Bi-directional reflectance measurements with the CCD line camera WAAC

Abstract

In order to determine the Bi-directional Reflectance Distribution Function (BRDF) of natural surfaces under natural conditions with high angular resolution a CCD line camera is used. We demonstrate the principle of the method with the Wide Angle Airborne Camera (WAAC) which was developed at the DLR for airborne stereo imaging purposes. The camera is mounted on a rotating device and an extendible boom provides an aerial platform. The arrangement is such that one end of the CCD line looks in nadir direction and the other end stretches to a view zenith angle of 80 degrees. Rotating the camera around a vertical axis makes it possible to measure almost the complete reflectance distribution of the surface below in a very short time. First measurements were carried out in the NIR spectral region to determine the BRDF of several surface types including various crops. We discuss smoothing strategies to obtain the BRDF from the raw data and present some examples of the results. We plan to continue the BRDF measurements for a variety of natural surfaces in order to understand the diurnal and seasonal variations and to compile an archive of BRDF data.