Field measurement of reflectance: some major considerations.

Success in determining when, whether, and in what conditions to acquire remote sensing data for describing a given target (e.g., vegetation) is contingent on understanding the reflectance properties of the target and its surroundings. Unfortunately, relatively little information on the reflectance properties of the earth's surface exists in the literature. Field measurements of a target's reflectance are usually made with single-beam instruments by sequentially viewing the target and a white standard reflector, which is assumed to be Lam-bertian. Because variations in atmospheric transmission can occur between the times of measuring the target and reflector, substantial errors in reflectance calculated from these measurements may result. To avoid these errors the irradiance on and radiance from the target must be measured simultaneously. Measurements of the spectral hemispherical-conical reflectance of vegetative canopies were made by simultaneously measuring irradiance and radiance with pairs of portable spectroradiometers. The procedures for calibrating the instruments and for collecting and analyzing spectral reflectance data are described. Major instrumental sources of error and their magnitude are discussed as are problems involved in making such measurements.