Patent Grant
9448167
The present application claims priority to European Patent Application No. 12 167 800.7, filed May 12, 2012, the disclosure of which is hereby incorporated by reference herein in its entirety.
The present invention pertains to a measurement device for the detection of airborne particles, the device comprising a nozzle designed to produce a laminar flow of air with a flow path beyond the nozzle, a first laser and a second laser, the first laser and the second laser being positioned for emitting laser light with intersecting laser beams with a laser beam intersection region in a region of the air flow path, and a first photodetector and a second photodetector positioned for the detection of laser light scattered in the laser beam intersection region by airborne particles contained in the laminar air flow.
In general, the present invention is situated in the context of precaution and/or protection means for detecting hazardous airborne particles. The growing concentration of different types of aerosols in the atmosphere becomes more and more an important issue. Their specific influence on the climate change and air quality is still an open question. More than that, recently new threats like bio-terrorism that employs biological particles as an arm of massive destruction have appeared. Reliable and cost-effective aerosol detectors, fast and with a high discrimination power, are still highly desirable in the market.
Particular species of aerosol particles, like pollens and spores, have a large impact on human health. Some of them are responsible for health problems like allergies affecting, according to statistical analysis, about 20% of the European population. The size of potentially hazardous airborne particles spreads over a range situated approximately between parts of a micrometer and some hundred micrometers.
A variety of different measurement devices and methods for a detection of airborne particles are known, which are mainly based on light scattering measurements for estimating the size of particles contained in an aerosol and, in some cases, detection of the position of the particle at a given moment.
A first known, simplest and in industry still most applied, measurement device and method for this purpose is based on one laser emitting a spatially shaped laser light beam for obtaining an elliptical laser light spot in a measurement region, where an air flow from a nozzle crosses the laser beam, and one photodetector for detecting scattered light from a particle contained in the air flow and crossing the laser beam is provided. An output signal from the photodetector is then used to calculate the particle size.
Another known approach is based on the use of one laser in combination with two photodetectors for acquiring, in different angular orientation with respect to the laser beam, scattering signals from a particle crossing the laser beam.
Because of signal acquisition in two different light scattering angles, this approach allows for a more precise determination of a particle size thanks to simultaneous acquisition of two scattering patterns than the first approach described above. However, this approach is not adequate for a sufficient determination of the exact location of a detected particle.
A further known approach is based on the use of two lasers emitting laser light at different wavelengths in combination with two photodetectors provided with different spectral filters adapted to the two different laser emission wavelengths for acquiring scattered laser light from a particle crossing the laser beams, selectively for the emission wavelengths of the two lasers and central transmission wavelengths of optical band pass filters provided to the photodetectors designed for selective detection of laser light from an assigned laser. However, this approach suffers from limitations in particle size measurement because, for both photodetectors, only scattered light from one laser at a detection wavelength assigned to the related light-emitting laser is detected. Consequently, also this known approach is not adequate for solving imprecisions of particle size determination, particularly because of a potentially chromatically based error, and a correct determination of an actual particle location is also impaired by the inherent disadvantages of this approach.
The solutions according to prior art therefore all suffer from an insufficient precision of a determination of size and location of airborne particles.
It is the object of the present invention to overcome the above mentioned difficulties and to realize an improved measurement device and measurement method as described in the following.
To this effect, the present invention proposes a measurement device and a measurement method, which are characterized by the features enumerated in claims 1 and 10, respectively, and which allow achievement of the objectives identified above.
The problems associated with known measurement devices and methods as described above are solved, according to the invention, by a measurement device for the detection of airborne particles, the device comprising a nozzle designed to produce a laminar flow of air with a flow path beyond the nozzle, a first laser and a second laser, which are time-amplitude modulated, the first laser and the second laser being positioned for emitting laser light with intersecting laser beams with a laser beam intersection region in a region of the air flow path, and a first photodetector and a second photodetector positioned for the detection of laser light scattered in the laser beam intersection region by airborne particles contained in the laminar air flow.
In particular, the measurement device according to the present invention distinguishes from prior art in that the first and the second laser are designed for emission of laser light of identical laser emission wavelength, and the first photodetector and the second photodetector are positioned and designed each for the detection of scattered laser light from both the first laser and the second laser.
The lasers may be any known light sources capable of emitting coherent laser light, such as gas lasers, solid-state lasers, dye lasers, or laser diodes. “Identical laser emission wavelength” shall mean that the same spectral laser emission line or band is used, or laser diodes similar to one another are used, wherein small wavelength deviations or shifts, which may be in the range of some nanometers, are neglected.
The object of the invention is also solved by a measurement method for the detection of airborne particles by means of a measurement device according to the invention, the measurement device comprising a nozzle designed to produce a laminar flow of air with a flow path beyond the nozzle, a first laser and a second laser which are time-amplitude modulated, the first laser and the second laser being positioned for emitting laser light with intersecting laser beams with a laser beam intersection region in a region of the air flow path, and a first photodetector and a second photodetector positioned for the detection of laser light scattered in the laser beam intersection region by airborne particles contained in the laminar air flow. The measurement method comprises the step of producing by means of the nozzle a laminar flow of air potentially containing airborne particles to be detected. In particular, the measurement method is characterized by the further steps of emitting laser light of identical wavelength from the first and the second laser, and detecting laser light scattered by an airborne particle located in a superposition region of the region of the air flow path with the intersection region of the laser beams from the first laser and the second laser both by the first photodetector and by the second photodetector.
The measurement device and the measurement method according to the invention presents significant advantages over the known approaches discussed above. Each one of the two photodetectors is sensitive for and detects scattered light from both lasers, resulting in four different optical response signals from one detected airborne particle/aerosol particle. As a consequence, the particle size can be determined with a significantly higher precision than with the known approaches, for example by determining a ratio of size values derived from the signals from the two detectors for scattered light from one laser, taken at different angles of light scattering. According to the invention, the same is done concerning the scattering signals generated by the beam of the second laser. Furthermore, using the same emission wavelength for both lasers eliminates chromatic effects on particle size determination, that would have to be expected when using two lasers of different emission wavelengths.
Preferably, the first laser and the second laser are positioned for emission of laser beams intersecting on an angle of approximately 90°. Thereby, “approximately 90°” shall mean that the intersection angle does not deviate more than 20°, preferably not more than 10°, and still more preferred not more than 5° from 90°. This allows for determination of the particle size in two planes nearly perpendicular to one another according to the so-called Mie scattering theory. Two scattering light traces generated by light from a first laser, but taken at different scattering angles, allow for a particle size determination in a plane perpendicular to the optical axis of the first laser beam. Similarly, two scattering light traces generated by light from a second laser, also taken at different scattering angles, allow for a particle size determination in a plane perpendicular to the optical axis of the second laser beam. The first laser beam and the second laser beam being oriented approximately perpendicular to one another, the particle size determination can thus be based on four independent scattering signals in two different planes nearly perpendicular to one another. This further improves the precision of the particle size determination by a favorable choice of the geometrical arrangement.
According to one embodiment of the measurement device according to the invention, the first laser and the second laser are designed for emitting modulated laser light, particularly with a phase shift of approximately 90° between the laser beam of the first laser and the laser beam of the second laser. A “phase shift of approximately 90°” shall mean a phase shift deviating not more than 20°, preferably not more than 10°, and still more preferred not more than 5° from 90°. Thus, the scattering light signals from the two photodetectors can be distinguished with respect to the generating laser beams, and an assignment of the scattering signals to the specific laser beam generating the light scattering is facilitated.
Preferably, the first photodetector and the second photodetector are provided each with a narrow-band optical filter with a central transmission wavelength identical to the laser emission wavelength. Thus, a discrimination between signals of scattered light at the laser emission wavelengths and possible other, potentially disturbing, light signals is facilitated.
According to one embodiment, the measurement device is provided with a calculation and control unit designed for correlation of signals of scattered laser light from the first photodetector and the second photodetector for determination of the size of airborne particles scattering the laser light and preferably also for a determination of the location of detected airborne particles from a known intersection region of the laser beams. Thus, by correlating the four photodetector signals and also the known intersection region of the laser beams, not only the size but also the position of an airborne particle in the air flow path can be precisely determined.
According to a further embodiment, the measurement device is provided with a calculation and control unit designed for controlling an adjustment of laser intensities, particularly by adjusting amplitude of analog laser light modulation. Thus, detector saturations potentially caused by too strong signals of scattered light can be avoided. In fact, for particles larger than about 5 μm, the scattering signal is roughly proportional to the second power of the particle diameter. Thus, detector saturation due to scattering signals from large particles is avoided by adjusting amplitude of analog laser light modulation.
According to another embodiment, the measurement device is provided with an imaging detector, particularly a CCD or CMOS camera, designed and positioned for taking an image of a detected airborne particle after receiving an initiating trigger signal. The additional image enables to recognize shape and surface aspects of the detected particle.
According to a further embodiment, the measurement device is provided with an imaging detector, particularly a CCD or CMOS camera, combined with a diverging lens in an optical reception path designed and positioned for taking an image of an airborne particle, after receiving an initiating trigger signal, for determination of a scattering pattern caused by the airborne particle. Thus, besides information about the particle shape, especially about its symmetry, also about the trace of the particle in the air flow when crossing the two intersecting laser beams can be extracted.
According to a still further embodiment, the measurement device is provided with a flash light source for emitting short-wavelength light, particularly ultra-violet light, designed and positioned for exciting fluorescence or phosphorescence from a detected airborne particle, and with an additional, third photodetector, particularly a multi-anode photomultiplier or an intensified CCD camera, combined with an objective for fluorescence or phosphorescence light collection and a spectrally resolving optical element in an optical reception path for fluorescence or phosphorescence emitted by the airborne particle, designed and positioned for the detection of an optical spectrum of the fluorescence or phosphorescence emitted by the airborne particle. This embodiment allows for a precise identification of the species of the detected airborne particle, as its fluorescence and/or phosphorescence spectrum represents a unique signature disclosing at least the associated family and preferably, also the specific species of the particle itself.
The different embodiments of the measurement method according to the invention present analogous advantages as the embodiments of the measurement device as described above, according to similar specific features.
According to one embodiment of the measurement method, the first laser and the second laser are positioned for emission of laser beams intersecting on an angle of approximately 90°.
Preferably, the first laser and the second laser are operated for emitting modulated laser light, particularly with a phase shift of approximately 90° between the laser beam of the first laser and the laser beam of the second laser.
According to a further embodiment, signals of scattered laser light from the first photodetector and the second photodetector are correlated by a calculation and control unit of the measurement device for determination of the size of airborne particles scattering the laser light, and preferably also for a determination of the location of detected airborne particles from a known intersection region of the laser beams.
Preferably, the laser intensities are controlled by a calculation and control unit of the measurement device, particularly by adjusting amplitude of analog laser light modulation.
According to one embodiment, following an initiating trigger signal, an image of a detected airborne particle is taken by an imaging detector, particularly a CCD or CMOS camera.
According to a further embodiment, following an initiating trigger signal, an image of a detected airborne particle is taken by an imaging detector, particularly a CCD or CMOS camera, combined with a diverging lens in an optical reception path designed and positioned for taking an image of an airborne particle, after receiving an initiating trigger signal, for determination of a scattering pattern caused by the airborne particle.
According to still a further embodiment of the measurement method according to the invention, following an initiating trigger signal, detected airborne particles capable of fluorescence and/or phosphorescence are excited by a light of short, particularly ultra-violet, wavelength from a flash light source for emission of fluorescence and/or phosphorescence, and emitted fluorescence and/or phosphorescence is detected by a third photodetector, particularly a multi-anode photomultiplier or an intensified CCD camera, combined with an objective collecting the emitted fluorescence and/or phosphorescence, and spectrally resolved by a spectrally resolving element for projection on a photo-sensitive element of the third photodetector for generating an optical spectrum of the fluorescence and/or phosphorescence from the airborne particle.
A subject of the invention is also the use of a measurement device according to one of the related embodiments described above and/or the application of a measurement method according to one of the related embodiments described above for determination of size and/or location, as well as identification, of particles in a gas.
Other features and advantages of the present invention are mentioned in the dependent claims as well as in the description disclosing in the following, particularly with reference to the figures, the invention in more detail.
The attached figures exemplarily and schematically illustrate the principles as well as several embodiments of the present invention.
In the following, the invention shall be described in detail with reference to the above mentioned figures.
In general, the total size distribution of the group of non-biological airborne particles NB is larger, between 10 nm and 1 mm. The largest ones are so-called “heavy” particles 105 with a size between roughly 50 μm and 1 mm. Ash particles 106 typically have a size between 1 μm and 100 μm, oil smoke particles 107 between 0.5 μm and 10 μm, so-called “nanoparticles” 108 between 10 nm and 1 μm, similar to typical sizes of smog particles 109. Finally, soot particles 110 typically have a size between 10 nm and 100 nm.
Given this situation, the device and the measurement method according to the present invention aim to realize a real-time detection of individual aerosols in the size range of approximately 1 μm to 100 μm, which includes in particular all pollens and spores, some bacteria, virus clusters, as well as ash and oil smoke particles and thus allows to cover a very large range of different aerosols.
This arrangement may be regarded as the simplest one for a detection of aerosol particles and is still most used in aerosol science and industry. Every particle crossing the laser beam 201a will scatter the laser light, and a small part 205a of the scattered light will then be detected by the photodetector 205. An output signal of the detector 205 may then be used for a determination of the particle size. However, this approach is only adequate for a determination of the particle position and size with a precision close to the size of the laser spot.
This second known method allows for a particle size determination with a higher precision than the method according to
Referring particularly to
This approach is adequate for delivering a good localization of a detected particle and, therefore, useful for a combination with other measurement methods relying on a precise particle localization, like particle imaging. However, this method suffers from non-precise particle size determination, because for detection of scattered light from one laser only one photodetector assigned to this laser is used, and the differences between the two laser emission wavelengths and the related detection wavelengths will additionally lead to chromatically induced differences in size determination.
The lasers 1, 2 may be any known light sources capable of emitting coherent laser light, such as gas lasers, solid-state lasers, dye lasers or laser diodes. “Identical laser emission wavelength” shall mean that the same spectral laser emission line or band is used, or laser diodes similar to one another are used, wherein small wavelength deviations or shifts, which may be in the range of some nanometers, are neglected.
According to one embodiment of the invention, the photodetectors 5 and 6 are provided with narrow-band optical filters with a transmission wavelength identical to the laser emission wavelength, thus allowing for suppression of disturbing signals not originating from scattered laser light from particles to be detected.
The measurement device 500 and the related measurement method according to the invention present significant advantages over the known approaches discussed above. Each one of the two photodetectors 5, 6 is sensitive for and detects scattered light from both lasers 1, 2, resulting in four different optical response signals from one detected airborne particle. The photodetectors 5 and 6 are positioned at a small angle in a forward or backward direction with respect to one of the laser beams 1a, 2a, for example at angles between about 0° to 30°, preferentially between 10° to 25° and most preferably at about 20°. As a consequence, the particle size can be determined with a significantly higher precision than with the known approaches, for example by determining a ratio of size values derived from the signals from the two photodetectors 5, 6 for scattered light from lasers 1, 2, taken at different angles of light scattering. Furthermore, using the same emission wavelength for both lasers 1 and 2 eliminates chromatic effects on particle size determination, that would have to be expected when using two lasers of different emission wavelengths.
The positioning of the two lasers 1 and 2 for generating laser beams intersecting at an angle of approximately 90° allows for determination of the particle size in two planes nearly perpendicular to one another. Two scattering light traces generated by light from the first laser 1, but taken at different scattering angles, allow for a particle size determination in a plane perpendicular to the optical axis of the first laser beam 1a. Similarly, two scattering light traces generated by light from the second laser 2, also taken at different scattering angles, allow for a particle size determination in a plane perpendicular to the optical axis of the second laser beam 2a. The first laser beam 1a and the second laser beam 2a being oriented approximately perpendicular to one another, the particle size determination can thus be based on four independent scattering signals in two different planes nearly perpendicular to one another. Such favorable choice of the geometrical arrangement of the light sources and light detectors used further improves the precision of the particle size determination.
For particles larger than 5 μm, the intensity of scattered light being proportional to the second power of the particle diameter, a detector saturation may occur in case of detecting scattering signals from large particles. In order to avoid such detector saturation, the calculation and control unit 9 is designed for controlling an adjustment of laser intensities, particularly by adjusting amplitude of analog laser light.
The principle of the adjustment of laser power/intensity is as follows: The calculation and control unit 9 receives, triggered by a system clock 601, signals from the photodetectors 5 and 6, processed by fast analog to digital converters 5b and 6b. As a response, in case of a pre-determined threshold intensity indicating a potential detector saturation, the calculation and control unit 9 sends control signals, via fast digital to analog converters 1b and 2b, to the lasers 1 and/or 2 for an adjustment of laser power/intensity to be emitted.
According to this embodiment of the invention, which is enabled by the capability for precise localization of a particle, the two lasers 1 and 2 are initially modulated for alternating laser emission (see intensity traces I(1) and I(2)), until a particle is detected. Then both lasers 1 and 2 are shut off, and a trigger signal is sent to the imaging detector 11 for opening the camera shutter CS in order to start image collection/integration on all pixels for a pre-defined period of time. This is followed at a time T by initiating a short laser flash to be emitted from both lasers 1 and 2 simultaneously. This should be the shortest flash time possible for the applied type of laser and should not exceed 100 ns. The taken image may be used for determining the shape and also specific surface aspects of the detected particle.
Also according to this embodiment of the invention, the two lasers 1 and 2 are initially modulated for alternating laser emission (see intensity traces I(1) and I(2)), until a particle is detected. Then both lasers 1 and 2 are shut off, and a trigger signal is sent to the imaging detector 13 for opening the camera shutter CS in order to start image collection/integration for a predefined period of time and in a way that the camera sensor is completely filled out by a detection light beam 13a resized by the diverging lens 14. The opening of the camera shutter is followed at a time T by initiating a short laser flash to be emitted from both lasers 1 and 2 simultaneously, as described above referring to
This approach is particularly useful for determination of an on-fly scattering pattern for a detected airborne particle. Besides information about the particle shape, especially about its symmetry, also information about the trace of the particle in the air flow when crossing the two intersecting laser beams 1a, 2a can be extracted. This approach is not associated with size limitations (concerning the related detection region) like particle imaging, but it is more difficult to extract the information about the particle shape.
As for the embodiments described above with reference to
This embodiment allows, additionally to particle size and localization determination, for a precise identification of the species of a detected airborne particle, as its fluorescence and/or phosphorescence spectrum represents a unique signature disclosing at least the associated family and preferably, also the specific species of the particle itself.
This approach is characterized by the highest particle identification power. It combines size measurement, localization, on-fly-imaging and fluorescence and/or phosphorescence acquisition for one and the same detected particle. This approach is particularly useful for determination of particles with a size larger than 10 μm. Consequently, this approach offers, for example, an ideal method for individual pollen identification and counting in real-time.
Finally, the present invention is also related to the use of a measurement device according to one of the related embodiments described above and/or the application of a measurement method according to one of the related embodiments described above for determination of size and/or location, as well as identification, of particles in a gas.
All disclosed embodiments of the invention can be combined with one another, if not explicitly stated otherwise.
In light of the above description of the structure and of the operating mode of the present invention, its advantages are clear. Primarily, the invention enables a precise determination of the size and the location of airborne particles, particularly by use of two time-amplitude modulated lasers and two photodetectors for detecting scattering signals of light from both lasers, the lasers being positioned for light emission in two approximately perpendicular planes, thus allowing the correlation/use of four signals for one detected particle; avoiding any chromatic effect on the particle size determination, as two lasers of identical emission wavelength are used; a precise localisation of a detected particle based on a known intersection point of the two laser beams; and further acquisition of information relating to particle structure and particle identification, particularly enabled by the precise particle localization.
The invention is particularly useful for real-time detection and/or determination of airborne particles with a refractive index different from the one of air, especially for detection and/or determination of pollens, spores, some bacteria, virus clusters, ash and oil smoke particles, as well as soot aggregates.
| Number | Date | Country | Kind |
|---|---|---|---|
| 12167800 | May 2012 | EP | regional |
| Number | Name | Date | Kind |
|---|---|---|---|
| 7127356 | Nicoli | Oct 2006 | B2 |
| 20030223063 | Hill | Dec 2003 | A1 |
| 20040125371 | Chang et al. | Jul 2004 | A1 |
| 20060152722 | Northby | Jul 2006 | A1 |
| Number | Date | Country |
|---|---|---|
| 2 346 445 | Aug 2000 | GB |
| Entry |
|---|
| Pan, Yong-Le, et al., “Dual-excitation-wavelength fluorescence spectra and elastic scattering for differentiation of single airborne pollen and fungal particles,” Atmospheric Environment, vol. 45, pp. 1555-1563 (2011). |
| Pan, Yong-Le, et al., “Rapid classification of aerosols by single-laser-shot Fluorescence spectra and elastic scattering patterns,” Conference on Lasers and Electro-Optics, pp. 401-402 (Jun. 6, 2003). |
| Pan, Yong-Le, et al., “Fluorescence spectra of atmospheric aerosol particles measured using one or two excitation wavelengths: Comparison of classification schemes employing different emission and scattering results,” Optics Express, vol. 18, No. 12, pp. 12436-12457 (Jun. 7, 2010). |
| Kiselev, Denis, et al., “Individual bioaerosol particle discrimination by multi-photon excited fluorescence,” Optics Express, vol. 19, No. 24, pp. 24516-24521 (Nov. 21, 2011). |
| Number | Date | Country | |
|---|---|---|---|
| 20130301047 A1 | Nov 2013 | US |
