Wide-angle Image Analysis for Sky Luminance Measurement

A sky scanner is an important tool for measuring sky luminance and establishing daylight conditions of a location. However, it is expensive, difficult to set-up and the process is time consuming. Digital imaging with wide-angle lenses can be a cost and time effective alternative to sky scanners. The use of photographic techniques in luminance measurement is not new and CCD (charged couple device)/ vidicon based still and video camera luminance meters have been previously developed for different tasks. Among the camera systems developed for daylight measurement only a few have concentrated on entire sky luminance measurement. As a part of ongoing research on digital camera calibration for sky luminance measurement, this paper investigates a procedure to determine the distortion that occurs in two-dimensional fisheye images from sky scanner aperture angles. The artificial sky at the University of Sydney is used for the experiments. Digital images are captured using a fisheye lens, following the 145-part CIE standard sky segmentation. The digital images are analysed, then converted to image filters to extract RGB (Red, Green and Blue) values from a coloured image of the sky for luminance calculation.