A standard high-speed, field-portable spectroradiometric measurement system built around a programmable microprocessor has been adapted to the form of a Reflectometer/Comparator. In this configuration, the instrument makes passive measurements of the absolute reflectance of agricultural plant canopies over a spectral range of 0.4 to 2.5 micrometers. Realtime absolute measurements are made possible by a unique optical chopper which constantly compares the target with the sun and derives the ratio, which is the reflectance. Extensive measurements of solar reflectance from a variety of these targets have been made. The paper describes the instrumentation and measurement procedures, reviews the software programming and discusses the results.