Measurements in five different passbands in the spectral region below 20 ${\mathrm{cm}}^{-1\mathrm{}}$ have been made with a liquid-helium-cooled radiometer in two flights at approximately 40-km altitude. The equivalent temperatures are ${2.7}_{-0.2\mathrm{}}^{+0.4\mathrm{}}$, 2.8±0.2, 2.8±0.2, ≤2.7, ≤3.4°K in passbands of 1 to 5.4, 7.8, 7.9, 11.1, and 18.5 ${\mathrm{cm}}^{-1\mathrm{}}$. The results are consistent with a 2.7°K thermal radiation background. Atmospheric radiation of uncertain magnitude dominates the region above 11 ${\mathrm{cm}}^{-1\mathrm{}}$.

• Received 2 March 1972

DOI:https://doi.org/10.1103/PhysRevD.7.326