EVALUATION OF THE APPLICABILITY OF SOLAR AND LAMP RADIOMETRIC CALIBRATIONS OF A PRECISION SUN PHOTOMETER OPERATING BETWEEN 300 AND 1025 NM

Over a period of 3 years a precision Sun photometer (SPM) operating be tween 300 and 1025 nm was calibrated four times at three different hig h-mountain sites in Switzerland, Germany, and the United States by mea ns of the Langley-plot technique. We found that for atmospheric window wavelengths the total error (2 sigma-statistical plus systematic erro rs) of the calibration constants V-0 (lambda), the SPM voltage in the absence of any attenuating atmosphere, can be kept below 1.6% in the W -A and blue, 0.9% in the mid-visible, and 0.6% in the near-infrared sp ectral region. For SPM channels within strong water-vapor or ozone abs orption bands a modified Langley-plot technique was used to determine V-0 (lambda) with a lower accuracy. Within the same period of time, we calibrated the SPM five times using irradiance standard lamps in the optical labs of the Physikalisch-Meteorologisches Observatorium Davos and World Radiation Center, Switzerland, and of the Remote Sensing Gro up of the Optical Sciences Center, University of Arizona, Tucson, Ariz ona. The lab calibration method requires knowledge of the extraterrest rial spectral irradiance. When we refer the standard lamp results to t he World Radiation Center extraterrestrial solar irradiance spectrum, they agree with the Langley results within 2% at 6 of 13 SPM wavelengt hs. The largest disagreement (4.4%) is found for the channel centered at 610 nm. The results of these intercomparisons change significantly when the lamp results are referred to two different extraterrestrial s olar irradiance spectra that have become recently available. (C) 1998 Optical Society of America.