WO2004008117A2

WO2004008117A2 - Method and apparatus for monitoring particles flowing in a stack

Info

Publication number: WO2004008117A2
Application number: PCT/GB2003/003073
Authority: WO
Inventors: Michael Rigby; Michael Wakefield; John Michael Blackwell; Victor Frederick Thomas
Original assignee: Pcme Limited
Priority date: 2002-07-15
Filing date: 2003-07-15
Publication date: 2004-01-22
Also published as: AU2003251334A8; EP1535049A2; AU2003251334A1; GB0216381D0; GB2390893A; WO2004008117A3

Abstract

A particle monitor for monitoring particles flowing in a stack comprises: (i) a light source (90) for generating a measurement beam (60) on a first side of the particle flow (70); (ii) an optical system and a tube connected thereto for directing the measurement beam (60) through said tube towards a second side of the particle flow (70) without the measurement beam (60) scattering from the particles, (iii) a reflector (210) arranged to reflect the measurement beam (60) back towards the first side of the particle flow (70), (iv) an optical system for directing the measurement beam (60) into the particle flow (70) such that light from the measurement beam (60) is scattered by the particles (70); and (v) a scattered-light detector (80) for detecting, on the first side of the particle flow (70), the scattered light.

Description

Method and apparatus for monitoring particles flowing in a stack

This invention relates to the field of monitoring particles flowing in a stack.

When light interacts with particles, the particles may reflect, refract, diffract or absorb the light, the nature of the interaction depending on the size, refractive index and surface profile of the particles and the wavelength of the interacting light.

For objects that are small compared with the wavelength of the light, the light will undergo Rayleigh scattering and a proportion of the light may be redirected in all directions. As the object size increases such that it is comparable to the wavelength of the light a larger proportion of the scattered light is redirected in the forward direction within well-defined angular lobes. This phenomenon is known as forward light scattering or Mie scattering. As the object size increases still further such that it is much greater than the wavelength of light, classical geometric optics begin to dominate.

Particle monitors that use forward light scattering techniques can, in general, be divided into two different categories: in-line and out-of-line. An in-line system utilises a light source, an interaction volume and a detector system on a common optical axis whereas the out-of-line approach places the detector system to one side of the light source axis.

The light scattering approach to dust measurement brings with it problems associated with reliable discrimination between scattered light and stray light or residues from the incident beam. Particle monitoring systems are installed in many dirty processes and hence contamination of optical surfaces is an important issue. Similarly, calibration of existing monitoring systems does not optimally take into account the effects of contamination. Contamination of optical surfaces of the monitor itself may produce unwanted scattered light that is measured by the monitor, giving a false reading. As discussed above, particles of different size scatter light in different directions and so the amount of light received by a prior art probe depends on the size of particles in the flow, as well as their number and mass density; that can lead to errors in measurement. Prior-art monitors use relatively short interaction lengths but short interaction lengths make measurements vulnerable to local inhomogeneities in the particle flow. Readings from prior-art monitors may be affected by the harsh conditions that are prevalent in many stacks. For example, probes using glass-fibres cannot operate above 350°C- 400°C and fibres based on sapphire, which may be able to operate at those temperatures, are very expensive. A particular disadvantage of some prior art designs is that, although they provide mechanisms for checking the calibration of a probe, they do so by moving one or more parts of the system that is to be calibrated in a way that may cast doubt on the reliability of the calibration measurements.

An object of the invention is to provide a particle monitor, and a method of monitoring particles, that provides information about the contamination level of the monitor. Another object of the invention is to provide a particle monitor, and a method of monitoring particles, that provides information about calibration of the monitor. Another object of the invention is to provide a particle monitor, and a method of monitoring particles, in which the effects of unwanted scatter from surfaces of the monitor itself are ameliorated. Another object of the invention is to provide a particle monitor, and a method of monitoring particles, in which the signal detected from particles of different sizes is controlled. Another object of the invention is to provide a particle monitor, and a method of monitoring particles, that monitors a long traverse of the particle flow. Another object of the invention is to provide a particle monitor, and a method of monitoring particles, that keeps sensitive parts of the monitor out of the particle flow. Another object of the invention is to provide a particle monitor and a method of monitoring particles, in which there are few or no changes between a measurement phase and a contamination and/or calibration check phase of operation. Another object of the invention is to provide a light source exhibiting low levels of peripheral light.

According to the invention there is provided a particle monitor for monitoring particles flowing in a stack comprising: (i) a light source for generating a measurement beam on a first side of the particle flow; (ii) an optical system for directing the measurement beam towards a second side of the particle flow without the measurement beam scattering from the particles, (iii) a reflector arranged to reflect the measurement beam back towards the first side of the particle flow, (iv) an optical system for directing the measurement beam into the particle flow such that light from the measurement beam is scattered by the particles; and (v) a scattered-light detector for detecting, on the first side of the particle flow, the scattered light.

Also according to the invention there is provided an installation comprising a stack and such a particle monitor installed in the stack.

Also according to the invention there is provided a method of monitoring particles flowing in a stack, comprising: (i) generating a measurement beam on a first side of the particle flow; (ii) directing the measurement beam towards a second side of the particle flow without the measurement beam scattering from the particles, (iii) reflecting the measurement beam back towards the first side of the particle flow, (iv) directing the measurement beam into the particle flow such that light from the measurement beam is scattered by the particles; and (v) detecting, on the first side of the particle flow, the scattered light.

Such a ^reflective' probe has several advantages. One of the greatest difficulties in designing a monitor is accommodating the high temperatures experienced when the monitor is inserted into a hot stack. In the reflective arrangement, sensitive parts of the monitor may be positioned outside the stack, thereby eliminating the need for significant (or preferably any) cooling or other special measures. Similarly, more bulky parts of the probe may also be located outside the stack; indeed, only minimal optics, in particular the reflector and focusing lens, need to be inside the stack. The folded beam/probe arrangement removes the electronics from the heat of the stack. It is not necessary to use an optical fibre to guide light in the monitor.

Preferably, the scattered light detector and the light source are housed in a common housing. Such an arrangement is more convenient than, for example, the source and detector being arranged independently on opposite sides of the stack (an arrangement which is likely to require significantly more alignment) .

Preferably, the measurement beam is directed through a tube to the second side of the flow. Provision of tube ensures that the measurement beam does not impinge on flowing particles prior to reflection from the reflector. The tube may also provide a convenient means of supporting the reflector in the stack.

Preferably, the light source, the optical system for directing the measurement beam, the reflector, and the scattered-light detector are housed in a common housing; that is, in one or more chambers in a single unit. Advantageously, use of a common housing allows the monitor to be installed in one piece in a stack, significantly reducing the amount of alignment and calibration required.

The reflector may comprise a prism. Of course, any other suitable form of reflector such as a retro reflector comprising a mirror (for example a retro-reflecting corner-cube) may be used.

Also according to the invention there is provided a particle monitor for monitoring particles flowing in a stack, comprising: (i) a light source for generating a measurement- phase beam; (ii) an optical system for directing the measurement-phase beam into a scattering zone in the particle flow; (iii) a transmitter window, arranged between the optical system and the scattering zone, through which the measurement- phase beam passes, the transmitter window being exposed to contamination by the particle flow; (iv) a scattered-light detector for detecting light scattered, from the measurement- phase beam, by the particles as they flow in the scattering zone; (v) a receiver window, arranged between the scattering zone and the scattered-light detector, through which the scattered light to be detected passes, the receiver window being exposed to contamination by the particle flow; (vi) a light source for generating a compensation-phase beam; (vii) a reference detector for detecting the compensation-phase beam and (viii) an optical system for directing the compensation-phase beam from the light source through the transmitter window and the receiver window to the reference detector, wherein the optical system for directing the measurement-phase beam and the optical system for directing the compensation-phase beam are arranged such that the measurement-phase beam and the compensation-phase beam will pass through substantially coincident and co-extensive regions of the transmitter window and the scattered light and the compensation-phase beam will pass through substantially coincident and co-extensive regions of the receiver window, such that changes in intensity detected by the reference detector provide an indication of contamination of the transmitter window and/or the receiver window that affects the intensity of scattered light detected by the scattered-light detector.

Also according to the invention there is provided a method of monitoring particles flowing in a stack, the method comprising: a measurement phase in which: (i) a measurement-phase beam is generated by a light source; (ii) the measurement-phase beam is directed into a scattering zone in the particle flow; (iii) the measurement-phase beam passes through a transmitter window, the transmitter window being exposed to contamination by the particle flow; (iv) light is scattered, from the measurement- phase beam, by the particles as they flow in the scattering zone, (v) the scattered light passes through a receiver window, the receiver window being exposed to contamination by the particle flow, and (vi) the scattered light is detected by a scattered-light detector; and a compensation phase in which (a) a compensation-phase beam is generated by a light source and (b) the compensation-phase beam is directed through the transmitter window and the receiver window to a reference detector; wherein, the measurement-phase beam and the compensation-phase beam pass through substantially coincident and co-extensive regions of the transmitter window and the scattered light and the compensation-phase beam pass through substantially coincident and co-extensive regions of the receiver window, such that changes in intensity detected by the reference detector provide an indication of contamination of the transmitter window and/or the receiver window that affects the intensity of scattered light detected by the scattered-light detector.

Thus the monitor may be operated in a compensation phase in which the reference beam passes through the same parts of the transmitter and receiver windows as the measurement-phase beam and the scattered light in the measurement phase. Thus any contamination of the windows will be detected as a reduction in signal detected at the reference beam detector. Some contamination of the probe may then be tolerated; for example, if the reference signal drops by only 5% then measurements may still be taken by assuming that the amount of scattered light detected will also be reduced by 5% and increasing measured values accordingly. The transmitter window will generally be the last optical surface that the measurement beam passes through before reaching the scattering zone and the receiver window will generally be the first optical surface that the scattered light passes through after leaving the scattering zone because those optical surfaces will generally be the surfaces that are exposed to contamination by the particle flow. Preferably, all other optical surfaces are sealed behind those surfaces and are safe from contamination. As the measurement beam and the reference beam both pass through the same, and only the same, areas of the optical surfaces that are exposed to contamination, contamination-check measurements will be more reliable than in prior art devices.

Preferably, the transmitter window is a transparent optical surface. Preferably, the receiver window is a transparent optical surface. Either window may, however, be associated with a filter or other means of reducing its transparency. "Light" here includes the electromagnetic spectrum from ultra-violet through to near-infra-red wavelengths. One or both of the light sources may be lasers. Preferably, the light source for generating the measurement-phase beam and the light source for generating the compensation-phase beam are the same light source. Preferably, the reference detector is also the scattered-light detector.

Preferably, the optical system for directing the compensation-phase beam comprises a means for diverging the compensation-phase beam so that it spreads across the regions of the transmitter window and/or receiver window through which the scattered light passes. A convex lens or any other suitable means for spreading the beam out radially to cover that region may be provided.

Preferably, the monitor is arranged such that the intensity of the compensation-phase beam is varied to provide a check on the calibration of the probe. Preferably, the calibration check is a span check, in which the signal output span of the probe is checked.

Preferably, the same light source is used to generate the measurement-phase beam and the compensation-phase beam, the scattered-light detector is also the reference detector and that detector is in the same position when the 'apparatus is in a measurement configuration as when it is in a check configuration.

Using the same light source and the same detector and keeping the detector in the same position in the measurement phase and the check phase is particularly advantageous for calibration measurements because it means that the calibration is likely to be significantly more reliable. Preferably, no parts of the monitor through which the measurement beam or the reference beam pass are altered between the measurement phase and the check phase.

The check phase may be a compensation phase or a calibration-check.

The monitor may give a signal indicative of mass concentration of the particle flow. The monitor may give a signal indicative of particle sizes in the particle flow. Mass concentration and particle size are two important parameters of particle flow.

Also according to the invention there is provided a particle monitor for monitoring particles flowing in a stack, comprising: (i) a light source for generating a normal-mode measurement-phase beam; (ii) an optical system for directing the normal-mode measurement-phase beam into a scattering zone in the particle flow; (iii) a scattered-light detector for detecting light scattered, from the directed normal-mode measurement-phase beam, by the particles as they flow in the scattering zone; (iv) a receiver window, arranged between the scattering zone and the scattered-light detector, through which the scattered light to be detected passes, the receiver window being exposed to contamination by the particle flow; (v) a light source for generating a calibration-mode measurement-phase beam; (vi) a reference detector for detecting the calibration-mode measurement-phase beam and (vii) an optical system for directing the calibration-mode measurement-phase beam from the light source through the receiver window to the reference detector, wherein the optical system for directing the normal-mode measurement-phase beam and the optical system for directing the calibration-mode measurement-phase beam are arranged such that the scattered light and the calibration-mode measurement-phase beam trace the same optical path through all optical elements between the scattering zone and the scattered-light detector, such that changes in intensity detected by the reference detector provide a means to calibrate the probe.

Also according to the invention there is provided a method of monitoring particles flowing in a stack, the method comprising: a normal-mode measurement phase in which: (i) a normal-mode measurement-phase beam is generated by a light source; (ii) the normal-mode measurement-phase beam is directed into a scattering zone in the particle flow; (iii) light is scattered, from the measurement beam, by the particles as they flow in the scattering zone, (iv) the scattered light passes through a receiver window, the receiver window being exposed to contamination by the particle flow, and (v) the scattered light is detected by a scattered-light detector; and a calibration-mode measurement-phase in which (a) a calibration-mode measurement-phase beam is generated by a light source and (b) the calibration-mode measurement-phase beam is directed through the receiver window to a reference detector; wherein, the scattered light and the calibration-mode measurement-phase beam trace the same optical path through all optical elements between the scattering zone and the scattered- light detector, such that changes in intensity detected by the reference detector provide a means to calibrate the probe.

Such an arrangement makes possible a calibration check that provides a very good indicator of the calibration of the probe because the calibration-mode measurement-phase beam and the scattered light pass along the same optical path through all optical elements between the scattering zone and the scattered- light detector. It is particularly advantageous that this can be achieved without moving those optical elements.

Preferably, the light source contains a means for expanding the beam (such as a diverging lens or any other means of providing a wide beam) , a converging lens and a central stop, being arranged such that the calibration-mode measurement-phase beam is expanded in diameter and passes through the converging lens and such that parts of the beam outside the radius of the stop are focused onto the scattering zone.

Thus the reference beam may be focused on the scattering zone in a beam the shape of a hollow cone and the hollow cone may be arranged such that it is a back projection of light scattered from the scattering region. For example, if light is scattered from the scattering region at angles between 1 degree and 8 degrees from the direction of incidence of the reference beam, the light source, converging lens and central stop are preferably arranged such that the calibration-mode measurement- phase beam is brought to a focus in the scattering zone and then diverges at all angles between 1 degree and 8 degrees.

Preferably, the monitor further comprises a light source for generating a calibration-mode compensation-phase beam and an optical system for directing the calibration-mode compensation- phase beam from the light source through a transmitter window and the receiver window to the reference detector, the optical system being arranged such that the calibration-mode compensation-phase beam and the calibration-mode measurement- phase beam pass through substantially co-incident and coextensive regions of the transmitter window and the receiver window, such that changes in intensity detected by the reference detector provide an indication of contamination of the transmitter window and/or the receiver window.

Similarly, preferably, the method further comprises a calibration mode compensation phase in which (i) a calibration- mode compensation-phase beam is generated, (ii) the calibration- mode compensation-phase beam is directed through a transmitter window and the receiver window to the reference detector; wherein, the calibration-mode measurement-phase beam and the calibration-mode compensation-phase beam pass through substantially co-incident and co-extensive regions of the transmitter window and the receiver window, such that changes in intensity detected by the reference detector provide an indication of contamination of the transmitter window and/or the receiver window.

Preferably, the optical system for directing the calibration-mode compensation-phase beam comprises a means for diverging the calibration-mode measurement-phase beam, such that the calibration-mode compensation-phase beam spreads across the regions of the transmitter and/or receiver window through which the scattered light passes.

Preferably, all optical elements between the scattering zone and the scattered-light detector are sealed behind the receiver window such that those elements are safe from contamination by the particles. Preferably, the same light source is used to generate the measurement beam and the reference beam. Preferably, the scattered-light detector is also the reference detector.

Preferably, an air buffer is provided in front of the transmitter window and/or the receiver window.

Preferably, a circularly symmetric air curtain is provided at the windows.

Preferably, a moveable optics holder is provided for inserting and removing optical components to change between normal and calibration modes and/or measurement and compensation phases. The optical components may be pivotally mounted such that they may be swung into the measurement beam. Preferably, the optical components are mounted on a rotatable carousel. Preferably the insertion is controlled automatically and remotely, for example by pneumatics or electronics.

The reference beam will generally be of a much higher intensity than any scattered light.

Also according to the invention there is provided a particle monitor for monitoring particles flowing in a stack, comprising: (i) a light source for generating a measurement beam; (ii) an optical system, for directing the measurement beam into a scattering zone in the particle flow; (iii) a transmitter window, through which the measurement beam passes before it reaches the scattering zone; (iv) a scattered-light detector for detecting light scattered, from the measurement beam, by the particles as they flow in the scattering zone; (v) an element that acts as the smallest stop aperture between the scattering zone and the scattered-light detector, that smallest-stop element and the transmitter window being in line-of-sight of each other, and (vi) an optical system that prevents light scattered by the transmitter window and passing through the smallest stop aperture from reaching the scattered-light detector.

Also according to the invention there is provided a method of monitoring particles flowing in a stack, comprising: (i) generating a measurement beam; (ii) passing the measurement beam through a transmitter window; (iii) directing the measurement beam into a scattering zone in the particle flow; (iv) preventing light that is scattered by the transmitter window, and that passes through the smallest stop aperture between the scattering zone and a scattered-light detector, from reaching the scattered-light detector; (v) detecting, at the scattered- light detector, light scattered from the measurement beam by the particles as they flow in the scattering zone. The smallest stop aperture is the region (which may be a transparent solid or a hole) whose spatial extent limits the maximum angle at which light is scattered from the scattering zone and reaches the scattered-light detector.

Advantageously, the scattering zone is long. Such an arrangement has the advantage that any inhomogeneities in the particle flow are averaged out because light is scattered from all parts of the scattering zone. A long scattering zone will generally result from small scattering angles. A particular problem of small scattering angles is that light scattered from contamination on the transmitter window can pass through the smallest stop aperture between the scattering zone and the scattered-light detector; indeed, it often has to be allowed to pass through, because it overlaps with (i.e., it is not resolved from) the light scattered from the particle flow. Preventing the unwanted scattered light from reaching the detector may thus improve the accuracy of monitoring.

Preferably, the optical system that prevents light scattered by the transmitter window and passing through the smallest stop aperture from reaching the scattered-light detector is arranged behind the smallest stop aperture.

Preferably, the optical system that prevents light scattered by the transmitter window and passing through the smallest stop aperture from reaching the scattered-light detector comprises a filter that absorbs light scattered by the transmitter window. Preferably, the filter is sufficiently transmitting to allow for light from a compensation or calibration phase beam to reach the scattered light detector.

Preferably, the optical system that prevents light scattered by the transmitter window and passing through the smallest stop aperture from reaching the scattered-light detector comprises a lens that focuses scattered light on the scattered light detector, a first mirror arranged behind the scattered-light detector, a second mirror with a hole arranged in front of the scattered light detector and a filter arranged between the first and second mirrors and preferably next to the second mirror, the mirrors and filter being arranged such that scattered light from the scattering zone passes directly to the scattered-light detector through the hole in the second mirror and the light scattered by the transmitter window is reflected by at least one of the mirrors and is substantially blocked by the filter. Of course, the filter may be combined with one or more mirrors, to provide "black mirrors".

Of course, the lens' may be any suitable means for focusing the scattered light.

Light scattered from contamination on the transmitter window will generally be scattered from a point further from the lens than light scattered in the scattering zone. The lens may be chosen such that the light scattered from the transmitter window is focused in front of the scattered-light detector. It then diverges away from and possibly past the scattered light detector and may be reflected back by the first mirror to the filter, where it is absorbed.

Preferably, the filter is arranged between the second mirror and the detector and has a hole through which the scattered light passes on its way to the detector.

Preferably, the monitor further comprises a light source for generating a reference beam (for example, a compensation- phase beam) and an optical system for directing the reference beam to the scattered-light detector, the first and second mirrors and the filter being arranged such that the reference beam is reflected by at least one of the mirrors, passes through the filter and reaches the detector at an intensity level sufficiently high for detection by the detector. Such an arrangement may be preferred when the reference beam does not follow the same optical path as light scattered from the scattering zone; for example, when it is arranged to coincide with that light at one or more optical surfaces that are exposed to contamination but not elsewhere. As the reference beam is much stronger than the light scattered from the transmitter window, it is able to pass through the filter where the unwanted scattered light is reduced to negligible intensities.

Also according to the invention there is provided a particle monitor for monitoring particles flowing in a stack, comprising: (i) a light source for generating a measurement beam; (ii) an optical system, for directing the measurement beam into a scattering zone in the particle flow; (iii) a scattered- light detector for detecting light scattered, from the measurement beam, by the particles as they flow in the scattering zone; (iv) a filter having a transmission function that varies radially across the filter and is selected to alter the proportion of light intensity detected at the scattered- light detector resulting from scattering from particles of a first size within a range of sizes relative to the light intensity detected at the scattered-light detector resulting from scattering from particles of a second size within the range of sizes, such that the intensity of light detected after scattering from a set of particles of the first size having a total mass M is substantially the same as the intensity of light detected after scattering from a set of particles of the second size also having a total mass M. Thus the same intensity of light will be detected from a given weight of particles of the first size as from the same weight of particles of the second size . Also according to the invention there is provided a method of monitoring particles flowing in a stack, comprising: (i) generating a measurement beam from a light source; (ii) directing the measurement beam into the particle flow; (iii) passing at least some light scattered from the particle flow through a filter having a transmission function that varies radially across the filter selected to alter the proportion of light intensity detected at a scattered-light detector resulting from scattering from particles of a first size within a range of sizes relative to the light intensity detected at the scattered- light detector resulting from scattering from particles of a second size within the range of sizes, such that the intensity of light detected after scattering from a set of particles of the first size having a total mass M is substantially the same as the intensity of light detected after scattering from a set of particles of the second size also having a total mass M; (iv) detecting the scattered light that has passed through the filter.

Particles of different sizes in the flow will generally scatter light at different intensities at different angles. Smaller particles generally scatter light at larger angles and lower intensities, whereas larger particles generally scatter light at smaller angles and higher intensities. A filter having a transmission function that varies across its surface may be used to exploit or to reduce differences in light reaching the detector from particles of different size. The exact relationship between particle size and scattering angle (and hence the exact configuration of the filter) will generally vary according to the nature of the particles and it is expected that different filters will be used in different particular applications. Such an arrangement may be used to provide a more accurate measure of properties of the particle flow by reducing or eliminating differences in detected light intensity due to different particle sizes.

The intensity pattern scattered from particles will usually be circularly symmetric.

Preferably, the filter has a transmission function selected to reduce the proportion of light intensity detected at the scattered-light detector resulting from scattering from a set of particles of a third size relative to the light intensity resulting from scattering from particles of the first size. Thus a lower intensity of light may be detected from a given weight of particles of the third size than from the same weight of particles of the first size.

Preferably, the filter is arranged to cut off light scattered from particles that are larger or, preferably, smaller than a selected particle size, for example, 10 microns.

Preferably, the filter blocks all scattered light reaching the filter outside an annular area of the filter. The filter may block all scattered light travelling to the filter from the scattering zone at an angle of less than 1 degree and all scattered light travelling to the filter from the scattering zone at an angle of more than 8 degrees, more preferably all scattered light travelling to the filter from the scattering zone at an angle of less than 2 degrees and all scattered light travelling to the filter from the scattering zone at an angle of more than 4 degrees.

Preferably, the filter transmits scattered light reaching an annular area of the filter but blocks all other scattered light. Such an arrangement may be used, for example, to give an approximate measure of the number of particles that are present in the flow in a particular range of sizes. Preferably, the annular area subtends at the scattering zone all angles between 1 degree and 8 degrees, more preferably between 2 degrees and 4 degrees.

Preferably, the light intensity detected at the scattered- light detector provides an indication of the proportion of particles in the flow that are of the first size. Thus the filter may have a transmission function selected to enhance a scattered-light signature characteristic of a particular particle size (or range of sizes) .

Preferably, the transmission function is selected such that the intensity of light detected after scattering is substantially the same for all particle sizes in the range. Thus, preferably, the filter is selected to provide a detection response that is substantially uniform across the range, i.e. that does not vary with particle size.

Also according to the invention there is provided a particle monitor for monitoring particles flowing in a stack, comprising: (i) a light source for generating a measurement beam; (ii) an optical system, for directing the measurement beam into a scattering zone in the particle flow; (iii) a scattered- light detector for detecting light scattered, from the measurement beam, by the particles as they flow in the scattering zone; (iv) a filter having a transmission function selected to alter the proportion of light intensity detected at the scattered-light detector resulting from scattering from particles of a first size relative to the light intensity detected at the scattered-light detector resulting from scattering from particles of a second size.

Also according to the invention there is provided a method of monitoring particles flowing in a stack, comprising: (i) generating a measurement beam from a light source; (ii) directing the measurement beam into the particle flow; (iii) passing at least some light scattered from the particle flow through a filter having a transmission function selected to alter the proportion of light intensity detected at a scattered- light detector resulting from scattering from particles of a first size relative to the light intensity detected at the scattered-light detector resulting from scattering from particles of a second size and (iv) detecting the scattered light that has passed through the filter.

Also according to the invention there is provided a particle monitor for monitoring particles flowing in a stack comprising: (i) a light source for generating a measurement beam; (ii) an optical system for directing the measurement beam into the particle flow; and (iii) a scattered-light detector for detecting only light scattered at angles of between 1 and 8 degrees from the direction of the measurement beam when it is incident on the particle flow.

Also according to the invention there is provided a method of monitoring particles flowing in a stack comprising: (i) generating a measurement beam; (ii) directing the measurement beam into the particle flow; and (iii) detecting only light scattered at angles of between 1 and 8 degrees from the direction of the measurement beam when it is incident on the particle flow.

Detecting only light scattered at such a small angle has the advantage of providing a long scattering volume, which smoothes out inhomogeneities due to local variations in particle properties in the flow.

Preferably, only light scattered at angles of between 2 and 4 degrees is detected.

Preferably, an optical element is inserted into the measurement beam to form the reference beam. Preferably, the optical element is inserted into the measurement beam upstream of the transmitter window. More preferably, the optical element is a lens, still more preferably a diverging lens.

Also according to the invention there is provided a laser source for providing a laser beam having low levels of peripheral light, comprising a laser, a lens and a first reflector arranged between the laser and the lens, the first reflector defining an aperture through which light emitted from the laser passes, the aperture of the first reflector being sized so as to limit the cross-section of the light passing through that aperture, and the first reflector being arranged to reflect light not passing through that aperture, the source further comprising an absorber arranged between the first reflector and the laser such that peripheral light not passing through the aperture in the first reflector is absorbed.

Preferably, the laser source further comprises a second reflector arranged between the laser and the lens, the second reflector defining an aperture through which light emitted from the laser passes, the first reflector being arranged to reflect light not passing through the aperture in the first reflector to the second reflector and the second reflector being arranged to reflect light reflected by the first reflector back to the first reflector, the absorber being arranged between the first reflector and the laser.

Thus, the unwanted peripheral light is absorbed by the absorber either directly or after one or more reflections between the first reflector and the laser.

Preferably, the lens is a convex lens. More preferably, the lens collimates the beam.

Preferably, the absorber is adjacent to the first or second reflector. More preferably, the first or second reflector is itself the absorber, such that it is partially reflecting and partially absorbing at the output wavelength of the laser. Still more preferably, one or both reflector is, or is associated with, an absorber.

Preferably, one or both reflectors are planar. More preferably, one or both reflectors are parallel to each other. Still more preferably, one or both reflectors are parallel to the collimating lens.

Preferably, the first reflector is adjacent to the collimating lens. More preferably, the second reflector is adjacent to the laser. Preferably, the laser is a diode laser. Preferably, light not absorbed is specularly reflected by the reflectors and substantially none is scattered.

The laser source may further comprise an aperture arranged adjacent to the laser. Preferably, the aperture is a pin hole, more preferably of transverse dimensions between 0.5 mm and 1 mm. We have found that provision of a pin hole in front of the laser greatly reduces unwanted peripheral light, we believe by blocking light scattered from objects close to the laser diode chip, such as mounting elements and any protective window covering the diode chip. The pin-hole is preferably placed as close to the laser diode (which has a highly divergent beam) as possible. The aperture may be comprised in the second reflector. Alternatively, the aperture may be comprised in an element other than the second reflector.

Embodiments of the invention will now be described, by way of example only, with reference to the accompanying figures, of which:

Fig. 1 is a particle monitor according to the invention, Fig. la being a side view and Fig. lb being a plan view;

Fig. 2 is a plan view of an external enclosure of the monitor of Fig. 1;

Fig. 3 is a plan view of an internal enclosure of the monitor of Fig. 1; Fig. 4a is a diagram showing ray paths in a measurement phase of operation of the monitor of Fig. 1;

Fig. 4b is a diagram showing ray paths in a compensation phase of operation of the monitor of Fig. 1;

Fig. 5 is a diagram showing ray paths in a laser source unit in the monitor of Fig. 1;

Fig. 6 is a plan view of an external enclosure of a preferred version of the probe of Fig. 1;

Fig. 7 is a plan view of an internal enclosure of the preferred version of the probe of Fig. 1;

Fig. 8a is a diagram showing ray paths in a calibration- mode measurement phase of operation of the preferred version of the probe of Fig. 1;

Fig. 8b is a diagram showing ray paths in a calibration- mode compensation phase of operation of the preferred version of the probe of Fig. 1; and

Fig. 9 is a diagram showing ray paths in a laser source unit, which has an alternative configuration to that of Fig. 5, in the monitor of Fig. 1.

First, the principal elements of probe 10 will be described. Probe 10 consists of four main assemblies (Fig. 1) : the external enclosure 20, which is external to the stack, the internal enclosure 30, which is inside the stack, the receiver 40 and the body 50. A laser beam 60 is projected from the external enclosure 20 down the body 50 of the probe 10. The laser beam 60 is reflected back through the stack gas flow 70 by optical elements in the internal enclosure 30 at the remote end of the probe 10. Scatter produced by dust carried by the stack gas flow 70 causes light to enter the receiver 40. A detector 80 in the receiver 40 generates a signal sent to the main electronics in the external enclosure 20. In a measurement phase, the signal generated from the dust 70 gives an indication of particle mass concentration rates in the stack. In a compensation phase, a reference beam is used to determine the effects of contamination on exposed optical surfaces 190, 200 of the probe 10 and that information is then used in a subsequent measurement phase to compensate for those effects on the measurement signal.

The external enclosure 20 (Fig. 2), mounted on the end of the probe body 50, houses a laser source unit 90, all electronics (including a printed circuit board (PCB 100)), and power supplies derived from a 24-volt input (not shown) . The receiver assembly 40, mounted within the probe body 50, stops just inside the enclosure 20 (Fig. 1) .

The enclosure 20 is a rectangular box. The width of the laser optics and depth of the PCB 100 determine the length, while the side dimension is largely dictated by the area needed for the electronics PCB 100. PCB 100 can have the full area of the end of the enclosure 20.

The main electronics PCB 100 is mounted at right angles to the longitudinal axis of probe 10. Connections to the outside world are via connectors on the main PCB 100 with cables routed through the walls of the external enclosure 20. There is also an air supply connection 110 from an external blower unit, placed on a stub 120 between the enclosure 20 and a fixing flange 130. The stub 120 allows the enclosure 20 to stand off from the stack flange 130 permitting the flange fixing bolts to be fitted and tightened in stack wall 150.

External enclosure 20 also comprises a solenoid 22 which rotates a rod 23, via arm 24, which in turn rotates a swinging lens assembly 150 in internal enclosure 30 (Fig. 3) .

The probe internal enclosure 30 (Fig. 3) contains a reflecting prism 210, transmitter air buffer 160 (Fig. 1) and transmitter window 190, together with a swinging lens assembly 150, which is used during check-phase operation of probe 10. The reflector 210 is mounted directly off the probe internal enclosure 30. It is important to note that light in the periphery of the collimated output beam is undesirable, even if it is of low intensity, because such light may lead to error in measured scattered light levels (scattered light intensities are generally very low) .

Tube 180 carries air to the probe body 50, internal enclosure 20, external enclosure 30 and receiver enclosure 40. It also allows passage of the laser beam 60 to the internal enclosure 30 and the reflecting prism 210 in such a manner as to avoid scattering of the laser beam 60 by particles 70. A second tube 185, travelling the length of the probe parallel to tube 180, from the flange 130 to the internal enclosure baffle 390 admits air to the air buffers 160, 170 in front of both transmitter and receiver windows 190, 200. Air buffers 160, 170 are designed to reduce the contamination of windows 190, 200. The buffers are designed with contoured flanges so as to increase turbulence at the end of the buffers and thus reduce accumulation of contamination at the ends of the buffers. The tube 185 also carries a long rod, actuated by a solenoid, to the swinging lens holder 212.

The laser beam 60 enters enclosure 30 via tube 180 and is reflected back in the direction from which it came (along a displaced parallel path) by reflector 210. The swinging lens assembly 150 comprises concave lens 151 and a lens holder 152. The whole is pivoted so that, by rotation of rod 23, lens 151 is swung into the path of the laser beam 60 emerging from the reflector 210 before it travels to the transmitter window 190 via exit 153. The lens is swung in place during the contamination compensation phase of measurement (holder position 152') .

The probe receiver 40 (shown most clearly in Fig. 4b) consists of a single tube 270. Near the front of the receiver 40 is an annular iris 280 that is clear between two diameters. Iris 280 is coupled with a radially graded neutral density filter 240. (Alternatively there may be two narrower annuli of differing radii) The function of the graded filter (or annuli) is to alter the response of the monitor 10 with respect to particle size. The two most important arrangements are a response independent of particle size and a response having a cut-off at a defined particle size. Immediately behind the annular aperture 280 is a lens 220.

In the measurement phase of operation of the probe 10 (Fig. 4a) beam 60 scatters from region 230 at small angles, which is advantageous for increasing the extent of scattering region 230 and hence averaging-out more effectively any variations in flow 70 (i.e. the length over which beam 60 interacts with flow 70). However, a potential disadvantage with the arrangement of probe 10 is that the transmitter window 190 and the iris 280 (which is the limiting optical stop between window 190 and detector 80) are in line of sight with each other. Hence light scattered from the edge of the iris associated with transmitter window 190 or from contamination on that window can potentially reach detector 80, as it is not spatially distinct from the light scattered from region 230.

Thus, it is desirable for light from the interaction volume 230 to come to a focus on the photodetector 80, whilst light from scatter derived from dust settled on the transmitter window 190 and from the edge of iris 280 comes to a focus substantially before the detector 80 so that unwanted light skirts round the diode 80 and misses it altogether. In probe 10, the transmitter window image is reduced in magnitude by the receiver lens 220 (the window 190 being further away than the interaction volume 230, hence giving a shorter image distance) .

The diameter, position, and focal length of lens 220 are chosen such that all the contamination/compensation light coming through the annular iris 280 is received by the detector 80. The single centrally mounted detector 80 is arranged in an aperture in a plane mirror 290 forming the end face of the receiver 40, together with the photodiode amplifier (not shown) .

Some way in front of detector 80 is a neutral density filter 300, (Fig. 4b) , with a central hole to allow scattered light from the interaction volume 230 to reach the detector 80 unhindered. Immediately behind this is a concave mirror 310, again with a central hole, designed to reflect both light scattered from the transmitter window 190 and compensation-phase light direct from the diverged laser beam onto the detector 80, both of which will first have been reflected from mirror 290. Unwanted scattered light is reduced to a negligible level by the filter 300 while direct divergent laser light present in the compensation phase provides a readily measurable signal despite two traverses of the neutral density filter 300. By this means both light scattered from interaction-volume 230 and compensation phase light are measured by the same photodiode 80 and amplifier. The position of the neutral density filter 300 and concave mirror 310 are obtained from consideration of the limiting rays from both interaction volume 230 and transmitter window 190 contamination scatter.

Attached to the front of the receiver 40 is a 45-degree mirror 320 (Fig. 1) , which deflects the incident laser beam to the side. A second 45-degree mirror 330 deflects this beam to the rear of the receiver 40. It passes through windows into the external enclosure 20 where it impinges on a screen 340 imprinted with a graticule.

Fig. 5 shows laser source 90 comprises a laser diode 91 mounted on a separate small PCB 92 having the laser drive circuits on its rear. The main PCB 100 is mounted in front of the laser source 90 (viewed from the outside) but fixings for the sheet are still accessible. Both sheet and PCB mount directly off the probe body 50.

Light output from laser diode 91 is collimated by lens 93. In order to eliminate peripheral light, laser source 90 is arranged such that light in the output from diode 91 that is too far from the centre of the output beam 60 to pass through lens 93 is blocked by black mirrors 94, 95, which specularly reflect any light that is not absorbed and which exhibit very little scatter. Surface 94 is mounted directly in front of diode 91 and substantially all output light passes from the diode 91 through a hole in the centre of surface 94. Lens 93 is mounted in surface 95 and some of the peripheral light is absorbed in that surface. Unwanted light that is not absorbed by surface 95 is reflected back to surface 94, where more is absorbed and the remainder reflected forwards again to surface 95, where still more is absorbed. That arrangement of multiple, highly absorbing reflectors greatly reduces unwanted peripheral light levels in beam 60. Beam 60 leaves enclosure 20 along tube 180 after reflection by a 45° mirror 21.

The probe body 50 is built around two tubes 180, 185 running the length of the probe 10. One carries air to pressurise the internal enclosure 20, external enclosure 30 and receiver 40, and the incident laser beam 60; the other carries the air buffer air and a rod to control the compensation lens as already described. Operation of probe 10 will now be described. Normal operation of probe 10 is a measurement phase of operation (Fig. 4a) , in which the particle concentration and/or particles sizes of flow 70 are measured. The probe 10 is switched to a compensation phase (Fig. 4b) approximately every hour; operation returns to the measurement phase after the compensation check is completed.

In the measurement phase, laser beam 60 is admitted to the internal enclosure 30 via tube 180 from the external enclosure 20, through body 50 to the internal enclosure 30. Reflecting prism 210 turns the laser beam 60 through 180 degrees and directs it back to the receiver unit 40. A proportion of the beam 60 is scattered by flow 70; the residual beam is directed to graticule 340 by mirrors 320, 330.

The scattered light passes through an annularly graded neutral density filter 240 and iris 280 to lens 220, which focuses it onto photodiode 80. The filter 240 is transparent in an annulus of inner radius 10 mm and outer radius 20 mm and opaque elsewhere. That transparent annulus transmits light scattered by particles of size 1 micron to 10 microns and substantially blocks light scattered by particles of other sizes. The signal from photodiode 80 is processed to give an indication of particle concentration.

Windows 190, 200 are the last and first surfaces respectively through which beam 60 and light scattered from flow 70 respectively pass between internal enclosure 30 and receiver 40. Windows 190, 200 are in this embodiment glass panes arranged to protect other optical elements from contamination. As they themselves are exposed to particle flow 70, windows 190, 200 are vulnerable to contamination by a build up of particles on their surfaces. Air buffers 160, 170 provide some protection but contamination is still likely to occur in many stack installations. The compensation phase of operation is used to determine the level of contamination. As long as contamination levels remain reasonably low, a reduction in the intensity of a reference beam can be used to calculate true scattered-light intensity values from the values actually measured during the measurement phase; thus, the effects of contamination are compensated for.

In the compensation phase (Figs. 3, 4b), a concave lens 151 is swung into the beam just prior to the transmission window 190. Lens 151 causes the beam to diverge so that the annular iris 280 at the entrance to the receiver section is uniformly lit. Since the exit pupil from the transmission window 190 will be virtually unchanged from that of the measurement phase, the beam traverses just those areas of both windows which are used during the measurement phase. Thus, any obstruction to either window will cause the same attenuation of the light in both measurement and compensation phases. The beam 60 is substantially identical in both phases. Thus, any disturbance or drift in either the laser diode 91 or its alignment will affect both phases of operation to the same degree. Furthermore, the same photodetector, amplification and digital train are used in both phases of operation. Any change in those will again affect both phases of operation to the same degree. The sole difference between the two phases of operation is the presence of the concave lens 151 prior to the transmitter window 190 and additional fixed optic elements encountered in the receiver. Thus, any difference observed in measurements made during the contamination-check phase can be used to correct the measurement phase values.

In a preferred embodiment, the concave lens 151 is associated with a neutral density filter with continuously radially varying neutral density. Such an arrangement enables the reference beam to illuminate the receiver window in the same manner as the measurement beam.

Varying the laser intensity upwards from 0 per cent should give an invariant result when likewise corrected with the value at 100 per cent. Thus a check of the calibration of the probe 10 may be carried out by varying the laser intensity. The calibration check or span check is preferably carried out during the compensation phase. Out-of-tolerance values can be used to flag a failure of the probe 10 to span the same range. Thus a "span-check" may be carried out. An alternative to varying the laser intensity is to use neutral density filters inserted into the incident beam. The value recorded in either phase of operation when the laser diode 91 is switched off will reveal any drift in the offset or "zero" values of the electronics.

An iris (not shown) acting on the incident beam on the inner face of the transmitter window is provided if the laser source 90 fails to generate a sufficiently clean-edged beam. If present, the iris may take the form of a "black" mirror, preferably anti-reflection coated at the appropriate wavelength. Treatment of the inner surface/baffling of the tube 180 carrying the laser beam 60 reduces grazing angle reflections.

The signal received at detector 80 is largely independent of changes in particle flow 70 and any significant reduction in the detected signal is attributable to contamination of windows 190, 200.

The alignment of the laser 90 can be checked and adjusted at any time by reference to the screen 340 in the external enclosure 20, with the probe 10 still in the stack and functioning.

As indicated above, the probe design provides a means of measuring the degree of contamination affecting both transmitter window 190 and receiver window 200 in precisely those areas through which light passes in the dust measurement process. That measurement of the effects of contamination is used to compensate for the error in dust measurement caused by window fouling. Compensation is achieved by causing the transmitted beam 60 to diverge slightly as it leaves the transmitter window 190 such that the beam subsequently strikes the receiver window 200 in that area used to receive the scattered light. Measuring the signal produced in this phase of operation gives a direct measure of the transmissivity of both transmitter and receiver windows 190, 200. That value is then used to compensate the dust reading signal.

In different applications the probe 10 will be of different size according to the stack size. The physical dimensions of the probe 10 should be modified and the optical components (i.e. focal length of lenses etc) changed, in order to produce a probe of the required dimensions.

A preferred embodiment of the invention will now be discussed with reference to Figures 6 to 8.

In a preferred embodiment of the invention, in order to meet the requirements of legislation for auto- calibration at defined intervals, the small diverging lens 151 is replaced with a compound lens 151', which is made up of a large diameter convex lens and a small diameter concave lens of greater power.

As a matter of nomenclature the following terms will be adopted. Two modes of operation are defined for the probe of the preferred embodiment; "normal" mode and "calibration" mode. In each mode there are two phases defined; "measurement" phase and "compensation" phase (the embodiment described above thus operates only in "normal mode measurement phase" and "normal mode compensation phase"). Ray paths in calibration mode, measurement phase and calibration mode, compensation phase are shown in Figs. 8(a) and (b) respectively.

The auto-calibration probe has the same features as the auto-compensation probe described above with the following modifications. First, when calibration mode is selected, solenoid 29 in external enclosure 20' inserts a weak diverging lens immediately in front of the laser source 90 (Fig. 6) . That causes the laser beam 60 to widen by the time it reaches the transmitter window 190. That window has associated with it a compound iris (not shown) clear in the central area, a black mirror region, a further clear region and a further black mirror region. (Alternatively, or additionally, there may be a central stop in the middle of the laser source diverging lens causing beam 60 to have an annular intensity cross-section) Secondly, the essentially two-position swinging lens assembly 150 (in or out) is replaced by a three-position optics holder 150'.

When the normal mode is selected (no lens at laser exit) , the optics holder 150' (Fig. 7) presents a clear area 158 (or an iris) in the measurement phase. In the compensation phase the optics holder 150' is moved so that the beam 60 passes through a diverging lens 151' as described in the auto-compensation probe above.

When the calibration mode is selected (lens at laser exit) , the optics holder 150' presents a converging lens 159 in measurement mode (which may have a central stop) . Lens 159 causes the calibration beam to pass through the outer annulus of the transmission window 190 and focus in the interaction volume 230 of the gas stream 70 such that the rays leaving the interaction volume 230 are travelling along paths identical to those generated by scatter from dust during the measurement phase in normal mode. In the compensation phase the optics holder 150' again presents a diverging lens 151' so that the annular iris 280 at the entrance to the receiver section 40 is uniformly lit. This position in the holder 150' is the same position as used in the compensation phase in normal mode. Either a single diverging lens is used, or preferably a compound lens consisting of a diverging lens with an additional central diverging lens is used. Such use of a common position in the optics holder makes the use of a central stop with the laser source exit diverging lens desirable.

(A two-position optics holder 150 may be used if a central stop is used with the laser source exit diverging lens. That would be possible by combining the optics holder 150 positions used in both measurement phases. The converging lens 151' used in calibration mode measurement phase would have a central hole. Of course, there would no longer be the possibility of an iris in the normal mode measurement phase. Provision of such an iris is preferable because the normal mode measurement phase generates extremely weak signals, orders of magnitude lower than the other three selections. Any possible peripheral light rays generated by the laser diode 91 and perhaps reflected by the inner wall of the tube 180 down which it travels from source 90 to internal enclosure 30 should be stopped as far as possible by the "black mirror" iris, since if those passed through the transmitter window 190 in the clear annular area, they would directly reach the photodetector 80 in the receiver 40.)

Better understanding of calibration-mode operation (Fig. 8a) may be had by considering the limiting light rays produced by the scattered light generated by the dust 70 in the stack (the wanted scattered light) . If those rays are projected back towards the transmitter window 190 they strike it in an annulus of a certain diameter. We can therefore precisely mimic the wanted scattered light signal by removing the normal incident laser beam 60 and substituting light of the correct angles directed from the annulus just described; measuring that light is exactly the same as measuring dust-scattered light, as it traverses exactly the same path. The two signals will track together. Put another way, the signal measured in calibration check mode at any time, corrected by the contamination compensation measurement, will be invariant within the error tolerance figure allowed for the dust measurement. It should be noted that unlike in normal dust measurements, light scattered by window contamination in the area of the transmitted beam is seen by the receiver, but since it is the incident beam itself which is being measured the signal from the window contamination is orders of magnitude less than the wanted reference-beam signal .

The analysis for the calibration mode measurement and compensation phases can be made in exactly the same way as for the normal mode measurement and compensation phases, as follows. Since the exit pupil from the transmission window in compensation phase will be virtually unchanged from that in the measurement phase, the compensation phase beam is seen to traverse just those areas of both windows which are used during the measurement phase. Thus any obstruction to either window will cause the same attenuation of the light in both measurement and compensation phase. The incident beam is identical in both phases; thus any disturbance of drift in either the laser or its alignment will affect both phases to the same degree. The same photodetector, amplification and digital train are used in both phases; any change in these will again affect both phases to the same degree. The sole difference between the two is the presence of the concave lens prior to the transmitter window and the additional fixed optic elements encountered in the receiver. Assuming those elements do not change, any difference observed in measurements made during the calibration mode compensation phase can be used to correct the calibration mode measurement phase values.

In both modes the principal differences between the measurement and compensation phases relate to the extra fixed elements encountered in the receiver in the compensation phase. Both modes use the same extra elements during the compensation phase, although the regions used will not be identical. If there is some change in those elements, say a misalignment or degradation in reflectivity, this will apply equally to both modes. Thus, any error introduced by an inadequacy in the compensation measurement in normal mode will be cancelled out by re-calibration using the calibration mode.

Span and offset checks may be made in exactly the same way as for the auto-compensation probe. These checks may be made in compensation phase in normal mode and in both measurement and compensation phases in calibration mode. Making the check in measurement phase in calibration mode is particularly apposite owing to the parallel with the measurement phase in normal mode.

The preferred embodiment of the probe thus provides a reliable calibration mechanism whilst requiring little change to the design of probe 10 previously described.

A means of expanding the laser beam from the normal mode, measurement phase to a significantly greater diameter in calibration mode is provided at source. That change (Fig. 6), though requiring a mechanical motion by solenoid 29, is nevertheless entirely confined to the electronics housing external to the stack, and requires no remote linkages. In the expanded beam 60, the central area may be stopped so that in calibration mode there is less dust-induced scattered light interfering with the measurement. The absorption/reflection characteristic of the stop eliminates as far as possible off-axis light escaping from the beam generator.

Compensation measurements made in this mode will give a measure of contamination of the receiver window 200 and the outer area of the transmitter window 190. That gives a more accurate figure with which to correct the calibration results. In one embodiment a more complex concave mirror is provided in the receiver with an outer region of different radius to the inner area, the central hole is wider and a different position is chosen for this mirror.

The contribution which dust scatter makes to all these compensation/calibration measurements is allowed for by calibrating the effect for a given dust loading as part of the design and subtracting an allowance from any such measurement dependent on the current dust measurement.

Fig. 9 shows an alternative embodiment of the laser source, labelled 90'. In this embodiment, the angular spread of light from diode 91 that forms beam 60 is substantially less than the maximum available spread (which is desirable, for example, to form a circular beam without using a cylindrical lens) . Consequently, absorbing surfaces 94, 95 have to absorb a significant fraction of the total output power of diode 91. Lens 93' is positioned further from diode 91 than lens 93 in the first embodiment (and hence lens 93' has a larger diameter) and the shape of beam 60 is determined by an iris 99 positioned just in front of lens 93'.

Claims

1. A particle monitor for monitoring particles flowing in a stack comprising: (i) a light source for generating a measurement beam on a first side of the particle flow; (ii) an optical system for directing the measurement beam towards a second side of the particle flow without the measurement beam scattering from the particles, (iii) a reflector arranged to reflect the measurement beam back towards the first side of the particle flow, (iv) an optical system for directing the measurement beam into the particle flow such that light from the measurement beam is scattered by the particles; and (v) a scattered-light detector for detecting, on the first side of the particle flow, the scattered light.

2. A monitor as claimed in claim 1, in which the scattered light detector and the light source are housed in a common enclosure .

3. A monitor as claimed in claim 1 or claim 2 , in which the measurement beam is directed through a tube to the second side of the flow.

4. A monitor as claimed in any of claims 1 to 3 , in which the light source, the optical system for directing the measurement beam, the reflector, the optical system for directing the measurement beam and the scattered-light detector are housed in a common housing.

5. A monitor as claimed in any of claims 1 to 4 , in which the reflector comprises a prism or a retro-reflector.

6. A method of monitoring particles flowing in a stack, comprising: (i) generating a measurement beam on a first side of the particle flow; (ii) directing the measurement beam towards a second side of the particle flow without the measurement beam scattering from the particles, (iii) reflecting the measurement beam back towards the first side of the particle flow, (iv) directing the measurement beam into the particle flow such that light from the measurement beam is scattered by the particles; and (v) detecting, on the first side of the particle flow, the scattered light.

7. A particle monitor for monitoring particles flowing in a stack, comprising: (i) a light source for generating a measurement-phase beam; (ii) an optical system for directing the measurement-phase beam into a scattering zone in the particle flow; (iii) a transmitter window, arranged between the optical system and the scattering zone, through which the measurement- phase beam passes, the transmitter window being exposed to contamination by the particle flow; (iv) a scattered-light detector for detecting light scattered, from the measurement- phase beam, by the particles as they flow in the scattering zone; (v) a receiver window , arranged between the scattering zone and the scattered-light detector, through which the scattered light to be detected passes, the receiver window being exposed to contamination by the particle flow; (vi) a light source for generating a compensation-phase beam; (vii) a reference detector for detecting the compensation-phase beam and (viii) an optical system for directing the compensation-phase beam from the light source through the transmitter window and the receiver window to the reference detector, wherein the optical system for directing the measurement-phase beam and the optical system for directing the compensation-phase beam are arranged such that the measurement-phase beam and the compensation-phase beam will pass through substantially coincident and co-extensive regions of the transmitter window and the scattered light and the compensation-phase beam will pass through substantially coincident and co-extensive regions of the receiver window, such that changes in intensity detected by the reference detector provide an indication of contamination of the transmitter window and/or the receiver window that affects the intensity of scattered light detected by the scattered-light detector.

8. A monitor as claimed in claim 7, in which the optical system for directing the compensation-phase beam comprises a means for diverging the compensation-phase beam so that it spreads across the regions of the transmitter window and/or receiver window through which the scattered light passes.

9. A monitor as claimed in claim 7 or claim 8 arranged such that the intensity of the compensation-phase beam is varied to provide a check on the calibration of the probe.

10. A monitor as claimed in claim 9, in which the calibration check is a span check.

11. A monitor as claimed in any of claims 7 to 10 in which the same light source is used to generate the measurement-phase beam and the compensation-phase beam.

12. A monitor as claimed in any of claims 7 to 11, in which the scattered-light detector is also the reference detector.

13. A monitor as claimed in claim 12, in which the detector is in the same position when the apparatus is in a measurement configuration as when it is in a check configuration.

14. A monitor as claimed in any of claims 7 to 13, in which the beams are generated by the same light source.

15. A monitor as claimed in any of claims 7 to 14, in which an air buffer is provided in front of the transmitter window and/or the receiver window.

16. A monitor as claimed in any of claims 7 to 15, further comprising a moveable optics holder for inserting and removing optical components to change between the measurement phase and the compensation phase.

17. A method of monitoring particles flowing in a stack, the method comprising: a measurement phase in which: (i) a measurement-phase beam is generated by a light source; (ii) the measurement-phase beam is directed into a scattering zone in the particle flow; (iii) the measurement-phase beam passes through a transmitter window, the transmitter window being exposed to contamination by the particle flow; (iv) light is scattered, from the measurement- phase beam, by the particles as they flow in the scattering zone, (v) the scattered light passes through a receiver window, the receiver window being exposed to contamination by the particle flow, and (vi) the scattered light is detected by a scattered-light detector; and a compensation phase in which (a) a compensation-phase beam is generated by a light source and (b) the compensation-phase beam is directed through the transmitter window and the receiver window to a reference detector; wherein, the measurement-phase beam and the compensation- phase beam pass through substantially coincident and coextensive regions of the transmitter window and the scattered light and the compensation-phase beam pass through substantially coincident and co-extensive regions of the receiver window, such that changes in intensity detected by the reference detector provide an indication of contamination of the transmitter window and/or the receiver window that affects the intensity of scattered light detected by the scattered-light detector.

18. A method as claimed in claim 17, in which the intensity of the compensation-phase beam is varied to check the calibration of the probe.

19. A method as claimed in claim 18, in which the intensity of the compensation-phase beam is varied to check the signal output span of the probe.

20. A method as claimed in any of claims 17 to 19 in which the same light source is used to generate the measurement-phase beam and the compensation-phase beam, the scattered-light detector is also the reference detector and that detector is in the same position in the measurement phase and the compensation phase.

21. A particle monitor for monitoring particles flowing in a stack, comprising: (i) a light source for generating a normal- mode measurement-phase beam; (ii) an optical system for directing the normal-mode measurement-phase beam into a scattering zone in the particle flow; (iii) a scattered-light detector for detecting light scattered, from the normal-mode measurement-phase beam, by the particles as they flow in the scattering zone; (iv) a receiver window, arranged between the scattering zone and the scattered-light detector, through which the scattered light to be detected passes, the receiver window being exposed to contamination by the particle flow; (v) a light source for generating a calibration-mode measurement-phase beam; (vi) a reference detector for detecting the calibration-mode measurement-phase beam and (vii) an optical system for directing the calibration-mode measurement-phase beam from the light source through the receiver window to the reference detector, wherein the optical system for directing the normal-mode measurement-phase beam and the optical system for directing the calibration-mode measurement-phase beam are arranged such that the scattered light and the calibration-mode measurement-phase beam trace the same optical path through all optical elements between the scattering zone and the scattered-light detector, such that changes in intensity detected by the reference detector provide a means to calibrate the probe.

22. A monitor as claimed in claim 21, in which the optical system for directing the calibration-mode measurement-phase beam comprises a means for expanding the beam, a converging lens and a central stop, being arranged such that the reference beam is expanded in diameter and passes through the converging lens and parts of the beam outside the radius of the stop and are focused onto the scattering zone.

23. A monitor as claimed in claim 21 or claim 22, further comprising a light source for generating a calibration-mode compensation-phase beam and an optical system for directing the calibration-mode compensation-phase beam from the light source through a transmitter window and the receiver window to the reference detector, the optical system being arranged such that the calibration-mode compensation-phase beam and the calibration-mode measurement-phase beam pass through substantially co-incident and co-extensive regions of the transmitter window and the receiver window, such that changes in intensity detected by the reference detector provide an indication of contamination of the transmitter window and/or the receiver window.

24. A monitor as claimed in claim 23, in which the optical system for directing the calibration-mode compensation-phase beam comprises a means for diverging the calibration-mode measurement-phase beam, such that the calibration-mode compensation-phase beam spreads across the regions of the transmitter and/or receiver window through which the scattered light passes.

25. A monitor as claimed in any of claims 21 to 24, in which the scattered-light detector is also the reference detector.

26. A monitor as claimed in any of claims 21 to 25, in which the beams are generated by the same light source.

27. A monitor as claimed in any of claims 21 to 26, in which an air buffer is provided in front of the transmitter window and/or the receiver window.

28. A monitor as claimed in any of claims 21 to 27, further comprising a moveable optics holder for inserting and removing optical components to change between normal and calibration modes and/or measurement and compensation phases .

29. A method of monitoring particles flowing in a stack, the method comprising: a normal mode measurement phase in which: (i) a normal-mode measurement-phase beam is generated by a light source; (ii) the normal-mode measurement-phase beam is directed into a scattering zone in the particle flow; (iii) light is scattered, from the normal-mode measurement-phase beam, by the particles as they flow in the scattering zone, (iv) the scattered light passes through a receiver window, the receiver window being exposed to contamination by the particle flow, and (v) the scattered light is detected by a scattered-light detector; and a calibration mode measurement phase in which (a) a calibration-mode measurement-phase beam is generated by a light source and (b) the calibration-mode measurement-phase beam is directed through the receiver window to a reference detector; wherein, the scattered light and the calibration-mode measurement-phase beam trace the same optical path through all optical elements between the scattering zone and the scattered- light detector, such that changes in intensity detected by the reference detector provide a means to calibrate the probe.

30. A method as claimed in claim 29, further comprising a calibration mode compensation phase in which (i) a calibration- mode compensation-phase beam is generated by a light source,

(ii) the calibration-mode compensation-phase beam is directed through a transmitter window and the receiver window to the reference detector; wherein, the calibration-mode measurement-phase beam and the calibration-mode compensation-phase beam pass through substantially co-incident and co-extensive regions of the transmitter window and the receiver window, such that changes in intensity detected by the reference detector provide an indication of contamination of the transmitter window and/or the receiver window.

31. A particle monitor for monitoring particles flowing in a stack, comprising: (i) a light source for generating a measurement beam; (ii) an optical system, for directing the measurement beam into a scattering zone in the particle flow;

(iii) a transmitter window, through which the measurement beam passes before it reaches the scattering zone; (iv) a scattered- light detector for detecting light scattered, from the measurement beam, by the particles as they flow in the scattering zone; (v) an element that acts as the smallest stop aperture between the scattering zone and the scattered-light detector, that smallest-stop element and the transmitter window being in line-of-sight of each other, and (vi) an optical system that prevents light scattered by the transmitter window and passing through the smallest stop aperture from reaching the scattered-light detector.

32. A monitor as claimed in claim 31, in which the optical system that prevents light scattered by the transmitter window and passing through the smallest stop aperture from reaching the scattered-light detector comprises a filter that absorbs light scattered by the transmitter window.

33. A monitor as claimed in claim 32, in which the filter is sufficiently transmitting to allow for light from a compensation or calibration phase beam to reach the scattered light detector.

34. A monitor as claimed in any of claims 31 to 33, in which the optical system that prevents light scattered by the transmitter window and passing through the smallest stop aperture from reaching the scattered-light detector comprises a lens that focuses scattered light on the scattered light detector, a first mirror arranged behind or around the scattered-light detector, a second mirror with a hole arranged in front of the scattered light detector and a filter arranged between the first and second mirrors, the mirrors and filter being arranged such that scattered light from the scattering zone passes directly to the scattered-light detector through the hole in the second mirror and filter and the light scattered by the transmitter window is reflected by at least one of the mirrors and is substantially blocked by the filter.

35. A monitor as claimed in claim 34, in which the filter is arranged between the second mirror and the detector and has a hole through which the scattered light passes on its way to the detector.

36. A monitor as claimed in claim 34 or claim 35, further comprising a light source for generating a reference beam and an optical system for directing the reference beam to the scattered-light detector, the first and second mirrors and the filter being arranged such that the reference beam is reflected by at least one of the mirrors, passes through the filter and reaches the detector at an intensity level sufficiently high for detection by the detector.

37. A method of monitoring particles flowing in a stack, comprising: (i) generating a measurement beam; (ii) passing the measurement beam through a transmitter window; (iii) directing the measurement beam into a scattering zone in the particle flow; (iv) preventing light that is scattered by the transmitter window, and that passes through the smallest stop aperture between the scattering zone and a scattered-light detector, from reaching the scattered-light detector; (v) detecting, at the scattered-light detector, light scattered from the measurement beam by the particles as they flow in the scattering zone.

38. A particle monitor for monitoring particles flowing in a stack, comprising: (i) a light source for generating a measurement beam; (ii) an optical system, for directing the measurement beam onto a scattering zone in the particle flow;

(iii) a scattered-light detector for detecting light scattered, from the measurement beam, by the particles as they flow in the scattering zone; (iv) a filter having a transmission function that varies radially across the filter and is selected to alter the proportion of light intensity detected at the scattered- light detector resulting from scattering from particles of a first size within a range of sizes relative to the light intensity detected at the scattered-light detector resulting from scattering from particles of a second size within the range of sizes, such that the intensity of light detected after scattering from a set of particles of the first size having a total mass M is substantially the same as the intensity of light detected after scattering from a set of particles of the second size also having a total mass M.

39. A monitor as claimed in claim 38, in which the filter transmits scattered light reaching an annular area of the filter.

40. A monitor as claimed in claim 39, in which the annular area subtends at the scattering zone all angles between 1 degree and 8 degrees.

41. A monitor as claimed in any of claims 38 to 40, in which the filter blocks all scattered light reaching the filter outside an annular area of the filter.

42. A monitor as claimed in claim 41, in which the filter blocks all scattered light travelling to the filter from the scattering zone at an angle of less than 1 degree and all scattered light travelling to the filter from the scattering zone at an angle of more than 8 degrees.

43. A monitor as claimed in any of claims 38 to 42, in which the monitor gives a signal indicative .of mass concentration of the particle flow.

44. A monitor as claimed in any of claims 38 to 43, in which the monitor gives a signal indicative of particle sizes in the particle flow.

45. A method of monitoring particles flowing in a stack, comprising: (i) generating a measurement beam from a light source; (ii) directing the measurement beam into the particle flow; (iii) passing at least some light scattered from the particle flow through a filter having a transmission function that varies radially across the filter selected to alter the proportion of light intensity detected at a scattered-light detector resulting from scattering from particles of a first size within a range of sizes relative to the light intensity detected at the scattered-light detector resulting from scattering from particles of a second size within the range of sizes, such that the intensity of light detected after scattering from a set of particles of the first size having a total mass M is substantially the same as the intensity of light detected after scattering from a set of particles of the second size also having a total mass M; (iv) detecting the scattered light that has passed through the filter.

46. A method as claimed claim 45, in which light scattered at angles of less than 1 degree or more than 8 degrees from the direction of the measurement beam when it is incident on the particle flow is not detected during measurement.

47. A laser source for providing a laser beam having low levels of peripheral light, comprising a laser, a lens and a first reflector arranged between the laser and the lens, the first reflector defining an aperture through which light emitted from the laser passes, the aperture of the first reflector being sized so as to limit the cross-section of the light passing through that aperture, and the first reflector being arranged to reflect light not passing through that aperture, the source further comprising an absorber arranged between the first reflector and the laser such that peripheral light not passing through the aperture in the first reflector is absorbed.

48. A laser source as claimed in claim 47, further comprising a second reflector arranged between the laser and the lens, the second reflector defining an aperture through which light emitted from the laser passes, the first reflector being arranged to reflect light not passing through the aperture in the first reflector to the second reflector and the second reflector being arranged to reflect light reflected by the first reflector back to the first reflector, the absorber being arranged between the first reflector and the second reflector.

49. A source as claimed in claim 47 or claim 48, in which the absorber is adjacent to the first or second reflector.

50. A source as claimed in any of claims 47 to 49, in which the first or second reflector is itself the absorber, such that it is partially reflecting and partially absorbing at "the output wavelength of the laser.

51. A source as claimed in any of claims 47 to 50, in which the first or second reflector is, or is associated with, an absorber.

52. A source as claimed in any of claims 47 to 51, in which the first or second reflector is planar.

53. A source as claimed in claim 52, in which the reflectors are parallel to each other.

54. A source as claimed in claim 53, in which the reflectors are parallel. to the collimating lens.

55. A source as claimed in any of claims 47 to 54, in which the first reflector is adjacent to the collimating lens.

56. A source as claimed in any of claims 47 to 55, in which the second reflector is adjacent to the laser.

57. A source as claimed in any of claims 47 to 56, in which the laser is a diode laser.

58. A source as claimed in any of claims 47 to 57, in which light not absorbed is specularly reflected by the first or second reflector and substantially none is scattered.

59. A source as claimed in any of claims 47 to 58, further comprising an aperture, preferably a pin hole, arranged adjacent to the laser.

60. A source as claimed in claim 59, in which the aperture is comprised in the second reflector.