Because the wavelengths of interest are dispersed on a detector that consists of a series of pixels arranged along an axis that can be converted to frequency, the necessity of carrying out a Fourier transformation of the data has been eliminated.
This not only eliminates the time delay inherent in Fourier Transform calculations, it also reduces the need to carefully match sample optics to spectrometer optics to reduce detector noise. Every part of the detector in a PA-IR spectrometer contributes to signal, as well as noise, while in an interferometer, only the illuminated part of the detector contributes to signal but the entire detector surface contributes to noise. Thus, if the optics are not carefully matched to the detector size, either signal is lost by “overfilling” the detector – or noise in increased by “underfilling” the detector.
Another important point not immediately obvious is that detectors used for FT-IR systems are limited in the number of photons they can accept. So, increasing the source output – or improving optical efficiencies in an FT-IR tend to saturate the detector, giving no improvement in performance. A planar array system can easily take advantage of an increase in the number of photons reaching the detector because it is a dispersive system. Thus, improvements in the optical efficiency of sampling apparatus can yield advantages to the user by increasing signal. With noise remaining constant, this improvement results in better S/N ratios.
Relative to a single channel measurement, a multichannel measurement (such as PA-IR) will always show a gain in sensitivity relative to a single channel measurement, independent of the type of limiting noise. This is not true for a multiplex measurement such as FT-IR. When there is any source fluctuation or flicker noise, the FT-IR measurements show a dramatic loss in S/N. Under similar conditions, the PA-IR measurement is unaffected. Because of this, a PA-IR instrument maintains the very high sensitivity of an FT-IR and in fact, exceeds it under different limiting noise conditions
Finally, sampling accessory alignment is more critical in FT-IR than in PA-IR since any signal lost only worsens the signal to noise ratio.