Rayleigh scattering

Rayleigh scattering is the elastic scattering of light by particles much smaller than the wavelength of the light. That is the case for gas phase molecules and, therefore, this method is suited for laser imaging in gases. Rayleigh scattering of sunlight by atmospheric molecules is the reason for the observed blue colour of the sky, because the scattering efficiency varies inversely with the fourth power of the wavelength. 

For a single component gas with known scattering cross section the Rayleigh signal is directly proportional to the gas density. The scattered light is almost at the same wavelength as the incident light, i. e. Rayleigh scattering is not species selective. Rayleigh scattering requires either constant gas composition or known mole fractions of all major species for the density measurement of a gas mixture. In some cases Rayleigh scattering is stronger for one species than another, and it can be used to image mixing processes such as fuel – air mixing.

When gas composition and pressure are known Rayleigh imaging allows to measure planar temperature fields (Rayleigh Thermometry). Rayleigh scattering is much weaker than Mie scattering but more than two orders of magnitude stronger than Spontaneous Raman Scattering. Incandescence from soot and Mie scattering are processes that can totally obscure the Rayleigh signal.

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