Optics: measuring and testing – For size of particles – By particle light scattering
Patent
1994-07-29
1995-11-28
Rosenberger, Richard A.
Optics: measuring and testing
For size of particles
By particle light scattering
356338, 356341, 356343, G01N 1502
Patent
active
054712998
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field of the Invention
The ability to measure particles by shape and size is of importance to many groups of people. The food and chemical industries are concerned from a quality control point of view; biologists are interested in characterising cells and monitoring changes in and differences between cells; environmental scientists are concerned with airborne particles and their effect on air quality and health. This list is by no means exhaustive, it is merely intended to illustrate the driving force behind the attempts to develop accurate and reliable measurement instrumentation, and to theoretically understand the nature of the problem.
2. Discussion of Prior Art
There are currently two main optical scattering methods in use in commercially available particle measurement systems. The first method attempts to size particles by measuring their static or dynamic behaviour in fluid. These systems generally measure deposition rate, acceleration in a let stream, or Brownian motion. The second method attempts to size particles by measuring the light scattered from an illuminated particle or ensemble of particles either at a few specific angles or over a large solid angular range. Apart from image analysis systems, none of the commercial instruments is capable of characterising particles by shape, non-spherical particles being sized by assigning an equivalent spherical diameter, although this diameter depends on the measurement method used. What is worse, is that some instruments are known to become inaccurate when tested with non-spherical particles of regular shape, so measurements taken with particles of arbitrary shape have to be treated with some caution.
Instruments which attempt a shape classification are based on image analysis, which requires taking an image of a small number of particles and performing complex image processing. The particle sample has to be prepared beforehand so that it is in a form suitable for image processing, i.e. it has to be processed so that individual particles can be seen with minimal overlapping. Thus there is a considerable time delay before the results are available. The method also requires fast computers in order to do the analysis reasonably quickly. Some of the other instruments also suffer a time delay before measurements are available, and whether this is important depends on the application. It is not necessarily important for batch testing powders for example, but it is of potential importance when monitoring a working environment for asbestos fibres or micro-organisms.
Several commercial laser based instruments are available which will size particles, as disclosed in "Particle Size Analysers Product Roundup", Powder and Bulk Engineering, Feb 1991; pp 42, and other research instruments have been built to investigate various aspects of particle sizing. For example, an instrument has been developed to size particles using the oscillation in intensity of the scattered light, as disclosed in "Drop Sizing by Laser Light Scattering Exploiting Intensity Angular Oscillation in the Mie Regime" by Ragucci, R., Cavaliere, A. and Massoli, P. Particle and Particle Systems Characterisation, Vol 7, 1990; pp 221. Most of the instruments analyze an ensemble of particles and, as stated previously, they assume a spherical particle or particles, and do not give any indication of non-sphericity.
Research reported in "Light Scattering Instrument to Discriminate and Size Fibres Part 2: Experimental System". Particle and Particle Systems Characterisation, Vol 6, 1989; pp 144, has been reported using an instrument designed to discriminate and size fibrous material. In this research, particles are passed through a laser beam in single file using a laminar airflow system similar to the design described below. The forward scattered light is collected by a lens and passed through a polarizing beamsplitter. The intensity of the light in two orthogonal polarizations is then recorded using photo-multiplier tubes. Results show that near spherical particles can be d
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Hirst Edwin
Kaye Paul H.
Eisenberg Jason D.
Rosenberger Richard A.
The Secretary of State for Defence in Her Britannic Majesty's Go
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