AU2023201469A1

AU2023201469A1 - Crystalline Particle Detection

Info

Publication number: AU2023201469A1
Application number: AU2023201469A
Authority: AU
Inventors: Debabrata Bhattacharyya; Joseph Lord
Original assignee: TROLEX Ltd
Current assignee: TROLEX Ltd
Priority date: 2022-04-29
Filing date: 2023-03-09
Publication date: 2023-11-16
Also published as: GB202206267D0

Abstract

There is described herein a method for determining the presence of a crystalline particle in a sample comprising a plurality of particles, the method comprising 5 illuminating the sample with a polarized radiation beam having a first polarisation state, the polarized radiation beam propagating along an optical axis and illuminating the sample at a sensing volume, detecting a portion of the polarized radiation beam that has been transmitted through a crystalline particle in the sample and which has a second polarisation state, wherein the portion of the polarized radiation beam 10 comprises a portion of an Airy pattern comprising a diffraction order greater than a zeroth order, and determining the presence of the crystalline particle based upon the detection. [Figure 2] CC 4N.O C) -4 NOD CC mn 4NO XN

Description

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Crystalline Particle Detection

Cross-reference to related applications

This application is related to PCT application PCT/GB2016/051698, filed on 8 June 2016, which claims priority from GB 1509926, filed on 8 June 2015, both of which are incorporated herein by reference.

Technical Field

The present invention relates to a method for determining the presence of a crystalline particle in a sample comprising a plurality of particles. More particularly, but not exclusively, the present invention relates to a method for determining whether a particle in a sample is a Respirable Crystalline Silica (RCS) particle.

Background

Crystalline particles such as RCS, quartz and calcite are a major occupational health and safety issue in industries such as mining, sandblasting, foundry work, agriculture, and construction. Minute shard-like particles of RCS (or other crystalline particles) can be carried in air currents for considerable distances and, if inhaled, are small enough to enter the deepest parts of the lung (alveoli) where they can become trapped. The resistance of crystalline particles to the body's attempts to remove them or chemically break them down means that they remain in the lungs for considerable periods, during which time they continue to cause irritation and damage.

Fracking operations have in recent times seen substantial and rapid expansion and have raised new concerns over the release of RCS into the atmosphere around fracking sites and the consequent potential for not only worker exposure but also, because of the proximity of many sites to residential areas, members of the general public. Fracking involves high pressure injection of large volumes of water and sand, and smaller quantities of well treatment chemicals, into a gas or oil well to fracture shale or other rock formations, allowing more efficient recovery of hydrocarbons from a petroleum-bearing reservoir. The generation of RCS may occur throughout the fracking process, from the initial delivery of the bulk fracking sand by road or rail, through to the mechanical unloading and storage of the fracking sand, and to the ultimate mixing of the sand with water and treatment chemicals. RCS is a significant health hazard if inhaled, and can cause health problems such as silicosis of the lungs and a variety of other life threatening conditions.

PCT/GB2016/051698 describes an apparatus for detecting crystalline particles such as RCS. In particular, the apparatus uses the optical scattering and birefringent properties of crystalline particles to detect the presence of said particles in a sample of ambient airborne particles. Ambient airborne particles, which may or may not contain crystalline particles, are drawn by an air pump into the apparatus. The particles pass through a sensing volume before leaving the apparatus. The apparatus comprises a diode laser module and a first polarizer, configured to produce a polarized laser beam. The polarized laser beam intersects particles at the sensing volume.

The apparatus comprises a birefringence detector arranged along the laser beam axis and a second polarizer positioned between the sensing volume and the birefringence detector. The second polarizer is arranged to filter out any radiation in the polarized beam which has not experienced a change in polarisation. In the absence of anisotropic crystalline particles, the polarised laser beam, having passed through the first polarizer, will be incident on, but will generally not be transmitted through the second polarizer such that the laser beam will not reach the birefringent detector. However, if the polarised laser beam is incident on, and refracted through, an anisotropic crystalline particle, such as RCS particle or calcite, a component of the refracted laser beam's polarisation will be modified as it is subjected to double refraction when traversing through the anisotropic crystalline particle. As such, the modified polarized laser scattered beam will be transmitted through the second polarizer and is incident on the birefringence detector, where the birefringence detector will register a detection of photon radiation. Detection of photon radiation incident on the birefringence detector causes an output electrical signal to be generated indicating that a particle is birefringent.

The apparatus further comprises two scatter detectors located away from the laser beam and configured to collect light which has scattered from a particle at an angle of, for example, between 60 and 90 degrees from the laser beam axis. In the event that the laser beam is incident on asymmetrical (e.g. non-spherical, irregular shape) crystalline particles, such as RCS, the radiation will generally be scattered asymmetrically such that a detection of more radiation in one of the scatter detectors than the other indicates that the radiation has been scattered by a particle in an asymmetrical manner and hence has an asymmetrical shape.

The apparatus may determine that a sample contains RCS based on measurements from the scatter detectors and the birefringence detector. The apparatus can operate in real-time so as to continuously monitor levels of RCS. By detecting levels of RCS in real-time, warnings of exceedance can be provided almost immediately allowing personnel to take protective measures such as putting on protective equipment or evacuating the work site.

It would be beneficial to provide an improved method and apparatus for detecting crystalline particles, such as RCS.

A reference herein to a patent document or any other matter identified as prior art, is not to be taken as an admission that the document or other matter was known or that the information it contains was part of the common general knowledge as at the priority date of any of the claims.

Summary of the Invention

In a first aspect of the invention there is provided a method for determining the presence of a crystalline particle in a sample comprising a plurality of particles, the method comprising, illuminating the sample with a polarized radiation beam having a first polarisation state, the polarized radiation beam propagating along an optical axis and illuminating the sample at a sensing volume, detecting a portion of the polarized radiation beam that has been transmitted through a crystalline particle in the sample and which has a second polarisation state, wherein the portion of the polarized radiation beam comprises a portion of an Airy pattern comprising a diffraction order greater than a zeroth order, and determining the presence of the crystalline particle based upon the detection.

It has been found that crystalline particles (which are typically anisotropic), such as calcite, quartz and RCS, can be detected based on detecting photon radiation that has propagated through the crystalline particle. In particular, the intrinsic birefringent properties of anisotropic crystalline particles cause a change in polarisation of radiation that is transmitted though said crystalline particles. For example, when an anisotropic crystalline particle is illuminated by a polarised radiation beam, such as an elliptically polarised laser beam, a portion of the beam is transmitted (and double refracted) through the crystalline particle, where two mutually perpendicular polarisation states of the radiation beam are generated as ordinary and extraordinary rays. Detecting a change in the polarisation state of the radiation beam (which is extraordinary rays) can therefore be used to identify crystalline particles.

The radiation beam may comprise a laser beam. Typically, an incident polarized radiation beam used to illuminate the crystalline particles has a larger beam waist than the crystalline particle which results in weak output signal (e.g. a detected change in polarization) due to the relatively small cross-section of the particle. As such, any detected signal indicating a change in polarisation that lies on the optical axis (e.g. within a region containing the zeroth order) may be masked by the original radiation beam. This can be true even when using an appropriate polarisation filter. For example, if a polarisation filter which is configured to block radiation having the same polarisation state as the original polarised radiation beam (e.g. the first polarisation state) is placed in front of a detector, a portion of the radiation having the first polarisation state will still be transmitted through the filter as such filters are not 100% efficient. Therefore, attempting to detect a change in polarisation state on the optical axis of the original laser beam can lead to a relatively low signal to noise ratio and may give rise to false positive or negative events, therefore, lower accuracy when determining the presence of a crystalline particle.

It is therefore advantageous to look for a detection signal which does not coincide with the optical axis of the original radiation beam (e.g. the optical axis of the incident laser beam). In particular, it has been found by the inventors that crystalline particles can be detected based on Airy patterns, and in particular based on a portion of the Airy pattern at the first diffraction order or greater, as higher order diffraction orders are off the optical axis. Furthermore, additional material properties of the crystalline particle (e.g. the size and shape of the crystalline particle) can be evaluated based on said portion of the Airy pattern.

The sample may be a sample of ambient airborne particles. The sample may be drawn by an air pump or the like so as to pass through the sensing volume.

The first polarisation state may be a first linear, circular or elliptical polarisation and the second polarisation state may a second linear, circular or elliptical polarisation, the first polarisation being different from the second. The first and second polarisation states may be the same type of polarisation (e.g. both may be elliptical) but may both have different states (e.g. opposite states).

Detecting a portion of the polarized radiation beam may comprise receiving data generated based upon the portion of the polarized radiation beam landing on a detector. The data may be an electrical output from a photodetector.

The portion of the Airy pattern may comprise the first diffraction order of the Airy pattern.

Using the first order of diffraction may provide a high signal to noise compared to the higher diffraction orders, whilst also providing additional information regarding physical properties of the crystalline particle. This is in contrast to using the zeroth diffraction order which, while higher in intensity than the first diffraction order, overlaps with the optical axis of the incident polarized radiation beam with which the crystalline particle has been illuminated with, so contains significant optical noise. Furthermore, zeroth order for each crystalline particle is substantially an overlap of maximum peak intensity, regardless of the particle's physical properties, whereas the maxima and minima of the higher diffraction orders are shifted radially on an image plane depending on the properties (e.g. size) of the crystalline particle.

The portion of the Airy pattern may not comprise the zeroth order of the Airy pattern. That is, the detected portion of the Airy pattern may correspond to a portion that does not contain the zeroth order (or at least does not contain a substantial portion of the zeroth order).

Detecting the portion of the polarized radiation beam may comprise detecting the portion of the polarised radiation beam at a position offset from the optical axis.

For example, the position may coincide with a position comprising the portion of the Airy pattern comprising a diffraction order greater than the zeroth order. For example, a detector may be placed or otherwise configured to detect the portion of the polarised radiation beam at the position offset from the optical axis. The detector may output data indicative of the portion of the diffraction pattern. That is, radiation incident on the detector may cause an electric signal to be generated and output which can then be analysed. The position offset from the optical axis may be offset significantly enough that the original radiation beam travelling along the optical axis is not collected by a detector placed at the position. The magnitude of said offset may depend on the collection area (e.g. size of the area capable of collecting radiation) of the detector. The position offset from the optical axis may be offset from the optical axis by an angle greater than the solid angle in which the radiation beam propagates.

The method may comprise blocking radiation propagating along the optical axis after the sensing volume.

As used herein, the phrase "after the sensing volume" is intended to mean between the sensing volume and a detector (or a plane in which a detector sits, such a plane being orthogonal to the optical axis). In other words, the original radiation beam propagating along the optical axis is substantially blocked after the radiation beam has illuminated the particles at the sensing volume.

Blocking may comprise absorption, reflection or any other means of inhibiting onwards propagation of the radiation beam along the optical axis. Blocking radiation propagating along the optical axis reduces the amount of unwanted stray radiation which may be otherwise collected by a detector, thereby further increasing signal to noise ratio of the data. Radiation may be blocked over a solid angle which is approximately equal to the beam waist (e.g. the 1/e, 1/e2 or FWHM beam size) of the original radiation beam in the plane in which the blocking occurs.

Blocking radiation propagating along the optical axis after the sensing volume may comprise providing a beam stop on the optical axis after the sensing volume.

A beam stop may absorb radiation. Alternatively, a beam stop may reflect radiation away towards a safe location in which the radiation cannot further propagate in the system, for example such that it cannot be incident upon a detector. Providing a beam stop provides a relatively simple method to reduce noise associated with "leaked" radiation. In known systems, radiation propagating along the optical axis after the sensing volume may be reduced by providing filters, for example polarizing filters configured to prevent radiation having the first polarisation from being transmitted through said filter. However, such filters are not perfect absorbers and a proportion of radiation having the first polarisation will continue to propagate along the optical axis after passing through such filters. By providing a beam stop, the proportion of unwanted radiation that continues to propagate beyond the filter can be inhibited from onward propagation.

The method may further comprise controlling the amount of radiation to be blocked by selecting a position of the beam stop.

Due to the diverging nature (Gaussian) of laser beams, by positioning the beam stop on the optical axis closer towards the sensing volume, or further away from the sensing volume, the solid angle of radiation which is blocked may be controlled. In this way, a controllable blocking means can be provided.

By controlling the solid angle of radiation which is blocked, more or less of the Airy pattern will also be blocked. The radial size and location of the first and higher orders of the Airy pattern are dependent on the size of the crystalline particle. As such, it may be beneficial to select the solid angle of radiation which is to be blocked in dependence on the size (e.g. the expected size) of the crystalline particle being detected. Furthermore, each successive diffraction order of the diffraction pattern is radially spaced from a preceding order. As such, it may be beneficial to select the solid angle of radiation to be blocked in dependence on the portion of the diffraction pattern which is to be used for determining the presence of the crystalline particle.

Controlling the amount of radiation to be blocked may comprise selecting a position of the beam stop and arranging the beam stop in that position. Controlling the amount of radiation may comprise moving the beam stop along the optical axis from a first position to a second position.

Detecting the portion of the polarised radiation beam at the position offset from the optical axis may comprise configuring a detector to detect the portion of the polarised radiation beam.

The detector may be any suitable detector capable of detecting radiation. For example, the detector (i.e. an effective surface area of the detector that receives radiation and coverts to an electrical signal) may be placed at the position offset from the optical axis.

Configuring the detector to detect the portion of the polarised radiation beam may comprise providing the detector within an image plane orthogonal to the optical axis at the position offset from the optical axis.

The position offset from the optical axis may correspond to an expected maximum of the first diffraction order of the Airy pattern, or a higher diffraction order of the Airy pattern. The plane may be an image plane.

By collecting radiation at a maximum of a diffraction order, the signal to noise ratio may be increased, thereby resulting in more accurate determinations of the presence of a crystalline particle.

The intensity and linewidth of an Airy pattern (e.g. the peak maxima of its diffraction orders) depend at least in part on the size of a crystalline particle that generated the Airy pattern. In some applications, it may be beneficial to determine the presence of a crystalline particle of a particular size (or range of sizes). For example, it may be known or assumed that the crystalline particles in a particular sample have a particular size (or range of sizes). In such instances, the approximate profile Airy pattern produced by radiation being transmitted through such crystalline particles may be known theoretically. As such, the detector can be provided (e.g. positioned) at a position which corresponds to a particular portion of the diffraction pattern, wherein that particular portion corresponds to the maximum of a diffraction order (a first or higher diffraction order) of the known Airy pattern.

The position offset from the optical axis may be at a position in which an intensity of the portion of the polarized radiation beam is maximised.

For example, a detector may be positioned at a location where the intensity of radiation is greater than any other position within that plane. Such a location may correspond to a maximum of a diffraction order, for example a maximum of the first diffraction order, of the Airy pattern. The method can therefore be used to increase the signal to noise by moving a detector to a position in which an increased signal is detected. This process may be particularly useful when the size of crystalline particles is unknown, such that the location of a maximum is not theoretically known. In other words, the detector may be moved between different positions over time to determine a location which corresponds to a maximum of a diffraction order.

The method may further comprise determining a size of the crystalline particle based upon the position.

The relationship between the size of a particle and the profile of the diffraction pattern formed by radiation transmitted through the particle is known (or can be theoretically derived). When the detector is located at a maximum, this may correspond to a maximum of an order of an Airy pattern for a particular crystalline particle. By comparing the location of the detector to the location of known maxima of Airy patterns of a range of sizes, the size of a particle which has generated such an Airy pattern may be determined. As such, using this method, the size of the crystalline particle can be determined. The size may be determined based on a number of detections of crystalline particles. That is, an average size and/or size distribution may be determined based on a number of detections of different crystalline particles in the sample.

The method may further comprise configuring the detector to be moveable between two or more positions within the plane orthogonal to the optical axis.

By configuring the detector to moveable between two or more positions, the detector can be moved within the plane. The detector can be used to collect radiation (or the absence thereof) from a variety of different portions of the Airy pattern. Alternatively, by moving the detector within this plane, the detector can be adjusted for the detection of different sizes of crystalline particles.

Moving the detector while the sample is being illuminated allows a determination to be made as to the position in which the intensity of the portion of the polarized radiation beam is maximised at a position off-axis.

The movement may be performed as part of a calibration step either manually or automatically. For example, an apparatus arranged to carry out the method may be calibrated to detect a particular particle having a particular size. Particles of said size may be illuminated and the location in which the intensity of the portion of the polarized radiation beam is maximised may be determined by moving the detector between multiple positions on the plane.

Alternatively, the movement may be automatic during normal use. That is, the detector may be moved multiple times over a period of time when carrying out normal observations. The movement may be provided in response to data output from the detector. The movement may be predefined, for example the movement may be between two positions covering a range of size off particles.

The method may further comprise moving the detector between the two or more positions within the plane orthogonal to the optical axis.

Detecting the portion of the polarised radiation beam at the position offset from the optical axis may comprise detecting a first portion of the polarised radiation beam at a first position offset from the optical axis and detecting a second portion of the polarised radiation beam at a second position offset from the optical axis.

For example, the detector may comprise a first and second detector, therein the first detector is placed at the first position and the second detector is placed at the second position. For example, the second detector may output second data indicative of the portion of the diffraction pattern. That is, radiation incident on the second detector may cause an electric signal to be generated and output which can then be analysed. The first and, if any, second data output by the first and second detectors may be analysed to determine a characteristic of the crystalline particle. The second detector may be provided in the same plane orthogonal to the optical axis as the first detector is provided in, and is offset from the optical axis in a manner which is symmetrical to the offset of the first detector. As such, the first and second detectors may be arranged to collect radiation associated with opposing portions of the Airy pattern. While first and second positions are described, it will be appreciated that many more positions may be monitored.

If the first detector is configured to move within the plane as described above, the second detector may also be configured to move within the plane. The movement of the first detector may mirror the movement of the second detector.

The first position may be located at a position having the same radial distance from the optical axis as the second position.

The first and second positions may be symmetrically opposed about the optical axis.

The method may further comprise determining a characteristic of the crystalline particle based on a comparison of the detected first portion and second portion.

That is, radiation intensity detected at the first position may be compared with radiation intensity detected at the second position. The characteristic may be a shape of the particle. For example, a symmetry or lack thereof, of the Airy pattern may be determined. The presence of symmetry may indicate the crystalline particle is spherical, whereas the absence of symmetry may indicate that the crystalline particle is non-spherical.

An amorphous or isotropic crystalline particle may be considered generally symmetrical in shape, for example it may have spherical geometry giving a major and minor axis which are similar. Asymmetric crystalline particles shape, for example non-spherical, may have major and minor axis which are substantially different. An amorphous or isotropic spherical crystalline particle may produce a generally symmetrical diffraction pattern when radiation is transmitted through it. An asymmetric crystalline particle may produce a diffraction pattern which is asymmetrical. By providing a first and second detector at symmetrically opposed locations, a symmetry of the diffraction pattern may be measured and used to determine the shape of the particle. Asymmetrical particles having polygon shape with sharp corners can be particularly hazardous, for example they may do increased damage to the lungs if inhaled, so identifying asymmetrical particles may be particularly advantageous.

The characteristic may be an indication of shape of the crystalline particle.

The method may further comprise outputting a warning when the presence of the crystalline particle is determined.

For example, an audible and/or visual warning may be output by a speaker and/or device screen when crystalline particles are detected. The warning may be output when a threshold number of crystalline particles have been detected. The warning may be output when a threshold number of crystalline particles have been detected within a specific time frame.

In a second aspect of the invention there is provided a computer-readable medium comprising instructions which, when the program is executed by a computer, causes the computer to carry out the method as described herein.

In a third aspect of the invention there is provided a system for determining the presence of a crystalline particle in a sample comprising a plurality of particles, the system comprising: a radiation emitter configured to emit a polarized radiation beam along an optical axis, a sensing volume configured to: receive the sample and receive the polarized radiation beam thereby enabling illumination of the sample within the sensing volume with the polarized radiation beam, and a detector configured to detect a portion of the polarized radiation beam that has been transmitted through a crystalline particle in the sample and which has a second polarisation state, wherein the portion of the polarized radiation beam corresponds to a portion of an Airy pattern comprising a diffraction order greater than a zeroth order, and a processor configured to receive data output by the detector, said data indicative of the portion of the polarized radiation beam, the processor further configured to determine the presence of the crystalline particle based on the data.

Various optional features of each aspect may be combined with one another. For example, the system of the third aspect may be configured to carry out any optional features of the first aspect.

Brief Description of Drawings

The invention will now be described, by way of non-limiting example only, with reference to the following Figures, in which:

Figure 1A schematically illustrates an Airy pattern; Figure 1B depicts the radial intensity of the Airy pattern of Figure 1A; Figure 2 schematically illustrates a sampling and detection apparatus; Figure 3 illustrates a method for determining the presence of a crystalline particle in a sample comprising a plurality of particles;

Figure 4 depicts the radial intensity of two Airy patterns associated with different sized particles; and Figure 5 depicts a schematic illustration of a device suitable for use in the apparatus of Figure 2.

Detailed Description

As described above, when a polarized radiation beam, such as a polarized laser beam, is incident on and refracted through an anisotropic crystalline particle (e.g. a birefringent particle such as RCS), a component of the laser beam's polarization will be modified as it is refracted through the anisotropic crystalline particle. In particular, the laser beam undergoes double refraction (e.g. birefringence) as it travels through anisotropic crystalline particles. The modified component of the laser beam can be imaged onto a detector, for example using an imaging lens. When radiation travels through an aperture such as the anisotropic crystalline particle, diffraction occurs. As such, an image formed by the modified polarized laser beam which has refracted through the anisotropic crystalline particle comprises a diffraction pattern in an image plane (i.e. in the far field). Note reference herein to radiation means photon radiation. The photon radiation may be light in the visible spectrum.

Figure 1A schematically depicts a diffraction pattern 10 formed when radiation which has refracted through an anisotropic crystalline particle is imaged in the far field. The diffraction pattern 10 is known as an Airy pattern 10. The Airy pattern 10 comprises regions of high intensity (referred to as maxima) and low intensity (referred to as minima) corresponding to different diffraction orders and can be generally represented using Bessel functions. The Airy pattern 10 comprises a central spot 100, typically referred to as an Airy disk 100, which corresponds to the zeroth order 100. The Airy pattern 10 also comprises concentric rings 101, 102 which correspond to the first diffraction order 101 and second diffraction order 102 respectively. The intensity of each diffraction order decreases compared to the previous order. In Figure 1A, three diffraction orders (i.e. the zeroth, first and second diffraction orders 100, 101, 102) are depicted but the Airy pattern 10 may comprise any number of diffraction orders. In fact, typically there are many more diffraction orders present than can be detected by the human eye. For simplicity, diffraction orders may be referred to simply as orders (i.e. the first order rather than the first diffraction order).

The size, shape, intensity and location (e.g. radial spacing from the central Airy disk) of an Airy pattern may be referred to as the profile of the Airy pattern. The profile typically depends on the size and shape of the particle from which the Airy pattern is formed.

Figure 1B depicts the radial intensity of the Airy pattern 10 in a radial direction (i.e. along the dashed line A in Figure 1A). The intensity graph follows a series of peaks, corresponding to different orders of the Airy pattern. The peaks each have a central maximum. Each order is typically described at being located at a radial distance (i.e. angular distance in the radial direction from the nominal centre of an image plane containing the Airy pattern) at which the maximum intensity of that order is located. The radial distance may be referred to as the radial angle or scattered angle. A central peak of high intensity, centred at the radial distance of 0, corresponds to the zeroth order 100. Either side of the peak corresponding to the zeroth order 100 there are smaller peaks corresponding to the first order 101 (centred at a radial distance of approximately 50) and the second order 102 (centred at a radial distance of approximately 8). The intensity of the zeroth order 100 is significantly higher than the intensity of the first order 101. Similarly, the intensity of the first order 101 is higher than the intensity of the second order 102.

The present invention is based upon the realisation that using diffraction orders higher than the zeroth order can be used in the classification of particles as crystalline particles. In particular, using diffraction orders higher than the zeroth order can be used to classify particles as crystalline particles with a higher accuracy than prior methods. Furthermore, by using diffraction orders higher than the zeroth order, additional information can be gained regarding the physical properties of the crystalline particles.

Figure 2 schematically illustrates a sampling and detection apparatus 200, also referred to as an apparatus or a system, for determining the presence of crystalline particles, such as RCS. In use, the apparatus 200 may be deployed in a region that may contain RCS, such as a fracking worksite. A sample of ambient airborne particles 201, which may or may not contain crystalline particles, are drawn by an air pump (not shown) into the apparatus 200 through a sample delivery tube (not shown). The air pump may be replaced and/or further enhanced by any suitable means that allows air to move through the apparatus 200, such as a fan or cyclone sampler. The sample of particles 201 passes through a sample location 202, which may be referred to as a 'sensing volume' 202. The particles 201 then leave the apparatus through a vent tube

(not shown). The direction of flow of particles 201 in the sample is indicated by arrows Z.

To ensure the particles 201 in the 'sample flow' are constrained when passing through the apparatus 200, a so-called 'sheath-flow' (not shown) of filtered clean air may be added to surround the particle flow. A sheath flow helps to prevent particles 201 at the edges of the flow being drawn out of the flow due to turbulence and into the chamber where they can contaminate, and over time, degrade the optical surfaces. The combined sample and sheath flows may be aerodynamically configured to ensure that particles 201 contained within the sample flow are essentially travelling in single file in the direction of flow. The skilled person will of course recognise that there are other ways to configure the sample flow such that the particles 201 contained within the sample flow travel in single file, where some of these ways do not require a sheath flow.

A polarized radiation source 204 produces a polarized radiation beam comprising polarized radiation, which is referred to herein as a polarized beam. In a preferred embodiment, the polarized radiation source 204 is a laser which produces polarized radiation. Note that the polarised radiation source 204 may be replaced with a suitable polarising element used in conjunction with an unpolarized radiation source. The polarised radiation may be linearly, circularly or elliptically polarized. The polarized beam is directed to a collimating lens 206, and subsequently to the sensing volume 202. As such, the polarized beam illuminates the particles 201 at the sensing volume 202. It should be understood that, because the particles 201 essentially travel in single file, at any one point in time the polarized beam may illuminate only a single particle 201, but over time a plurality of particles 201 in the sample will be illuminated. The sensing volume 202 is not fixed in space, but rather is defined by the intersection of the polarized beam and the sample flow of particles 201. The radiation beam has an optical axis 207, indicated by a dash-dotted line, along which the polarized beam propagates.

The polarized beam may be otherwise modified prior to arriving at the sensing volume 202. For example, one or more lenses, mirrors, prisms may be used to modify the beam shape and/or size. The beam may be shaped into a thin ribbon shape, which may be elliptical in cross-section. The cross-section may have any suitable dimensions.

In an example, the dimensions are 1 mm by 0.5 mm at the focus, which is coincident with the cylindrical sample flow of airborne particles.

Anisotropic crystalline particles, such as RCS, are intrinsically birefringent. As such, radiation which has propagated through an anisotropic crystalline particle by refraction can be altered due to the birefringent properties of the crystalline particle, as described above. In particular, when polarized radiation is transmitted through an anisotropic crystalline particle, at least a portion of the radiation's polarization is changed due to double refraction arising from refractive index variation along the crystal axes. Therefore, detecting the change in polarisation state of the polarized laser beam can be used to identify anisotropic crystalline particles.

In more detail, when an anisotropic crystalline particle is illuminated by a polarized radiation beam, a portion of the beam is transmitted (double refracted) through the crystalline particle, forming two distinct radiation beams with mutually perpendicular polarization states. The two mutually perpendicular polarization states of the radiation output are generated under the condition of the incident beam propagating along non equivalent crystal axes (e.g. at least one axis not being along the optical axis 207). Phase retardance arises between the two beams, which are referred to as ordinary and extraordinary rays.

A polarizing filter 208 is arranged to receive the polarized beam after it has passed through the sensing volume 202. The polarizing filter 208 is arranged to filter the same polarization as the radiation generated by the polarized radiation source 204. For example, if the polarized radiation source 204 produces radiation having a first polarization, the polarizing filter 208 is arranged to filter radiation having the first polarization (e.g. the same polarization as what is generated by the source 204). As such, any component of the polarized beam which has travelled through the sensing volume 202 and has not experienced a change in polarization will generally be blocked by the polarizing filter 208. However, if the polarized beam propagates through an anisotropic crystalline particle, at least a portion of the beam (e.g., extraordinary rays) will change polarization state and hence be transmitted through the polarizing filter 208. Therefore, the detection of radiation which has travelled through the polarizing filter 208 typically corresponds to presence of an anisotropic crystalline particle in the sensing volume 202.

Polarizing filters are typically not 100% effective. That is, a small portion of the polarized beam which has not changed polarization state may still be transmitted through the polarizing filter 208. This can be referred to as optical leakage. Such leakage can cause a significant signal at a detector positioned on the optical axis 207 to collect radiation which has travelled through the polarizing filter 208. The particles 201 are typically small compared to the polarized radiation beam, for example their radius may be significantly smaller than the incident beam waist at the sensing volume 202. As a result, only a small portion of the polarized beam is transmitted through a crystalline particle and hence any detected signal due to the radiation transmitted through the crystalline particle will be weak. As such, even a small amount of optical leakage through the polarizing filter 208 may result in a detected signal due to optical leakage which overwhelms any signal due to a change in polarized radiation. The optical leakage therefore increases the noise in any form of detection. Said optical leakage hence reduces the signal to noise ratio of the apparatus 200. This can lead to false positive events where the detection of a signal after the polarizing filter 208 does not correspond to the presence of an anisotropic crystalline particle in the sensing volume 202, but instead corresponds to optical leakage. The optical leakage typically presents itself as a low-frequency DC signal at a detector, and can hence be removed, for example electronically. However, due to the low relative amplitude of any signal due to radiation with a changed polarization state (e.g. radiation that indicates birefringence and hence RCS), such signals may be erroneously removed, leading to false negative events.

In order to improve the detection of anisotropic crystalline particles, in the apparatus 200, a detector 210 is provided that is configured to detect a portion of an Airy pattern comprising a diffraction order greater than a zeroth order. For example, the detector 210 may be offset from the optical axis 207 so as to detect the first order. The detector 210 is positioned after the polarizing filter 208 (i.e. downstream along the optical axis 207) such that the detector 210 can receive radiation which has passed through the polarizing filter 208). In the particular example shown, the detector 210 is positioned such that it is offset from the optical axis 207 so as to receive radiation which is travelling at an angle with respect to the optical axis 207. The detector's 210 positioning may be referred to as offset or off-axis.

As described above and with reference to Figures 1A and 1B, when radiation travels through a crystalline particle, an Airy pattern 219 is formed. The Airy pattern 219 has a significant radial extent (which contains the first order 221 and orders higher than the first order 221) in addition to the central Airy disk (which comprises the zeroth order 220). As such, the detector 210 in its off-axis position can detect portions of the radial extent of the Airy pattern 219 (that is, portions of the Airy pattern 219 other than the zeroth order 220), for example the orders higher than the zeroth order 220.

In the example shown in Figure 2, the detector 210 is positioned so as to detect a portion of the first order 221 of the Airy pattern 219. However, in other arrangements, the detector 210 can be positioned so as to detect any other portion of the Airy pattern 219, such as the second order.

While the zeroth order 220 of the Airy pattern 219 has the highest intensity compared to other orders in the Airy pattern, it (at least partially) spatially overlaps with the high intensity polarized radiation beam generated by the laser 204. That is, the zeroth order 220 spatially overlaps any radiation of the unperturbed laser beam that has leaked through the polarizing filter 208 along the optical axis 207. Therefore, despite the high intensity of the zeroth order, the location at which the zeroth order is detected (e.g. along the optical axis 207) can lead to a relatively poor signal to noise ratio. Preferably, the detector 210 should be arranged so as to detect any order higher than the zeroth order 220, as higher orders are not as affected by leakage along the optical axis 207. Any order of diffraction greater than the zeroth order 220 may be used, for example the first, second, third or fourth order. In a preferable arrangement, however, and as illustrated in Figure 2, the first order 221 is used (that is, the signal recorded by the detector 210 corresponds to a portion of the Airy pattern 219 which comprises the first order 221). The first order 221 is typically higher in intensity compared to higher orders (second, third, etc. orders). As such, detecting the first order 221 can result in a stronger signal compared to using the second or higher orders.

The polarized radiation beam from the polarized source 204 travels along the optical axis 207 and typically does not deviate therefrom. Therefore, by positioning the detector 210 off-axis, it can preferentially detect radiation from the first order 221 and not detect leaked radiation from the optical leakage of the unperturbed polarized laser beam (or at least detect a significantly reduced amount of leaked radiation). In other words, by positioning the detector 210 off-axis, it can preferentially detect radiation which has interacted with a crystalline particle and not detect radiation which has not interacted with a crystalline particle (or at least detect a significantly reduced amount of radiation which has not interacted with a crystalline particle). This preferential detection of radiation in an off-axis position can increase the signal to noise ratio of the apparatus 200 and enhance the accuracy of the method over prior art methods. The detector 210 may be offset from the optical axis 207 by an angle greater than the solid angle in which the polarized laser beam propagates such that, little to no leaked radiation will be detected.

The detection of a crystalline particle using polarized radiation can depend on the orientation of its crystal planes, because the extraordinary ray is generated under the condition of the polarized beam being propagated along an axis not equivalent to the crystalline particle's optical axis (referred to herein as its crystalline axis). However, particle motion under gravity and air drag plays a key role in selecting the crystal plane of interest. When a particle falls under gravity, its trajectory is dispersed and subjected to turbulence (Stokes' law) owing to Brownian motion. Since the particle is travelling in free space in an ambient environment, its crystalline axis cannot be fixed at a particular orientation relative to the optical axis 207. Furthermore, its crystalline axis can be oscillatory in nature over a period of time. Control of the sheath flow may partially restrict the lateral dispersion due to hydrodynamic focussing, but rotation of the particle may still occur during free fall. Such rotation may be particularly unpredictable due to its random nature for inhomogeneous particles of irregular shape (i.e. asymmetric particles). A 45 angle between incident beam and crystalline axis of a crystalline particle provides an ideal condition to produce polarised radiation output with a maximum intensity. With other relative angles between the optical axis 207 and the crystalline axis, the polarized radiation intensity may be reduced. Therefore, it is especially advantageous to provide a method as described herein which can increase the signal to noise ratio. This is achieved by providing (e.g. placing) the detector 210 off-axis (or having a detector able to detect off-axis), enabling better detection of particles even if their orientation is not the ideal orientation.

Figure 3 illustrates a method 300 for determining the presence of a crystalline particle in a sample comprising a plurality of particles. The method can be used with apparatus 200 shown in Figure 2. The particles may, for example, contain one or more of the following crystalline particles: silica particles (e.g. RCS, quartz), and non-silica crystalline particles (such a calcite). The particles may, for example, contain one or more non-crystalline particles such as amorphous silica. One or more of the particles may be anisotropic. An anisotropic particle has a non-uniform refractive index profile along its crystal axes (uniaxial or biaxial). For example the value of refractive index along the z-axis is different from x- and y-axes in a uniaxial crystal. One or more of the particles may be isotropic. Isotropic particles have the same refractive index in all directions that provides a spherical wavefront after scattering unlike ellipsoidal in anisotropic material. Both isotropic and anisotropic particles may have either spherical or non-spherical shape (e.g. may be symmetric or asymmetric in shape) that depends on the material processing, grinding and environmental conditions of rock formation.

At step 301, the sample is illuminated with a polarized radiation beam having a first polarisation state, the polarized radiation beam propagating along an optical axis and illuminating the sample at a sensing volume.

At step 302, a portion of the polarized radiation beam that has been transmitted through a crystalline particle in the sample and which has a second polarisation state is detected, wherein the portion of the polarized radiation beam comprises a portion of an Airy pattern comprising a diffraction order greater than a zeroth order. The first polarization state is different from the second polarization state and where the second polarization state is generated due to the birefringent properties of an anisotropic crystalline particle that the beam has been transmitted through.

At step 303, the presence of the crystalline particle based upon the detection is determined. For example, using the apparatus 200, a detection by detector 210 of any radiation indicates that there is a birefringent particle (e.g. anisotropic crystalline particle) in the sample.

The method 300 can be performed using any processing means, for example a processor of a computer. The method can be performed in conjunction with an apparatus such as the apparatus 200 described with reference to Figure 2.

When used for monitoring purposes, the method may include additional steps such as providing an indication in response to determining the presence of a crystalline particle. The indication may, for example, be a type of alarm such as an audio or visual cue to indicate the presence of crystalline particles. Such an indication may be provided when a single detection of an anisotropic crystalline particle is made (e.g. whenever the detector 210 outputs a signal), or when a threshold signal has been exceeded, such as a predefined number of detection events within a specific time period. A graded warning system may also be provided, the output being appropriate to the levels of RCS. For example, if a relatively low level of anisotropic crystalline particles are detected, a low level warning may be output - e.g. an amber light without an audible alarm. If a relatively high level of anisotropic crystalline particles are detected, a high level warning may be output, such as a red light and audible alarm.

The Airy pattern formed by a particle is dependent on its physical properties (e.g. size, shape and orientation). For all particles, the zeroth order will appear at a central position, for example a nominal 0 upon an image plane. However, the maxima associated with higher orders (first order and higher) occur at different radial distances depending on the physical properties of the particle. Figure 4 depicts the radial intensity of two Airy patterns associated with different sized particles. A first Airy pattern 40 is associated with a particle with a radius of 10 pm. A second Airy pattern 42 is associated with a particle with a radius of 2pm. The intensities of each Airy pattern 40, 42 are shown in a radial direction from the nominal 00 upon an image plane (i.e. the centre of the image plane). The nominal 0 is typically coincident with the optical axis 207, although it may be located elsewhere as described in more detail further below. The maximum intensity of the zeroth order is not shown for either Airy pattern 40, 42, but it can be seen that the maximum for the zeroth order is significantly higher than the maximum for the higher orders.

The first Airy pattern 40 has multiple peaks centred at 4.8, 7.2, 10.70, 15.40, 19.30, 23.30 and 27.40, corresponding to the maxima of the first, second, third, fourth, fifth, sixth and seventh orders, respectively. The first Airy pattern 40 correspondingly has a first minimum at 3.50 and successive minima at 8.80, 13.10, 17.30, 21.50, 25.40 and 29.20 between the higher order peaks. The second Airy pattern 42, on the other hand, has a single peak centred at 21.30, which corresponds to the first order, and a first minimum at 14.60. Any higher orders in the second Airy pattern 42 are at a radial distance greater than the range depicted in Figure 4 (i.e. 300). By utilising this information, the physical properties of detected crystalline particles can be determined. For example, if the detector 210 was positioned (or otherwise configured to detect a signal) at a radial distance of approximately 15.40 from the nominal centre of the image plane, the detection of a signal at a radial distance of approximately 15.40 from the nominal centre of the image plane would indicate the presence of a 10 pm particle rather than a 2 pm particle, because this radial distance corresponds to a maximum (in particular the maximum of the fourth order) of the first Airy pattern 40 associated with the 10 pm particle, whereas it does not correspond to a maximum of the second Airy pattern 42 associated with the 2 pm particle.

Returning to Figure 2, the detector 210 of the apparatus 200 may be positioned at any radial distance from the optical axis 207 within a plane. The plane can be considered an image plane in which an image of an Airy disk is formed, and is typically orthogonal to the optical axis 207. The plane is typically hemispherical (i.e. it extends in a nominal x, y and z direction). However, for simplicity the plane will be described as if it were flat and extending in the nominal x-y direction indicated in Figure 2. Generally, the extent of the plane in the z-direction is small within the field of view typically used in apparatus 200 (e.g. < 90), so approximating the hemispherical plane as a two-dimensional plane provides a good approximation for typical photodetectors (e.g. detector 210).

The detector 210 can be provided at a desired location within the plane, for example as part of a manufacturing or calibration step. Additionally or alternatively, the detector 210 can be configured to be movable within the plane, for example by mounting the detector 210 on a movable stage. In this way, the detector 210 can be moved between multiple (i.e. two or more) locations within the plane. Any means of moving the detector 210 within the apparatus 200 can be used and various means will be readily apparent to the skilled person, for example a piezoelectric actuator or stepper motor.

In a first example, the detector 210 is positioned at a location which corresponds to an expected maximum for a particular type of particle. For example, it may be of interest to detect a particular size of particle. Crystalline silica particles of 4.25 pm or smaller are particularly dangerous due to their size making them more likely to be inhaled and cause damage to the lungs. As such, the detector 210 can be positioned at an angular distance which corresponds to the maximum of a first or higher order peak of silica particles of, for example, 4.25 pm. In this way, the presence of a particular type of particle (e.g. dangerous particles) can be preferentially detected. By positioning the detector 210 in a location which corresponds to a maximum of a particular type of particle, that particle can be identified with improved signal to noise compared to detecting at a random location which may not coincide with a maximum of the Airy pattern.

In a second example, it may be unknown what type of particles are present in a sample. As such, the desired position of the detector 210 is unknown. Instead, the detector 210 is mounted on a movable stage or other means for moving the detector 210, and can be freely moved within the plane. The detector 210 can then be moved, in real time, while a sample of particles is delivered to the sensing volume 202. The signal output from the detector 210 is monitored as the detector 210 is moved, and the signal will increase and decrease in intensity depending on its location in the plane, due to the varying maxima and minima of Airy patterns caused by anisotropic crystalline particles present in the sensing volume 202. Information relating to the varying maxima and minima of Airy patterns can be used to determine properties of the anisotropic crystalline particles, such as their size. Additionally, a position can thus be found where the signal output from the detector 210 is maximised (such as the location of the first order maxima). Once a location for the detector 210 which corresponds to a maximised signal has been found, the location of the detector 210 can be recorded. The location can then be compared to data (e.g. empirical or theoretical) concerning the Airy pattern of different types of particles. The location of the detector 210 will correspond to a particular radial distance, which corresponds to a maximum of an order of an Airy pattern for a particle of a particular size. For example, if the detector 210 is moved to a location which corresponds to a maximised signal, and that location has a radial distance of 21.3, it may be determined that the particle size is 2 pm because the first order maximum of a 2 pm particle has a radial distance of 21.3 as described above. Determination of size may also be based on a statistical analysis of many detections by the detector over a given time period. For example, an average size of the crystalline particles may be determined, and/or a size distribution of the crystalline particles may be determined.

Of course, in any sample there may be an array of different particle sizes. As such, while a first position may be selected (e.g. during calibration and/or in real-time during use), the detector 210 may be subsequently moved to detect a different sized crystalline particle.

When considering a maximised signal, it should be understood that there are typically fluctuations in intensity in any given measurement. For example, in this method and apparatus, fluctuations in intensity may be caused by phase retardation of birefringent signals, or from noise associated with the detector 210 or electronics. The output signal may hence comprise an output envelope with jitter rather than a sharp peak. As such, a maximum signal may be determined based upon a signal which is averaged over time. Alternatively, a maximum signal may be determined based upon when the signal exceeds a threshold output. Alternatively, a correlation function may be developed from the jittery envelope.

As previously discussed, the detector 210 in Figure 2 is arranged to coincide with the first order of the pictured Airy pattern 219. In many cases, the first order is a preferred order upon which to position the detector 210 due to its increased maximum intensity compared to the higher orders. However, returning to Figure 4, it can be seen that orders of different sizes of crystalline particles can overlap. For example, the first, second and third orders of the 10 pm particle overlap with the zeroth order of the 2 pm particle. Similarly, the fifth, sixth and seventh orders of the 10 pm particle overlap with the first order of the 2 pm particle. As such, when attempting to distinguish between 10 pm and 2 pm particles in a sample, it may be beneficial to detect the fourth order of the Airy pattern of the 10 pm particle, because this does not overlap with any features (i.e. order maxima) of the Airy disk of the 2 pm particle. This example is specific to the 2 pm and 10 pm silica particles depicted in Figure 4, and other orders will be selected in other situations depending on the relative position of order maxima of other particles in these situations.

The above method has so far described determining the presences of anisotropic crystalline particles. It can typically be assumed that when operating in an environment likely to contain RCS, any detection of an anisotropic crystalline particle is likely to be hazardous. However, not all anisotropic crystals are asymmetric (e.g. irregular shaped with sharp corners). It can therefore be further beneficial to discriminate between asymmetric anisotropic crystalline particles and symmetric anisotropic crystalline particles, as symmetric particles may not be as hazardous as asymmetric particles. The method described in PCT/GB2016/051698 uses two scatter detectors, symmetrically opposed, and configured to detect reflected radiation from a particle. However, three detectors are required in this arrangement, one for detecting birefringence, and two for detecting asymmetry.

The shape (or an indication of the shape) of crystalline particles can be determined by the apparatus 200 by providing a second detector. The second detector is positioned in the same plane as the (first) detector 210, but is symmetrically opposed about the optical axis 207 with respect the first detector 210. For example, in the arrangement depicted in Figure 2, the first detector 210 is positioned to detect the lowest point of the first order 221. A second detector (which is not shown) symmetrically opposed about the optical axis 207 would therefore be positioned to detect the highest point of the first order 221, at a position indicated by the dotted circle on Airy pattern 219. The terms "lowest" and "highest" are intended to mean the most negative and positive position reached by a maximum of the first order in the y-direction in Figure 2, respectively. Of course, other arrangements will be apparent to the skilled person.

A spherical particle will typically produce a generally symmetric Airy pattern. As such, the radiation detected by the first detector and the second detector for a spherical particle will be substantially equal, e.g., having identical Gaussian beam profile of equal area. However, a non-spherical particle, for example an asymmetrical particle of polygon shape, will typically produce an Airy pattern which is asymmetric. That is, the asymmetry will introduce tailing in the Gaussian beam with a modification of its linewidth and the individual peak of a diffraction order will be shifted from a position relative to an opposing position about the optical axis. As such, the signal output by the first detector (referred to as first data) and the signal output by the second detector (referred to as second data) for an anisotropic non-spherical particle will be different. A symmetry or lack thereof can therefore be determined between the first and second data, and the shape of the crystalline particle can be determined based upon said symmetry or lack thereof. For example, if the difference between the first and second data is small, the determined symmetry will be high, indicating that the particle has a low irregularity and is likely to be generally symmetric. Additionally, when the difference between the first and second data is small, the particle's size can be predicted more accurately. On the other hand, if the difference between the first and second data is large, the determined symmetry will be low, indicating that the particle has a high non uniformity (e.g. is non-spherical) but there will be greater uncertainty about the particle's size. The magnitude of the determined symmetry may be used as an indication of particle shape. An alarm warning of potentially dangerous airborne particles, such as RCS, may be output when there is a determination that there is both a birefringent and asymmetric particle in the sample. Again, such alarm may be output when a threshold number of detections have been made within a predetermined time period.

A method of determining the shape of the crystalline particle comprises the following steps.

The particles (i.e. the plurality of particles in the sample) are illuminated with radiation. The radiation has a first polarization, for example as provided by the polarized radiation source 204 of Figure 2. The particles are illuminated at the sensing volume 202 which is coincident with the optical axis 207 of the apparatus 200.

Radiation which has transmitted through a crystalline particle within the sample is then collected using a first detector, for example using the detector 210 of Figure 2. Radiation which has transmitted through the crystalline particle is also collected using a second detector. The first detector is offset from the optical axis 207 and the second detector is symmetrically opposed about the optical axis 207 with respect the first detector. Alternatively, a single detector could be used which is positioned or configured to detect symmetrically opposing portions of the Airy pattern.

First data indicative of a portion of the Airy pattern is output by the first detector. Second data indicative of the Airy pattern is output by the second detector. A symmetry is determined based on the first and second data. For example, the symmetry may be determined based on a comparison between the first and second data. The comparison may be a simple difference calculation, ratio of the areas (normalised integral) calculation, or any other method (for example, statistical analysis) of comparing the first and second data. A characteristic of the crystalline particle is then determined based on the symmetry. The characteristic may be a shape of the crystalline particle, or a physical characteristic related to the shape. The crystalline particle may be determined to be asymmetric if the determined symmetry exceeds a threshold value.

The above methods and apparatus have described the determination of the presence of a crystalline particle in a sample comprising a plurality of particles. In particular, this method and apparatus can achieve higher signal to noise ratios with improved accuracy than previous systems and methods, at least in part due to the use of data associated with the first or higher orders of the Airy pattern. The signal to noise ratio of the systems and methods can be further improved by providing means for blocking radiation propagating along the optical axis as described below.

The apparatus 200 depicted in Figure 2 further depicts a beam stop 230. The beam stop 230 is an optional feature which can be incorporated into the apparatus 200 to improve the signal to noise ratio. The beam stop 230 is positioned along the optical axis 207 so as to block radiation propagating along the optical axis 207 after the sensing volume 202 (i.e. between the sensing volume 202 and the plane in which the detector 210 is located). That is, incident laser radiation which has not been diffracted off the optical axis 207 is blocked. In this way, radiation propagating along the optical axis 207 is further inhibited from being detected by the detector 210. This can be particularly useful, because the polarized laser beam and/or the zeroth order of the Airy pattern can have a significant extent in the radial direction, resulting in the detector 210 collecting some of this radiation despite the detector 210 being positioned off-axis. Incident laser light travelling along the optical axis 207 is typically relatively higher in intensity compared to the higher (first and higher) orders of the Airy pattern generated by diffraction, and hence can readily interact with optical elements within the apparatus 200 thereby spreading, reflecting (back reflection) and scattering further away from the optical axis 207, resulting in 'optical noise' due to increased levels of ambient radiation. Blocking this radiation prior to the detector 210 can reduce the noise received by the detector 210.

The polarized beam typically diverges from the optical axis 207 as it propagates. As such, the beam stop 230 blocks a particular solid angle of radiation depending on its size and position along the optical axis 207. That is, while the beam stop 230 is said to block radiation propagating 'along' the optical axis 207, it also blocks radiation spaced from the optical axis 207 by a particular solid angle. To block a larger solid angle of radiation, a larger beam stop 230 can be provided or the beam stop 230 can be positioned closer to the sensing volume 202. In a specific example, the beam stop 230 is configured (i.e. its size, shape and location are selected) so as to block a solid angle approximately equal to the beam waist of the polarized laser beam at the location of the beam stop 230. The beam waist may be measured in any known way, for example the 1/e, 1/e2 or FWHM (linewidth) beam size.

The solid angle blocked by the beam stop 230 may also be selected at least partly depending on the size of the crystalline particles to be detected, and hence upon the arrangement of the Airy pattern. For example, the beam stop 230 may be positioned so as to allow propagation of the first order of diffraction but block all radiation travelling closer to the optical axis 207 (e.g. blocking the zeroth order and any leaked radiation travelling within the same solid angle as the zeroth order). Considering, for example, the intensity of the Airy pattern for a 10 pm particle as depicted in Figure 4, to block the zeroth order for the 10 pm particle, a solid angle extending to a radial distance of 3.5°

(i.e. the radial distance of the first minimum) must be blocked. On the other hand, considering the intensity of the Airy pattern for a 2 pm particle as depicted in Figure 4, to block the zeroth order for the 2 pm particle, a solid angle extending to a radial distance of 14.60 must be blocked (i.e. the radial distance to the first minimum). However, if blocking to a radial distance of 14.6, the first, second and third orders of the Airy pattern for the 10 pm particle would also be blocked. It follows that, when detecting more than one size of particle, there is a trade-off between blocking undesired radiation which can lead to noise, and blocking desired radiation which would result in an improved signal for determining the presence of a crystalline particle. More generally, there is a trade-off between generally blocking radiation close to the optical axis, which improves the signal to noise ratio, and permitting propagation of the desired signal used for determining the presence of crystalline particles.

Due to the changing requirements for different particle sizes, the beam stop 230 can be moved between multiple positions along the optical axis 207. For example, for the detection of a first size of crystalline particle, the beam stop 230 may be arranged at a first position where it blocks a first solid angle of radiation. The beam stop 230 may then be moved to a second position where it blocks a second solid angle of radiation (different to the first solid angle) so as to better detect a second size of crystalline particle. This process may be done in combination with, or in isolation from, moving the detector 210 to different positions within the plane of the detector 210.

In some arrangements the polarized beam may be collimated (e.g. using one or more collimating lenses) after the sensing volume 202 and hence will not deviate from the optical axis 207 in any significant way. When the polarized beam has been collimated, rather than blocking a solid angle of radiation, an angular cross-section of a generally cylindrical beam of radiation will be blocked by the beam stop 230. In this embodiment, the angle blocked by the beam stop 230 will be substantially determined by the size of the beam stop 230.

In addition to the size and location (along the optical axis 207) of the beam stop 230, the shape of the beam stop 230 may be selected. For example, a circular beam stop 230 may be appropriate for a Gaussian beam. However, for other beam shapes, other beam stop shapes may be more appropriate. It will be apparent to those skilled in the art how baffle geometry may be tailored depending on beam shape.

Other means of blocking the radiation other than using a beam stop will be apparent to one skilled in the art. For example, mirrors or additional filters can be used to prohibit the propagation of radiation along the optical axis towards the plane containing the detector rather than, or in addition to, a beam stop.

Reference is made herein to a portion of the Airy pattern comprising a specific order. For example, the detector may be arranged to receive radiation associated with a specific (e.g. the first) order of the Airy pattern for a specific particle and output data associated with said specific order. However, in some instances the detector may receive radiation associated with more than one order. In a first example, the detector may receive radiation from neighbouring orders (e.g. receiving radiation from the first and second order). In a second example, the detector may receive radiation from a specific order of a first particle and a specific order of a second particle (for example, if two crystalline particles are present in the sensing volume at the same time). In such circumstances, processing can be performed to extract information relating to a single order (or a single particle). An example of such processing is to use the Michel-Levy colour chart which relies upon phase retardance between two modes of a refracted beam resulting in an interference pattern. This may lead to a radial shift between the peaks of two different orders. The radial shift may be used to isolate a first signal (associated with a first order or first particle) from a second signal (associated with a second order or second particle). As such, when reference is made to data comprising a portion of the Airy pattern comprising a diffraction order, this is to be construed as comprising primarily of that diffraction order. The presence of radiation from any other orders in that data is coincidental and can be removed in post processing. Other processing means may be used, for example by fitting a correlation function to the data based upon known particle sizes.

The size of the detector (i.e. the size of the sensing/pixel area or collection aperture of the detector) can be selected so as to preferably detect radiation from only one diffraction order. For example, a detector with a small sensing area may be less likely to receive overlapping orders. Additionally or alternatively, the plane containing the detector can be selected so as to preferably detect radiation from only (or primarily) a single desired order. For example, due to the divergence of radiation (in the absence of collimating lenses), orders may have a larger radial spacing further from the point of origin (i.e. the sensing volume), so placing the detector in a plane further from the sensing volume may reduce the overlap between neighbouring diffraction orders.

The detector used in the above described apparatus and methods is a photodetector. The detector may comprise, for example, a photodiode. Alternatively, the detector may comprise a 2D array of image sensors, for example a charge coupled device (CCD). In particular, in the arrangement where two detectors are used so as to determine a symmetry of the crystalline particle, rather than two separate detectors a single detector comprising a 2D array of sensors may be used, which can detect radiation in locations symmetrically opposed about the optical axis. For example, a single CCD may be used which can collect the entirety of the first order of the Airy pattern, and potentially higher orders too. In this example, the detector is considered to be offset from the optical axis because it is primarily detecting at a position not coinciding with the optical axis (e.g. it is configured to detect the first orderorahigherorder). Similarly, the 2D array can be arranged to collect part or all of any other order of the Airy pattern from different scattered angles over the hemispherical image plane of larger area. When the detector comprises a 2D array, it may be particularly beneficial to incorporate the means for blocking radiation propagating along the optical axis so as to avoid saturating a central portion of the array. Alternatively, the detector can be formed so as to have a central cut-out (i.e. no sensors in a pixel array region positioned at the optical axis) such that radiation propagating along the optical axis is not detected. Alternatively, the detector may be configured to ignore signals from certain locations within the 2D array. For example, the central portion of the 2D array may coincide with the optical axis, and so any signal received from sensors in this region may be ignored by the detector, for example using digital masking.

The methods and apparatus described herein can be used and/or incorporated in the apparatus for detecting crystalline particles described in PCT/GB2016/051698. That is, the apparatus for detecting crystalline particles described in PCT/GB2016/051698 can be adapted using the disclosures herein. For example, one or more detectors of the apparatus described in PCT/GB2016/051698 can be positioned as described herein, and/or the data referred to herein can be collected and used to determine the presence of, and physical properties of, crystalline particles.

Although specific embodiments of the invention have been described above, it will be appreciated that various modifications can be made to the described embodiments without departing from the spirit and scope of the present invention. That is, the described embodiments are to be considered in all respects exemplary and non- limiting. In particular, where a particular form has been described for particular processing, it will be appreciated that such processing may be carried out in any suitable form arranged to provide suitable output data.

While it has been described that individual detection of crystalline particles may take place, it will be appreciated that multiple measurements from the sample may be taken and used to determine average characteristics of the crystalline particles. For example, when using two detectors opposed about the optical axis, a determination of shape (or degree of asymmetry) of crystalline particles in the sample may be determined based on statistical analysis of many detections of many crystalline particles by the detectors.

It will be appreciated that aspects of the invention can be implemented in any convenient form. For example, the invention may be implemented by appropriate computer programs which may be carried on appropriate carrier media which may be tangible carrier media (e.g. disks) or intangible carrier media (e.g. communications signals). Aspects of the invention may also be implemented using suitable apparatus which may take the form of programmable computers running computer programs arranged to implement the invention.

Embodiments of the subject matter and the operations described in this specification can be implemented in digital electronic circuitry, or in computer software, firmware, or hardware. Embodiments of the subject matter described in this specification can be implemented as one or more computer programs, i.e., one or more modules of computer program instructions, encoded on computer storage medium for execution by, or to control the operation of, data processing apparatus. Alternatively or in addition, the program instructions can be encoded on an artificially-generated propagated signal, e.g., a machine-generated electrical, optical, or electromagnetic signal, that is generated to encode information for transmission to suitable receiver apparatus for execution by a data processing apparatus. A computer storage medium can be, or be included in, a computer-readable storage device, a computer-readable storage substrate, a random or serial access memory array or device, or a combination of one or more of them. Moreover, while a computer storage medium is not a propagated signal, a computer storage medium can be a source or destination of computer program instructions encoded in an artificially-generated propagated signal. The computer storage medium can also be, or be included in, one or more separate physical components or media (e.g., multiple CDs, disks, or other storage devices).

The operations described in this specification can be implemented as operations performed by a processor on data stored on one or more computer-readable storage devices or received from other sources (e.g. from the detector(s) of the apparatus 200).

The term "processor" encompasses all kinds of apparatus, devices, and machines for processing data, including by way of example a programmable processor, a computer, a system on a chip, or multiple ones, or combinations, of the foregoing. The apparatus can include special purpose reprogrammable logic circuitry, e.g., an FPGA (field programmable gate array) or an ASIC (application-specific integrated circuit). The apparatus can also include, in addition to hardware, code that creates an execution environment for the computer program in question, e.g., code that constitutes processor firmware, a protocol stack, a database management system, an operating system, a cross-platform runtime environment, a virtual machine, or a combination of one or more of them. The apparatus and execution environment can realize various different computing model infrastructures, such as web services, distributed computing and grid computing infrastructures.

Processors suitable for the execution of a computer program include, by way of example, both general and special purpose microprocessors, and any one or more processors of any kind of digital computer. Generally, a processor will receive instructions and data from a read-only memory or a random access memory or both. The essential elements of a computer are a processor for performing actions in accordance with instructions and one or more memory devices for storing instructions and data. Devices suitable for storing computer program instructions and data include all forms of computer-readable media and memory devices, including by way of example semiconductor memory devices, e.g., EPROM, EEPROM, and flash memory devices; magnetic disks, e.g., internal hard disks or removable disks; magneto-optical disks; and CD-ROM and DVD-ROM disks. The processor and the memory can be supplemented by, or incorporated in, special purpose logic circuitry and fiber-optic platform for faster data transfer remotely.

To provide for interaction with a user, embodiments of the subject matter described in this specification can be implemented on a computer having a display device, e.g., a CRT (cathode ray tube) or LCD (liquid crystal display) monitor including audio, for displaying information (e.g. an indication and/or alert) to the user. Other kinds of devices can be used to provide for interaction with a user as well; for example, feedback provided to the user can be any form of sensory feedback, e.g., visual feedback, auditory feedback, or tactile feedback.

Figure 5 shows an example computer 720 which could be used to perform the method described herein. The computer 720 may be coupled to the apparatus 200. It can be seen that the computer comprises a CPU 901a which is configured to read and execute instructions stored in a volatile memory 901b which takes the form of a random access memory. The volatile memory 901b stores instructions for execution by the CPU 901a and data used by those instructions. For example, in use, data such as data received from the one or more detectors may be stored in volatile memory 901b.

The computer 720 further comprises non-volatile storage in the form of a hard disc drive 901c. It will be appreciated by the skilled person that any non-volatile storage may be used, such as a solid state drive. Data such as data received from any of detectors 709, 709a, 709b may be stored on hard disc drive 901c and may for example be analysed to generate an output signal from the one or more detectors. The computer 720 further comprises an 1/O interface 901d to which are connected peripheral devices used in connection with the computer 720. More particularly, a display 901e is configured so as to display output from the computer 720 such as output 711 in the form of a warning indication such as an indication of a risk level associated with the detection of RCS and/or of physical characteristics such as an indication of the particle's size. The computer may also comprise one or more speakers (not shown) which can be used to provide an audio alert. Input devices are also connected to the 1/O interface 901d. Such input devices may include a keyboard 901f and a mouse 901g which allow user interaction with the computer 720. It will be appreciated that the computer may have other input interfaces, for example a touch screen. A network interface 901h allows the computer 720 to be connected to an appropriate communications network so as to receive and transmit data from and to other computers. The input devices can be used such that the computer is able to receive the data captured by any one of the detectors and/or the acquisition data. The CPU 901a, volatile memory 901b, hard disc drive 901c, 1/O interface 901d, and network interface 901h, are connected together by a bus 901i. It will be appreciated that the foregoing description of the computer 720 is an example computer set up suitable for carrying out aspects of the invention, and the skilled person will recognise that various modifications may be made to the computer 720.

Where any or all of the terms "comprise", "comprises", "comprised" or "comprising" are used in this specification (including the claims) they are to be interpreted as specifying the presence of the stated features, integers, steps or components, but not precluding the presence of one or more other features, integers, steps or components.

Claims

The claims defining the invention are as follows:

1. A method for determining the presence of a crystalline particle in a sample comprising a plurality of particles, the method comprising: illuminating the sample with a polarized radiation beam having a first polarisation state, the polarized radiation beam propagating along an optical axis and illuminating the sample at a sensing volume; detecting a portion of the polarized radiation beam that has been transmitted through a crystalline particle in the sample and which has a second polarisation state, wherein the portion of the polarized radiation beam comprises a portion of an Airy pattern comprising a diffraction order greater than a zeroth order; and determining the presence of the crystalline particle based upon the detection.

2. The method of claim 1, wherein the portion of the Airy pattern comprises the first diffraction order of the Airy pattern.

3. The method of claim 1 or 2, wherein the portion of the Airy pattern does not comprise the zeroth order of the Airy pattern.

4. The method of any one of claims 1, 2 or 3 wherein detecting the portion of the polarized radiation beam comprises detecting the portion of the polarised radiation beam at a position offset from the optical axis.

5. The method of any one of the preceding claims, further comprising blocking radiation propagating along the optical axis after the sensing volume.

6. The method of claim 5, wherein blocking radiation propagating along the optical axis after the sensing volume comprises providing a beam stop on the optical axis after the sensing volume.

7. The method of claim 6, further comprising controlling the amount of radiation to be blocked by selecting a position of the beam stop.

8. The method of any one of claims 4 to 7, wherein detecting the portion of the polarised radiation beam at the position offset from the optical axis comprises configuring a detector to detect the portion of the polarised radiation beam.

9. The method of claim 8, wherein configuring the detector to detect the portion of the polarised radiation beam comprises providing the detector within an image plane orthogonal to the optical axis at the position offset from the optical axis.

10. The method of any one of claims 4 to 9, wherein the position offset from the optical axis is at a position in which an intensity of the portion of the polarized radiation beam is maximised.

11. The method of claim 10, further comprising determining a size of the crystalline particle based upon the position.

12. The method of any one of claims 8 to 11, further comprising configuring the detector to be moveable between two or more positions within the plane orthogonal to the optical axis.

13. The method of claim 12, further comprising moving the detector between the two or more positions within the plane orthogonal to the optical axis.

14. The method of any one of claims 4 to 13, wherein detecting the portion of the polarised radiation beam at the position offset from the optical axis comprises detecting a first portion of the polarised radiation beam at a first position offset from the optical axis and detecting a second portion of the polarised radiation beam at a second position offset from the optical axis.

15. The method of claim 14, wherein the first position is located at a position having the same radial distance from the optical axis as the second position.

16. The method of claim 15, wherein the first and second positions are symmetrically opposed about the optical axis.

17. The method of any one of claims 14, 15 or 16 further comprising determining a characteristic of the crystalline particle based on a comparison of the detected first portion and second portion.

18. The method of claim 17, wherein the characteristic is an indication of shape of the crystalline particle.

19. The method of any one of the preceding claims, further comprising outputting a warning when the presence of the crystalline particle is determined.

20. A computer-readable medium comprising instructions which, when the program is executed by a computer, causes the computer to carry out the method of any one of claims 1 to 19.

21. A system for determining the presence of a crystalline particle in a sample comprising a plurality of particles, the system comprising: a radiation emitter configured to emit a polarized radiation beam along an optical axis; a sensing volume configured to: receive the sample and receive the polarized radiation beam thereby enabling illumination of the sample within the sensing volume with the polarized radiation beam; and a detector configured to detect a portion of the polarized radiation beam that has been transmitted through a crystalline particle in the sample and which has a second polarisation state, wherein the portion of the polarized radiation beam corresponds to a portion of an Airy pattern comprising a diffraction order greater than a zeroth order; a processor configured to receive data output by the detector, said data indicative of the portion of the polarized radiation beam, the processor further configured to determine the presence of the crystalline particle based on the data.