The effect of photobleaching and velocity fluctuations on single-point LIF measurements

 This study investigates the effect of photobleaching on laser-induced fluorescence (LIF) measurements. Photobleaching causes the fluorescence to be velocity-dependent, which is undesirable if quantitative measurements are being made. To quantify this effect, simultaneous and coincident measurements of fluorescence and velocity were made within the measuring volume of a three-beam laser-Doppler anemometer (LDA), using both fluorescein and rhodamine 6G dyes in water. In addition, analytical expressions were developed for photobleaching in the LDA measuring volume, and a parameter was identified which predicts the degree of velocity sensitivity. Fluorescein was found to be far more susceptible to photobleaching than rhodamine 6G. Finally, the impact of photobleaching on statistical quantities (such as scalar fluxes) obtained from simultaneous LDA/LIF measurements is discussed. Received: 28 October 1996/Accepted: 17 March 1997