GB2513366A

GB2513366A - Improvements in or relating to optical beam detectors

Info

Publication number: GB2513366A
Application number: GB1307424.0A
Authority: GB
Inventors: Sean Quilliam
Original assignee: Fire Fighting Enterprises Ltd
Current assignee: Fire Fighting Enterprises Ltd
Priority date: 2013-04-24
Filing date: 2013-04-24
Publication date: 2014-10-29
Anticipated expiration: 2033-04-24
Also published as: GB2513366B; GB201307424D0

Abstract

An arrangement is provided for reducing the occurrence of false alarms in optical beam detectors caused by ambient electromagnetic radiation. The arrangement comprises: a photosensitive device 10; an amplifier 20; a filter 30; a bias control 40; an output 50; and an ambient monitor 60. The photosensitive device 10 receives radiation including both background radiation and a reference modulated radiation, and provides a signal representative of such to the amplifier 20. The amplifier 20 amplifies and alters the phase of the signal by 180 degrees before providing it to the filter 30. The filter 30 removes a portion of the signal attributable to background radiation and further provides a signal to the photosensitive device 10 and the ambient monitor 60. The signal fed back to the photosensitive device 10 corresponds to the background radiation having a phase and amplitude equal and opposite to the background portion of the original signal. By feeding back this signal, cancellation of the ambient portion of the photosensitive device 10 is achieved. The filter 30 may be a band-stop or notch filter.

Description

Improvements in or Relating to Optical Beam Detectors The present invention relates to a method for reducing the occurrence of false alarms in beam detectors caused by ambient electromagnetic radiation. The present invention also relates to a system for reducing the occurrence of false alarms in beam detectors caused by ambient electromagnetic radiation and electronic circuitry for reducing the occurrence of false alarms in beam detectors caused by ambient electromagnetic radiation.

Optical beam smoke detectors are known in the art. Typically, light is transmitted across a volume to be mpnitored and detected using a photosensitive device. The level of light detected is compared to previous measurements. Any reduction in the level of light detected is typically caused by obscuration in the path of the light beam. If the level of obscuration has certain characteristics, then it can be determined that the cause of the obscuration is smoke, and systems can respond to such an identification.

A problem faced by known optical bean smoke detectors is that the light transmitted across the volume to be monitored is not the only source of light that can be detected and detecting other sources can lead to false alarms, when no smoke is present in the volume being monitored but an alarm has triggered, and by which is also meant no alarm despite smoke being present in the volume being monitored.

Interference with the detector from other sources of light may be caused by just about any source of light and, at lower frequencies, an example is sunlight which provides a continuous level of light and, at higher frequencies, sources may be lighting or other sources of modulated light. To date, a known method of reducing the effect of interference of ambient light is for all of the light to be sampled and passed through various filters which then rejects unwanted portions of the signals.

Unfortunately, this known method can reject some of the wanted signal. Further, this method is not infallible as the unwanted portions of the signals -when in high levels -cause the detection stage to saturate, which has the effect that the method can no longer reject unwanted portions of the signals as it cannot differentiate between the noise and the wanted signal portion. Accordingly, the known method can fall foul of false alarms in these circumstances.

I

There is, therefore, a need for an improved method for preventing interference of a detector caused by ambient light. Further, there is a need for a method which reduces the occurrence of false alarms.

Accordingly, in a first aspect of the present invention, there is provided a method for reducing the occurrence of false alarms in beam detectors caused by ambient electromagnetic radiation, the method comprising: detecting electromagnetic radiation to produce a signal representative thereof; treating the signal to determine a portion thereof which is attributable to ambient electromagnetic radiation and to cancel the ambient portion from the signal; and preferably analysing the treated signal.

Preferably, treating the signal comprises filtering thereof. Further preferably, treating the signal comprises filtering with a band-stop filter or notch filter. Treating may further comprise treating the signal to provide a phase and amplitude equal and opposite.

Preferably, the method comprises feeding back into the detector a portion of the signal attributable to ambient electromagnetic radiation. Further preferably, the method comprises combining a signal, of a phase and amplitude equal and opposite, with the signal representative of the electromagnetic radiation.

The method may also comprise monitoring performance of the method by monitoring an output of the circuit. The method may also comprise analysing the ambient portion to determine whether detecting a saturation level of electromagnetic radiation and, optionally, reacting if a threshold is reached.

The detected electromagnetic radiation may comprise ambient electromagnetic radiation and a reference modulated electromagnetic radiation. The method may further comprise attenuating a portion of the signal attributable to the reference modulated electromagnetic radiation. Further, the method may comprise attenuating the modulated portion thereof with a notch filter or a band-stop filter.

Preferably, the method comprises combining the signal representative of detected electromagnetic radiation with the remaining signal after attenuation.

Most preferably, the method comprises any one or more of: a) receiving a current signal from a photosensitive device, converting the current signal, and preferably amplifying it, to provide a voltage signal having a phase and amplitude opposite the current signal; b) passing the voltage signal through a filter so as to attenuate a reference frequency; c) converting the voltage signal, and preferably amplifying it, to provide a current signal having a phase and amplitude opposite the current signal of a); and/or d) combining the current signal from the photosensitive device and the current signal from c) to provide a current signal representative of the reference frequency.

The method may further comprise: e) analysing the so produced reference frequency current signal to determine whether obscuration has occurreft The method may also further comprise: f) analysing the signal from b) to determine whether saturation of the circuit has occurred. Alternatively, the method may also further comprise analysing the signal from c) to determine whether saturation of the circuit has occurred.

Preferably, step a) comprises: a) receiving a current signal from a photodiode and converting the current signal and amplifying it to provide a voltage signal having a phase and amplitude opposite the current signal; or a) receiving a current signal from a photodiode and converting the current signal and amplifying it through a transimpedance amplifier providing a voltage signal having a phase and amplitude opposite the current signal.

Preferably, step b) comprises: b) passing the voltage signal through a notch filter or band-stop filter so as to attenuate a reference frequency.

Preferably, step c) comprises: c) converting the voltage signal and amphfying it through a transconductance amplifier providing a phase and amplitude opposite the current signal of a).

Preferably, step d) comprises: d) combining the current signal from the photodiode and the current signal from c) to provide a current signal representative of the reference frequency.

According to a second aspect of the present invention, there is provided a system for reducing the occurrence of false alarms in beam detectors caused by ambient electromagnetic radiation, the system comprising: a detector, for detecting electromagnetic radiation and producing a signal representative thereof; a filter, for treating the signal to determine a portion thereof which is attributable to ambient electromagnetic radiation; means for cancelling the ambient portion from the signal; and preferably means for analysing the treated signal.

Preferably, the detector is a photosensitive device, further preferably a photodiode.

Preferably, the filter comprises a band-stop filter or notch filter.

Preferably, the system comprises means for feeding back into the detector a portion of the signal attributable to ambient electromagnetic radiation. Further preferably, the system comprises means for combining a signal, of a phase and amplitude equal and opposite, with the signal representative of the electromagnetic radiation.

The system may comprise means for monitoring performance of the method by monitoring an output of the circuit against a threshold. The system may further comprise means for analysing the ambient portion to determine whether detecting a saturation level of electromagnetic radiation and, optionally, reacting if a threshold is reached.

Preferably, the system comprises means for attenuating a portion of the signal attributable to the reference modulated electromagnetic radiation and, most preferably, the means comprises a notch filter or a band-stop filter. Further, the system may comprise means for combining the signal representative of detected electromagnetic radiation with the remaining signal after attenuation.

Preferably, the system comprises any one or more of: a) a photosensitive device, for receiving electromagnetic radiation and providing a current sLgnal; b) means for converting the current signal, and preferably amplifying it, to provide a voltage signal having a phase and amplitude opposite the current signal; c) a filter, through which the voltage signal is passed so as to attenuate a reference frequency; d) means for converting the voltage signal, and preferably amplifying it, to provide a current signal having a phase and amplitude opposite the current signal of a); and/or d) means for combining the current signal from the photosensitive device and the current signal from the amplifier in d) to provide a current signal representative of the reference frequency.

Preferably, the system further comprises: e) means for analysing the so produced reference frequency current signal to determine whether obscuration has occurred.

Preferably, the system further comprises: f) means for analysing the signal from b) to determine whether saturation of the circuit has occurred. Alternatively, the system may also further comprise means for analysing the signal from c) to determine whether saturation of the circuit has occurred.

The photosensitive device may be a detector, such as a photodiode. The means for converting of a) may be a transimpedance amplifier. The filter may be a notch filter or band-stop fitter. The means for converting of c) may be a trarisconductance amplifier.

In a yet further aspect, the present invention provides electronic circuitry for reducing the occurrence of false alarms in beam detectors caused by ambient electromagnetic radiatton, the circuitry comprising: a detector, for detecting electromagnetic radiation and producing a signal representative thereof; a filter, for treating the signal to determine a portion thereof which is attributable to ambient electromagnetic radiation; means for cancelling the ambient portion from the signal; and preferably means for analysing the treated signal.

Preferably, the electronic circuitry comprises any one or more of the system features defined in relation to the second aspect.

Advantageously1 cancelling unwanted signals in line with the present invention provides a reduction in the occurrence of false alarms. Further, cancelling provides improved operation over known methods using rejection of unwanted signals, as cancelling is provided at source (i.e. at the detector or thereabouts). A notch filter provides a targeted frequency for attenuation with a relatively high degree of accuracy, as the notch is tuned to have steep roll off. Further, use of a notch in the method, apparatus and circuitry of the present invention has the effect of cancelling portions of the signal having both higher and lower modulated frequency.

Therefore, there is no loss of wanted signal and only the unwanted signal is cancelled. As such, the notch filter provides an ideal tool for determining a portion of a signal which is representative of ambient electromagnetic radiation as compared to a reference modulated electromagnetic radiation. Additionally, use of a notch is advantageous in wide spectral applications. Further, the notch is tuned to the reference modulated electromagnetic radiation. Analysing the output from the notch is advantageous as it can lead to a determination that the circuit is becoming or has become saturated and may no longer be able to perform its function properly. This analysis leads to a fault condition being signalled and not a fire condition and, therefore, has the advantage of further preventing false alarms.

As used herein, the term reference modulated electromagnetic radiation' will be understood by those skilled in the art to pertain to electromagnetic radiation having a modulated frequency which is preferred or indeed chosen for beam detection purposes and, therefore, has a reference frequency' of modulation.

The invention will now be disclosed, by way of example only, with reference to the following drawings, in which: Figure 1 is a schematic block diagram of a system for reducing the occurrence of false alarms in beam detectors caused by ambient electromagnetic radiation; and Figure 2 is a circuit diagram representative of a circuit which reduces the occurrence of false alarms in beam detectors caused by ambient electromagnetic radiation.

Figure 1 shows a schematic block diagram of a system for reducing the occurrence of false alarms in beam detectors caused by ambient electromagnetic radiation. The system, identified in general by reference 1, includes a photosensitive device 10, an amplifier 20, a notch filter 30, a bias control 40, an output 50 and an ambient monitor 60.

The photosensitive device 10 receives electromagnetic radiation, including ambient electromagnetic radiation and a reference modulated electromagnetic radiation, and provides a signal representative of the electromagnetic radiation to the amplifier 20. The amplifier 20 amplifies the signal and alters the phase of the signal by 180 degrees, and provides a signal to the notch filter 30. The notch filter 30 receives the signal and filters it to remove a portion of the signal which relates to the.

reference modulated electromagnetic radiation, and thereby determines a portion which is attributable to ambient electromagnetic radiation, and provides a further signal to the photosensitive device 10 and the ambient monitor 60. The signal received at the photosensitive device 10 (from the notch filter) relates to only the portion attributable to ambient electromagnetic radiation, which has a phase and amplitude equal and opposite to the ambient portion of the original signal provided by the photosensitive device 10, and, by feeding back in the signal from the notch filter 30, cancellation of the ambient portion of the signal provided by the photosensitive device is achieved. The modified signal, after further amplification through the amplifier 20, passes to the output 50, where further processing or monitoring may take place. The signal from the notch filter 30 received at the ambient monitor 60 is analysed I monitored for performance of the circuit, in particular the notch filter 30. The bias control 40 is provided to ensure correct operation of the photosensitive device 10.

Figure 2 shows a circuit diagram of a circuit which reduces the occurrence of false alarms in beam detectors caused by ambient electromagnetic radiation.

Specifically, Figure 2 shows a first-stage of an infra-red receive circuit and includes the following features: an infra-red photodiode D5, responsible for converting light into an electrical current source; a transimpedance amplifier, including U2C, R36, RIS, and C5, that converts the current output from the photodiode 05 into a voltage signal; a notch filter, including a gyrator circuit, which includes U2B, R14, R15, and C3 and simulates an inductor to ground, and a series Ri 3 and C4; and a basic transconductance amplifier, including U2D and R17, that converts a voltage signal into a current source, wherein those skilled in the art will understand that C designates a capacitor, D designates a diode, R designates a resistor and U designates an operational amplifier.

An output of the circuit, Output 2, may be further processed, for instance by passing it through a band-pass filter (not shown).

In an exemplary operation of the circuit, a pulse train of light is transmitted at a desired (reference) modulated frequency and strikes the photodiode D5 as well as any unwanted surrounding ambient light. Both tight sources (ambient and desired modulated frequency) are then converted into an electrical current signal by the photodiode D5. A convoluted current signal is produced which is then converted and amplified into a voltage signal, via the transimpedance amplifier U2C, R36, R18 and C5, having a 180 degrees phase shift -an opposite polarity of the original current signal. This voltage signal is now fed back into the photodiode D5; however, it first passes via the notch filter. The purpose of the notch filter is to attenuate the desired (reference) modulated frequency of the pulse train of light. By doing so, all other signals that are 180 degrees out of phase are converted into an electrical current signal via the transconductance amplifier U2D and Ri7, which cancels a portion of the current that is produced from the photodiode. Substantially all bar the modulated light signal is cancelled from the current signal of the photodiode D5, as the modulated light signal was attenuated by the notch filter and not fed back into the photodiode D5. Accordingly, signals relating to ambient light are cancelled. The remaining modulated light signal, Output 2, (i.e. the output of the first-stage of an infra-red receive circuit) may then be further processed, perhaps by passing through a band-pass filter to further attenuate any unwanted signals, and may be used in latter stages of the infra-red receive circuit for detecting obscuration of the pulse train of light.

Monitoring of performance of the circuit may be conducted at a point where the unwanted light signals are still represented as a voltage source, for example TP24 or Output 1 on Figure 2, and this monitoring may be conducted by another circuit. If the voltage signal at this point TP24 reaches a threshold, then a fault condition can be signalled, which suggests that the circuit may start to saturate and no longer be a reliable representation of the light pulse train. Such fault conditiOns are only signalled when very extreme amplitudes of ambient light strike the photodiode D5.

As will be understood by those skilled in the art, the frequency of the notch is chosen to correspond to the reference frequency' of modulation of the beam used for detection. Purely as examples, the frequency of the notch may be any value from 1 to 20 kHz or preferably from 1 to 10kHz.

Claims

Claims: 1.) A method for reducing the occurrence of false alarms in beam detectors caused by ambient electromagnetic radiation, the method comprising: detecting electromagnetic radiation to produce a signal representative thereof; and treating the signal to determine a portion thereof which is attributable to ambient electromagnetic radiation and to cancel the ambient portion from the signal.
2.) A method as claimed in claim I wherein treating the signal comprises filtering thereof.
3.) A method as claimed in claim 2, wherein treating the signal comprises filtering with a band-stop filter or notch filter.
4.) A method as claimed in any preceding claim, comprising treating the signal to provide a phase and amplitude equal and opposite.
5.) A method as claimed in any preceding claim, comprising feeding back into the detector the portion of the signal attributable to ambient electromagnetic radiation.
6.) A method as claimed in claim 4 or claim 5 when appended to claim 4, comprising combining a signal, of a phase and amplitude equal and opposite, with the signal representative of the electromagnetic radiation.
7.) A method as claimed in any preceding claim, comprising monitoring performance of the method by monitoring an output of the circuit.
8.) A method as claimed in any preceding claim, comprising analysing the ambient portion to determine whether detecting a saturation level of electromagnetic radiation and, optionally, reacting if a threshold is reached.
9.) A method as claimed in any preceding claim, wherein the electromagnetic radiation comprises ambient electromagnetic radiation and a reference modulated electromagnetic radiation.
10.) A method as claimed in claim 9, comprising attenuating a portion of the signal attributable to the reference modulated electromagnetic radiation.
11.) A method as claimed in claim 10, comprising attenuating the modulated portion thereof with a notch filter or a band-stop filter.
12.) A method as ciaimed in claim 10 or claim 11, comprising combining the signal representative of detected electromagnetic radiation with the remaining signal after attenuation.
13.) A method as claimed in any preceding claim, the method comprising: a) receiving a current signal from a photosensitive device, converting the current signal to provide a voltage signal having a phase and amplitude opposite the current signal; b) passing the voltage signal through a filter so as to attenuate a reference frequency; c) converting the voltaga signal to provide a current signal having a phase and amplitude opposite the current signal of a); d) combining the current signal from the photosensitive device and the current signal from c) to provide a current signal representative of the reference frequency.
14.) A method as claimed in claim 13, further comprising: e) analysing the so produced reference frequency current signal to determine whether obscuration has occurred.
15.) A method as claimed in claim 13 or clam 14, further comprising: f) analysing the signal from b) to determine whether saturation of the circuit has occurred.
16.) A method as claimed in any one of claims 13 to 15, wherein step a) comprises: a) receiving a current signal from a photodiode and converting the current signal and amplifying it to provide a voltage signal having a phase and amplitude opposite the current signal; or a) receiving a current signal from a photodiode and converting the current signal and amplifying it through a transimpedance amplifier providing a voltage signal having a phase and amplitude opposite the current signal.
17.) A method as claimed in any one of claims l3to 16, wherein step b) comprises: b) passing the voltage signal through a notch filter or band-stop filter so as to attenuate a reference frequency.
18.) A method as claimed in any one of claims 13 to 17, wherein step c) comprises: c) converting the voltage signal and amplifying it through a transconductance amplifier providing a phase and amplitude opposite the current signal of a).
19.) A method as claimed in claim 16, wherein step d) comprises: d) combining the current signal from the photodiode and the current signal from c) to provide a current signal representative of the reference frequency.
20.) A method for reducing the occurrence of false alarms in beam detectors caused by ambient electromagnetic radiation substantially as herein described, with reference to the accompanying description and/or any example described herein.
21.) A system for reducing the occurrence of false alarms in beam detectors caused by ambient electromagnetic radiation, the system comprising: a detector, for detecting electromagnetic radiation and producing a signal representative thereof; a filter, for treating the signal to determine a portion thereof which is attributable to ambient electromagnetic radiation; and means for cancelling the ambient portion from the signal.
22.) A system as claimed in claim 21 wherein the detector is a photosensitive device, preferably a photodiode.
23.) A system as claimed in claim 22, wherein the filter comprises a band-stop filter or notch filter.
24.) A system as claimed in any one of claims 21 to 23, comprising means for feeding back into the detector a portion of the signal representative of ambient electromagnetic radiation.
25.) A system as claimed in any one of claims 21 to 24, comprising means for combining a signal, of a phase and amplitude equal and opposite, with the signal representative of the electromagnetic radiation.
26.) A system as claimed in any one of claims 21 to 25, comprising means for monitoring performance of the method by monitoring an output of the circuit.
27.) A system as claimed in any one of claims 21 to 26, comprising means for analysing the ambient portion to determine whether detecting a saturation level of electromagnetic radiation and, optionally, reacting if a threshold is reached.
28.) A system as claimed in any one of claims 21 to 27, comprising means for attenuating a portion of the signal attributable to a reference modulated electromagnetic radiation.
29.) A system as claimed in claim 28, wherein the means comprises a notch filter or a band-stop filter.
30.) A system as claimed in claim 28 or claim 29, comprising means for combining the signal representative of detected electromagnetic radiation with the remaining signal after attenuation.
31.) A system as claimed in any one of claims 21 to 30, comprising any one or more of: a) a photosensitive device, for receiving electromagnetic radiation and providing a current signal; b) means for converting the current signal to provide a voltage signal having a phase and amplitude opposite the current signal; c) a filter, through which the voltage signal is passed so as to attenuate a reference frequency; d) means for converting the voltage signal to provide a current signal having a phase and amplitude opposite the current signal of a); and/or d) means for combining the current signal from the photosensitive device and the current signal from the amplifier in d) to provide a current signal representative of the reference frequency.
32.) A system as claimed in claim 31, further comprising: e) means for analysing the so produced reference frequency current signal to determine whether obscuration has occurred.
33.) A system as claimed in claim 31 or claim 32, further comprising: f) means for analysing the signal from b) to determine whether saturation of the circuit has occurred.
34.) A system as claimed in any one of claims 31 to 33, wherein the photosensitive device is a detector such as a photodiode.
35.) A system as claimed in any one of claims 31 to 34, wherein the means for converting of a) is a transimpedance amplifier.
36.) A system as claimed in any one of claims 31 to 35, wherein the filter is a notch filter or band-stop filter.
37.) A system as claimed in any one of claims 31 to 36, wherein the means for converting of c) is a transconductance amplifier.
38.) A system for reducing the occurrence of false alarms in beam detectors caused by ambient electromagnetic radiation substantially as herein described, with reference to Figure 1 or Figure 2 of the accompanying drawings and/or any example described herein.
39.) Electronic circuitry for reducing the occurrence of false alarms in beam detectors caused by ambient electromagnetic radiation, the circuitry comprising: a detector, for detecting electromagnetic radiation and producing a signal representative thereof; a filter, for treating the signal to determine a portion thereof which is attributable to ambient electromagnetic radiation; and means for cancelling the ambient portion from the signal.
40.) Electronic circuitry as claimed in claim 39, further comprising any one or more of the system features defined in any one of claims 21 to 38.
41.) Electronic circuitry for reducing the occurrence of false alarms in beam detectors caused by ambient electromagnetic radiation substantially as herein described, with reference to Figure 2 of the accompanying drawings and/or any example described herein.Amendments to the claims have been made as follows: Claims: 1.) A method for reducing the occurrence of false alarms in beam detectors caused by ambient electromagnetic radiation, the method comprising: detecting electromagnetic radiation to produce a signal representative thereof; and treating the signal, by filtering with a band-stop filter or notch filter, to determine a portion thereof which is attributable to ambient electromagnetic radiation and treating the signal portion to provide a phase and amplitude equal and opposite the ambient portion of the signal, to cancel the ambient portion from the signal.2.) A method as claimed in claim 1, comprising feeding back into a detector the portion of the signal attributable to ambient electromagnetic radiation.3.) A method as claimed in claim 1 or claim 2 comprising combining a signal, of a phase and amplitude equal and opposite the ambient portion of the signal, with the o signal representative of the electromagnetic radiation. Co4.) A method as claimed in any preceding claim, comprising monitoring performance of the method by monitoring an output of a circuit for implementing the method, and analysing the ambient portion to determine whether detecting a saturation level of electromagnetic radiation.5.) A method as claimed in any preceding claim, wherein the electromagnetic radiation comprises ambient electromagnetic radiation and a reference modulated electromagnetic radiation.6.) A method as claimed in claim 5, comprising attenuating a portion of the signal attributable to the reference modulated electromagnetic radiation.7.) A method as claimed in claim 6, comprising attenuating the modulated portion thereof with a notch filter or a band-stop filter.8.) A method as claimed in claim 6 or claim 7, comprising combining the signal representative of detected electromagnetic radiation with the remaining signal after attenuation.9.) A method as claimed in any preceding claim, the method comprising: a) receiving a current signal from a photosensitive device, converting the current signal to provide a voltage signal having a phase and amplitude opposite the current signal; b) passing the voltage signal through a filter so as to attenuate a reference frequency; c) converting the voltage signal to provide a current signal having a phase and amplitude opposite the current signal of a); d) combining the current signal from the photosensitive device and the current signal from c) to provide a current signal representative of the reference frequency.10.) A method as claimed in claim 9, further comprising: 0 e) analysing the so produced reference frequency current signal to co determine whether obscuration has occurred.11.) A method as claimed in claim 9 or clam 10, further comprising: analysing the signal from b) to determine whether saturation of the circuit has occurred.12.) A method as claimed in any one of claims 9 to 11, wherein step a) comprises: a) receiving a current signal from a photodiode and converting the current signal and amplifying it to provide a voltage signal having a phase and amplitude opposite the current signal; or a) receiving a current signal from a photodiode and converting the current signal and amplifying it through a transimpedance amplifier providing a voltage signal having a phase and amplitude opposite the current signal.13.) A method as claimed in any one of claims 9 to 12, wherein step b) comprises: b) passing the voltage signal through a notch filter or band-stop filter so as to attenuate a reference frequency.14.) A method as claimed in any one of claims 9 to 13, wherein step c) comprises: c) converting the voltage signal and amplifying it through a transconductance amplifier providing a phase and amplitude opposite the current signal of a).15.) A method as claimed in claim 9, wherein step d) comprises: d) combining the current signal from a photodiode and the current signal from c) to provide a current signal representative of the reference frequency.16.) A method for reducing the occurrence of false alarms in beam detectors caused by ambient electromagnetic radiation substantially as herein described, with reference to the accompanying description and/or any example described herein.17.) A system for reducing the occurrence of false alarms in beam detectors o caused by ambient electromagnetic radiation, the system comprising: co a detector, for detecting electromagnetic radiation and producing a signal representative thereof; a band-stop or notch filter, for treating the signal to determine a portion thereof which is attributable to ambient electromagnetic radiation; means for combining a signal, of a phase and amplitude equal and opposite the portion of the signal which is attributable to ambient electromagnetic radiation, with the portion of signal representative of the electromagnetic radiation; and means for cancelling the ambient portion from the signal.18.) A system as claimed in claim 17, wherein the detector is a photosensitive device.19.) A system as claimed in claim 18, wherein the detector is a photodiode.20.) A system as claimed in any one of claims 17 to 19, comprising means for feeding back into the detector a portion of the signal representative of ambient electromagnetic radiation.21.) A system as claimed in any one of claims 17 to 20, comprising means for monitoring performance of the system by monitoring an output of a circuit for implementing the system, and analysing the ambient portion to determine whether detecting a saturation level of electromagnetic radiation.22.) A system as claimed in any one of claims 17 to 21, comprising means for attenuating a portion of the signal attributable to a reference modulated electromagnetic radiation.23.) A system as claimed in claim 22, wherein the means comprises a notch filter or a band-stop filter.24.) A system as claimed in claim 22 or claim 23, comprising means for combining the signal representative of detected electromagnetic radiation with the remaining o signal after attenuation. Co25.) A system as claimed in any one of claims 17 to 24, comprising any one or more of: a) a photosensitive device, for receiving electromagnetic radiation and providing a current signal; b) means for converting the current signal to provide a voltage signal having a phase and amplitude opposite the current signal; c) a filter, through which the voltage signal is passed so as to attenuate a reference frequency; d) means for converting the voltage signal to provide a current signal having a phase and amplitude opposite the current signal of a); and/or e) means for combining the current signal from the photosensitive device and the current signal from an amplifier in d) to provide a current signal representative of the reference frequency.26.) A system as claimed in claim 25, further comprising: f) means for analysing the so produced current signal representative of the reference frequency of e) to determine whether obscuration has occurred.27.) A system as claimed in claim 25 or claim 26, further comprising: means for analysing the signal from b) to determine whether saturation of the circuit has occurred.28.) A system as claimed in any one of claims 25 to 27, wherein the photosensitive device is a detector such as a photodiode.29.) A system as claimed in any one of claims 25 to 28, wherein the means for converting of b) is a transimpedance amplifier.30.) A system as claimed in any one of claims 25 to 29, wherein the filter is a notch filter or band-stop filter.31.) A system as claimed in any one of claims 25 to 30, wherein the means for o converting of d) is a transconductance amplifier. Co32.) A system for reducing the occurrence of false alarms in beam detectors caused by ambient electromagnetic radiation substantially as herein described, with reference to Figure 1 or Figure 2 of the accompanying drawings and/or any example described herein.33.) Electronic circuitry for reducing the occurrence of false alarms in beam detectors caused by ambient electromagnetic radiation, the circuitry comprising: a detector, for detecting electromagnetic radiation and producing a signal representative thereof; a band-stop or notch filter, for treating the signal to determine a portion thereof which is attributable to ambient electromagnetic radiation; means for combining a signal, of a phase and amplitude equal and opposite the portion of the signal which is attributable to ambient electromagnetic radiation, with the portion of signal representative of the electromagnetic radiation; and means for cancelling the ambient portion from the signal.34.) Electronic circuitry as claimed in claim 33, further comprising any one or more of the system features defined in any one of claims 17 to 32.35.) Electronic circuitry for reducing the occurrence of false alarms in beam detectors caused by ambient electromagnetic radiation substantially as herein described, with reference to Figure 2 of the accompanying drawings and/or any example described herein. a) Co