Spontaneous Raman Scattering is the inelastic counterpart to Rayleigh scattering. Raman scattering shows a spectral response that is shifted from the laser line. This shift is characteristic for the Raman active molecules and allows to measure all major species concentrations at the same time. Major gas phase constituents like O2, N2, hydrocarbon fuels, CO2, H2 and H2O can be measured simultaneously together with the gas temperature.
The drawback of Raman scattering in the gas phase is the weakness of the signal, roughly six orders of magnitude less than Mie scattering. For 1D Raman imaging along a line focus multiple laser shots are averaged and high power laser are used. Raman like Rayleigh signals scale linearly with the molecular species’ density and do not suffer from collisional quenching effects involved in LIF processes.