An application of the multiple-exposure speckle photography to the Rayleigh-Benard flow is described. A fluid seeded with a minute concentration of diffusing particles is illuminated by a laser light sheet. The multiple-exposure photograph of this sheet is analysed by the well-known techniques of speckle photography to obtain a bidimensional map of the instantaneous velocities.
"Flow Velocity Measurement By A Speckle Method", Proc. SPIE 0210, 2nd European Congress on Optics Applied to Metrology, (20 May 1980); doi: 10.1117/12.958310; https://doi.org/10.1117/12.958310