Comparison of atmospheric spectral radiance measurements from five independently calibrated systems.

A variety of instruments have been developed over the past 50 years to measure spectral radiance in absolute units at UV and visible wavelengths with high spectral resolution. While there is considerable experience in the measurement of spectral irradiance, less emphasis has been given to the reliable measurement of spectral radiance from ground observations. We discuss the methodology and calibration procedures for five instruments capable of making such measurements. Four of these instruments are based on double monochromators that scan each wavelength in turn, and one is based on a single monochromator with a charged coupled device (CCD) allowing the recording of all wavelengths simultaneously. The measured spectral radiance deviates between 3% and about 35% depending on the instruments. The results are compared with radiative transfer calculations when the aerosol characteristics of the atmosphere are known.