PARTICLE SHAPE

Abstract: Apparatus (10) is used to obtain information relating to the shape of particles in a sample (12). The sample is illuminated with polarized radiation from a source (14). Scattered radiation (16) leaves the sample (12) and is measured (at 18) by sensors (18a, 18b) which measure intensity at different polarization states. Measurements from the sensors (16a, 16b) are provided to a control arrangement (20), which may be in the form of a computer, which combines data to produce a data value for the corresponding scattering angle. The set of data so produced is characteristic of the particle shape.

Description: Particle Shape The present invention relates to determination of particle shape. Particle shape can be important in many industries, particularly for determining the suitability of materials for particular purposes. In the paint industry, particle shape in an emulsion can affect flow characteristics and the area coverage obtainable by a paint. In the pharmaceutical industry, particle shape can affect the stability of tablets and the rate of release of the pharmaceutical when ingested. In the china clay industry, clay slurries may be formed, and will have characteristics which are dependent on particular shape. The present invention provides a method of obtaining information relating to the shape of particles in a sample, in which the sample is illuminated with polarized radiation, and scattered radiation is measured at a plurality of scattering angles to produce a set of data which is characteristic of the particle shape, the measurements including measurement of the intensity of at least two polarization states of the scattered radiation at each scattering angle. Preferably, the intensity measurements at each scattering angle are combined to produce a data value for the corresponding scattering angle. Illuminating radiation is preferably polarized and preferably coherent. Measurements are preferably taken of two orthogonal polarization states and measurements at each scattering angle are preferably taken simultaneously. Measurements at each scattering angle are preferably time averaged. Measurements are preferably taken over a period sufficiently long for substantially all possible particle orientations to have been expected to be present in the sample during the measurement period. Measurements at a scattering angle are preferably combined by forming a correlation coefficient, preferably a normalised correlation coefficient. Preferably, the data value for a scattering angle is given by one of the following formulae: where I θ is the data value for the scattering angle θ; and I j and I. are the sampled intensity measurements for respective polarization states at the measurement angle θ. The invention also provides apparatus for obtaining information relating to the shape of particles in a sample, comprising illumination means operable to illuminate the sample with polarized radiation, and measurement means operable to measure scattered radiation at a plurality of scattering angles, including measurement of the intensity of at least two polarization states of the scattered radiation at each scattering angle to produce a set of data which is characteristic of the particle shape. Preferably control means combine the intensity measurements at each scattering angle to produce a data value for the corresponding scattering angle. Preferably the illumination means produces polarized radiation, preferably coherent radiation. Preferably the measurement means are operable to take measurements of two orthogonal polarization states and are preferably operable to take simultaneously the measurements to be combined. Preferably the control means is operable to time average measurements. Preferably measurements are time averaged over a period sufficient long for substantially all possible particle orientations to have been expected to be present in the sample during the measurement period. Preferably the control means combines measurements by forming a correlation coefficient, preferably a normalised correlation coefficient. The control means may be operable in accordance with one of the following formulae: where I θ is the data value for the scattering angle θ; and I ( and I 2 are the intensity measurements for respective polarization states at the measurement angle θ. Preferably the control means comprises a computer programmed to receive intensity measurements from the measurement means and to combine measurements as aforesaid. Examples of methods according to the present invention, and apparatus for implementing the methods, will now be described in more detail, by way of example only, and with reference to the accompanying drawings, in which: Fig. 1 is a schematic diagram of apparatus for taking measurements in accordance with the present invention; and Fig. 2 is a plot of results obtained in accordance with the invention. The apparatus 10 shown in Fig. 1 is used to obtain information relating to the shape of particles in a sample 12. The sample is illuminated, as will be described, with polarized radiation from a source 14. Scattered radiation 16 leaves the sample 12 and is measured at 18 by sensors 18a, 18b which measure intensity at different polarization states. Measurements from the sensors 16a, 16b are provided to a control arrangement 20, which may be in the form of a computer, which combines data to produce a data value for the corresponding scattering angle. The set of data so produced is characteristic of the particle shape, as will be described. In more detail, the sample 12 is shown in a closed cell 22 and will consist of the particles to be measured, suspended in a generally transparent fluid. Alternatively, the cell 22 could be a section of pipe through which fluid and suspended particles are flowing. In either example, the volume of the sample 12 should be sufficiently large for random particular orientation to be expected to give rise to an even distribution of particles in all possible orientations. The source 14 is a polarized laser producing polarized, coherent illuminating radiation 24 through a lens 26 focusing the radiation 24 into the sample 12. Radiation entering the sample 12 is scattered by the particles. A pin hole 28 selects light scattered to a particular angle relative to the illuminating radiation 24, according to the position of the pin hole 28. The radiation passing through the pin hole 28 enters a polarizing beam splitter (a device which is well known in itself) which simultaneously divides the radiation into two beams 32 which pass respectively to the sensors 18a, 18b. Both beams 32 are polarized, the polarization states being orthogonal to each other and at 45° to the polarization state of the illuminating radiation 24. The sensors 18a, 18b are photo-multiplier tubes which measure light intensity by counting photons arriving over a period of time. The computer 20 is able to control the period of time over which measurements are taken, and then to receive the measurements so produced. The arrangement is also such that measurements from both sensors 18a, 18b are taken simultaneously, over the same period of time. Thus, with the set-up of the source 14 and lens 26 as shown in bold lines in Fig. 1, the computer 20 can instruct the sensors 18a, 18b to begin measuring. Measurement continues during a period of time sufficiently long for substantially all possible particle orientations to have been expected to be present in the sample 12, for instance by virtue of Brownian motion in a static sample, or as a result of the random orientation of particles flowing through a pipe. It is important to the analysis of the results that all possible particle orientations are equally likely to be encountered by the radiation. Measurements are passed from the sensors 18a, 18b to the computer 20 for further analysis as will be described below. These measurements allow the computer to produce a data value corresponding to the scattering angle selected by the pin hole 28. The process will then be repeated to acquire data corresponding to a different scattering angle. This can be done either by moving the illuminating arrangements relative to the sample, or by moving the detecting arrangements relative to the sample, or both. In Fig. 1, the first of these options is illustrated in broken lines, indicating that the source 14 and lens 26 have moved to illuminate from a different angle, so that the radiation passing through the pin hole 28 has been scattered at a different angle. In this way, readings can be taken for a range of scattering angles. It is found that very little radiation scatters by more than about 75°. The two intensity measurements obtained by the sensors 18a, 18b for a particular scattering angle are combined to produce a single data value for that scattering angle, as follows. The value I θ , which may be termed the correlation coefficient, is given as follows: Ie = (W or = CA) ( ΛXVΛ) fl " ( ιX 2 ) where I θ is the data value for the scattering angle θ; and I ; and I 2 are the intensity measurements for respective polarization states at the measurement angle θ. The use of the bar superscript in this formula (e.g. I) is to indicate that the value is time averaged. The left formula set out above (in which measurements are squared in the denominator) is used to account for pin hole averaging when using a relatively large pin hole. The other formula (without denominator measurements being squared) is used when a relatively small pin hole is being used. The value of I e can be plotted against scattering angle to produce a plot similar to those shown in Fig. 2. If every particle in the sample 12 is perfectly spherical, I θ will be unity for all scattering angles, giving the flat profile 34. If the particles are distorted from a pure spherical shape, to a spheroidal shape, profiles like those shown at 36a,b,c,d are produced. Initially, for low scattering angles, these have a value close to unity, but the value drops away for higher scattering angles. It has been found that the angle at which this drop occurs is a characteristic of the aspect ratio of the spheroidal particles and in particular, the closer they are to a spherical shape, the higher the scattering angle at which the profile 36 begins to drop away. Consequently, the shape of the profile 36 produced from a real sample can be used to deduce the average shape of the particles in the sample. It is apparent that by using the formula set out above, I θ is independent of the strength of the incident radiation (and thus of any variation which may occur during the measurement period); is independent of the absolute transparency of any elements of the system; is independent of the sensitivity of the sensors and of any differences between them; and is independent of the size and shape of the illuminated volume containing the particles of interest. Many variations and modifications can be made to the apparatus set out above, without departing from the scope of the invention. In particular, the skilled man may wish to select other, perhaps more complex optical systems for illuminating a sample, for isolating light scattered to a particular angle, for splitting that light and for measuring it. It is possible to envisage arrangements which simultaneously make measurements at more than one scattering angle. Whilst endeavouring in the foregoing specification to draw attention to those features of the invention believed to be of particular importance it should be understood that the Applicant claims protection in respect of any patentable feature or combination of features hereinbefore referred to and/or shown in the drawings whether or not particular emphasis has been placed thereon.

Claims: Claims 1. A method of obtaining information relating to the shape of particles in a sample, in which the sample is illuminated with polarized radiation, and scattered radiation is measured at a plurality of scattering angles to produce a set of data which is characteristic of the particle shape, the measurements including measurement of the intensity of at least two polarization states of the scattered radiation at each scattering angle. 2. A method according to claim 1 wherein the intensity measurements at each scattering angle are combined to produce a data value for the corresponding scattering angle. 3. A method according to claim 1 or claim 2 wherein the illuminating radiation is polarized. 4. A method according to any preceding claim wherein the illuminating radiation is coherent. 5. A method according to any preceding claim wherein measurements are taken of two orthogonal polarization states. 6. A method according to any preceding claim wherein measurements at each scattering angle are taken simultaneously. 7. A method according to any preceding claim wherein measurements at each scattering angle are time averaged. 8. A method according to claim 7 wherein measurements are taken over a period sufficiently long for substantially all possible particle orientations to have been expected to be present in the sample during the measurement period. 9. A method according to any preceding claim wherein measurements at a scattering angle are combined by forming a correlation coefficient. 10. A method according to claim 9 wherein the correlation coefficient is a normalised correlation coefficient. 11. A method according to any preceding claim wherein the data value for a scattering angle is given by one of the following formulae: where I θ is the data value for the scattering angle θ; and I j and I 2 are the sampled intensity measurements for respective polarization states at the measurement angle θ. 12. Apparatus for obtaining information relating to the shape of particles in a sample, the apparatus comprising illumination means operable to illuminate the sample with polarized radiation, and measurement means operable to measure scattered radiation at a plurality of scattering angles, including measurement of the intensity of at least two polarization states of the scattered radiation at each scattering angle to produce a set of data which is characteristic of the particle shape. 13. Apparatus according to claim 12 wherein control means combine the intensity measurements at each scattering angle to produce a data value for the corresponding scattering angle. 14. Apparatus according to claim 12 or claim 13 wherein the illumination means produces polarized radiation. 15. Apparatus according to any of claims 12 to 14 wherein the illumination means produces coherent radiation. 16. Apparatus according to any of claims 12 to 15 wherein the measurement means are operable to take measurements of two orthogonal polarization states. 17. Apparatus according to any of claims 12 to 16 wherein the measurement means are operable to take simultaneously the measurements to be combined. 18. Apparatus according to any of claims 12 to 17 wherein the control means is operable to time average measurements. 19. Apparatus according to claim 18 wherein measurements are time averaged over a period sufficient long for substantially all possible particle orientations to have been expected to be present in the sample during the measurement period. 20. Apparatus according to any of claims 12 to 19 wherein the control means combines measurements by forming a correlation coefficient. 21. Apparatus according to claim 20 wherein the correlation coefficient is a normalised correlation coefficient. 22. Apparatus according to any of claims 12 to 21 wherein the control means is operable in accordance with one of the following formulae: where I θ is the data value for the scattering angle θ; and I. and I 2 are the intensity measurements for respective polarization states at the measurement angle θ. 23. Apparatus according to any of claims 12 to 22 wherein the control means comprises a computer programmed to receive intensity measurements from the measurement means and to combine measurements as aforesaid. 24. A method substantially as herein described with reference to the drawings. 25. Apparatus substantially as herein described with reference to the drawings. 26. Any novel subject matter or combination including novel subject matter disclosed herein, whether or not within the scope of or relating to the same invention as any of the preceding claims.

Inventors: HOPCRAFT KEITH IAIN, JAKEMAN ERIC, PITTER MARK CHARLES, SMITH PETER RICHARD, WALKER JOHN GRAHAM, HOPCRAFT, KEITH, IAIN, JAKEMAN, ERIC, PITTER, MARK, CHARLES, SMITH, PETER, RICHARD, WALKER, JOHN, GRAHAM
Applicants: HOPCRAFT KEITH IAIN, JAKEMAN ERIC, PITTER MARK CHARLES, SMITH PETER RICHARD, UNIV LOUGHBOROUGH, UNIV NOTTINGHAM, WALKER JOHN GRAHAM, HOPCRAFT, KEITH, IAIN, JAKEMAN, ERIC, LOUGHBOROUGH UNIVERSITY INNOVATIONS LIMITED, PITTER, MARK, CHARLES, SMITH, PETER, RICHARD, THE UNIVERSITY OF NOTTINGHAM, WALKER, JOHN, GRAHAM
Application year: 1999
Main classification: G01N15
IPC codes: G01N15