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Airborne particle shape and size classification from spatial light scattering profiles
J. Aerosol Sci., Vol. 23, No. 6, pp. 597 611, 1992 Printed in Great Britain.
0021 8502/92 $5.00+0.00 © 1992 Pergamon Press Ltd
AIRBORNE PARTICLE SHAPE AND SIZE CLASSIFICATION FROM SPATIAL LIGHT SCATTERING PROFILES P. H. KAYE,* E. HIRST,* J. M. CLARKt and F. MICHELI* * Electronics Research and Development Centre, Hatfield Polytechnic, Hatfield, Hertfordshire, ALl0 9AB, U.K. and t CBDE, Porton Down, Salisbury, Wiltshire, U.K. (Received 2 March 1992; and in final form 7 May 1992)
new instrument for the study of airborne particles is described. The instrument incorporates a laminar flow sampledeliverysystemwhich constrains airborne particles to traverse a focussed laser beam in single file. Particle transits through the beam are of 2-3 #s duration. The transient spatial intensity distributions of light scattered from individual particles are recorded by an intensified charge-coupled-device(CCD) camera as two-dimensional photon distributions. These distributions, or scattering profiles, represent the light scattered from the particles over approximately 84% of the total sphere of scattering. Preliminary results using standard latex spheres in the range 14.45/zm diameter have shown good agreement with Mie theory for both particle singletsand doublets. Preferential orientations of the doublets towards alignment with the sample delivery airflow have been observed. Further data recorded from samples of precision micromachined fibre particles of 12 #m length are also shown, and these further illustrate the orientational behaviour of the particles in the sample delivery airflow. The potential of the instrument for the classificationof airborne particles on the basis of shape and size is briefly discussed.
Abstract--A
INTRODUCTION
Since most airborne particles, either naturally occurring or m~n-made, are not perfect spheres, particle shape is an important parameter by which particle species may often be classified. Fibres or accicular particles, flakes, and deformable liquid droplets represent examples of such possible classifications with importance in occupational hygiene and environmental monitoring. Optical scattering techniques are widely used as a means of counting and sizing airborne particles on an individual basis, and are embodied in a number of commercial instruments. However, in general these instruments do not attempt to assess particle shape but rather attribute a spherical volume equivalent size to each measured particle, based upon an empirical or theoretical calibration function. The accuracy and validity of such results has been the subject of some considerable research by workers in the field (for example: Gebhart and Anselm, 1988; Bottlinger and Umhauer, 1988; Marshall et al., 1991). Recent work by the authors has attempted to assess the feasibility of using the spatial intensity distribution of light scattered by individual airborne particles to derive some indication of particle shape and correspondingly to allow particle sizing with increased confidence. This work resulted in the construction of an instrument (Kaye et al., 1990, 1991), in which airborne particles, primarily in the size range 1-10/~m equivalent sphere volume diameter, were constrained by laminar air-flow to pass in single-file through a focused laser beam at rates of up to 10,000 particles per second. A substantial proportion of the spatial light distribution scattered by each particle was captured by a system of reflective and refractive optics and directed to three photomultiplier detectors arranged symmetrically around the axis of the beam. The optical intensities received by these detectors were then interpreted to yield a semi-empirical factor dependent primarily on the shape and orientation of the scattering particle. Though the instrument proved useful in the classification of certain particle types, notably liquid droplets and fibres, it was evident that potentially valuable information relating to particle morphology was being lost by virtue of the low spatial resolution of the detector arrangement. This paper describes preliminary results on a new research instrument which overcomes this limited spatial resolution restriction. The instrument is designed to allow fundamental studies of light scattering from 597
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airborne particles with a view to establishing principles upon which real-time monitoring/ classification techniques may be based.
APPARATUS Figure 1 shows a simplified schematic diagram of the new laser scattering instrument. The radiation source (not shown) is a linearly polarized 15 mW helium-neon laser operating at 633 nm wavelength arranged with the beam axis perpendicular to the paper in Fig. 1. Radiation from the laser is directed through a beam expander, iris diaphragm, and cylindrical lens into the scattering chamber where it strikes a 45 ° front silvered mirror (labelled 1) with the plane of polarization of the beam being perpendicular to the plane of incidence at the mirror. The radiation is focused to an approximately ellipsoidal crosssection (3 mm width by 120 #m depth) at a point within the scattering chamber coincident with the transverse flow of incoming sample air. The unscattered beam then passes on to a second 45 ° mirror and is subsequently absorbed within a baffle chamber. Particle laden air is drawn in through the scattering chamber in laminar flow at a rate of approximately 1.51 min- 1, and is ensheathed by filtered air drawn in through lateral ports as in Fig. 1. This composite laminar flow is subsequently aerodynamically focused such that the sample air is constrained to a column of approximately 0.8 mm diameter at the intersection with the laser beam, thus defining a scattering volume. Individual particles in the sample air traverse the laser beam and produce pulses of scattered light of ~ 3/~s duration. Light scattered into angles between 30 ° and 141 ° to the beam direction is incident upon an ellipsoidal reflector whose first principal focus is coincident with the scattering volume. The reflected light thus is refocused to the second principal focus where it passes through an iris diaphragm before being collimated by a lens assembly and directed onto the photocathode of an intensified CCD (charge-coupled device) camera. The light falling on the photocathode is therefore a two-dimensional transform of the three-dimensional spatial intensity distribution falling onto the ellipsoidal reflector. This image is referred to as the scattering profile. Light scattered from the particle at angles between 5° and 30° to the beam direction passes through an aperture in the rear of the ellipsoidal reflector and is refocused onto a miniature photomultiplier tube (PMT) whose output is used to trigger image acquisition by the intensified CCD camera.
Image acquisition The intensified CCD camera utilizes a microchannel plate with an overall photon gain of ~ 200,000. This allows the CCD imaging device to register (well above CCD read-out noise) individual photons arriving at the photocathode of the intensifier. Since airborne particle transits through the scattering volume are obviously random in time, the camera is required to be of an asynchronous type, i.e. images are captured only when initiated by the trigger pulse derived from the PMT. For an airborne particle of, for example, 3/~m equivalent sphere diameter, the total photon flux arriving at the intensifier faceplate during the 3 #s beam transit may be estimated to be the order of several thousand photons. The CCD imaging device has a pixel resolution of 385 x 288, i.e. 110,880 pixels--far in excess of the number of photons, and thus the image of the spatial scattered light intensity profile which is captured may more accurately be described as a photon distribution map, and indeed the results illustrate this. Once an image of a scattering profile has been captured, the information is transferred in video format to a commercial frame-grabber processor card within a host computer. The data are subsequently stored to hard disc within the computer. Timing constraints on this data transfer result in an image capture rate of only 3-4 images per second, which, whilst adequate for experimentation purposes, is recognised to be inappropriate for a real-time monitoring application. (See Discussion section.)
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RESULTS To facilitate theoretical verification of scattering behaviour and establish the perfo finance of the instrument, preliminary results have been based upon the measurement of standard particulates such as polystyrene latex spheres and silicon dioxide fibres~ For each particle type, an aerosol was generated using a TSI Tri-Jet Aerosol Generator. The output from the generator was delivered via a short flexible polythene hose to a ballast chamber of 100 litres capacity. Sample air was then immediately drawn through the light scattering instrument from the ballast chamber at the aforementioned rate of 1.5 i rnin ~. For the preliminary results included in this paper no additional particle drying (beyond that achieved by the TSI generator) and no particle neutralization were undertaken. Whilst the results obtained from polystyrene latex spheres and silicon dioxide fibres indicated that they were effectively dry when sampled, the authors recognise that particle dryness and charge have an important effect on the particles' behaviour and are currently investigating t'urther these parameters and their effect on the instrument response.
Sphere singlets Figure 2a shows the experimental scattering profile image from the transit of a 1 l~m diameter polystyrene latex sphere. The laser beam propagation axis is orthogonal to the paper towards the centre of the image with the plane of polarization vertical. The outer circumference of the image corresponds to light scattered at 141 to the laser beam propagation axis. The innermost dark circle results from the aperture in the rear of the eUipsoidal reflector and its circumference corresponds to a scattering angle of 30: to the beam axis. The image was captured in a period of 2 #s. The dark vertically-oriented 'dumb-bell' shadow is an artifact of the instrument; the circular regions are caused by the ends of the airflow inlet and outlet tubes respectively and as such their centres correspond to scattering angles 90 ° directly above and below the particle respectively. The linear shadow connecting these circles is caused by light scattered from the particle being reflected from the ellipsoidal reflector before impinging upon the inlet and outlet tubes. Figure 2b shows theoretical Mie scattering intensities (Kerker. 1969} from a l~m diameter polystyrene sphere assuming a refractive index of 1.58. The figure shows both parallel and perpendicular polarization components, corresponding, respectively, to vertical and horizontal lines drawn through the centre of the image in Fig. 2a. The deep perpendicular polarization minimum predicted theoretically al approximately 70 ~ scattering angle is not present in the parallel polarization plane, and this is clearly demonstrated in the image in Fig. 2a as the minima to the left and right of the central shadow circle. These minima can be seen to virtually disappear at angles bisecting the horizontal and vertical planes of polarization, as would be expected. Whilst the matching between experiment and theory is good, the different formats of the data make direct comparison difficult. For this reason, computer models of theoretical greyscale scattering profile images were produced for specified scattering particles by computing the full angular intensity functions from Mie scattering theory and modifying these to take into account the geometric distortion produced by the optical configuration of the scattering chamber. A result of this theoretical mapping for a 1/zm polystyrene sphere is shown in Fig. 2c which more clearly illustrates the good match between theoretical and experimental data. Figure 2d shows the same data in the form of a combined experimental/theoretical profile, facilitating direct comparison. One aspect of importance which may be observed in the experimental data and has been verified by theoretical ray tracing of the scattered light within the instrument is the critical dependence of the scattered intensity profile images on the exact trajectory of the particle through the scattering volume. Although the scattering volume is only 0.8 mm in diameter centred on the focus of the ellipsoidal reflector, particle trajectories close to the edge of the volume result in a measurable asymmetry in the scattering profile, particularly at scattering angles below 90 °. This is illustrated in Figs 2d and 3, the latter showing a combined
Airborne particle classification
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Angle Fig. 2a. Transient spatial light scattering profile recorded from a 1 •m diameter polystyrene latex sphere. The image was recorded in 2 ~s, and each white dot corresponds to a single scattered photon arrival at the image intensifier. The laser beam propagation axis is orthogonal to the paper towards the centre of the image. The inner dark circle circumference corresponds to scattering at 30 ° to the beam axis, and the outer circumference to scattering at 141 °. The vertical 'dumb-bell' shadow is caused by particle airflow inlet and vent tubes, with the centres of the circular ends corresponding to 90 ~ scattering above and below the particle, respectively. Fig. 2b. Theoretical Mie scattering from a 1 ,um polystyrene sphere for vertically polarized illumination at 633 nm. The deep minimum at 70 ° scattering for perpendicular polarization, virtually absent in the parallel polarization case, can clearly be seen in the experimental data of Fig. 2a.
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Fig. 2c. Theoretical model of the scattering profile from a I pm diameter polystyrene la ‘1he sealttering intensity data were computed from Mie theory and then corrected for I distribution variation resulting from the optical geometry of the instrument. Fig. 2d. Combined experimental and theoretical profiles for a 1 Mm polystyrene sphere direct comparison.
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Airborne particle classification
Fig. 3. Combined experimental and theoretical scattering profiles for a 4.3/~m polystyrene sphere. The failure of the patterns to match on the right of centre is a result of geometric distortion in the experimental data caused by the particle traversing the scattering volume fractionally to the left of the exact focus of the ellipsoidal reflector. Fig. 4a. Experimental scattering profile recorded from a single droplet sampled from an aerosol of distilled water (image capture time 3 ~s.) Fig. 4b. Best-fit theoretical profile to the experimental data of Fig. 4a. The theoretical data correspond to a spherical droplet of 4.0 ~tm diameter.
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Fig. 5a. Experimental scattering profile recorded from a particle drawn from an aerosol of 2.95 #m polystyrene spheres (image capture time 3 #s). The particle was identified as a doublet of 2.95 #m spheres aligned almost parallel to the vertical sample airflow, as verified by the theoretical data in Fig. 5d. Fig. 5b. Theoretical scattering profile corresponding to two 2.95 #m polystyrene spheres aligned vertically with centre separation of 50 #m.
Airborne particle classification
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Fig. 6a. Experimental scattering profile recorded from a doublet i n an aerosol of 1 ~m diameter polystyrene spheres (image capture time 3 9s). Fig. 6b. Theoretical scattering profile corresponding to a vertically aligned 1 #m sphere doublet, cf. Fig. 6a.
Airborne particle classification
Fig. 7. Scanning electron micrograph of particles of silicon dioxide manufactured by the process of silicon micromachining and used in the scattering profile investigations. The particles are of 12 #m length (nominally +/-0.2/am) and 1.5/am by 1.5/zm cross-section.
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Fig. 8a. Experimental scattering profile recorded from a single 12/tm silicon dioxide fibre (image capture in 2 #s). The horizontal scattering implies close alignment of the particle with the vertical sample airflow. Fig. 8b. Experimental scattering profile recorded from a single 12/~m silicon dioxide fibre (image capture in 2 #s). In this case the conic section form of the scattering implies a particle orientation approximately 10° clockwise to the vertical in the plane orthogonal to the laser beam axis, and 1 5 tilted forward (towards the laser) in the plane of the beam axis.
Airborne particle classification
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experimental/theoretical scattering profile image for a 4.3 #m diameter polystyrene sphere. Whilst experimental and theoretical data in both figures show good agreement on one side of the central region, there exists a mismatch of the patterns on the opposite side. This is due in both cases to geometric distortion in the experimental data caused by particle trajectories marginally to the side of the geometric focus of the ellipsoidal reflector. Figure 4a shows the experimental profile of light scattered from a single droplet sampled from an aerosol of distilled water. (In this case the aerosol was generated directly into the ballast chamber using a high-pressure Venturi spray.) The pattern exhibits a relative intensity of scattering at high angles (>90 °) greater than would be expected from a polystyrene sphere of equivalent size. Figure 4b shows the best-fit theoretical scattering profile which corresponds to a sphere of 4.0 #m diameter. The absence of measurable distortion or radial asymmetry of the ring pattern in the experimental data suggests a corresponding lack of any distortion of the droplet in the air-flow delivery system. This is counter to earlier results by the authors (Kaye et al., 1991) which suggested that such distortion did occur with similar flow conditions. Sphere doublets A small fraction (typically -~ 8 %) of profiles recorded from each of the polystyrene sphere aerosols were of strikingly different form. Figure 5a shows such a profile recorded from a sample aerosol of 2.95 #m polystyrene spheres. These profiles were intuitively attributed to sphere doublets, and this was confirmed by theoretical modelling, again using Mie computations. Figures 5b, 5c and 5d show theoretical profiles for two 2.95 #m spheres vertically aligned and with centre separations of 50 #m, 5 #m and 2.95 #m, respectively. The figures show graphically the increasing profile perturbation with decreasing separation until the final profile in Fig. 5d, corresponding to particles in contact, closely matches the experimental data of Fig. 5a. Figures 6a and 6b show similar experimental and theoretical profiles for doublets of 1/~m polystyrene spheres. (It should be noted here that the theoretical model used to compute the scattering from doublets was simplified in that it ignored interaction phenomena between the spheres and considered only far-field phase differences in the scattering from each sphere. The close match between this theoretical model and experimental data suggests that any perturbation caused by interaction phenomena within a doublet would be difficult to detect experimentally.) One feature of the results obtained with particle doublets of all sizes used was the degree of orientation exhibited by the doublets in the air-flow. To a first approximation 90% of all doublets, including 1 #m doublets, exhibited vertical orientation (parallel to the air-flow) to within a cone of semi-angle 30 °. The relative ease with which the orientation of doublets could be determined suggests that these particle types may prove a convenient vehicle for investigating the optimisation of non-spherical particle orientation by aerodynamic or electrostatic means. This is one of the objectives of future work since the effective control of non-spherical particle orientation would considerably enhance the interpretation of particle shape and size characteristics. Silicon dioxide fibres Silicon dioxide fibres have been manufactured by the process of silicon micromachining, a modification of microchip fabrication technology which offers unparalleled uniformity and reproducibility for non-spherical particle geometries. This technique for precision particle production was first employed by Hoover et al. (1990), and has been marginally modified for our own purposes (Kaye, 1991). Figure 7 shows an electron micrograph of a sample of fibres produced by silicon micromachining. The fibres are of 12/~m length (nominally + / - 0.2/~m), with an essentially square cross-section of 1.5 #m by 1.5 #m. Figures 8a and 8b show scattering profile images recorded from individual fibres in the sample airflow. In Fig. 8a the horizontal linear scattering implies particle orientation with the long axis closely aligned to the airflow. Intensity maxima and minima are visible in the scattering profile at
610
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comparable angular positions to those that would be expected to a first order for a circular cross-section fibre of similar dimensions. Figure 8b shows a scattering profile image for the transit through the beam of a fibre with non-vertical orientation. In this case the scattering profile is modified to a conic section as predicted by theory (see, for example, Bohren and Huffman, 1983). In this example, the fibre orientation is approximately t0 c' clockwise to the vertical in the plane orthogonal to the laser beam axis, and tilted 15° (upper end of the fibre towards the laser) in the plane containing the beam axis. To a first approximation, 90% of" the fibres of this size were oriented vertically to within a cone of 20 ° semiangle. FJbres of other sizes down to 3 #m length and aspect ratios (down to 2: l) have also been produced by silicon micromachining, and these will provide a basis for further studies into the interrelationship between particle aspect ratio and orientational behaviour irt the instrument's sample delivery system (see below). DISCUSSION This paper describes preliminary results obtained from a new instrument which records the major portion of the spatial intensity distribution of light scattered from individual airborne particles in flow. As such it offers potential for more fully understanding the optical scattering behaviour of airborne particles in flow and provides a platform to address the development of systems capable of real-time particle classification on the basis of particle shape and size. The authors are about to embark on a programme of experimentation examining the scattering profiles of a range of high precision non-spherical particles produced by silicon micromachining. It is intended to investigate the interrelationships, both experimentally and by theoretical modelling, of the alignment behaviour of elongated particles of increasing aspect ratio, and the behaviour of other particle species such as flat round discs, so as to optimize the flow conditions of the sample delivery system in the instrument and develop classification methodologies. These results will form the basis of a further communication. The instrument has been designed to allow, if desired, the positional interchange of the camera and the photomultiplier detector so as to enable the camera to record scattering profiles corresponding to the lower angular range of 5 ° and 30 ° to the beam direction. With the photomultiplier detector removed altogether and a different method of particle detection triggering employed, the instrument may be made to operate with a camera at each end of the scattering chamber, thus covering the wider scattering angle range of 5°-14t °~ This modification would offer potentially improved particle characterization capability (though at the expense of greater data processing). Finally, the use of a high-resolution CCD camera to image the scattering profiles provides valuable information detail but reduces the particle data processing rate to an extent that would not be appropriate for a real-time monitoring instrument. A principal objective of future work is to use the data produced by the camera system to allow the design of simplified custom discrete detector configurations (of typically 20--30 photodetector elements) which would replace the CCD camera and allow efficient particle classification at much higher particle throughput rates, thus providing the basis of a real-time monitoring capability. Acknowledaements--This work has been supported by a grant under the Joint Scienceand EngineeringResearch Council/Ministryof Defencescheme.The authors wish to expresstheir gratitude for this support. The manufacture of the silicon microparticles was supported by a Science and Engineering Research Council grant under their Nanotechnology Initiative. The majority of the manufacturing process was carried out under contract by A. Gundlach at the Edinburgh University Microfabrieation Unit, whose expertiseis greatly appreciated by the authors.
REFERENCES Bohren, C. F. and Huffman,D. R. (1983)Absorption and Scattering of Light by Small Particles. Wiley,New York, Bottlinger, M. and Umhauer, H. (1988) Optical Particle Sizin#: Theory and Practice. Int. Syrup., pp. 363-369. Plenum Press, New York.
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Gebhart, J. and Anselm, A. (1988) Optical Particle Sizing: Theory and Practice. Int. Symp., pp. 393-409. Plenum Press, New York. Hoover, M. D., Casalnuovo, S. A., Lipowicz, P. J., Hsu Chi Yeh, Hanson, R. W. and Hurd, A. J. (1990) J. Aerosol Sci. 21, 569-575. Kaye, P. H. (1991) Proc. 5th Annual Conf. of the Aerosol Society. Loughborough Univ., U.K. 25-27 March, pp. 223-228. Kaye, P. H., Eyles, N. A. and Clark, J. M. (1990) Proc. 2nd Int. Congress on Optical Particle Sizing. Univ. of Arizona, Pheonix, 5-8 March, pp. 501-510. Kaye, P. H., Eyles, N. A., Ludlow, I. K. and Clark, J. M. (1991) Atmos. Envir. 25A, 645~554. Kerker, M. (1969) The Scattering of Li#ht and other Electromagnetic Radiation. Academic Press, New York. Marshall, I. A, Mitchell, J. P. and Griffiths, W. D. (1991) J. Aerosol Sci. 22, 73-89.
0021 8502/92 $5.00+0.00 © 1992 Pergamon Press Ltd
AIRBORNE PARTICLE SHAPE AND SIZE CLASSIFICATION FROM SPATIAL LIGHT SCATTERING PROFILES P. H. KAYE,* E. HIRST,* J. M. CLARKt and F. MICHELI* * Electronics Research and Development Centre, Hatfield Polytechnic, Hatfield, Hertfordshire, ALl0 9AB, U.K. and t CBDE, Porton Down, Salisbury, Wiltshire, U.K. (Received 2 March 1992; and in final form 7 May 1992)
new instrument for the study of airborne particles is described. The instrument incorporates a laminar flow sampledeliverysystemwhich constrains airborne particles to traverse a focussed laser beam in single file. Particle transits through the beam are of 2-3 #s duration. The transient spatial intensity distributions of light scattered from individual particles are recorded by an intensified charge-coupled-device(CCD) camera as two-dimensional photon distributions. These distributions, or scattering profiles, represent the light scattered from the particles over approximately 84% of the total sphere of scattering. Preliminary results using standard latex spheres in the range 14.45/zm diameter have shown good agreement with Mie theory for both particle singletsand doublets. Preferential orientations of the doublets towards alignment with the sample delivery airflow have been observed. Further data recorded from samples of precision micromachined fibre particles of 12 #m length are also shown, and these further illustrate the orientational behaviour of the particles in the sample delivery airflow. The potential of the instrument for the classificationof airborne particles on the basis of shape and size is briefly discussed.
Abstract--A
INTRODUCTION
Since most airborne particles, either naturally occurring or m~n-made, are not perfect spheres, particle shape is an important parameter by which particle species may often be classified. Fibres or accicular particles, flakes, and deformable liquid droplets represent examples of such possible classifications with importance in occupational hygiene and environmental monitoring. Optical scattering techniques are widely used as a means of counting and sizing airborne particles on an individual basis, and are embodied in a number of commercial instruments. However, in general these instruments do not attempt to assess particle shape but rather attribute a spherical volume equivalent size to each measured particle, based upon an empirical or theoretical calibration function. The accuracy and validity of such results has been the subject of some considerable research by workers in the field (for example: Gebhart and Anselm, 1988; Bottlinger and Umhauer, 1988; Marshall et al., 1991). Recent work by the authors has attempted to assess the feasibility of using the spatial intensity distribution of light scattered by individual airborne particles to derive some indication of particle shape and correspondingly to allow particle sizing with increased confidence. This work resulted in the construction of an instrument (Kaye et al., 1990, 1991), in which airborne particles, primarily in the size range 1-10/~m equivalent sphere volume diameter, were constrained by laminar air-flow to pass in single-file through a focused laser beam at rates of up to 10,000 particles per second. A substantial proportion of the spatial light distribution scattered by each particle was captured by a system of reflective and refractive optics and directed to three photomultiplier detectors arranged symmetrically around the axis of the beam. The optical intensities received by these detectors were then interpreted to yield a semi-empirical factor dependent primarily on the shape and orientation of the scattering particle. Though the instrument proved useful in the classification of certain particle types, notably liquid droplets and fibres, it was evident that potentially valuable information relating to particle morphology was being lost by virtue of the low spatial resolution of the detector arrangement. This paper describes preliminary results on a new research instrument which overcomes this limited spatial resolution restriction. The instrument is designed to allow fundamental studies of light scattering from 597
598
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Airborne particle classification
599
airborne particles with a view to establishing principles upon which real-time monitoring/ classification techniques may be based.
APPARATUS Figure 1 shows a simplified schematic diagram of the new laser scattering instrument. The radiation source (not shown) is a linearly polarized 15 mW helium-neon laser operating at 633 nm wavelength arranged with the beam axis perpendicular to the paper in Fig. 1. Radiation from the laser is directed through a beam expander, iris diaphragm, and cylindrical lens into the scattering chamber where it strikes a 45 ° front silvered mirror (labelled 1) with the plane of polarization of the beam being perpendicular to the plane of incidence at the mirror. The radiation is focused to an approximately ellipsoidal crosssection (3 mm width by 120 #m depth) at a point within the scattering chamber coincident with the transverse flow of incoming sample air. The unscattered beam then passes on to a second 45 ° mirror and is subsequently absorbed within a baffle chamber. Particle laden air is drawn in through the scattering chamber in laminar flow at a rate of approximately 1.51 min- 1, and is ensheathed by filtered air drawn in through lateral ports as in Fig. 1. This composite laminar flow is subsequently aerodynamically focused such that the sample air is constrained to a column of approximately 0.8 mm diameter at the intersection with the laser beam, thus defining a scattering volume. Individual particles in the sample air traverse the laser beam and produce pulses of scattered light of ~ 3/~s duration. Light scattered into angles between 30 ° and 141 ° to the beam direction is incident upon an ellipsoidal reflector whose first principal focus is coincident with the scattering volume. The reflected light thus is refocused to the second principal focus where it passes through an iris diaphragm before being collimated by a lens assembly and directed onto the photocathode of an intensified CCD (charge-coupled device) camera. The light falling on the photocathode is therefore a two-dimensional transform of the three-dimensional spatial intensity distribution falling onto the ellipsoidal reflector. This image is referred to as the scattering profile. Light scattered from the particle at angles between 5° and 30° to the beam direction passes through an aperture in the rear of the ellipsoidal reflector and is refocused onto a miniature photomultiplier tube (PMT) whose output is used to trigger image acquisition by the intensified CCD camera.
Image acquisition The intensified CCD camera utilizes a microchannel plate with an overall photon gain of ~ 200,000. This allows the CCD imaging device to register (well above CCD read-out noise) individual photons arriving at the photocathode of the intensifier. Since airborne particle transits through the scattering volume are obviously random in time, the camera is required to be of an asynchronous type, i.e. images are captured only when initiated by the trigger pulse derived from the PMT. For an airborne particle of, for example, 3/~m equivalent sphere diameter, the total photon flux arriving at the intensifier faceplate during the 3 #s beam transit may be estimated to be the order of several thousand photons. The CCD imaging device has a pixel resolution of 385 x 288, i.e. 110,880 pixels--far in excess of the number of photons, and thus the image of the spatial scattered light intensity profile which is captured may more accurately be described as a photon distribution map, and indeed the results illustrate this. Once an image of a scattering profile has been captured, the information is transferred in video format to a commercial frame-grabber processor card within a host computer. The data are subsequently stored to hard disc within the computer. Timing constraints on this data transfer result in an image capture rate of only 3-4 images per second, which, whilst adequate for experimentation purposes, is recognised to be inappropriate for a real-time monitoring application. (See Discussion section.)
600
~. H, KAYE et aL
RESULTS To facilitate theoretical verification of scattering behaviour and establish the perfo finance of the instrument, preliminary results have been based upon the measurement of standard particulates such as polystyrene latex spheres and silicon dioxide fibres~ For each particle type, an aerosol was generated using a TSI Tri-Jet Aerosol Generator. The output from the generator was delivered via a short flexible polythene hose to a ballast chamber of 100 litres capacity. Sample air was then immediately drawn through the light scattering instrument from the ballast chamber at the aforementioned rate of 1.5 i rnin ~. For the preliminary results included in this paper no additional particle drying (beyond that achieved by the TSI generator) and no particle neutralization were undertaken. Whilst the results obtained from polystyrene latex spheres and silicon dioxide fibres indicated that they were effectively dry when sampled, the authors recognise that particle dryness and charge have an important effect on the particles' behaviour and are currently investigating t'urther these parameters and their effect on the instrument response.
Sphere singlets Figure 2a shows the experimental scattering profile image from the transit of a 1 l~m diameter polystyrene latex sphere. The laser beam propagation axis is orthogonal to the paper towards the centre of the image with the plane of polarization vertical. The outer circumference of the image corresponds to light scattered at 141 to the laser beam propagation axis. The innermost dark circle results from the aperture in the rear of the eUipsoidal reflector and its circumference corresponds to a scattering angle of 30: to the beam axis. The image was captured in a period of 2 #s. The dark vertically-oriented 'dumb-bell' shadow is an artifact of the instrument; the circular regions are caused by the ends of the airflow inlet and outlet tubes respectively and as such their centres correspond to scattering angles 90 ° directly above and below the particle respectively. The linear shadow connecting these circles is caused by light scattered from the particle being reflected from the ellipsoidal reflector before impinging upon the inlet and outlet tubes. Figure 2b shows theoretical Mie scattering intensities (Kerker. 1969} from a l~m diameter polystyrene sphere assuming a refractive index of 1.58. The figure shows both parallel and perpendicular polarization components, corresponding, respectively, to vertical and horizontal lines drawn through the centre of the image in Fig. 2a. The deep perpendicular polarization minimum predicted theoretically al approximately 70 ~ scattering angle is not present in the parallel polarization plane, and this is clearly demonstrated in the image in Fig. 2a as the minima to the left and right of the central shadow circle. These minima can be seen to virtually disappear at angles bisecting the horizontal and vertical planes of polarization, as would be expected. Whilst the matching between experiment and theory is good, the different formats of the data make direct comparison difficult. For this reason, computer models of theoretical greyscale scattering profile images were produced for specified scattering particles by computing the full angular intensity functions from Mie scattering theory and modifying these to take into account the geometric distortion produced by the optical configuration of the scattering chamber. A result of this theoretical mapping for a 1/zm polystyrene sphere is shown in Fig. 2c which more clearly illustrates the good match between theoretical and experimental data. Figure 2d shows the same data in the form of a combined experimental/theoretical profile, facilitating direct comparison. One aspect of importance which may be observed in the experimental data and has been verified by theoretical ray tracing of the scattered light within the instrument is the critical dependence of the scattered intensity profile images on the exact trajectory of the particle through the scattering volume. Although the scattering volume is only 0.8 mm in diameter centred on the focus of the ellipsoidal reflector, particle trajectories close to the edge of the volume result in a measurable asymmetry in the scattering profile, particularly at scattering angles below 90 °. This is illustrated in Figs 2d and 3, the latter showing a combined
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Angle Fig. 2a. Transient spatial light scattering profile recorded from a 1 •m diameter polystyrene latex sphere. The image was recorded in 2 ~s, and each white dot corresponds to a single scattered photon arrival at the image intensifier. The laser beam propagation axis is orthogonal to the paper towards the centre of the image. The inner dark circle circumference corresponds to scattering at 30 ° to the beam axis, and the outer circumference to scattering at 141 °. The vertical 'dumb-bell' shadow is caused by particle airflow inlet and vent tubes, with the centres of the circular ends corresponding to 90 ~ scattering above and below the particle, respectively. Fig. 2b. Theoretical Mie scattering from a 1 ,um polystyrene sphere for vertically polarized illumination at 633 nm. The deep minimum at 70 ° scattering for perpendicular polarization, virtually absent in the parallel polarization case, can clearly be seen in the experimental data of Fig. 2a.
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Fig. 2c. Theoretical model of the scattering profile from a I pm diameter polystyrene la ‘1he sealttering intensity data were computed from Mie theory and then corrected for I distribution variation resulting from the optical geometry of the instrument. Fig. 2d. Combined experimental and theoretical profiles for a 1 Mm polystyrene sphere direct comparison.
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Airborne particle classification
Fig. 3. Combined experimental and theoretical scattering profiles for a 4.3/~m polystyrene sphere. The failure of the patterns to match on the right of centre is a result of geometric distortion in the experimental data caused by the particle traversing the scattering volume fractionally to the left of the exact focus of the ellipsoidal reflector. Fig. 4a. Experimental scattering profile recorded from a single droplet sampled from an aerosol of distilled water (image capture time 3 ~s.) Fig. 4b. Best-fit theoretical profile to the experimental data of Fig. 4a. The theoretical data correspond to a spherical droplet of 4.0 ~tm diameter.
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Fig. 5a. Experimental scattering profile recorded from a particle drawn from an aerosol of 2.95 #m polystyrene spheres (image capture time 3 #s). The particle was identified as a doublet of 2.95 #m spheres aligned almost parallel to the vertical sample airflow, as verified by the theoretical data in Fig. 5d. Fig. 5b. Theoretical scattering profile corresponding to two 2.95 #m polystyrene spheres aligned vertically with centre separation of 50 #m.
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Fig. 5c. Theoretical scattering profile corresponding to two 2.95/tm polystyrenespheres aligned vertically with centre separation of 5 #m. Fig. 5d. Theoretical scattering profile corresponding to two 2.95/~m polystyrenespheres aligned vertically with centre separation of 2.95/~m,i.e. the spheres are in contact.The nine central fringes match the experimentaldata of Fig. 5a.
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Fig. 6a. Experimental scattering profile recorded from a doublet i n an aerosol of 1 ~m diameter polystyrene spheres (image capture time 3 9s). Fig. 6b. Theoretical scattering profile corresponding to a vertically aligned 1 #m sphere doublet, cf. Fig. 6a.
Airborne particle classification
Fig. 7. Scanning electron micrograph of particles of silicon dioxide manufactured by the process of silicon micromachining and used in the scattering profile investigations. The particles are of 12 #m length (nominally +/-0.2/am) and 1.5/am by 1.5/zm cross-section.
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Fig. 8a. Experimental scattering profile recorded from a single 12/tm silicon dioxide fibre (image capture in 2 #s). The horizontal scattering implies close alignment of the particle with the vertical sample airflow. Fig. 8b. Experimental scattering profile recorded from a single 12/~m silicon dioxide fibre (image capture in 2 #s). In this case the conic section form of the scattering implies a particle orientation approximately 10° clockwise to the vertical in the plane orthogonal to the laser beam axis, and 1 5 tilted forward (towards the laser) in the plane of the beam axis.
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experimental/theoretical scattering profile image for a 4.3 #m diameter polystyrene sphere. Whilst experimental and theoretical data in both figures show good agreement on one side of the central region, there exists a mismatch of the patterns on the opposite side. This is due in both cases to geometric distortion in the experimental data caused by particle trajectories marginally to the side of the geometric focus of the ellipsoidal reflector. Figure 4a shows the experimental profile of light scattered from a single droplet sampled from an aerosol of distilled water. (In this case the aerosol was generated directly into the ballast chamber using a high-pressure Venturi spray.) The pattern exhibits a relative intensity of scattering at high angles (>90 °) greater than would be expected from a polystyrene sphere of equivalent size. Figure 4b shows the best-fit theoretical scattering profile which corresponds to a sphere of 4.0 #m diameter. The absence of measurable distortion or radial asymmetry of the ring pattern in the experimental data suggests a corresponding lack of any distortion of the droplet in the air-flow delivery system. This is counter to earlier results by the authors (Kaye et al., 1991) which suggested that such distortion did occur with similar flow conditions. Sphere doublets A small fraction (typically -~ 8 %) of profiles recorded from each of the polystyrene sphere aerosols were of strikingly different form. Figure 5a shows such a profile recorded from a sample aerosol of 2.95 #m polystyrene spheres. These profiles were intuitively attributed to sphere doublets, and this was confirmed by theoretical modelling, again using Mie computations. Figures 5b, 5c and 5d show theoretical profiles for two 2.95 #m spheres vertically aligned and with centre separations of 50 #m, 5 #m and 2.95 #m, respectively. The figures show graphically the increasing profile perturbation with decreasing separation until the final profile in Fig. 5d, corresponding to particles in contact, closely matches the experimental data of Fig. 5a. Figures 6a and 6b show similar experimental and theoretical profiles for doublets of 1/~m polystyrene spheres. (It should be noted here that the theoretical model used to compute the scattering from doublets was simplified in that it ignored interaction phenomena between the spheres and considered only far-field phase differences in the scattering from each sphere. The close match between this theoretical model and experimental data suggests that any perturbation caused by interaction phenomena within a doublet would be difficult to detect experimentally.) One feature of the results obtained with particle doublets of all sizes used was the degree of orientation exhibited by the doublets in the air-flow. To a first approximation 90% of all doublets, including 1 #m doublets, exhibited vertical orientation (parallel to the air-flow) to within a cone of semi-angle 30 °. The relative ease with which the orientation of doublets could be determined suggests that these particle types may prove a convenient vehicle for investigating the optimisation of non-spherical particle orientation by aerodynamic or electrostatic means. This is one of the objectives of future work since the effective control of non-spherical particle orientation would considerably enhance the interpretation of particle shape and size characteristics. Silicon dioxide fibres Silicon dioxide fibres have been manufactured by the process of silicon micromachining, a modification of microchip fabrication technology which offers unparalleled uniformity and reproducibility for non-spherical particle geometries. This technique for precision particle production was first employed by Hoover et al. (1990), and has been marginally modified for our own purposes (Kaye, 1991). Figure 7 shows an electron micrograph of a sample of fibres produced by silicon micromachining. The fibres are of 12/~m length (nominally + / - 0.2/~m), with an essentially square cross-section of 1.5 #m by 1.5 #m. Figures 8a and 8b show scattering profile images recorded from individual fibres in the sample airflow. In Fig. 8a the horizontal linear scattering implies particle orientation with the long axis closely aligned to the airflow. Intensity maxima and minima are visible in the scattering profile at
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comparable angular positions to those that would be expected to a first order for a circular cross-section fibre of similar dimensions. Figure 8b shows a scattering profile image for the transit through the beam of a fibre with non-vertical orientation. In this case the scattering profile is modified to a conic section as predicted by theory (see, for example, Bohren and Huffman, 1983). In this example, the fibre orientation is approximately t0 c' clockwise to the vertical in the plane orthogonal to the laser beam axis, and tilted 15° (upper end of the fibre towards the laser) in the plane containing the beam axis. To a first approximation, 90% of" the fibres of this size were oriented vertically to within a cone of 20 ° semiangle. FJbres of other sizes down to 3 #m length and aspect ratios (down to 2: l) have also been produced by silicon micromachining, and these will provide a basis for further studies into the interrelationship between particle aspect ratio and orientational behaviour irt the instrument's sample delivery system (see below). DISCUSSION This paper describes preliminary results obtained from a new instrument which records the major portion of the spatial intensity distribution of light scattered from individual airborne particles in flow. As such it offers potential for more fully understanding the optical scattering behaviour of airborne particles in flow and provides a platform to address the development of systems capable of real-time particle classification on the basis of particle shape and size. The authors are about to embark on a programme of experimentation examining the scattering profiles of a range of high precision non-spherical particles produced by silicon micromachining. It is intended to investigate the interrelationships, both experimentally and by theoretical modelling, of the alignment behaviour of elongated particles of increasing aspect ratio, and the behaviour of other particle species such as flat round discs, so as to optimize the flow conditions of the sample delivery system in the instrument and develop classification methodologies. These results will form the basis of a further communication. The instrument has been designed to allow, if desired, the positional interchange of the camera and the photomultiplier detector so as to enable the camera to record scattering profiles corresponding to the lower angular range of 5 ° and 30 ° to the beam direction. With the photomultiplier detector removed altogether and a different method of particle detection triggering employed, the instrument may be made to operate with a camera at each end of the scattering chamber, thus covering the wider scattering angle range of 5°-14t °~ This modification would offer potentially improved particle characterization capability (though at the expense of greater data processing). Finally, the use of a high-resolution CCD camera to image the scattering profiles provides valuable information detail but reduces the particle data processing rate to an extent that would not be appropriate for a real-time monitoring instrument. A principal objective of future work is to use the data produced by the camera system to allow the design of simplified custom discrete detector configurations (of typically 20--30 photodetector elements) which would replace the CCD camera and allow efficient particle classification at much higher particle throughput rates, thus providing the basis of a real-time monitoring capability. Acknowledaements--This work has been supported by a grant under the Joint Scienceand EngineeringResearch Council/Ministryof Defencescheme.The authors wish to expresstheir gratitude for this support. The manufacture of the silicon microparticles was supported by a Science and Engineering Research Council grant under their Nanotechnology Initiative. The majority of the manufacturing process was carried out under contract by A. Gundlach at the Edinburgh University Microfabrieation Unit, whose expertiseis greatly appreciated by the authors.
REFERENCES Bohren, C. F. and Huffman,D. R. (1983)Absorption and Scattering of Light by Small Particles. Wiley,New York, Bottlinger, M. and Umhauer, H. (1988) Optical Particle Sizin#: Theory and Practice. Int. Syrup., pp. 363-369. Plenum Press, New York.
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Gebhart, J. and Anselm, A. (1988) Optical Particle Sizing: Theory and Practice. Int. Symp., pp. 393-409. Plenum Press, New York. Hoover, M. D., Casalnuovo, S. A., Lipowicz, P. J., Hsu Chi Yeh, Hanson, R. W. and Hurd, A. J. (1990) J. Aerosol Sci. 21, 569-575. Kaye, P. H. (1991) Proc. 5th Annual Conf. of the Aerosol Society. Loughborough Univ., U.K. 25-27 March, pp. 223-228. Kaye, P. H., Eyles, N. A. and Clark, J. M. (1990) Proc. 2nd Int. Congress on Optical Particle Sizing. Univ. of Arizona, Pheonix, 5-8 March, pp. 501-510. Kaye, P. H., Eyles, N. A., Ludlow, I. K. and Clark, J. M. (1991) Atmos. Envir. 25A, 645~554. Kerker, M. (1969) The Scattering of Li#ht and other Electromagnetic Radiation. Academic Press, New York. Marshall, I. A, Mitchell, J. P. and Griffiths, W. D. (1991) J. Aerosol Sci. 22, 73-89.