HIGH SENSITIVITY PARTICLE DETECTION

Abstract: A smoke detector is shown in which smoke particles are detected by the collection and detection of blue light emitted from a radiation source (3) and scattered by the particles. The emitted radiation (7) is passed into a volume (9) and is scattered by any smoke particles present. Scattered radiation is collected by an ellipsoidal mirror (13) and focussed on a silicon photodiode (15). The light collected by the mirror (13) has been scattered through angles substantially less than 45 DEG and preferably between about 10 DEG and 35 DEG .

Description: HIGH SENSITIVITY PARTICLE DETECTION The invention relates generally to high sensitivity particle detection. Embodiments of the invention to be described in more detail, by way of example only, are for detecting the presence of smoke particles. According to the invention there is provided a particle detector for detecting particles of sizes of less than 1 micron, comprising radiation emitting means for emitting radiation along a predetermined path through a scattering volume, and radiation detection means for receiving and detecting radiation scattered at a predetermined forward scattering angle from the scattering volume by the presence of particles, the angle being less than 45° to the predetermined path of radiation, and the wavelength of the radiation being between about 400nm and about 500 nm. According to the invention, there is also provided a particle detecting method for detecting particles of sizes of less than 1 micron, comprising the steps of emitting radiation along a predetermined path through a scattering volume, and receiving and detecting radiation scattered at a predetermined forward scattering angle from the scattering volume by the presence of particles, the angle being less than 45° to the predetermined path of radiation, and the wavelength of the radiation being between about 400nm and about 500 nm. High sensitivity particle detection apparatus and methods according to the invention will now be described, by way of example only, with reference to the accompanying diagrammatic drawings m which: - Figure 1 is a schematic diagram of one form of the apparatus; Figures 2,3 and 4 are graphs for explaining the operation and advantages of the apparatus of Figure 1 ; Figure 5 corresponds to Figure 1 but shows a modified form of the apparatus; and Figures 6 and 7 are graphs for explaining the operation and advantages of the apparatus of Figure 5. The apparatus and methods to be described are for detecting smoke in air using light scattering techniques, although it will be appreciated that other particles can be detected using the same apparatus and methods . The apparatus and methods aim to detect the presence of smoke particles at smoke densities at least as low as 0.2% per metre. The primary use of such apparatus is for detecting incipient fires. The apparatus 1 (Figure 1) comprises a radiation source 3 emitting radiation along a path 5. Radiation 7 passes through a volume 9 towards a beam dump 11. An ellipsoidal mirror 13 is positioned for collecting radiation scattered by the presence of smoke particles m the volume 9 (within a predetermined range of forward scattering angles to be discussed below ' and focussing such radiation on a silicon photodiode 15. It will be appreciated that the collection means for the scattered radiation need not be an ellipsoidal mirror 13 but may be any suitable collection means. Additionally, it will also be appreciated that any suitable detector means may be used and the detector need not be silicon photodiode 15. In use, radiation 7 from the radiation source 3 is emitted along the path 5 through the scattering volume 9. The presence of any smoke particles m the scattering volume 9 will cause the radiation 7 to be scattered through a predetermined range of angles. The ellipsoidal mirror 13 is positioned such that any light scattered at forward scattering angles of less than 45°, and more particularly at scattering angles between about 10° and 35° will be collected by the ellipsoidal mirror 13. The ellipsoidal mirror 13 focuses the light scattered at these angles from the scattering volume in all planes perpendicular to the incident radiation direction on to the silicon photodiode 15. This arrangement maximises the radiation incident on the photodiode 15. The signal produced by the silicon photodiode 15 may be used to trigger a suitable alarm system and/or a fire extinguishing system. Any radiation which is not scattered will be incident on and be trapped substantially by the beam dump 11 and no corresponding signal will be produced by the silicon photodiode 15. The radiation source 3 emits radiation 7 at relatively short wavelengths between about 400nm and 500nm, that is, blue visible light; preferably, the radiation source 3 is an LED producing radiation at 470 nm wavelength. It is found that the use of this relatively short wavelength, combined with the use of relatively small forward scattering angles, produces increased sensitivity of particle detection, at least for smoke particles This is explained m more detail with reference to Figures 2 to 4. Curve A in Figure 2 shows the output of the detector 15 for different degrees of smoke obscuration expressed as a percentage of light obscured per metre. Curves B, C, D and E show the corresponding detector outputs at the same scattering angle but for different (longer) radiation wavelengths . Curve B shows the detector output where the radiation is in the green part of the spectrum. Curve C shows the detector output where the radiation is in the red part of the spectrum. Curve D shows the detector output when the radiation is in the infra-red part of the spectrum and of the order of 880 nm. Finally, curve E shows the detector output when the radiation is in the infra-red part of the spectrum and of the order of 950 nm. In each case, the range of forward scattering angles is the same (between about 10° and 35°) . The smoke for the tests illustrated was produced by smouldering cotton. Figure 2 clearly snows the increased detector output, and thus the increased sensitivity of detection, which is obtained by using a radiation source producing blue visible light of the order of 470 nm. Figure 2 shows how detectable signals can be produced from the photodiode 15 at smoke densities as low as 0.2% per metre. Radiation at the other wavelengths (curves B, C, D and E) produces significantly lower outputs. Shorter wavelength light also has the advantage that it has a lower reflectivity from typical matt black surfaces. By suitable design of the detecting apparatus, therefore, the output from the photodiode 15 due to background scattered light signals (primarily signals reflected from internal surfaces of the apparatus and not due to smoke) can be made very small - and significantly less than when light of longer wavelengths is used. Figure 3 plots the calculated scattering gain for a particle size distribution typical of smoke against the forward scattering angle using light at different wavelengths. Scattering gain is the amount of light scattered into a unit solid angle as a fraction of the light falling on an individual particle. Curve A corresponds to blue visible light, curve B to green visible light, curve C to red visible light, curve D to infra-red radiation of the order of 880 nm, and curve E to infra-red radiation of 950 nm. Figure 3 shows how the use of blue visible light (curve A) produce significantly more scattering gain than radiation at the other wavelengths (curves B to E) at scattering angles up to about 155°, although the increase m scattering gain is much more pronounced at scattering angles less than 45°. Curves A in Figures 2 and 3 therefore show how the combination of the use of blue visible light (radiation between 400 and 500nm) and the use of low scattering angles (between about 10° and 35°) produces a significant increase in sensitivity. Smoke detectors may be susceptible to false alarms in the presence of larger aerosol particles such as condensed water mist or dust. Figure 4 corresponds to Figure 3 except that the particles used are particles having a size distribution typical of condensed water mist, and calculations were carried out for only two wavelengths: blue visible light at 450 nm (curve A) , and infra-red radiation at 950 nm (curve E) . Curves A and E m Figure 4 show that the scattering gain is substantially the same at both the wavelengths tested, at least for scattering angles between about 15° and 30° . A comparison of Figures 3 and 4 therefore shows that the ratio (signal to noise ratio) between the output of the photodiode in response to smoke particles and the corresponding output for "nuisance" aerosols, such as water mist particles, will be higher when blue light is used than when radiation at the other wavelengths is used. Figure 5 shows a modified arrangement of Figure 1 which uses the principle illustrated by comparing Figures 3 and 4. In Figure 5 items corresponding to items m Figure 1 are similarly referenced. In Figure 5, the source 3 of Figure 1 is supplemented by a source 3A. Source 3 produces blue light, as before, m the range 400 to 500 nm. Source 3A produces infra-red radiation at about 880 nm and may (like source 3) be an LED. The radiation emitted by both sources is passed via a beam splitter 17 and thence through the volume 9. As before, radiation forward-scattered (at the appropriate angles) by obscuration m the volume 9 is collected by the ellipsoidal mirror 13 and focussed an detector 15. As before, detector 15 is a silicon photodiode. Such a detector is sensitive to blue light and also infra-red radiation at about 880nm. A control system indicated generally at 19 and 20 enables the detector 15 to produce separate outputs on lines 21 and 23 corresponding respectively to the scattered blue light and the scattered infra-red radiation as received by the detector. The control system 19,20 may take any suitable form. For example, it may arrange to pulse the sources 3 and 3A alternately and to switch the detector output synchronously between the lines 21 and 23. Instead, the sources 3 and 3A can be energised separately at different frequencies and separate narrow band or lock-in amplifiers can be used for responding to the output from the detector and for respectively energising the lines 21 and 23. The outputs of the detector 15 on lines 21 and 23 are processed by a comparison unit 25. Figures 6 and 7 illustrate the operation of the arrangement of Figure 5 . In Figures 6 and 7, the horizontal axis represents time, the left hand vertical axis represents visible obscuration expressed as a percentage of light obscured per metre, and the right hand vertical axis represents the output of the detector 15 in Figure 5. The left and right hand axis are to a logarithmic scale. Figure 6 shows results obtained when obscuration is caused by smoke (in this case, grey smoke produced by smouldering cotton) . the smoke being released for 5s at 100s and then for 100s between 200 and 300s. In Figure 7, the obscuration is caused by a non-smoke source, in this case by a hairspray aerosol. A one second spray is released at 100s and a 10s spray at 200s. In Figure 6, curve I plots the obscuration. Curve II plots the output of the detector 15 in response to the blue light emitted by the source 3. Curve III plots the output of detector 15 in response to the infra-red radiation emitted by source 3A. It will be seen that the detector output in response to the scattered infra-red radiation (Curve III) is much less than the detector output in response to the scattered blue light (curve II) . Curve IV shows the ratio of the detector output when the emitted radiation is blue light (curve II) to the output when the emitted radiation is infrared (curve III) . The ratio is significantly greater than one. In Figure 7, the curves I, II, III and IV have the same identities as in Figure 6. It will be noted that the ratio shown by curve IV is significantly less than one. The unit 23 is therefore arranged to measure the ratio of the output of detector 15 to the output of detector 15A. If this ratio is more than one, obscuration by smoke is signalled. If the ratio is less than one, smoke obscuration is not signalled. The infra-red radiation used in the embodiment of Figure 5 does not need to be at 880nm.

Claims: 1. A particle detector for detecting particles of sizes of less than 1 micron, comprising radiation emitting means (3) for emitting radiation along a predetermined path through a scattering volume (9), and radiation detection means (15) for receiving and detecting radiation scattered at a predetermined forward scattering angle from the scattering volume by the presence of particles, characterised m that the angle is less than 45° to the predetermined path of radiation, and the wavelength of the radiation is between about 400nm and about 500 nm. 2. A detector according to claim 1, characterised m that the particles are smoke particles. 3. A detector according to claim 1 or 2 , characterised m that the detecting means (3) is a photodiode. 4. A detector according to claim 1, characterised by second radiation emitting means (3A) for emitting infra-red radiation along the predetermined path through the scattering volume (9), second radiation detecting means (15A) for receiving and detecting the infra-red radiation scattered at the predetermined forward scattering angle from the scattering volume (9) by the presence of the particles, and output means (23) for comparing the outputs of the two detecting means (15,15A) whereby to produce a warning output when the comparison indicates that the particles are of a predetermined type but not when the comparison indicates otherwise. 5. A detector according to claim 4, characterised in that the output means (23) measures the ratio between the outputs of the detecting means (15.15A) . 6. A detector according to claim 4 or 5 , characterised in that the particles of the predetermined type are smoke particles . 7. A detector according to any preceding claim, characterised in that the or each radiation emitting means 8. A detector according to any preceding claim, characterised m that the predetermined scattering angle lies in a range between about 10° and 35°. 9. A detector according to any preceding claim, characterised by collecting and focussing means (13) for collecting the scattered radiation and focussing it on the or each detection means (15,15A) . 10. A detector according to claim 9, characterised in that the collecting and focussing means is an ellipsoidal mirror (13) . 11. A detector according to any preceding claim, characterised by beam dump means (11) positioned in the predetermined path and further from the radiation emitting means (3,3A) than the scattering volume (9) . 12. A particle detecting method for detecting particles of sizes of less than 1 micron, comprising the steps of emitting radiation along a predetermined path through a scattering volume (9) , and receiving and detecting radiation scattered at a predetermined forward scattering angle from the scattering volume (9) by the presence of particles, characterised in that the angle is less than 45° to the predetermined path of radiation, and the wavelength of the radiation is between about 400nm and about 500 nm. 13. A metnod according to claim 12, characterised in that the particles are smoke particles. 14. A method according to claim 12, characterised by the step of emitting infra-red radiation along the predetermined path through the scattering volume (9) , receiving and detecting the infra-red radiation scattered at the predetermined forward scattering angle from the scattering volume (9) by the presence of the particles, and comparing outputs respectively corresponding to the detected radiation between about 400nm and about 500nm and the detected infra-red radiation whereby to produce a warning output when the comparison indicates that the particles are of a predetermined type but not when the comparison indicates otherwise. 15. A detector according to claim 14, characterised in that the comparison step comprises the step of measuring the ratio between the compared outputs. 16. A detector according to claim 14 or 15, characterised in that the particles of the predetermined type are smoke particles . 17. A method according to any one of claims 13 to 16, characterised in that the predetermined scattering angle lies in a range between about 10° and 35°. 18. A method according to any one of claims 12 to 17, characterised by the step of collecting and focussing the scattered radiation.

Inventors: POWELL BRIAN, POWELL, BRIAN
Applicants: KIDDE FIRE PROTECTION LTD, POWELL BRIAN, KIDDE FIRE PROTECTION LIMITED, POWELL, BRIAN
Application year: 1998
Main classification: G08B17
IPC codes: G01N21, G08B17