Fixed pattern noise is removed from images by a technique called flat fielding, where a computer adjusts pixel sensitivities to be equal. Fixed pattern noise severely limits S/N performance for CCD and CMOS imagers, which will be discussed in Chapter 10. Fortunately, simple computer algorithms can remove FPN and achieve the shot noise limit, thereby significantly improving S/N performance.
The flat-fielding technique demonstrated in Fig. 8.1 shows two raw sinusoidal video traces that are FPN limited (labeled as S RAW ). Also presented is a flat-field trace used to remove FPN (labeled as S FF ). The solid dark curves shown are the corrected traces after the raw traces are divided (pixel-by-pixel) by the flat-field level and the result multiplied by Î¼ FF according to Eq. (8.1). Note that S/N performance improves significantly after flat fielding. Figure 8.2 presents images for the lowest-contrast sinusoidal shown in Fig. 8.1 before and after flat fielding is performed. The improvement in image quality is obvious.
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