STUDIES OF ERRORS IN FIELD-MEASUREMENTS OF THE BIDIRECTIONAL REFLECTANCE FACTOR

Errors in the determination of the bidirectional reflectance factor (BRF) were studied in a measuring campaign in the desert using one radiometer, a sprayed barium sulfate panel as reference and solarimeters. The following sources of error were evaluated: (1) specific characteristics of instruments, (2) interpolations due to non-simultaneous measurement of panel and object, (3) optical properties of the panel and (4) dependence of panel reflectance on atmosphere. The most significant errors occurred if the solar zenith angle dependent reflectance of the panel was ignored. Moreover, the assumption that all irradiance is direct, was discovered to be an important source of error, in particular pertaining to panel reflectance at large solar zenith angles, at short wavelengths and non-lambertian panels. The use of simultaneous solarimeter readings when object measurements were taken under nonideal atmospheric conditions may be partly of help.