GB2404731A

GB2404731A - Smoke detector with a surface-mount-device (SMD) light source

Info

Publication number: GB2404731A
Application number: GB0318006A
Authority: GB
Inventors: Roger Barrett
Original assignee: Apollo Fire Detectors Ltd
Current assignee: Apollo Fire Detectors Ltd
Priority date: 2003-07-31
Filing date: 2003-07-31
Publication date: 2005-02-09
Anticipated expiration: 2023-07-31
Also published as: GB2404731B; GB0318006D0

Abstract

A smoke detector comprising: a surface-mount-device (SMD) light source 26; a light sensor 30 receiving light scattered by smoke; a Printed Circuit Board (PCB) 24; and a light-directing means 38 which is adapted to project light as beam in a direction parallel to the surface of the PCB. The sensor 30 may be mounted on the PCB 24. The light source's emission direction may be in a normal direction to the PCB's surface. The light-directing means may include a reflecting surface 56, and include a prism with a beam forming lens 52. This lens may be formed as a part of the prism 38, and it may be formed on light entry face of prism (104, fig. 7). The reflecting surface of the light-directing means may act both as reflective and focussing part (96, fig. 6). A lens may be positioned downstream of light exit face of the prism (75, fig. 5). Light-directing means may have a first and a second aperture 60, 48. A real image of the source may be formed in the field of view of the sensor.

Description

2404731

SMOKE DETECTOR WITH COMPACT LIGHT SOURCE

The present invention relates to a smoke detector, and in particular to a scattered-light smoke detector with a compact light source.

Scattered-light smoke detectors normally use a light-emitting diode (LED) as their light source because of the low cost and high reliability of LEDs. In a typical such detector, a light source and light sensor are arranged to function within a dark optical chamber such that there is no direct line of sight between them, but such that their respective emission and reception fields intersect at a point inside the chamber. In such an arrangement, one of the practical difficulties is to minimize the amount of stray light from the light source that reaches a receiving surface of the light sensor.

The largest contribution to stray light is reflections from the chamber walls. Although the walls may be black, there is still a substantial amount of reflected light compared with the levels scattered from smoke. The easiest way to reduce the effect of such reflected light is to make the dimensions of the chamber quite large. However, that of course results in a detector of overall large size, which has the disadvantages of increased cost and reduced aesthetic appeal.

An alternative way to reduce the effect of reflected light is to use a light source with a narrow beam having small divergence, since the reflection from a narrow beam

can be more easily controlled within a small optical chamber. This allows the detector size to be kept small.

The LEDs normally used in smoke detectors have polar emission patterns that have a very strong central lobe; such emission patterns are typically only 8° wide. Although a substantial portion of the light energy is contained within the central lobe, there always exist side lobes that can contribute significantly to the stray light inside the chamber. Such side lobes are normally blocked by one or more apertures placed in front of the LED. Unfortunately, the complete removal of the side lobes is very difficult unless a series of aperture stops is used. However, inclusion of aperture stops may result in the overall size of the detector being increased to an unacceptable level.

It is desirable to use a light beam in which the light is made as parallel as possible. A common method of forming a near-parallel light beam is to use a separate beam-forming lens, with the light source in the focal plane of the lens. In that case, the divergence of the light beam is fixed by the ratio of the lens' focal length to the diameter of the source. If the normal 5mm-diameter LED is used as the source, the focal length of the lens must be approximately 25mm to achieve a beam divergence of 5° each side of the beam axis. The overall length of the assembly using such a lens becomes impractical in a small detector.

An object of the preferred embodiments of the subject invention is to produce a narrow light beam having low divergence within a small envelope. In one aspect of the

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invention this can be achieved by using a small light source (preferably in the form of a LED chip device) in Surface-Mount Device (SMD) technology, in association with a high-quality lens of relatively-short focal length. The term 5 'SMD LED' as used hereafter is intended to include a laser diode. The term 'light source' includes a source of non-visible light, to the extent such sources are usable in smoke detectors.

In this aspect, the invention is a smoke detector that 10 includes: a printed circuit board; a surface-mount-device (SMD) light source, preferably a light-emitting diode (LED), mounted on the printed circuit board; a light-directing means disposed over the light source and adapted to direct light emitted by the light source such that the directed 15 light comprises a beam projected in a direction generally parallel to the surface of the printed circuit board; and, a sensor for receiving light scattered by smoke from the projected beam.

Preferably, the sensor is mounted on the printed 20 circuit board.

Preferably, the light source is mounted on the printed circuit board to emit light in a direction generally normal to the surface of the board.

Preferably, the light-directing means includes a light-25 reflective surface. More preferably, the light-directing means includes a prism with a light-entry surface, the light-reflective surface and a light-exit surface, and wherein the light-reflective surface acts to both reflect

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and focus light.

Preferably, the light-directing means may include a. prism. More preferably, the light-directing means also includes a beam-forming lens. In a first preferred 5 arrangement, the lens is separately formed from the prism. In the first arrangement, the prism has a light-entry face, a light-reflective face, and a light-exit face, and the lens is positioned downstream of the light-exit face of the prism. In a second preferred arrangement, the lens is 10 formed as part of the prism, and more preferably, the lens is formed in one face of the prism. In one form of such more preferable arrangement, the prism has a light-entry face, a light-reflective face, and a light-exit face, and the beam-forming lens is formed in the light-exit face. In 15 another form of such more preferable arrangement, a laser diode is used as the SMD LED, and the beam-forming lens is formed in the light-entry face rather than the light-exit face.

Preferably, the light-directing means includes a 20 housing with a first side having a first aperture, the first side facing the printed circuit board. More preferably, the first aperture is circular and is centred in the first side of the housing. Even more preferably, the housing also has a second side with a second aperture, the second side being 25 a side through which the projected beam leaves the light-directing means. Yet more preferably, the second aperture is circular and is centred in the second side of the housing. Even still more preferably, the prism is

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positioned in the housing such that light that enters through the first aperture is directed to leave through the second aperture.

Preferably, the prism is positioned in the housing such 5 that the light-entry face extends generally parallel to the printed circuit board, the light-exit face extends generally normal to the printed circuit board, and the light-reflective face extends at an angle to, and between outer ends of, the light-entry and light-exit faces. More 10 preferably, the light-reflective face extends at an angle of 45° to the light-entry and light-exit faces of the prism.

Preferably, the housing has a pair of resilient legs each fitting into a respective aperture in the printed circuit board for retaining the light-directing means on the 15 board.

Preferably, the housing has an arm portion extending in the direction of the projected beam, the arm portion having an aperture positioned directly above the light sensor on the board, the aperture in the arm portion 20 allowing light scattered from the projected light to pass through and strike the light sensor.

To reduce further the undesirable divergence of the beam in the field of view of the sensor, preferably the beam effectively is focussed to provide an image of the light 2 5 source at a point above the sensor, rather than being nominally parallel. In the latter case the image of the light source is theoretically at infinity. More preferably, the lens is positioned and sized such that a real image of

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a die of the LED is formed by the projected light in a first plane which extends through the arm aperture. Even more preferably, a real image of the first circular aperture is formed by the projected light in a second plane which is 5 positioned downstream from the first plane.

This principle forms a second aspect of the invention, according to which there is provided a smoke detector comprising a light source, means for directing a beam of light from the light source across a field of view of a 10 sensor for receiving light scattered by smoke from the projected beam, the beam directing means being configured to form a real image of the light source in the field of view of the sensor.

The sensor field of view may have an axis which 15 intersects the beam substantially orthogonally, the beam-directing means being configured to form the real image of the light source substantially at the intersection of the beam and the axis of the sensor field of view.

The light-directing means may comprise a stop aperture, 2 0 the light-directing means being configured to form a real image of the aperture downstream of the real image of the light source. Preferably, a light trap is disposed at the location of the real image of the aperture; this principle forms a further aspect of the invention. According to that 25 aspect there is provided a smoke detector that includes a light source, means for directing a beam of light from the light source across a field of view of a sensor for receiving light scattered by smoke from the projected beam,

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and a light trap downstream of the sensor for receiving the projected beam. The light-directing means includes a stop aperture and is configured to form a real image of the aperture at the light trap.

5 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 a first embodiment of the smoke detector embodying two aspects of 10 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 first embodiment of the smoke detector of Figure 1;

Figure 3 is a side view through the light-directing 15 unit of the first embodiment, the view schematically illustrating, for the case where the lens is formed in the light-exit face of the prism, the path of a light beam from its emission by the Surface-Mount-Device(SMD) LED chip device on the printed circuit board to a point where the 2 0 light beam passes above the light sensor on the board;

Figure 4 is the side view of Figure 3, but illustrating the positions at which real images are formed of the LED die and the first circular aperture of the light-directing unit;

Figure 5 is a side view of a second embodiment of the 25 light-directing unit, in which the beam-forming lens is part of the unit but not formed as part of the prism;

Figure 6 is a side view of a third embodiment of the light-directing unit, in which the unit has no beam-forming

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lens and in which the light-reflective face of the prism directs the light;

Figure 7 is a side view of a fourth embodiment of the light-directing unit, in which a laser diode is used as the SMD LED, and a beam-forming lens is formed on the light-entry face of the prism;

Figure 8 is a side view of an embodiment of a further aspect of the invention; and,

Figure 9 is a schematic plan view of a light trap that is positioned to trap light at the plane Y in Figure 4.

In recent years small-sized LEDs have become available as surface-mounted devices in Surface-Mount Device (SMD) technology. Newer SMD LED devices, such as the one used in this invention, are chip devices in which the LED die is mounted on a small insulating substrate and is over-moulded with a small beam-forming lens. The optical axis of the device is normal to the surface of the printed circuit board (PCB) on which it is mounted.

Because the SMD LED lens is very small, typically 1.8mm in diameter, a supplementary beam-forming lens with a focal length of only 8mm will give a beam of the required diameter and divergence. The overall assembly is therefore only one third of the size that is possible using a 5mm-diameter LED. The fact that the LED can be placed by machine is also a great advantage when compared with through-hole devices that must be fitted manually.

The smoke detector of the first embodiment, shown in Figures 1 and 2, has a base 20, a cover 22 that fits on the

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base 20, and a circuit board 24 fitted between the base 20 and the cover 22. On the circuit board 24 is fitted a SMD LED chip package 26. A light sensor is generally designated as 30, and a pair of holes 32 each extend on an opposite 5 side of a line drawn between the light source 26 and the light sensor 30. The light sensor 30, which forms the basis of co-pending UK Patent Application No. 0311966.6, is not further described here.

The smoke detector further includes a prism carrier 34 10 which includes a carrier base 36, a prism 38 mounted in the carrier base 36, and a carrier cover 4 0 mounted on the carrier base 36 to hold the prism 38 on the carrier base 36. The prism carrier 34 is made of moulded plastic and has an integral pair of legs 42 each having a hooked end 44 15 extending through a respective hole 32 in the circuit board 24 and flexibly engaging with an underside of that board so as to secure the prism carrier 34 to the board 24. One part of the prism carrier 34 is formed as an arm 46 that extends above the light sensor 30. The arm 46 has an aperture 48 20 in its base to allow light scattered from the redirected light beam to pass through to the light sensor 30.

A disadvantage with the SMD LED chip package 2 6 is that the light beam it creates extends normal to the PCB 24, whereas the beam ideally should be parallel to the surface 25 so that a low profile can be achieved for the detector. To achieve this, the prism 38 is fitted above the LED chip package 26 to redirect the beam through 90°. The 90° beam redirection is achieved through use of a 45° light-

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reflective back face on the prism 38. Preferably, the prism 3 8 is moulded from a clear plastic material, and a beam-forming lens 50 is formed on its light-exit face 52. The prism 38 also has a planar light-entry face 54 adapted to 5 face toward the LED chip package 26. The 45° light-reflective face 56 connects to the outer ends of the light-entry face 54 and the light-exit face 52. The dimensions of the light-entry face 54 and the light-exit face 52, and the focal length of the beam-forming lens 50, are calculated 10 so as to produce a light beam that is parallel or near-parallel to the surface of PCB 24.

Although the arrangement described above is an improvement on previous systems that use 5-mm LEDs, the amount of light energy extending outside the main light beam 15 envelope is still quite high. This is because the LED chip package 2 6 allows light to escape in all directions, thereby expanding the nominal small source size. The stray light is partially removed by the placement of a first circular aperture 60 in a bottom first side 62 of the carrier base 20 36, the first side 62 extending between the LED chip package 26 and the prism light-entry face 54. The first circular aperture 60 blocks all light rays except those extending close to the optical axis of the LED chip package 26. A second circular aperture 64 is formed in the carrier cover 25 40 so as to sit in front of beam-forming lens 50 when the carrier cover 40 is mounted on the carrier base 36 with the prism 38 sitting therebetween; the second circular aperture 64 blocks any off-axis light rays that emerge from the beam-

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forming lens 50. The carrier cover 40 has front and back brackets 66 that are captured by respective detents (not shown) within the carrier base 36 to hold the cover 4 0 on the base 36.

5 In the side view of Figure 3, a light beam is emitted upward from the SMD LED 26. A portion of that light beam passes upward through the first circular aperture 60 of the prism carrier 34; any stray light in side lobes of the light beam are there blocked. The light beam is diverging as it 10 enters the light-entry face 54 of the prism 38, and is still diverging as it is reflected by the light-reflective face 56 and moves toward the light-exit face 52. A circular central portion of light-exit face 52 is formed as the beam-forming lens 50, and the light beam is caused to converge 15 by its passage through the lens 50. Only a portion of such converging light beam passes through the second circular aperture 64 of prism carrier 34; any off-axis light rays that emerge from the lens 50 are thereby removed. The resulting converging light beam extends above the arm 46 of 20 prism carrier 34, and crosses above the aperture 48 which sits directly above the light sensor 30. In operation of the smoke detector, light scattered from the light beam and passing above the aperture 4 8 is detectable by the light sensor 30. The field of view of the sensor, the principal 25 axis of which is shown at 31, is directed orthogonally of the light beam.

As shown in Figure 4, the focal length of the beam-forming lens 50 is calculated such that a die 68 of the LED

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2 6 is imaged in a plane X extending through the aperture 48 in the arm 46, aligned with the axis 31 of the sensor field of view. By imaging the die 68 in the region above the sensor, the effective diameter of the beam in this region 5 is minimized. This makes the detector less susceptible to false alarms from small insects or other foreign bodies that may find their way onto the top face of arm 46, within the field-of-view of the sensor 30. The first aperture 60 is imaged in a plane Y which is further downstream and at a 10 convenient distance from the front face of the beam-forming lens 50. The position of the image in plane Y is independent of the position of the output face of the LED chip package 26, and is fixed by the relative positioning of the first aperture 60 and the lens 50. The plane Y is 15 the location of a "light trap" (or "dump") designed to absorb all or almost all of the energy in the beam. The light trap in this smoke detector is an L-shaped chamber 12 0 (shown in schematic plan view in Figure 9), having a first passage 122 meeting, at an angle of about 45°, a second 20 shorter passage 124 with a closed outer end 126. The chamber shape is such that entering light is reflected multiple times within the chamber, with only a small amount of the light able to escape from the chamber. The provision of such a light trap is greatly simplified when the exact 25 location of the beam is known, and stray reflections can be eliminated. It is therefore important that the prism carrier 34 is assembled very precisely so that the beam position is always known within close limits.

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By focussing the beam to provide an image of the LED in plane X, the highest-intensity part of the beam is convergent to the plane of the image, and thus is most concentrated at this point. Unwanted divergence thus is 5 reduced.

A second embodiment of the subject invention utilizes the alternate prism carrier 70 shown in Figure 5. The prism carrier 70 operates in a similar way to prism carrier 34 of Figure 3 except that the beam-forming lens 50 on the light-10 exit face 52 of the prism 38 has been replaced by a separate beam-forming lens 72. The lens 72 is mounted in the prism carrier 70 so as to be just forward of a planar light-exit face 74 of prism 76. Other parts of the prism carrier 70 remain the same as in the prism carrier 34. As with the 15 lens 50 of the first embodiment, the position and focal length of lens 72 are such that the real images X and Y shown in Figure 4 are formed. A real image of the lens die 77 is formed in a first plane extending through arm aperture 78, and a real image of the first circular aperture 80 is 2 0 formed in a second plane (not shown) downstream of the first plane.

An advantage of the first and second embodiments is that a portion of the distance (focal length) between the lens and the LED is in the horizontal axis, allowing a 25 reduction in the height of the prism carrier.

A third embodiment of the subject invention utilizes the alternate prism carrier 90 shown in Figure 6. In this embodiment, the light-reflective face 92 of the prism 94 has

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a central section 96 that acts to both reflect the light and change its focus, i.e. a beam-forming lens is not required. Thus a diverging light beam from the LED 98 not only has its direction changed by the light-reflective face 92 but is 5 also refocused into a converging beam with images at planes X and Y as in figure 4.

A fourth embodiment of the subject invention utilizes a prism carrier 100 as shown in Figure 7. In this fourth embodiment, a laser diode 102 is used as the SMD LED; such 10 laser diode may be, for instance, a VCSEL laser device. The use of a laser diode, with a reduced emitting area compared to a non-laser diode, as the light source allows for the beam-forming lens to be placed closer to the light source than was possible with the other embodiments while still 15 achieving a small beam divergence. The beam-forming lens may be formed on the light-entry surface of the prism. In Figure 7, the beam-forming lens is shown as a bump 104 on the light-entry surface 106 of the prism 108. If a VCSEL or other type of laser diode is used as the source, there 20 is still an advantage to forming an image of the emitting area in the region over the sensor. There is certainly an advantage in imaging the first aperture in the plane of the light trap in that the position of the light source thereby becomes less critical. Also, even with a laser diode there 25 exists stray, off-axis light, but the effect of that off-axis light can be minimized using the lens and the arrangement of stops.

In Figure 8 the light source is provided by a sideways-

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looking through-hole LED 110 mounted on the PCB 24.

This source has a LED die 112 and emits a beam of light sideways and parallel to the PCB 24 through an integral lens 114 and stop aperture 116, and thence through beam-forming 5 lens 50 and circular aperture 64 as already described with reference to Figures 3 and 4. The lens 50 produces real images of die 112 and aperture 116 at planes X and Y respectively as already described with reference to Figures 3 and 4. Parts corresponding to those of Figures 3 and 4 10 carry the same reference numerals in Figure 8.

The embodiment of Figure 8 thus provides the beam-concentrating advantages of imaging the light source at plane X, on the axis of the field of view of sensor 30, without the need for a prism or other device to turn the 15 light beam through 90° from a SMD LED on the PCB 24. However, the use of a through-hole LED may increase assembly costs. The narrow beam and compactness of a SMD LED can still be obtained if the through-hole LED 110 is replaced by a SMD LED on a small PCB set orthogonally to the board 20 24. However, there will be again a penalty in greater complexity and assembly costs; thus the arrangement of Figures 1 to 7 is generally to be preferred.

While the present invention has been described in its preferred embodiments, it is to be understood that the words 25 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.

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Each feature disclosed in this specification (which term includes the claims) and/or shown in the drawings maybe 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 scattered-light smoke detector with compact light source includes a surface-mount-device (SMD) light-emitting diode (LED) mounted on a printed circuit board, and a light-directing unit mounted on a surface of the board so as to sit over the LED. The light-directing unit is adapted to direct light emitted from the LED such that a central axis of the directed light extends in a direction generally parallel to the surface of the printed circuit board. The directed light extends above a sensing surface of a light sensor mounted on the board. The light-directing unit may include a prism and a beam-forming lens, with the lens being formed in a light-exit face of the prism. Alternatively, the beam-forming lens may be separate from the prism, and positioned downstream of the light-exit face of the prism. Another alternative is to have a light-reflective face of the prism perform the function of the beam-forming lens, so that no separate lens is required. The use of a SMD LED in association with a proximate means for redirecting and refocusing light allows for construction of a compact low-profile smoke detector.

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Claims

CLAIMS:

1. A smoke detector comprising:

a printed circuit board;

a surface-mount-device (SMD) light source mounted on the printed circuit board;

a light-directing means disposed over the light source and adapted to direct light emitted by the light source such that the directed light comprises a beam projected in a direction generally parallel to the surface of the printed circuit board; and,

a sensor for receiving light scattered by smoke from the projected beam.

2. The smoke detector of claim 1, wherein the sensor is mounted on the printed circuit board.

3. The smoke detector of claim 1, wherein the light source is mounted on the printed circuit board to emit light in a direction generally normal to the surface of the board.

4. The smoke detector of claim 1, 2 or 3, wherein the light-directing means includes a light-reflective surface.

5. The smoke detector of claim 1, 2 or 3, wherein the light-directing means includes a prism.

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6. The smoke detector of claim 5, wherein the light -directing means also includes a beam-forming lens.

7. The smoke detector of claim 6, wherein the lens is formed as part of the prism.

8. The smoke detector of claim 7, wherein the lens is formed in one face of the prism.

9. The smoke detector of claim 4, wherein the light-directing means includes a prism with a light-entry surface, the light-reflective surface and a light-exit surface, and wherein the light-reflective surface acts to both reflect and focus light.

10. The smoke detector of claim 6, wherein the prism has a light-entry face, a light-reflective face, and a light-exit face, and wherein the lens is positioned downstream of the light-exit face of the prism.

11. The smoke detector of claim 8, wherein the prism has a light-entry face, a light-reflective face, and a light-exit face, and wherein the lens is formed in the light-exit face.

12. The smoke detector of claim 8, wherein the prism has a light-entry face, a light-reflective face, and a light-exit face, wherein the lens is formed in the light-

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entry face, and wherein the SMD light source is a laser diode.

13. The smoke detector of any one of claims 9 to 12, wherein the light-directing means includes a housing with a first side having a first aperture, the first side facing the printed circuit board.

14. The smoke detector of claim 13, wherein the first aperture is circular and is centred in the first side of the housing.

15. The smoke detector of claim 13 or 14, wherein the housing also has a second side with a second aperture, the second side being a side through which the projected beam leaves the light-directing means.

16. The smoke detector of claim 15, wherein the second aperture is circular and is centred in the second side of the housing.

17. The smoke detector of claim 15 or 16, wherein the prism is positioned in the housing such that light that enters through the first aperture is directed to leave through the second aperture.

18. The smoke detector of claim 13, wherein the prism is positioned in the housing such that the light-entry face

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extends generally parallel to the printed circuit board, the light-exit face extends generally normal to the printed circuit board, and the light-reflective face extends at an angle to, and between outer ends of, the light-entry and light-exit faces.

19. The smoke detector of claim 18, wherein the light-reflective face extends at an angle of 45° to the light-entry and light-exit faces of the prism.

20. The smoke detector of any one of claims 13 to 19, wherein the housing has an arm portion extending in the direction of the projected beam, the arm portion having an aperture positioned directly above the light sensor on the board, the aperture in the arm portion allowing light scattered from the projected light to pass through and strike the light sensor.

21. The smoke detector of any one of claims 13 to 20, wherein the housing has a pair of resilient legs each fitting into a respective aperture in the printed circuit board for retaining the light-directing means on the board.

22. The smoke detector of any preceding claim, wherein the light-directing means is configured to form a real image of the light source in the field of view of the sensor.

23. The smoke detector of any preceding claim,

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wherein the sensor is positioned with an axis of its field of view directed so as to intersect the projected beam substantially orthogonally.

24. The smoke detector of claims 22 and 23, wherein the light-directing means is such as to form the real image of the light source substantially at the intersection of the projected beam and the axis of the sensor field of view.

25. The smoke detector of claims 13 and 22, wherein a real image of the first circular aperture is formed by the projected light at a position which is downstream from the image of the light source.

26. A smoke detector comprising a light source, means for directing a beam of light from the light source across a field of view of a sensor for receiving light scattered by smoke from the projected beam, the beam directing means being configured to form a real image of the light source in the field of view of the sensor.

27. A smoke detector substantially as herein described with reference to and as shown in any of the accompanying figures.

27. The smoke detector of claim 24, wherein the sensor field of view has an axis which intersects the beam substantially orthogonally; the beam-directing means being configured to form the real image of the light source substantially at the intersection of the beam and the axis of the sensor field of view.

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28. The smoke detector of claim 2 6 or 27, wherein the light-directing means comprises a stop aperture, the light-directing means being configured to form a real image of the aperture downstream of the real image of the light source.

29. A smoke detector comprising a light source, means for directing a beam of light from the light source across a field of view of a sensor for receiving light scattered by smoke from the projected beam, and a light trap downstream of the sensor for receiving the projected beam, the light-directing means comprising a stop aperture and being configured to form a real image of the aperture at the light trap.

30. The smoke detector of any preceding claim, wherein the light source is a LED.

31. A smoke detector substantially as herein described with reference to and as shown in any of the accompanying figures.

Amendments to the claims have been filed as follows

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CLAIMS:

1. A smoke detector comprising a light source, means for directing a beam of light from the light source across a field of view of a sensor for receiving light scattered by smoke from the projected beam, the beam directing means being configured to form a real image of the light source in the field of view of the sensor.

2. The smoke detector of claim 2, wherein the sensor field of view has an axis which intersects the beam substantially orthogonally; the beam-directing means being configured to form the real image of the light source substantially at the intersection of the beam and the axis of the sensor field of view.

3. The smoke detector of claim 1 or 2, wherein the light-directing means comprises a stop aperture, the light-directing means being configured to form a real image of the aperture downstream of the real image of the light source.

4. A smoke detector comprising a light source, means for directing a beam of light from the light source across a field of view of a sensor for receiving light scattered by smoke from the projected beam, and a light trap downstream of the sensor for receiving the projected beam, the light-directing means comprising a stop aperture and

'ZM

being configured to form a real image of the aperture at the light trap.

5. The smoke detector of any preceding claim, wherein the light source is a LED.

6. The smoke detector of any of claims 1 to 4, comprising:

a printed circuit board;

the light source being a surface-mount-device (SMD) light source mounted on the printed circuit board;

the light-directing means being disposed over the light source and adapted to direct the light beam such that the beam is projected in a direction generally parallel to the surface of the printed circuit board.

7. The smoke detector of claim 6, wherein the sensor is mounted on the printed circuit board.

8. The smoke detector of claim 6, wherein the light source is mounted on the printed circuit board to emit light in a direction generally normal to the surface of the board.

9. The smoke detector of claim 6, 7 or 8, wherein the light-directing means includes a light-reflective surface.

10. The smoke detector of claim 6, 7 or 8, wherein the light-directing means includes a prism.

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11. The smoke detector of claim 10, wherein the light -directing means also includes a beam-forming lens.

12. The smoke detector of claim 11, wherein the lens is formed as part of the prism.

13. The smoke detector of claim 12, wherein the lens is formed in one face of the prism.

14. The smoke detector of claim 9, wherein the light-directing means includes a prism with a light-entry surface, the light-reflective surface and a light-exit surface, and wherein the light-reflective surface acts to both reflect and focus light.

15. The smoke detector of claim 11, wherein the prism has a light-entry face, a light-reflective face, and a light-exit face, and wherein the lens is positioned downstream of the light-exit face of the prism.

16. The smoke detector of claim 13, wherein the prism has a light-entry face, a light-reflective face, and a light-exit face, and wherein the lens is formed in the light-exit face.

17. The smoke detector of claim 13, wherein the prism has a light-entry face, a light-reflective face, and a light-exit face, wherein the lens is formed in the light-

entry face, and wherein the SMD light source is a laser diode.

18. The smoke detector of any one of claims 14 to 17, wherein the light-directing means includes a housing with a first side having a first aperture, the first side facing the printed circuit board.

19. The smoke detector of claim 18, wherein the first aperture is circular and is centred in the first side of the housing.

20. The smoke detector of claim 18 or 19, wherein the housing also has a second side with a second aperture, the second side being a side through which the projected beam leaves the light-directing means.

21. The smoke detector of claim 20, wherein the second aperture is circular and is centred in the second side of the housing.

22. The smoke detector of claim 20 or 21, wherein the prism is positioned in the housing such that light that enters through the first aperture is directed to leave through the second aperture.

23. The smoke detector of claim 18, wherein the prism is positioned in the housing such that the light-entry face

2-}

24. The smoke detector of claim 23, wherein the light-reflective face extends at an angle of 45° to the light-entry and light-exit faces of the prism.

25. The smoke detector of any one of claims 18 to 24, wherein the housing has an arm portion extending in the direction of the projected beam, the arm portion having an aperture positioned directly above the light sensor on the board, the aperture in the arm portion allowing light scattered from the projected light to pass through and strike the light sensor.

26. The smoke detector of any one of claims 18 to 25, wherein the housing has a pair of resilient legs each fitting into a respective aperture in the printed circuit board for retaining the light-directing means on the board.