Editing Infrared spectroscopy (section)

===FTIR===
{{Main|Fourier transform infrared spectroscopy}}
[[File:FTIR-interferogram.svg|thumb|An interferogram from an [[FTIR]] measurement. The horizontal axis is the position of the mirror, and the vertical axis is the amount of light detected. This is the "raw data" which can be [[Fourier transform]]ed to get the actual spectrum.]]
'''[[Fourier transform]] infrared (FTIR) spectroscopy''' is a measurement technique that allows one to record infrared spectra. Infrared light is guided through an [[interferometer]] and then through the sample (or vice versa). A moving mirror inside the apparatus alters the distribution of infrared light that passes through the interferometer. The signal directly recorded, called an "interferogram", represents light output as a function of mirror position. A data-processing technique called [[Fourier transform]] turns this raw data into the desired result (the sample's spectrum): light output as a function of infrared [[wavelength]] (or equivalently, [[wavenumber]]). As described above, the sample's spectrum is always compared to a reference.{{citation needed|date=February 2024}}

An alternate method for acquiring spectra is the "dispersive" or "scanning [[monochromator]]" method. In this approach, the sample is irradiated sequentially with various single wavelengths. The dispersive method is more common in [[Ultraviolet-visible spectroscopy|UV-Vis spectroscopy]], but is less practical in the infrared than the FTIR method. One reason that FTIR is favored is called "[[Fellgett's advantage]]" or the "multiplex advantage": The information at all frequencies is collected simultaneously, improving both speed and [[signal-to-noise ratio]]. Another is called "Jacquinot's Throughput Advantage": A dispersive measurement requires detecting much lower light levels than an FTIR measurement.<ref name=white>[https://books.google.com/books?id=t2VSNnFoO3wC&pg=PA7 ''Chromatography/Fourier transform infrared spectroscopy and its applications'', by Robert White, p7]</ref> There are other advantages, as well as some disadvantages,<ref name=white/> but virtually all modern infrared spectrometers are FTIR instruments.