Evaluation of forest-canopy photographs with diode-array scanner OSIRIS

Abstract

Fish-eye pictures are often used to characterize forest canopies. The sky-obscuration factor (SOF), defined as the relative area of sky obscured by vegetational elements for a certain solid angle when observed from below, is evaluated by means of the interactive, computer-supported, diode-array scanner OSIRIS (now commercially available), with high geometric and photometric resolution. The SOF in each picture element is calculated by linear interpolation between the frequency peaks of completely covered and uncovered sky. This method gives good results when the two peaks show small skewness and limited width and are clearly separated. Compared with OSIRIS results (from nonideally exposed pictures), manual evaluation and radiation measurements gave a somewhat higher SOF, while precipitation–throughfall analysis agreed in the zenith-angle interval 0–10°. To facilitate and improve evaluation, a standardized photographic technique is recommended including usage of high contrast film, carefully controlled exposure, and identical development. An artificial horizon and north position indicated by lamps mounted within the depth of field of the fish-eye lens is also recommended.