Particle detector apparatus and method for detecting particles in gaseous medium

Abstract

A particle detector apparatus comprises a light source (1001) emitting a light beam (1002) through a detection zone (1003), a channel (1004) directing a flow (1005) of gaseous medium through said detection zone, a plurality of light detectors (901, 902, 1006, 1008) detecting scattered light, and a processing unit (1203) generating detection and classification results indicative of detected particles on basis of detection signals received from said plurality of light detectors (901, 902, 1006, 1008). Said plurality of light detectors (901, 902, 1006, 1008) comprise at least three light detectors (901, 902, 1006, 1008), of which at least two (901, 902, 1006) are on a common plane, said optical axis being parallel to said common plane.

Description

PARTICLE DETECTOR APPARATUS AND METHOD FOR DETECTING PARTICLES IN GASEOUS MEDIUMTECHNICAL FIELD

[0001] The following disclosure is related to the detection of particles in a gaseous medium, like impurities in air . In particular, the disclosure is related to hardware, software, and methods that can be used to detect and classify particles more accurately than earlier, using devices of relatively low complexity.BACKGROUND

[0002] Many applications would benefit from an ability to detect and classify small particles floating in and transported by gaseous media . In this text, "detecting" means obtaining information that particles exist in a sample of gaseous medium. It can be further split into qualitative and quantitative detecting. The former means simply noting that the gaseous medium is not clean or pure but contains impurities in the form of particles in amounts larger than a detection threshold. Quantitative detecting means obtaining more accurate information about how much impurities there are per unit volume of gaseous medium. "Classifying" means obtaining information about some characteristics of the detected particles : for example of their size and shape .

[0003] In this text, the concept of particles covers solid particles, liquid particles (i . e . droplets) , and mixed-phase solid / liquid particles . Of special interest are particles with their size in the range from 0.1 to20 micrometres, especially from 1 to 10 micrometres . In most cases of interest, the gaseous medium is air .

[0004] The size and shape of particles in air affect their hazardousness to human and animal health. Particles in the size range mentioned above are small enough to find their way into the smallest structural parts of lungs, where they may get trapped. Particularly hazardous are fibrous particles such as asbestos dust, because they may become permanently trapped and lead to serious consequences such as fibrosis, cancer, and others .

[0005] As the hazards of particles in breathing air have been acknowledged, there are various standards related to e . g. occupational health that require monitoring the quality of air at potentially dangerous work like mining, sandblasting, sanding, construction work, crop dusting, and the like, and taking action to protect users against particle-related hazards . Problems exist, however, because the task of detecting and classifying particles in gaseous media is technically and methodologically complex and has conventionally required large, bulky, and expensive measurement devices . There is a considerable need for hardware and methods that would enable detecting and classifying particles in air and other gaseous media with devices that could be small, lightweight, and power-efficient enough to be carried along as a part of personal protective equipment .SUMMARY

[0006] This summary is provided to introduce a selection of concepts in a simplified form that are further described below in the detailed description. Thissummary is not intended to identify key features or essential features of the claimed subj ect matter, nor is it intended to be used to limit the scope of the claimed subj ect matter .

[0007] It is an obj ective to provide hardware and method type solutions that would enable making reasonably accurate detection and classification of particles in gaseous media with a device that is simple and cost-effective enough to qualify as a personal safety device for everyday work.

[0008] According to a first aspect, there is provided A particle detector apparatus for detecting and classifying particles in gaseous media . The apparatus comprises a light source configured to emit a light beam ( 1002 ) in a direction of an optical axis through a detection zone, a channel configured to direct a flow of gaseous medium through said detection zone, and a plurality of light detectors configured to detect scattered light propagating from said detection zone into directions other than the direction of the light beam. A processing unit is coupled to said plurality of light detectors and configured to generate detection and classification results indicative of detected particles on basis of detection signals received from said plurality of light detectors . The plurality of light detectors comprises at least three light detectors in an at least two-dimensional spatial distribution, of which at least two are on a common plane, said optical axis being parallel to said common plane .

[0009] According to an embodiment, said at least two light detectors are a first and a second one of said at least three light detectors, and additionally a thirdone of said at least three light detectors is on said common plane . This involves at least the advantage that a common substrate, like a circuit board for example, can be used for many of the functional components, leading to a simple structure that has low manufacturing costs .

[0010] According to an embodiment, said first one of said plurality of light detectors is closer to said detection zone than said second and third ones of said plurality of light detectors . This involves at least the advantage that a relatively large amount of scattered light can be received at the first light detector, leading to an advantageously high signal to noise ratio and improved ability of using the detection signals as indications of the size of the detected particle .

[0011] According to an embodiment, said first one of said plurality of light detectors is located on said common plane centred on a perpendicular proj ection of said optical axis on said common plane . This involves at least the advantage that a relatively large amount of scattered light can be received at the first light detector, leading to an advantageously high signal to noise ratio and improved ability of using the detection signals as indications of the size of the detected particle .

[0012] According to an embodiment, said second and third ones of said plurality of light detectors are located on said common plane on opposite sides of said perpendicular proj ection of said optical axis on said common plane . This involves at least the advantage that useful information about the shape of the scattering pattern of a particle can be obtained.

[0013] According to an embodiment, the first, second, and third ones of said plurality of light detectors are located on said common plane at corners of an isosceles triangle located symmetrically with respect to the perpendicular projection of said optical axis on said common plane . This involves at least the advantage that useful information about the shape of the scattering pattern of a particle can be obtained.

[0014] According to an embodiment, each of said plurality of light detectors has a uniform light-sensitive detection area of more than two square millimetres, preferably more than five square millimetres, and more preferably about eight square millimetres . This involves at least the advantage that an advantageously large amount of scattered light may fall upon each light detector, allowing the use of relatively simple processing electronics for the detection signals .

[0015] According to an embodiment, said channel is configured to direct said flow of gaseous medium through said detection zone in a direction perpendicular to the direction of said light beam. This involves at least the advantage that each particle will pass through the detection zone in a relatively short time interval, allowing reliable detection and classification of particles also when there are a lot of them suspended in the gaseous medium.

[0016] According to an embodiment, the apparatus comprises a fan configured to maintain said flow of gaseous medium through said channel . This involves at least the advantage that the flow rate of the gaseous medium through the detection zone may remain advantageouslyhigh for reliable detection and classification of particles .

[0017] According to an embodiment, said fan is controllable by said processing unit, and the particle detector apparatus comprises one or more pressure sensors coupled to said processing unit for providing said processing unit with indications of a pressure difference between inside and outside of said channel . This involves at least the advantage that information is available for calculating a flow rate and, as a further conclusion, the density of particles in a volume of the gaseous medium.

[0018] According to an embodiment, the particle detector apparatus comprises a self-contained power source for operation. This involves at least the advantage that the apparatus can be made easily portable and / or wearable .

[0019] According to an embodiment, the particle detector apparatus is a personal portable device . This involves at least the advantage that occupational health and safety of workers can be monitored under potentially hazardous conditions .

[0020] According to an embodiment, the particle detector apparatus comprises a communications unit coupled to said processing unit . The processing unit can then be configured to communicate information indicative of said detection and classification results through said communications unit to at least one external device . This involves at least the advantage that a relatively simple user interface may suffice for the particle detector apparatus itself .

[0021] According to a second aspect, there is provided a method for detecting and classifying particles in gaseous media . The method comprises passing a light beam in a direction of an optical axis through a detection zone, directing a flow of gaseous medium through said detection zone, and detecting scattered light that propagates from said detection zone into a plurality of directions other than the direction of the light beam. Based on said detecting, detection and classification results can be generated indicative of detected particles . Said detecting is done at locations of which there are at least three in an at least two-dimensional spatial distribution, of which at least two are on a common plane, said optical axis being parallel to said common plane .

[0022] According to an embodiment the method comprises evaluating detection signals generated at said at three locations within a common time interval that corresponds to a single particle passing through said detection zone, and - in response to finding that said detection signals are similar in form but differ from each other with respect to their timing within said time interval - generating a classification result indicative of an elongate particle having been detected. This involves at least the advantage that information is available for evaluating the hazardousness of the detected particles .

[0023] According to an embodiment, the length of said time interval is more than 10 microseconds but less than 100 microseconds . This involves at least the advantage that large ranges of possible particle densities in the gaseous medium may be covered.

[0024] According to an embodiment, the method comprises measuring a pressure difference between said flow of gaseous medium and an environment, providing rate information indicative of a rotation rate of a fan that maintains said flow of gaseous medium, and based on said measured pressure difference and said rate information, calculating a flow rate of the flow of gaseous medium. This involves at least the advantage that quantitative detection results can be obtained about the density of particles in the gaseous medium.BRIEF DESCRIPTION OF THE DRAWINGS

[0025] The accompanying drawings, which are included to provide a further understanding of the disclosure and constitute a part of this specification, illustrate practical embodiments and together with the description help to explain the principles of the disclosure . In the drawings :figure 1 illustrates a particle detector apparatus ,figure 2 illustrates a detection zone, figure 3 illustrates two scattering patterns, figure 4 illustrates two scattering patterns in relation to two light detectors,figure 5 illustrates a scattering pattern in relation to a line of detectors,figure 6 illustrates an arrangement for detecting a scattering pattern,figure 7 illustrates examples of scattering patterns ,figure 8 illustrates a scattering pattern in relation to two light detectors,figure 9 illustrates a scattering pattern in relation to two light detectors,figure 10 illustrates a particle detector apparatus ,figure 11 illustrates a detail of a particle detector apparatus,figure 12 illustrates a functional block diagram of a particle detector apparatus,figure 13 illustrates an amplifier circuit, figure 14 illustrates voltages in an amplifier circuit like that in fig. 13,figure 15 illustrates variations in placing light detectors,figure 16 illustrates detection signals obtained from an example measurement,figure 17 illustrates detection signals obtained from an example measurement, andfigure 18 illustrates assumed detection signals from the detection of an elongate particle .DETAILED DESCRIPTION

[0026] In the following description, reference is made to the accompanying drawings, which form part of the disclosure, and in which are shown, by way of illustration, specific aspects in which the present disclosure may be placed. It is understood that other aspects may be utilised, and structural or logical changes may be made without departing from the scope of the present disclosure . The following detailed description, therefore, is not to be taken in a limiting sense, as the scope of the present disclosure is defined in the appended claims .

[0027] For instance, it is understood that a disclosure in connection with a described method may also hold true for a corresponding device or system configured to perform the method and vice versa. For example, if a specific method step is described, a correspondingdevice may include a unit to perform the described method step, even if such unit is not explicitly described or illustrated in the figures . On the other hand, for example, if a specific apparatus is described based on functional units, a corresponding method may include a step performing the described functionality, even if such step is not explicitly described or illustrated in the figures . Further, it is understood that the features of the various example aspects described herein may be combined with each other, unless specifically noted otherwise .

[0028] In this text, air is used as an example of a gaseous medium for conciseness of notation. However, references to air must be understood generally as references to any gaseous medium, the particle contents of which are to be examined. Similarly, the radiation used for the detection and classification of particles is referred to concisely as light . This term should be understood to cover not only visible light but also extensions thereof to spectral ranges not visible for the human eye but similarly applicable for optical measurements . Typically, but not mandatorily, laser light is meant because its extremely limited wavelength band, which makes it easier to draw unambiguous conclusions of measured patterns of scattered light .

[0029] Fig. 1 illustrates schematically a device for detecting particles in air . A light source 101, typically a laser, is used to generate a beam of light 102 and direct it to a detection zone 103. Reference designator 104 illustrates the flow of air through the detection zone 103. In a close vicinity of the detection zone 103, one or more light detectors 105 are placed fordetecting light that becomes scattered by a particle that was brought to the detection zone 103 by the flow of air 104. Electronic circuitry 106 is used to drive the light source 101. A support structure or housing 107 can be used for self-evident purposes . Beam forming structures such as pinholes, baffles, lenses, reflectors, and / or the like can be used to ensure an optimal form of the beam of light 102. Examples of these appear as the collimator holes in the intermediate walls 108 and 109 in fig. 1 .

[0030] Fig. 2 is a schematic illustration of some concepts related to the detection zone . A light ray 102 comes from the lower left and is directed toward the upper right in fig. 2. Focusing of the light ray 102 makes it slightly hourglass-shaped, with a smallest width d at a focal point . In fig. 2, a detector 201 is placed adjacent to the detection zone so that the focal point is directly above a centre point of the detector 201, which is assumed to be at length L from the light source . If the length of the detector 201 in the propagation direction of the light beam 102 is b, the detection zone may be said to consist of a b-long section of the hourglass shape of the light beam with its centre at the focal point .

[0031] Light that hits a particle can become absorbed, reflected, refracted, or diffracted. Which of the three last-mentioned (which result in detectable light patterns around the detection zone) is the most important, depends on a number of factors, among which is the size of the particle . For the very smallest particles, in the size range of individual molecules, Rayleigh scattering dominates . Particles that small are not consideredrelevant for the purposes of this text . In the size range from approximately 0.01 to 1.0 micrometres, Mie scattering is the dominant interaction mechanism. This is a very important size range for the purposes of this text . Towards larger particle sizes, geometric and optical (in macroscopic scale) interaction mechanisms become more and more important, and even dominant for particle sizes above 10 micrometres . For the purposes of this text, particle sizes above 1.0 micrometre are important up to a certain maximum limit, like 10 or 20 micrometres .

[0032] Fig. 3 illustrates the cross sections of two simulated scattering patterns of light, which here is assumed to be a laser beam at a visible wavelength and come horizontally from the left to hit a particle at the origin. The numbers at the outer rim mark the scattering angle in degrees, with angle zero coinciding with the propagation direction of the light beam. The numbers at the inner circles mark relative intensity in arbitrary units; notably the radial scale is not the same for the two patterns .

[0033] The pattern on the left in fig. 3 illustrates the scattering from a spherical particle with diameter 4 micrometres, and the pattern on the right illustrates the scattering from a spherical particle with diameter 0.9 micrometres . There are significant differences between the two patterns in both the overall intensity of scattered light and the angular distribution of scattered light . As a first conclusion therefrom, it may be possible to detect and classify, with reference to their size, particles in a gaseous medium by examining the pattern of light scattered by said particles .

[0034] Fig. 4 illustrates schematically two instances of an arrangement in which two light detectors 401 and 402 are placed close to a detection zone . On the left in fig. 4, a larger spherical particle appears in the detection zone . On the right, a smaller spherical particle appears in the detection zone . Light scattered by both particles is shown primarily with arrows in fig.4. Scattering patterns copied from fig . 3 are overlaid with the arrows, although it should be noted that the relative difference in size between the particles in fig. 4 is not as big as in fig. 3 and, consequently, the scattering patterns are indicative only. Additionally, neither the length of the arrows nor the size of the scattering patterns is fully comparable between the left and right sides in fig. 4, as the right half of the drawing has been scaled larger than actual to improve graphical clarity.

[0035] In both instances shown in fig. 4, both detectors 401 and 402 will detect some scattered light . This means that in both instances, at least in principle the appearance of a particle in the detection zone may be detected by monitoring output signals given by at least one of the detectors 401 and 402. In the following, some methods of using the detected light to additionally classify the detected particle are considered.

[0036] The overall amount of light scattered into a particular direction (to be exact : into a solid angle covered by the detector used) can be measured with a single detector, like detector 401 in fig . 1 for example, simply by observing the amplitude of an output signal given by the detector . The larger the amplitude, the larger the particle . Such a classification methodis, however, not very reliable because of several factors . For example, the whole particle may have passed cleanly through the detection zone, or a "partial hit" may have occurred so that only some edge of the particle went through the light beam, with obvious consequences to the overall amount of light that becomes scattered. As the detection zone has a length in the propagation direction of the light beam and (typically) a focal point somewhere therein (see fig. 2 above) , also the point at which the particle occurred in relation to the length of the detection zone has an effect . For larger particles, also reflection and absorption become important, which means that the surface topology and "colour" of the particle affect the overall intensity of scattered light . Further, ageing of the light source and / or contamination in the system may affect the detection of overall amount of scattered light in ways that are difficult to predict or compensate for .

[0037] At 1 east some of the problems mentioned above may be avoided or at least mitigated by using a plurality of detectors, monitoring the amplitude of their output signals, and comparing detection events that occur simultaneously (and, hence, were caused by light scattered by the same particle) . In fig. 4, the two detectors 401 and 402 can be used for such purpose . As explained above, in the case shown on the left in fig. 4 the overall amount of scattered light will be larger, so the sum of amplitudes of the output signals given by detectors 401 and 402 will be larger than in the case shown on the right . Additionally, the relative difference between the output amplitude of detector 401 and that of detector402 will be different, obeying the known laws of how the scattering patterns relate to particle size .

[0038] The two detectors 401 and 402 are located on a line parallel to the propagation direction of the light beam. Fig. 5 shows how the form of the scattering pattern can be detected more accurately by using a large plurality of detectors 501 in a similar line . A drawback of such a solution is, however, the relatively smaller solid angle covered by each individual detector . To make sufficiently many of the individual detectors 501 receive a meaningful amount of scattered light to make deductions about the scattering pattern, a relatively powerful light source must be used. Additionally, or alternatively, the signal processing electronics that are needed to collect and process the output signals of the detectors 501 may become rather complicated.

[0039] Above in the description of figs . 3, 4, and 5 the light-scattering particle was assumed to be spherical and become detected on the optical axis of the light beam. As a result of such assumptions, the scattering pattern can be assumed to exhibit rotational symmetry around the optical axis . This, in turn, means that reasonably reliable conclusions about the form of the scattering pattern can be made using detectors 401, 402, or 501 located on a single line parallel to the optical axis .

[0040] Fig. 6 illustrates an alternative detection principle, according to which there is a two-dimensional array (here : a 6x6 square array) of detectors 601 located behind the detection zone, on a plane perpendicular to the optical axis . In fig. 6, the light beam comes from the lower left as shown with the lone arrow602 and hits a particle 603 in the detection zone . Compared to figs . 3, 4, and 5, an arrangement like that in fig. 6 can be used to detect also scattering patterns that are not rotationally symmetrical .

[0041] Fig. 7 shows examples of scattering patterns that have been detected with an arrangement like that in fig . 6. Detected particles that generated the scattering patterns 701 to 710 were : ellipsoidal particle 701, rough mineral dust particle 702, fibre particle 703, cuboidal salt particle 704, water droplet 705, fungal spore particle 706, droplet containing a solid inclusion 707, large gypsum particle 708, curved fibre 709, and nearly spherical pollen particle 710. Fig. 7 and the explanatory list of particles have been presented in a patent document EP 3 304 039 B .

[0042] A basic drawback of an arrangement like that in fig. 6 is, however, the same as that of fig. 5. As can be seen from the relatively fine resolution of the scattering patterns in fig. 7, the array of detectors 601 must actually be a CCD imaging screen consisting of a large plurality of pixels . A powerful light source as well as complicated imaging electronics must be used.

[0043] Fig. 8 illustrates an alternative approach to placing the light detectors of a particle detector apparatus . In fig. 8, light comes from the lower left according to arrow 602 and hits a particle 603 in the detection zone . In place of the two-dimensional array of detectors 601 in fig. 6, only a conceptual, imaginary evaluation plane 801 is shown. While in fig. 6 the plurality of small light detectors (the pixels of the CCD imaging screen) were located on a plane perpendicular to the optical axis, in fig. 8 two large light detectors802 and 803 are located with their flat, light-sensitive detection areas parallel to the optical axis . In the direction of the optical axis, both light detectors 802 and 803 are equally far from the detection zone where the detected particle 603 is shown to appear . The light detectors 802 and 803 have their flat, light-sensitive detection areas pointing at perpendicular directions .

[0044] While the particle 603 is schematically represented with just a white sphere in fig. 8, it is here assumed to have a significantly elongate form, like a fibre . Comparing to the sample image 703 in fig. 7, this means that its scattering pattern comprises a flat, fanshaped main lobe 604. If the main lobe 604 is oriented horizontally like in fig. 8, the first light detector 802 will receive very little scattered light propagating thereto from the detection zone . On the other hand, the second light detector 803 will receive a significant amount of light . If a suitable processing unit is coupled to the two light detectors 802 and 803 and configured to generate detection and classification results indicative of detected particles, it would receive a weak detection signal from the first light detector 802 and, simultaneously, a strong detection signal from the second light detector 803. Based on these, it could generate a detection and classification result indicating that an elongate particle has been detected. A similar detection and classification result could have been generated if, due to a different orientation of the elongate particle 603 when detected, the main lobe of its scattering pattern would have been oriented vertically. In such a case, the processing unit would receive a strong detection signal from the first light detector802 and, simultaneously, a weak detection signal from the second light detector 803.

[0045] Fig. 9 illustrates another alternative approach to placing the light detectors of a particle detector apparatus . Reference designators 602, 603, 604, and 801 are the same as in fig. 8 . Two light detectors 901 and 902 are configured to detect scattered light propagating from the detection zone into directions other than the direction of the light beam. In fig. 9, the two light detectors 901 and 902 are located on a common plane parallel to the optical axis of the light beam. They are, however, not located on the perpendicular proj ection 903 of the optical axis on said common plane like the detectors 401, 402, and 501 in figs . 4 and 5 above . Looking into the direction of the light beam in fig. 9, one light detector 901 is to the left and the other light detector 902 to the right of the perpendicular proj ection 903 of the optical axis on said common plane .

[0046] If the main lobe 604 of the scattering pattern of the (assumedly elongate) particle 603 is oriented horizontally as in fig. 9, neither of the two light detectors 901 and 902 detects a very large amount of scattered light . However, if - due to a different orientation of the elongate particle 603 when detected -the main lobe of the scattering pattern would occur rotated by either about +45 degrees or about -45 degrees from the orientation shown in fig. 9, one of them would again produce a strong detection signal and the other, simultaneously, a weak detection signal .

[0047] Fig. 10 is an axonometric illustration of some parts of a particle detector apparatus for detecting andclassifying particles in gaseous media . A light source 1001 is configured to emit a light beam 1001 in the direction of an optical axis through a detection zone 1003. A channel 1004 is configured to direct a flow 1005 of gaseous medium through the detection zone 1003. To better illustrate the detection zone 1003 and the location of light detectors, a middle section of the channel 1004 is not shown in fig. 10. A line of white arrows illustrates the flow 1005 of gaseous medium through the section of the channel that is not shown.

[0048] A plurality of light detectors are configured to detect scattered light propagating from the detection zone 1003 into directions other than the direction of the light beam 1002. In particular, the plurality of light detectors comprise three light detectors 901, 902, and 1006 in fig. 10. These are at least two-dimension-ally distributed. In fig. 10, they are two-dimensionally distributed on a common plane . The optical axis of the light beam 1002 is parallel to said common plane but lies at a distance from it . The optical axis is above said common plane if the orientation shown in fig. 10 is considered.

[0049] Each light detector is here assumed to have an essentially planar light-sensitive detection area . When such a light detector is located on a plane, like the common plane mentioned above, this means that in most cases the essentially planar light-sensitive detection area of that light detector is parallel to the plane . Assuming, as an example, that the common plane is defined by a planar surface of a circuit board, a light detector on the common plane may be a slab-like or chiplike component soldered onto electric connection patcheson the circuit board with its light-sensitive detection area facing out of the common plane . It is possible, but not mandatory, to additionally equip the light-sensitive detection area of the light detector with an optical element such as a lens for improving the directing of light propagating from the detection zone to the lightsensitive detection area .

[0050] In fig. 10, a perpendicular proj ection 1007 of the optical axis of the light beam 1002 is shown. Geometrically taken, the perpendicular proj ection 1007 is the intersecting line of said common plane and another, imaginary plane that is perpendicular to said common plane and contains the optical axis of the light beam 1002. Fig . 11 is a side view of the light source 1001, light beam 1002, detection zone 1003, light detectors 1006 and 902, and perpendicular proj ection 1007, the side view being taken against said other, imaginary plane . In such a side view, the light detector shown with reference designator 901 in figs . 9 and 10 is behind the light detector shown with reference designator 902.

[0051] As most clearly seen in fig. 11, the first light detector 1006 may be closer to the detection zone 1003 than the two other light detectors 901 and 902. While in fig. 11 the first light detector 1006 is not directly below the detection zone 1003 but somewhat to the lower left, it could also be directly below the detection zone 1003. Being closer to the detection zone 1003 than the two other light detectors 901 and 902 means that the solid angle 1101 that the first light detector 1006 covers as seen from the centre point of the detection zone 1003 is larger than the solid angle1102 covered by any of the two other light detectors 901 and 902 .

[0052] Further, as seen in fig. 10, the first light detector 1006 is located on said common plane centred on the perpendicular proj ection 1007 of the optical axis . The two other light detectors 901 and 902 are located on said common plane on opposite sides of the perpendicular proj ection. The arrangement shown in fig.10 may be described to that the first, second, and third light detectors 1006, 901 , and 902 are located on said common plane at corners of an isosceles triangle located symmetrically with respect to the perpendicular projection 1007 of said optical axis on said common plane .

[0053] This is not the only possible alternative of placing at least three light detectors on the common plane . For example, all three could be equally far from the detection zone 1003. The first light detector 1006 could be displaced sideways from the perpendicular proj ection 1007 of the light beam on the common plane . The two other detectors 901 and 902 could be located at different distances from the perpendicular proj ection 1007. It would even be possible to have two light detectors about at the level of the detection zone 1003 (as seen along the direction of the light beam 1002 ) and a single one further away in the direction of the light beam 1002 .

[0054] Shown with dashed lines in figs . 10 and 11 is a possible fourth light detector 1008, which could be placed and used in accordance with the principle shown above in fig. 8. A particle detector apparatus of the kind described here could have e . g. three light detectors 902, 1006, and 1008, so that at least two of themhere for example light detectors 902 and 1006 are on the common plane . Alternatively, a particle detector apparatus of the kind described here could have four light detectors 901, 902, 1006, and 1008, of which three - here light detectors 901, 902, and 1006 - are on the common plane .

[0055] Furthest down along the direction of the light beam, a light trap 1009 is seen in fig. 10. The purpose of a light trap, if used is to ensure that any light that passed straight through the detection zone 1003 will permanently disappear from all evaluation, i . e . become "trapped" and not produce any reflections, not even diffuse ones, back towards the detection zone 1003. The use of light traps for such purposes is a technique known in the art and does not need to be described here in more detail .

[0056] While not the only possibility, the two-dimensional distribution shown in fig. 10 of the light detectors 901, 902, and 1006 on the common plane has certain advantages . First, the relative magnitudes of the solid angles 1101 and 1102 is at least partly compensated by the provision of the larger light-sensitive detection area that comes from the sum of the lightsensitive detection areas of the two detectors 901 and 902 (at least if all three detectors are of the same size) . The symmetry with respect to the perpendicular proj ection 1007 of the optical axis on said common plane may simplify the task of analysing the detection signals : if a detected particle gives rise to a symmetrical scattering pattern, this can be noted from corresponding similarity of the detection signals of the two detectors 901 and 902 on opposite sides of the perpendicularproj ection 1007. Also if the scattering pattern is not symmetrical, its asymmetrical form may be easier to analyse and recognize if the detection signals nevertheless represent the amount of detected scattered light at symmetrically located positions . The symmetrical location of two detectors with respect to the perpendicular proj ection 1007 may come from physical symmetry, meaning that the two detectors are physically there . It may also refer to optical symmetry, so that optical elements such as one or more lenses, mirrors, refractors, or the like are used to ensure that the detected light represented those portions of the scattering pattern that propagated from the detection zone towards such symmetric locations .

[0057] Compared to e . g. figs . 5 and 6, the light detectors shown in fig. 10 are remarkably large . Preferably, each light detector 901, 902, 1006, or 1008 has a uniform light-sensitive detection area of more than two square millimetres, or even more than five square millimetres . At the effective date of this text, an example of suitable light detectors comes in the size of approximately 2 times 2.5-4 millimetres . A large, uniform light-sensitive area at each detector means that even a relatively weak light source 1001, such as a laser with its optical power between 1 and 5 milliwatts or even just 1 and 3 milliwatts, may be sufficient to generate scattering patterns of sufficient intensity. On the other hand, a small number of detectors (here three or four; advantageously not more than six, or at least not more than ten) means that the complicatedness of required processing electronics can be kept reasonable .

[0058] In fig. 10, the channel 1004 is configured to direct the flow 1005 of gaseous medium through the detection zone 1003 in a direction perpendicular to the direction of the light beam 1002. While not the only possible choice, this may again simplify the task of deducting, from a set of detection signals, what would have been the original scattering pattern and what, if anything, it would tell about the size and / or shape of the detected particle .

[0059] Referring back to figs . 8 and 9, it is known that an elongate particle gives rise to a scattering pattern that has a characteristically flat main lobe . As a notable difference to the schematic representation in figs . 8 and 9, that main lobe may be wide enough so that at least a part of it would hit even the first light detector 1006 in fig. 10. An elongate, fibrous particle typically does not float steadily along in a flow of gaseous medium but rotates . Its rotation may be fast enough so that it may cover a significant rotation angle during the time interval the particle spends in the light beam, i . e . within the detection zone .

[0060] Now a plurality of two-dimensionally distributed light detectors on a common plane, like the four light detectors 901 , 902, 1006, and 1008 of fig . 10 or any set of three of them, may be considered. An elongate particle brought along with the flow 1005 of gaseous medium and rotating may give rise to a characteristically flat scattering pattern, the intensity maximum of which hits each of the three or four light detectors 901, 902, 1006, and / or 1008 in turn. If one evaluated the detection signals generated by the light detectors within a common time interval that corresponds to asingle particle passing through the detection zone, one may find that the detection signals are similar in form but differ from each other with respect to their timing within said time interval . As a consequence, a classification result may be generated indicating that an elongate particle was detected.

[0061] Fig. 12 illustrates a functional block diagram of a particle detector apparatus . It comprises a light source 1001, like a laser for example, and possibly some driver circuitry 1201 for the light source . As explained above, the light source 1001 is configured to emit a light beam in a direction of an optical axis through a detection zone . The channel that is configured to direct a flow of gaseous medium through the detection zone is a structural rather than functional feature and is therefore not separately illustrated in fig. 12.

[0062] The particle detector apparatus comprises a plurality of light detectors, of which light detector 1006 is singled out in fig. 12. The light detectors are configured to detect scattered light propagating from said detection zone into directions other than the direction of the light beam. As also explained above already, the light detectors comprise at least three light detectors, of which at least two are on a common plane, wherein the optical axis of the light beam is parallel to said common plane .

[0063] Possibly, some interfacing circuitry 1202 is provided for the plurality of light detectors . As an example, there may be an analogue, low-noise front-end amplifier and a dedicated channel of an analogue to digital converter for each individual light detector . The interfacing circuitry 1202 may comprise bufferingstages for temporarily holding (i . e . buffering) a piece of the detection signal, and there may be a synchronization unit capable of reading out such buffered pieces of detection signals mutually synchronized so that it is possible to recognize detection signals that are similar in form but differ from each other with respect to their timing within the time interval it took for a particle to pass through the detection zone .

[0064] The particle detector apparatus comprises a processing unit 1203 coupled to the plurality of light detectors . The processing unit 1203 is configured to generate detection and classification results indicative of detected particles on basis of detection signals received from the plurality of light detectors . The processing unit 1203 may also be configured to control the operation of the light source 1001, possibly through the driver circuitry 1201 therebetween.

[0065] While the interfacing circuitry 1202 and the processing unit 1203 are shown separately in fig. 12, it should be noted that at least some (or even all) interfacing circuitry may be provided as an integral part of the processing unit . As an example, if the processing unit 1203 is a microcontroller, it may comprise a sufficient number of built-in analogue to digital converters for digitizing the received (amplified) detection signals for further processing and evaluation.

[0066] Basically, the flow of gaseous medium through the detection zone could be maintained naturally, for example by gravity, relying on the tendency of warmer gaseous medium to become lighter and flow upwards in a suitable directed channel . However, significant advantages may be achieved if the particle detectorapparatus comprises means for forcibly maintaining the flow of gaseous medium. Two basic approaches for providing such means involve the use of a pump or a fan.

[0067] A(n electrically driven) pump is an electromechanical apparatus that produces a known displacement, i . e . moves a known amount of gaseous medium per unit time . A (n electrically driven) fan is a simpler device and may comprise just a bladed wheel with a rotating motor . For its simplicity, a fan 1204 may be preferred as shown in fig. 12.

[0068] As a difference to a pump, a fan does not produce any known displacement . Hence, even if one would know the rotation rate of a fan, it is not straightforward to deduce therefrom the flow rate of gaseous medium in a channel where the fan is used to maintain the flow. This does not matter, if one it only to make some kind of qualitative detection and classification of particles in the gaseous medium.

[0069] If also quantitative results would be desirable, it becomes more important for the processing unit 1203 to also know the flow rate of gaseous medium. For this purpose, one or more sensors 1206 may be used. According to an advantageous embodiment, the particle detector apparatus comprises one or more pressure sensors 1206 coupled (possibly through some interfacing circuitry 1207 ) to the processing unit 1203 for providing said processing unit 1203 with indications of a pressure difference between inside and outside of the channel . If, additionally, rate information is provided indicative of a rotation rate of the fan 1204 that maintains the flow of gaseous medium, it may be possible to calculate a flow rate of the flow of gaseous mediumbased on the measured pressure difference and the rate information. Rate information of such kind may be readily available if the fan 1204 is controllable

[0070] For such a measurement to produce reliable results, the channel should be tight against leaks so that only gaseous medium that enters the channel through its intended inlet may eventually pass through and exit the channel through its intended outlet . One advantageous way to ensure sufficient tightness against leaks is to use, as one wall of the channel, the circuit board on which e . g. the light detectors are mounted. Other walls of such a channel may be provided as hollow sections of a cover piece that comes against the circuit board. For additional tightness, an elastic seal like a band of rubber or silicon can be used between the circuit board and the cover piece . One pressure sensor may then be placed on the circuit board so that it comes inside the channel or some auxiliary space that is linked to the channel and forms part of the tightly sealed enclosure, and another pressure sensor somewhere where it has fluid connections only to the surrounding environment and not to the inside of the channel .

[0071] As shown in fig. 12, the particle detector apparatus may comprise a user interface 1208 for one- or bidirectional exchange of information with a user . At its simplest, the user interface 1208 may consist of a single indicator light that tells something about the operating status of the apparatus . The user interface 1208 may also comprise e . g. a display, one or more keys or knobs, a touch-sensitive surface, an audio interface (microphone and / or sound-emitting unit (s) ) or the like .

[0072] In the embodiment of fig. 12, the particle detector apparatus comprises a communications unit 1209 coupled to the processing unit 1203. In such a case, the processing unit 1203 may be configured to communicate information indicative of the detection and classification results through said communications unit to at least one external device . Such an arrangement is particularly advantageous for example if one wants to keep the user interface 1208 of the apparatus itself as simple and robust as possible and use e . g. the smartphone of the user as a user interface . Additionally, or alternatively, a communications unit 1209 may exchange information with remote control stations, such as a monitoring station responsible for tracking the safety of conditions under which the person (s) closest to the particle detector apparatus work (s) .

[0073] Following the structural and functional principles explained above, it may be possible to make the particle detector apparatus a personal portable device to be worn e . g. as a mandatory safety feature in the uniform of workers who may become exposed to hazardous amounts of particles in breathing air . For this purpose, it is useful if the particle detector apparatus comprises a self-contained power source 1210, such as a rechargeable or disposable battery for example, for operation .

[0074] A particle detector apparatus of a kind described above can be made to execute a variety of methods . According to an embodiment, such a method comprises passing a light beam in a direction of an optical axis through a detection zone, directing a flow of gaseous medium through said detection zone, and detectingscattered light that propagates from said detection zone into a plurality of directions other than the direction of the light beam. Based on said detecting, the method may comprise generating detection and classification results indicative of detected particles . The detecting should be done at locations of which there are at least three, two-dimensionally distributed on a common plane, said optical axis being parallel to said common plane .

[0075] For reliable detection and classification, only scattering events that take place at the detection zone 1003 and that produce a detection signal from at least two, preferably all, light detectors should be considered. Hence, a method may comprise a pre-filtering step in which such detection signals are excluded from further processing that only came from a subset of the light detectors within a time interval that would correspond to a particle passing through the detection zone . The method may comprise deciding the length of such an assumed time interval on basis of acquired knowledge about the flow rate of gaseous medium through the detection zone .

[0076] As already indicated earlier in this text, the method may comprise evaluating detection signals generated at the detection locations within a common time interval that corresponds to a single particle passing through the detection zone . The method may comprise, in response to finding that said detection signals are similar in form but differ from each other with respect to their timing within said time interval, generating a classification result indicative of an elongate particle having been detected. For such evaluation to be possible, the method may comprise buffering detection signalsthat originate from different light detectors and reading such buffered detection signals synchronously with respect to their relation to said time interval .

[0077] For evaluating detection signals from a relatively small number of light detectors, like three for example, it may be possible to use a time-multiplexing analogue to digital converter that repeatedly scans through the small number of channels in rapid succession, like at a rate of a million measurements or more per second. As a possible drawback, increasing the scanning rate may lead to decreasing resolution and, hence, worse accuracy of detection, and / or to a manufacturing cost that may be unacceptably high for a particle detector apparatus that is meant to be a part of personal safety equipment of wear-on type .

[0078] It is also possible to use dedicated, parallel analogue to digital converters for the individual channels and to make them operate in exact synchronism in time . A possible drawback of such a solution is again related to manufacturing costs . Microcontrollers of the kind that could typically be used as processing units in personal safety equipment of wear-on type are likely to include built-in analogue to digital converters anyway, so adding external circuitry of similar purpose could be considered as an unnecessary additional cost .

[0079] As another example, it is possible to use sam-ple-and-hold circuits and / or suitably timed peak detector circuits for the individual channels . Conceptually, such circuits may be characterized as relatively simple analogue circuits that generate kind of artificial delay to the detection signal .

[0080] Fig. 13 illustrates a simplified amplifier circuit that can be used as a peak detector circuit . The input signal coupled to the circuit from the left is a potential vinof an output node of a light detector . It is coupled to the non-inverting input of an operational amplifier 1301, the output of which is coupled to a floating electrode of a capacitor 1303 through a forward biased diode 1302. From said floating electrode of the capacitor 1303 is a feedback coupling to the inverting input of the operational amplifier 1301. The other electrode of the capacitor 1303 is grounded. A reset switch 1304, such as a MOSFET or some other semiconductor switch, is coupled across the capacitor 1303. The output signal voutis the voltage across the capacitor 1303.

[0081] Fig . 14 shows how, as long as the switch 1304 remains non-conductive, the output signal voutfollows the highest value so far of the input signal vin. The value of the output signal voutcan be reset to zero by momentarily making the switch 1304 conductive .

[0082] Advantageously, two ADC channels can be coupled to the output of each light detector, at least as long as there are so few light detectors that their number does not exceed one half of the number of available parallel ADC-equipped inputs of the microcontroller used. One of the light-detector-specific ADC channels could sample and buffer the vinsignal, i . e . the (possibly pre-amplified) direct output signal of the light detector . The other of the light-detector-specific ADC channels could sample and hold the voutsignal, i . e . the highest value detected so far during the ongoing time interval . The microcontroller or other processing unit may then use the sampled and held peak values (the voutvalues) to generate detection and classification results indicative of a size and possibly shape of the detected particle . By comparing the sampled and buffered vinvalues to each other, the processing unit may derive additional information about the shape of the detected particle, for example by noting that the vinsignals are similar in form but differ from each other with respect to their timing within the ongoing time interval, which would then indicate that the detected particle had an elongate form. After a predetermined maximum time period like some tens of microseconds for example, or when the processing algorithm notes that the received detection signals have fallen below a threshold (because the particle has left the detection zone and does not scatter any light anymore) , the processing unit may reset the peak detectors of all vout_coupled channels by making their respective reset switches briefly conductive .

[0083] In practice, it may prove difficult to make fully analytical and accurate conclusions about the size and shape of a detected particle simply by comparing the samples of its scattering pattern that were obtained through the detection signals . However, the principles described above may be utilized so that the processing unit is trained to detect and classify particles of certain kind by using specifically selected, essentially homogenous particle streams . After a large number of particles of some exactly known kind have passed through the detection zone, the processing unit has gathered a library of detection and classification results that it can then be programmed to associate with particles of this kind also later .

[0084] If the method involves measuring a pressure difference between the air flow and the environment and providing rate information indicative of a rotation rate of a fan that maintains the air flow, it may further involve calculating, based on the measured pressure difference and the rate information, a flow rate of the air flow. When the processing unit then generated detection and classification results, it may announce these results in a form that directly tells the measured number of particles in a volume of air .

[0085] An advantage of possibly having a controllable fan or air pump as means for maintaining the air flow is the possibility of changing the flow rate of the air flow according to need. For example, if the processing unit notes that too many detection events take place very close to each other, so that the detection and classification algorithms run a risk of saturating, the processing unit may slow down the controllable fan or air pump . Or the other way round, if it seems that only very few particles become detected in a given time frame, the flow rate of the air flow can be increased.

[0086] Fig. 15 illustrates some variations of placing and using light detectors, however so that to the extent meant here they are still on a common plane . In fig. 15, light detector 1006 is placed on a pedestal 1501 that lifts it up from the surface of the substrate (like circuit board) that otherwise supports the components . Light detector 901 is shaped like a wedge, so that its light-sensitive detection area is not parallel to the surface of the substrate . Light detector 902 is equipped with a mirror 1502 that increases the solid angle into which scattered light must propagate from the detectionzone 1003 to be detected by the light detector 902. In all cases, a common factor is the basic principle of placing a relatively small number of light detectors, each with a relatively large light-sensitive detection area, in a distribution that enables drawing advanced conclusions about the scattering pattern when light from the light source 1001 hits a particle within the detection zone 1003.

[0087] In the description above, a particle has been characterized as "elongate" if its scattering pattern exhibits the significant divergence from rotationally symmetric form that, among others, enables its classification based on detection signals that are similar in form but differ from each other with respect to their timing within the time interval . Such a particle could alternatively be characterized as non-spherical or highly irregular, as a difference to more regularly shaped particles the outer dimensions of which approach a spherical form and the scattering patterns of which are characteristically rotationally symmetric .

[0088] In addition to elongate or non-spherical particles, the principles explained above lend themselves to the detection and classification of crystalline or chiselled particles . Characteristic of such particles is their tendency to generate relatively concentrated reflections and / or refractions of light, which typically differ significantly from any rotationally symmetric scattering patterns of ordinary, spherical or nearly spherical particles . Thus, what has been said above with reference to the detection and classification of elongate particles can be generalized to cover all sorts of particles that due to their nature and appearance giverise to scattering patterns that differ significantly from rotational symmetry.EXAMPLES

[0089] For evaluating the feasibility of the technologies explained above, a prototype of a particle detector apparatus was constructed with a general structure corresponding to that shown above in fig. 10. The prototype comprised three light detectors placed on a common planar substrate (circuit board) that also supported a holder for a laser source so that the laser beam emitted by the laser source was parallel to the surface of the substrate . The three light detectors were located on said substate at corners of an isosceles triangle located symmetrically with respect to a perpendicular proj ection of the laser beam on said substrate . One of the light detectors was located on said substrate directly below the detection zone, centred on the perpendicular proj ection of the laser beam on said substrate . The two other light detectors were located further away from the detection zone in the direction of the laser beam, symmetrically displaced on opposite sides of the perpendicular proj ection of the laser beam on said substrate .

[0090] In the first example, an aerosol sample consisting of synthetically produced fatty acid particles suspended in air was made to pass through the detection zone . In the second example, an air flow containing rock dust was made to pass through the detection zone . The fatty acid particles can be assumed to have been practically spherical, while the rock dust particles (which consisted essentially of quartz) were irregularly shapeddue to their crystalline nature, with characteristically j agged edges, reflective surfaces, and internal refractive characteristics .

[0091] Graphs 1601 and 1602 in fig. 16 represent a short portion of detection signals received in the first example from those two light detectors that were symmetrically displaced on opposite sides of the perpendicular proj ection of the laser beam on the substrate . In fig . 17, graphs 1701 and 1702 represent the same in the second example . In both cases, the horizontal axis represents time, and the vertical axis represents signal strength, both in arbitrary units . The arbitrary time units equal the sampling periods used in the measurement .

[0092] From graphs 1601 and 1602 it can be deduced that three fatty acid particles have passed through the detection zone during the time between 1 and 80 time units : one between about 18 and 26 time units, another between about 61 and 64 time units, and yet another between about 70 and 76 time units . In fig. 17, a single rock dust particle has passed through the detection zone during the time between about 11 and 19 time units .

[0093] A notable feature of graphs 1601 and 1602 is their close resemblance to each other . This appears to be a consequence of the spherical form of the fatty acid particles . Above the peak between about 18 and 26 time units, the schematic illustration shows an imagined perspective view along the direction of the laser beam, with an imaginary screen perpendicular to the laser beam to illustrate an example of a (simplified) rotationally symmetrical scattering pattern typical of a spherical particle (compare to 705 in fig. 7 ) . The two blacktrapezoids show how the two detectors 901 and 902 ( see figs . 9 and 10) would relate to the scattering pattern. As both of them receive an essentially equal amount of scattered light, they also produce an essentially equal detection signal .

[0094] Above the single peak in fig. 17, a similar schematic illustration is shown. As the rock dust particle does not give rise to a rotationally symmetric scattering pattern (compare to 706 or 707 in fig. 7 ) , the two detectors 901 and 902 receive unequal amounts of scattered light . Hence, from the simultaneousness in time but difference in height of the peaks in graphs 1701 and 1702, it may be deduced that the detected particle was non-spherical .

[0095] In fig. 16, the different relative heights (or different time integrals) of the three peaks can be used to deduce that the first detected particle was the largest and the second detected particle the smallest of the three . The detection signal from the light detector located directly below the detection zone in the prototype apparatus is not shown here . It would most likely give a significant amount of additional information concerning the size of the detected particles .

[0096] In fig. 17, the fact that the baseline ( zero level) of the graphs 1701 and 1702 is slightly below zero can be explained with a slightly inaccurate calibration of the mapping of detection signals to numerical values . Taken the difference in the signal strength scale (up to 1800 in fig. 16, up to 300 in fig. 17 ) , it is likely that the same would be seen in fig. 16 if the vertical scale was enlarged close to the zero line .

[0097] In both figs . 16 and 17, the time scale is relatively rough, with less than 10 samples (i . e . analogue to digital conversions ) taken during the time it took for a particle to pass through the detection zone . This explains the broken-line appearance of the graphs, and possibly also contributes to the small differences between graphs 1601 and 1602 in the second and third peaks . A higher sampling rate would probably give smoother results and, possibly, also even closer resemblance between the graphs in the case of a spherical particle being detected. Also, a higher sampling rate would probably be needed if one was to draw conclusions about an elongate, rotating particle being detected because a higher sampling rate would make it easier to show similarity in form but difference in timing during the time interval that the particle spends in the detection zone .

[0098] Fig. 18 does not represent an example measurement but a comparable thought experiment of how the detection of a particle and its classification as an elongate particle would compare to the example measurements of figs . 16 and 17. For this reason, no time or signal strength scales are given in fig. 18. Due to the elongate form of the particle, its scattering pattern is assumed to comprise a dominant, elongate, characteristically bright lobe (compare to 703 in fig. 7 ) that in fig. 18 is rotated with reference to the optical axis so that a significant part thereof hits detector 902, while detector 901 receives almost no scattered light at all . Graph 1801 shows how almost no increase at all can be seen in the detection signal from detector 901,while - as shown by graph 1802 - the detection signal from detector 902 exhibits a remarkable peak.

[0099] As shown by fig. 18, classification of a detected particle as an elongate particle may be based on a mere significant asymmetry between detection signals from two symmetrically (with reference to the proj ection of the optical axis) located detectors, without having to be additionally based on any similarity in form but difference in timing. It may be, namely, that the elongate particle was not rotating while it passed through the detection zone, or it may have rotated at a rate that did not make its scattering pattern hit both symmetrically located detectors . As explained above, in figs . 16 to 18 the detection signal from the detector directly below (or otherwise significantly closer to) the detection zone is not shown. It may be assumed, though, that also in the case of fig. 18 there would be a significant amount of scattered light hitting the closest detector, so that the simultaneousness of the detection signals from at least two detectors would suffice for the event to be considered a true detection event and not just some random noise peak from one detector .

[0100] Referring to figs . 16 to 18, while the relatively large size of the light-sensitive detection area in each detector is an advantage (in the sense of signal to noise ratio, for example) , there may be a limit above which an even larger area would not be advantageous anymore . Namely, if the symmetrically (with reference to the proj ection of the optical axis) located detectors cover too large a portion of the solid angle into which scattered light from the detection zone may propagate,their ability to generate detection signals of significantly different strength in response to a scattering pattern that is not rotationally symmetrical may suffer . Of an other-than-rotationally-symmetric scattering pattern, too large a portion could hit an excessively large detector, causing an undesired averaging effect . As a rough rule of thumb, the scattering pattern on an imaginary screen of the kind shown in figs . 16 to 18 can be considered. The imaginary screen meant here is an imaginary square perpendicular to the optical axis of the light beam, with its two lower corners at the outer front corners of the symmetrically located detectors 901 and 902 . No one of the symmetrically located detectors should cover more than about 10 % of the solid angle corresponding to the imaginary screen. On the other hand, for the purpose of achieving the advantageous signal to noise ration as explained above, each of the symmetrically located detectors should cover more than about 2 %, and preferably more than 5 % of the solid angle corresponding to the imaginary screen.

Claims

CLAIMS1 . A particle detector apparatus for detecting and classi fying particles in gaseous media, comprising :- a light source ( 1001 ) configured to emit a light beam ( 1002 ) in a direction of an optical axis through a detection zone ( 1003 ) ,- a channel ( 1004 ) configured to direct a flow ( 1005 ) of gaseous medium through said detection zone ( 1003 ) , - a plurality of light detectors ( 901 , 902 , 1006 , 1008 ) configured to detect scattered light propagating from said detection zone ( 1003 ) into directions other than the direction of the light beam ( 1002 ) , and- a processing unit ( 1203 ) coupled ( 1202 ) to said plurality of light detectors ( 901 , 902 , 1006 , 1008 ) and configured to generate detection and classi fication results indicative of detected particles on basis of detection signals received from said plurality of light detectors ( 901 , 902 , 1006 , 1008 ) ;characterized in that :- said plurality of light detectors ( 901 , 902 , 1006 , 1008 ) comprise at least three light detectors ( 901 , 902 , 1006 , 1008 ) in an at least two-dimensional spatial distribution- of said at least three light detectors ( 901 , 902 , 1006 , 1008 ) , at least two ( 901 , 902 , 1006 ) are on a common plane , said optical axis being parallel to said common plane .2 . A particle detector apparatus according to claim 1 , wherein said at least two light detectors are a first ( 1006 ) and a second ( 901 ) one of said at least three light detectors ( 901 , 902 , 1006 , 1008 ) , and additionally a third ( 902 ) one of said at least three light detectors is on said common plane .3 . A particle detector apparatus according to claim 2 , wherein said first one ( 1006 ) of said plurality of light detectors is closer to said detection zone ( 1003 ) than said second ( 901 ) and third ( 902 ) ones of said plurality of light detectors .4 . A particle detector apparatus according to claim 3 , wherein said first one ( 1006 ) of said plurality of light detectors is located on said common plane centred on a perpendicular proj ection ( 1007 ) of said optical axis on said common plane .5 . A particle detector apparatus according to any of claims 2 to 4 , wherein said second ( 901 ) and third ( 902 ) ones of said plurality of light detectors are located on said common plane on opposite sides of said perpendicular proj ection ( 1007 ) of said optical axis on said common plane .6 . A particle detector apparatus according to any of claims 2 to 5, wherein the first ( 1006 ) , second ( 901 ) , and third ones ( 902 ) of said plurality of light detectors are located on said common plane at corners of an isosceles triangle located symmetrically with respect to the perpendicular proj ection ( 1007 ) of said optical axis on said common plane .7 . A particle detector apparatus according to any of the preceding claims , wherein each of said plurality of light detectors ( 901 , 902 , 1006 , 1008 ) has a uni form light-sensitive detection area of more than two square millimetres , preferably more than five square millimetres , and more preferably about eight square millimetres .8 . A particle detector apparatus according to any of the preceding claims , wherein said channel ( 1004 ) is configured to direct said flow ( 1005 ) of gaseous medium through said detection zone ( 1003 ) in adirection perpendicular to the direction of said light beam ( 1002 ) .

9. A particle detector apparatus according to any of the preceding claims, comprising a fan ( 1204 ) configured to maintain said flow ( 1005) of gaseous medium through said channel ( 1004 ) .

10. A particle detector apparatus according to claim 9, wherein:- said fan ( 1204 ) is controllable by said processing unit ( 1203) ,- the particle detector apparatus comprises one or more pressure sensors ( 1206) coupled ( 1207 ) to said processing unit ( 1203) for providing said processing unit ( 1203) with indications of a pressure difference between inside and outside of said channel ( 1004 ) .

11. A particle detector apparatus according to any of the preceding claims, comprising a self-contained power source ( 1210) for operation.

12. A particle detector apparatus according to claim 10, wherein the particle detector apparatus is a personal portable device .

13. A particle detector apparatus according to any of the preceding claims, wherein:- the particle detector apparatus comprises a communications unit ( 1209) coupled to said processing unit ( 1203) , and- the processing unit ( 1203) is configured to communicate information indicative of said detection and classification results through said communications unit to at least one external device .

14. A method for detecting and classifying particles in gaseous media, the method comprising: - passing a light beam ( 1002 ) in a direction of anoptical axis through a detection zone ( 1003) ,- directing a flow ( 1005) of gaseous medium through said detection zone ( 1003) ,- detecting scattered light that propagates from said detection zone ( 1003) into a plurality of directions other than the direction of the light beam,- based on said detecting, generating detection and classification results indicative of detected particles;characterized in that said detecting is done at locations of which there are at least three in an at least two-dimensional spatial distribution, of which at least two are on a common plane, said optical axis being parallel to said common plane .

15. A method according to claim 14, comprising :- evaluating detection signals generated at said at three locations within a common time interval that corresponds to a single particle passing through said detection zone ( 1003) , and- in response to finding that said detection signals are similar in form but differ from each other with respect to their timing within said time interval, generating a classification result indicative of an elongate particle having been detected.

16. A method according to claim 15, wherein the length of said time interval is more than 10 microseconds but less than 100 microseconds .

17. A method according to any of claims 14 to 16, comprising:- measuring a pressure difference between said flow of gaseous medium and an environment,- providing rate information indicative of a rotation rate of a fan that maintains said flow of gaseous medium, and- based on said measured pressure difference and said rate information, calculating a flow rate of the flow of gaseous medium.

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