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Image quality is generally either photon limited or noise limited. Both are concerned with fluctuations in image irradiance, which may derive from photon statistics and/or random motion of charge carriers. The basic concept of noise is considered here. Extension to actual imaging systems is delayed to Chapter 11 where spatial frequency effects are considered too. Electronic noise in photoelectronic imaging systems derives primarily from two sources: quantum noise and Johnson noise. The former results from fluctuations in the arrival rates of photons into the imaging system, which gives rise to fluctuations in the generation rates of optically generated current carriers. Fluctuations in generation rate of current carriers exist also with regard to bias current or dark current and are known as shot noise. All such fluctuations exhibit similar statistics and are lumped together here under the general heading of quantum noise. Note that both cases are concerned with variations in current, the first deriving from fluctuations in generation rate and the second from random motion. Generally, nonsignal direct currents themselves are not considered "noise" because they can be removed via a capacitor. However, variations in direct currents are time varying and therefore not removable with a capacitor. For this reason, noise current is defined as standard deviation of current from its average value. Both types of noise are considered in turn here for a single detector. This is extended in Chapter 11 to imaging systems. The material in this chapter and the next is basic to optical communication systems as well as both passive and active imaging.
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