Advancements in Rayleigh scattering thermometry by means of structured illumination

Abstract:
Laser-induced Rayleigh scattering is commonly employed for two-dimensional temperature measurements and offers benefits such as high accuracy, easily interpreted data and low experimental complexity. Yet the approach suffers from an interference often referred to as stray light, an umbrella term used for all spurious light being detected. As Rayleigh scattering is an elastic scattering phenomenon, distinguishing between stray light and the signal of interest is not straightforward. In high-temperature environments, Rayleigh signals are weak due to low molecular densities, which make stray light interferences particularly cumbersome, impairing both the reliability and accuracy of Rayleigh thermometry, especially when applied in harsh combustion environments. In this paper we present an experimental solution to greatly mitigate this issue. The method, Structured Laser Illumination Planar Imaging (SLIPI), employs an intensity modulated laser light sheet to add a recognizable signature to the signal photons. This unique signature allows utilization of a post-processing algorithm that isolates and extracts the desired Rayleigh signal, thereby minimizing measurement uncertainties caused by stray light. The fidelity of the proposed method is first verified by comparing with conventional Rayleigh thermometry under ideal, i.e., stray light-free, measurement conditions. The technique is then employed under more realistic measurement conditions, where the results conclusively illustrate that the current operating range for Rayleigh thermometry can be increased significantly by means of SLIPI. (C) 2014 The Combustion Institute. Published by Elsevier Inc. All rights reserved.