To determine the infrared spectrum of a sample is a fine beam of infrared light shone through the sample. By comparison with a reference beam with the same light intensity is measured how much of the light of each frequency is absorbed by the sample. One thus determines the absorbance of waves having a wavelength of 2.5 micrometers to 17 micrometers (in the infrared spectroscopy it is often expressed as the number of waves per centimeter: 600 cm-1 to 4000 cm-1).
With the aid of modern infrared spectrometers, chemometrics and spectral data bases, a compound usually be identified uniquely from a fully infra-red spectrum.
By following the absorption of a certain frequency over time, the amount may be followed or the precise nature of a chemical bond. This can, for example, be used to follow a polymerization reaction. A modern device can be a whole range of up to 30 times per second, if desired together with other analyzes.
By making use of the characteristic absorption of specific substances, infra-red spectroscopy can also be used for the continuous measurement of the presence of certain components, such as the concentration of nitrogen oxides or carbon monoxide in flue gases.