Evaluation of a ground-based sky camera system for use in surface irradiance measurement

This paper describes the evaluation of a ground-based sky camera system for studying the effect of clouds on the level of the ambient ultraviolet radi ation. The system has been developed for research in the characterization o f the effect of clouds around the sun. It is the first sky camera system to be used for the assessment of cloud conditions in the vicinity of the sun, rather than a whole-sky assessment. The system features a sun-tracking sky camera with an integrated measurement of horizontally received radiation a t the same location. The image-processing algorithm uses solar radiation re adings to reduce reflections from die sun on the camera system being mistak en for cloud in the images. Cloud amount was estimated in an angular region of between 12.5 degrees;md 37.5 degrees around the sun. The algorithm also estimates the amount of solar obstruction by cloud (sun not covered, parti ally or totally covered). The system was evaluated during September 1997 at Toowoomba, Australia (27.6 degrees S latitude). Compared to manual assessm ent of 592 images, 76.5% were identified correctly by the algorithm for the degree of solar obstruction and 81.9% for cloud amount. The behavior of ul traviolet radiation levels with cloud conditions is discussed.