Reliable smoke detectors are available at moderate cost, but they are often easily triggered by a range of false alarm stimuli, such as steam and aerosol sprays.

More advanced detectors exist, better able to discriminate, for example via the specific detection of carbon monoxide by a (relatively expensive) platinum detection cell.

There is a need for a simple device able reliably to distinguish between combustion products and other airborne material.

The present invention is such a device.

It is known (eg US2013234856) to use unpolarised light of two different wavelengths and measure the ratio of scattering at each frequency.

It is known (eg WO2013121192) to use unpolarised light at a first wavelength and to detect light at another wavelength, relying on luminescence for detection.

It is known (eg GB2277589) to compare the scattering of two beams of light differently polarised, a first beam polarised in the plane of scattering and a second beam polarised perpendicularly to the plane of scattering.

It is known (eg GB2267342) to use two light sources "alternately energised" to measure scattering.

It is known (US5280272) to measure polarisation caused by scattering of unpolarised light, by the use of differently aligned polarising filters in front of multiple detectors.

The present invention is a smoke detector whose operational parts comprise a single laser light source, at least two photo-detectors, a plurality of optical polarising filters and a processing unit, where the laser produces a beam of light which passes through a first said filter to generate a beam of polarised light; the beam of polarised light is scattered by airborne material in a scattering region, resulting in scattered light; at least two photo-detectors are disposed at different scattering angles to receive scattered light;

each photo-detector has a respective secondary said filter aligned perpendicularly to the first said filter through which secondary filter scattered light must pass in order to reach the respective photo-detector; each photo-detector generates a signal in response to light incident on it; the processing unit processes signals from at least two photo- detectors to generate a scattering coefficient; and the processing unit undertakes a computation using the scattering coefficient and pre-defined conditions to determine whether to generate an alarm signal.

There is also provided a smoke detector whose operational parts consist essentially of the features specified above.

The device may have a first photo-detector disposed with a scattering angle in the range 0 to 90 degrees, and a second photo-detector disposed with a scattering angle in the range 90 to 180 degrees.

The device may have said photo-detectors disposed with scattering angles in the ranges zero to 30 degrees and 150 to 180 degrees.

The device may have said photo-detectors disposed with scattering angles of 18 and 162 degrees to within manufacturing tolerance.

The laser may comprise a blue laser. The laser may comprise a 405 nanometre wavelength laser.

The laser may comprise a solid state laser. The laser may comprise a semiconductor laser.

The device may be able to distinguish between combustion products and other airborne material. This distinction may be based on the value of a scattering coefficient. The scattering coefficient may be the ratio of outputs from two photo- detectors.

The output of each photo-detector may be amplified. The output of each photo- detector may be normalised by the subtraction of a background value.

Figure 1 shows a schematic representation of an embodiment of the present invention.

Figure 2 shows characterisation plots for several airborne materials measured by an embodiment of the present invention.

In the present invention (100) an unpolarised beam of light (104) from a laser (102) passes through a polarising filter (106) to create a polarised beam (108) which enters a scattering region, where it can interact with airborne materials (110). Some of the beam (108) passes straight through unscattered (112) and enters a light trap (114) to prevent unwanted reflection. Some of the beam (108) is scattered in many directions.

The laser (102) may be a solid state laser. The laser (102) may be a semiconductor laser.

The laser (102) may operate at any suitable wavelength. A suitable wavelength may be equal to or less than the diameter of airborne particles from which it is desired to detect scattering. The laser (102) may be a red laser. The laser may be a laser of wavelength 650 nanometres.

The laser (102) may be a green laser. The laser may be a laser of wavelength 532 nanometres.

The laser (102) may be a blue laser. The laser may be a laser of wavelength 405 nanometres.

The laser may be an ultra-violet laser. Preferably the laser beam (104 and 108) is narrow and non-divergent.

The laser (102) may be operated continuously.

The laser (102) may be operated in pulsed mode. This offers the possibility of reduced energy consumption versus continuous operation; and/or there may be higher power during the pulse (giving better sensitivity). The invention comprises a container (not shown) which allows air and airborne material (110) to enter (for example by convection from below) and to pass into the region where scattering occurs. As is well known, entry and exit ports may take any suitable form. The entry and exits ports may be substantially spiral and internally matt black so that physical material (110) may enter, but light access from outside is minimised.

The invention is concerned with light scattered in at least two directions. For clarity Figure 1 shows two directions with respective scattering angles (201a, 201b). The scattered light (203a, 203b) passes through secondary polarising filters (205a, 205b) oriented perpendicularly to the first polarising filter (106). Thereafter the light (207a, 207b) passes into a respective photo-detector (209a, 209b).

The use of crossed polarising filters (106 versus 205a and 205b) ensures that no light is detected by a photo-detector (209a, 209b) unless it has undergone a change of polarisation, for example by being scattered.

Suitable photo-detectors (209a, 209b) may be selected to match the wavelength of the laser (102) as is well known. At least one photo-detector (209b) may be located at a forward scattering position (ie with a scattering angle (eg 201b) between zero and ninety degrees).

At least one photo-detector (209a) may be located towards the backscattering position (ie with a scattering angle (eg 201a) between 90 and 180 degrees). The laser beams (104, 108, 112, 203a, 203b, 207a, 207b) may be co-planar.

The photo-detectors (209a, 209b) are operably connected (211a, 211b) to a processing unit (301). The connections (211a, 211b) may be electrical. Electrical connections (211a, 211b) may contain active electronic circuitry (for example: amplifiers, and/or analogue-to-digital converters).

The output from each photo-detector (209a, 209b) may be amplified before being used for computation. Electrical amplification may be obtained by using well known electronic circuit techniques, for example using an operation amplifier.

The processing unit (301) samples the output of the photo-detectors (209a, 209b) from time to time. A suitable sampling rate may be once per second. It (301) performs a computation involving at least those outputs, at least one threshold level and optionally a repeat count and/or time interval. When suitable conditions are satisfied the processing unit (301) generates an alarm signal (303).

The processing unit (301) may be digital. The processing unit (301) may be a single chip computer with embedded software. The processing unit (301) may use an analogue-to-digital converter to sample the outputs of the photo-detectors (209a, 209b).

The processing unit (301) may subtract a background value from the outputs of the photo-detectors (209a, 209b) before performing further computation. The background value may be a pre-defined value. The background value may be automatically generated by the processing unit (301) storing the minimum signal value recorded from each photo-detector (209a, 209b) while the laser (102) is emitting light. As is well known, the alarm signal (303) may be communicated to other devices, and/or may be used directly by the invention (100) to generate a warning. A suitable warning may be an audible warning and/or a visible warning, or any other suitable means. In order to reduce false positives, the processing unit (301) may ignore sample data if the output of at least one photo-detector (for example 209b) is smaller than or equal to a pre-configured threshold value. If the output of the said selected photo-detector (for example 209b) is greater than the said pre-configured value, the processing unit (301) calculates a scattering coefficient. A suitable scattering coefficient may be the ratio of the outputs of two photo- detectors. For example, referring to Figure 1, a suitable scattering coefficient may be the ratio of the output of a first photo-detector (eg 209a) to the output of a second photo-detector (eg 209b). Taking a ratio has the advantage of removing dependency on the amount of substance (110) causing scattering. Generally the more substance (110) present, the greater is the amount of scattered light (203a, 203b), but the ratio is broadly unchanged.

If the scattering coefficient lies within a pre-determined range, this is a selective indication of the presence of a combustion product (110) within the device (100) and the processing unit (301) may generate an alarm signal (303).

Certain embodiments of the present invention may generate an alarm signal (303) promptly on each such selective indication.

To reduce false positives, certain other embodiments may generate an alarm signal (303) only after multiple such indications.

Certain such embodiments may generate an alarm signal (303) only after a predefined number (for example five) of successive indications. The number may be selected as appropriate for a particular application.

Certain such embodiments may generate an alarm signal (303) when a pre-defined number (eg six) of indications occur within a larger number (eg ten) of successive samples. The numbers may be selected as appropriate for a particular application. Certain embodiments may generate an alarm signal (303) only when a positive indication coincides with a positive indication from at least a second device. Such a device may be a heat sensor or another smoke detector.

The device (100) may be powered by an internal battery and/or an external power supply.

The device (100) preferably contains miniature components such that the whole device (including a battery, if required) fits inside a container of about 12 centimetres in diameter.

One embodiment of the present invention (100) is described in detail. The laser (102) is a blue 405 nanometre wavelength laser from Odic Force Lasers, product number OFL229/001 (wavelength 405 ± 5 nanometres, output power 30 milliwatts, laser class 3B). The width of the laser beam is approximately one millimetre. In this embodiment the laser (102) is operated in continuous mode. The polarising filters (106, 205a, 205b) are made from polarising film from Edmund Optics (Stock number #86-186, film thickness 750 microns, wavelength range 400 to 700 nanometres, polarisation efficiency greater than 99%, extinction ratio 9,000:1). The first polarising filter (106) polarises the light horizontally. The secondary polarising filters (205a, 205b) pass only vertically polarised light.

This embodiment uses two photo-detectors (209a, 209b) disposed co-planar with the incident laser beam (104, 108) with a first scattering angle (201b) of eighteen degrees, and a second scattering angle (201a) of one hundred and sixty two degrees. The photo-detectors (209a, 209b) are Osram SFH 203PIN photodiodes sourced from RS Components Ltd (RS stock number 654-8154).

The path length from the scattering substance (110) to each photo-detector (209a, 209b) is about 40 millimetres. This embodiment records data from the photo-detectors (209a, 209b) once per second. Data points where the signal from any photo-detector is less than or equal to a threshold value are discarded. In this embodiment the threshold level was set to ten times the noise level observed in the absence of substance (110).

In this embodiment the output of each photo-detector was amplified by an operational amplifier circuit (as is well known) before being used for computation. The device (100) is initially operated with no scattering material (110) present, in order to obtain background levels from each photo-detector (209a, 209b), and these levels are then subtracted from the data before computation of scattering coefficient.

The processing unit (301) calculates a scattering coefficient as the ratio of the signal (smaller) from the photo-detector (209a) with scattering angle 162 degrees to the signal (larger) from the photo-detector (209b) with scattering angle 18 degrees.

The inventors used this embodiment to characterise a range of combustion products and control materials. For each substance the scattering coefficients recorded showed some limited statistical variation. These data are represented in Figure 2 as ranked cumulative plots. Each line represents one material, either a combustion product (5xx) or a control material (6xx).

Line 501 represents smoke from burning paper (554 data points)

Line 502 represents smoke from burning rubber (326 data points)

Line 503 represents smoke from burning cotton (205 data points)

Line 504 represents smoke from burning wood (330 data points)

Line 601 represents SURE ^® (a commercial aerosol product) (205 data points) Line 602 represents GLADE ^® (a commercial aerosol product) (220 data points) Line 603 represents OUST ^® (a commercial aerosol product) (242 data points) Line 604 represents steam (298 data points)

Cooking oil, hexane and ethanol each produced no smoke and no signal. In Figure 2 the x-axis represents the scattering coefficient as described above. The y- axis represents the fraction of data points for each material falling above the respective scattering coefficient. Figure 2 thus represents a cumulative chart of the experimental results. The co-ordinates of a point on any line indicate the probability that data has a respective probability greater than the respective scattering coefficient.

It can be seen that materials (5xx) for which a positive signal from a smoke detector is required lie to the left of the chart, and control materials (6xx) to the right.

The data presentation of Figure 2 allows a suitable warning range of scattering coefficients for this embodiment to be determined. In this case it can be seen that the warning range is to extend from zero to an upper bound around 0.16. The vertical dashed line in Figure 2 represents this position.

Too low a value of this upper bound threshold gives false negatives (ie failures to detect smoke), whereas too high a threshold gives false positives (ie false alarms due to non-smoke control materials). As can be seen from Figure 2, choosing an upper bound of 0.16 for the scattering coefficient for this embodiment gives a good compromise with a small number of false negatives for wood smoke (about 6% of samples); negligible false negatives for the other types of smoke; and a small number of false positives for control substances (the largest value being about 7% for SURE ^® ).

Selection of the upper bound may be varied according to application, depending on the relative application-specific weighting given to the undesirability of false positives versus false negatives. Different choices of laser and/or scattering angle give different characterisation plots, and so may allow suitable embodiments to be created with good discrimination for a range of specific applications. While the present invention has been described in generic terms, those skilled in the art will recognise that the present invention is not limited to the cases described, but can be practised with modification and alteration within the scope of the appended claims. The Description and Figures are thus to be regarded as illustrative instead of limiting.

Where reference is made to numerical values, it is intended that a range from 10% less than the numerical value to 10% more than the numerical value is intended. Throughout the description and claims of this specification, the words "comprise" and "contain" and variations of the words, for example "comprising" and "comprises", mean "including but not limited to", and are not intended to (and do not) exclude other components, integers or steps. All documents referred to herein are incorporated by reference.

Various modifications and variations of the described aspects of the invention will be apparent to those skilled in the art without departing from the scope and spirit of the invention. Although the invention has been described in connection with specific preferred embodiments, it should be understood that the invention as claimed should not be unduly limited to such specific embodiments, indeed, various modifications of the described modes of carrying out the invention which are obvious to those skilled in the relevant fields are intended to be within the scope of the following claims.