Optical Communications, 2nd Edition

2.6 BACKGROUND RADIATION

In addition to the desired source power, a receiving space system viewing a lighted background also collects undesirable strong background radiation falling within the spatial and frequency ranges of the detector. The collected background radiation is processed along with the desired signal background, and presents a basic degradation to the overall system performance. Of particular importance is the actual amount of background radiation power that is collected. The determination of the power, however, requires an accurate model for the source of this radiation. The accepted model is to consider the background to be generated from uniformly radiating sources. These sources divide into two basic types: (1) the extended background, assumed to occupy the entire background, and therefore is present in any receiver field of view, and (2) discrete or point sources that are more localized but are more intense and may or may not be in the receiver field of view. In a space system, the sky is the primary extended background, and the localized sources may correspond to stars, planets, moon, sun, and so on. In an indoor environment, reflecting walls become the extended background, and localized sources may be room lights, reflecting surfaces, and the like. In this section, we review the analysis of general background optical noise.

Extended background radiators are most often described by their spectral radiance function, W(λ), defined as the power radiated at wavelength ...