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 slit-width), and inversely proportional to the time-constant of the filter and to a constant characterizing the maximum tolerable distortion.
© 1953 Optical Society of America
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