We describe bistatic scattering measurements at 230 GHz, in the 330-490 GHz range and, at 650 GHz onvarious surfaces. These include a series of eight reference targets constructed from alumina grit embeddedin an absorptive epoxy matrix, and a set of conventional outdoor building materials. The samples’ surfacetopographies were measured by focus-variation microscopy (FVM) and their autocorrelation lengths andRMS roughness levels extracted. All bistatic measurements were performed in the principal plane, atincidence angles of 25°, 45°, and 65°, in s and p polarization. The reference samples’ normalizedroughness levels cover the range 0.040 ≤ / ≤ 0.60, and their normalizedautocorrelation lengths cover the range 0.086 ≤ / ≤ 1.14. The measurements are described interms of bidirection reflectance distribution function (BRDF) or normalized radar cross section (nRCS),and include regimes of both diffuse scattering and specular reflectance. The reference samples’measurements are compared to two ab initio scattering theories, the Modified Integral Equation Method(IEM-B) of A. Fung, and the Generalized Harvey-Shack (GHS) model, that have no free parameters.Although there are several individual cases where either the IEM or GHS theory (or both) provide a goodmatch to measurement, their overall agreement across the entire dataset is poor. In addition, the diffuseBRDF in each bistatic scan has been fit to a Lambertian (constant) dependence of scattering angle, and apurely empirical model developed for the dependence of Lambertian scattering on frequency, roughness,polarization, and incidence angle. The empirical model provides the best match to measurement acrossthe full dataset, and can be used for reliable phenomenology studies of submillimeter imaging or wirelesstelecommunication. Nearly all the outdoor building materials, like the roughest of the reference samples,fall in a regime where L/σ is not large, and therefore where ab initio scattering theories can’t be expectedto apply.
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