Over a period of 3 years a precision Sun photometer (SPM) operating between 300 and 1025 nm was calibrated four times at three different high-mountain sites in Switzerland, Germany, and the United States by means of the Langley-plot technique. We found that for atmospheric window wavelengths the total error (2ς—statistical plus systematic errors) of the calibration constants V0 (λ), the SPM voltage in the absence of any attenuating atmosphere, can be kept below 1.6% in the UV-A and blue, 0.9% in the mid-visible, and 0.6% in the near-infrared spectral region. For SPM channels within strong water-vapor or ozone absorption bands a modified Langley-plot technique was used to determine V0 (λ) 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 Group of the Optical Sciences Center, University of Arizona, Tucson, Arizona. The lab calibration method requires knowledge of the extraterrestrial spectral irradiance. When we refer the standard lamp results to the World Radiation Center extraterrestrial solar irradiance spectrum, they agree with the Langley results within 2% at 6 of 13 SPM wavelengths. 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 solar irradiance spectra that have become recently available.
© 1998 Optical Society of America
(010.1290) Atmospheric and oceanic optics : Atmospheric optics
(120.0120) Instrumentation, measurement, and metrology : Instrumentation, measurement, and metrology
(120.5240) Instrumentation, measurement, and metrology : Photometry
(120.5630) Instrumentation, measurement, and metrology : Radiometry
Beat Schmid, Paul R. Spyak, Stuart F. Biggar, Christoph Wehrli, Jörg Sekler, Thomas Ingold, Christian Mätzler, and Niklaus Kämpfer, "Evaluation of the Applicability of Solar and Lamp Radiometric Calibrations of a Precision Sun Photometer Operating Between 300 and 1025 nm," Appl. Opt. 37, 3923-3941 (1998)