We present three different methods for the absolute calibration of direct spectral irradiances measured with a Brewer spectroradiometer, which are shown to agree to within ±2%. Direct irradiance spectra derived by Brewer and Bentham spectroradiometers agree to within 4 ± 3%. Good agreement was also found by a comparison of the aerosol optical depth and Angstrom exponent retrieved by the two instruments and a multifilter rotational shadowband radiometer. The spectral aerosol optical depth (300–365 nm) derived from six years of direct irradiance measurements at Thessaloniki shows a distinct seasonal variation, averaging to ~0.3 at 340 nm in winter and ~0.7 in summer.
© 2005 Optical Society of America
Original Manuscript: August 9, 2004
Revised Manuscript: October 30, 2004
Manuscript Accepted: November 8, 2004
Published: March 20, 2005
Stelios Kazadzis, Alkiviadis Bais, Natalia Kouremeti, Evangelos Gerasopoulos, Katerina Garane, Mario Blumthaler, Barbara Schallhart, and Alexander Cede, "Direct spectral measurements with a Brewer spectroradiometer: absolute calibration and aerosol optical depth retrieval," Appl. Opt. 44, 1681-1690 (2005)