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Detector noise is probably the most studied and well-known source of error in spectroscopy, including Fourier-transform spectroscopy. However, contrary to popular belief, detector noise may not always be the limiting factor to the signal-to-noise ratio obtainable by a Fourier-transform spectrometer. It is the goal of this chapter to provide an understanding of the relationship between the magnitude of the detector noise and the spectral SNR allowable in FTS. In addition, various types of photo detectors and the general guidelines for selecting them are described. It is important to understand that detector noise has a unique characteristic: unlike noise from other sources in the system, it is independent of the signal level. Thus, with the conventional definition of the signal-to-noise ratio, SNR=signal noise , the SNR obtainable is proportional to the amount of signal available. What this means is that the SNR due to detector noise can be extremely high when the radiation energy that falls on the detector element is high. In this case, the actual instrumental SNR is limited by other components, which may be the mechanical drive error, the A/D digitization error, the sampling error, etc. Therefore, detector noise generally becomes significant only when the radiation power incident on the detector is very low.
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