Abstract

The wavelength dependence of both the total back-scatter cross section σ<sub><i>T</i></sub> and the depolarized back-scatter cross section σ<sub><i>D</i></sub> for rough metallic surfaces of known statistical characteristics has been determined experimentally by comparing data at 0.63-, 3.39- and 10.6 µ wavelengths. The rms height from the mean, <i>h</i>, and the mean scale size l of the two surfaces used in the experiment are: <i>h</i> ≈ 1 µ, l ≈ 10 µ; <i>h</i> ≈ 7 µ, l ≈ 50 µ. At or near normal incidence, results show that the total cross section per beam area σ<sub><i>T</i></sub>/<sub><i>A</i></sub><sub>o</sub> is independent of wavelength λ, provided that <i>h</i>/λ is greater than approximately ¼. When <i>h</i>/λ≤¼, σ<sub><i>T</i></sub>/<i>A</i><sub>0</sub> increases rapidly with decreasing <i>h</i>/λ. Previous microwave data suggest that the metallic surface reflects nearly as a perfectly smooth surface without significant scattering losses when <i>h</i>/λ≲1/40. At or near normal incidence the ratio of σ<i>D</i> to σ<i>T</i> varies as (<i>h</i>/l)<sup>4</sup>λ/4πδ for all values of <i>h</i>/λ studied, where δ is the skin depth of the metallic surface. For incident angles ψ in the range from 20° to 80° and <i>h</i>/λ>¼, bothσ<i>T</i>/<i>A</i><sub>0</sub> and σ<i>D</i>/A<sub>0</sub> vary as λ<sup>-n</sup> where <i>n</i> increases with increasing ψ. <i>n</i> has a value of 0.40 (±0.2) at ψ = 20° and 0.8 (±0.2) at 80°.

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