Editing Fourier-transform spectroscopy (section)

===Measuring an absorption spectrum===
[[File:FTIR-interferogram.svg|thumb|An "interferogram" from a Fourier-transform spectrometer. This is the "raw data" which can be [[Fourier transform|Fourier-transformed]] into an actual spectrum. The peak at the center is the ZPD position ("zero path difference"): Here, all the light passes through the [[Michelson interferometer|interferometer]] because its two arms have equal length.]]
The method of Fourier-transform spectroscopy can also be used for [[absorption spectroscopy]]. The primary example is "[[Fourier-transform infrared spectroscopy|FTIR Spectroscopy]]", a common technique in chemistry.

In general, the goal of absorption spectroscopy is to measure how well a sample absorbs or transmits light at each different wavelength. Although absorption spectroscopy and emission spectroscopy are different in principle, they are closely related in practice; any technique for emission spectroscopy can also be used for absorption spectroscopy. First, the emission spectrum of a broadband lamp is measured (this is called the "background spectrum"). Second, the emission spectrum of the same lamp ''shining through the sample'' is measured (this is called the "sample spectrum"). The sample will absorb some of the light, causing the spectra to be different. The ratio of the "sample spectrum" to the "background spectrum" is directly related to the sample's absorption spectrum.

Accordingly, the technique of "Fourier-transform spectroscopy" can be used both for measuring emission spectra (for example, the emission spectrum of a star), ''and'' absorption spectra (for example, the absorption spectrum of a liquid).