Photoacoustic spectroscopy (PAS) is a spectroscopic method that makes use of the photoacoustic effect. In this process, a sample, such as a gas, is irradiated with modulated light of a predefined wavelength. A certain portion of this light energy is absorbed by the sample and converted into acoustic waves. These signals can be detected with a microphone and then evaluated. IR laser diodes are often used as the light source, as for many applications the specific wavelength (color) of the material to be examined lies in the infrared range. The light is modulated electronically or mechanically, e.g., by using a chopper. Gas molecules absorb part of the light when the light frequency matches an absorption band of the gas in the cell. The higher the gas concentration, the more light is absorbed. This generates heat, which leads to a pressure change in the measurement cell. Normally, such a pressure difference equalizes again immediately. This is different with light modulated by a chopper. Here, when it strikes the molecules, a pressure wave—and thus an acoustic signal—is generated, which can be detected with a microphone. Using photoacoustic spectroscopy, nitrogen dioxide (NO2) concentrations, among others, can be determined very precisely.