Relating to the measurement of particle size distribution

A particle size distribution analysis apparatus wherein there are provided a sample measurement zone adapted to contain a sample of particles, a light emitting means adapted to provide a source of light incident upon the measurement zone, and a detection means adapted to measure light levels at different scattering angles and to output signals to a computation means, enabling the size of particles contained within the sample to be determined, wherein the light emitting means comprises a first light source emitting a substantially monochromatic first wavelength of light and a second light source emitting a substantially monochromatic second, different, wavelength of light.

This invention relates to an improved apparatus and method for measuring
 particle size distribution,
 It is known to provide particle size distribution analysis apparatus which
 rely on the scattering of light incident upon a sample of particles and
 have multiple laser sources at different angles relative to a cell
 containing the sample. An example of such a system is shown in EP 0 559
 529 wherein a further beam of laser produced light is introduced by an
 optical fibre at an angle to the main laser beam. In such systems the
 minimum size of particle which can be detected is reduced by transforming
 the information from the extra, angled, lasers in to further angular
 information. It is also known to introduce an extra light source and
 filter to achieve further information as shown in U.S. Pat. No. 5,164,787.
 It will be appreciated that the term particle may mean any phase of a
 discontinuous material contained within a continuous phase of a supporting
 medium. Either phase may be gaseous, liquid or solid. The only physical
 limitation is that the particle must have a different refractive index to
 the medium and further, that the medium must be substantially transparent
 at any illuminating wavelength of light.
 According to a first aspect of the invention there is provided a particle
 size distribution analysis apparatus wherein there are provided a sample
 measurement zone adapted to define a sample of particles, a light emitting
 means adapted to provide a source of light incident upon the measurement
 zone, and a detection means adapted to measure light levels at different
 scattering angles and to output signals to a computation means, enabling
 the size of particles contained win the sample to be determined, wherein
 the light emitting means comprises a first light source emitting a
 substantially monochromatic first wavelength of light and a second light
 source emitting a substantially monochromatic second, different,
 wavelength of light.
 An advantage of such a system is that the range of particle sizes which can
 be determined by the calculation means is increased over a system with
 only a single light source.
 In the preferred embodiment at least the first source is a laser, possibly
 a He/Ne laser and preferably a red light laser.
 Preferably at least the second light source is an LED (light emitting
 diode). This has the advantage that a cheap and robust light source is.
 provided which has a longer life than other light sources, is physically
 small, and does not produce a large amount of heat.
 In another embodiment at least the second light source may be a laser
 diode.
 Some prior art systems have provided a plurality of light sources. However,
 such prior an systems have tended to use a second source which was not
 mono-chromatic, for example a tungsten halogen source. The provision of
 such a light source is disadvantageous because it is bulky, does not have
 a long life, produces a large amount of heat, must be left energised for
 long periods to ensure thermal equilibrium is achieved and for application
 to the arrangement of the present intention would require filtering means
 to ensure that only a substantially monochromatic light were output.
 The second light source may output light with a wavelength substantially in
 the range 350 nm to 550 nm. The second light source may output light with
 a wavelength substantially in the range 400 nm to 500 nm and possibly the
 second light source may output light with a wavelength of substantially
 466 nm. The skilled person will appreciate that in the preferred
 embodiment the second light source should output light with as small a
 wavelength as possible but that practical considerations may mean that a
 compromise wavelength is used. Such considerations include cost of
 manufacture, availability of suitable light sources, the stability of
 available sources, etc.
 The second light source may emit light that is monochromatic enough so that
 it does not need filtering to achieve analysable scattering results (i.e.
 no monochromatic filter may be provided). Alternatively, for some
 applications we may provide a filter.
 The first light source may output light with a wavelength substantially in
 the range 533 nm to 2 .mu.n. The first light source may output light with
 a wavelength substantially in the range 583 nm to 683 nm and in one
 embodiment the first light source may output light with a wavelength of
 substantially 633 nm. The skilled person will appreciate that the choice
 of wavelength of the first light source may be influenced by practical
 considerations.
 The light sources may emit light with wavelengths differing by
 substantially 170 nm or may be by substantially 300 nm, 250 nm, 200 nm,
 150 nm, 100 nm or 50 nm. However, the skilled person will appreciate that
 the it is desirable to have a larger wavelength difference than this and
 also that the device may well work with a wavelength difference smaller
 than this.
 Preferably the light sources are arranged so that beams of light emitted
 substantially superpose (or substantially superimpose) one another on the
 measurement zone. This has the advantage that the structure of the
 apparatus is simplified.
 The second light source may be arranged so that a beam of light that it
 emits is inclined at an angle to a beam of light emitted from the first
 light source. Again, this has the advantage that the structure of the
 apparatus is simplified. The second light source may be arranged at an
 angle substantially in the range 0.degree. to 30.degree. to the beam of
 light emitted from the first light source. The second light source may be
 arranged at an angle substantially in the range 10.degree. to 20.degree..
 In the most preferred embodiment the second light source may be arranged
 at substantially 10.degree. to 15.degree., most preferably at
 15.degree..+-.1.degree..
 It is advantageous to allow the second light source to pass directly
 through the sample by inclining the beam at an angle to the beam emitted
 from the first light source as this means that no beam splitter is
 required, thus simplifying the optical components of the system.
 Preferably light emitted from the second light source and the optical axis
 of the first light source lie in a plane which is inclined at an angle
 .phi. to a plane in which the detection means (which may include a large
 angle detector, a forward angle detector, a focal plane detector, s back
 scatter detector) is situated. Preferably the angle .phi. is substantially
 a right angle. For the avoidance of doubt this is illustrated in FIG. 1 of
 the accompanying drawings wherein the optical axis of the first light
 source is along the z axis, light emitted from the second light source is
 inclined at angle .phi. to the z axis in the yz plane and wherein the
 detection means is provided in the xz plane.
 The second light source may be adapted, in use, to be pulsed. This has the
 advantage that the signal to noise ratio of the system may be increased
 and also the peak intensity of the system may be increased.
 At least one light output stabilisation means may be provided to ensure
 that the light emitted from either (or both) of the light sources is
 constant. Preferably a first light source stabilisation means is provided
 to monitor the first light source and/or an a second light source
 stabilisation means is provided to monitor the second light source.
 The stabilisation means may comprise a primary monitoring means and primary
 processing means. The processing means may be connected in a closed loop
 which uses the detected signal from the primary monitoring means to
 control the output power of the light emitted from the respective light
 source. Alternatively, or additionally, instead of controlling the output
 of the light source to be stable we may allow it to fluctuate and the
 primary processing means may output a signal representative of the light
 power emitted from the respective light source to enable provision for
 fluctuations in the light power emitted from the light source in
 subsequent calculations relating to the particle sizes.
 One of tie light sources may have the processing means connected to a
 closed loop in order to control the output power of the light source and
 the power from the other of the light sources is allowed for in subsequent
 calculations relating to the particle sizes. Preferably it is the second
 light source which is monitored with a closed loop monitor and has its
 output power so controlled. Further, it is preferable for the laser to be
 monitored and the signal representative of the of the light power emitted
 from the light source to be allowed for in subsequent calculations
 relating to the particle sizes.
 The stabilisation means may further comprise temperature, stabilisation
 means providing provision to take into account the variation in
 temperature of the apparatus. The temperature stabilisation means may
 comprise a monitoring means and processing means, which may be
 substantially identical to the primary monitoring and primary processing
 means, and the primary processing means may be arranged so that no light
 is received by the monitoring means. Temperature variation compensation
 may be provided only for one light source (e.g. the second light source),
 or for both. We may prefer to arrange for the first light source to be
 left on substantially continuously during an operational programme, with
 the second light source being turned on and off periodically, which will
 minimise temperature changes for the first light source.
 The detection means may comprise a plurality of detectors arranged
 substantially in a plane. The second light source and beam emitted from
 the second light source may be provided in a plane transverse to the plane
 of the detectors of the detection means. Preferably, the plane of the
 second light source and beam emitted from the second light source may be
 substantially perpendicular to the plane of the detectors of the detection
 means.
 Preferably the beam of light output from the first light source is
 collimated. Collimation may be achieved by the use of lenses.
 A second light source transmission detector may be provided to measure the
 level of light from the second light source transmitted through the
 measurement zone. Preferably the second light source transmission detector
 is situated substantially directly opposite the measurement zone from the
 second light source (along a straight line).
 A first light source transmission detector may be provided to measure the
 level of light from the first light source tramsmitted through the
 measurement zone. Preferably the first light source transnmission detector
 is situated substantially directly opposite the measurement zone from the
 first light source.
 The detection means may comprise one or more of the following in addition
 to the second light source and the first light source transmission
 detectors: a large angle detector adapted to detect light reflected at
 large angles by the sample, a bat scatter detector adapted to detected
 light reflected back towards the light source by the sample, forward angle
 detectors adapted to detect light reflected at medium angles by the sample
 and a focal plane detector adapted to detect light reflected at small
 angles by the sample.
 The large angle detectors may detect light reflected from a light source by
 the sample at angles substantially in the range of 80.degree. to
 30.degree. from the axis of the laser beam with the direction of travel of
 the incident laser light taken as 0.degree.. There may be two large age
 detectors possibly situated substantially at 45.degree. and 60.degree..
 The back scatter detectors may detect light reflected from a light source
 by the sample at angles substantially in the range of 100.degree. and
 150.degree. from the axis of the laser beam with the direction of travel
 of the incident laser light taken as 0.degree.. There may be two back
 scatter detectors possibly situated substantially at 120.degree. and
 135.degree..
 The forward angle detector may be situated substantially in the range of
 15.degree. to 45.degree. from the axis of the laser beam with the
 direction of travel of the incident laser light taken as 0.degree.. The
 forward angle detector may comprise an array of detectors and may in the
 preferred embodiment comprise nine detectors in an array.
 The focal plane detector may be situated substantially in the range of
 0.degree. to 30.degree. from the axis of the laser beam with the direction
 of travel of the incident laser light taken as 0.degree.. The focal plane
 detector may comprise an array of detectors and in the preferred
 embodiment comprises an array of thirty three detectors.
 Preferably the large angle detectors and the back scatter detectors receive
 light from both of the light sources. Conversly the second light source
 and first light source transmission detectors may be designed to received
 light only from the second light source and first light source
 respectively. The readings from the focal plane and forward angle
 detectors may only be valid for scattering of light from the laser.
 A computational element (e.g. a computer or microprocessor) may be adapted
 to determine the obscuration of the light emitted from the second light
 source. The computational element may also be adapted to determine the
 obscuration of the light emitted from the first light source.
 The large angle detectors may be provided at substantially a first angle
 relative to the measurement zone and the back scatter detectors may be
 provided at 180.degree. minus the first angle relative to the measurement
 zone.
 That is the back scatter detector may be provided as a mirror image of the
 large angle detectors.
 According to a second aspect of the invention there is provided a method of
 determining particle size distribution comprising illuminating a sample of
 particles with first and second beams of light, emitted from first and
 second light sources respectively, the beams having different wavelengths
 and being substantially monochromatic and the method further comprising
 measuring light levels around the sample to determine the particle size
 distribution within the sample.
 Preferably the first light source is an LED and most preferably a blue LED.
 The second light source may be a laser.
 The method may comprise illuminating the sample with light from each source
 sequentially and may comprise using substantially the same detection means
 to measure light levels produces by each light source.
 The obscuration of the light emitted by the first light source and passing
 through the sample may be calculated giving a first light source
 obscuration signal. Further, the obscuration of the light emitted by the
 second light source passing through the sample may be calculated giving a
 second light source obscuration signal. The first light source and the
 second light source obscuration signals may be used in conjunction to
 increase the range of particle sizes measured by the method.
 The method may comprise using the light level measurements taken using
 light from each detector and manipulating them so that the measurements
 comprise a single data set as if the measurements had been taken by a
 single wavelength of light.
 Detecting forward angle scattering signals may be used to compensate
 detected back scattering signals for reflections of forward scattering,
 thereby producing a processed back scattering signal that is not the same
 as the detected back scattering signal.
 The detected forward angle signal may be subtracted from the detected back
 scattering signal to produce the modified, processed, back scattering
 signal.
 There now follows by ways of example only a detailed description of an
 embodiment of the invention with reference to the accompanying drawings of
 which:
 FIG. 1 shows the co-ordinate system used;
 FIG. 2 shows a basic system configuration;
 FIG. 3 shows an enhanced system configuration;
 FIG. 4 shows a further enhancement of the system shown in FIG. 3;
 FIG. 5 shows an optical arrangement allowing the system to be calibrated;
 FIG. 6 shows an arrangement of detectors;
 FIG. 7 shows a schematic circuit for stabilising a light source of FIG. 4;
 FIG. 8 shows a schematic of the signal and collection circuits of the
 system; and
 FIG. 9 shows the reflections occurring from a sample cell of the system.

FIG. 2 shows a schematic of a basic particle size distribution measuring
 apparatus wherein a low power laser source 2 (light emitting means),
 typically a He--Ne laser is beam expanded and spatially filtered to
 produce a larger collimated beam containing only the TEM 00 mode of laser
 propagation. A lens 4 and a spatial filter component 6 positioned in the
 focal plane of the lens 4 achieves this.
 A beam splitter 8 is typically used in order to allow a small fraction of
 the laser power to be directed onto a laser monitor detector 10. This
 detector 10 allows the incident laser power to be monitored and any
 fluctuations corrected. It is important in the sub-micron measurement of
 particles to ensure that the laser power does not fluctuate between the
 sample and background measurement stages. For this reason optical sources
 are always either directly stabilised or monitored so that compensation
 can be performed. Non laser and semiconductor laser optical sources can be
 readily power controlled, however gas lasers need to operate in a steady
 state mode.
 Although intensity control of a gas laser can be achieved by feeding the
 detector signal into the measurement electronics as gain compensation (for
 example it can be used to modulate the ADC reference voltage so that the
 ADC conversion characteristic is constant regardless of laser power) it is
 preferred to use a different and better approach. The laser power is read
 as a data value and allowed for by performing a scale correction during
 subsequent data processing by a signal processing unit.
 A moveable shutter (not shown) is provided to turn off the laser and can be
 introduced under system control allowing the laser illumination to be
 blocked from impinging on a sample cell without in fact removimg power
 from the laser. This is a commonly employed approach for gas lasers, which
 do not respond well if turned off and on frequently. The purpose of the
 shutter is to allow the laser power to be removed in certain measurements
 of the detection system, for example, dark current and electronic offsets
 (and when using a second light source to scatter from the sample).
 A range lens 12 then focuses the beam 14 so that a diffraction limited spot
 is produced in a plane of a focal plane detector 22. The laser beam also
 passes through a sample region 16 (or measurement zone) into which sample
 particles will be introduced.
 In principle the sample region or measurement zone needs no physical parts
 to define it, since particles may be driven through the beam without any
 form of containment. However, it is preferred to provide a "sample cell"
 (acting as a sample continent means, or measurement zone) in order to
 provide protection of the optical system and containment of any particle
 carrier fluids. A sample cell would typically consist of two glass windows
 18, 20 spaced apart by a well defined distance which are built into a cell
 body (not shown) designed to contain and suspend/circulate the particles
 in a carrier medium.
 The windows 18, 20 allow the access and egress of the laser beam and of
 scattered light from the sample particles over the required practical
 range of the system. The carrier medium may be liquid or gas, the most
 common media being water and air. The sample region or cell 16 is the
 intersection of the laser beam diameter with the space between the
 containment windows 18, 20.
 The sample region 16 is positioned at a known distance F from the focus
 point of the laser beam. The dimension F is critical in that it defines,
 for a given set of components, the available size range of the system for
 the system shown in FIG. 2. If further detectors are added the particle
 size distribution which can be measured is extended.
 At the focal point of the laser beam a multi-element focal plane detector
 22 is positioned, conventionally constructed as a single silicon
 photodiode array and an example of such a multi-element detector layout is
 shown in FIG. 6.
 In the remaining description the following co-ordinate system has been used
 and is shown in FIG. 1. The direction of the first light source or laser
 light propagation is assumed to be the positive Z direction. The X-Z plane
 is assumed to be the "horizontal plane" and Y-Z the "vertical" plane. The
 arrows indicate the positive direction for all co-ordinates. Angle .theta.
 indicates the scattering angle away from the laser axis Z to an
 arbitrary-point P, in the plane containing both Z and P. The angle .phi.
 is the azimuthal angle from the X-Z plane around the Z axis to the point
 P.
 Performance enhancing improvements to the basic system of FIG. 2, allowing
 smaller particles to be detected, are shown in FIGS. 3 and 4.
 The skilled person will realise that in some prior art systems multiple
 laser sources at different cell entry angles are used allowing the
 detector to be transformed into apparently new collection angles. Also,
 some prior art system utilise multiple detection systems with a single
 laser source to achieve the same effect. Although not commonly employed
 any permutation of these principles is also possible.
 In the preferred embodiment a further array of detectors is used to extend
 the optics described earlier as shown diagrammatically in FIG. 3. The
 extra detectors decrease the minimum size of particle which can be
 measured pushing the detectable particle size down to approximately 0.2
 .mu.m.
 A series of nine forward angle detectors are provided although only five
 are shown in the FIGS. 24, 26, 28, 30, 32 on a PCB that runs the length of
 a cell-detector void. The detectors 24, 26, 28, 30, 32 are mounted
 physically aligned to face the cell 16. Each detector 24, 26, 28, 30, 32
 is located at a special distance and angle from the cell 16 which is
 selected to optimise the information content of the entire system.
 As the position of a detector depars from the focal plane of the lens 12
 the detector no longer perfectly integrates over .theta.. Instead the
 detector measures signals over an additional .DELTA..theta. which
 increases the nearer the detector is to the cell 16. This error in the
 angular collection of the detector can be predicted and therefore taken
 account of in a theoretical model of the system. In addition it is known
 that the light scattering characteristic changes more slowly as the angles
 become larger and thus the effect of the slight integration error becomes
 smaller and eventually negligible. The detectors 24, 26, 28, 30, 32 are
 provided and can be considered to be a simple angular continuation of the
 signal received on the focal plane detector 22. Indeed in the preferred
 embodiment the detectors 24, 26, 28, 30, 32 are routed into the same
 signal conversion electronics as the signals from the focal plane detector
 22,
 Because of the reducing angular dependence of the signal it is normal for
 the detectors 24, 26, 28, 30, 32 not to form a continuous angular sequence
 since no information can get "lost" in the gaps. Further, the detectors
 24, 26, 28, 30, 32 are rectangular or circular standard components and not
 the angular ring structure of the focal plane detector 22.
 At angles approaching 45.degree. or greater a new problem in signal
 detection becomes relevant. The detectors 24, 26, 28, 30, 32 are operating
 as simple line of sight detectors and thus they see also the stray
 reflections from the cell 16 (and multiple reflections). At forward angles
 the scattering from particles is strongly dominant and the stray multiple
 reflections are safely ignored. At the larger angles the tilt of the cell
 brings the cell walls more directly into view and the particle scattering
 intensity is typically masked. That is the detector field of view
 eventually brings the cell wall into view and the detector consequently
 collects light reflected from the cell wall as well as light scattered by
 the particles. This forces consideration of the proper spatial filtering
 of the received detector signals at larger angles.
 As a consequence for these larger angles (that is angle approaching
 45.degree. or greater) a detector is produced as a small element
 positioned in the focal plane of a collection lens. The combination forms
 a spatial filter, and ensures that the detector receives light only from a
 narrow collection angle. This allows the cell wall signals to be
 acceptably rejected and not interfere with the measurement. Because of
 this additional complexity these detectors are used sparingly and referred
 to as large angle detectors 34, 36 in FIG. 3.
 A further enhancement of the basic system of FIG. 2 is also shown in FIG. 3
 and comprises providing back scattering detectors 38, 40. These detect
 light scattered by particles from the rear of the cell; that is where
 90.degree.&lt;.theta.&lt;180.degree.. This detection is particularly important
 for the submicron determination of particle size. However light scattered
 at backward angles has poor angular variation and thus there is little
 need to sample at small angular increments. The back scatter detectors 38,
 40 have mirror symmetry with the large angle detectors 34, 36 about an X-Y
 plane passing through the sample cell, and are constructed in an identical
 manner. The mirror symmetry is a convenience that allows for a simple
 correction of the large angle 34, 36 and back scatter 38, 40 detectors for
 the high cell reflectivity that occurs at these large exit angles from the
 cell 16.
 Thus the basic approach of the preferred embodiment has a detection means
 comprising a focal plane detector 22 of thirty three elements, a
 transmission detector 50, an forward angle array of nine further elements,
 two large angle detectors 34, 36 and two back scatter 38, 40 detectors.
 The detectors are all disposed in a single plane with respect to the laser
 polarisation plane.
 FIG. 4 shows a schematic of a further enhancement of the system shown and
 described in relation to FIG. 2 which allows the system to measure
 particles to a size of less than 0.1 .mu.m and to improve the resolution
 for particle sizes of less than 1 .mu.m.
 A second light source with a shorter wavelength than the laser 2 is
 provided and the large angle and back-scatter responses to this light are
 measured to gain the extra resolution. The use of shorter wavelengths is
 the key to the bottom end size reduction and the combination of the two
 wavelengths increases the available sub-micron resolution of the system.
 The shorter the wavelength the greater the enhancement to the sub-micron
 range and resolution. However there are practical considerations that
 prevent a substantial reduction in wavelength at effective cost. Thus only
 visible light wavelengths may be a practical option. In the system of FIG.
 4 an LED 42 (a light emitting means) which emits blue light has been used.
 Other sources such as laser diodes may be used.
 The sub-micron performance is only enhanced by the large angle scattering
 from the additional optical measurement and small angle scattering is
 largely redundant as it is duplicating data in the original measurement.
 Thus there is no need to produce a reduced wavelength light source 42
 capable of the same stringent spatial filtering as the main optical
 system. This means that the requirements for collimation of the second
 beam are reduced significantly since only large angle scattering needs to
 be measured and the light output of an LED or laser diode is sufficiently
 collimated and monochromatic.
 Further, the source 42 need only be near monochromatic rather than have the
 wavelength purity of the laser source 2. This is because small wavelength
 errors are equivalent to small angular errors. For the large angles of
 interest the angular dependence of particle scattering is reduced and thus
 the wavelength spreading effect becomes negligible.
 Thus the blue LED 42 or laser diode emitting a blue beam of light are two
 examples of suitable sources provided they have a narrow spectrum of
 output light, typically .+-.50 nm. The ability to use an LED or laser
 diode as the source significantly reduces the cost of implementation and
 improves the long-term robustness.
 The shorter wavelength second light source 42 is provided off-axis to the
 laser source 2 and in a plane perpendicular to the plane formed by the
 detectors 24, 26, 28, 30, 32 and the large angle 34, 36 and the back
 scatter 38, 40 detectors. (That is FIG. 4 has been rotated 90.degree.
 about the z axis relative to FIG. 3). If the angle of the beam from the
 second light source 42 relative to the z axis is kept small it is possible
 to re-use the large angle 34, 36 and the back scatter 38, 40 detectors in
 measurements using both light emitted from the second light source 42 and
 the laser 2. This has the benefit of avoiding the cost of further
 detectors specifically for measurement of signals produced by the second
 light source 42. Although feasible it may not be considered necessary to
 utilise the larger angle elements of the detectors 24, 26, 28, 30, 32 for
 the short wavelength requirements (or we may re-use them in this way in
 other embodiments).
 In the preferred embodiment the second light source 42 is provided at a
 shallow angle, typically 10-15.degree. to the main beam path, sufficient
 to allow the optical components to co-reside without mechanical
 interference.
 In one embodiment the second light source is an LED 42 and has a wavelength
 of typically 466 nm and with a narrow spectral range, typically +/-30 nm
 half width, half height. The light emitted from the LED 42 is collected
 and collimated by a single lens 44 which projects a beam through the cell
 windows 18, 20 at the same physical location as the beam from the laser 2.
 Thus the same cell 18, 20 windows are effective for measurements from both
 the LED 42 and the laser 2 and there is no need for a dual cell
 configuration.
 The LED 42 projects the blue beam through the cell 16 so that it superposes
 the area where the beam from the laser 2 intercepts in the cell 16
 (preferably exactly superposes). The unscattered beam from the LED 42
 exits the cell 16 and is collected by an LED transmission detector 45 that
 measures the transmission of the blue beam through the cell. This LED
 transmission detector 45 also requires spatial filtering in order to
 improve the angular resolution of the measurement. This is achieved by use
 of a detection scheme (identical to that of the large angle 34, 36 and
 back scatter 38, 40 detectors described earlier), having a small detector
 element 45 in the focal plane of a collection lens 43.
 The structure of the focal plane detector 22 is shown in FIG. 6. A centre
 of the focal plane detector 22 comprises a structure that is designed to
 allow the monitoring of the unscattered laser beam power. This may be
 implemented in one of three forms, a hole drilled through the wafer from
 which the detector is fabricated, a detector structure built on the wafer
 surface, or a mirror-like element that reflects the laser beam off the
 surface to an auxiliary detector mounted elsewhere. Each solution is aimed
 at allowing a measure of the power of the focused spot, which gives the
 power of the unscattered laser 2 output power.
 In the preferred embodiment the wafer 46 from which the focal plane
 detector 22 is fabricated is drilled completely through from front to back
 with a small hole 48 of exact diameter and positioning. The focal plane
 detector 22 is aligned with the laser beam so that the diffraction limited
 spot falls down through the hole and out of the rear of the focal plane
 detector 22. A P.C.B (not show) supporting the wafer 46 is provided with
 suitable clearance holes to allow the beam to expand from the rear of the
 focal plane detector 22 and to fall onto a transmission detector 50.
 In addition to the central hole 48 the focal plane detector comprises a
 series of annular ring detectors (51 to 62). Each detector is defined by
 an inner R.sub.1 and an outer R.sub.0 radial boundary, and an azimuthal
 angle .DELTA..phi.. The detectors may be constructed with a wide variation
 in the number of ring detectors provided and spacing in the spacing of
 each of the detectors, each different design trying to optimise the
 system. [For clarity, only twelve ring detectors are shown in FIG. 6, but
 there may be any other number. In one embodiment there are thirty three
 detectors.]
 If the focal plane detector 22 has no hole and the transmission detector
 were provided on the surface of the focal plane detector 22 then it is
 possible to construct the transmission detector 50 as three or four
 sub-elements. The laser beam is then adjusted until it equalises the
 signal contribution from each sub-element, the sum of all elements being
 used as the reading. Whilst this is a convenient layout it suffers from
 specific disadvantages. The first is that it depends on the beam having
 circular symmetry, whereas the real beam may have aberration. The second
 is that the detector structures are very small and are thus subject to
 significant error in dimensions through photolithography limitations.
 In another embodiment the focal plane detector 22 has a hole at a centre
 portion and an auxiliary transmission detector 50 to measure the initial
 laser beam intensity and it is possible to similarly split this detector
 into sub elements. Again the beam is adjusted to balance the relative
 powers on the sub-elements with the entire signal being used as the
 transmission measure or level of initial laser beam power. This approach
 eliminates the problem of having detector elements of small sizes since
 the beam has expanded considerably by the time it hits a transmission
 detector 50 beyond the focal plane detector 22 and typically expands to
 substantially 3 mm. Thus, the sub-detectors of the transmission detector
 50 can be larger with the same discrimination.
 However it adds new difficulties in that the transmission detector 50 is
 normally mounted to the focal plane detector 22 by hand and thus its
 alignment with the centre of the focal plane detector 22 has to be
 calibrated during assembly in some way. Given the extreme precision
 involved this requires extra expense. In addition since in some systems
 multiple ranges are achieved by changing the distance F, the alignment of
 the detector normal to the Z direction must be exact to avoid the
 alignment point apparently moving at different range positions.
 A single detector element provides an integration (over time) of the light
 scattered by particles into those angles received between the R.sub.I and
 R.sub.o boundaries. These angles are also determined by the cell 16--focal
 plane detector 22 distance F (as shown in FIG. 2). It may be possible to
 arrange the lens 12 so that cell 16 to focal plane detector 22 length F is
 varied. Such a variable arrangement allows a wider range of angles to be
 covered by the instrument in a series of size ranges determined by the
 distance F. In the preferred embodiment a single range lens 12 and a fixed
 cell 16--focal plane detector 22 distance, F, with range extension being
 achieved by the use of the large angle 34, 36, the back 25 scatter 38, 40
 and detectors 24, 26, 28, 30, 32.
 In order to measure the largest possible particle sizes the inner detector
 51 wants to measure the smallest angles possible and in practise means
 that the size of the central hole 48 needs to as small as is practical
 whilst collecting all of the beam from the laser 2 with the first detector
 as close to the boundary of the hole 48 as possible. The practical limits
 of photolithography and micromachining determine this smallest detector
 dimension.
 In order to measure the smallest particle Sizes the detectors need to
 subtend larger angles, eventually even back-scatter angles are required.
 There is a clear practical limit to the range of angles that can be
 covered by a system having only a focal plane detector 22. The limit is
 determined by the largest physical dimensions that can be integrated into
 the planar focal plane detector array and such systems are typically
 limited to angles up to 30.degree., which prevents accurate sizing below
 0.3 .mu.m.
 As discussed hereinbefore the light sources must be stabilised or
 corrections made for the power fluctuation. Unlike the laser 2 the LED 42
 has desirable properties with regard to light power control in that it can
 be turned on and off at will and rapidly stabilises in output power. It
 can also be readily temperature stabilised, and outputs relatively little
 heat. The output power can therefore be controlled by modulation of the
 LED 42 current. For these reasons the power control of the LED is
 accomplished by a closed loop electronic control system shown in FIG. 7.
 The LED 42 output power is monitored by a stabilisation means which
 comprises a photodiode monitor 68 (primary monitoring means). The monitor
 68 requires no beam splitter or special optics since the LED 42 has
 sufficient optical losses that the detector can monitor using the stray
 light 70 lost from the LED plastic body. The stray light is represented in
 the Figure by the outer region around the main beam 72 from the LED 42.
 The monitor 68 provides a feedback signal to a current control circuit 74
 (or primary processing means) that varies current to the LED 42 until an
 output power is achieved that matches an input control demand.
 In order to provide temperature compensation for the LED stabilisation
 means a further identical detector held in blackout conditions but under
 the same temperature environment way be used to provide a temperature
 stabilsation means. This allows the monitor signal to be compared
 differentially with the signal from the blacked out detector giving common
 mode rejection of temperature variation.
 This provides a stable known output light intensity from the LED 42 that is
 entirely electronically servo controlled. This implies that the LED 42
 power monitor does not require that it be fed into the computing element
 (as is done with the reading from the laser monitor detector 10), since it
 can be taken as pre-calibrated.
 An alternative/different approach to providing the same control effect
 would be to feed the signal from the monitor 68 into the computational
 element 77 so that the gain compensation could be performed by digitally
 re-scaling the data obtained using the LED 42. (That is as is done with
 the signal from the laser monitor detector 10). This would avoid the need
 for closed loop control of the LED 42 power supply, which could instead
 then work as a constant current source. We prefer to stabilise the LED
 since it is an elegant solution and avoids unnecessarily complicated
 computational calculations.
 In order to enter the data into a computational element 77 it is
 fundamentally necessary to multiplex parallel data produced by the system
 to a serial stream that can be read through a common interface. There are
 many conventional ways to perform this using electronic systems, analogue,
 digital or bus based multiplexers an all commonly utilised singly or
 mixed.
 In the preferred embodiment the arrangement in FIG. 8 is used to process
 the data produced by the system. Each detector in the system (the
 detectors are represented on the right hand side of the Figure and are
 connected via busses to the remaining circuitry of the Figure), is
 provided with its own dedicated transimpedance gain amplifier followed by
 a sample and hold stage (represented by S/H in the Figure). The gain stage
 of the amplifier lifts the signal levels to a sufficient level to allow
 the following signal processing to introduce negligible error.
 The sample and hold circuits S/H are connected to a common timing line 78.
 An address line 104 is connected to the multiplexers 76, 106, 108, 110.
 The analogue to digital converter 82 is connected to a data line 102 and a
 timing lines 100.
 The parallel outputs of the detectors (apart from the LED stabilization
 means) are fed to a multiplexing element 76 that is conventionally
 implemented as a cascade of analogue multiplexers. Equally the
 multiplexing is sometimes achieved digitally using control of output
 enables of bus connected drivers.
 The single channel of output is further gain and offset adjusted by an
 amplifier 80 and then input to an Analogue to Digital Converter (ADC) 82.
 The ADC will, on command, convert the signal value to a digital
 representation at a specified level of precision. This value is then read
 by a computational device (not shown), typically a microprocessor and read
 into a memory location.
 The computational element operates on the scattering data to fit it to
 known Fraunhofer and/or Mie Scattering theories to evaluate the particle
 size distribution by evaluating the best fit distribution that would
 produce the detected scattering.
 In prediction of the back scatter signal expected from theory it has been
 found necessary to take account of the reflection properties of the cell
 windows 18, 20 and those of any other plane surfaces in the scattering
 region, such as protection windows, etc.
 The back-scatter signals are relatively weak at all particle sizes and only
 become significant when the particle sizes become small and scattering
 becomes more isotropic. Thus as size reduces the strongly dominant forward
 scattering becomes weaker until it has reached the same intensity as the
 back-scatter.
 This situation means that if forward scatter light were to be back
 reflected, even inefficiently (for example from a cell window), then it
 would significantly corrupt the back-scatter signals The mechanisms for
 cell reflection in the standard form of cell are as shown below in FIG. 9.
 The principle illumination, or incident beam, is shown above at 84 on the
 main axis, propagating left to right. The main scattering components are
 shown from a notional single particle 86. These rays are those predicted
 by the use of conventional light scattering theories, such as Fraunhofer
 or Mie, etc.
 Additional ray paths 88 exist if each plane surface of the cell 16 is
 considered to have finite reflectivity. These are shown above the incident
 beam 84 and displaced higher on the figure to aid visibility. Sing from
 the centre and working upwards we have, the 0.degree. reflection of the
 unscattered beam 90,91 that is back-reflected and then forward scattering
 off the particle. The forward scattering produces two beams since there
 are two cell window surfaces. Then there are the two components 92, 93
 caused by reflection of the forward-scattered light emerging from the cell
 without further scattering.
 Any other plane surfaces in the system demonstrate the same basic
 behaviour, and therefore any further surfaces between cell and detector
 generate similar signals. Because the detectors have limited collection
 range caused by the typical apertures present in the optical system it is
 generally only the 0.degree. back reflections that needs to be accounted
 for with these additional windows. The 0.degree. reflected beam propagates
 back through the system until it again passes through the cell and any
 subsequent scattering is then directly in the line of sight of the
 detectors.
 Each reflection depends upon the reflectivity of the surface involved and
 the angle of incidence of the beam. By the appropriate use of
 anti-reflection coatings it is possible to minimise reflections,
 particularly those effects involving 0.degree. reflections. However for
 high angle of incidence reflectivity values of up to 10% can be
 experienced, even with optimised surfaces/coatings.
 The original back-scatters light can also be forward scattered although
 this is usually a small effect due to the strong dominance of the forward
 signal. However for the smallest sizes, where light has become almost
 completely isotropic it is useful to take account of the reflection of the
 back scattered components. In the diagram above this is shown at R, and
 displaced downward from the diagram centre.
 Reflections are cumulative since the first reflected beam might suffer
 further reflection before exiting the cell. However whilst reflectivities
 are &lt;10% any second reflection will have reduced to a &lt;1% effect and thus
 can be safely ignored. For the purposes of the models it is normally
 sufficient to include only first reflection behaviour for each mechanism.
 The overall effect of these multi-component reflections is simple, a
 practical reflectivity R.sub.1 and R.sub.2 can be determined for the cell
 which describe the grossed up cell behaviour at the scattering angle
 concerned. Thus R.sub.1 describes the effect where light originally
 scattered to angle .theta. will be reflected into the back-scatter angle
 180-.theta.. Generally because of their different composition R.sub.1 and
 R.sub.2 are not identical, despite the apparent symmetry of the cell.
 This makes clear that the back-scatter angle 180-.theta. is corrupted by a
 signal component from forward angle .theta.. To take account of this
 theoretically it is necessary to integrate the appropriate light
 scattering theory over the back-scatter angles, and those forward angles
 mirrored by the cell. These two signals can then be combined according to
 the reflectivity R.sub.1. The need to account for the mirror angles
 doubles the computational load for the calculation of the scattering
 matrices, essential to the analysis of data to particle size.
 An advantage accrues in this computation if the large angle detectors 34,
 36 and back scatter detectors 38, 40 operate at angles that are mirror
 symmetric and are preferably identical in construction. If for example
 back scatter detector 40 is mirror symmetric with large angle detector 36
 then the integration of reflection angles appropriate to back scatter
 detector 40 has already been accomplished in computing the forward scatter
 at large angle detector 36. Thus the necessary reflection corrections can
 be performed using the standard theory. For the case mentioned the
 correction would be
EQU BS.sub.2 *=(1-R.sub.2)(BS.sub.2 +R.sub.1,FS.sub.1)
 BS.sub.2 =reading at back scatter detector 40
 FS.sub.1 =reading at large angle detector 36
 Where BS.sub.2 " is the corrected scattering matrix signal under any given
 condition where BS.sub.2 and FS.sub.1 were the original theoretical
 predictions without reflections assumed.
 Thus we compensate the detected back scatter signals by an amount dependent
 upon the detected forward scatter signals to take into account
 reflections.
 The effect of accounting for cell reflections in the performance of the
 size analysis is extremely beneficial at all particle sizes. Without cell
 reflection correction the system under-predicts the amount of back-scatter
 light present for a given material. The excess causes the instrument to
 assume that sub micron particles are present too, since they can give rise
 to back-scatter signals whilst barely altering the forward scatter data.
 By accounting for the cell reflections the system is able to correctly
 predict the back-scatter signal excess, thus improving accuracy of
 measurement for sub micron sizes.
 The mirror symmetry of the large angle detectors 34, 36 and the back
 scatter detectors 38, 40 detectors offers a computational advantage in the
 calculation of scattering matrices only, it does not affect the sizing
 performance directly.
 Although compensation for reflections from cell surfaces have their major
 use in back scattering we may additionally or alternatively compensate the
 detected signals for forward scattering using input from detected back
 scatter signals, but this is likely to be far less significant.
 In the absence of particles in the cell 16 there is no scattering of the
 laser 2 beam or beam from the LED 42 and thus in theory the entire beam
 will pass through the hole 48 in the wafer 46 and onto the transmission
 detector 50. If particles are introduced to the cell 16 of the system then
 light is both absorbed by the particles and scattered into other angles
 resulting in a reduction in the signal received on the transmission
 detector 50. It is normal to measure the transmission before the
 introduction of particles T.sub.RE and when particles are present in the
 cell 16, T.sub.RS. This is used to calculate the "obscuration" of the
 laser beam, which is given by:
EQU O.sub.R =1-T.sub.Rs /T.sub.RE
 The obscuration of the laser beam O.sub.g is used both in the data
 processing to obtain particle size and as a diagnostic to assist in
 setting up a suitable particle concentration range for a particular
 measurement.
 The scattered light from particles present in the cell 16 spreads out into
 all angles, having a size dependent angular intensity distribution
 S(d,.theta.,.phi.). The d represents the particle size, .theta. the
 scattering angle and .phi. the azimuth angle. Because particles are
 normally in random orientation within the cell, and many thousands of
 particles are scattering simultaneously, the azimuthal dependence of
 scattering is lost. It is normal practise to sacrifice any potential size
 information in the .phi. variation to avoid the problems that would arise
 in needing to align particles practically within the cell. Thus these
 systems typically concern themselves with measurement of the .theta.
 variation, assuming .phi. symmetry, reducing scattering dependence to S(d,
 .theta.)
 Broadly, very small particles scatter light isotropically whereas large
 particles scatter into a very small angle around the unscattered beam.
 There are a number of theories available that allow complete prediction of
 this .theta. variation for a known sized particle and thus by measuring it
 the size of the scattering particles can be inferred.
 The LED transmission detector 45 is used in an identical manner to the
 transmission detector 50 for the laser beam, which is to find the sample
 obscuration of the blue light.
EQU O.sub.B =1-T.sub.Bs /T.sub.BE
 Using the same terminology as described earlier for the red obscuration.
 Because the blue beam path is offset in the plane perpendicular to the
 detector plane by a relatively small amount the effect of the offset is
 negligible in altering the scattering angles subtended by the large angle
 38, 40 and back scatter 34, 36 detectors. As a consequence these detectors
 can be considered as occupying identical angles of detection in
 measurement of the blue beam (beam from LED 42) when compared to
 measurement of the red beam (beam from the laser 2). Alternatively, some
 compensation may be applied but we do not believe it to be necessary. The
 detectors 34, 36, 38, 40 have gain characteristics that are different at
 the wavelengths emitted by the laser 2 and the LED 42. In addition the
 data from each detector 34, 36, 38, 40 is weighted differently in the
 analysis for the data obtained for the light emitted from the laser 2 and
 the LED 42. For these reasons the large angle 34, 36 and back scatter 38,
 40 detectors have two gain calibrations recorded, one for light emitted
 from the laser 2 and the other for light emitted from the LED 42.
 The measurement of the transmission of the light emitted from the laser 2
 (and light emitted from the LED 42 in our system) is generally used in
 prior art systems to ensure that the particle concentration in the cell 16
 is in an optimum range. The particle concentration must lie within a
 specified range if the signal processing is to be effective. At high
 concentrations it is important that multiple scattering does not occur,
 and if too low there is inefficient signal created on the detectors for
 reliable measurement. These two limits are usually expressed as an
 obscuration range so that the user can easily determine from the data
 display that both criteria are satisfactory. For example in one prior art
 system it is required to ensure that the obscuration signal lies in the
 following range:
EQU 0.01&lt;O.sub.R &lt;0.5
 In the preferred embodiment a further use has been made of these signals.
 Each attenuation is converted to a synthetic data point, the "Extinction".
 The extinction is related to transmission by the following formula
EQU E.sub.B =-A. ln (T.sub.Bs /T.sub.Be)
 Where E.sub.B is the extinction of the blue beam and A is an arbitrary
 constant.
 The important property of the extinction data that is useful is that it
 behaves linearly with concentration whereas the original transmission and
 obscuration behave non-linearly. This allows the extinction to be treated
 as a data point and the scaling constant A can be set to scale the signal
 so that it fits into the data set with an appropriate significance. Thus
 two additional data points are derived from the transmission values and
 added to the data set that is analysed, the Extinction points for light
 emitted from the laser 42 and emitted from the LED 42.
 These data points are useful in that they are sensitive to the detection of
 small particles. Such materials generate weak scattering signals but are
 effective at reducing the beam transmission, effectively generating high
 extinction. The combination therefore of high extinction and low
 scattering is indicative of fine materials. The disparity between the
 extinction of light emitted from the laser 2 and the LED 42 values also
 contains useful small size information. For larger sizes the extinction
 values are identical, for fine particle sizes the extinction differs. The
 difference increases as size reduces within a useful size range. The
 extinction data points therefore provide size discrimination information
 for small particles. They are approximately equivalent in information
 terms to the back-scatter detectors 38, 40.
 In the preferred embodiment there are back scatter 38, 40 detectors
 provided and it is valid to question why the extinction points are also
 included, if they provide the same size information. There is a further
 benefit to sub-micron capability given by these points however, when the
 sample concentration is low the back-scatter data becomes small, poorly
 resolved and hence subject to substantial experimental error. The absence
 of extinction points would affect the ability of the preferred embodiment
 to repeatable measure small sizes. The transmission measurements are much
 easier to make and remain precise after the back-scatter signals have
 become unreliable.
 Thus the extinction data points enhance the performance extending the size
 range over that obtained using the back scatter 38, 40 signals alone.
 The transmission measurements of the light emitted from the laser 2 and LED
 42 are made one after the other and not simultaneously. During background
 or sample measurements the sequence is the same. The shutter in front of
 the laser is turned on (passing the light from the laser 2), the blue LED
 42 is switched off and measurement using light from the laser 2 is
 performed. When the measurement is completed (for example immediately
 afterwards) the shutter is introduced to block light from the laser 2 and
 the LED 42 is switched on. The same measurement process can then take the
 data obtained from the light emitted from the LED 42.
 The measurement points using light emitted from the LED 42 are extracted
 from this second measurement and inserted into the measurement data
 obtained from measurements taken from light emitted from the laser 2,
 ending it. As the data sets are combined the respective gains of both
 systems are adjusted to comply with a previous system calibration. The
 combined data set becomes the resultant experimental data that is analysed
 to obtain particle size using the computational element 77 and
 computational device.
 As particles pass through the cell 16 many thousands of particles are
 simultaneously illuminated and the signal received on a detector is a
 continuous optical summation of the scattering from all particles within
 the cell 16. As the particles pass through the cell 16 the sample volume
 population fluctuates statistically and thus the signal develops a noise
 like fluctuation reflecting the local population variation.
 It is normal for a detector signal to be integrated over a significant time
 period in order to ensure that the angular intensity curve analysed is
 representative of a large number of particles. The integration process
 thus removes the statistical noise and ensures that the average is
 representative of the entire population of the material. This integration
 can be performed conventionally either by the analogue electronics, by
 digital electronics, by summation in a microprocessor or stand-alone
 computer such as a PC. In the preferred embodiment it is normally
 performed by a microprocessor built into the system.
 In any event the detector data is produced simultaneously from all angles
 during any measurement from either one of the light sources 2, 42.
 The parallel data produced by the system is fed to the circuitry of FIG. 8
 which produces a serial stream that can be read through a common
 interface.
 The Sample and Hold function is operated by a control signal on the common
 timing line 78 and effectively freezes the signal at a single time
 instant. By ensuring that data from all detectors is frozen at the same
 time before conversion it is ensured that no concentration fluctuations of
 the sample in the cell 16 become converted into apparent angular
 fluctuations by the serialisation process that follows.
 At the time at which measurement is required the control signal goes from
 tracking to hold mode which locks the signal readings on the outputs of
 the sample and hold circuits. It is important that data is then converted
 quickly so that signal droop does not occur.
 The computational device connected to the output of the ADC runs an
 algorithm that accesses each detector channel in sequence until all valid
 channels for that wavelength has been collected, digitised and held in
 memory. The complete data set from this single sample and hold event is
 called either a "sweep" or a "snap" and is the smallest unit of
 measurement of data.
 These snaps of complete system data are then taken successively and summed
 by the computational element 77 to form an experiment. A snap requires a
 defined minimum time interval to complete, and multiple snaps are
 performed at the fastest rate that the computational device can
 accommodate. Thus the measurement time is determined by the number of
 snaps requested, usually controllable by the user.
 Because of the time sequential nature of the measurements using light
 emitted by the laser 2 and the LED 42 the measurement is in fact
 accumulated in two sub-experiments. When the beam from the laser is
 incident upon the cell 16 the requested number of snaps of integration are
 first summed, the first experiment. Then the instrument switches
 automatically so that light emitted from the LED 42 is incident upon the
 cell 16 and performs the same number of snaps again, accumulating a new
 record, the second experiment. For the experiment using the LED 42 to emit
 the light most of the data accumulated is unused in our preferred
 embodiment since only the large angle 34, 36 and the back scatter 38, 40
 signals are valid. These data points are extracted by the computational
 element 77 from the second experiment and interleaved with the first
 experiment extending it (extending the angular range of scattering over
 which reliable meaningful signals have been collected). At this point any
 scale compensation required between the optical components for light
 emitted from the laser 2 and the LED 42 is applied. Thus the computational
 device uses experimental results accumulated from the use of light emitted
 from both the laser 2 and the LED 42.
 The obscuration signals for light emitted from both the laser 2 and the LED
 42 from the transmission detector 50 and the LED transmission 45 detectors
 are also read by being passed through the multiplexer and ADC to the
 computational device. Similarly the signal from the laser power monitor 10
 is fed through to allow the signals using the light emitted from the laser
 2 to be scaled to be adjusted for any laser power variation. An obvious
 extension of this approach would be, as mentioned earlier, the reading of
 the blue monitor signal for the same purpose on blue data.
 The apparatus may be provided with a visible light means situated at a top
 most region of the apparatus acting as a power on/off display. That is the
 light means may be adapted to emit light when the apparatus is in a
 powered on situation, and off when there is no power to the apparatus. The
 light means may be situated such that it is visible from substantially any
 angle around the machine which is advantageous in that it allows a user to
 readily determine whether or not there is power to the apparatus.
 The light level measurements taken using light from each detector may be
 manipulated so that the measurements comprise a single data set as if the
 measurements had been taken by a single wavelength of light.