GB2203542A - Measuring particle size distribution - Google Patents

Abstract

Particle size distribution in low concentration particle fields is measured by forward light scattering using a high power light source such as a pulsed gallium arsenide laser. With such a powerful source, beam stopping is necessary and vignetting inevitable and means for compensating for losses due to beam stopping and vignetting are employed. As shown, a Fraunhofer diffraction pattern is formed on a ring detector in the focal plane of a Fourier transform lens. The undiffracted light is blocked by a beam stop 122, or alternatively a hole is left in the centre of the detector to allow the undiffracted light through. A blocking factor is calculated to compensate for beam stopping and vignetting before the particle size distribution is determined. <IMAGE>

Description

MEASURING PARTICLE SIZE DISTRIBUTION This invention relates to measuring particle size distribution in suspended particle fields.

A conventional method comprises sampling the particle field at a point in space and time in a filter assembly of pr,gressively decreasing pass size through which the partcile-containing air is drawn as by a pump. The particles caught, on the filter are then assessed in the laboratory. This method is time comsuming and inconvenient, and in any event serves to give but a single evaluation of particle size distribution for single point in space and time, notwithstanding the tine and trouble that must be expended to evaluate the result of the sampling.

A faster and more convenient, indeed substantially autornated method for evaluating particle size in suspended particle fields has been developed in recent years. This method uses a technique known as forward light scattering. Such technique is discussed in J Swithenbank, J M B-r, D S Taylor, D Abbott, C G itcCreath, "A Laser Diagnostic Technique for the Measurement of Droplet and Particle Size Distribution", Experimental Diagnostics in Gas Phase Combustion Systems, Progress in Astronautics and Aeronautics, 53, 421, (1977).

Fast, automated laser diffraction particle sizers are available from Malvern Instruments, Malvern, Worcestershire, England.

Such instruments form a Fraunhofer diffraction pattern on a detector. The detector is a ring diode array located in the focal plane of a Fourier transform lens. A parallel beam of monochromatic light from a low power He Ne laser is passed through the particle field which is between the light source and the lens.

The instruments currently available are intended for laboratory use, where the particles to be examined can be introduced for example from an aerosol spray or a fuel injection nozzle. Particle sizes down to 1 micron in concentrations as low as lOOmg/m can be measured, and the lialvern instruments are claimed to be capable of measuring particles as small as 0.5 or even 0.3 microns.

At these particle sizes and concentrations, however, the instruments zero at the limit of their useful range for a number of reasons. For oti thing, the unscattered light passing directly through the lens on to the control part detector swamps it. Extending the particle field by separating the light source and the Fourier transform lens, in order to include more particles in the beam path, necessitates a irger aperture lens if all the scattered light is to be collected, and this makes the instrument bulkier and more expensive.

Tne available instruments are already bulky, so that it is impracticable to use them for certain applications outside the laboratory.

The invention comprise; a method for measuring particle size distribution in low concentration suspended particle fields by forward light scattering, comprising using a high power light source.

The light source nay comprise a pulsed laser, for example a gallium arsenide laser, and may have a mean power of the order of 20mW or more. The conventional He Ne laser has a continuous power of lmF, by way of comparison.

beam stop may be ud to prevent unscattered light reaching te detector - this avoids the detector being swamped by this unscatter-d light. The beam stop will also cut off some of the scattered light and distort the intensity distribution of the dif-raction pattern, which will have to be corrected by computation based on the system geometry or by calibration.

The beam stop may comprise a black body cavity.

The detector may comprise a plurality of concentric ring detectors arranged coaxially with the beam, and the measured intensity at each ring is then corrected by dividing it by (l-B) where B is a blockage factor defined . (l-Ep/Ea) where Ep and Ea are respectively the energy incident on the ring when the stop is present and that when the stop is absent.

The blockage factor B for a ring having inner and outer radii R1 and R2 may be approximated, however, by assuming that the @ergy scattered by a particle to the ring, expressed as a fraction of the energy incident on the particle, is constant over the ring radius between R1 and R .

For a spherical particle (or a field of equal size spherical particles) it can be shown that the energy incident on a ring is given by I0 # a2 N [@ J R1, R2] where # J R1, R2 = J02 (K R /f) + J12 (Ka R2 /f) - @@ (KaR1/f) + J12 (KaR1/f) where lQ = number of particles f = focal length of lens I0 = incident light intensity K = 2#/#, A = wavelength of light a = particle radius and J0 and J1 are zero and first order Bessel functions.

If w is the volume of particles of radius a then N = 3w/4#a# ad the incident energy on ring R1, R2 is lo 3/4 w/a [# JRl R2-7 For a distribution of particle sizes, divided into a m size bands the inc dent energy on ring sl-s2 is

Each of the rings of the detector is associated with a characteristic particle size range depending on the focal length of the collection lens.

Table 1 shows the inner and outer radii of the thirty rings of a particle sizer available from Malvern Instruments of Worcestershire, England.

The total light energy distribution is the sum of the products of the energy distribution for each size range and the volume fractions in that size range. This can be expressed in matrix notation as E(I) = W(J) T(I,J) T(I,J) is a square matrix the elements of which define the scattered light energy distribution for each size class.

Concersely W(J) = E(I) T1 (I,J) where T is the inverse matrix consisting of a set of coefficients that must be multiplied by measured light energy to obtain the size fraction distribution.

In practice, this task is effected by a microcomputer interfaced to the detector system.

The beam stop alters the R(I) in a calculable way, namely by the blocking factor B referred to above. The E(I) must therefore be corrected by dividing the measured energy for each ring by the corresponding value (1-Bj, and the size distribution computation is the:i carried out in the usual way using the corrected energy distributon.

The length of the particle field from which light is scattered, ie the distant between the collimating lens and the Fourier transform lens may be such, having regard to the diameter of the latter and of the detector, that all scattered light is collected and formed into a r:; ffraction pattern at the detector. However, it is possible also for the geometry to be such that vignetting occurs, namely not all the scattered light is formed into the diffraction pattern, and a correct on must then be made in evaluating the measurement to account for the light lost. As with a beam stop, the correction may be computed from the system geometry or it may be effected by calibration.

The detector may comprise a plurality of concentric ring detectors arranged coaxially with a circular beam, and the measured intensity at each ring is corrected by dividing it by (1-HP) where HP is an efficiency defined for any point P on the detector by Hp = Energy collected on ring at point P divided by the Energy collected in ring at point P but using an infinite lens aperture.

The efficiency H may be approximated for a ring having inner and outer radii R1 arid R2 by assuming that the energy scattered by a particle to the ring, expressed as a fraction of the energy incident on the particle, is constant over the ring radius between R1 and R2.

For a spherical particle (or a field of equal size shperical particles it can be shown that the energy incident on a ring is given by I0 # a2 N [# J R1 ,R2] where # J R1 ' R2 = J02 (Ka R2/f) + J12 (Ka R2/f) - J02 (KaR1/f) + J12 (Ka R1/f) where N = number of particles f = focal length of lens I0 = incident light intensity K = 2 # \ = wavelength of light a = particle radius and J and J1 ar- zero and first order Bessel functions.

0 1 w w@s @s thc volume of particles of radius a then N = 3w/4 a3 and the incident energy o@ ring R1, R2 is I0. 3/4. w/a. [@J@1s2] Fo. a distribution of particle sizes, divided into m size bands the incident energy on ring sl-s2 is

Each of the rings of the detector is associated with a characteristic particle size range depending on the focal length of the collection lens.

Table 1 shows the inner and outer radii of the thirty rings of the particle sizer available from Malvern Instruments of Worcestershire, England.

The total light energy distribution is the sum of the products of the energy distribution for each size range and the volume fractions in that size range. This can be expressed in matrix notation as E(I) = W(J) T(I,J) T(I,J) is a square matrix the elements of which define the scattered light energy distribution for each size class.

Conversely W(J) = E(I) T (I,J) where T is the inverse matrix consisting of a set of coefficients that must be multiplied by the measured light energy to obtain the size fraction distribution.

In practice, this task is effected by a microcomputer interfaced to the detector system.

The vignetting effect alters the (I) in a calculable way, namely by the efficiency factor H referred to above. The E(I) must therefore be corrected by dividing the measured energy for each ring by the corwesponding value (l-H) and the size distribution computation is then carried out in the usual way using the corrected energy distribution.

A measurement may be made of the diffraction pattern from a single i)ulse of light, and a series of measurements may be made of the diffraction patterns from a series of pulses of light so as to show how particle size distribution is varying with time or to provide a time average.

The measurement from a single pulse of light may be stored for later recall and evaluation.

Particle size distribution over a large region, for example for purposes of identifying or monitoring sources or particulate airborne pollution such as chemical plants, can be measured by measuring the distribution at a number of places in the region simultaneously by taking successive measurements, le change in distribution with tine can be observed.

The invention also comprises apparatus for measuring particle size distribution in low concentration suspended particle fields by forward light scattering, comprising a high power light source.

The apparatus may have data processing and storage means processing and storing the measurement effected by the detector.

The measurements from successive pulses of a pulsed light source such as a gallium arsenide laser may be processed and stored, for example digitised and stored in random access memory (REd) or on a disc. The stored data can be recalled later for analysis in a computer. Conventional gas laser particle detectors have limited data processing power. This invention enables the user to evaluate the data from a single observation or from a number of observations.

In addition data from many of observations of each of a number of apparatus can effect analysis of the behavious of a cloud of particles over a period of time.

The apparatus may also comprise control means effecting pulsing of the light source, and such control means may comprise remote, such as radio control.

The apparatus may also be contained in a portable unti having an internal power source.

Embodiments of apparatus and methods for measuring particle size distribution according to the invention will now be described with reference to the accompanying drawings, of which: Figure 1 is a diagrammatic representation of a self-contained portable apparatus, Figure 2a is a diagrammatic representation of a part of a modified apparatus having a beam stop, Figure 2r is a view along the arrow 3 of Figure 2a, Figure 2c is a diagrammatic il)ustration like Figure 2 but witha small-extent particle field.

Figure 2d is a view like Figure 3 along the arrow 5 of Figure 4, Figures 2e ad 2f are graphs plotting values of (l-Bj for a specific field ccinfiguration.

Figure 3 is a diagrammatic representation of a system in which the geometry gives rise to vignetting, Figure 3a is a diagrammatic illustration like Figure 1 of an extended field arrangement according to the invention, Figure 3b is a view along the arrow 3 of Figure 2, and Figures 3c, d, e and f are graphs of the efficiency H of a ring diode of a detector at different extents of the particle field.

Figure 4 is a diagrammatic representation of an arrangement of a plurality of measuring apparatus deployed to analyse a particle cloud.

The drawings illustrate apparatus and methods for measuring size distribution in low concentration suspended particle fields by forward light scattering.

The apparatus comprises a moriochromatic light source 11, which is a pulsed gallium arsenide laser of 20mW power. A collimating lens 12 produces a parallel beam 13 aimed at a Fourier transform lens 14.

A ring diode detector 15 is located in the focal plane of the lens 14 on the side remote from the light source 11.

Particles (such as, liquid droplets) in the path of the beam 13 scatter light in directions which depend on the particle size. The scattered light is collected by the lens 14 and formed into a diffraction pattern on the ring triode detector 15.

The voltage outputs from the rings of the detector 15 are converted into digital form in a D/A converter 16 and stored in RAM or disc 17.

The laser 11 is driven from a self-contained power pack 18 and pulsed on a signal from a radio control receiver 19.

In a practical arrangement, a Fourier transform lens 14 of Focal length 63mum is employed. The distance of from the collimating lens 12 to the Fourier transform lens 14 is 77mm or 1.2 times the focal length of the lens 14. The diameter of the lens 14 is 28mm, the diameter of the outer radius of the largest detector ring is 18mm and the beam diameter is 6mm. With this geometry, the distance d is the maximum distance before vignetting occurs.

To avoid execessive brightness a beam stop 122 (Figures 2a-2d) is usd according to the another aspect of the invention to effectively block off undiffracted light from reaching the centre of the detector 15. Alternatively a hole can be left in the centre of the-detector to allow undiffracted light to pass through.

At the same time, however, it reduces the amount of diffracted light reaching ring diodes 15a, b by effectively blocking off part of the particle field from the view, so to speak, of the diode.

The geometry of this is apparent from Figures 2a-2d. The beam stop 122 comprises a hollow cylinder of radius Rs formign a black body cavity for absorbing the beam. R denotes the radius of the D beam, and Z 1 arid Zp2 are distances measured from the edge of the beam stop along the optical axis denoting the boundaries of the optical field.

The blockage fact or B at a point P on any detector ring 21a, b is defined by BP = 1 - EP/Ea where EP is the energy collected at point P with the stop present, and Ea is the energy collected at point P with the stop absent.

Incident light intensity at any beam radius r is given by -2r2/w2 1=1 e 2 where w = radius at which I falls to lie of its maximum I = constant o The energy scattered to any point P on the detector from an incremental portion of the measurement volume -2r2/w2 = S.A.I0 e .r.# where r, o and z are polar coordinates whose orientation is shown in Figure 2 S = particle cross sectional area per unit volume A = energy scattered by a particle at an angle 0 expressed as a fraction of the energy incident on the particle.

If Rd is the radial position of point P on the detector, it can be seen from Figure 2 that tan &commat; = Rd/f A is a function of the angle e and s, the particle size distribution.

The energy incident at the point P on the detector is obtained by integrating the incremental energies with respect to the polar coordinates z, o and r over the limits determined by the geometry of the optics. If these limits are given as z1 and Z2, M1 and 2, and r1 and r2 when the beam stop is present and z1 and z2, 1 and 2 and r1 al.d r; when the beam stop is present then the blockage factor B for the point P is simply the ratio of the definite integrals.

Since the energy is measured over a ring, it is useful to define a ring, rathern than a point blockage factor. A difficulty is that A is a function of Rd which depends on particle size distribution, which is nto known initially. However, if it be assumed that A is constant over the range of ring diameter between the outside and inside edges, a good approximation is obtained from the ratio of the definite integrels of re-2r2 with respect to z, , r and Rd.

Assuming Zpl is small and Zp2 large, as shown in Figure 2a the integration is solved numerically over the range Zpl # f/Rd2 (R5 - Rb) Zp2 # f/Rd1 (Rs + Rb) With no beam stop, the measurement volume is the cylinder radius Rb and length Zp2 - Zp1. The limits of integration are then Z1 = Zp1 1 = 0 r1 = 0 Z2 = Zp2 2 = 2 r2 = Rb The measurement volume blocked out by the stop is a cylinder truncated by a curved surface, shown by the shaded area in Figures 2 and 3.

The Z limits for this become Z1 = Zpl Z2 = f/Rd [(Rs2 P r2 sin 2) - r cos If the Zp1 and Zp2 inequalities above de not apply, because the particle field is too small for example as shown in Figures @ and@@ the limits of the particle field cut the truncated part of the blocked out measurement volume, as shown by the shaded area in Figure ; The discontinuous nature of the function means that a numerical solution is required for each ring.

Typical evaluated blocking factors are illustrated in Figures 2e and @@ which give values of 1-B for a beam radius 5mm and a stop radius 5.5mm for different values of Zp1/f and two different values of Zp2/f.

From the given values of 1-B for these geometries, a correction can be made to the measured energy on each ring of the detector by dividing it by 1-B, after which the particle size distribution can be obtained in the usual way using the corrected values for the measured energy.

Alternatively, the instrument can be calibrated using fields of known size distribution and calculating the blocking factors which should be used to correct the measured intensities to those needed to give the correct (known) size distribution. For purposes of calibration, the distribution of a similar field of particles can be measured by conventional means such for example as filtration through a series of filters of progressively decreasing pass size.

Apparatus can be provided with a longer beam path, as shown in Figure 3a. Here the beam path between lenses 12 and 14 is substantially in excess of 1.2 times the focal length of the Fourier transform lens 14. As a result, although more particles are available for light scattering, some of the scattered light, eg ray i', misses the collecting lens i4 and does not contribute to the diffraction pattern at the detector 15.Such losses, uke the beam stop losses, can also be compensated by corrective measures in evaluation of the results of the measurement, again in accordance with the system geometry, or by calibration as aforementioned. The larger field which can give rise to vignetting is useful in connecting with sno thing our non-homogeneity of field density.

When the field it: extended, as in Figure 3a, so as to ensure the field contains at least a reasonable number of particles available for scattering, some of the light scattered by particles beyond a c-rtain distance away from the lens will be diffracted at such a wide angle as to miss the lens altogether and will not contribute to the Fraunhofer diffraction pattern formed by the lens.

The geometry of this is apparent from Figure 3@. The onset of vignetting can be defined as the maximum distance of the particle field from th- lens before which size information is lost due to the finite lens aperture. This distance is denoted by Z.

At the onset of vignetting, X = f(RL - Rb)/Rd where Rd is the outer radius of the largest detector ring in the detector and RL is the radius of the collector lens, and f is its focal length.

For a commercially available instrument - a particle size analyser available fron Malvern Instruments, Worcestershire, England, f=100mm, RL=22.5mm, Rd=14.3mm and Rb=5mm. The maximum distance of a particle from the lens before vignetting occurs is thus 122mm.

It is possible to correct the measured energy ditribution for the effect of vignetting.

An efficiency ii is defined at a point P on any detector ring 2la, b, ... as tip = energy collected at point P divided by the Energy wnich would be collected at point P from the same particle field but using an infinite lens aperture.

incident light intensity at any beam radius r is given by I = I0 e-2r2/w2 where w = radius et which I falls to 1/e2 of its maximum I = constant o The energy scattered to any point P on the detector from an incremental portion of the measurement volume = S.A.I0 e-2r2/w2 .r. # where r, o and z are polar coordinates whose orientation is shown in Figure 3a.

S = particle cross sectional area per unlit volume A = energy scattered by a particle at an angle o expressed as a fraction of the energy incident on the particle.

If Rd is the radial position of point P on the detector, it can be seen fro Figure 2 that tan # = Rd/f A is a function of the angle # and s, the particle size distribution.

Tne energy incident at the point P on the detector is obtained by integrating the incremental energies with respect to the polar coordinates z, o and r over the limits determined by the geometry of the optics. If these limits are given as z1 and z2, 1 and 2, and r1 and r2 when the beam stop is present and z1 and z2, 1 and 2 and r1 and r2 when the stop is absent, then the blockage factor B for the point P is simply the ratio of the definite integrals.

Since tne energy is measured over a ring, it is useful to define a ring1 rather than point, olocKage factor. A difficulty is that A is a function of Rd which depende on particle size distribution, whicn is not known intially. However, if it be assumed that h is constant over the range of ring diameter between the outside and inside edges, a good approximation is obtained fron the ratio of the definite integrals of re-2r2/w2 with respect to z, o, r and Rd Assuming Zp1 is small and Zp2 large, as shown in Figure 3a, tne integration is solved numerically over the range Zp1 # f/Rd2 (Rs - Rb) Zp2 # f/Rd1 (Rs + Rb) ith n vignetting, tne measurement volume is the cylinder radius Rb and lengtn Zp2 -Zp1. The limits of integration are then z1 = zp1 1 = 0 r1 = # 2 2 b The measurement volume lost by vignetting is a cylinder truncated by a curved surface, shown by the shaded area in b The Z linits for this become Z1 = Zp1 Z2 = f/Rd [(RL2 - r2 sin2 #) -r cos ] The efficiency @ can now be calculated by solving the integral numerically for the various values of RL.

Figures 3c, d, e and f show efficiencies calculated for ring 30 of a Malvern Instruments particle size analyser having the geometry described above. Efficiencies can be calulated for other rings in like fashion.

From the given values of H a correction can be made to the measured energy on each ring of the detector, after which the particle size distribution can be obtained in the usual way using the corrected values for the measured energy.

The detector could, alternatively, be experimentally calibrated for the vignetting effects of different extents of particle fields of known size distribution, the efficiencies being estimated from the measurements either by substituting a larger lens (one with no vignetting effect at the field size) or by comparing the size distribution as measured with the known distribution or the distribution as measured by conventional means, such for example as filtration through a series of filters of prgressively decreasing pass size.

Because, in any event, of the high power (in the order of 20mew as compared to the conventional 1mew) of the gallium arsenide laser 11, the arrangement is suitable for the measurement of particle size distribution in low concentration particle fields, and is especially useful where small (ie below 0.5 micron and even below O.j micron) particles. It is found, with the apparatus of the invention, that particles smaller than 1 micron can be examined down to concentrations of less then 20mg/m3 and even, possibly, to as low as 2mg/m3.

Because, also, of the high rate and high power at which the laser 11 can be pulsed, if, especially, it comprises a gallium arsenide laser, a large number of readings can be taken of a continuously changing environment. For example, ten readings per second can be taken.

The readings can be controlled or initiated in response to radio signals picked up by radio detector 15. Such detector 15 is of especial utility when a number of instruments are to be used together as illustrated in Figure 4 to investigate or evaluate tie output of a contaminant source 41. The instruments 42 are arranged in a preselected pattern as suggested in Figure 4 and controlled from a remote radio transmitter 43 to fire all at the same time for a predetermined number of readings, or otherwise as desired.

The stored data can later be interrogated by or fed into a computer (not shown) which can analyse the individual detector or compare the output from different detectors as a function of time.

In this way, for example, or the course of airborne particle or droplet contamination can be monitored.

Detector Inner Outer Ring No Radius Radius (mm) (mm) 1 .149 .218 2 .254 .318 3 .353 .417 4 .452 .518 5 .554 .625 6 .660 .737 7 .772 .856 8 .892 .986 9 1.021 1.128 10 1.163 1.285 11 1.321 1.461 12 1.496 1.656 13 1.692 1.880 14 1.915 2.131 15 2.167 2.416 16 2.451 2.738 17 2.774 3.101 18 3.137 3.513 19 3.459 3.978 2u 4.013 4.501 21 4.536 5.085 22 5.121 5.738 23 5.733 6.469 24 6.505 7.282 25 7.318 8.184 8.219 9.185 27 9.220 10.287 10.323 11.501 29 11.537 12.837 30 12.873 14.300 Table 1 Dimensions of the annular detector elements of the Malvern particle sizer photodiode array.

Claims (35)

CLAIiflS

1. A method for measuring particle size distribution in low concentration suspended particle fields by forward light scattering comprising using a high power light source.

2. A method according to claim 1, in which the light source comprises a pulsed laser.

3. A method according to claim 2, in which the laser is a gallium arsenide laser.

4. A method according to any one of claims 1 to 3, in which the mean power of the light source is of the order of 20 or more.

5. A method a(cord3rg to any one of claims 1 to 4 comprising forming a Fraunhofer diffraction pattern from a parallel beam of monochromatic light passed through the particle field on a detector in the focal plane of a fourier transform lens, stopping unscattered light from reaching the detector by beam stop means, and correcting the measured intensity of the diffracted beam at different radii to compensate for the blockage of the diffracted beam by the beam stop.

6. A method according to claim 5, for an arrangement in which the beam is of circular cross-section, in which the beam stop is circular and coaxial with the beam and has a diameter equal to or greater than the bean diameter.

7. A method according to claim 5 or claim 6, in which the beam stop comprises a black body cavity.

d. A method according to clair 5 or claim 6 in which the beam stop comprises a hole in the detector.

9. A method according to claim 6, claim 7, or claim 8 in which the detector comprises a plurality of concentric ring detectors arranged coaxially with the beam, and the measured intensity at each ring is corrected by dividing it by (1-B) where B is a blockage factor defined as (l- /E ) where E and E are respectively the energy pa p a incident or, the ring when the stop is present and when the stop is absent.

10. A method according to claim 3, in which the blockage factor B for a ring having inner and outer radii Rl and R2 is approximated by assurning that the energy scattered by a particle to the ring, expressed as a fraction of the energy incident on the particle, is constant over the ring radius between R1 and R2.

11. A method according to claini 9, in which the blocking factors are determined by calibration against a field of known particle size distribution.

12. A method according to any one of claims 1 to 11 in which the length of the particle field from which light is scattered is such that all scattered light is collected and forded into a diffraction pattern at the detector.

13. A method according to any one of claims 1 to 11 wherein the extent of the particle field is such as to give rise to vignetting, comprising correcting the measured intensity of the diffracted beam to compensate for the vignetting effect of the extended field.

14. A method according to claim 13 in which the detector comprises a plurality of concentric ring detectors arranged co-axially with the beam, and the measured intensity at each ring is corrected by dividing it by (if ) where H an efficiency defined for any point P on the p p detector by H = Energy collected on ring at point P divided by p Energy collected on ring at point P but using an infinite lens aperture.

15. A method according to claim 14, in which the efficiency H for a ring having inner and outer radii R1 and R2 is approximately by assuming that the energy scattered by a particle to the ring, expressed as a fraction of the energy incident on the particle, is constant over the ring radius between Ri and R2.

16. A method according to claim 13 in which the intensity of the diffracted beam is corrected by calibrating the measured intensity of the diffracted beam against a particle field of known size distribution.

17. A method according to any- one of claiins 1 to 6, in which a measurement is made of the diffraction pattern from a single pulse of light.

18. A method according to any one of claims 1 to 7, in which a series of measurements is made of the diffraction patterns from a series of pulses of light so as to show how particle size distribution is varying with time.

19. A method according to claim 17 or claim 18, in which the measurement frol a single pulse of light is stored in a memory arrangement for later recall and evaluation.

20. A method for measuring particle size distribution over a large region comprising measuring the distribution at a number of places in the region sinultaneously by a method according to any one of claims 1 to 19.

21. A method for measuring particle size distribution substantially as hereinbefore described with reference to the accompanying drawings.

22. Apparatus for measuring particle size distribution in low concentration suspended particle fields by forward light scat--ering, comprising a high puwer light source.

23. Apparatus according to claim 22, said light source comprising a pulsed source.

2. Apparatus according to claim 22 or claim 14, said light source comprising a laser.

25. Apparatus according to claim 24, said laser being a gallium arsenia- laser.

26. Apparatus according to any one of claims 22 to 25, in which the l:ght source has a mean power in the order of 20mW or more.

27. Apparatus according to any one of claims 22 to 26, comprising beam stop means preventing unscattered light reaching the detector.

2b. Apparatus according to any one of claims 22 to 27, in which the length of the particle field from the source of collimated light to the Fourier transform lens is about 1.2 times the focal length of the said lens.

29. Apparatus according to any one of claims 2 to 28, having data processing aid storage means processing and storing the measurement effected by the detector.

30. Apparatus according to claim 29, in which the light source is pulsed and the measurements from successive pulses are processed and stored.

31. Apparatus according to any one of claims 22 to 30, comprising control means effecting pulsing of the light source.

32. Apparatus according to claim 31, said control neans comprising remote control means.

33. Apparatus according to claim 32, said remote control means comprising radio control means.

3c. Apparatus according to any one of claims 22 to 33, contained in a portable unit having an internal power source.

35. Apparatus substantially as hereinbefore described with reference to the accompanying drawings.