From a discussion in the previous chapter on the interaction of radiation with materials, we now turn to the subject of how radiation is generated, and the roles that emission, reflection, and other processes play.
Sources of optical radiation can be classified in a variety of ways. Active sources emit optical radiation due to their temperature (thermal sources) or as a result of atomic transitions (luminescent sources). Passive sources reflect optical radiation from active sources or from other passive sources. Passive sources can also be classified as thermal or luminescent, depending upon the process that generated the radiation initially. Examples of thermal sources include blackbody radiation simulators, tungsten-filament lamps, gases, the sun, the moon, and you and I. Examples of luminescent sources include lasers, fluorescent lamps, mercury arcs, sodium lamps, electroluminescent panels, LEDs, and gases. Some sources combine both thermal and luminescent mechanisms, and some may be both active and passive, reflecting in one spectral region and emitting in another.
Other means of classification have also been used. Some authors distinguish between artificial (man-made) and natural sources. Lamps are artificial sources, whereas the earth, the sun, and stars are natural sources. Still another practice is to divide sources according to their output spectral characteristics. Continuous sources have a spectral radiance that is slowly varying with wavelength, typical of thermal radiation, while line sources emit in narrow, well-defined spectral regions. Yet another attempt to distinguish sources is by their degree of spatial andâor temporal coherence.
Thermal radiation has been extensively studied since the late nineteenth century. Stefan was the first to experimentally examine the relationship between radiation and temperature in 1879. He analyzed data from Tyndall and found that the total radiation is proportional to the fourth power of temperature T4. Boltzmann derived this T4 relationship from the Carnot cycle in 1884.
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