Infrared spectroscopy (IR) is based on the excitation of vibrations and rotations in molecules by irradiation with infrared light in the wavelength range from 0.8 to 1000 μm. Particularly the average infrared in the wavelength range from 2.5 to 25 μm (4000 to 400 wave numbers) is used in the analysis since in this area concrete statements about functional groups in the molecules can be made. These absorb the irradiated IR light at characteristic wavelengths, which makes identification possible. By comparing the recorded spectra with corresponding data base spectra, identity checks are also possible. The samples can be measured and characterized in reflection mode in the transmission mode and for materials that are impermeable to infrared radiation. The latter is performed either by ATR measurements (attenuated total reflection) or by the IR microscope.