Solar millimeter radiation, isolated by black filters, was received with a thermal detector in the focus of a 60-in parabola. The transmission of this radiation through woven copper wire mesh of various gauge was measured. Since neither theoretical nor experimental data on the filter characteristics of mesh were available, a microwave method was used to obtain this information. The resulting filter curves together with the solar measurements made an evaluation of the millimeter spectrum possible.
Also, a theoretical spectral curve was drawn multiplying Rayleigh-Jeans’ λ-4. intensity with Van Vleck attentuation data which were computed for the atmospheric conditions during the solar measurements, but raised according to earlier experimental attenuation results of the authors. This curve was in good agreement with the measured spectrum showing that the observed short-wave cutoff near 0.9 mm is due to sharply rising attenuation; the observed peak between 1.0 and 1.1 mm lies in the predicted window, after which the spectral intensity falls off at longer wavelengths according to the λ-4. law.
H. H. THEISSING and P. J. CAPLAN, "Measurement of the Solar Millimeter Spectrum," J. Opt. Soc. Am. 46, 971-978 (1956)