On the use of light sheet methods in the presence of refraction by flames

Abstract

When a light sheet traverses a flame, refraction caused by optical density variations alters theillumination intensity downbeam. This can be a serious limitation for optical methods that presuppose a uniform intensity initially (or a distribution otherwise established by prior calibration) for the purposes of recording, for example, Mie or Rayleigh scattering, or planar laser-induced fluorescence. The theory of the light deflection is analyzed in terms of premixed flame parameters, and the consequent intensity redistribution is predicted as a function of distance downbeam. This indicates the magnitude of the problem and corresponds to observations on the experimental records that first drew attention to the phenomenon. The effects of temperature and pressure are predicted. The use of the theory to correct the results is assessed and, since that may prove a somewhat difficult process, various methods of correcting or avoiding the problem are put forward. They include optimizing the beam insertion, defining regions over which averaging conserves incident light energy, as well as a simple experimental technique based on breaking the sheet into individual thin beams, for example by introducing interference fringes in its plane. Their spacing may be varied so as to avoid overlap, and the recording system then operates on the total output from each pencil of light.