Radiant energy – Photocells; circuits and apparatus – Optical or pre-photocell system
Reexamination Certificate
1998-09-08
2001-03-06
Le, Que T. (Department: 2878)
Radiant energy
Photocells; circuits and apparatus
Optical or pre-photocell system
C250S573000, C356S336000
Reexamination Certificate
active
06198110
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to apparatus for and methods of classifying particle shape in a fluid. It is particularly applicable, but in no way limited to, the real-time classification of particle shape in the atmosphere.
BACKGROUND TO THE INVENTION
In investigations of the composition and dynamics of the earth's particulate atmosphere, particle shape is an important parameter by which classification and possibly identification of particles may often be achieved. Spherical droplets, cuboidal crystals typical of marine aerosols, and the wide variety of morphologies assumed by ice crystals, are examples where the determination of shape may be used in combination with size spectra measurements to provide experimental data upon which theoretical models of macroscopic and microscopic physical behaviour of clouds and aerosols may be developed and tested. A specific example of this involves the study of ice microphysics and the behaviour of droplets and ice crystals which can occur simultaneously within clouds. The radiative properties of these mixed-phase clouds can be radically dependent upon the relative proportions and size spectra of the two phases, as well as the orientations of the ice crystals present, and this has a profound effect upon the proportion of incident sunlight reaching the lower atmosphere and earth surface. To be able to understand the radiative transfer properties of ice and mixed phase clouds, a detailed knowledge of the particles' shapes and sizes is required, along with measurements of the number concentration of ice and super-cooled liquid water particles. Furthermore, measurement of the total ice crystal number is important to facilitate the testing of theories of the nucleation of ice crystals and their role in climate change.
Existing Atmospheric Particle Measurement Techniques
Whilst there are several commercially available aircraft-mounted instruments designed to measure the size spectra of atmospheric particles. The FSSP—Forward Scatter Spectrometer Probe, from Particle Measurement Systems Inc. Boulder Colo.—is perhaps the most widely used in airborne platforms. These instruments cannot provide information relating to particle shape. They are generally calibrated on the assumption that all particles are spherical and they are thus incapable of discriminating between, for example, ice crystals and water droplets of equivalent optical scattering size.
In regard to particle shape measurement, the 2D-OAP-2D—Optical Array Probe from Particle Measurement Systems Inc.—is commonly employed for examining airborne particles greater than ~30 &mgr;m in size. This instrument records a silhouette of individual particles as they pass through a light sheet and occlude elements within a linear detector array arranged orthogonally to the particle trajectory. However, the 2D-OAP suffers a number of limitations, principally:
(i) it provides only limited instantaneous information of the range or distribution of particle sizes within a measured atmosphere (post-processing in the laboratory is normally undertaken to assess shape spectra);
(ii) it suffers from a number of artefacts produced in the recorded data by events such as the collection and subsequent release of liquid water drips from the leading points of the probe arms or the ‘splashing’ of large droplets on these arms producing artificially high populations of small droplets; and most importantly;
(iii) it cannot accurately resolve the shapes of particles which occlude less than the order of five array pixels, corresponding to particle sizes below about 125 &mgr;m [Moss S. J. and Johnson D. W.
Atmospheric Research
34 pp. 1-25, 1994]. The inability to analyze and categorize the shapes of smaller particles makes it impossible for the instrument to differentiate water droplets from ice crystals for these sizes. Therefore the instrument is unable to provide data which can answer the microphysical questions concerning the radiative transfer properties of, for example, cirrus clouds in which the ice particle and water droplet sizes are frequently well below the limit of resolution of the 2D-OAP.
The shapes of smaller particles, theoretically down to ~2-4 &mgr;m but in practice down to ~10 &mgr;m because of aircraft vibration, can be assessed using a holographic technique [Brown, P. R. A. j. Atmos.
Oceanic Technol
., vol. 6, pp. 293-306, 1989]. This technique involves using a pulsed Nd:YAG laser and photographic film recording system to acquire holographic ‘snapshots’ of particle populations within a measurement space. The processed holograms are later interrogated using a CW laser to recreate the images of the particles, allowing detailed analysis. This process is extremely slow and manually intensive, taking up to a day for each hologram, and again, the smaller particles of interest are beyond the instrument's limit of resolution.
Spatial Light Scattering Techniques
The applicants have developed several ground-based instruments for the classification and identification of airborne particles by analysis of the manner in which individual particles spatially scatter incident laser illumination. These are described in ‘Portable Particle Analysers’. Ludlow, I. K. and Kaye P. H. European Patent EP 0 316 172, July 1992; ‘Particle Asymmetry Analyser’. Ludlow, I. K. and Kaye, P. H. European Patent EP 0316 171, September 1992 which represent the closest prior art known to the applicant. In these instruments, airborne particles are drawn from the ambient atmosphere by a suction pump and are constrained by narrow delivery tubes, typically 1 mm in diameter, and a surrounding layer of filtered sheath air, to pass through an incident laser beam within a laser scattering chamber. The intersection of the particle flow with the beam defines the measurement space through which all particles in the sample flow will pass. Particle flow is such that statistically, particle coincidences within the measurement space are rare. Each particle passing through the measurement space will scatter light in a manner which is governed inter alia by the size, shape, and structure of the particle.
FIG. 1
shows typical light scattering patterns recorded from individual microscopic airborne particles. The black circle at the centre of each pattern is caused by a beam stop, and the outer circumference of the patterns corresponds to scattering at an angle of approximately 35° to the direction of the incident beam. As can be seen in
FIG. 1
, spherical particles such as droplets produce a regular concentric ring scattering patterns, whilst elongated particles such as fibres or long crystals produce linear scattering angled according to the orientation of the particle. Irregular shaped particles may produce more complex patterns with few easily discernible features. In the instruments described in the aforementioned prior art, the scattering patterns as shown in
FIG. 1
are collected by the three detectors arranged symmetrically about the laser beam axis. By measurement of the difference in magnitude of the signals received from the three detectors, a crude estimate of the shape of the scattering particle may be deduced. However, the type of instrument described above is not suitable for measuring atmospheric particles such as ice crystals or super-cooled water droplets for the following reason: in the measurement of atmospheric particles it is essential that neither the phase (ie: ice or water) nor the orientation (which governs radiative behaviour) of the particles is affected by the measurement process. This precludes the use of a pumped sample delivery system in which the particles are drawn from the atmosphere via narrow tubes into a measurement chamber. Such a pumped system would certainly change the orientation of the particles and would be likely to melt or partially melt smaller ice crystals present in the sample.
With the foregoing argument in mind, the present invention has the objective of providing a means by which the sizes, shapes, and orientations of fluid-borne particles may be
Hirst Edwin
Kaye Paul H
Le Que T.
Nixon & Vanderhye P.C.
The Secretary of State for Defence in Her Britannic Majesty&apos
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