[1] When measuring a signal whose noise is dominated by detector noise, a multiplexed measurement such as the signal generated by a Fourier transform spectrometer can produce a relative improvement in SNR, compared to an equivalent scanning monochromator, of the order of the square root of m, where m is the number of sample points comprising the spectrum.
[2] Sellar and Boreman have argued that this SNR improvement can be considered as a result of freedom from needing an exit slit inside the spectrometer, since an exit slit reduces the light collected by the detector by the same factor.
At the peak of the emission line, a monochromator measurement will be noisy, since the noise is proportional to the square root of the signal.
Thus, multiplexed measurements can achieve higher SNR at the emission line peaks.
Shot noise is the main reason Fourier transform spectroscopy has never been popular for UV and visible light spectrometry.