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An experimental procedure for the measurements of the magnetostrictive coefficients of planar, anisotropic, polycrystalline magnetic structures was established. A new optoelectronic method has been conceived and realized by a dynamic modulation of a Fraunhofer diffraction pattern which results when a collimated, monochromatic light beam passes through a slit, one edge of which is simply the end of the given magnetostrictive sample, whereas the other is a fixed, opaque reference, formed by a displaced photographic negative of the sample. The width of the slit changes as a result of an externally applied, sinusoidal magnetic field. A calibrated photomultiplier circuit, followed by an electronic filter and wave analyzer, which are tuned about twice the frequency of the applied field, detects the magnetostrictive displacement. The high-Q characteristics of the vibrating magnetoelastic system enable an easy interpolation for the determination of the static parameters. For a 50 : 50 permalloy in the form of 2-mil thick foil, 1 by 3.1 cm, the magnetoelastic strains were found to be 0.97 × 10-5and 1.93 × 10-5, depending upon the preferred direction of magnetization which was induced principally via annealing in the presence of a magnetic field. This fact supports the author's previous phenomenological treatment of the subject .