In the context of the source–medium–sensor system model, the medium is everything between the source and the sensor. The optical medium affects the radiance field by flux attenuation, flux amplification (in the case of lasers), flux increase (path radiance) and refractive wave distortions (e.g., turbulence). The medium effects can be either static and temporally constant or temporally and spatially dynamic, such as in turbulent flow. In keeping with the theme of the book, this chapter investigates the effects of the atmosphere as a component in the system rather than on the physical processes in the atmosphere - these are covered elsewhere.
The atmospheric index of refraction varies with pressure and temperature. The turbulence atmospheric air movements result in eddy currents with varying temperature and pressure, resulting in cells of varying indices of refraction. These variations cause a number of different effects, depending on the magnitude of the variation, the physical area of the variation, the nature of the optical flux, and so forth. Some of the effects that can occur are beam steering, arrival-angle variations, scintillation (variation in signal strength), and visual mirage effects. This chapter considers only static media; turbulence is well documented elsewhere.
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