Analysis of signal attenuation for quantification of a planar imaging technique

Abstract

An optical patternator can measure the fuel mass distribution of a spray using laser induced fluorescence (LIF) images obtained from a planar imaging technique. In the dense spray region, however, attenuation by scattering from particles in the path of the detector is so significant that the true spray pattern might be altered into a false distribution. Therefore, the method to find the geometric mean value of the intensities obtained from two cameras was evaluated and verified in a solid-cone spray under the assumption that the optical depth is constant throughout the spray. The variation of the averaged optical depth detected by the two cameras was negligible in the present study, so that the deviation of signal attenuation was within ±5% of the average value. The measurement of the mass distribution at Z = 25 mm using a single camera was estimated to have a large range of signal attenuation from 0 to 17%, while the method using two cameras has a small range of signal attenuation from 6.5% to 12.5%. Therefore, the effect of signal attenuation can be dealt with by the two-camera method. This method was also applied to measure the size distribution by taking the ratio of fluorescence to Mie-scattering signals. It was found that Mie-scattering and LIF signals were attenuated at a similar rate within the range of the signal wavelength used in this measurement. Hence, the effect of signal attenuation on the size measurement was cancelled in the single image detection and agreed well with other techniques, such as PDPA.

Application of structured illumination for multiple scattering suppression in planar laser imaging of dense sprays

“…Another solution is to experimentally use bidirectional illumination by means of sequential excitations from counter-propagating laser sheets. Such techniques have been applied for both Planar Liquid Laser Induce Fluorescence and planar Mie imaging [13][14][15]. In addition to these corrections, other methods have been tested to account for signal attenuation [10,15].…”

“…Such techniques have been applied for both Planar Liquid Laser Induce Fluorescence and planar Mie imaging [13][14][15]. In addition to these corrections, other methods have been tested to account for signal attenuation [10,15]. However, due to the complexity in predicting and understanding effects introduced by multiple scattering, no experimental technique has been proposed, so far, for the correction of the secondary emission.…”

Planar Drop-Sizing in Dense Fuel Sprays Using Advanced Laser Diagnostic Techniques

“…Talley et al [58] proposed a correction for laser extinction using counterpropagating laser sheets. Koh et al [59] corrected signal attenuation using the geometric mean value of the intensities obtained using two cameras. The loss in the fluorescence signal due to auto-absorption of the fluorescence signal and absorption of the laser sheet can be corrected using Beer-Lambert law [52].…”

Corrections to facilitate planar imaging of particle concentration in particle-laden flows using Mie scattering, Part 1: Collimated laser sheets

“…Laser diagnostics of conical sprays has motivated much of this early work, and many preliminary and key papers appear in the proceedings of technical conferences rather than the archival literature. Great success has been demonstrated in correcting attenuation and signal trapping in conical sprays because the degree of axial symmetry enables out-of-plane information to be well inferred from inplane information [15][16][17][18]. This approach is limited in more complex cases with less instantaneous axisymmetry.…”

“…Koh et al [15] use another method based on the Beer-Lambert absorption within a conical spray. The Beer-Lambert corrections require a path length and attenuation coefficient for each pixel.…”