourier Transforms, and Optics'' (Wiley Series in Pure and Applied Optics) Jack D. Gaskill</ref><ref>''Introduction to Fourier Optics'', Joseph W. Goodman.</ref> but it has been applied extensively in the field of [[ultrasound]]. The technique can predict an acoustic pressure field distribution over a plane, based upon knowledge of the pressure field distribution at a parallel plane. Predictions in both the forward and backward propagation directions are possible.<ref>[http://www.egr.msu.edu/~fultras-web/background/asa.php ''Angular Spectrum Approach'', Robert J. McGough]</ref>
Modeling the diffraction of a CW (continuous wave), monochromatic (single frequency) field involves the following steps:
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