Measurement of the All-Sky Spectral Radiance Distribution Using a Fisheye Camera and Principal Component Analysis
Abstract
A new method was developed to measure the spectral radiance of all sky elements using a general purpose digital camera with a circular fisheye lens. The method provides sufficient accuracy for solar energy applications such as daylighting design, photosynthesis analyses and photovoltaic evaluations. The instrument is portable for field measurements in various climates. The digital camera with the fisheye lens is colorimetrically calibrated using a xenon lamp and a spectroradiometer to obtain the calibration functions which transfer the video signal values RGB on a digital color image into the absolute values of CIE 1931 tristimulus values XYZ pixel by pixel. The calibrated fisheye digital camera measures the all sky distribution of the XYZ tristimulus values which are also useful for measuring all sky distributions of luminance and correlated color temperature. The XYZ tristimulus values are transferred into the all sky spectral radiance distribution using the eigenvectors obtained by principal component analysis of spectral radiance data of sky elements measured by a spectroradiometer under various weather conditions. The accuracy of the method is experimentally validated by comparing with the spectral radiance of sky elements measured by the spectroradiometer.