WO2007113567A1

WO2007113567A1 - Optical beam dump

Info

Publication number: WO2007113567A1
Application number: PCT/GB2007/001253
Authority: WO
Inventors: John Rogers; Michael Rigby
Original assignee: Pcme Ltd.
Priority date: 2006-04-04
Filing date: 2007-04-04
Publication date: 2007-10-11
Also published as: EP2008083A1; GB2436871A; CN101416045A; GB0606800D0

Abstract

An optical beam dump (30), (51) having a passage defined by a passage wall and an optical filter (33), (55) located in an opening in that passage wall. The optical filter (33), (55) being arranged to direct light reflected from that filter (33), (55) along the passage away from an optical input (32), (65) to the beam dump. The passage may be formed as a tube (31), or may be cut within a block of material (52). The passage may have a plurality of bends. Provision for the flow of purging air through the beam dump is made by providing holes (36), (61) in the passage wall.

Description

Optical Beam Dump

This invention relates to an optical beam dump, and in particular an optical beam dump for use in a particle monitoring system.

When light interacts with particles, the particles may reflect, refract, diffract or absorb the light; the nature of the interaction depends on inter alia the size, refractive index and surface profile of the particles and the wavelength of the interacting light. Particulate monitoring systems can utilise that interaction to monitor the particulate content of a gas, for example in a stack.

Monitoring devices are known that monitor the particulate content of a gas by directing a collimated beam of light (typically from a laser) through a sample of the gas and observing the proportion of the light that is scattered by the gas. In a gas with no particulate content the beam will continue in a straight line, but when particles are present in the gas a proportion of the light is scattered on to a path at an angle to the main beam. The proportion of the main beam that is scattered is indicative of the particulate content of the gas, and so by detecting the magnitude of the scattered light it is possible to determine the particulate content of the gas.

Figure 1 shows a monitoring device 1 operating according to these principles. Such a device is disclosed in UK patent Application No. 0502150.6. A beam of light 2 (typically from a laser) is directed into a monitoring region 3 where a small proportion 4 of the beam 2 is scattered by particles in the gas in the monitoring region 3. The monitoring device is located and arranged (for example in a stack) such that the gas being monitored passes through the monitoring region 3.

At the opposite side of the region 3, a curved mirror 5, having a central hole 6, is arranged to reflect only the scattered light 4 to a waveguide 8, while any un-scattered light 7 (the direct beam) passes through the central hole 6. The scattered light 4 is captured by the waveguide 8 which directs it to a sensor which detects the magnitude of that scattered light 4, enabling the particulate content of the gas in the monitoring region to be determined.

As the proportion of the beam 2 that is scattered is very small (typically the ratio of incident light to scattered light is of the order of 40,000,000 to one, for a dust signal of 1 mg/m³t) even small amounts of stray light entering the waveguide 8 can affect the accuracy of the measurement of particulate content. Ideally, only scattered light should be captured by the waveguide 8, and not any other light, such as stray reflections of the main, unscattered, beam 7. A means to absorb the main beam 7 and prevent or significantly reduce unwanted reflections is therefore required.

In GB0502150.6, main-beam reflections are reduced by placing a Neutral Density (ND) filter 10 beyond the mirror 5 such that the direct beam 7 is absorbed by that filter 10. The ND filter 10 absorbs the majority of the light incident upon it, preventing the absorbed light from entering the waveguide 8. However, a small proportion (typically around 4%) of the light incident on the filter 10 is reflected from the filter' s surface and at least some of that light may- then enter the waveguide 8. Moreover, the monitoring device 1 is generally used in environments with a high particulate content, and particles can be deposited onto the filter. Such deposits increase the reflectivity of the filter 10, thereby increasing the amount of reflected light. Furthermore, light is reflected diffusely from particles on the surface of the filter and is therefore scattered in many directions. Particle contamination can thereby increase the amount of stray light that enters waveguide 8.

An anti-reflection coating can be applied to the surface of the filter but, while that reduces the reflectivity when the surface is clean, the reflectivity may still be increased substantially when the filter surface becomes contaminated by particles .

A further type of beam dump is shown in Figure 2. A conical surface 20 is constructed of absorbent material such that light incident on the interior of that surface 20 is absorbed. The surface 20 is arranged such that any reflected light is not directed directly out of the device. However, light can still be reflected out of the device after a number of reflections and, although greatly attenuated, that reflected light may be sufficient to affect measurements. The surfaces are again susceptible to deposits which will increase the reflectivity of the surfaces and thereby increase the amount of reflected light.

An improved beam dump could be used in an environment in which there is a high particulate content, without the beam dump' s performance being affected to the same extent as in prior-art beam dumps, or even at all.

According to the present invention there is provided an optical beam dump, comprising a passage defined by a passage wall, one end of the passage being open and forming an input for a light beam, and an optical filter, mounted in an opening in the passage wall such that a beam of light entering the passage through the input will strike the filter, the filter being aligned such that any of the light striking the filter that is reflected is reflected along the passage, in a direction away from the input end.

Such a beam dump is especially advantageous particularly when used in environments with a high particulate content, for example in stacks.

The distal end of the passage (i.e., the end away from the input end) may be closed, the inner surface of the closed end being arranged at one or more angles to a plane that is perpendicular to the axis of the passage immediately adjacent to the closed end (i.e., the inner surface of the closed end is not in that plane) . The amount of light that is reflected back along the passage to the input end is thereby reduced. Preferably, the inner surface of the closed end forms a cone shape.

Reflecting light off the filter in a direction away from, rather than towards, the input end (that is, back along the passage) is advantageous, because it reduces the likelihood of the reflected light escaping from the beam dump and affecting measurements.

Preferably, the passage has at least one opening in the passage wall through which purging air can be injected to prevent the ingress of particles and/or to remove particles deposited on the interior surface of the passage.

Such an arrangement reduces the accumulation of particles on the optical surfaces of the beam dump, thereby improving performance in particle-rich environments.

Preferably, the passage is U-shaped in a plan view.

The passage may have at least 2 bends of 90°.

The optical filter is preferably a neutral density filter; that allows the beam dump to be used with a wide range of light wavelengths.

The passage preferably has a diameter of between 5mm and 15mm.

The optical filter preferably has an optical density in the range 2-6.

The passage wall may be elongate. The passage wall may be a wall of a tube, or alternatively the passage may be defined within a solid block of material. A mounting for an optical mirror adjacent the input end of the passage may be provided. A particle monitoring device may be provided having an optical beam dump, the optical beam dump comprising a passage defined by a passage wall, one end of the passage being open and forming an input for a light beam, and an optical filter, mounted in an opening in the passage wall such that a beam of light entering the passage through the input will strike the filter, the filter being aligned such that any of the light striking the filter that is reflected is reflected along the passage, in a direction away from the input end.

The improved performance of the beam dump allows the particulate content of the monitored air to be measured more accurately.

The present invention also provides a method of monitoring particles flowing in a stack, comprising the steps of: (1) scattering a light beam from the particles; (2) detecting light scattered from the light beam by the particles and monitoring a property of the particles using the detected light; (3) directing light from the light beam not scattered by the particles into an input end of a beam dump; (4) absorbing in an optical filter a majority of the light entering the beam dump; (5) reflecting in a direction away from the input end of the beam dump substantially all of the light entering the beam dump that is not absorbed by the filter. Examples of embodiments of the current invention will now be described with reference to the drawings, of which:-

Figure 1 shows a prior-art monitoring device which utilises light scatter from particles to monitor the particulate content of a gas/

Figure 2 shows a prior-art beam dump;

Figure 3 is a plan view of a beam dump according to a first embodiment of the present invention;

Figure 4 shows a cross-section of the beam dump of Fig. 3 showing the path of a light beam entering the beam dump; and

Figure 5 shows a cross-section of a beam dump according to a second embodiment of the invention.

The current invention provides an improved beam dump to improve the performance of light-scatter-based particle detectors .

A beam dump 30 (Fig. 3) , which is an example embodiment of the current invention, consists of a substantially U-shaped tube 31, one end 32 being open for a light beam 34 (to be absorbed) to enter the beam dump 30.

The tube 31 has a hole located on the outside of the first corner of the tube 31, and the hole is closed by a neutral density filter 33. The filter 33 is located such that a light beam 34 entering the beam dump 30 will strike the filter 33 (as shown in Figure 4) . The filter 33 is aligned such that any light 40 reflected from the surface of the filter 33 is directed further along the tube 31, away from the input end 32, thereby preventing light from being reflected directly out of the beam dump 30.

The opposite end of the tube 31 to the input end 32 has a cone-shape 35 designed to minimise the reflection of light arriving at that end back towards the input end 32. Other end-shapes could also be utilised which would have the same effect as the cone-shape. The shape should not have any surfaces that are perpendicular to the axis of the tube, so that light is not reflected directly back along the tube.

Adjacent the closed end are a set of holes 36 to allow the injection of purge air into the tube 31 in order to clean the internal surfaces to remove deposited particles. Clean air is injected and passes along the tube 31 and out of the optical input 32, carrying with it at least some of any particles in the tube 31. The air can be applied continuously, or at intervals. The holes 36 allow sufficient air to enter the tube 31 to perform this function, while only an insignificant quantity of light can escape from the beam dump 30 through the holes 36.

In use, the majority of light 34 entering the beam dump 30 is absorbed by filter 33. A small proportion of the beam 34 will pass through the filter 33, and a small proportion will be reflected from the filter 33. By choosing a filter with a sufficiently low transparency (for example an optical density of 4) the transmitted light can be reduced to a level that will not affect the operation of the measurement device. The internal walls of the tube 31 are coated with a light- absorbing material, for example absorbent paint, or they may^¬ be anodised. A majority of any light striking the walls (for example light reflected from the filter 33) will be absorbed, with a small amount being diffusely reflected. The majority of the reflected light 40 from the filter 33 will therefore be absorbed when it next falls on the wall 41. The small reflection from that point 43 is directed further along the tube 31, and again a still smaller quantity is reflected when it strikes the wall at 42. In Figure 4 only the direct, main, reflection is shown, as described above the surfaces create a diffuse reflection and therefore light is reflected in all directions. Due to the design of the tube 31 light must reflect from surfaces a number of times to be reflected back out of the beam dump 30 and so the amount of light leaving the beam dump 30 is negligible .

The deposition of particles on the surfaces will increase their reflectivity and also lead to more diffuse reflections, but even with greatly increased reflectivity, the attenuation along and back out of the tube 31 is sufficient to reduce the level of light leaving the tube 31 to a negligible amount.

Figure 5 shows a beam dump 51 according to a second embodiment of the current invention, in which Figure dotted lines indicate passages formed within the body 52. The second embodiment operates according to the same principles as the first embodiment - having an optical filter 55 arranged at an angle to the light beam to be absorbed and a passageway 57, 58, 59 to absorb light reflected from the filter 55 - but is made by a different construction technique.

The beam dump 51 is constructed by cutting the passages 57, 58, 59, 62 into a solid block of material 52. This method of construction leads to openings in the surface of the body 52 (since the passages 57, 58, 59 must be cut from the surface) that are not desired; the openings are closed by the insertion of set screws or plugs 53. Other methods of construction are also suitable, as will be understood by the person skilled in the art.

Opening 65 forms the optical input of the beam dump 51, and a mirror 54, as previously described, is mounted in that opening.

A neutral density filter 55 is mounted such that it is struck by the direct beam of the monitoring device once that beam has passed through the hole in mirror 54. Neutral density filter 55 is held in place by a plate 56 attached to the beam dump body 52. The plate 56 is formed of an opaque material and reflects or absorbs any light striking it. Light passing through filter 55 is thereby prevented, by plate 56, from escaping from beam dump 51 through filter 55. Light reflected from plate 56 passes back through filter 55 and therefore undergoes two passes through filter 55 before re-entering the passage from where it could pass back out of the beam dump 51. The attenuation of the filter 55 is such that the amount of light ultimately leaving the beam dump 51 through the input 65 is negligible. As described in relation to the first embodiment, an optical density of 4 is suitable.

Light reflected from the surface of the filter 55 is dissipated in the passages 57, 58, 59 in the same manner as described above in connection with the first embodiment of the invention.

The ends of the passages 57, 58, 59 have a cone shape 64 in order to provide the functionality described above in relation to the cone-shaped end 35 of the tube 31 of the first embodiment.

Open passage 60 is provided to mount the beam dump 51 to the particle monitoring device (or other device) in which it is being utilised. The beam dump 51 is secured to the end of a hollow structural element 63 of the particle monitoring device such that the mirror 54 is correctly aligned with the laser beam to be absorbed.

Passage 60 is linked 61 to the other passages 57, 58, 59, 62 of the devices to allow the introduction of purge air to those passages 57, 58, 59, 62. The maze-like layout of the passages ensures that only an insignificant amount of light can escape from the beam dump 51 through passage 61 from where it may interfere with the performance of the monitoring device.

Purge air is pumped through the support member 63 (within which it can also be used to cool the monitoring device) and from there passes through the passages 61, 59, 58, 57, 62 of the beam dump 51 and out of the optical input 65. The purge air carries with it at least some of any particles that have entered the beam dump 51 or been deposited on the surfaces within the beam dump 51. The purging helps to control any increase in the reflectivity of the surfaces as described above with reference to the first embodiment of the invention. As described previously, the purge air may be passed continuously or at intervals.

The interior surfaces of the beam dump 51 which may be struck by light are coated with absorbent paint, or may be anodised to provide a similar absorbent characteristic.

As discussed, the embodiments of a beam dump according to the invention described herein are designed for use in conjunction with a particulate content monitoring device and so are likely to be used in particularly dirty atmospheres. The design of the beam dumps, having enclosed passageways, reduces the likelihood of particles being deposited onto the optical surfaces of the beam dump, thereby reducing the problems associated with the increased reflectivity of dirty surfaces .

The embodiments of the invention described herein are constructed with passages of a circular cross section, however other cross sections are equally applicable.

As will be understood by the person skilled in the art a beam dump having fewer, or more, corners and/or passages than are provided by each embodiment described herein would operate as described in relation to the first and second embodiments of the invention described herein. The optical filter in each embodiment has been described as being of the neutral density type, but any filter having a high absorption at the wavelength of the laser beam (or indeed any light source) to be absorbed would be equally suitable.

Whilst the present invention has been described and illustrated with reference to particular embodiments, it will be appreciated by those of ordinary skill in the art that the invention lends itself to many different variations not specifically illustrated herein. Some examples of such variations and alternatives have been described above.

Where in the foregoing description, integers or elements are mentioned which have known, obvious or foreseeable equivalents, then such equivalents are herein incorporated as if individually set forth. Reference should be made to the claims for determining the true scope of the present invention, which should be construed so as to encompass any such equivalents. It will also be appreciated by the reader that integers or features of the invention that are described as preferable, advantageous, convenient or the like are optional and do not limit the scope of the independent claims.

Claims

1. An optical beam dump, comprising a passage defined by a passage wall, one end of the passage being open and forming an input for a light beam, and an optical filter, mounted in an opening in the passage wall such that a beam of light entering the passage through the input will strike the filter, the filter being aligned such that any of the light striking the filter that is reflected is reflected along the passage, in a direction away from the input end.

2. An optical beam dump according to claim 1 wherein a distal end of the passage from the input end is closed, the inner surface of the closed end being arranged at one or more angles to a plane that is perpendicular to the axis of the passage immediately adjacent to the closed end.

3. An optical beam dump according to claim 1 wherein a distal end of the passage from the input end is closed, the inner surface of the closed end forming a cone shape.

4. An optical beam dump according to any preceding claim wherein the passage has at least one opening in the tube wall through which purging air can be injected to prevent the ingress of particles and/or to remove particles deposited on the interior surface of the tube.

5. An optical beam dump according to any preceding claim wherein the passage is U-shaped in a plan view.

6. An optical beam dump according to any preceding claim wherein the passage has at least 2 bends of 90°.

7. An optical beam dump according to any preceding claim wherein the optical filter is a neutral density filter.

8. An optical beam dump according to any preceding claim wherein the passage has a diameter of between 5mm and 15mm.

9. An optical beam dump according to any preceding claim wherein the optical filter has an optical density in the range 2-6.

10. An optical beam dump according to any preceding claim wherein the passage wall is a wall of a tube.

11. An optical beam dump according to any one of claims 1-7 wherein the passage is defined within a solid block of material .

12. An optical beam dump according to any preceding claim further comprising a mounting for an optical mirror adjacent the input end of the passage.

13. A particle monitoring device having a beam dump according to any preceding claim.

14. A method of monitoring particles flowing in a stack, comprising the steps of: (1) scattering a light beam from the particles;

(2) detecting light scattered from the light beam by the particles and monitoring a property of the particles using the detected light; (3) directing light from the light beam not scattered by the particles into an input end of a beam dump;

(4) absorbing in an optical filter a majority of the light entering the beam dump;

(5) reflecting in a direction away from the input end of the beam dump substantially all of the light entering the beam dump that is not absorbed by the filter.

15. An optical beam dump as hereinbefore described or as shown in Figures 2-5.