FTIR is a material analyses technique aimed at obtaining the infrared absorption spectrum (typically in the 5 to 20 mm range) of solids, liquids, or gasses. The main advantage of a FTIR system versus a normal transmission-reflection measurement is the fact that a whole wavelength range can be measured at the same time instead of just on wavelength or a very narrow wavelength range requiring scanning and consequently significantly longer measurement times. Figure 1 shows the configuration of a typical FTIR system. Light is emanating from a light source that is approximately a black body radiator. This light is then collimated and directed to a beam splitter. Part of the light is transmitted to a fixed mirror, while the other part is transmitted towards a moving mirror. Light reflects back from both mirrors and part of the light is transmitted into the sample compartment of the FTIR system and focussed onto the sample. The resolution of the measurement can be changed by changing the optical path distance between the two light paths, whereby a larger optical path distance results in a higher measurement resolution at the cost of a longer measurement time. The detector records an interferogram which is a function of the moving mirror’s position. After the interferogram signal is collected, a Fourier transform is done by a computer to reconstruct the original infrared spectrum. This spectrum is typically expressed in wavenumbers (reciprocal wavelength with a unit of cm-1) as this unit is linear in energy.
In contrast to light absorption in the visible part of the spectrum, the infrared absorption of materials is mainly determined by molecular absorptions.
 “https://www.newport.com/n/introduction-to-ftir-spectroscopy,” 2018.