On the errors and their mitigation in planar laser induced fluorescence

In planar laser induced fluorescence (PLIF), scalar concentration fields are obtained from fluorescence images of the flow where the scalar is 'tagged' with a suitable fluorescent dye and illuminated with a laser sheet. While the fluorescence occurs, the intensity of the laser sheet is attenuated along its passage through the dye field. This paper discusses the effect of correcting for the attenuation of the laser by the dye while obtaining the scalar concentration field from the fluorescent images of the flow. If the attenuation correction cannot be done, the paper discusses three possible calibration scenarios to minimise the error. It is shown that the error in ignoring the attenuation due to turbulent concentration fluctuations cannot be corrected by any means. However, the remaining error can be exactly corrected if it were possible to calibrate with the mean concentration field. It is shown that a uniform concentration field, centred around the mean field, greatly reduces the error when compared to having no attenuation correction but still is significantly more when compared to the use of the average field. Relying on these results, we propose an iterative technique which ideally achieves the accuracy obtainable when calibrating with the mean concentration field, but which requires no reference cell. In addition, the paper explores the removal striations, of a common artefact during PLIF image acquisition with origins in the image optics. A technique to remove the same is proposed and its advantages discussed when compared to available techniques.