CHARACTERIZATION AND CALIBRATION OF RHODAMINE 6G PLANAR LASER-INDUCED FLUORESCENCE FOR CONCENTRATION MEASUREMENTS

Abstract

Planar laser-induced fluorescence (PLIF) is commonly used to measure the transport of a fluorescent tracer in turbulent flows. To convert the recorded fluorescence intensity into concentration, a calibration is required for the optical arrangement and concentration range used in the experiment. In this study, Rhodamine 6G solutions of known concentration were used to calibrate a PLIF system for measurements in a laboratory flume. Twelve concentrations between 0.025 and 0.300 mg/L were tested under 532 nm pulsed illumination at eleven laser control settings. At the moderate and high laser settings, the response was approximately linear over 0.025–0.250 mg/L; the lower settings showed greater scatter, and the two highest concentrations departed from the restricted-range fits. Over the fitted range, the calibration slopes increased from 379.17 to 934.93 a.u. per mg/L and the corresponding  values ranged from 0.9606 to 0.9973. Setting 9 gave the smallest concentration-equivalent residual scale, whereas setting 20 gave the highest sensitivity. At setting 20, for example, the 0.300 mg/L standard was recovered as 0.2679 mg/L using the calibration fitted over 0.025–0.250 mg/L. Based on these results, 0.025–0.250 mg/L was taken as the practical working range for the present system, with separate calibration coefficients for each laser setting. The fitted equations describe the recorded intensity scale; applying them to plume images requires matching the calibration processing. Regression uncertainty is evaluated separately from the additional uncertainty in instantaneous concentration fields.