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Printed scattering particles for LDA calibration


For the first time printed scattering particles have been used for the calibration of Laser Doppler Anemometers (LDA). They are to contribute to a better reproducibility of calibration results of various LDA systems.

For the measurement of fluid velocities in a gas flow with Laser Doppler Anemometers (LDA) a preferred procedure is to spray liquid fluids into the flow. The surface tension of the liquid creates spherical droplets used as a scatterer by the LDA. When the LDA system needs calibration, solid scatterers are used that move with a well-known velocity on the surface of a moving body. These scatterers scatter light with variable quality because their shape and structure are not uniform. When they are selected for the calibration process, the selection criteria are not stringent and may change with the next calibration. Now for the first time lithographically printed scattering particles have been examined for better reproducibility.

Because plane particles reflect rather than scatter light, the artificial particles were provided with a randomized nanostructure which shows a strong light-scattering characteristic (Fig. 1). The particles were lithographically printed on a glass slide that was subsequently attached to a rotating glass disk for examinations. The instrument constant, i.e. the fringe distance of the LDA, can be evaluated from the set rotational frequency and the measured radial distance of the particles to the centre of the disk but also from the precise distance between the particles itself given by the lithographic process.

Due to the fixed position of the scattering particles all further LDAs can use the same selection for calibration and the dissemination of velocity stays consistent. Because the particles initially have different random structures - similar to real particles on the rotating disk - in a next step identical scattering particles with a rotational symmetric nanostructure will be used to reduce further the variance of velocity measurement which is now 0.15%.

Fig. 1: Backscattering and angular scattering of an artificial 10 µm scattering particle


[1] V. Strunck, N. Pape, T. Weimann, H. Müller: "Lithographie-Streuteilchen für die LDA-Kalibrierung", 20. Fachtagung, Lasermethoden in der Strömungsmesstechnik, veranstaltet von der Deutschen Gesellschaft für Laser-Anemometrie (GALA' 2012), 4. - 6. September 2012, Universität Rostock, ISBN 978-3-9805613-8-9, Beitrag 21, S. 1 – 6 (in German)

Contact person:

Volker Strunck, Dept. 1.4, WG 1.41, E-Mail: volker.strunck@ptb.de