Infrared spectroscopy (IR spectroscopy) is the subset of spectroscopy that deals with the infrared region of the electromagnetic spectrum. It covers a range of techniques, the most common being the "mid infrared" (MIR) region and the absorption spectroscopy. Infrared spectroscopy exploits the fact that molecules have specific frequencies at which they absorb the excitation energy and rotate or vibrate corresponding to discrete energy levels. The mid-infrared, approximately 6,000-700 cm-1 (1.7–14 µm), is typically used to study the fundamental vibrations and associated rotational-vibration molecular structures and is the most widely used FT-IR region for a wide variety of industrial applications. When infrared light is passed through an organic compound, some of the frequencies are absorbed, while other frequencies are transmitted through the sample without being absorbed. Plotting absorbance or transmittance against frequency or wave number will result in an infrared spectrum. All the peaks of the spectrum are not of analytical importance. The characteristic peaks are of main interest. Comparison of the peaks to previously run materials in a library can provide positive identification. The goal of any absorption spectroscopy (FTIR, UV-Vis spectroscopy, etc.) is to measure how well a sample absorbs light at each wavelength. Fourier Transform Spectroscopy shines a beam containing many frequencies of light at once, and measures how much of that beam is absorbed by the sample. Next, the beam is modified to contain a different combination of frequencies, giving a second data point. This process is repeated many times. Afterwards, a computer takes all this data and works backward to infer what the absorption is at each wavelength. The beam described above is generated by starting with a broadband light source—one containing the full spectrum of wavelengths to be measured. The light shines into a Michelson interferometer — a certain configuration of mirrors, one of which is moved by a motor. As this mirror moves, each wavelength of light in the beam is periodically blocked and transmitted by the interferometer, due to wave interference. Different wavelengths are modulated at different rates, so that at each moment, the beam coming out of the interferometer has a different spectrum. A computer is used to turn the raw data (light absorption for each mirror position) into the desired result (light absorption for each wavelength). The processing required turns out to be a common algorithm called the Fourier transform (hence the name, Fourier Transform Spectroscopy). The raw data is an interferogram.

Thanks to a decade of continuous technical development of FT-IR spectroscopy (increasing throughput, dynamic alignment, more sensitive detectors, brighter sources, increasing scanning speed, development of focal plane array detectors, flexible spectral manipulations and data handling, etc.), this method has developed a great number of new and unique applications. For example, the in situ IR absorption measurements, such as remote sensing using the sun, the sky, or natural hot objects as IR sources of radiation. These include a number of originally hot gaseous samples such as volcanic plumes, automobile gases, stack gas plumes, or flames related to environmental analysis of the atmosphere. There is a wide range of atmospheric environmental applications of FTIR spectroscopy; such as power plants, petrochemical and natural gas plants, waste disposals, agricultural sites, and industrial sites; as well as the detection of gases produced in flames, in biomass burning, and in flares.