Radiometry may be strictly defined as the measurement of optical radiant energy, but the context in its broader sense is useful here, referring to the generation, propagation, and detection of optical radiant energy, as well as its measurement. Measurements, in turn, may be relative or absolute: the former refers to the value obtained from a target with reference to other objects in the scene, and the latter relates a target’s output value to a physical standard. These absolute measurements require that a sensor be radiometrically calibrated with traceability to a standards authority, such as the U. S. National Institute of Standards and Technology (NIST). This chapter reviews emission, reflection, and related subjects; a more detailed consideration of radiometric terminology, laws, and approximations is presented in Chapter 3.
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