GB2401936A

GB2401936A - Light sensor for smoke detector

Info

Publication number: GB2401936A
Application number: GB0311966A
Authority: GB
Inventors: Roger Barrett
Original assignee: Apollo Fire Detectors Ltd
Current assignee: Apollo Fire Detectors Ltd
Priority date: 2003-05-23
Filing date: 2003-05-23
Publication date: 2004-11-24
Also published as: GB0311966D0

Abstract

A smoke detector has a light sensor 56 positioned such that a light-receiving surface of the sensor extends generally parallel to a light beam generated by a light source 50 of the smoke detector. The light sensor has an enclosure 58 surrounding the light-receiving surface, which enclosure has an aperture in a side facing the light beam. The front 64 and rear edges 62 of the aperture (with reference to the direction of the light beam), as well as the two side edges extending normal to the front and rear edges, are positioned so as to affect the amount of scattered light from the light beam that reaches the light-receiving surface. In particular, the position of the front and rear edges of the aperture can be used to create a balance of sensitivity to forward-scattered pale-coloured smoke and backward-scattered dark-coloured smoke. The light-receiving surface may be a light-sensitive surface of a plane silicon PIN photodiode.

Description

240 1 936

LIGHT SENSOR FOR SMOKE DETECTOR

The present invention relates to a smoke detector, and in particular to a light sensor for detecting light scattered from smoke particles in such detector.

In smoke detectors based on sensing light scattered from a light beam by smoke particles, the light scattered onto a sensor is a tiny fraction of the total light emitted by a source of the light beam. The fraction depends on the smoke density, and typically is in the order of 0.001% at the alarm point of a general-purpose fire detector.

Reliably sensing such a signal is difficult. It is there- fore highly desirable to design the smoke detector so as to maximize the amount of scattered light falling on the light sensor.

One way to achieve such aim is to increase the amount of energy emitted by the light source. However, this has two disadvantages. Firstly, it requires that the operating power of the smoke detector be increased. Secondly, it reduces the lifespan of the light source. Because of these disadvantages, it is preferable to improve the amount of scattered light falling on a light sensor by, if possible, improving the light collection efficiency of the light sensor through passive means.

A further aim in the design of smoke detectors is to achieve a lightsensor response that is uniform for a range of smoke colours and types. It is well-known that pale- coloured smoke ("pale smoke") produced by shouldering fires gives higher levels of scattered light than the darkcoloured smoke ("dark smoke") produced by flaming fires.

It is also well-known that the fraction of light scattered in different directions varies with the colour of the smoke.

Pale smoke and liquid aerosols entering a light beam will cause light in the light beam to scatter predominantly in forward directions, i.e. those directions having a component that is parallel to the light beam and in an outward direction (away from the light source); on the other hand, dark smoke will cause light to scatter proportionally more in directions with a component that is parallel to the light beam and in an inward direction (toward the light source).

The two foregoing types of scattered light are hereafter respectively referred to as 'forward scattered light' and 'backward scattered light'.

Most smoke detectors presently available are able to sense forward scattered light, and are designed to maximize the amount of such light sensed. As a result, most such detectors have a poor response to dark smoke relative to their response to pale smoke.

In regard to either forward-scattered light or backward-scattered light, the amount of scattered light reduces with an increase in the angle formed with the direction of the light beam. Thus the amount of scattered light is at a minimum in a direction normal to the direction of the light beam. Moreover, in such normal direction the difference in the level of scattered light produced by pale smoke and that produced by dark smoke tends to be at a minimum. There is thus the advantage in measuring the scattered light extending normal to the light beam in that a uniform response to different-coloured, i.e. both pale and dark, smoke entering the light beam is possible. However, since the scattered light is at a minimum in the normal direction to the light beam, the design of a suitable light sensor for sensing in such normal direction is difficult.

A first object of the preferred embodiment of the present invention is to achieve a balance between the foregoing factors, i.e. using a sensing direction that allows a uniform response to the presence of differentcoloured smoke, but optimizing the amount of scattered light receivable by a sensor in that sensing direction.

A second object is to achieve an optical arrangement that is compact, so as to facilitate a smoke detector with a low-profile design.

The light sensor of the subject invention is intended to detect scattered light extending at a normal to the light beam, and also to detect forwardscattered light and backward-scattered light each extending within a respective defined angle on a respective opposite side of the normal; the sum of the two defined angles is termed 'the sensor acceptance angle'.

The subject invention is a smoke detector having a light sensor, the light sensor being positioned such that its optical axis extends generally perpendicular to a light beam generated by a light source of the smoke detector, the sensor having first and second light shields positioned between the light beam and the light-receiving surface for restricting the amount of scattered light from the light beam that strikes the light-receiving surface, the position of the shields being chosen in accordance with a desired ratio of forward scattered light striking the light- receiving surface to backward scattered light striking that surface.

Preferably, the first and second light shields are formed as a first pair of opposed edges of an aperture positioned between the light beam and the light-receiving surface, scattered light passing from the light beam to the light-receiving surface through the aperture. More preferably, the aperture is a rectangular or square aperture of a frame, with the first pair of opposed edges extending generally normal to the light beam, and with a second pair of opposed edges extending generally parallel to the light beam. Even more preferably, the frame is spaced from the lightreceiving surface such that, and the pair of first edges are positioned relative to the light-receiving surface such that, light scattered from the light beam strikes the light-receiving surface at an angle of 45 or more. Still more preferably, the second edges of the frame are positioned such that each second edge is approximately on a respective line extending between a respective side boundary of the light beam and a respective corresponding side edge of the light-receiving surface.

Preferably, the frame is formed as part of an enclosure surrounding the light-receiving surface.

Preferably, the light sensor may be a photodiode. More preferably, the photodiode is a silicon Positive-Intrinsic- Negative (PIN) photodiode.

Preferably, the frame is spaced approximately 3mm from the longitudinal axis of the light beam.

Preferably, the light-receiving surface is spaced 10 mm or less from the longitudinal axis of the light beam.

More preferably, the light-receiving surface is spaced 8mm or less from the longitudinal axis of the light beam. Even more preferably, the lightreceiving surface is spaced 6mm or less from the longitudinal axis of the light beam.

Preferably, the smoke detector has a base, a cover mounted for covering the base, and a circuit board carrying the light source. When the cover is covering the base, the base and cover together define an internal chamber adapted to house the circuit board and to receive smoke but not external light.

Preferably, the smoke detector also includes a cylindrical condenser lens positioned between the light beam and the light-receiving surface, the axis of the cylindrical surface of the lens extending parallel to the light beam.

For an optimized result, the light-receiving surface of the light sensor needs to be positioned near to the light beam. An analysis of the geometry of light scatter from a narrow light beam shows that the scattered-light energy density in a plane extending parallel to the light beam varies approximately reciprocally with distance from the center of the light beam. This suggests that, for intercepting as much scattered light as possible, the light receiving surface of the light sensor should be positioned as near to the center of the light beam as structural and other considerations allow.

Also, the energy density is found to be a function of the sine of the angle of incidence, i.e. the angle which the scattered light forms with the light-receiving surface. For angles of incidence between 45 and 135 , the sine value varies more slowly with angle than for angles outside that range. This suggests that it may not be critical to mount the light sensor such that its light-receiving surface in normal to the direction in which the scattered light is being measured. However, in practice there are significant benefits in mounting the light sensor such that its light receiving surface is parallel to, or approximately parallel to, the axis of the light beam. By mounting the sensor parallel to the longitudinal axis of the light beam, the sensor can be placed closer to the light beam. Such close spacing more than compensates for the loss in sensitivity caused by the reduced angle of incidence. The close spacing also assists in maintaining very compact sensor geometry, even when a wide field-of-view is used. This allows a designer to achieve a compact smoke detector design that is attractive to building owners and occupiers.

It is known in smoke detectors to attempt to increase collection efficiency by positioning a condenser lens in front of a light sensor. Such condenser lens will collect light over a large area for concentration onto the light receiving surface of the sensor, and allows for significant gains. However, the acceptance angle over which the scattered light is collectable varies inversely with the optical gain, and it is not therefore appropriate to use a spherical condenser in the present arrangement since it would then be difficult to achieve the desirable wide angular field in the plane parallel to the light beam. It is possible, however, to use a cylindrical condenser having a longitudinal axis parallel to the light beam. Since the angular field-of- view in the plane normal to the light beam is limited in comparison to that in the plane parallel to the light beam (as will be further described), a useful increase in collectable light is achievable with use of the cylindrical condenser lens.

Preferred features of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 is an exploded perspective view of an embodiment of the smoke detector of the invention, the view showing a base, a cover, a circuit board and components mountable on the circuit board; Figure 2 is an exploded side view of the smoke detector of Figure 1; Flqure 3 is a schematic side view of a light source, a light beam produced by the light source, and a light sensor in the preferred embodiment of the invention; Figure 4 is a schematic end view of the light source, light beam and light sensor of Figure 3; Figure 5 is a schematic side view similar to Figure 3 but also illustrating a cylindrical condenser lens; and, Figure 6 is a schematic end view similar to Figure 4 but also illustrating the cylindrical condenser lens.

The smoke detector in Figures 1 and 2 has a base 20, a cover 22 adapted to fit on the base 20, and a circuit board 24 adapted to be fitted between the base 20 and the; cover 22. On the circuit board 24 is fitted a compact light source 26; this could be a light-emitting diode (LED) or similar compact light source. A light sensor is generally designated 30, and a pair of holes 32 each extend on an opposite side of a line drawn between the light source 26 and the light sensor 30. The smoke detector further includes a lens carrier 34 which includes a carrier base 36, a lens 38 mountable into the carrier base 36, and a carrier cover 40 mountable on the carrier base 36 to hold the lens 38 on the carrier base 36. The lens carrier 34 is moulded plastic and has an integral pair of legs 42 each having a hooked end 44 intended to extend through a respective hole i 32 in the circuit board 24 to flexibly engage with an underside of that board so as to secure the lens carrier 34 to the board 24.

The light source in Figures 3 to 6 is generally shown as the box 50, and is not further described. Although the box 50 may represent a LED whose light is turned 90 by a lens to form a light beam 52, such as in the arrangement shown in Figures 1 and 2, the box 50 may also represent a LED or other light source that is oriented so as to directly transmit its output as the light beam 52. The light beam 52 is of small diameter and low divergence, such that very little light strays outside the main envelope of the beam.

The light sensor 30 has a light-receiving surface 56 enclosed within an enclosure 58. The light-receiving surface 56 is a sensitive surface of a plane silicon Positive-Intrinsic-Negative (PIN) photodiode that has an approximately-Lambertian variation of sensitivity with angle of incidence. The surface 56 is planar, though a surface with slight curvature might also be used. The planar surface 56 extends generally parallel to the axis 60 of the light beam 52. The enclosure 58 has a rear edge 62 and a front edge 64 that, together with side edges 66 and 68 (see Figure 4) define an aperture 70 of the light sensor 30.

The aperture 70 is sized to restrict the angles over which the light sensor 30 receives light scattered from the light beam 52. The position of the rear edge 62 of the aperture 70, forming an angle "A" with a front edge of the light-receiving surface 56, determines the field-of-view for forward-scattered light; forward-scattered light at any greater angle is prevented from reaching the light-receiving surface 56. Similarly, the position of the front edge 64 of the aperture 70, forming an angle "B" with a rear edge of the light-receiving surface 56, determines the fieldof view for backward-scattered light; backward-scattered light at any greater angle is prevented from reaching the light- receiving surface 56. The value of the two fields-of-view can be set independently of each other by the positioning of the rear edge 62 and the front edge 64 of the enclosure 58, and this provides a means for the smoke detector to achieve a balance of sensitivity to both pale smoke and dark smoke.

As shown in Figure 4, the position of the side edges 66 and 68 of the aperture 70, forming an angle "C" with the respective side edges of the light-receiving surface 56, determine the lateral field- of-view for both the forward scattered light and the backward-scattered light. The lateral field-of-view extends just outside the side boundaries of the light beam 52 so as to exclude, as far as possible, any light scattered outside the light beam 52 or any stray reflected light from reaching the light-receiving surface 56.

The light sensor 30 is mounted on the circuit board 24.

The chamber formed by placing the cover 22 on the base 20 of the smoke detector houses the circuit board 24 and has dark walls to reduce the possibility of reflected light from the light source 50 reaching the light sensor 30. The cover 22 and base 20 each has perforations and baffles arranged on its periphery such that external light is unable to enter the chamber they form when connected, but smoke carried by the outside air is able to easily enter the chamber.

In this preferred embodiment, the diameter of light beam 52 is 3mm, and the upper face of the enclosure 58 is situated approximately 3mm from the longitudinal axis of the light beam 52. This separation distance ensures that small insects walking across the upper face of the enclosure 58 will not enter the path of the light beam 52 to cause light scattering that will trigger a false alarm.

The distance between the light-receiving surface 56 and the aperture 70 is made as small as possible so as to achieve the field-of-view values discussed above. In this embodiment, the light-receiving surface 56 is positioned approximately 3mm below the aperture 70. Thus the lightreceiving surface 56 of light sensor 30 is situated only approximately 6mm from the longitudinal axis of the light beam 52. Such close spacing increases the amount of scattered light received by the light-receiving surface 56, and also allows for a compact smoke-detector design.

Although the light-receiving surface 56 is ideally a sensitive planar surface of a plane silicon PIN photodiode, that surface could have a curvature; however, a curved surface would have a degraded performance in comparison to the planar surface. The area of the light-receiving surface in the preferred embodiment is 3mm2. The light sensor 58 may be implemented in Surface-Mounted Device (SMD) technology by mounting the plane silicon PIN photodiode on the circuit board 24.

Figures 5 and 6 illustrate the same arrangement as in Figures 3 and 4, but with a cylindrical condenser lens 72 added to improve the amount of light collected by the light receiving surface 56. The axis of the cylindrical surface of the lens 72 extends parallel to light beam 52. Since the angular field-of-view can be very limited in the plane normal to the light beam 52, a useful increase in collected light is achieved by using the lens 72. In Figure 6, the presence of the lens 72 allows the width of the aperture to be increased without increasing the angle "C". The spacing between the side edges 66 and 68 of the enclosure 58 in Figure 6 could be increased over the spacing between those edges shown in Figure 4.

While the present invention has been described in its preferred embodiments, it is to be understood that the words which have been used are words of description rather than limitation, and that changes may be made to the invention without departing from its scope as defined by the appended claims.

Each feature disclosed in this specification (which term includes the claims) and/or shown in the drawings may be incorporated in the invention independently of other disclosed and/or illustrated features.

The text of the abstract filed herewith is repeated

here as part of the specification.

A smoke detector has a light sensor positioned such that a lightreceiving surface of the sensor extends generally parallel to a light beam generated by a light source of the smoke detector. The light sensor has an enclosure surrounding the light-receiving surface, which enclosure has an aperture in a side facing the light beam.

The front and rear edges of the aperture (with reference to the direction of the light beam), as well as the two side edges extending normal to the front and rear edges, are positioned so as to affect the amount of scattered light from the light beam that reaches the light-receiving surface. In particular, the position of the front and rear edges of the aperture can be used to create a balance of sensitivity to forwardscattered pale-coloured smoke and backward-scattered dark-coloured smoke. The light-receiving surface may be a light-sensitive surface of a plane silicon PIN photodiode.

Claims

CLAIMS: 1. A smoke detector having a light sensor, the light sensor being

positioned such that its optical axis extends generally perpendicular to a light beam generated by a light source of the smoke detector, the sensor having first and second light shields positioned between the light beam and the light-receiving surface for restricting the amount of scattered light from the light beam that strikes the light- receiving surface, the position of the shields being chosen in accordance with a desired ratio of forward scattered light striking the light- receiving surface to backward scattered light striking that surface.
2. The smoke detector of claim 1, wherein the first and second light shields are formed as a first pair of opposed edges of an aperture positioned between the light beam and the light-receiving surface, scattered light passing from the light beam to the light-receiving surface through the aperture.
3. The smoke detector of claim 2, wherein the aperture is a rectangular or square aperture of a frame, with the first pair of opposed edges extending generally normal to a path of the light beam, and with a second pair of opposed edges extending generally parallel to the path of the light beam.
4. The smoke detector of claim 3, wherein the frame is spaced from the light-receiving surface such that, and the pair of first edges are positioned relative to the light-receiving surface such that, light scattered from the light beam strikes the light-receiving surface at an angle of 45 or more.
5. The smoke detector of claim 4, wherein the second edges of the frame are positioned such that each second edge is approximately on a respective line extending between a respective side boundary of the light beam and a respective corresponding side edge of the light-receiving surface.
6. The smoke detector of any one of claims 3 to 5, wherein the frame is formed as part of an enclosure surrounding the light-receiving surface.
7. The smoke detector of any preceding claim, wherein the light sensor is a photodiode.
8. The smoke detector of claim 7, wherein the photodiode is a silicon Positive-Intrinsic-Negative (PIN) photodiode.
9. The smoke detector of any one of claims 3 to 8, wherein the frame is spaced approximately 3mm from the longitudinal axis of the light beam.
10. The smoke detector of any preceding claim, wherein the lightreceiving surface is spaced lOmm or less from the longitudinal axis of the light beam.
11. The smoke detector of any preceding claim, wherein the lightreceiving surface is spaced 8mm or less from the longitudinal axis of the light beam.
12. The smoke detector of any preceding claim, wherein the lightreceiving surface is spaced 6mm or less from the longitudinal axis of the light beam.
13. The smoke detector of any preceding claim, wherein the smoke detector has a base, a cover mounted for covering the base, and a circuit board carrying the light source, and wherein, when the cover is covering the base, the base and cover together define an internal chamber adapted to house the circuit board and to receive smoke but not external light.
14. The smoke detector of any preceding claim, and also comprising a cylindrical condenser lens positioned between the light beam and the light-receiving surface, the axis of the cylindrical surface of the lens extending parallel to the light beam.
15. A smoke detector substantially as herein described with reference to and as shown in the accompanying figures.