When photons strike a detector, interactions immediately produce a signal variance or noise from pixel-to-pixel. This chapter introduces four fundamental noise sources important to PT work. The first two sources, signal shot noise and Fano noise, are related to photon interaction. The third noise source, fixed pattern noise, is associated with pixel-to-pixel sensitivity nonuniformity. The fourth source, read noise, encompasses all other noise sources that are not dependent on signal strength. Shot noise increases by the square root of signal, whereas FPN increases proportionally with signal. Fano noise increases by the square root of photon energy (or quantum yield).
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