WO2017090026A1

WO2017090026A1 - Apparatus for counting particles in air and an illuminator therefor

Info

Publication number: WO2017090026A1
Application number: PCT/IL2016/051229
Authority: WO
Inventors: Zvi Lapidot; Ehud Tirosh; Oded Arnon
Original assignee: Veeride Ltd.
Priority date: 2015-11-24
Filing date: 2016-11-15
Publication date: 2017-06-01

Abstract

An apparatus (15) for counting air particles includes a casing (22) and an illuminator (28, 29) inside the casing that substantially uniformly illuminates a slice (26) of air inside a volume of the casing located within a depth of focus of a camera, which simultaneously images particles (27) within the slice, allowing the image to be subsequently analyzed to count the number of particles. The apparatus may fit on to an external device such as a smartphone that includes the camera and whose flash may serve as a light source for the illuminator.

Description

Apparatus for Counting Particles in Air and an Illuminator Therefor

FIELD OF THE INVENTION

This invention relates to a sensor and apparatus for quantifying particulate contamination in air.

BACKGROUND OF THE INVENTION

Air pollution has been a growing major risk factor in public health care. This includes respiratory infections, heart diseases, Chronic Obstructive Pulmonary Disease (COPD) and lung cancer. Of special concern is particulate contamination. Studies have demonstrated and confirmed the long-term and short-term adverse effects of particulate contamination. As a result, there is growing awareness of the need for real-time monitoring and mapping the particulate contamination level, especially in urban areas.

Particulate pollution is usually characterized by the mass concentration in air measured in μg/m³. PM_JO and PM2.5 refer to the mass concentration of particles between ΙΟμπι and 2.5μπι, and smaller than 2.5μπι, respectively. Although mass concentration is commonly used as reference, it has been suggested that the number of particles per volume, rather than the mass concentration, is more closely correlated to health effects.

In recent years, attempts have been made to map air contamination in large cities using particle monitors and traditional stationary monitors using mass measurements combined with data communication. Such activities have been reported, for example, in Beijing, Antwerp and London. One example is a pilot UK system using stations known by the acronym MESSAGE: Mobile Environmental Sensing Systems Across Grid Environments htffi://www.treehugger oin/d

A large number of sensors are spread across the city. These sensors are equipped with a location sensor such as GPS, to correlate the measurements to their location and enable a creation of a pollution map. Also, to increase the coverage, these sensors should be mobile (i.e. carried by people in bags or on bicycles).

The requirement to map pollution in real time and in high density grid creates the need for personal, portable particles monitors.

Sensors for particle measurements were traditionally developed for in-door, such as clean room monitoring for air. Such sensors usually employ focused laser/diode laser illumination and a photodiode as described, for example, in US Patent Nos. 5,282,151 and 4,571,079 and in EP 1 271 126. A similar approach is adopted by Anna Morpurgo et al. "A low-cost instrument for environmental particulate analysis based on optical scattering" published in Instrumentation and Measurement Technology Conference (I2MTC), 2012 IEEE International, pages 2646 - 2650, May 2012. Such approaches need to count each particle one at a time and care must be taken to avoid counting the same particle more than once. Also, air flow rate should be controlled to a high accuracy to reliably determine the volume corresponding to the particle count.

The flow of liquid or air through such devices is controlled by a fan/pump. Typical air flow is 0.1 CFM (app. 3.8 i/m). Particles crossing the laser illumination scatter light to the sensor. Particles are counted and their sizes are estimated using the intensity of the scattered light. Some sensors include 2D sensors for increased sensitivity by analyzing the trajectory of the flowing particles, thus differentiating them from noise. For example, reference is made to the Wikipedia article "Particle Counter" at htt ;//en^ where mention is also made of the use of a high-resolution camera and a light to detect particles. Vision based particle sizing units obtain two-dimensional images that are analyzed by computer software to obtain particle size measurement in both the laboratory and online. It has also been proposed to use the Sharp GP2Y1010AU0F sensor or its modifications in dust sensors such as the one demonstrated by Karlsruhe Institute of Technology (KIT), Karlsruhe, Germany. Use of the sensor in conjunction with a smart- phone is described by Matthias Budde et al, "Retrofitting Smartphones to be Used as Particulate Matter Dosimeters" Proceedings of the 2013 International Symposium on Wearable Computers pp 139-140 (2013), from which it emerges that the dust sensor does not count discrete particles but rather measures the total scattered light intensity and uses calibration to determine the air dust density in μg/m . However, this type of measurement can provide information neither on the number of particles nor on their sizes and is also prone to errors when size distribution changes from the one for which it was calibrated. Further details can also be seen on https: 7www.you ube.com/watch?%ⁱ'-l^'4jH 6RTPino&feature~youai.be, which shows an animated representation of the device.

The Sharp sensor has also been included in the experimental Evaboard developed by Karlsruhe Institute of Technology for use in large cities such as Beijing. Other sensors include MET ONE laser particle sensor, Etech VPC300 particle counter, Samyoung DUST SENSOR MODULE DSM501 and ARTI HHPC-6.

It has also been proposed to attach such portable sensors to smart phones to enable data upload via the cellular or WiFi network. Such an approach is described, for example, in a graduate M.Sc. thesis by Srisivapriya Ganesan titled "Smartphone Application for m-health and environmental monitoring systems", Arizona State University, May 2012. Various approaches are outlined, but all use conventional particle detectors and counters, whose data is fed to a mobile phone, mainly for display and communication purposes.

Likewise, US 2014/0277624 discloses a personal air quality monitoring system worn or carried by a user. Such a system can be used to monitor the local air quality and alert a user should air quality deteriorate below some acceptable desired or configurable threshold. In one embodiment, the device is housed as an adjunct to or as a holder for a mobile or smart phone. The personal counter device is attached to the docking port of the mobile phone in order to provide a wired interface. It may be molded into a phone holder and communicate with the phone wirelessly via Bluetooth or WiFi for example. The air may enter from the top and exit from the bottom. In all these examples, the mobile phone merely serves as a means for communicating data from the counter. It is not physically integrated with the counter, and plays no part in the counting which is performed only by the counter.

The University of California, San Diego, has developed a sensor called CitiSense, which measures the local concentrations of ozone, nitrogen dioxide and carbon monoxide. The sensor transmits the data via the mobile communication network. (litlp://w .gimiag ofn/citisense-air-qua¾ity-monitor/25512 Other such sensors are being developed such as Air. Air! (hi-^Plii ^yi . kA .QD ^and Tzoa (h ft : //www, my tzoa. com) . Both of these devices use portable sensors to measure air quality and transmit the data to a smartphone, which serves to display the measured data graphically.

It is apparent that the sensor comprises a low volume housing that fits over the camera/flash combination of the smartphone. An optical fiber inside the sensor directs high intensity light from the flash on to a small sampling volume inside the sensor housing, within which the air particles are imaged and processed. However as noted above, the sensor does not count particles but rather measures a signature which is related through calibration to the dust density in microgram/m^" . Additionally, the optical fiber is highly directional such that the illumination in the sampling volume is non-uniform.

Known particle counters typically operate on the principle of streaming air through a focused beam which illuminates the air as it passes and a sensor which counts the number of particles thus sensed. Therefore, in order to determine the number of particles in a given volume of air, the volume of air passing the sensor in a given time must be known. Consequently, the processing algorithms in known particle counters must take into account the flow rate through the device i.e. the rate at which its pump draws the sample air through the sample chamber and also ensure that only one particle passes through the beam at a time.

CN 104502292 discloses an illumination system comprising opposing front and rear concave spherical reflectors, the mirror surface of the front reflector being provided with spaced apart incidence and emergence through holes. Incidence and emergence optical fiber collimators are coaxial to the incidence and emergence through holes, and serve to irradiate light beams emitted by the incidence optical fiber collimator to the rear reflector through the incidence through hole of the front reflector. The light beams are reflected by the front and rear reflectors multiple times, and then are irradiated out through the emergence optical collimator by virtue of the emergence through hole in the front reflector. Owing to multiple reflection of the light beams between the front and rear reflectors, very high absorption optical path can be achieved within limited space.

SUMMARY OF THE INVENTION

It is an object of the invention to provide an improved particle counter that images particles in a defined volume of air on to an area sensor, such as a camera, thus allowing the number of particles in the imaged volume to be repeatedly determined using 2-d image processing without the need for a pump or knowledge of air flow rate.

It is a further object of the invention to provide a particle counter which is portable and affordable.

It is a further object to provide such a counter as an add-on accessory to a smartphone having a camera/flashlight combination, thus utilizing these components in the smartphone and obviating the need to provide them in the accessory.

These objects are realized by a personal air particle counter having the features of the respective independent claims.

In accordance with another aspect that may find independent application there are provided two variations of illuminator having toroidal reflecting surfaces. BRIEF DESCRIPTION OF THE DRAWINGS

In order to understand the invention and to see how it may be carried out in practice, embodiments will now be described, by way of non-limiting example only, with reference to the accompanying drawings, in which:

Fig. 1 is a schematic representation of a particle counter assembly according to a first embodiment of the invention fitted as an external accessory to a smartphone;

Fig. 2 is a schematic representation of a particle counter assembly according to a second embodiment of the invention;

Figs. 3a and 3b are flow diagrams showing the principal operations during use of the particle counter assembly, Fig. 4 shows schematically the principles of an illuminator according to a first embodiment producing uniform illumination within a cylindrical volume;

Fig. 5 is a ray diagram showing the path of optical rays propagated by an object being imaged by a concave mirror having a circular profile;

Fig. 6 is a pictorial view showing in perspective an illuminator according to a first embodiment suitable for use in the particle counter assembly of Fig. 1 ;

Fig. 7 is a ray tracing showing light propagation through the illuminator of Fig. 6;

Fig. 8 shows schematically the principles of an illuminator according to a second embodiment producing uniform illumination within an annular volume having a central gap that is light-free;

Fig. 9 is a ray tracing showing light propagation through the illuminator of Fig 8;

Fig. 10 shows schematically use of the illuminator depicted in Fig. 8 in the particle counter accessory of Fig. 2 with the camera but without the light source; and

Fig. 11 shows schematically shows a layout of an illuminator depicted schematically in Fig. 10.

DETAILED DESCRIPTION OF EMBODIMENTS

In the following description of some embodiments, identical components that appear in more than one figure or that share similar functionality will be referenced by identical reference symbols.

Fig. 1 shows schematically a Particle Counter Assembly (PC A) 10 according to a first embodiment in the form of an accessory 15 for a smartphone 20. The smartphone 20 can be any phone having a camera having a lens and a flash, of which only the camera lens 21 is shown for the sake of simplicity. The accessory 15 has a casing 22 adapted for fitting in predetermined alignment on to the smartphone 20. To this end, the casing 22 may be fixed to a preferably rigid protective cover 24 that fits snugly onto the smartphone in known manner. Typically, the casing 22 contains a lens 25 that is so located that when the accessory is attached to the smartphone, the lens 25 is aligned with the smartphone camera lens (not shown). Air containing particles of different sizes fills the casing 22, but only those particles that are located within the slice 26 and illuminated as explained below and located within the depth of field of the optics defined by the combination of the smartphone camera lens and lens 25 will be sharply observed. The focal lens of the lens 25 is typically approximately 100mm, thus defining the location of the slice 26 within the accessory 15 so that any particles 27 within this slice 26 will be sharply imaged by the optics defined by the combination of the smartphone camera lens and lens 25. Alternatively, depending on the casing geometry, the slice may be located within the depth of field of the camera lens without the need for the lens 25, in which the lens 25 may be omitted.

The accessory 15 also includes a fiber bundle, which splits into multiple fibers 28, only two of which are shown for simplicity. The bundle is arranged so that one end of the fibers is aligned with the phone flash (not shown) and the fibers 28 are used to illuminate the air contained in the slice 26 through peripheral illuminators 29 (only their side views being shown in Fig. 1). In such an arrangement, the illuminators 29 define the slice 26 since it is only those particles within the slice 26 that are illuminated and subsequently imaged and therefore counted. Furthermore, since many optical fibers may be evenly distributed around the complete periphery of the peripheral illuminators 29, the illumination within the sampling volume may be rendered substantially uniform. Within the context of the description and the appended claims, the terms "uniform" or "substantially uniform" or "substantially uniformly" when applied to the illumination imply uniformity within ±10% i.e. a variance of no more than ±10%. In such an embodiment, the peripheral illuminators 29 are passive since they merely serve as conduits for relaying light from the smartphone 's flash. In other embodiments, uniform illumination may be achieved by providing a plurality of active light sources, such as LEDs, around the periphery of the peripheral illuminators 29. Since the height of the illuminators 29 is small, typically less than 10 mm, the number of particles in the slice 26 is low enough and therefore amenable to simultaneous one-shot imaging of all the particles. This avoids or significantly reduces the risk of overlap between particles, which might produce a false low number and an error in particle size determination. Nevertheless, any residuals errors resulting from overlapping of particles may be significantly reduced by image processing. It is to be noted that this arrangement is distinguished in two major respects over known particle counters, which constantly pump air into a chamber all of which is sequentially illuminated. First, in such systems the pump flow-rate must be accurately known since it determines the volume of the air in which particles are counted. Secondly, simultaneous illumination of more than one particle will result in a single count, meaning loss of detected particles as well as misinterpretation of their sizes. This is avoided or significantly reduced in the invention by using image processing techniques.

Also shown are an inlet valve 30a and an outlet valve 30b that are used to change the air volume within the accessory. Typically, this is done by pushing down on the top surface 31 of the accessory, thereby folding the sidewalls 32, which are flexible and elastic. Thus, on pressing the top surface 31, the accessory 15 contracts and air exits from the outlet valve 30b. With expansion of the accessory 15, a fresh volume of air flows into the accessory through the inlet valve 30a. The flexibility of the accessory walls is also used to fold the accessory 15 into a compact package when not in use and to unfold it when needed. Alternatively, the airflow may be controlled by a fan, as is known per se. The inlet and outlet valves may be normally closed one-way valves so that when the accessory contracts, the outlet valve allows air to exit, while the inlet valve is inoperative. The same applies in reverse when the accessory is expanded. In this case, the air enters through the air inlet valve, while the outlet valve is inoperative.

Alternatively, in embodiments that use a rigid accessory, a miniature pump can be used in order to pump air into and out of the accessory. Alternatively, the accessory may expand and contract automatically, for example by use of a motor. Nevertheless, in all these variants, it is the volume of the slice that determines the volume in which particles are counted and this volume is defined by the geometry of the illuminators 29.

Fig. 2 shows schematically a second embodiment wherein a mirror 35 is provided inside the accessory, so that the optical path is folded, thereby allowing the height of the accessory to be halved, as compared to Fig. 1. The light scattered from a particle 27 strikes the mirror 35 and is reflected to the lens 25 and to the smartphone camera, as before. The smartphone camera actually "sees" a virtual image 27' of the particle 27 through the mirror 35. Baffles 36 are introduced to prevent light from peripheral illuminators 29 reaching volume 38 located in the immediate vicinity of the camera while allowing the imaging of particles residing in the rest of slice 26.

In order to gather meaningful information, a minimal air volume should be scanned, as explained by the following example. Table 1

The higher the index the more hazardous the particle pollution. It is seen that an air pollution level of 50μg/m is regarded as unhealthy for sensitive groups.

5 With reference to Table 1, we will assume air pollution level of 50μg/m . We will assume also that only particles larger than 2.5μπι are measured.

Table 2 below is taken from A. A. Tittarelli et al. "Estimation of particle mass concentration in ambient air using a particle counter" in Atmospheric Environment, Volume 42, Issue 36, p. 8543-8548. It is seen that a mass density of 15.93μg m^" corresponds to particles larger than 2.5μπι or a mean density of 206x10^"-3 cm^"-3

10 , which thus establishes a level of moderate air pollution as determined by the EPA in Table 1. It thus follows that a mass density of 50μg m^" corresponds to particles larger than

2.5μπι with a mean density of 646.5x10 -^"3 cm -^"3 or 646,000 m -^"3.

A typical dimension of the illuminated volume within the slice 26 in Fig. 1 is

3 -5 3

15 115x65x10 mm or 7.46x10^" m . This volume will contain, on average, -50 particles.

With ten different air volumes, 500 particles on average will be gathered, resulting in a particle count measurement with better than 5% accuracy. Assuming that each measurement takes 5 sec per cycle (including air replacement), this would take less than a minute.

20 Table 2

It is important to note that this assumes that only particles larger than 2.5μπι are measured. As seen from Table 2, the number of smaller particles is significantly larger for a given mass concentration, so in practice for smaller particles fewer measurements will be required to establish a statistically reliable count. The volume of the slice 26 determines the average number of particles counted in each measurement, and therefore it determines the number of sequential measurements required to count on average a sufficient number of particles to achieve a desired accuracy. Thus, in the example above, if we assume that a total count of 500 particles achieves an acceptable margin of error and that the slice 26 is liable to contain only about 50 particles, then as noted above, we need to make ten consecutive measurements. In order to do this at sufficiently high speed, after each measurement we need to replace the air in the slice 26 with fresh air and repeat this process. The larger is the volume of the slice 26, the fewer measurements are required, and vice versa.

As the volume of the slice 26 is reduced, thus requiring more measurement cycles, a fan may be required to replace the air in order to reduce the time for each measurement cycle. Where the volume of the slice is larger or time is less critical, the air may be replaced manually by folding and unfolding the accessory, as explained above. It is important to note that the airflow rate need not be either controlled or measured to a high accuracy. It is only important to ensure that substantially all of the air volume is replaced between consecutive measurements in order to avoid counting the same particle more than once.

Fig. 3a depicts the method of using the PCA. The accessory 15 is first attached to the smartphone and then unfolded as explained above. Then measurements are performed. When measurements are completed, the accessory is folded and detached.

The measurement process is now further explained with reference to Figs, land

3b. With the accessory attached to the smartphone, particles in the observed volume of the slice 26 are counted as follows:

• Camera flash illuminates the slice 26 through fibers 28 and illuminator 29;

• Smartphone camera acquires picture or pictures of the slice 26;

· Light scattered from a particle 27 is collected by the camera lens and an image of the particle is created at the focal plane of the phone camera. • The camera images are analyzed to determine the number of particles and their sizes.

• Alternatively, pictures may be sent to an off-line computer via the cellular or Wi-Fi network for processing.

· Alternatively, only partial processing is done on the smartphone and the rest is done off-line. Particle sizes are estimated using their spatial distribution and their scattered light intensity. Also since the smaller the particles, the more pronounced is their light scattering at shorter wavelengths, camera color information is also used to estimate particle sizes. In the embodiment shown in Fig. 1 as described above, the observed slice 26 is located at a specific distance from the camera that is pre-defined by the illuminators 29, to produce a focused image of all particles in a specified volume defined by the geometry of the illuminators 29. However, other embodiments are possible as follows:

• The slice 26, including the peripheral illuminators 29, may be extended to include most of the chamber height and volume. In this case, multiple images using different values of focus are needed to cover the whole accessory volume. In this case, the volume of the slice is determined by the optics depth of field and images of particles located outside the depth of field of the camera will be blurred and will later be removed using image processing.

· Alternatively, the peripheral illuminator 29 can be moved up and down as shown by arrows 32a, 32b in steps using a motor (not shown), thereby moving the slice 26 while adjusting the focus accordingly. This requires some motion mechanism but is more efficient than illuminating the whole volume in terms of illumination and also avoids scattering light from portions that are not in focus. In this case, slice width may also be adjusted for a given focal distance, to compensate for the changing depth of focus.

The measurement sequence is now explained with reference to Figs. 3b. First, the flash illuminates the selected slice 26 and an image is acquired. This is done by moving illumination while changing focus or in the case where all volume is illuminated by changing the focus only. This image is then analyzed in the smartphone. Now one of the following scenarios may happen:

• Replace the air by folding and unfolding the accessory and repeat the process; • In the case where the illuminated volume includes most of the volume height, the focus is changed and a sequence of images is acquired until the whole volume is acquired. Only then is the air replaced.

• In the case where the illuminated volume can move up and down, a new volume can be illuminated and acquired until all the volume is acquired. Only then is the air replaced.

In addition to adding a fan, the size of the accessory may be significantly reduced by using the flash in a strobe mode. In this mode, the air volume is illuminated with short light pulses while air flows. These short pulses "freeze" the particle images so that large volumes of flowing air passing through a relatively small casing can be measured and analyzed. In this connection, it is again noted that since a smaller casing has a smaller volume, to maintain the air volume per minute, faster airflow is required. In order to acquire images of fast-flowing air, short light pulses are required, as explained above. Every time the air volume in the accessory is replaced by a new air volume, an image is acquired. The faster the air flows, the faster is the changing rate of the air volumes, and a faster flash rate is required.

Furthermore, other sensors may be added to the PCA. Such sensors as provided, for example, by Sensirion, http://www. sensjrkm.com include, for example, pressure, humidity temperature and gases content such as VOCs (Volatile Organic Compounds).

The accessory 15 may include a wired connection such as USB connection to the smartphone to allow connection of an external power supply, if necessary, i.e. to supply power to the fan, pump and the motor that moves the illuminator 29. Alternatively, the accessory may include an independent power supply. The wired connection can also be used for data transmission between the accessory and the smartphone.

Replacement of air within the accessory can also be done using physical motion of the entire PCA, i.e. by moving it by hand or attaching it to a moving vehicle such as bicycle, motorcycle or a car. In this case, the replacement of the volume of air within the accessory in ensured by the physical motion of the PCA. Air replacement can be verified by using motion information as recorded, for example, by the vehicle speedometer, phone GPS and phone gyro. In cases in which some of the air volume is not replaced between measurements, image processing may be used to eliminate double readings. While the particle counter assembly has been described with particular reference to accessories that are attachable to a smartphone, the invention also contemplates an independent standalone device similar to those described above but which also includes its own light source, camera, power supply and transmitter/receiver, which may employ a short-range wireless protocol such as Bluetooth™. This obviates the need to attach the device to a smartphone. Operation is similar to that described above, but images are transmitted to the smartphone via wireless communication for processing or to another remote processor such as a computer or, if the device includes a CPU some image processing can be done in the CPU and only reduced information need then be transferred.

Regardless of whether an independent standalone device or an accessory is used, particles are imaged and counted in parallel without the need for a pump or fan.

Fig. 4 shows schematically the principles of an illuminator according to a first embodiment producing uniform illumination within a cylindrical volume. There are shown a pair of identical concave reflectors 40, 41 having a circular profile and displaced from each other by a distance equal to their focal length /with their reflecting surfaces facing toward each other and their optical axes collinear. At the center of one of the reflectors a point source of light 42 is introduced, for example via an optical fiber 43 disposed through an aperture at the optical center of the reflector. Light is thereby injected through the optical center of the second reflector 41 into the space between the two concave reflectors and strikes the first reflector 40. It will be understood that the light source spreads out in all directions and for the sake of explanation the paths of only a single light ray is shown. Thus, a first ray strikes the reflector 40 and being at the focal plane of the first reflector 40 is reflected thereby as a second ray that is parallel to the optical axes of the two reflectors. A third ray will be directed to the focal plane of the second reflector 41, which is the center of the first reflector 40 and will be reflected as the fourth ray, and so on and so forth as is clear to one skilled in the art. Only the first six rays are shown, and then the sequence repeats itself.

In order to understand why this is, reference is made to Fig. 5 showing the ray diagram for light emitted by a distant object O located at infinity. Rays of light emitted by such an object may be regarded as mutually parallel. When they fall on a concave mirror along its axis C, they are reflected by the mirror and meet at a point F in front of the mirror called its principal focus. Conversely, light rays that pass through the principal focus, strike the mirror are reflected thereby parallel to the optical axis. Since the two mirrors in Figure 4 are displaced by a distance equal to the focal length of the mirrors, the light source 42 is located at the focal plane of the first mirror 40. Therefore, wherever light emitted by the light source strike the first mirror 40, they will be reflected parallel to the optical axis.

Light rays from the object that pass through the geometric center A of the mirror will strike the mirror radially and are thus reflected along exactly the same propagation path. It may be shown that if Rj is the radius of curvature of the mirror and /is its focal length, then:

For a full derivation of this relationship see htffi://www.t«tomsta om/conte^

Reverting to Figure 4, since the two mirrors 40 and 41 are each located at the focal plane of the other mirror, any light reflected by the first mirror to the second mirror will always be reflected by the second mirror parallel to the principal axis and vice versa. As a result the space between the two mirrors will be filled with uniform illumination.

Consider now what happens when we geometrically spin the profile of the two mirrors about a vertical line mid- way between the two mirrors. This will create a volume of revolution forming a toroidal reflector 45 that resembles a tire as shown in Fig. 6. It is no longer meaningful to speak of "first" and "second" mirrors since what we now have is a toroidal reflector having a continuous curved reflecting surface whose cross-sectional elevation forms a segment of a circle. However, the diameter of the toroid must be equal to the distance between the two reflecting surfaces when seen in cross-section i.e. the same as/. From this we can derive that:

f = = 2R₂

where Rj is the radius of curvature of the mirror profile (as described in Figure 4) and R₂ is the radius of the ring. Light emitted by the optical fiber 42 will now propagate in all directions, as shown in Fig. 7 i.e. when seen in cross-section in the x-y or the y-z plane the light rays fan out toward the upper and lower edges of the reflecting surface and are reflected parallel to the principal axis i.e. parallel to the x-z plane. When seen in the x-z plane the rays fan out in the plane of the circle and strike the reflecting surface at arbitrary angles of incidence. The angle of reflection will therefore also be arbitrary thus filling the complete volume of the toroidal reflector. Such an illuminator is suitable for use in the particle counter assembly described above with reference to Fig. 1.

Fig. 8 shows an arrangement similar to that shown in Fig. 7 in that there is also provided a first toroidal reflector 50 having a curved reflecting surface whose cross- sectional elevation forms a segment of a circle. However, in this case there is disposed a second toroidal reflector 51 of smaller diameter in the center of the first toroidal reflector and coaxial therewith. It will be seen that the direction of curvature of the two toroidal reflectors is mutually opposed so that reflection always take place between the concave surfaces of the first and second reflectors. In such an arrangement it can be shown that:

R = 2 (R₂ - R₁)

where:

R] and 7¾ are the respective radii of the inner and outer toroidal reflectors in the x-y plane as seen in Fig. 8; and

R is the radius of curvature of the concave reflecting surfaces of both inner and outer reflector in the x-z (or y-z) plane as seen in Fig. 8. The arrangement shown in Fig. 8 defines two concentric outer and inner toroidal reflecting surfaces having respective circular cross-sections of radii Rj and R2 in a first dimension and open circular sections of radius R in a second dimension orthogonal to the first dimension where R= 2(Ri - R2). At least one point source of illumination is located at any point of either or both of the reflecting surfaces.

Fig. 9 shows schematically the path of light rays as they are reflected between the two mutually opposing concave reflectors thus uniformly filling the annular volume between the reflectors while retaining a central portion through which no light passes. Such an illuminator is therefore suitable for use in the particle counter assembly described above with reference to Fig. 2. It is to be noted that Fig. 9 is schematic and the gaps in the concave reflectors have no significance.

Fig. 10 shows schematically use of the illuminator depicted in Fig. 8 in the particle counter accessory of Fig. 2 with the camera but without the light source. Only one half of the cross section of the toroid mirrors is depicted.

Fig. 11 shows a layout of an embodiment of the invention with the toroid illumination of Fig. 8 having outer and inner toroids, 50 and 51 respectively. The light is inserted through a slit 52 in the outer toroid. The slit has typical dimensions of 10mm (H) by 3mm (W). The illumination uses the camera flash 53, as explained below. Cylindrical lenses LV1 and LV2 are used to generate the vertical image of the flash 53 on the slit 52 and cylindrical horizontal lenses H are used to generate the horizontal image. The horizontal and vertical lenses have different magnifications so that an elongated image of the square-shaped flash is created on the slit 52. A field lens 53 is used to shape the beam to ensure that all rays illuminate the inner toroid. Prisms PI, P2 and P3 are used each to divert the light at 90°. Also shown in Fig. 11 is the mirror 35 through which the camera (whose lens 21 is shown) observes the illuminated area between the two toroids.

Light from the camera flash is diverted by the prisms and is directed through the slit 52 into the outer toroid 50. The light is reflected between the spherical concave reflecting surfaces of the two toroids 50 and 51 to produce an annular uniform illumination as explained above. Particles in the chamber are thus illuminated uniformly by the toroid illuminator while leaving a central gap through which the lens images the particles via the light scattered by the particles and reflected to the lens by the mirror.

It will be appreciated that while the toroid illuminator has been described with particular reference to its use in the particle counter and accessory, it may find independent use as a source of uniform illumination also in other applications. The scope of protection is therefore not to be limited to any specific use or application of the toroidal illuminator.

Claims

CLAIMS:

1. An apparatus (15) for counting air particles , the device comprising:

a casing (22); and

an illuminator (28, 29) inside the casing for directing light from a light source to a slice (26) of air inside a volume of the casing, said slice being located within a depth of focus of a camera;

particles (27) within said slice being simultaneously imaged by the camera to create an image of the particles, allowing the image to be subsequently analyzed to count the number of particles;

characterized in that:

the illuminator is configured to illuminate the slice substantially uniformly.

2. The apparatus according to claim 1, wherein:

the illuminator determines the volume of said slice, and

the casing is collapsible and there are further provided an air outlet valve (30b) for releasing air from the casing when the casing is collapsed and an air inlet valve (30a) for filling the casing with air when the casing is opened, or

a fan or a pump is provided for replacing the air in the casing.

3. The apparatus according to claim 1 or 2, wherein the casing includes said camera.

4. The apparatus according to claim 1 or 2, wherein:

the casing is adapted for fitting in predetermined alignment on to an external device (20) that includes said camera, and

the slice is located within a depth of focus of the camera when the apparatus is attached to the external device.

5. The apparatus according to any one of the preceding claims, wherein the light source is external to the illuminator and the illuminator (28, 29) includes at least one passive element that directs illumination from the light source.

6. The apparatus according to claim 5, wherein the light source is located inside the apparatus.

7. The apparatus according to any one of claims 4 to 6 when dependent on claim 4, wherein the light source is located inside the external device.

8. The apparatus according to claim 7, wherein the external device is a smartphone to which the apparatus is removably attachable and the external light source is a flash

5 unit in the smartphone.

9. The apparatus according to any one of claims 1 to 8, wherein:

the illuminator (28, 29) comprises a toroidal reflecting surface having a closed circular cross-section of radius R in a first dimension and open circular sections of radius 4R in a second dimension orthogonal to the first dimension, and

10 illumination from the light source is directed to at least one point of the reflecting surface.

10. The apparatus according to any one of claims 1 to 8, wherein:

the illuminator (28, 29) comprises two concentric outer and inner toroidal reflecting surfaces having respective circular cross-sections of radii Rj and R2 in a first 15 dimension and open circular sections of radius R in a second dimension orthogonal to the first dimension where R= 2(Rj - R2), and

illumination from the light source is directed to at least one point of either or both of the reflecting surfaces.

11. The apparatus according to any one of claims 1 to 10, wherein the illuminator 20 (28, 29) includes at least one optical fiber for conveying light from the light source.

12. The apparatus according to any one of the preceding claims, wherein the light source is an active element within the illuminator (28, 29).

13. The apparatus according to claim 12, wherein the at least one active light source is an LED.

25 14. The apparatus according to any one of the preceding claims, further including a folding mirror (35) for reflecting an image (27) of a particle.

15. The apparatus according to any one of the preceding claims, further including a processor coupled to the camera and being programmed to analyze the image.

16. The apparatus according to any one of claims 1 to 14, further including a communications port coupled to the camera and adapted for connection to an external processor that is programmed to analyze the image.

17. The apparatus according to claim 15 or 16, wherein the processor is configured 5 to analyze the image to count the number of particles in a specified size range.

18. The accessory according to any one of claims 12 to 17, wherein the illuminator has a circular periphery.

19. An illumination system for illuminating a volume, the illumination system comprising:

10 a toroidal reflecting surface having a closed circular cross-section of radius R in a first dimension and open circular sections of radius 4R in a second dimension orthogonal to the first dimension, and

at least one point source of illumination located at a point of the reflecting surface.

15 20. An illumination system for illuminating a volume having a central circular void, the illumination system comprising:

two concentric outer and inner toroidal reflecting surfaces having respective circular cross-sections of radii Ri and R₂ in a first dimension and open circular sections of radius R in a second dimension orthogonal to the first dimension where R= 2(Rj -

20 R₂), and

at least one point source of illumination located at a point of either or both of the reflecting surfaces.