HK1208265B - Optical detection of particle characteristics - Google Patents

Description

Optical detection of particle properties

RELATED APPLICATIONS

The present application is a divisional application of a patent application having an application date of 2009, 9/4, application No. CN200980144262.2, entitled "optical detection of microparticle characteristics".

Technical Field

The present invention relates to optical particle detection.

In one form, the invention relates to an optical smoke detector that uses multiple wavelengths of electromagnetic radiation to enable detection of particles of a range of sizes. In a preferred form, the invention will be described as being carried out using 4 wavelengths of light for particle detection, although it is not to be considered that the invention is limited to such typical applications or implementations.

Background

Various methods of detecting particles in air are known. One method includes projecting a light beam through a detection chamber (containing an air sample therein), and measuring an amount of light scattered from the light beam at a particular scattering angle. Such a particle detector may be of the air-breathing type, in that it actively draws air into it, or alternatively it may rely on a natural air flow to move air into the detection chamber.

It is well known that the angular scattering properties of particles depend on the wavelength of the incident light compared to the particle size. Thus, smoke and particle detectors have been fabricated using multiple scattering angles and/or multiple wavelengths to detect particles of a predetermined size of interest. For example, ultraviolet light is scattered relatively strongly by small particles (e.g., smoke), while infrared light is scattered less by such particles. On the other hand, ultraviolet and infrared light are also sensitive to variations in the received light intensity, which are caused by factors such as drift of the system, contamination of the system optics, or the introduction of large particles (e.g., dust) into the detection chamber.

Particle detection systems having such multiple wavelengths or multiple scattering angles need to be able to accurately determine whether signals received at multiple scattering angles or wavelengths are due to particles of interest or interfering particles (e.g., dust). This wavelength sensitivity or angle sensitivity of light scattering can also be used to track the characteristics of the population of particles over time, for example to track the development of a fire as the size distribution of the smoke particles changes.

It is therefore an object of the present invention to provide a method for determining whether a particle of interest has entered a detection chamber using more than one wavelength of electromagnetic radiation.

The reference to any prior art in this specification is not, and should not be taken as, an acknowledgment or any form of suggestion that prior art forms part of the common general knowledge in australia or any jurisdiction or that this prior art could reasonably be ascertained, understood and regarded as relevant by a person skilled in the art.

Disclosure of Invention

In one aspect of the invention, there is provided a method of detecting particles in an airflow, comprising receiving signals indicative of the intensity of light scattered from the airflow at a plurality of wavelengths; the signal indicative of the intensity of light received at each of the wavelengths and the corresponding wavelength dependent parameter are processed to produce an output signal indicative of at least one characteristic of particles in the airflow.

The step of processing may comprise fitting at least one function to the signal representative of the intensity of the received light and its corresponding wavelength dependent parameter; and determining an output signal representative of at least one characteristic of the particles in the gas flow from at least one function fitted to the plurality of plotted values. Advantageously, this function can be used in the following way to characterize the particles in the gas flow.

Multiple functions or a single function may be fitted to the signal representing the intensity of received light and its corresponding wavelength dependent parameter.

The function may be, for example, a polynomial.

In some examples, the output signal may be determined from a gradient of at least one point on at least one of the functions. A plurality of linear functions may be fitted to two or more signals and their corresponding wavelength dependent parameters. In this case, the output signal may be determined from the gradient of the plurality of linear functions (e.g. by taking the average of the gradients).

The two or more signals used to fit each linear function may represent scattered light at adjacent wavelengths. Alternatively, the two or more signals used to fit each linear function may comprise signals corresponding to a common wavelength.

In an alternative embodiment, the step of processing the signal indicative of the intensity of light received at each of the wavelengths and the corresponding wavelength dependent parameter comprises applying a statistical method to the signal indicative of the intensity of light received at each of the wavelengths and the corresponding wavelength dependent parameter to produce an output signal indicative of a characteristic of particles in the airflow.

In these methods, the wavelength-dependent parameter may be wavelength, energy, or frequency; or a wavelength, energy or frequency based parameter; or some other parameter.

In some embodiments, the method may include comparing the at least one function to one or more predetermined signatures corresponding to one or more characteristics of particles in the airflow.

The step of processing the signals indicative of the intensity of light received at each of the wavelengths and the corresponding wavelength dependent parameter to generate output signals indicative of the characteristics of the particles in the air sample may comprise comparing the signals indicative of the intensity of light received at a plurality of wavelengths with one or more predetermined signatures corresponding to one or more characteristics of particles in the air flow.

Preferably, the comparison is performed at least 3 wavelengths. In an exemplary embodiment, the comparison is performed at 4 wavelengths.

One or more of the predetermined signatures may relate to characteristics of the particles in the airflow including, but not limited to, particle concentration, particle size distribution, particle color, particle composition, particle type, particle shape, fire type, stage of fire development, type of particulate-producing combustion material.

By way of example, the identification indicia may correspond to the following types of particles: smoke particles, dust particles, lint particles, other interfering particles, macroscopic foreign objects, particles in a predetermined size range.

The method may comprise normalizing any or all of the signals representative of the intensity of the received light and their corresponding wavelength dependent parameters, e.g. the values may be normalized with respect to a highest or lowest value.

The method may further comprise repeating the method one or more times to track over time at least one characteristic of the particles in the air sample characterized by the output signal.

The method may include comparing the at least one time tracked characteristic to a time based signature. This allows certain events or conditions with specific temporal characteristics to be identified. In this regard, the method may identify a condition based on the comparison. For identification, suitable conditions or events may include, but are not limited to: a fire, a change in fire (e.g., an increase in scale), a change in fuel or combustion conditions, a progression through a fire stage (e.g., from slow burning to bear burning), a fire type (e.g., type of burning material, cigarette smoke, or electrical fire, etc.), a dust generating event (e.g., an event that generates or causes dust), a transient interference condition (e.g., a dust event), a detector failure (e.g., a light source or light detector failure), an intrusion of a foreign object into a detection chamber (e.g., an insect or lint piece into a detection chamber).

The method may include pre-processing the signal indicative of the intensity of light received at a wavelength to remove the effects of background light.

The method may include determining a concentration of particles in the size range from the output signal.

In another aspect, the invention provides a particle detection system comprising a detection chamber adapted to receive an air sample, means for illuminating the air sample at a plurality of wavelengths, means for receiving scattered light from the air sample at the plurality of wavelengths and outputting a signal indicative of the intensity of light received at each of the plurality of wavelengths, processing means for processing the signal indicative of the intensity of light received at each of the plurality of wavelengths and a corresponding wavelength dependent parameter to generate an output signal indicative of at least one characteristic of particles in the air sample using the method described herein.

In yet another aspect, a particle detection system is provided, comprising a detection chamber adapted to contain an air sample; a first particle detection device comprising a first light source for illuminating a first volume of the air sample at least a first wavelength and a first light receiver having a field of view intersecting the first volume for receiving scattered light from the detection chamber and outputting a first signal indicative of the received scattered light; a second particle detection arrangement comprising a second light source for illuminating a second volume of the air sample at least a second wavelength and a second light receiver having a field of view intersecting the second volume for receiving scattered light from the detection chamber and outputting a second signal indicative of the received scattered light; a light source activation device adapted to selectively activate a first light source for a first time period and a second light source for a second time period; processing means adapted to receive a first signal from the first optical receiver and a second signal from the second optical receiver corresponding to a first time period and to process the received signals to produce a first output modified for background light corresponding to the first time period and adapted to receive the first signal from the first optical receiver and the second signal from the second optical receiver corresponding to a second time period and to process the received signals to produce a second output modified for background light corresponding to the second time period.

Preferably, the detection chamber comprises at least one wall within the field of view of each of the first and second light receivers, and wherein the first and second light receivers are positioned such that the same portion of the chamber wall is substantially within the field of view of each of the first and second light receivers.

The first and second light receivers are preferably positioned relative to their respective first and second light sources such that the field of view and the second volume of the first light receiver do not intersect and the field of view and the first volume of the second light receiver do not intersect.

The first and second wavelengths are preferably different wavelengths, but may be the same wavelength. The principle can also be extended to additional light sources and light receivers.

The first output corresponding to the first time period is preferably modified for background light by a process comprising subtracting a second signal representative of scattered light received corresponding to the first time period from the first signal corresponding to scattered light received corresponding to the first time period.

Similarly, the second output corresponding to the second time period is preferably corrected for background light by a process that includes subtracting a first signal representative of scattered light received corresponding to the second time period from a second signal representative of scattered light received corresponding to the second time period.

As used herein, unless the context requires otherwise, the term "comprise" and variations thereof, such as present participatory forms, third person referred to as singular forms and past participatory forms, are not intended to exclude additional additives, components, integers or steps.

Drawings

Preferred embodiments of the present invention will now be described, by way of non-limiting example only, with reference to the accompanying drawings. In the figure:

FIG. 1 illustrates a perspective view of a particle detection chamber made in accordance with an embodiment of the present invention;

FIG. 2 shows a cross-sectional view of the detection chamber of FIG. 1 taken along line 2-2;

FIG. 3 shows a cross-sectional view of the lower portion of the airflow path through the detection chamber along line 4-4;

FIG. 4 shows a cross-sectional view of the upper portion of the airflow path through the detection chamber along line 3-3;

FIG. 5 illustrates a partial perspective view of a portion of a detection chamber showing a volume illuminated by a light source of a detector and a field of view of a light receiver in an embodiment of the invention;

FIG. 6 shows a cross section through the upper part of the detection chamber, further showing the intersection between the field of view of the light receiver and the illumination area of the light source in an embodiment of the invention;

FIG. 7 illustrates an adjustment mode for an embodiment of the present invention;

FIG. 8 shows a flow chart of a particle detection process for use in an embodiment of the invention;

9A, 9B and 9C illustrate exemplary outputs at 4 wavelengths and 3 processing methods associated therewith using an embodiment of the present invention;

figure 10 is a graph showing the drift of the smoke detection system over time, caused by contamination of the optical surfaces of the detection chamber.

Detailed Description

Figure 1 shows a perspective view of a detection chamber 100 of a smoke detector. The detection chamber 100 is adapted to contain a sample of air drawn therein by an aspiration system (not shown) and to detect the presence of particles in the air stream. The particle detection alarm may sound if one or more alarm conditions are met. In the detection chamber of fig. 1, an air sample is drawn into an inlet 102, flows through an airflow path through the detection chamber, and is exhausted from the detection chamber 100 via an exhaust 104. A portion of the exhaust air flow may be filtered to provide a cleaning flow for cleaning the optical surfaces of the detector that are prone to particulate buildup.

Fig. 2 shows a cross-sectional view through the detection chamber 100 along line 2-2 shown in fig. 1, while fig. 3 and 4 show orthogonal cross-sections through the lower and upper arms of the flow path, respectively.

Initially, air is introduced into the inlet 102. The air then passes through a flow sensor 106. In this embodiment, the flow sensor is an ultrasonic flow sensor operating in accordance with international patent publication WO/2004/102499 filed in the name of Vision Fire & Security Pty Ltd, australia. The flow sensor ultrasonic transducers 108 and 110 are disposed on opposite sides of the centerline of the flow channel and are diagonally offset across the centerline of the flow path. The cross-section of the flow path in the ultrasonic flow sensor 106 is generally rectangular. The dimensions of the flow path may then be selected to maintain the cross-section of the flow path within the flow sensor equal to the cross-sectional area of the inlet 102, if desired. By matching these cross sections, no pressure variations are induced by the ultrasonic flow sensor, and furthermore by selecting a relatively flat rectangular shape (or other elongated shape, such as an oval shape), the vertical length of the ultrasonic flow sensor can be minimized. More particularly, this enables one of the dimensions of the flow sensor perpendicular to the direction of propagation of the ultrasonic signal to be kept small to prevent unwanted echoes in the sensor (multipath effects). The airflow then proceeds along the lower portion of the flow path and flows around the curved portion 112 and into a region of interest in the upper portion of the detection chamber.

At this time, fig. 4 shows a sectional view through the detection chamber 100. This portion of the detection chamber 100 includes some apertures, e.g., 114 and 116, in its walls, enabling the light source associated with the detection chamber to illuminate the airflow, and also enabling the light receiver to receive scattered light from within the illuminated volume. In the cross-sectional views of fig. 2 and 4, the optical components 118, 120 can be seen. These optical components 118 and 120 include one or more light sources and associated optics for illuminating the volume within the detection chamber 100. The optics associated with each of the optical components 118 and 120 may include one or more lenses and a spatial aperture to define a desired illumination volume. For obvious reasons, the light source units 118 and 120 are adapted to emit light at an angle to the centerline of the detection chamber 100, and thus the light source units 118 and 120 are at an angle to the centerline.

This is better illustrated in fig. 5 and 6, which show the illumination light cone caused by each of the light sources and the field of view of the light receiver. Referring first to FIG. 5, a partial cross-sectional view of a portion of the detection chamber 100 is shown. In this figure, a portion of the detection chamber housing 125 is shown. One of the optical components 118, including a surface mounted LED on a circuit board 130, is mounted in the rear 126 of the housing. A lens 132 is also provided. The aperture through which the light shines defines a collimating aperture to create a first illuminated volume 134. In addition, a mirrored light source component (not shown in this figure) creates a second illuminated volume 136.

As shown in fig. 6, the illuminated volumes 134 and 136 partially overlap and do not intersect the outer wall of the chamber housing 125 due to the angular offset of the light source relative to the centerline of the flow path. The illuminated volumes 134 and 136 are terminated by the angled back wall 127 of the housing 125. The wall 127 may be configured to include one or more baffles to control reflections from the surface 127.

The illuminated volumes 134 and 136 intersect the fields of view 138 and 140 of the respective aligned light receivers (e.g., photodiodes 142). The other photodiode is not shown in this figure. In this figure, the fields of view 138 and 140 are shown as cones to aid understanding. The intersection of the illuminated volume 134 and the corresponding field of view 138 creates a corresponding region of interest 144, while the illuminated volume 136 and the corresponding light receiver field of view 140 form a second region of interest 146. In this embodiment, the center line of the field of view of the photodiode is set at an angle of 67 degrees with respect to the center line of the illuminated volume formed by its corresponding light source.

In use, when a particle in the airflow passing through the detector 100 enters the field of view of one of the light receivers, a portion of the light impinging on the particle within the illuminated volume will be scattered into the field of view of the corresponding light receiver. The intensity of the received light can be used to determine the concentration of particles in the airflow.

In a preferred embodiment of the present invention, light sources 118 and 120 emit radiation at different wavelengths. Most preferably, the light source is capable of emitting at a large number of wavelengths. For example, the light source 120 may be adapted to emit light at an infrared wavelength, while the other light source 118 may be adapted to emit light at multiple wavelengths (e.g., at 3 wavelengths, one in the blue portion of the electromagnetic spectrum, one in the green portion of the electromagnetic spectrum, and one in the red portion of the electromagnetic spectrum). Other light emitting means may also be used, for example one or more broadband light sources may be used, as will be appreciated by those skilled in the art.

The light receivers, such as photodiodes 142, are preferably positioned with respect to their respective first and second illuminated volumes 134 and 136 such that the field of view of the first light receiver 142 and the volume 136 illuminated by the second light source do not intersect, and vice versa. However, as will be appreciated by those skilled in the art, the illuminated volume will not be a well-defined cone because the light intensity from the light source will be radially attenuated from the center of the illuminated volume. The exact profile will depend on the light source used and the optical configuration of the system. Thus, it should be understood that the preference for non-intersection of the field of view of the first light receiver and the second illuminated volume (and the field of view of the second light receiver and the first illuminated volume) should not be understood as requiring that there be no light from the light source within the field of view of the light receiver, but merely that the light level from the light source within the field of view of the receiver is below some acceptable threshold, e.g., the light level has dropped below the-3 dB point, or to some other percentage (e.g., 1%) of the peak intensity. Similarly, the edge of the illuminated volume may be determined based on the level of light energy.

The fields of view 138 and 140 of the two photodiodes are also arranged such that they substantially overlap at the surface of the detection chamber where they impinge. This enables a background subtraction algorithm to be performed in the detector as described below.

Returning again to light sources 118 and 120, in a preferred form, the light sources include an infrared LED 120 and a red, green, and blue (RGB) LED 118. This allows 4 wavelengths of light to be in a relatively compact physical space. Of course, in other embodiments of the invention, more or fewer wavelengths of light or electromagnetic radiation within or outside the visible range may be used.

In a preferred form, the sample volume is illuminated by each of the 4 wavelengths in turn. For example, the LED switching scheme may be performed as listed in table 1.

Table 1: light source switch and receiver state

This cycle is repeated every 8.8 msec. A graphical representation of an 8.8msec cycle for a 4 wavelength drive waveform is shown in fig. 7.

When 4 indications of light scattering (reading) are used, one indication corresponding to each wavelength of light emitted by the light source pair 118 and 120 will be detected sequentially according to the above adjustment scheme. Fig. 8 shows a flow chart 800 of how 4 indications of light scattering are processed to generate smoke detection levels according to an aspect of the invention.

In the following discussion:

each of the signals from b (t) to h (t) includes infrared, red, green, and blue components. For example C (t) having component C_IR(t)、C_R(t)、C_G(t)、C_B(t), corresponding to 4 wavelengths respectively: infrared, red, green and blue.

The signal i (t) may comprise 3 signals corresponding to a band of wavelengths, for example a band extending from red to infrared, or from blue to green.

Signals L (t), K (t), J (t) are all single signals.

Initially, the light source is illuminated as described above, and light is scattered by the particles in the corresponding region of interest. The scattered light a (t) is sensed by the corresponding primary photodetector at step 801 and then amplified. Either the high gain or low gain amplifier may be selected depending on the received light intensity.

Next, in step 802, the amplified signal b (t) is digitized by an analog-to-digital converter. In one form of the invention, 8 indications at each wavelength are noted when the corresponding LED is on, and then added to the running total. The 8 indications at each wavelength are also noted when the corresponding LED is off, and then subtracted from the accumulated sum. Narrow spikes of positive value (i.e. transient high level signals) can also be deleted in this step, since it can be assumed that these spikes are due to dust particles flowing through the region of interest. This sum is accumulated for 128 cycles (1126.4msec) giving the original smoke level at each wavelength c (t).

The background level is then subtracted at step 805 in the following manner. The result is a signal g (t).

Next, at 806, if a dust discrimination model is configured, steps 807 to 811 are performed. If the dust discrimination is off, steps 812 to 814 are performed.

In the dust discrimination path, the signal g (t) is multiplied by the measured dust normalization factors NIR, NR, NG, NB (where NIR, NR, NG, NB are the measured normalized values, which are substantially equal if the particle size is large, i.e. in the case of dust) in step 807. The result is a signal h (t).

At step 808, the "raw smoke" level is calculated for a plurality of bands in the following manner.

In one case, the raw smoke value is calculated as follows:

raw smoke in the red to infrared band (R ═ R)_R-R_IR)/(λ_R-λ_IR) (1) (wherein. lambda.)_RIs the wavelength of red light)

Green to infrared band of raw smoke (R ═ R)_G-R_IR)/(λ_G-λ_IR) (2)

Raw smoke in the blue to infrared band (R ═ R)_B-R_IR)/(λ_B-λ_IR) (3)

Alternatively, the raw smoke value may be calculated as follows:

original smoke in the blue to green band (R ═ R)_B-R_G)/(λ_B-λ_G) (4)

Green to red band original smoke ═ (R)_G-R_R)/(λ_G-λ_R) (5)

Raw smoke in the red to infrared band (R ═ R)_R-R_IR)/(λ_R-λ_IR) (6)

These raw smoke values are the slopes of the plots of signal level h (t) versus wavelength. In fig. 8, these signals are signals i (t).

The raw smoke signal in each band, e.g. "blue to green raw smoke", "green to red raw smoke", etc., can be considered as a measure of the particle concentration in the particle size range most strongly scattered by the blue to green wavelength, green to red wavelength, respectively. The raw smoke values in the band may be used to distinguish between the type of particles or smoke (or other characteristics of the particles or events that cause the emission of particles), for example if there are more particles in the size range measured by the signal "blue to green raw smoke" than in the "green to red raw smoke" range, it may be concluded that the fire is in a severe burning phase.

Fig. 9A and 9B graphically illustrate an alternative processing scheme described in connection with step 808 of fig. 8. In these figures, a plot of h (t) is shown at each of the 4 wavelengths (blue, green, red, infrared).

In fig. 9A, the slopes of 3 lines 901, 902, and 903 are calculated using equations (1), (2), and (3), respectively.

In fig. 9B, the slopes of the 3 lines 904, 905, and 906 are calculated using equations (4), (5), and (6), respectively.

In each case j (t) is generated by noting the average of 3 raw smoke signals, followed by combining 3 slope values at step 809. Alternatively, signal j (t) may be calculated as the gradient of a least squares error fit line passing through the image of the infrared, red, green, blue components of signal h (t) with respect to wavelength, as described below with respect to fig. 9C. This form of the process essentially combines steps 808 and 809 into a single step. Fig. 9C is a plot of h (t) at each of 4 wavelengths (blue, green, red, infrared). In the example shown in fig. 9C, a line is fitted to the 4 intensity indications and the slope of the line is determined.

In other embodiments, different functions may be fitted for the measurements. Such as a parabola or other function, and one or more parameters of the function may be used to determine the presence of smoke. For example, a gradient of a tangent to the curve may be determined and used to determine whether a particle of interest or interfering particle is a cause of scattered light. In some implementations, the fitting of the function does not really occur, however equivalent mathematical operations may be performed to form an approximation of the behavior of the system characterized by an ordered pair (x, y), where x is a signal representing the intensity of light received at a wavelength and y is a corresponding wavelength-dependent parameter. In an alternative embodiment, the algorithm may apply a statistical method to the plurality of measurements. For example, the average scattering level can be determined via a number of wavelengths, and the standard deviation from the mean is determined.

At step 810, the signal j (t) is multiplied by a calibration gain factor to produce a smoke level in units of "percent dim per meter". The result is a signal k (t). The negative values of signal k (t) are removed in step 811. If K (t) is negative, the result L (t) is set to zero. Otherwise L (t) is set equal to K (t). Preferably, the signal l (t) is also limited to a maximum smoke level, i.e. 32%/m.

At step 806, if a dust rejection model (dust rejection mode) is not configured, the smoke level is calculated as follows:

at step 812, the signal g (t) is multiplied by a calibration gain factor (preferably one for each wavelength). At step 813, the 4 values may be combined, e.g., superimposed or averaged (with or without scaling), and any negative values removed at step 814 and the smoke level may be output.

The output smoke level may then be further processed in any known manner to raise an alarm, according to alarm criteria.

In the above calculations, it is advantageous to normalize one or both of the following:

a signal indicative of the intensity of light received at each wavelength; and

its corresponding wavelength dependent parameter.

For example, the raw smoke levels may be normalized to the smoke level at one of the wavelengths or in one of the bands. In one example, the smoke values may be normalized to the smoke value at the longest wavelength. Similarly, the wavelength dependent parameter may be normalized for one of the parameters, e.g. to the parameter corresponding to the longest wavelength.

As described above, in preferred embodiments of the invention, the particle detection indication may be compared to an identification mark to characterize a particle in the chamber or an event that generates a particle. Such comparison with signatures may be performed at many different points in the process described above, for example any of the signals b (t) to l (t) or variants of these signals may be compared with corresponding signatures to characterise particles or particle generating events in the chamber.

In a preferred form of the invention, the slope of one or more functions fitted to the particle detection indication (at one or more points) is compared with known signatures.

In some cases, it is advantageous to track the characteristics of the detected particles over time. Such time-based data may be compared to a time-based signature. Advantageously, this allows certain events or conditions with specific temporal characteristics to be identified.

The signatures corresponding to a plurality of particle characteristics or events may be determined empirically.

At step 805, a background removal step is performed on the smoke indication e (t). This step is performed because contamination of the chamber wall will over time cause an increase in the background light to be received by the light detector.

Fig. 10 illustrates how this principle of background subtraction can be implemented in an embodiment of the present invention. Due to the physical structure of the chamber of the illustrated embodiment, each light receiver may be used to provide an indication of the "background" light level for the other light receiver. To this end, the geometry of the system is configured as:

the fields of view of the two light receivers overlap to the greatest extent at the wall of the chamber-this ensures that the measured background light is comparable.

The field of view of each light receiver should not coincide with the illumination field of the light source corresponding to the other detector-this ensures that light is not scattered directly from the light beam associated with one detector into the other receiver.

In this way, each light receiver operates as a "primary detector" to detect primary light scatter when its associated light source is illuminated, and also operates as a "secondary detector" to detect background light when its associated light source is illuminated. The design is shown in table 1, which represents the photodetector states corresponding to each illumination state in a system comprising 2 LEDs, one infrared LED and the other RGB LED, as shown in fig. 4-6.

Advantageously, this allows performing background cancellation for all measurements without including additional hardware.

The image 1000 of fig. 10 shows a long term light scatter indication or smoke indication over several years from the detector. Plot 1010 shows the level of scattered light received from the detector before any compensation is applied (e.g., b (t)). As shown, plot 1010 steadily increases over time as contaminants of the field of view and illuminated surfaces of the detection chamber appear, increasing the level of background light.

During the manufacturing process, the background light level may be measured with the main sensor in clean air to obtain a "background of main sensor manufacturing" value 1011. The background level may also be measured in clean air with the secondary sensor to obtain a "background of secondary sensor manufacture" value 1021.

In use, the values of the secondary sensors may be measured periodically (e.g., once per minute or hour) to determine the "secondary sensor in field background" values plotted by image 1020. As shown, plot 1020 also increases over time as contaminants of the field of view and illuminated surfaces of the detection chamber occur. The background value in the predicted field can then be determined as follows:

prediction in field background-background of main sensor manufacturing + (background of sub sensor-sub sensor manufacturing in field background)

If there is clean air in the room, the value of "prediction in field background" predicts the value that the primary sensor will see in the field — this is shown by plot 1030.

Thus for a single color, the signal g (t) of fig. 8 is given by:

g (t) ═ b (t) prediction in the main sensor-field background, which is plotted by image 1040 on fig. 10.

It will be understood that the invention disclosed and defined in this specification extends to all alternative combinations of two or more of the individual features mentioned or evident from the text or drawings. All of these different combinations constitute various alternative aspects of the present invention.

Claims (8)

1. A particle detection system comprising:

a detection chamber adapted to contain an air sample;

a first particle detection device comprising a first light source for illuminating a first volume of an air sample at least a first wavelength and a first light receiver having a field of view intersecting the first volume for receiving scattered light from the detection chamber and outputting a first signal representative of the received scattered light;

a second particle detection device comprising a second light source for illuminating a second volume of the air sample at least a second wavelength and a second light receiver having a field of view intersecting the second volume for receiving scattered light from the detection chamber and outputting a second signal representative of the received scattered light;

a light source activation device adapted to selectively activate a first light source for a first time period and a second light source for a second time period; and

processing means adapted to receive a first signal from the first optical receiver and a second signal from the second optical receiver corresponding to the first time period and to process the received signals to produce a first output modified for background light corresponding to the first time period; and adapted to receive a first signal from the first optical receiver and a second signal from the second optical receiver corresponding to the second time period and to process the received signals to produce a second output modified for background light corresponding to the second time period.

2. A particle detection system as claimed in claim 1 wherein: the detection chamber includes at least one wall within the field of view of each of the first and second light receivers, and wherein the first and second light receivers are positioned such that the same portion of the chamber wall is substantially within the field of view of each of the first and second light receivers.

3. A particle detection system as claimed in claim 1 or 2 wherein: the first and second light receivers are positioned relative to their corresponding illuminated first and second volumes such that the field of view of the first light receiver and the second volume do not intersect, and the field of view of the second light receiver and the first volume do not intersect.

4. A particle detection system as claimed in claim 1 or 2 wherein: the first and second wavelengths are different wavelengths.

5. A particle detection system as claimed in claim 1 or 2 wherein: a first output corresponding to the first time period is modified for background light by a process that includes subtracting a second signal representative of scattered light received corresponding to the first time period from a first signal representative of scattered light received corresponding to the first time period.

6. The particle detection system of claim 5, wherein: a second output corresponding to the second time period is corrected for background light by a process that includes subtracting a first signal representative of scattered light received corresponding to the second time period from a second signal representative of scattered light received corresponding to the second time period.

7. A particle detection system as claimed in claim 1 or 2 wherein: a second output corresponding to the second time period is corrected for background light by a process that includes subtracting a first signal representative of scattered light received corresponding to the second time period from a second signal representative of scattered light received corresponding to the second time period.

8. A particle detection system as claimed in claim 1 or 2 wherein: the first and second wavelengths are the same wavelength.