WO2017174978A1

WO2017174978A1 - Combining laser doppler velocimetry and spectroscopy for particle characterisation

Info

Publication number: WO2017174978A1
Application number: PCT/GB2017/050944
Authority: WO
Inventors: Odd Ketil Andersen
Original assignee: Particulate As
Current assignee: Particulate As
Priority date: 2016-04-04
Filing date: 2017-04-04
Publication date: 2017-10-12
Anticipated expiration: 2018-10-04

Abstract

A method and apparatus for characterising individual particles is disclosed. The apparatus comprises first and second sources (6) of electromagnetic radiation and a hyperspectral detector (42). An interference pattern (22) is generated by overlapping two beams (11, 13) of first electromagnetic radiation from the first source, where the region of overlap defines a probe volume (15) such that the interference pattern (22) is generated in the probe volume. One or more beams (10, 12) of second electromagnetic radiation having a range of different wavelengths are produced from the second source. A single particle (26) in the probe volume (15) is illuminated with the second electromagnetic radiation, thereby causing said single particle (26) to scatter the second electromagnetic radiation to produce scattered radiation (28). The scattered radiation (28) is detected using the hyperspectral detector (42), and hyperspectral data derived from the detected radiation is used to determine a property of the single particle (26).

Description

COMBINING LASER DOPPLER VELOCIMETRY AND SPECTROSCOPY FOR

PARTICLE CHARACTERISATION

This invention relates to apparatus and methods for characterising particles, for example in an aquatic mass or in air.

The characterisation of particles is important in many applications. For example, there are many circumstances in which it is necessary to monitor the extent of pollution in an ocean or other body of water. In the event of an oil spill, it is desirable to be able to measure the extent of the spill, and/or to monitor the effect of the use of chemicals (e.g. dispersants) on spills to determine the fate of the oil. It may also be necessary to monitor particles in discharges, for example, drilling mud from drilling operations or produced water from oil production, as well as the discharge of ballast water, which might be a major pathway for introducing species to new environments. Ballast water may be checked to ensure that particles are removed according to protocol, and remaining particles are not alive. This is important to ensure agreement with regulations regarding discharges before water is discharged into an ocean. As another example, there is a need to monitor algae in coastal regions. Many coastal regions are affected by harmful algal blooms (HABs), which are caused by blooms of microscopic algae that may be toxic to humans, fish, birds and other life in and near oceans.

There are also many applications where it would be advantageous to be able to characterise airborne particles. One example of this is analysing air pollution. By being able to identify particles it would facilitate identification of the source of pollution, determine whether the particles are potentially harmful to humans or wildlife etc. Known methods of monitoring particles include exciting fluorescence (i.e. the emission of radiation in response to stimulating radiation) in particles in a body of water, and then studying the spectral properties of the emitted radiation to try to characterise the particles. However, while some particles of interest exhibit fluorescence, it is desirable to be able to study particles that do not. Furthermore, fluorescence radiation from a small particle is typically very weak compared with the intensity of radiation necessary to excite fluorescence. This can limit the

information that can be reliably obtained using fluorescence-based methods, even if the particles of interest do exhibit fluorescence. In addition, in many cases, the water or air contains not only the particles of interest, but many other particle types. For example, in a region where it is desired to monitor oil contamination, there may also be algal particles. Consequently, the measured fluorescence signal is the sum of many individual particle fluorescence signals, emitted by many different particle types. As a result, it can be difficult to determine from the spectral characteristics of the measured fluorescence signal what types of particles are present, or whether a particular particle type of interest is present. This is because it may not be easy or possible to separate out the fluorescence signals emitted by each type of particle. For example, data deconvolution may be required to extract information relating to individual particle species.

Accordingly, the Applicant has identified a need for an improved method and apparatus for the characterisation of particles in body of water or air which contains a mixture of different particle types, particularly but not exclusively for particles that do not exhibit fluorescence. It has also identified that it would be desirable to be able to obtain more information, and more useful information, regarding the characterised particles.

According to the invention there is provided a method of characterising individual particles, the method comprising:

generating an interference pattern by overlapping two beams of first electromagnetic radiation from a first source, wherein the interference pattern is generated in a probe volume defined by a region of overlap of the two beams; producing one or more beams of second electromagnetic radiation having a range of different wavelengths from a second source ;

illuminating a single particle in the first probe volume with the second electromagnetic radiation, thereby causing said single particle to scatter the second electromagnetic radiation to produce scattered radiation;

detecting the scattered radiation using a hyperspectral detector; and using hyperspectral data derived from the detected radiation to determine a property of the single particle.

The invention extends to an apparatus for characterising individual particles, the apparatus comprising:

a first source of first electromagnetic radiation arranged to generate an interference pattern by overlapping two beams therefrom, wherein the interference pattern is generated in a probe volume defined by a region of overlap of the two beams;

a second source of second electromagnetic radiation having a range of different wavelengths arranged to produce one or more beams and to illuminate a single particle in the first probe volume to cause said single particle to scatter the second electromagnetic radiation to produce scattered radiation;

a hyperspectral detector arranged to detect the scattered radiation; and processing means configured to use hyperspectral data derived from the detected radiation to determine a property of the single particle.

The invention may enable more detailed and more useful information to be obtained about particles because i) the particles are characterised individually and ii) the use of a hyperspectral detector allows hyperspectral analysis of individual particles to be obtained from a very broad and high resolution spectrum of the scattered radiation. The spectrum of the scattered radiation is determined by what the particle is, e.g. its substance/material, and so hyperspectral data derived from the scattered radiation in accordance with the invention allows the determination of the particle type for individual particles, including particles that do not exhibit fluorescence.

In contrast, the prior art only allows the determination of bulk properties of large numbers of particles in water, e.g. the bulk properties of many thousands of particles, which may provide less information and/or less useful information. In addition, in bulk measurements, the measured signal of radiation emitted from or scattered by particles is the sum of many individual particle signals, which includes not only signals from the particles of interest, but also signals from other particles. The signals from particles that are not of interest may therefore obscure the signals of interest. It will be appreciated that, in accordance with the invention, the ability to look at individual particles arises because the first source is arranged to overlap two beams in a probe volume, i.e. the spatial extent of the illuminated volume is limited. Due to, for example, properties of the source, such as radiation beam size, the probe volume may be very small, allowing the second radiation in the probe volume to illuminate one particle at a time.

The second source is preferably a broadband source, i.e. producing broadband electromagnetic radiation. It will be understood that a broadband source of electromagnetic radiation refers to a source that emits a broad range (e.g. a continuous spectrum) of wavelengths. Preferably the source has a wavelength range covering several hundred nanometers, e.g. a range greater than 500 nm, or greater than 1000nm. In a particularly preferred embodiment, the spectrum of the second source extends from approximately 450nm to approximately 2400nm.

In a set of embodiments the second source is a supercontinuum light source, e.g. a supercontinuum laser. An example of a suitable supercontinuum laser is SuperK Power from NKT Photonics, which produces a quasi-continuous output in the range 470nm to 2400nm. Supercontinuum lasers are particularly suitable as they can provide a broad wavelength range with a substantially flat spectrum over the wavelength range (i.e. no significant features in the laser emission spectrum). They can also produce high energy radiation giving a stronger signal than is the case with fluorescence which improves resolution and sensitivity.

It will be understood that the term "hyperspectral" as used herein does not refer to hyperspectral imaging, for example, as is used in applications such as remote sensing, but refers to a detector, i.e. a spectrometer, that measures spectral components of incident radiation over a broad range of wavelength with high resolution. The wavelength range of the detector may cover all or substantially all of the visible electromagnetic spectrum. The wavelength range may extend substantially into the infrared and/or ultraviolet range. In one set of embodiments, the hyperspectral detector range encompasses the range of approximately 450 nm to approximately 2400 nm. The hyperspectral detector may have a very narrow spacing of measured wavelength bands. For example, the wavelength bands may be separated by less than about 10 nm, although resolution can be chosen according to the application depending on the required sensitivity, size, cost power comsumption etc.. The hyperspectral data may comprise data representing a spectrum of the scattered radiation detected using the hyperspectral detector.

It will be appreciated that a hyperspectral detector differs from a multispectral detector or imager. In multispectral imaging, a relatively small number of wavelength bands (typically around 10) are measured. Hyperspectral detectors (e.g. hyperspectral analysers) measure many more bands (e.g. typically hundreds). It is therefore possible to obtain much more detailed spectral information using a hyperspectral detector.

The hyperspectral detector may comprise one or more spectrally dispersing elements (e.g. prism(s), grating(s) and/or grism(s)) for separating the radiation components according to wavelength. The hyperspectral detector may comprise an array of detector elements (e.g. CCDs or CCD pixels) for receiving respective components of radiation. The hyperspectral detector may comprise one or more optical elements (e.g. lens(es) and/or mirror(s)) arranged to direct the separated radiation components onto the detector elements (e.g. to direct each wavelength band onto a respective detector element).

In a set of embodiments , the second source comprises a laser, e.g. a

supercontimuum laser as previously mentioned, arranged (e.g. using suitable optical elements) to produce a laser beam that is focussed on the probe volume. The second source may comprise more than one laser. The beam(s) may be focussed e.g. using focusing optics.

In another set of embodiments a single laser beam focussed on the probe volume is provided.

In a set of embodiments , the probe volume has a maximum dimension that is smaller than a mean separation between particles in the aquatic mass. The maximum dimension of the probe volume may be, for example, less than half of the mean particle separation, less than one fifth of the mean particle separation, or less than one tenth of the mean particle separation. It will be appreciated that if the probe volume is small compared to the mean particle separation, it is likely that only one particle will be in the probe volume at any one time. Preferably, the method comprises illuminating just one particle of interest at a time. However, there may be occasions when more than one particle is present in the probe volume. In such cases, the detected radiation may be scattered by more than one particle in the probe volume. In a set of embodiments the method comprises the step of discarding or disregarding hyperspectral data corresponding to radiation detected when more than one particle is simultaneously present in the probe volume. It will be appreciated that when particle density is high, a larger amount of hyperspectral data may be discarded, as the presence of more than one particle in the probe volume may occur with greater frequency. Such data may be discarded automatically by the apparatus. It will be appreciated that characterising individual particles refers to determining a property of a single particle, i.e. so that individual properties of individual particles may be determined separately. It will be appreciated that a property of the particle may be determined from hyperspectral data that is characteristic of (e.g. unique to) to the single particle that is illuminated. Determining a property of the particle may comprise determining one or more of the particle type (i.e. the material or substance the particle is made from), the particle volume, and/or the particle size. In a set of embodiments the method comprises determining at least one property of the particle other than its velocity, speed, or direction of motion. Nevertheless, it may be desirable to determine a speed of the particle in addition to determining a characteristic of the particle using the hyperspectral data.

Accordingly, in In a set of embodiments, the speed of the particle is also

determined. In some embodiments, a single particle in the probe volume may be illuminated by the interference pattern, thereby causing the particle to scatter the first, typically narrowband, radiation, thereby producing scattered first radiation.

The scattered first radiation may then be detected and used to determine a speed of the particle. For example, the apparatus may comprise a Laser Doppler Velocimeter arranged to determine a speed of the particle using Laser Doppler Velocimetry (LDV), e.g. the Laser Doppler Velocimeter may comprise the second source. For example, as a particle passes through the interference pattern in the probe volume, the amount of light it scatters will depend on whether the particle is in a region of constructive or destructive interference. In a region of constructive interference, i.e. a bright region of the interference pattern, the particle will tend to scatter more light to the detector than when it is in a region of destructive interference, i.e. a dark region of the interference pattern. As the particle moves through the interference pattern, the detected radiation will vary in intensity. For example, if the interference pattern is a sinusoidally varying fringe pattern, the intensity of detected radiation will also be sinusoidally varying. The frequency of variation in intensity will depend on the speed of the particle and the fringe separation. The speed of the particle perpendicular to the fringes may thus be related to the fringe spacing and the frequency of variation in the intensity of the detected radiation.

In accordance with the invention the probe volume is illuminated with the second, broader wavelength band radiation. This allows both types of scattered radiation (e.g. second, broadband and first, narrowband) to be collected from the same particle. In preferred embodiments, an optical arrangement is provided to collect the second, broadband scattered radiation and/or the first, narrowband scattered radiation and to direct it to the hyperspectral detector and/or to a further detector for detecting the first, narrowband scattered radiation.

In a set of embodiments the second, broadband scattered radiation and the first, narrowband scattered radiation are separated (e.g. by a filter) to enable separate analysis of each portion of radiation. The wavelength or wavelength range of the first, narrowband source preferably falls outside of the wavelength range of the second source used with the hyperspectral detector. This allows the collected radiation to be separated by wavelength (e.g. using a dichroic mirror or filter) into the scattered radiation (for hyperspectral analysis, e.g. to determine particle type) and the narrowband scattered radiation (e.g. for Laser Doppler Velocimetry analysis to determine particle speed).

The first, narrowband source used to generate the interference pattern may be substantially monochromatic. It may be any suitable source of electromagnetic radiation but preferably a laser is used. The interference pattern is created using two beams from a single electromagnetic radiation source arranged (e.g. using optical components) so that they overlap to define the probe volume, thereby generating an interference pattern.

The first, narrowband source may emit radiation in the blue-violet frequency region of the visible spectrum; for example, the radiation emitted by the narrowband source may have a wavelength between 300 nm and 500 nm. In some

embodiments, the narrowband source comprises a blue laser. It will be understood by one skilled in the art that a blue laser may refer to a laser having a wavelength spectrum covering part of the short wavelength end of the visible spectrum, e.g. part of the blue to violet spectrum. For example, a 405 nm laser may be used. A 405nm laser is particularly advantageous for several reasons. Suitable 405nm lasers produce a strong (i.e. intense) beam which is suitable for use in the sea. In addition, suitable dichroic mirrors are readily available to separate the mixed scattered radiation into the LDV Doppler signal and the hyperspectral signal.

However, other wavelengths may be used. The first, narrowband source's wavelength may fall outside of the visible spectrum. However, preferably the wavelength is a visible wavelength. This may be better from a health and safety viewpoint, e.g. to satisfy safety regulations relating to open radiation sources. In addition, it facilitates the alignment of the apparatus (e.g. optical components) which may be more easily achieved if the radiation is visible. It will be appreciated that the wavelength of a radiation source may refer to a peak or central wavelength of the spectrum of the radiation emitted by the source.

In a set of embodiments, the apparatus is mounted on a moving vehicle, e.g. an autonomous underwater vehicle (AUV), or a remotely operated vehicle (ROV). In some of these embodiments, in which particle speed is also determined, the apparatus may be advantageously positioned and configured so that the interference pattern comprises fringes aligned substantially perpendicular to a direction of propulsion of the vehicle. It is also preferred that the probe volume is sufficiently far from the vehicle to be in a region of laminar flow of the water. In that situation, the flow of particles relative to the vehicle will be substantially

perpendicular to the fringe direction, allowing the particle speed relative to the vehicle to be more accurately determined. The method may also comprise counting particles, e.g. counting the number of particles illuminated in the probe volume in a time period. Particle counting may be used to determine a particle density and thereby to determine a particle flow rate. The variation in the signal corresponding to the intensity of the detected first, narrowband radiation may be also be used to determine the reliability of the data collected. For example, as discussed above, the recorded intensity of the first, narrowband scattered radiation may vary according to the brightness of the fringes as the particle moves through the interference pattern, e.g. exhibiting a number of peaks corresponding to the number of fringes that the particle has moved through. The interference pattern may have, for example, between five and ten fringes. If a particle passes close to the centre of the probe volume, the recorded scattered radiation may exhibit several peaks, e.g. five or more peaks. In contrast, if the particle passes close to the edge of the probe volume, there might be only one or two peaks.

The reliability of the data may be greater if the particle has passed near the centre of the probe volume, as there is more scattered radiation that can be collected and used. Accordingly, data corresponding to a particle may be discarded if the recorded intensity of the first, narrowband scattered radiation exhibits fewer than a threshold number of peaks. The discarded data may include the hyperspectral data recorded for the particle. The threshold number may be, for example, three, four, five, six, or more than six. It will be understood, therefore, that the actual sampling volume in which particles may be characterised may be smaller than the probe volume, as data corresponding to particles passing through the probe volume extrema may be rejected. It will therefore also be appreciated that the size and shape of the actual sampling volume may be dynamically determined depending on, for example, particle and flow characteristics, the laser properties, and the threshold number of peaks.

As used herein, particle means any individual piece of matter or material, e.g. a piece of particulate matter or a droplet. For example, the particle may be an algae particle, a droplet, e.g. an oil droplet, zooplankton, fish eggs, larvae, minerals from drilling or re-suspension of sediments, marine snow (aggregates that decompose on touch or sampling), oil dispersion chemicals, organic chemicals attached to minerals or living organism etc. Airborne particles might be soot from fires and diesel combustion, mineral particles from asphalt or concrete, widely dispersed particles as sand, pollen etc. The distance of the probe volume from the apparatus may be chosen depending on the type of particles to be characterised and the situation in which the apparatus is used. The distance may also be chosen to optimise the amount of light collected for transmission to the hyperspectral detector. For example, if the beams of the first electromagnetic radiation are directed so that they overlap close to the apparatus, more of the scattered radiation may be collected. This is because of two factors. First, light from the probe volume will be incident on the detector over a greater area which allows for more sensitive measurements from a detector having a given sensitivity per unit detection surface area. Second, there is a reduced amount of absorption by the medium being monitored, especially in the case of water (e.g. sea water) if the light only travels a short distance through the medium. Accordingly, if the probe volume is far away from the apparatus, there may be reduced sensitivity.

However, there are additional factors that may be used to determine the distance of the probe volume from the apparatus. For example, if the probe volume is close to the apparatus (e.g. close to an optical arrangement for directing light onto the detector), good spatial resolution may be obtained for small particles. If the probe volume is farther away, good spatial resolution may be obtained for medium and large particles. If the apparatus is mounted on a moving vehicle (e.g. an

autonomous underwater vehicle AUV), there may be turbulence in the water or air around the vehicle when it is moving, with a region of laminar flow farther away from the vehicle. In the region of laminar flow, the particles may all move with approximately the same speed, while in the turbulence, the particle may have many different speeds. The distance to the probe beam may be advantageously chosen so that it is outside the turbulence and in the laminar flow, which may yield better results for the particle characterisation.

As mentioned above, in embodiments in which the particle speed is determined, the detected radiation may be separated according to its wavelength. For example, the detected radiation may be separated into two portions. A first portion of the separated radiation may comprise radiation that is below a threshold wavelength. The first portion may correspond to the first, narrowband scattered radiation. This portion may be subject to speed (e.g. LDV Doppler signal) analysis.. A second portion of the separated radiation may comprise radiation that is above the threshold wavelength. The second portion may correspond to the second scattered radiation which is subject to hyperspectral analysis

Any suitable means, e.g. optical component(s), may be used to separate the radiation. For example, a dichroic mirror or filter may be used. The radiation may be separated so as to send the second scattered radiation to the hyperspectral detector and to send the first, narrowband scattered radiation to an LDV processing engine, or other speed, velocity, and/or direction analysis engine.

A threshold wavelength (e.g. a single threshold wavelength in a case where the radiation is separated into two portions) may be slightly above the wavelength of the first, narrowband source. For example, it may be between 5 and 10 nm above. As an example, for a 405 nm radiation source, the threshold may be 410 nm. The threshold wavelength may be part-way between a wavelength of the first, narrowband source and a lower limit wavelength of the second source used with the hyperspectral detector. For example, if the first, narrowband source is a 405nm laser, and the second, hyperspectral source is a 450nm-2400nm supercontinuum laser, the threshold wavelength may fall in the gap between 405nm and 450nm, e.g. it may be around 420nm or 430nm. This may help to separate the radiation cleanly according to whether it originated from the 405nm laser or the

supercontinuum laser.

It will be appreciated that the size of the probe volume will be determined by the characteristics of the first, narrowband radiation source e.g. laser and/or any optical components used to focus the radiation, e.g. laser beam width, radiation

wavelength, focal length of optical components. For example, the size of the probe volume may be on the scale of tens of microns, e.g. between 50 μηι and 100 μηι, or between 20 μηι and 200 μηι. As a non-limiting specific example, the probe volume may be around 200μηι in length, around 50 μηι in width and around 50 μηι in depth. However, it will be understood that larger and smaller probe volume sizes may be used. The characteristic length scale of the interference pattern in the probe volume, e.g. the fringe spacing, depends on the wavelength of the first electromagnetic radiation. The characteristic length scale, e.g. fringe spacing, may be of the order of microns, e.g. between 5 μηι and 10 μηι, or between 2 μηι and 20 μηι, although it will be understood that the length scale/fringe spacing may be larger or smaller than this.

Any suitable detector may be used to detect the first, narrowband scattered radiation. For example, photomultiplier tubes may be used. Alternatively a charge coupled device may be used. A linear photo diode array may be used. The method of the invention may be applied e.g. in a laboratory setting on test samples that have been collected from an environment being monitored such as an aquatic mass or region of air,. However, preferably the method is applied in situ in the monitored environment, e.g. in an aquatic mass where particles of interest are found or in an area suffering from air pollution. As mentioned above, the apparatus may be mounted on a moving vehicle. In other embodiments, the apparatus is fixed at a location, e.g. at a monitoring station to monitor water or air flowing past the station. The method may be applied "in line", for example, in discharge outlets. For example, oil droplet concentration or type may be measured in produced water discharges. The method may be used for characterisation of ballast water, for example to determine the size of particles present to ensure agreement with regulations before discharging the water. It could also be used in ventilation ducts.

In situ in an aquatic mass, the method may be used to detect changes in water mass characteristics (e.g. which may be characterised by changes in algal composition). As another example, it may be used to detect oil droplets from leaks or spills, e.g. to measure the extent of a spill. It may be used to document the effects of the use of chemicals on spills, for example during clean-up operations and the weathering of oil or other chemicals. It may be used to validate distribution models for produced water and drilling mud. It may be used to detect microplastic beads.

The method may be used in combination with positioning data, e.g. GPS (global positioning system) and/or depth data, for mapping, e.g. to create maps showing locations of particle types. For example, if the apparatus is mounted on an AUV, the vehicle may record GPS and depth data, or if the vehicle is an ROV following a ship, the vehicle may record depth data, and acquire GPS data recorded on the ship. This may be used, for example, for mapping oil spills. It can also be used to map air pollution on land. The method may comprise comparing the hyperspectral data with recorded hyperspectral data in a database, and/or reviewing of the hyperspectral data by an operator. This may enable the processing means or an operator to identify or characterise a particle based on the comparison of the hyperspectral data with recorded hyperspectral data. The hyperspectral data may be used to create a record, e.g. library of database files, of hyperspectral data corresponding to particular particles or particle types or chemicals. These records may then be used subsequently to identify particles. It will be understood that different particles produce different spectra when they scatter radiation, and that some spectra are easier to distinguish/identify than others. The recorded data may added to the database to build up a library of different recorded spectra, irrespective of whether a particle or particle type has been identified as corresponding to the data. The data added to the database may include and/or be grouped according to the results of data analysis, e.g. multivariate and/or pattern recognition analysis. Such data may be used subsequently to help distinguish spectra and/or to identify particles.

The apparatus may comprise a user interface. The user interface may allow an operator to view hyperspectral data or data relating to particles in the aquatic mass. For example, the user interface may allow the operator to view data embodying the characterisation of particles, e.g. data indicating a particle's material or size. It may also allow the operator to view data indicating to a particle's velocity, speed or direction of motion, or data indicating a particle density or flow rate in the aquatic mass.

The user interface may allow an operator to select what data to display. The user interface may be capable of displaying data in real time, e.g. hyperspectral data or data indicating the properties of particles, as it is received, or shortly after it is received. It may also allow an operator to view stored data, e.g. previously recorded data, and/or reference data, e.g. library data indicating known spectral signatures of certain types of particles. For example, data collected on an AUV may be stored for later retrieval and analysis, while data on an ROV following a ship may transmit recorded data in real time for real time analysis.

Where optional features are described with reference to the method of the invention, corresponding optional features may be provided for the apparatus, and vice versa.

Certain preferred embodiments will now be described, by way of example only, with reference to the accompanying drawings, in which:

Figure 1 shows a hyperspectral analysis apparatus embodying the present invention;

Figure 2 shows a close-up view of a probe volume formed by an intersection of two laser beams;

Figure 3 shows a schematic representation of a typical spectrum of a

supercontinuum laser; Figure 4 shows a schematic representation of a scatter spectrum of oil;

Figure 5 shows a schematic representation of the optical components of the hyperspectral detector; Figure 6 shows the variation with time of the light intensity scattered from a particle in a probe volume of the Laser Doppler Velocimeter;

Figure 7 shows the light intensity of Figure 6 filtered using a bandpass filter to remove the Gaussian envelope; and

Figure 8 shows a schematic representation of signal and data handling according to embodiments of the present invention.

Figure 1 shows a hyperspectral analysis apparatus 2 for characterising individual particles in an aquatic mass embodying the present invention. Other embodiments may however be sued for characterising airborne particles. The hyperspectral analysis apparatus 2 comprises a main housing 4 containing a laser module 6 which includes both a supercontinuum laser with a spectrum extending from approximately 450nm to approximately 2400nm and a 405nm monochromatic laser. On the front of the housing 4 there is a lens 8 for coupling radiation in and out of the housing 4.

The hyperspectral analysis apparatus 2 incorporates a Laser Doppler Velocimeter for detecting particle presence and determining particle speed which uses the 405nm monochromatic laser. This laser is divided into two beams 1 1 , 13 which are 180 degrees apart and are directed through the edge of the lens 8. The 405nm beams 1 1 , 13 overlap to define a probe volume 15, which is encompassed by or substantially the same as the first probe volume 14, as shown in Figure 2. In this embodiment the output of the supercontinuum laser is divided into two beams 10, 12. The beams 10, 12 are directed by additional optical components (not shown) through the edge of the lens 8 so that the beams illuminate the probe volume 14. In alternative embodiments a single beam could be produced which is focussed onto the probe volume 14. It may be more difficult in practice however to focus such broadband radiation. Other embodiments may illuminate the probe volume 14 from outside the apparatus shown in Figure. 1.

As the 405nm beams 11 , 13 are coherent, when the beams 11 , 13 overlap, an interference pattern 22 is created. The interference pattern 22 consists of interference fringes localised in the second probe volume. It will be appreciated that as the beams 10, 12 from the supercontimuum laser are broadband, no interference pattern is created by them. A particle 26 in the probe volume 14wi 11 therefore scatter both the broadband and the narrowband radiation. Returning to Figure 1 , it can be seen that received scattered radiation 28 of all wavelengths is coupled in to one or more optical fibres 30 by the centre portion of the lens 8 and optical components 29. The optical components 29 are represented schematically by a lens, but it will be appreciated that the optical components may comprise any number of optical components possibly including, but not limited to, one or more lenses, diffraction gratings, mirrors, etc. The scattered radiation 28 is directed via the one or more optical fibres 30 to a dichroic mirror 38. The dichroic mirror 38 separates the radiation according to wavelength. Radiation that is above 430 nm is directed along a first optical fibre 40 towards a hyperspectral detector 42. The longer wavelength portion which passes along the fibre 40 contains

predominantly radiation that originated from the supercontinuum laser and which was scattered by the particle 26. The hyperspectral detector 42 is used to measure the spectral components of this radiation as will be described later. The component of the received radiation that is below 430 nm is directed along a second optical fibre 44 towards an LDV processing engine 46, where the intensity of the radiation is measured and the speed of the particle 26 is inferred. The shorter wavelength component comprises predominantly radiation from the 405 nm laser that was scattered from the particle 26. These can be used to detect presence and infer speed of the particle as explained below.

In use, the hyperspectral analysis apparatus 2 is disposed in an aquatic mass 24, e.g. an ocean, or other suitable environment. Particles 26 in the aquatic mass move through the probe volume 14 due to the flow of the aquatic mass 24 in which the particles 26 are suspended. Due to the size of the probe volume 14, typically only one particle 26 is in the probe volume at a time. If data should, unusually, be recorded indicating that more than one particle 26 is in the probe volume 14, that data may be discarded. The particle 26 is illuminated by the interference pattern 22. If the particle 26 enters a region of constructive interference (i.e. a fringe) it will scatter the 405nm radiation. The 405nm radiation that is scattered is collected by optical components 29 and directed to the optical fibre(s) 30. It is subsequently separated from any scattered supercontinuum radiation and passed to the LDV processing engine 46 as described above. Initially the LDV engine 46 detects the presence of the particle 26 and initiates its own analysis and that of the hyperspectral detector 42. The supercontinuum laser is running all the time but the data from the hyperspectrcal is only detected between the beginning and end of a Doppler detection event. Figure 3 shows a schematic representation of a spectrum of a supercontinuum laser which may be used with embodiments of the present invention. It can be seen that the spectrum extends from around 450nm to around 2400nm, and in substantially flat in this range. This makes it particularly suitable as it allows the scattering spectrum of a particles to be measured across a very broad range of wavelengths.

Figure 4 shows a schematic representation of a reflectance spectrum of oil (i.e. the spectrum of radiation scattered by oil). It can be seen that the spectrum has a number of characteristic peaks 31 corresponding to 1.2 μηι, 1.73 μηι and 2.3 μηι C- H features as a result of absorption of radiation in adjacent bands by the particle 26. It will be appreciated that different substances have different characteristic spectra, and that by determining the spectrum of radiation scattered by a particle, it is possible to determine what the particle is (e.g. its substance). The spectrum of scattered radiation measured by the hyperspectral detector 42 can therefore be used to determine the type of particle 26 that was in the probe volume 14. For example, if the scattering (reflectance) spectrum matches the characteristic scattering spectrum of oil (or a particular type of oil) shown in Figure 4, it can be determined that an oil droplet is in the probe volume 14, 15. Further information may be obtained from the spectrum. For example, depending on the intensity of the spectrum, the size of the particle 26 may be inferred.

Figure 5 shows a schematic representation of the optical components 32 of the hyperspectral detector 42. These comprise a spectrally dispersing element 33, represented by a prism. It will be appreciated that the spectrally dispersing element 33 may comprise a different type of dispersing component (e.g. a grating or grism), or it may comprise a plurality of dispersing components (e.g. one or more prism(s), grating(s) and/or grism(s)). The longer wavelength broadband radiation from the first optical fibre 40 is directed through the spectrally dispersing element 33 to disperse the wavelength

components of the radiation so that the components are spatially separated. The separated wavelength components are represented in Figure 5 by components 34a, 34b and 34c, where 34a is the longest wavelength, and 34c is the shortest wavelength. The hyperspectral detector also comprises focusing optics 35, represented by a lens. It will be appreciated that one or more other focusing optical component(s) (e.g. a mirror) may be used instead of or in addition to one or more lenses. The hyperspectral detector also comprises a charge-coupled device (CCD) 36 having an array of CCD pixels 37. The focusing optics 35 focus the wavelength components 34a, 34b, 34c so that each component (where each component corresponds to a wavelength band, e.g. having a width of 10nm) is incident on (and thus detected by) a respective CCD pixel 37. In this way, the intensity of each component in the scattering spectrum of the particle 26 can be determined.

As previously mentioned the LDV engine 46 infers the speed of the particle using the shorter wavelength narrowband radiation passed through the optical fibre 44. Figure 6 shows a typical time variation in the intensity of the narrowband radiation scattered from a particle moving through the probe volume 14. As the particle 26 moves, it is periodically illuminated by the interference fringes arising from constructive interference of the narrowband laser beams. This gives rise to a sinusoidal variation in the intensity of scattered light. The profile of the light intensity has a Gaussian envelope, which arises because of the Gaussian profile of the laser beams 11 , 13.

Between the peaks in Figure 6, the light intensity does not fall close to zero. This is because, in the example shown, the particle 26 is comparable in size to the fringe width. In such a case, as the particle moves through the fringes, it is partially illuminated by the next fringe before it has completely moved out of the previous fringe. If the particle were much smaller than the fringe width, the intensity would fall much closer to zero as there would be times when the small particle is almost entirely within a region of destructive interference. Nonetheless the variation in intensity can still be used to determine the speed of the particle.

Figure 7 shows the light intensity profile of Figure 6 filtered using a bandpass filter to remove the Gaussian profile. As the intensity of scattered light depends on whether the particle is in a region of constructive interference or destructive interference, the frequency of the sinusoidal variation of measured scattered light intensity will depend on how quickly the particle is moving through the fringes. The component of the particle velocity perpendicular to the fringes is related to the fringe spacing and the frequency f of the sinusoidal profile showed in Figure 6 by Perpendicular component of velocity = fringe spacing x f.

The frequency f is calculated from the bandpass filtered light intensity profile (such as shown in Figure 7). The frequency may be obtained from the sinusoidal profile using, for example, a Fast Fourier Transform.

It is thus possible to obtain information regarding the speed of the particle moving through the probe volume, in addition to information characterising the particle.

Figure 8 shows a schematic representation of the signal and data handling carried out in accordance with embodiments of the present invention.

The radiation 28 scattered from particles is separated by the dichroic mirror 38 (here represented simply by a functional block) into a first radiation component 48 (comprising radiation that is less than 430 nm in wavelength) and a second radiation component 50 (comprising radiation that is greater than 430 nm in wavelength) as previously described.

The first component 48 is detected by a detector measuring intensity, such as a photomultiplier tube, and the measured intensity is processed by the LDV processing engine 46 as described above to determine the speed of the particle.

The second component is directed to a hyperspectral detector 42 which measures the spectrum of the radiation.

The data obtained from the LDV processing engine 46 and the hyperspectral detector 42 are sent to a computer 52 for analysis and display. The computer 52 comprises a processor 54 and a memory 56. Stored in the memory is a database 58. In other embodiments the memory 56 (and therefore the database 58) may be remote from the computer 52 (for example, accessed via a network). The speed data and hyperspectral data obtained by the LDV processing engine 46 and the hyperspectral detector 42 are written to the database 58. Positioning information 59 is also provided to the database 58, e.g. for mapping purposes. Positioning data may include GPS (global positioning system) data and/or depth data (e.g. depth of an underwater vehicle below sea level). For example, an AUV may record GPS and depth data. An ROV following a ship may record depth data, and acquire GPS data recorded on the ship.

The hyperspectral data are compared with recorded hyperspectral data in the database 58 to allow the particle 26 to be identified according to its scattering (reflectance) spectrum. If the particle cannot be positively identified, the spectrum and/or a multivariate data analysis thereof may be recorded in the database for use in subsequent analysis or comparison. The computer 52 provides a possibility for the comparison to be automatic, although the comparison may be initiated by an operator via a user interface 60 provided on the computer 52, e.g. by inputting an instruction to activate a comparison with data stored in the database (e.g. via a search of stored reference spectra).

The computer 52 also provides the option to display the recorded hyperspectral data along with hyperspectral data from the database 58 for a visual comparison by the operator. The processor 54 is configured to carry out multivariate analysis or pattern recognition 62 to facilitate the comparison of the hyperspectral data with recorded hyperspectral data in the database 58. Once the corresponding spectrum or spectra have been identified in the database, information related to the identified spectrum is used in a further data analysis step 64 to present the data in a useful format (e.g. including a visual representation) in a 3D graphical information system (GIS) presentation 66 for the operator to study. The user interface 60 can be used to control the output on the 3D GIS presentation 66. For example, the user interface can get the GIS presentation 62 to display a continuous log of data obtained, or it can retrieve previously recorded data and/or reference data from the database for display, e.g. for comparison purposes.

It will be appreciated that only one possible embodiment has been described herein, and that variations are possible within the scope of the present invention. For example in the present embodiment, oil droplets are described. However in other embodiments, other particles may be characterised. For example, algae particles , zooplankton, fish eggs, larvae, minerals from drilling or re-suspension of sediments, marine snow (aggregates that decompose on touch or sampling), oil dispersion chemicals, organic chemicals attached to minerals or living organism etc. may be characterised.

The Applicant has discovered that the method of the present invention can be used to identify microplastic beads, which are becoming a significant environmental concern.

Airborne particles might also be characterised such as soot from fires and diesel combustion, mineral particles from asphalt or concrete, widely dispersed particles as sand, pollen etc.

Claims

Claims:

1. A method of characterising individual particles, the method comprising: generating an interference pattern by overlapping two beams of first electromagnetic radiation from a first source, wherein the interference pattern is generated in a probe volume defined by a region of overlap of the two beams; producing one or more beams of second electromagnetic radiation having a range of different wavelengths from a second source ;

illuminating a single particle in the probe volume with the second electromagnetic radiation, thereby causing said single particle to scatter the second electromagnetic radiation to produce scattered radiation;

2. The method as claimed in claim 1 , wherein the second source has a wavelength range greater than 500 nm.

3. The method as claimed in claim 2, wherein the second source is a supercontinuum light source.

4. The method as claimed in claim 1 , 2 or 3 wherein the wavelength range of the detector covers all or substantially all of the visible electromagnetic spectrum.

5. The method as claimed in any preceding claim, wherein the hyperspectral detector has a spacing of measured wavelength bands separated by less than 10 nm.

6. The method as claimed in any preceding claim, wherein the second source comprises a laser producing a laser beam that is focussed on the probe volume.

7. The method as claimed in any preceding claim, wherein the probe volume has a maximum dimension that is smaller than a mean separation between particles in the aquatic mass.

8. The method as claimed in any preceding claim, wherein the size of the probe volume is between 20 μηι and 200 μηι.

9. The method as claimed in any preceding claim, wherein a characteristic length scale of the interference pattern is between 2 μηι and 20 μηι.

10. The method as claimed in any preceding claim, comprising illuminating just one particle of interest at a time.

1 1. The method as claimed in any preceding claim, comprising the step of discarding or disregarding hyperspectral data corresponding to radiation detected when more than one particle is simultaneously present in the probe volume.

12. The method as claimed in any preceding claim, comprising determining at least one property of the particle other than its velocity, speed, or direction of motion.

13. The method as claimed in any preceding claim, comprising determining a speed of the particle using Laser Doppler Velocimetry.

14. The method as claimed in any preceding claim, comprising illuminating a single particle in the probe volume by the interference pattern, thereby causing the particle to scatter the first electromagnetic radiation, thereby producing scattered first radiation.

15. The method as claimed in claim 14, comprising discarding data

corresponding to a particle if a recorded intensity of the scattered first radiation exhibits fewer than a threshold number of peaks.

16. The method as claimed in any preceding claim, wherein the first source is substantially monochromatic.

17. The method as claimed in any preceding claim, wherein the first electromagnetic radiation has a wavelength between 300 nm and 500 nm.

18. The method as claimed in any preceding claim, wherein the method is applied in situ in the monitored environment.

19. The method as claimed in any preceding claim, wherein the apparatus is mounted on a moving vehicle, and wherein the apparatus is positioned and configured so that the interference pattern comprises fringes aligned substantially perpendicular to a direction of propulsion of the vehicle.

20. The method as claimed in any preceding claim, further comprising counting particles.

21. The method as claimed in any preceding claim, comprising separating the detected radiation according to its wavelength.

22. The method as claimed in any preceding claim, comprising separating the detected radiation into a portion for speed analysis and a portion for hyperspectral analysis.

23. The method as claimed in any preceding claim, comprising displaying data on a user interface in real time.

24. An apparatus for characterising individual particles, the apparatus comprising:

a second source of second electromagnetic radiation having a range of different wavelengths arranged to produce one or more beams and to illuminate a single particle in the probe volume to cause said single particle to scatter the second electromagnetic radiation to produce scattered radiation;

25. The apparatus as claimed in claim 24, wherein the second source has a wavelength range greater than 500 nm.

26. The apparatus as claimed in claim 25, wherein the second source is a supercontinuum light source.

27. The apparatus as claimed in claim 24, 25 or 26 wherein the wavelength range of the detector covers all or substantially all of the visible electromagnetic spectrum.

28. The apparatus as claimed in any of claims 24 to 27, wherein the

hyperspectral detector has a spacing of measured wavelength bands separated by less than 10 nm.

29. The apparatus as claimed in any of claims 24 to 28, wherein the second source comprises a laser arranged to produce a laser beam that is focussed on the probe volume.

30. The apparatus as claimed in any of claims 24 to 29, wherein the probe volume has a maximum dimension that is smaller than a mean separation between particles in the aquatic mass.

31. The apparatus as claimed in any of claims 24 to 30, wherein the size of the probe volume is between 20 μηι and 200 μηι.

32. The apparatus as claimed in any of claims 24 to 31 , wherein a characteristic length scale of the interference pattern is between 2 μηι and 20 μηι.

33. The apparatus as claimed in any of claims 24 to 32, configured to discard or disregard hyperspectral data corresponding to radiation detected when more than one particle is simultaneously present in the probe volume.

34. The apparatus as claimed in any of claims 24 to 33, configured to determine at least one property of the particle other than its velocity, speed, or direction of motion.

35. The apparatus as claimed in any of claims 24 to 34, comprising a Laser Doppler Velocimeter arranged to determine a speed of the particle using Laser Doppler Velocimetry.

36. The apparatus as claimed in any of claims 24 to 35, wherein a single particle in the probe volume is illuminated by the interference pattern, thereby causing the particle to scatter the first electromagnetic radiation, thereby producing scattered first radiation.

37. The apparatus as claimed in any of claims 24 to 36, configured to discard data corresponding to a particle if a recorded intensity of the scattered first radiation exhibits fewer than a threshold number of peaks.

38. The apparatus as claimed in any of claims 24 to 37, wherein the first source is substantially monochromatic.

39. The apparatus as claimed in any of claims 24 to 38, wherein the first electromagnetic radiation has a wavelength between 300 nm and 500 nm.

40. The apparatus as claimed in any of claims 24 to 39, wherein the apparatus is operable in situ in the monitored environment.

41. The apparatus as claimed in any of claims 24 to 40, wherein the apparatus is mounted on a moving vehicle, and wherein the apparatus is positioned and configured so that the interference pattern comprises fringes aligned substantially perpendicular to a direction of propulsion of the vehicle.

42. The apparatus as claimed in any of claims 24 to 41 , further arranged to count particles.

43. The apparatus as claimed in any of claims 24 to 42, arranged to separate the detected radiation according to its wavelength.

44. The apparatus as claimed in any of claims 24 to 43, arranged to separate the detected radiation into a portion for speed analysis and a portion for hyperspectral analysis.

45. The apparatus as claimed in any of claims 24 to 44, comprising a user interface arranged to display data in real time.