Abstract

What is believed to be a new concept for the measurement of micrometer-sized particle trajectories in an inlet air stream is introduced. The technique uses a light source and a mask to generate a spatial pattern of light within a volume in space. Particles traverse the illumination volume and elastically scatter light to a photodetector where the signal is recorded in time. The detected scattering waveform is decoded to find the particle trajectory. A design is presented for the structured laser beam, and the accuracy of the technique in determining particle position is demonstrated. It is also demonstrated that the structured laser beam can be used to measure and then correct for the spatially dependent instrument-response function of an optical-scattering-based particle-sizing system for aerosols.

© 2007 Optical Society of America

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References

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2006 (1)

2005 (1)

1999 (2)

1977 (1)

1970 (1)

Bartlett, K. G.

Bottiger, J. R.

Burnham, D. R.

Chang, R.

Chang, R. K.

Davitt, K.

Gherasimova, M.

Han, J.

Hill, S. C.

Holler, S.

Huffaker, R. M.

McGloin, D.

Niles, S.

Nurmikko, A.

Pan, Y. L.

Pan, Y.-L.

Patterson, W.

Pinnick, R. G.

Prather, K. A.

D. T. Suess and K. A. Prather, "Mass spectrometry of aerosols," Chem. Rev. 99, 3007-3035 (1999).
[CrossRef]

She, C. Y.

Song, Y.-K.

Suess, D. T.

D. T. Suess and K. A. Prather, "Mass spectrometry of aerosols," Chem. Rev. 99, 3007-3035 (1999).
[CrossRef]

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Figures (9)

Fig. 1
Fig. 1

(a) Illustration of the structured laser beam (SLB), which is propagating in the z direction. Particles moving in the minus y direction intersect four beams of light. The second and fourth beams of the SLB are projected to have a fixed spacing for all the particle trajectories and are used to measure particle velocity. The first and third beams have a variable spacing relative to the second and fourth beams and are used to encode the z axis and x axis positions of the particle, respectively. Three example trajectories (i), (ii), and (iii) are illustrated. (b) Illustrations of the elastic-scattering waveforms generated by particles traveling these three trajectories with equal velocities. The position of the first elastic-scattering peak shifts in time to encode the particle's z axis position, the third elastic-scattering peak shifts in time to encode the particle's x axis position, and the second and fourth peaks encode velocity and serve as reference peaks for the particle position along the y axis in time.

Fig. 2
Fig. 2

Diagram of the experimental setup used to test the SLB concept from two views: (a) side view and (b) top view. Light from a laser diode is collimated, and a portion of the collimated beam is deflected 2° using a wedge prism before illuminating the pattern on the mask. The mask is reimaged into the aerosol flow cell using a pair of achromats with 3x demagnification. Particles flowing in the flow cell traverse the SLB in the direction indicated by the arrow in the side view. The scattered light is collected by two independent detection systems as shown in the top view. An elliptical mirror collects a portion of the elastic scatter and images the light onto a PMT for detection of the SLB waveform as shown in the top view. A CCD camera and imaging lens collect images of the elastic scattering in the forward-scattering direction for verification purposes.

Fig. 3
Fig. 3

Images of the SLB as it propagates along the z axis inside the aerosol flow cell collected using the CCD camera with the beam block removed. The images correspond to the positions (a) z = 1.0   mm , (b) z = 0.0   mm , and (c) z = + 1.0   mm .

Fig. 4
Fig. 4

Trace of the photocurrent generated in the PMT due to the elastic scattering of a 3 .12   μm polystyrene sphere traversing the SLB. Decoding of the photocurrent waveform indicates the particle was moving at a speed of 5.77   m / s through the center of the SLB, z = 0.007   mm , and x = 0.005   mm .

Fig. 5
Fig. 5

Nominally in focus CCD image of the illumination volume as was seen in 3(b) displayed as gray. Overlaid on the CCD image is the CCD image collected for a 3 .12   μm PSL traversing the SLB seen as bright spots. The image of the particle traversing the SLB is binned in the vertical dimension to find the x axis location of the particle trajectory. The vertically binned image intensity is shown at the bottom of the figure.

Fig. 6
Fig. 6

Scatterplot of the x position of each polystyrene latex sphere traversing the SLB as determined by the PMT collected side scattering (abscissa) and by the CCD-camera-imaged forward scattering (ordinate). The rms difference between the SLB derived particle position and the forward-scattering imaging system position data is 8   μm , or 1.6 % of the 500   μm range.

Fig. 7
Fig. 7

Normalized histograms of time-average elastic-scattering signal for various locations in the SLB volume. The abscissa of (b) is plotted on a log scale. The dashed curve in (b) corresponds to the elastic-scatter signal for all the polystyrene latex (PSL) spheres in the data set displayed in (a). The solid curves in (b) marked (i) and (ii) are the elastic-scattering signal for the corresponding subsets of PSL spheres indicated by the boxes labeled (i) and (ii) in (a). For any given small subregion the scattering histogram is relatively tight compared to the overall scattering histogram.

Fig. 8
Fig. 8

Multiplicative instrument correction factor map for elastic scattering derived from the average of the three instrument-response function maps generated from the mean-normalized, time-average photocurrent signal of 0.966, 1.96, and 3 .12   μm diameter polystyrene latex spheres.

Fig. 9
Fig. 9

Normalized histograms of the elastic-scatter signal amplitude from the 0.966, 1.96, and 3 .12   μm diameter PSL spheres plotted on a log scale. The histograms labeled (a) are for the uncorrected, time-average photocurrent signals. The histograms labeled (b) show the benefit of correcting for the spatial dependence of the instrument-response function and each particle's residence time in the laser beam.

Equations (3)

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z = ( y 2 y 4 tan ( 6 ° ) ) ( y 1 y 2 y 2 y 4 1 2 ) = ( D tan ( 6 ° ) ) ( t 2 t 1 t 4 t 2 1 2 ) ,
x = ( y 2 y 4 0.2 ) ( y 2 y 3 y 2 y 4 1 2 ) = ( D 0.2 ) ( t 3 t 2 t 4 t 2 1 2 ) .
v = D t 4 t 2 .

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