The wavelength dependence of both the total back-scatter cross section σT and the depolarized back-scatter cross section σD 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, h, and the mean scale size l of the two surfaces used in the experiment are: h≈1 μ, l≈10 μ; h≈7 μ, l≈50 μ. At or near normal incidence, results show that the total cross section per beam area σT/A0 is independent of wavelength λ, provided that h/λ is greater than approximately . When , σT/A0 increases rapidly with decreasing h/λ. Previous microwave data suggest that the metallic surface reflects nearly as a perfectly smooth surface without significant scattering losses when h/λ≲1/40. At or near normal incidence the ratio of σD to σT varies as (h/l)4λ/4πδ for all values of h/λ studied, where δ is the skin depth of the metallic surface. For incident angles ψ in the range from 20° to 80° and , both σT/A0 and σD/A0 vary as λ−n where n increases with increasing ψ. n has a value of 0.40 (±0.2) at 4ψ=20° and 0.8 (±0.2) at 80°.
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