CN114041048A

CN114041048A - Particle sensing system, for example for an anti-fouling mask

Info

Publication number: CN114041048A
Application number: CN202080027595.3A
Authority: CN
Inventors: A·R·M·维许伦; D·P·凯莉; J·H·M·斯普瑞特; P·T·朱特
Original assignee: Koninklijke Philips NV
Current assignee: Koninklijke Philips NV
Priority date: 2019-04-11
Filing date: 2020-04-03
Publication date: 2022-02-11
Also published as: WO2020207908A1; JP2022527563A

Abstract

A system utilizes optical particle sensing and has a light source, a detector for detecting the presence of light reflected back to the detector, and a processor for determining a particle count or concentration based on received light reflected by particles. The optical unit follows an optical path that suppresses direct reflections back to the detector. The detector provides a detection signal based on interference between the emitted light and the light reflected to the detector. In this way, the size of the interference-based optical sensor can be reduced without compromising sensing capability.

Description

Particle sensing system, for example for an anti-fouling mask

Technical Field

The present invention relates to sensing of particulate matter, such as for use in a breathing assistance mask.

Background

Ventilation pollution is a worldwide concern. The world health organization (world health organization) estimates that 400 million people die of air pollution each year. Part of this problem is the outdoor air quality in cities. Nearly 300 cities experiencing haze do not meet the national air quality standards.

The official outdoor air quality standard defines particulate matter concentration as mass per unit volume (e.g., μ g/m 3). Of particular interest are particles with a diameter of less than 2.5 μm (called "PM 2.5", PM: Particulate Matter ") because they are able to penetrate into the gas exchange areas of the lungs (alveoli) and very small particles (<100nm) can pass through the lungs to affect other organs.

Since the problem does not improve significantly in a short time, a common method of dealing with the problem is to wear a mask that provides cleaner air by filtration, and a tremendous surge in mask markets has been seen in recent years.

Such masks may be made of a material that acts as a contaminant particle filter, or may have a filter for only a portion of the mask surface, and the filter may be replaceable when it becomes clogged.

However, during use, the temperature and relative humidity inside the mask increase and, in combination with the pressure differential inside the mask relative to the outside, this makes breathing uncomfortable. This can be partially mitigated by providing an outlet valve or check valve that allows exhaled air to escape the mask with little resistance, but requires that inhaled air be drawn through the filter. To improve comfort and effectiveness, a fan may be added to the mask that draws air through a filter.

The benefit to the wearer of the use of a fan-powered mask is the reduction of slight pressure caused by inhalation against the resistance of the filter in conventional unpowered masks. In addition, in conventional unpowered masks, inhalation also causes a slight pressure drop within the mask, which results in leakage of contaminants into the mask, which may prove dangerous if the contaminants are toxic substances.

Therefore, the fan-assisted mask can improve wearing comfort by reducing temperature, humidity and breathing resistance.

In one arrangement, an inlet (i.e., suction) fan may be used to provide continuous extraction of air. Thus, in conventional unpowered masks, the lungs relieve slight pressure due to inhalation against the resistance of the filter. A steady flow of air may then be provided to the face and a slight positive pressure may be provided, for example, to ensure that any leaks are outward rather than inward. However, this creates additional resistance to breathing when exhaling.

In another arrangement, an exhaust (i.e., exhalation) fan may be used to provide a continuous release of air. This instead provides breathing assistance during exhalation, but, as opposed to using a passive exhalation valve, has the disadvantage of a negative pressure in the volume, such that leakage around the edges of the mask results in leakage of contaminated air into the mask volume.

Another alternative is to provide inlet and outlet fans and to time the control of the fans in synchronism with the user's breathing cycle. The respiratory cycle may be measured based on a pressure (or differential pressure) measurement. This provides improved control of temperature and humidity and reduced resistance to breathing for inhalation and exhalation.

Thus, several types of masks are available for preventing routine exposure to airborne contaminants, including passive masks, passive masks having exhalation valves, and masks having at least one active fan.

Power consumption is a problem with fan assisted masks, as fan assisted masks are battery operated devices. One way to provide power savings is to monitor the pollution level and control the fan accordingly. The environmental pollution level may be monitored and/or the filtered air within the mask may be monitored to ensure filtration performance.

The most widely used sensor technology for measuring particulate matter concentrations is based on optical light scattering. Such sensor devices typically include a heater element or fan for generating a controlled air flow, an LED or laser as a light source, and a photodetector for receiving light affected by the presence of particulate matter.

Due to the above components and optical paths, the external dimensions of these devices are typically several centimeters in length, width and depth, and are therefore not suitable for embedding in air filtration masks.

Another particulate matter sensor technology is the SMI (Self Mixing Interference) sensor. Particle sensors of this type are disclosed, for example, in WO 2018/104153. It utilizes laser sensor modules using interference or self-mixing interference for particle density detection. An advantage of SMI sensors is their very compact size. EP 3401664 discloses another sensor based on self-mixing interference. There is a mirror in the path from the laser source to the detection volume. The mirror angle can be adjusted to change the optical path difference and optimize the detection of particle density.

However, a limitation of SMI sensors is that the optical path from the laser should be free of obstacles within a distance, which may be as large as 30 cm. Otherwise, light reflected by the obstruction is erroneously interpreted as being associated with the particle. While the SMI sensor itself is very compact, this restriction on the optical path makes it difficult to embed inside the air filtration mask, and is problematic in other applications where the sensor needs to measure particulate matter in a compact interior volume.

Therefore, there is a need for a solution that can integrate optical sensors into a compact volume so that they can be used in an anti-fouling mask.

Disclosure of Invention

The invention is defined by the claims.

According to an example of an aspect of the present invention, there is provided a particle sensing system comprising:

an air chamber;

an interference-based particle sensor, comprising:

a light source for emitting light along a light path to the gas cell;

a detector for providing a detection signal, the detection signal being dependent on interference between the emitted light and light reflected by the particulate matter at the detection region towards the detector; and

a processor for determining a particulate count or concentration based on the received light reflected by the particulate;

an optical unit along an optical path that suppresses direct reflections from the optical unit to the detector, wherein the optical unit is downstream of a detection region along the optical path.

The suppression of reflection by the optical unit means that the physical length of the optical path from the light source to the optical unit can be reduced. For example, the sensor requires the optical path to be free of obstacles for a predetermined distance, which may be several tens of centimeters. Obstacles within the predetermined distance may reflect light to be detected and the sensor may erroneously interpret these signals as relating to particulate matter. The optical unit is arranged such that the detector does not receive light directly reflected from the optical unit. Thus, the space occupied by the optical path can be reduced without increasing the risk of misinterpretation of the sensor. This makes the design of the air chamber more compact. This in turn allows for a more compact design of the anti-fouling mask when used in an anti-fouling mask.

By providing an optical unit downstream of the detection area, this is for preventing reflection of light source light that passes the detection area without being reflected by the particles. It is thus avoided that reflections from downstream of the detection zone are incorrectly interpreted as being caused by interaction with the particles to be sensed.

In one set of examples, the light source and the detector are co-located and integrated into a single unit. Thus, the reflected light to be analyzed returns to the light source where the interference between the emitted light and the reflected light occurs. This provides a compact structure.

The sensor is for example a self-mixing interference sensor.

The optical unit may comprise a light absorbing material.

The light absorbing material absorbs light emitted from the light source along the light path rather than reflecting it back to the light source. Thus, the optical path is terminated before the predetermined distance is reached. This can reduce the space required for the sensor.

The optical unit instead comprises a reflector facing away from the detector. In this way, the light path is directed by the reflector along a predetermined path, rather than being reflected back to the light source.

The optical unit may then comprise a plurality of reflectors for providing an optical path length of at least 30cm before the reflection is directed back to the detector. For example, the optical path follows a zigzag path between two reflective surfaces that extend over a sufficient length to encompass multiple reflections that together achieve the desired path length.

This enables the space required for the optical path to be compressed into, for example, a folded path without reducing the length of the optical path.

It is preferable to provide a focusing means for focusing light emitted from the light source to a focal point located between the light source and the optical unit.

In this way, the sensor determines the particle count or concentration at the focus. This optimizes the performance of the sensor.

The optical unit is located, for example, between 5cm and 10cm from the focal point along the optical path.

A minimum 5cm length of the light path reduces the number of misinterpretations determined by the sensor unit to less than 1 time per second, for example. A maximum length of 10cm ensures space saving compared to an effective path length of about 30 cm.

The light emitted from the light source has, for example, a near infrared wavelength in the range of 700nm to 1000nm, for example 850 nm. Therefore, the light source uses infrared light.

The particle sensing system may be applied to an anti-fouling mask.

In one set of examples, the air chamber defines a volume that is closed relative to the face of the wearer of the anti-fouling mask, and the anti-fouling mask further includes a filter that forms a boundary between the air chamber and the ambient environment outside the air chamber.

This defines one possible general type of mask that seals the face to define an enclosed volume, and the mask body itself defines a filter.

A fan may then be provided for drawing air from outside the plenum, through the filter, to the plenum and/or from the plenum to the outside.

In this way, the fan creates an airflow between the ambient environment and the air chamber, enabling the air drawn into the air chamber to be filtered. Different fan and filter options are possible, including an intake fan, an exhaust fan, or both.

In another set of examples, the air chamber defines a volume that is open relative to a face of a wearer of the anti-fouling mask, and the anti-fouling mask further includes a fan and a filter for maintaining a pressure of the filtered air in the open volume greater than atmospheric pressure.

This defines another possible general type of mask that maintains a positive pressure so that the filtered air flow remains into the volume and the air flow leaves the open volume to the ambient environment.

In all examples, a controller may be provided that is adapted to control the speed of the fan based on the determined particle count or concentration.

Thus, the speed of the fan can be controlled based on the level of contamination in the mask (caused by the surrounding environment). Thus, the fan speed is increased for increasing pollution levels. If the contamination level is below the threshold, it may be turned off, thereby saving power. Thus, there may be different fan speed settings for different pollution level ranges.

The second sensor may be located outside the air for determining a particulate count or concentration in the ambient environment.

In this way, the particulate matter count or concentration in the gas chamber and in the surrounding environment can be determined. The controller may then be adapted to determine a difference between the particulate matter count or concentration in the gas chamber and the particulate matter count or concentration in the ambient environment.

This comparison of the particle counts or concentrations in the gas chamber and the ambient environment enables determination of filter performance. The information may then be shared with the user using the user interface unit. For example, a small difference between the determined concentration of particulate matter in the air chamber and the ambient environment may indicate a problem with the mask leak and/or the filter. It is thus possible to determine when a filter cleaning or replacement is to be performed.

These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter.

Drawings

For a better understanding of the present invention, and to show more clearly how it may be carried into effect, reference will now be made, by way of example only, to the accompanying drawings, in which:

fig. 1 shows a subject wearing a first type of mask to which the present invention is applicable;

FIG. 2 illustrates the basic principle of an SMI sensor;

fig. 3 shows a first example of an optical element in the form of a black absorbing material;

fig. 4 shows a second example of an optical unit in the form of a specular reflector;

FIG. 5 shows a graph of error counts caused by obstacles (y-axis) as a function of absorber-to-focus distance (x-axis) for a black absorber and a white reflector;

FIG. 6 illustrates another mask design to which the present invention may be applied;

FIG. 7 shows an alternative design for an interference-based sensor, rather than an SMI sensor, that may benefit from an optical unit; and

fig. 8 shows another alternative design for an interference-based sensor instead of an SMI sensor, which may benefit from an optical unit.

Detailed Description

The present invention will be described with reference to the accompanying drawings.

It should be understood that the detailed description and specific examples, while indicating exemplary embodiments of the devices, systems and methods, are intended for purposes of illustration only and are not intended to limit the scope of the invention. These and other features, aspects, and advantages of the apparatus, systems, and methods of the present invention will become better understood with regard to the following description, appended claims, and accompanying drawings. It should be understood that the figures are merely schematic and are not drawn to scale. It should also be understood that the same reference numerals are used throughout the figures to indicate the same or similar parts.

The present invention provides a system utilizing optical particle sensing having a light source, a detector for detecting the presence of light reflected back to the detector, and a processor for determining a particle count or concentration based on received light reflected by particles. The optical unit follows an optical path that suppresses direct reflections back to the detector. The detector provides a detection signal based on interference between the emitted light and the light reflected to the detector. In this way, the size of the interference-based optical sensor can be reduced without compromising sensing capability.

The present invention will be described with reference to a preferred embodiment in an anti-fouling mask.

Fig. 1 shows a subject 10 wearing a mask having a mask body 12 covering the nose and mouth of the wearer. The purpose of the mask is to filter the ambient air before it is inhaled by the wearer and to provide active control of the air flow (i.e. mask volume) into the air chamber 14. The fan 16 causes ambient air to be drawn in through a filter 18 in series with the fan. A controller 20 controls the fan and is associated with a memory 22. The first contamination sensor 24 is used to measure the particle count or concentration in the mask volume. An optional second contamination sensor 26 is used to measure ambient particle count or concentration.

A portion of the remaining portion of the mask region is formed by an air-permeable filter so that a user can inhale and exhale through the mask. Thus, with the aid of the fan acting as an air pump, air is drawn into the air chamber 14 through the mask wall by pulmonary inhalation.

FIG. 1 is a schematic diagram of but one general example with a fan for drawing air into the plenum. During exhalation, air is expelled from the air chamber 14. In this example, the exhalation is through the same air pump device and therefore also through the filter 18, and through the air permeable portion of the mask body, even though the exhausted air need not be filtered.

Alternatively, a one-way check valve (not shown in fig. 1) in the mask wall may allow exhaled air to be expelled. Since the mask chamber 14 is closed by the face of the user, the pressure within the closed chamber when the mask is worn will vary as a function of the subject's breathing cycle. There will be a slight pressure increase when the subject is exhaling and a slight pressure decrease when the subject is exhaling. Thus, the check valve may avoid the need for the wearer to exhale through the mask wall. The check valve thus opens during exhalation and closes during inhalation.

If a fan is instead used to blow air out of the air chamber, the fan may be on top of the check valve. In this case, the filter 18 is not required, and the mask body itself filters the inhaled air.

Thus, in examples where there is only one fan 16, the fan may be controlled to operate only to expel air to aid in exhalation, or the fan may be controlled to operate only to ingest air to aid in inhalation. Alternatively, the fan may be bi-directionally operable, i.e. in opposite flow directions, to sequentially assist in the exhalation and inhalation of air through the filter 18. Alternatively, there may be a separate fan for drawing air into the plenum and exhausting the air.

In the case of bi-directional flow control, the operation of the fan assembly is also synchronized with the user's breathing cycle. The breathing cycle is identified by the controller 20, and the controller 20 receives a differential pressure signal, for example from a pressure sensor device (not shown). The controller may then use the breathing cycle timing to time the operating period of the fan.

Furthermore, the breathing cycle timing may be derived from the fan speed signal (rather than based on pressure sensing) because for a given fan drive level, fluctuations in fan speed will be observed that are related to the wearer's breathing. Thus, the fan speed signal may also be used as a proxy for the pressure difference, and the fan speed signal may be used to control the fan according to the breathing cycle (by detecting the end of inspiration then switching to expiration control, and detecting the end of expiration then switching to inspiration control).

The controller 20, memory 22, fan 16 and

sensors

24, 26 are shown as separate units at separate locations, but of course some of them may be formed together as a common module.

A power source, preferably rechargeable, such as a battery (not shown) is disposed in the mask to power the controller 20, the

sensor devices

24, 26 and the fan 16.

The invention relates in particular to particle sensing. In particular, an optical particle sensor is utilized that includes a light source for emitting light along a light path in the gas cell 14, a detector for detecting the presence of light reflected by the particulate matter received along the light path, and a processor for determining a particulate matter count or concentration based on the received light reflected by the particulate matter.

One particular example of an optical sensor is an SMI sensor, for example disclosed in WO 2018/104153.

Fig. 2 shows the basic principle of an SMI sensor. The SMI sensor comprises a light source (laser) 30 which also receives reflected light entering its laser cavity. The light source typically emits light along an optical path 31, and the light is focused by a focusing lens 34. Particles 36 passing through the beam near the focal position reflect light back to the laser. This is detected by detector 32. The detector signals are provided to a processor 38 which processes the signals to derive particle counts and/or particle concentrations.

The laser is for example a diode laser with a laser cavity. If a particle is present at the focal position, it scatters the beam. A part of the radiation of the beam is scattered in the direction of the illumination beam and this part is converged by the lens on the emitting surface of the laser diode and re-enters the cavity of the laser. Radiation re-entering the cavity of a diode laser causes a change in the gain of the laser and thus a change in the intensity of the radiation emitted by the laser, and it is this phenomenon that is known as the self-mixing effect in diode lasers.

The radiation emitted by the laser or the intensity variations of the optical wave in the laser cavity may be detected by a photodiode or a detector arranged to determine the impedance variations across the laser cavity. This is the function of the detector 32. A diode or impedance detector converts the radiation changes into an electrical signal and provides electronic circuitry for processing the electrical signal.

The self-mixing interference signal may be characterized by a short signal burst or multiple signal bursts, for example. The observed doppler frequency in these signals is a measure of the particle velocity along the optical axis. Thus, the particle signal also gives the doppler frequency used to determine the particle velocity. Using the number of particles detected per time unit (count rate) and the particle velocity, the particle concentration can be determined.

For example, to simplify signal detection and signal analysis, it is preferable to use a DC drive current. The duration and intensity of the detection signal may also optionally be used to determine particle size.

The distance (and optionally the velocity) may be determined within one measurement or based on a sequence of measurement steps. For example, the DC drive current may be used during a first time period to generate a measure of particle number and velocity, and the modulated drive current to determine an erroneous object in the beam.

The SMI sensor may be comprised of more than one laser beam in different directions (and more than one return path) in order to improve concentration and velocity measurement accuracy.

Other details of possible SMI sensor designs are known to those skilled in the art.

An advantage of the SMI sensor is its very compact size, occupying less than 5 x 5mm³Thus for embedding airFilter masks are attractive. For example, the diameter of the lens may be 1.7mm and the focal length may be of the same order. However, any reflections received over a distance of about 30cm may be processed by the system such that any object within that distance will distort the particle sensing measurements. This is especially important when the sensor-to-object distance changes, resulting in a doppler frequency.

Signal processing measures may be taken to detect these objects, which are to be filtered out, but this introduces complexity and requires additional processing power, thus requiring the use of a battery.

By using an optical unit along the optical path 31, the present invention enables the provision of a sensor in a smaller volume, which suppresses direct reflections from the optical unit back to the light source along the optical path. In this way, the optical path length is reduced (terminated) or redirected. This avoids the need for complex signal processing and enables more power efficient processing of the detected signal.

The optical unit is located downstream of the detection region or detection volume. In other words, the optical unit receives light source light that is not scattered or reflected by any present particles. If this light is reflected back to the detector it can be interpreted as a signal from the particle. Thus, by preventing light from returning directly from the optical unit to the detector, even if the optical unit misinterprets, even if the optical unit is close to the detector.

When used in an anti-fouling mask, the gas chamber through which the laser is directed comprises a portion of the interior volume of the mask. The focal point (i.e. focal volume) of the laser is located at the internal volume of the mask, i.e. within the air chamber, in the flow path of the filtered air delivered by the fan (or by the wearer's breath) through the filter.

Fig. 3 shows a first example of an optical element in the form of an absorbing material 40 in black.

An alternative material is a coating consisting of a vertically aligned nanotube array (VANTA). Such coatings are known to have excellent absorption properties over a wide wavelength range. A typical wavelength for an SMI sensor is, for example, 850 nanometers. Another absorbent material is known as "metal wool", for example under the trade name Acktar Black (trade mark). Any suitable light absorbing material may be used to absorb the wavelength of the laser light.

Fig. 4 shows a second example of an optical unit in the form of a specular reflector 50 to fold the optical path in the direction of the mask (e.g. along the contour of the face from ear to ear). There may be a plurality of reflectors to define the desired path. For example, there may be a top reflector and a bottom reflector to define a zigzag folded path therebetween. The first surface 50 ensures that the beam is off normal, preventing direct reflection of the laser beam from the reflector back to the laser. The offset created by the reflector causes the entire laser beam width (as the reflector passes through the focal point, and thus is a diverging beam) to be reflected out of the field of view of the lens so that the reflected light is not focused back into the laser cavity. The path is then confined to a longer distance of, for example, 30cm or more (if not folded), and fits within the mask due to the folded nature of the light path.

In both examples, the optical unit suppresses direct reflections from the optical unit back to the light source along the optical path. For the example of fig. 3, there is no reflection. For the example of FIG. 4, there is no direct reflection, so that the light reflected back to the light source is the light reflected from the medium in the gas cell. Light may be reflected back to the light source by the optical unit 50, but this is not direct light and therefore there is no optical path from the light source to the optical unit and back to the light source.

The effectiveness of the Acktar Black (trade mark) material was tested by placing absorbers at different distances along the SMI laser optical path. Fig. 5 shows a graph of the error count through an obstacle (in counts per second, y-axis) as a function of the absorber-to-focus distance (in mm, x-axis) for a black absorber (curve 60) and for a white reflector (curve 62).

It has been found that if the absorber is located at least 5cm from the focal point, the absorption is sufficient to reduce the number of false counts to less than 1/second. In the case of a white reflector, this low count rate is only achieved at distances above 150mm, which are too large to be used as a direct path in a mask.

The above example is based on an anti-fouling mask that creates a volume that is closed against the wearer's face.

Fig. 6 shows an alternative mask design to which the present invention may be applied. The air chamber defined by the mask is no longer sealed but the fan creates an overpressure to ensure clean air in the breathing zone. In this case, the mask body 12 forms a shield covering the nose and mouth, and an air chamber is defined between the shield and the face, but it is not sealed. In the open volume, the pressure of the filtered air is maintained at greater than atmospheric pressure.

The mask has two fan and

filter units

70, 72 mounted under the wearer's ears and which deliver an air flow across the face, maintaining a positive pressure in the region near the nose and mouth. Thus, the airflow always leaves this area, avoiding unfiltered air from reaching the nose or mouth of the wearer.

The sensor is again placed in the path of the clean air within the air chamber to indicate the level of contamination of the air available for breathing. The sensor is for example arranged such that the detection point is within the air flow from the fan on the side of the mask.

The above examples are based on a self-mixing interferometric sensor design, where the reflected light to be analyzed is returned to the laser cavity, where it interferes with the emitted laser light.

The interference is caused by particle motion through the emitted beam. In particular, at different locations along its trajectory, when the reflected light from a particle mixes with the emitted light itself, a pattern of destructive and constructive interference is produced. Such interference patterns indicate the presence of particles, making particle counting feasible. The interference pattern itself also allows the particle velocity to be determined. Thus, the detection is based on the detection of an interference pattern caused by the interference between the reflection from the moving particle and the static reference beam. In particular, the optical path length through the particle to the detector is a function of the particle position and therefore varies over time. Thus, interference-based sensors utilize detection and analysis of the resulting interference patterns rather than the instantaneous intensity levels.

In the case of an SMI sensor, the interference pattern (i.e. the evolution over time between the intensity peak and the intensity valley) can be detected by a photodiode integrally formed behind the laser or within the laser cavity.

However, other forms of interference-based detection may also be used.

One such alternative is shown in fig. 7.

The light source 30 and focusing lens 34 again direct the laser light along an optical path to the gas cell in which the sample to be analyzed is present. However, the light is transmitted through a window 80 that is partially transmissive and partially reflective to the laser light. The transmitted light passes through the gas cell 14 and the reflected light passes through the detector 32. In this design, the detector is no longer co-located with the light source, but is integrated with the light source. However, when the particles move, interference occurs again between the emitted light and the reflected light, and thus the position moves from where the reflection occurs, but in this case, interference occurs between the reflection of the emitted light and the light reflected from the particulate matter (indicated by a dotted arrow in fig. 7). Thus, a detection signal depending on the interference between the emitted light and the light reflected by the particles towards the detector may use the directly emitted light or the reflected emitted light in a known manner (irrespective of the particles).

Fig. 7 also shows an optical element in the form of a light absorbing material 40. Also shown at the input to detector 32 is a bandpass filter 82 for blocking ambient light.

Fig. 8 shows another possible design. Light from the laser 30 enters the polarizing beam splitter 90 where it is reflected to the gas cell 14. The light passes through a quarter wave plate 92 which effects a 90 degree polarization rotation and passes through a filter 94 before being focused by the lens 34.

Due to the new polarization, the light reflected from the particulate matter, after passing back through the filter and the quarter wave plate, can pass through the polarizing beam splitter to the detector 32.

There is also a filter indicated by arrow 96 that air converts the reflected light. This acts as a reference beam which interferes with the light reflected by the particulate matter, thereby detecting the particulate signal. Thus, the detection is again based on signal interference.

The detector comprises, for example, a photodiode.

As shown in fig. 1, there may be a sensor for the internal mask volume and a separate sensor for the ambient environment. By combining sensors inside the mask with sensors outside the mask, the system can calculate the efficacy of the filtering and share the efficacy of the filtering with the user. In this case, a low value indicates a possible problem with the filter.

With the self-mixing design, the ambient light entering the laser cavity is negligible and does not produce doppler frequencies.

The present invention is generally applicable to laser-based optical particle sensors, such as SMI sensors or other interference-based sensors, for particle detection (i.e., measuring particle count or concentration) within a compact volume.

This may apply to an anti-fouling mask as described above, but may also be applied to other applications, such as a robotic vacuum cleaner or air purifier. Thus, the present invention is not limited to the anti-fouling mask. In various air purification systems that require feedback to improve energy efficiency or filtration efficiency, there is a need to compactly measure pollution levels.

Variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality. A single processor or other unit may fulfill the functions of several items recited in the claims. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. If the term "adapted" is used in the claims or the description, it is to be noted that the term "adapted" is intended to be equivalent to the term "configured to". Any reference signs in the claims shall not be construed as limiting the scope.

Claims

1. A particle sensing system, comprising:

an air chamber (14);

an interference-based particle sensor, comprising:

a light source (30) for emitting light along a light path to the gas chamber;

a detector (32) for providing a detection signal, the detection signal being dependent on interference between the emitted light and light reflected by the particulate matter at a detection region towards the detector; and

a processor (38) for determining a particulate count or concentration based on the received light reflected by the particulate;

an optical unit (40; 50) along the optical path suppressing direct reflections from the optical unit (40, 50) to the detector (32), wherein the optical unit is downstream of the detection area along the optical path.

2. The system of claim 1, wherein the light source and the detector are co-located and integrated into a single unit.

3. The system of claim 2, wherein the sensor is a self-mixing interferometric sensor.

4. The system according to any one of claims 1 to 3, wherein the optical unit comprises a light absorbing material (40).

5. A system according to any one of claims 1 to 3, wherein the optical unit comprises a reflector (50) facing away from the detector (32).

6. The system of claim 5, wherein the optical unit comprises a plurality of reflectors for providing an optical path length of at least 30cm before reflections from obstacles are directed back to the detector (32).

7. System according to any one of claims 1 to 6, comprising focusing means (34) for focusing the light emitted from the light source to a focal point of the detection area located between the light source (30) and the optical unit (40; 50).

8. The system according to claim 7, wherein said optical unit (40; 50) is located between 5cm and 10cm from said focal point along said optical path (31).

9. An anti-fouling mask comprising:

a mask body (12); and

the system of any one of claims 1 to 8,

wherein the air chamber (14) is defined between the mask body (12) and the face of the wearer of the anti-fouling mask.

10. The mask of claim 9 wherein said air chamber (14) defines a volume that is closed relative to the face of the wearer of said anti-fouling mask and wherein said anti-fouling mask further comprises a filter that forms a boundary between said air chamber and the surrounding environment surrounding the exterior of said air chamber.

11. The mask of claim 10 further comprising a fan (18) for drawing air from outside the air chamber into the air chamber through the filter and/or from the air chamber to outside the air chamber.

12. The mask of claim 9 wherein said air chamber (14) defines a volume that is open relative to the face of the wearer of said anti-fouling mask and wherein said anti-fouling mask further comprises a fan and a filter for maintaining the pressure of the filtered air in said open volume greater than atmospheric pressure.

13. The mask of claim 12 including a controller (20) adapted to control the speed of the fan based on the determined concentration of particulate matter.

14. The mask of any of claims 9 to 12 further comprising a second sensor (26) located outside the air chamber (14) for determining the particulate count or concentration in the ambient environment.

15. The mask of claim 14 including a controller (20) adapted to determine a difference between a particle count or concentration in said gas chamber and a particle count or concentration in said ambient environment.