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Chapter 4:
Review of Radiometry
Radiometric calculations for the generation and transfer of radiation are reviewed in this chapter. Every infrared system senses the power that is incident upon the detector. This power is generated by a remote source and transmitted to the optical system, which in turn focuses it on the detector. All these aspects of signal generation come under the general heading of radiometry. Therefore, this chapter reviews the pertinent topics. These topics include the concepts of solid angles, projected solid angles, throughput, elements of radiometric transfer, definitions of radiometric quantities for both fields and materials, and some properties of blackbodies. It is necessary to define several radiometric terms to describe how radiation transfers from sources to receivers. There has been much ado about the words and symbols associated with these concepts, and we will spend no more time than is absolutely necessary with these excursions into semantics. There are four different geometric terms related to power transfer—in addition to power. They all have some relationship to a solid angle, so it is important to obtain a clear definition of solid angle. This may be done by reviewing first the definition of an ordinary or linear angle.
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