Optical Emission Spectrometer is an instrument measures the intensity of several spectral lines simultaneously in the light emitted by the sample, when the atoms that compose it are exited by an external energy source. The analysed light is located approximately into the 150nm to 800nm wavelength range. The whole measuring system is therefore based on the physical phenomenon that is summarised as follows:

When certain energy is applied to an atom, some of its electrons change their orbit. When these electrons return to their initial orbit, a precise energy is restored in the form of a light at a determined wavelength. This is an atomic phenomenon, and consequently it is practically unaffected by the chemical or crystalline form of the atom. This means the instrument can determine, for example the quantity of silicon in steel; but will not give information about the form under this silicon is to be found. The following figure gives a rough representation of this excitation.

A sample containing several different elements will therefore produce light composed of wavelength specific to each of the elements. By separating these wavelengths by a dispersion system, the spectrometer can determine which elements are present, the intensity of each of these wavelengths being a function of the concentration of the considered element. By measuring this luminous intensity (with a photomultiplier) and by processing this information with a computer, the instrument can thus determine the concentration of the considered element.