title: All-sky imaging: a simple, versatile system for atmospheric research.
abstract: A simple and inexpensive fully automated all-sky imaging system based on a commercial digital camera with a fish-eye lens and a rotating polarizer is presented. The system is characterized and two examples of applications in atmospheric physics are given: polarization maps and cloud detection. All-sky polarization maps are obtained by acquiring images at different polarizer angles and computing Stokes vectors. The polarization in the principal plane, a vertical cut through the sky containing the Sun, is compared to measurements of a well-characterized spectroradiometer with polarized radiance optics to validate the method. The images are further used for automated cloud detection using a simple color-ratio algorithm. The resulting cloud cover is validated against synoptic cloud observations. A Sun coverage parameter is introduced that shows, in combination with the total cloud cover, useful correlation with UV irradiance.
authors.affiliation: Division for Biomedical Physics, Department of Physiology and Medical Physics, Innsbruck Medical University, Innsbruck, Austria. axel.kreuter@i-med.ac.at
authors.forename: Axel
authors.initials: A
authors.lastname: Kreuter
authors.forename: Matthias
authors.initials: M
authors.lastname: Zangerl
authors.forename: Michael
authors.initials: M
authors.lastname: Schwarzmann
authors.forename: Mario
authors.initials: M
authors.lastname: Blumthaler