Planar laser-induced fluorescence (PLIF) is an optical measuring technique used to measure instant whole-field concentration or temperature maps in liquid and gaseous flows, reacting as well as non-reacting.
The Combustion-LIF system employs a laser light source with light sheet optics to illuminate a thin plane of the combustion process. The laser wavelength is tuned to correspond to an energy transition within the molecular species of interest, resulting in absorption of the light, thus leading to excitation of some of the molecules to a higher electronic energy state.
Due to collisions between the molecules in the gas, a redistribution of energy occurs immediately after the excitation, causing a population of closely lying energy states. In the subsequent return to a lower energy state a part of the excess energy is released as photons: commonly known as Fluorescence.
As a result of the energy redistribution, the fluorescence is to a large extent shifted towards longer wavelengths, relative to that of the excitation. Detection of the fluorescence is generally done at these red-shifted wavelengths so as to minimize interference from scattered laser light. A spectral filter is placed in front of the camera lens, allowing only the fluorescence to reach the intensified camera.