This invention relates to a fire detector, and in particular to a flame detector that detects the presence of a fire by sensing the presence of a flame.

A flame detector is often installed outdoors in a harsh environment. The sensor of the flame detector looks out through a window that can become dirty, and this tends to reduce the effectiveness of the flame detector. The dirt can blow onto the window of the flame detector either as wet or dry air-born particles. There is, therefore, a need to check that the window of a flame detector is clean enough to provide accurate flame detection, and to identify and report any condition where the window is too dirty for the detector to operate correctly.

Traditionally, the window of a flame detector is checked by shining a source provided within the detector through the window onto an external reflector, which reflects that signal back onto a single sensor mounted within the flame detector. The reflected signal is measured, and a reference signal level is taken for a clean window. If the window becomes dirty, then the amplitude of the reflected signal level will be reduced.

If the reflected signal level falls below a predetermined reference level, the window is considered to be too dirty for the flame detector to operate correctly.

Known flame detectors incorporate a precise optical system whose optical path gives an accurate, reliable and consistent measurement of the reference signal level. In particular, the external reflector needs to be carefully shaped, and rigidly mounted. The external reflector may also need to be tuned to the requirements of the particular flame detector concerned. An aim of the invention is to provide a flame detector having an inbuilt optical path monitoring facility for checking the cleanliness of its window.

The present invention provides a detector comprising a housing having a window, a source of electromagnetic radiation, a plurality of sensors, and a reflector, the electromagnetic radiation source and the sensors being mounted within the housing, and the reflector being mounted outside the housing, wherein the reflector is positioned

electromagnetic radiation passing through the window from the electromagnetic radiation source onto the sensors via the window, and wherein the sensors provide an output indicative of the level of electromagnetic radiation reaching the sensors.

Preferably, the detector comprises a measuring and processing unit, wherein the sensors are operatively associated with the measuring and processing unit to provide that unit with an input indicative of the level of electromagnetic radiation reaching the sensors. The measuring and processing unit may be mounted within the housing.

Advantageously, the window is made of a material that is substantially transparent to the electromagnetic radiation emitted by the electromagnetic radiation source.

In a preferred embodiment, the detector further comprises a further reflector associated with the electromagnetic radiation source for focusing radiation from that source onto the external reflector via the window.

Conveniently, an array of sensors constitutes the plurality of sensors. Preferably, a 16 x 16 array of sensors constitutes the plurality of sensors.

Preferably, each sensor is adapted to sense electromagnetic radiation of the frequency emitted by the electromagnetic radiation source.

In a preferred embodiment, the source of electromagnetic radiation is a light source. In this way, the light source may be arranged to emit light having a frequency which matches that of a flame.

Advantageously, the source of electromagnetic radiation is arranged to emit a pulsed output signal, the pulses preferably being emitted at a regular frequency.

There may be provided two, or more, of said sources of electromagnetic radiation.

ion thus provides a method of assessing the cleanliness of a window forming part of a detector, the detector housing a source of electromagnetic radiation, a plurality of sensors and a measuring and processing unit, the detector further comprises an external reflector positioned to reflect electromagnetic radiation passing through the window from the electromagnetic radiation source onto the sensors via the window, the sensors being operatively associated with the measuring and processing unit to provide that unit with an input indicative of the level of electromagnetic radiation reaching the sensors, the method comprising the step of comparing the input from the sensors to the measuring and processing unit when the window is known to be clean and when the detector has been operating for some time, whereby the measuring and processing unit provides an indication of the state of cleanliness of the window.

Preferably, the measuring and processing unit provides an output at a reference level when the window is known to be clean, and provides an output to indicate a first predetermined level of dirtiness when the input to the measuring and processing unit differs from the reference level by a first predetermined amount.

Advantageously, the measuring and processing unit provides a second output to indicate a second predetermined level of dirtiness when the input to the measuring and processing unit differs from the reference level by a second predetermined amount.

The invention will now be described in greater detail, by way of example, with reference to the drawing, the single figure which is a schematic representation of a flame detector constructed in accordance with the invention.

Referring to the drawing, a flame detector has a housing 1 provided with a flame detector measurement and processing unit 2 for detecting the presence of a flame external to the detector through a window 3. A lamp 4 is mounted within the detector housing 1, a concave reflector 5 being associated with the lamp 4 focusing light from the lamp through the window 3 onto an external reflector 6. The lamp 4 is electrically monitored by means of a circuitry (not shown) to confirm that it is working and is in a

ing condition. The circuitry measures the current flowing through the lamp once any surge current at switch on has subsided.

The reflector 6 is angled so as to reflect light from the lamp 4 through the window 3 onto a sensor array 7 mounted within the housing 1. Typically, the sensor array 7 is constituted by a grid of 16 x 16 radiation sensors. The lamp 4 uses the same part of the electromagnetic spectrum as the measurement and processing unit 2 uses for flame detection, so that the cleanliness of the window 3 is assessed at the operating wavelength. The lamp 4 is arranged to produce a pulsed output signal. In this way, the signal is distinguished from background radiation.

The light signal reflected by the reflector 6 is measured by each of the sensors in the array 7, whose outputs are combined in the measurement and processing unit 2 to provide an accurate measurement of the cleanliness of the window 3. During or after manufacture of the flame detector, and before the detector is installed, a measurement is performed when the window 3 is clean to provide a reference level indicative of a clean window. When the flame detector is positioned for operational use, measurements are performed, either manually or automatically, on a regular basis. If such a measurement provides a level that falls below a first, predetermined threshold, the window 3 is considered to be dirty. If, however, the measured signal level falls below a second, lower, predetermined threshold, the window 3 is considered to be obscured. In either case, the flame detector is arranged to provide a warning signal of the window condition. The warning signal can, for example, be provided by differently-coloured LEDs forming part of the flame detector, or can be transmitted to a central control unit via control circuitry.

It will be apparent that the use of an array 7 of sensors averages the light signal reflected by the reflector 6, thereby giving greater resilience to tolerances in the optical path. This is particularly important where the window 3 is subjected to varying degrees of dirtiness. The use of multiple sensors also ensures that the light signal reflected by the reflector 6 can be detected over a relatively wide area. The system can, therefore, cope with greater variations in the optical path, compared to the use of a system utilising a single sensor.

As the signal is detected over a large area, the cleanliness of the window 3 is also measured over a large area, thereby resulting in an improved test of the cleanliness of the window.

It is preferred to use two lamps rather than the single lamp described above, thereby giving resilience to the system in the event of one lamp failing.