The fluorescence which is stimulated by an ultraviolet radiation is a special form of the luminescence. Thereby, electrons of fluorescent molecules reach a higher energy level by absorbing photons (as shown in the picture). Such a stimulated condition is energetically unfavorable and unstable. The electrons return immediately into their ground state where the absorbed energy is released again. At the same time an emission of the fluorescence light takes place. Due to the fact that a part of the energy is dissipated into heat, the emitted radiation has less energy and thus, a longer wavelength. Organic materials which are exposed to the UV light, often show a high self-fluorescence. Therefore, it is possible to detect contamination such as oil films as well as surfactants on surfaces.
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