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This chapter is excerpted from Schott (2007). It contains a review of radiometric principles that are used extensively in later chapters as we develop governing equations that describe the reflective and emissive behavior of polarized signals reaching remote platforms.
2.1 Radiometric Terms
Radiometry is formally defined as the science of characterizing or measuring how much EM energy is present at, or associated with, some location or direction in space. It has evolved separately in the fields of physics, illumination or vision, and engineering, and as a result, a host of terms are used to describe various radiometric concepts. Often one concept has several different names; it is also common for the same term (e.g., intensity) to mean different things to different authors. To provide a common framework, we will briefly review the definitions of the relevant physical parameters and radiometric terms. An emphasis will be placed on the units of measure in this section and throughout the book to ensure a clearer understandingâunits are usually designated with square brackets ([ ]) for clarity, and, where relevant, a unit's cancellation analysis may be performed within square brackets. In reading other authors, particularly older works or work drawn from other disciplines, the reader should carefully evaluate the author's definitions and units of measure to determine what term is being applied to each radiometric concept. The definitions used throughout this volume are consistent with those established by the Commission Internationale de l'Eclairage (CIE) and adopted by most international societies [see CIE (1970)]. In addition, to the extent practical, the parameters, nomenclature, and symbology are consistent with the relevant reference material [see Grum and Becherer (1979) and Nicodemus (1976)].
2.1.1 Definition of terms
For most radiometric considerations, we can use the ray/particle simplification of optics. This approach is based on geometric optics and assumes that light travels in straight lines and transfers energy in discrete packets or quanta.
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