The magnitude of the noise present in the signal from a spectroscope depends strongly upon the frequency-band-pass width of the amplifier. It is, therefore, possible to reduce the noise by inserting a low-pass filter. At the same time a distortion is produced in the spectrum recorded. It is shown for two different kinds of filter that this distortion may be divided into two effects: (1) a displacement of the spectrum as a whole, which is easily corrected for, and (2) a real distortion depending upon the width of the spectral lines. It is shown that the only means to limit this distortion is to scan the spectrum at a sufficiently low speed. An expression is given for this speed, which is proportional to the width of the narrowest line (often equal to the effective slitwidth), and inversely proportional to the time-constant of the filter and to a constant characterizing the maximum tolerable distortion.
SVEND BRODERSEN, "Noise Filter Effects in Recording Spectroscopes," J. Opt. Soc. Am. 43, 1216-1220 (1953)