Measurement Principles of Planar-LIF

image of laser induced fluorescence overview

Planar Laser-Induced Fluorescence (PLIF) is an optical measuring technique used to measure instant whole-field concentration or temperature maps in liquid and gaseous flows.

For mixing studies, typically one of the fluids is marked with the tracer compound, whereas the other is fresh fluid. A laser light sheet illuminates a thin plane in the flow and the tracer absorbs some of the light; the tracer is excited to a higher electronic energy state. In the subsequent return to a lower energy state a part of the excess energy is released as light at a longer wavelengths: commonly known as Fluorescence.

Commonly used dyes for measurements in liquids are rhodamine 6G (for concentration measurements), rhodamine B (for temperature measurements). Commonly used tracers for measurements in gas phase flows are ketons such as acetone.

For fluorescence detection, a camera equipped with a sharp cut-off or narrow-band filter is used so that only the fluorescent light is recorded. The level of fluorescence is known to vary with the concentration or the temperature and several other experimental parameters.

image of laser induced flourescence graph
Typical single-pixel temperature and concentration calibration curves

At low concentration levels, absorption is negligible, which leads to a linear relationship between the fluorescence signal and the concentration or temperature. The calibration procedure consists of determining a calibration constant at every pixel of the camera. From such a calibration map, the recorded raw images of fluorescence are converted to concentration or temperature maps by signal processing.

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