A short, energetic laser pulse is used as the illumination source for LII. The illuminated soot particles absorb the light and are heated up by the laser pulse to very high temperatures (~4000 K).
As a consequence of the increased temperature, the soot particles emit increased levels of black body (Planck) radiation, Incandescence. The Incandescence is in turn related to the soot volume fraction, and with a proper calibration quantitative soot volume fraction measurements can be made.
The radiation is also wavelength shifted according to Wien’s displacement law. This makes it possible to spectrally distinguish the laser-induced incandescence from that occurring naturally at lower temperatures by means of appropriate camera filters.
Just like Laser-Induced Fluorescence (LIF), LII can be applied in a planar manner by expanding a laser beam into a light sheet and illuminating a thin plane of the process under investigation.