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Scanning laser speckle photography for temperature field measurement in fluid flows

This paper describes a scanning laser speckle photography (SLSP) measurement system which is an advancement of a standard laser speckle photography (LSP) technique. The new SLSP set-up enlarges the measuring area of a conventional LSP which is restricted to the used optical expansion/collimation system. This is done by a mechanical movement of a set of mirrors in the beam path of a small expanded laser beam by a precise computer-controlled XY- positioner. Thus the magnitude of the measuring area is enlarged from the relative small cross section of the expanded/collimated laser beam to the movement distance of the positioner. Furthermore an additional advantage of the SLSP is the decrease of the exposure time for a specklegram due to the increased laser light intensity of a small expanded laser beam. The established SLSP measurement set-up is used to investigate the temperature field in a thermal boundary layer of a natural convection flow of air around a slender heated vertical cylindrical tube. Therefore the axis-symmetrical temperature field of the heated tube is vertically scanned by the moved laser beam and a specklegram is fixed on a holographic film by a double-exposure. The optical SLSP arrangement is described in detail and first experimental results of temperature measurements with the SLSP are presented.