Spectral Measurements Field Guide

Abstract : Because of the rapid advance of airborne and satellite sensor technology in providing higher spectral resolution over progressively broader wavelength regions, there is a need for more (and more accurate) field measurements to complement overhead data. The purpose of this field guide is to facilitate such ground-based measurements, first through a review of the environmental factors affecting such measurements, second through an evaluation of the instrumentation involved, and third through a suggested approach to the measurement process. In evaluating environmental factors affecting spectral measurements in the field, the sources of radiance from a target are discussed in both the reflectance and emittance regions of the spectrum, as well as how those sources are modified by atmospheric attenuation and scattering, and the presence of clouds and wind. Another factor affecting all spectral measurements in the field is the computer typically used for instrument control and data storage. Computers tend to be the universal weak link in field spectrometers, because of their typical low tolerance for bright sunlight, temperature extremes, windblown dust, and rain. Various solutions to the computer problem are discussed, including the acquisition of hardened computers. The most commonly used field spectrometers are described, with advice on how to get the most out of each instrument. Then the pros and cons of each instrument are discussed with regard to different applications. Finally, how to approach field measurements is described, beginning with a thorough testing of a field instrument (and the field instrument user) in the laboratory. Approaches to data collection, record keeping, data reduction, and data analysis are discussed. A major conclusion is that much greater support for data analysis is necessary to reach the full potential of spectroscopic remote sensing for target identification.