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Atmospheric Measurement Techniques An interactive open-access journal of the European Geosciences Union

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Atmos. Meas. Tech., 9, 4279-4294, 2016
© Author(s) 2016. This work is distributed under
the Creative Commons Attribution 3.0 License.
Research article
05 Sep 2016
Sky camera geometric calibration using solar observations
Bryan Urquhart, Ben Kurtz, and Jan Kleissl Department of Mechanical and Aerospace Engineering, University of California, San Diego, USA
Abstract. A camera model and associated automated calibration procedure for stationary daytime sky imaging cameras is presented. The specific modeling and calibration needs are motivated by remotely deployed cameras used to forecast solar power production where cameras point skyward and use 180° fisheye lenses. Sun position in the sky and on the image plane provides a simple and automated approach to calibration; special equipment or calibration patterns are not required. Sun position in the sky is modeled using a solar position algorithm (requiring latitude, longitude, altitude and time as inputs). Sun position on the image plane is detected using a simple image processing algorithm. The performance evaluation focuses on the calibration of a camera employing a fisheye lens with an equisolid angle projection, but the camera model is general enough to treat most fixed focal length, central, dioptric camera systems with a photo objective lens. Calibration errors scale with the noise level of the sun position measurement in the image plane, but the calibration is robust across a large range of noise in the sun position. Calibration performance on clear days ranged from 0.94 to 1.24 pixels root mean square error.

Citation: Urquhart, B., Kurtz, B., and Kleissl, J.: Sky camera geometric calibration using solar observations, Atmos. Meas. Tech., 9, 4279-4294,, 2016.
Publications Copernicus
Short summary
A model relating the position of objects in the 3-D world to their pixel coordinates has been developed for a fixed-focal length fisheye lens camera. An associated automated method to calibrate model parameters has been developed for a daytime skyward-pointing camera. The position of the sun throughout the day is used as input to the calibration algorithm. The accuracy of the calibration was found to be on the same order as the accuracy of sun position detection in an image.
A model relating the position of objects in the 3-D world to their pixel coordinates has been...