This chapter opens by discussing the mechanics of generating a PTC. An ideal PTC response from a camera system exposed to a uniform light source is illustrated in Fig. 5.1. For a subarray of pixels, rms noise is plotted as a function of average signal at different light levels (or exposure times). Four distinct noise regimes are identified in a PTC. The first regime, read noise, represents the random noise measured under totally dark conditions, which often includes several different noise contributors (refer to Chapter 12). As the light illumination is increased, read noise gives way to photon shot noise, which represents the middle region of the curve. Since the plot in Fig. 5.1 is on log-log coordinates, shot noise is characterized by a line with a slope of 1/2. The third regime is associated with pixel FPN, which produces a characteristic slope of unity because signal and FPN scale together.
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