The filtered Rayleigh scattering (FRS) approach relies upon the interaction of light with molecules to obtain scalar information and velocity in gas flows. Particularly when particle seeding is not appropriate, molecular approaches, such as FRS, offer non-intrusive measurements not obtainable otherwise.  Our lab frequently employs molecular iodine cells as optical filters for velocimetry, and the same filters can be used to deduce spectral information about the Rayleigh-scattered signal collected from a volume excited by a probe laser beam. By varying the optical thicnkess of the filter, filtered signals can be made more sensitive to flow density or temperature. Further, the scattered light filtered signal over a range of frequencies may be compared to the filtered signal of the incident laser probe beam to measure Doppler shift--a directionally sensitive measure of flow velocity.

Relevant publications:

Cadel D* and Lowe KT 2016 “Investigation of measurement sensitivities in cross-correlation Doppler global velocimetry,” Optics and Lasers in Engineering, 86, 44-55.

Cadel DR* and Lowe KT 2015 “Cross-correlation Doppler global velocimetry (CC-DGV),” Optics and Lasers in Engineering, 71, 51-61.

Yeaton I*, Maisto P* and Lowe KT 2012 “Time resolved filtered Rayleigh scattering for temperature and density measurements,” 28th AIAA Aerodynamic Measurement Technology, Ground Testing, and Flight Testing Conference, New Orleans, LA, 25-28 June, paper AIAA-2012-3200.