-The measurement of spectral data in the field has an important role in remote sensing, and a long history, but instruments and methods to achieve this have serious limitations under all but the most ideal conditions. Problems arise from the instruments themselves, from the environment in which they are used, and from the methodologies that are commonly adopted.The variable most commonly sought from field measurements is spectral reflectance, or more strictly the bidirectional reflectance factor (BRF), but this is dependent to some extent on the instrument used to make the measurement, and the conditions of measurement, notably the sky irradiance distribution. In this paper we argue that field spectral measurements should be recorded in the appropriate SI units, which will normally be the derived units of radiance for the flux reflected from the target and irradiance for the incident energy. Reflectance data remain a convenient way to represent the energy interactions occurring at the surface, and they have value in generic spectral libraries, but ultimately they lack reproducibility unless accompanied by much more detailed metadata than is the norm in most spectral libraries.

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