GB2272760A

GB2272760A - Optical detection of combustion products

Info

Publication number: GB2272760A
Application number: GB9224328A
Authority: GB
Inventors: Stephen John Penney
Original assignee: Thorn Security Ltd
Current assignee: Thorn Security Ltd
Priority date: 1992-11-20
Filing date: 1992-11-20
Publication date: 1994-05-25
Also published as: GB9224328D0

Abstract

A detector (2) of combustion products from a fire comprises an enclosure (4) having an opening (6) and a focussing means (24, 26) arranged for converging light from the opening (6) to a focus (12). A detector (14) is prevented from detecting the light at the focus (12) by a screening member (16). When a combustion product is present in the enclosure (4) the light is either refracted or scattered off-focus where it can then be detected by the detector (14). <IMAGE>

Description

METHOD AND APPARATUS FOR THE OPTICAL DETECTION OF COMBUSTION PRODUCTS The present invention relates to a method and apparatus for the optical detection of combustion products and has particular, although not exclusive, relevance to detection of generally invisible combustion products developed during the initial stages of a fire.

Optical combustion product detectors are known which are generally only applicable to the detection of smoke, or the particulate products of a fire. This is so because such devices rely on the optical property of scattering caused when a beam of light is dispersed by virtue of passing through such smoke, the individual particles of which act as discrete scattering centres for the light.

Alternatively, there are available ion-chamber combustion product detectors.

These detectors utilise a source of ionising radiation to produce positive and negative charge. In the presence of such combustion products, the charge gradient will change and may then be sensed to provide an indication of the concentration of the combustion products in the atmosphere of the detector.

Although the ion-chamber detector is able to detect invisible products of a fire (unlike the optical detector), it is highly undesirable, for environmental reasons, to use a source of ionising radiation in such a detector.

The present invention concerns apparatus for the optical detection of combustion products of the type comprising an enclosure of optically non-transmissive material having an opening through which light may pass, the enclosure having a light detector therewithin. A known detector of this type is, for example disclosed in United Kingdom Patent Application No. GB-A-2,170,597.

In this document, there is described a smoke detector including a light source and a light sensor disposed within an enclosure. The sensor is positioned such that no direct light from the source may be sensed. Light from the source which strikes the walls of the enclosure is either absorbed or reflected to such an extent that the intensity thereof rapidly becomes insignificant.

When smoke enters the chamber, the particles of the smoke cause light from the source to be scattered and hence sensed by the sensor.

Such a smoke detector is, therefore, applicable to detection of particulate products of a fire and will generally be unable to detect invisible or colourless gases which may be developed in the early stages of a fire. This is because such gases cause no scattering of light from the source.

It is thus an object of the present invention to provide an optical detector of combustion products. Such a detector may be capable of detecting invisible nonparticulate combustion products, such as aerosols, as well as visible particulate products, such as soot, without the need for a source of ionising radiation and, therefore, the environmental hazards associated therewith. By such provision, the invention aims to alleviate the aforementioned shortcomings associated with the various prior art detectors.

Thus, according to a first aspect of the present invention, there is provided apparatus for the optical detector of combination products of the aforesaid type characterised in that the enclosure further has therewithin; focussing means arranged for converging, to a focus, light passing through the opening, in the absence of a combination product within the enclosure; and a screening member arranged to screen the detector from light incident at the focus, the light detector being positioned to receive only any light which falls outside the focus.

In apparatus in accordance with the invention the light reflected from the reflective surface may be caused to fall outside the focus either by scattering from a particulate combustion product, or due to a local phase object causing a change in refractive index in the form of a gas from a fire. Thus the apparatus can be capable of detecting both particulate and invisible products of a fire.

Preferably the focussing means comprise a light refractive surface.

Advantageously the light reflective surface comprises a concave mirror.

Preferably the screening member is formed from material rendered non-lighttransmissive by the action of light thereon. Thus the apparatus may be initially set up without the need for highly accurate optical alignment, this being achieved automatically on the introduction of light into the enclosure and thereby defining the screening member.

Also it is preferable that the enclosure includes an inner surface which, other than the light reflective surface, is arranged for absorbing substantially all light incident thereon.

According to a second aspect of the present invention there is provided a method for the optical detection of combustion products which comprises: introducing light through an opening into an enclosure of optically nontransmissive material; focussing light from the opening to a focus in the absence of a combustion product within the enclosure; screening the light at the focus from a detector; and detecting the light, by means of the detector, which is deviated to fall outside the focus by a combustion product within the enclosure.

Light may fall outside the focus by a combustion product acting as a local phase object within the enclosure. Additionally or alternatively light may fall outside the focus due to scattering by a combustion product within the focus.

The present invention will now be described, by way of example only, with reference to the accompanying drawings of which: Figure 1 shows schematically a section through a device according to the present invention; Figure 2 shows a section through the line A-A of Figure 1; Figure 3 illustrates selected ray diagrams for the section of Figure 1; and, Figure 4 shows schematically an enlargement of an optically active area of Figure 3; and, Figure 5 illustrates an alternative embodiment to that of Figure 1.

Referring firstly to Figures 1 and 2 apparatus for the optical detection of combustion products shown generally as 2, comprises an enclosure 4 having only one opening 6. The opening is no larger than the size of a pin-head so that only a very narrow beam of light may enter the enclosure.

Remote from the opening 6 and on the opposite side of the enclosure 4 therefrom is a focussing means, in this example a reflective surface, concave mirror 8. All remaining surfaces within the inner periphery of the enclosure 4 are for absorbing substantially all light incident thereon. This is achieved by an arrangement of optical baffles 10 along these surfaces. Light entering the enclosure 4 through the opening 6 beyond the angle subtended by the mirror 8 at the opening 6 is absorbed within these baffles by multiple reflections so that no reflection directly therefrom occurs which could then reach the mirror 8.

Only light within the angle subtended by the mirror 8 at the opening 6 is transmitted directly through the enclosure 4 to strike the mirror 8. The radius of curvature of the mirror 8 is chosen so as to reflect incident light to a focus, shown generally as 12.

It is important to note that all the light will be incident at this focus 12, only in the absence of combustion products within the enclosure.

Referring now also to Figure 3 it can be seen that light from the opening 6 which is reflected from the mirror 8 to focus 12 would strike an optical detector, in this example a photodiode 14,were it not for the fact that the photodiode 14 has a screening member, in this example a mirror 16, placed directly over it and in exact coincidence with the area which would be occupied at the detector 14 by the light incident at the focus 12.

By utilising such an arrangement, the light at the focus 12 is not detectable by the detector 14 because the mirror 16 is blocking the path of the light thereto.

Furthermore, as can be seen from the ray traces in Figure 3, light reflected by mirror 16 will then be either absorbed by the baffles 10 or, if reflected toward the mirror 8, reflected towards the focus 12 again. In any event, the mirror 16 prevents detection of light by detector 14 which enters the enclosure 4 via opening 6 in the absence of a combustion product.

Considering now Figure 4 alone, the effect produced by the action of a combustion product such as an aerosol entering the enclosure 4 will be explained. It will be noted that Figure 4 illustrates in detail only the area around the focus 12, for example, as shown in Figure 3. Furthermore it must be understood that, only in Figure 4, are there shown two different ray traces. The dotted ray trace 18 shows the path of light in the absence of an aerosol in the enclosure 4; whilst the solid ray trace 20 shows the path of light when an aerosol is present in enclosure 4. Considering firstly dotted trace 18, it can be seen that the light path is as has been described herebefore. That is, light from the mirror 8 is incident at focus 12 where the mirror 16 reflects this incident light away from the detector 14 such that it cannot be detected.

However, when an aerosol from a fire enters the enclosure 4, then a local region or regions of such aerosol act as local phase objects, that is in these localities the refractive index of the atmosphere within the enclosure 4 alters from what it is in the absence of such an aerosol. This may be because the aerosol is hot from the heat of the fire and, therefore, less dense than its environment, or it may be because the aerosol is inherently of a density different to that of the atmosphere within enclosure 4.

In any event, local phase objects such as this give rise to the so-called Schlieren (or "Streak") effect. As will be known by those skilled in the art, this effect has the property of being able to visualise inhomogeneities due to differences of density and refractive index in a medium.

However, unlike conventional applications of Schlieren optics, in this example it is not necessary to actually visualise the variations.

Thus by observing ray trace 20 of Figure 4, it is noted that with the presence of an aerosol locally in the enclosure 4, then the light is deviated slightly due to the change in local refractive index and no longer falls on the focus 12, but slightly outside this. Because the mirror 16 had been chosen, in the aerosol-free case, to only just obscure the focus 12 from the detector 14, this deviated light 20 will now be detectable by detector 14 because it falls outside the focus 12.

Once, and if, a uniform atmosphere of the aerosol occurs within the enclosure 4, it will be apparent that the condition according to ray trace 18 will once more be true.

This Schlieren effect in the present invention is only true for local phase objects.

It is, of course, usual in a situation where combustion products arise by virtue of a fire that, after the initial stages of the fire have occurred during which predominantly invisible aerosols are produced, then particulate products such as smoke and soot occur. It will be apparent that these, too, may be detected by the present invention (only until their density within the enclosure becomes too great to permit the passage of light therethrough) by way of scattering of the light from the mirror 8 by the particles therein. The detector 14 may usefully by arranged to have a larger detection area than is strictly necessary to detect the Schlieren effect described above.

An alternative embodiment to that of Figure 1 is illustrated in figure 5. In this figure, all components similar to those described previously are similarly numbered.

The enclosure 4 has an opening 6 at one end and the detector 14 and mirror 16 the other end. Between these are a focussing means, in this example, two convex lenses 24,26. The lenses are arranged to provide a means to focus the light from the opening 6 to the focus 12. Light passing through the opening 6 will be collimated by lens 24 and the collimated light then converged to focus 12 by lens 26. Clearly the focal length of both lenses 24,26 is suitably chosen to achieve this effect. The device 2 then operates in the same manner as has been described above.

Considering now the screening member, in the above example represented by mirror 16, this may alternatively be formed, not from a reflective material, but from an optically absorptive material such as a matt black surface. Furthermore, as will be appreciated, because accurate optical alignment as between the opening 6, mirror 8, detector 14 and the screening means is essential, it is particularly useful during fabrication of the device 2 to coat the entire optically-active surface of the detector 14 with a form of photographic film or emulsion which is rendered non-transmissive of light by the action of light being incident thereon. A film such as silver chloride 22 may be employed.

Thus, in the absence of a combustion product, when the opening 6 is exposed to light, then, in the manner described hereabove, a narrow focus 12 of light will be incident upon the silver chloride 22 and cure it into its non-light-transmissive state only within the area of the focus 12. In this way the initial optical alignment between the focus 12, the screening member 16,22 and the detector 14 may be achieved automatically.

If now the silver chloride 22 is set so as to be permanently in this state, i.e.

only non-transmissive at the focus 12 by exposure to light, then on introduction of an aerosol product into the enclosure 4, Schlieren or scattering effects will send the light along the path 20 and hence outside the focus 12 where it may then be detected.

In the above example parts of the inner periphery of the enclosure 4 have been described as including optical baffles 10. It is, of course, possible to have altematives to this multiple-reflective baffle structure, such as a matt black surface or a highly pitted surface, both of which reflect virtually none of the light incident thereon.

Furthermore, although the reflective surface has been illustrated in the above example as a circular concave mirror 8, any suitable arrangement of reflective surface is possible so long as light incident thereon within the angle subtended by the reflective surface at the opening is such as to be incident at the focus 12. Those skilled in the art will realise that the screening member need not necessarily reflect light incident thereon. Its function is to screen the detector 14 from light incident at the focus 12.

In the above example, the term combustion product is intended to encompass all gases, liquids or solids which are the result of a fire and which are likely to cause either scattering within the enclosure 4 or act as a local phase object therewithin. The former will usually be caused by solids such as soot and the latter usually by gases such as alcohols, aerosols and carbon monoxide or carbon dioxide.

It will be apparent to those skilled in the art that the invention will be equally applicable and utile if, for example more than one opening and/or focus are provided by suitable choice of optical geometry. Only one such opening and focus have been illustrated herebefore for clarity.

In the above example, the term light will be understood to encompass any suitable wavelength range. For example, to cure silver chloride, ultra-violet light may be useful. Whereas for detection of a combustion product after curing, infra-red light may be employed.

Claims

1. Apparatus for the optical detection of combustion products comprising: an enclosure of optically non-transmissive material having an opening through which light may pass, the enclosure having a light detector therewithin, the apparatus characterised in that the enclosure further has therewithin; focussing means arranged for converging, to a focus, light passing through the opening, in the absence of a combustion product with the enclosure; and a screening member arranged to screen the detector from light incident at the focus, the light detector being positioned to receive only any light which falls outside the focus.

2. Apparatus according to claim 1 wherein the focussing means comprises a light reflective surface remote from the opening for reflecting light from the opening to the focus.

3. Apparatus according to either one of the preceding claims wherein the screening member is formed from material rendered non-transmissive by the action of light incident thereon.

4. Apparatus according to any one of the preceding claims wherein the screening member is a mirror.

5. Apparatus according to any one of claims 24 wherein all surfaces within the enclosure, other than the light reflective surface, are arranged for absorbing substantially all light incident thereon.

6. Apparatus according to any one of claims 2-5 wherein the reflective surface comprises a concave mirror.

7. Apparatus as substantially hereinbefore described with reference to the accompanying drawings.

8. A method for the optical detection of combustion products which comprises: introducing light through an opening into an enclosure of optically nontransmissive material; focussing light from the opening to a focus in the absence of a combustion product within the enclosure; screening the light at the focus from a detector; and detecting the light, by means of the detector, which is deviated to fall outside the focus by a combustion product within the enclosure.

9. A method according to claim 8 wherein light is deviated to fall outside the focus by a combustion product acting as a local phase object within the enclosure.

10. A method as substantially hereinbefore described with reference to the accompanying drawings.

10. A method according to claim 8 wherein light is deviated to fall outside the focus by a combustion product causing scattering of the light.

11. A method according to any one of claims 8-10 wherein focussing of the light is achieved by a reflective surface reflecting light from the opening to the focus.

12. A method according to any one of claims 8-11 wherein the screening is achieved by incident light rendering material non-transmissive of the focus.

13. A method as substantially hereinbefore described with reference to the accompanying drawings.

Amendments to the claims have been filed as follows 1. Apparatus for the optical detection of combustion products comprising: an enclosure of optically non-transmissive material having an opening through which light may pass, the enclosure having a light detector therewithin, the apparatus characterised in that the enclosure further has therewithin; focussing means arranged for converging, to a focus, light passing through the opening, in the absence of a combustion product with the enclosure: and a screening member formed from material rendered non-transmissive by the action of light incident thereon, the screening member arranged to screen the detector from light incident at the focus, the light detector being positioned to receive only any refracted light which falls outside the focus due to a combustion product acting as a local phase object within the enclosure.

3. Apparatus according to any one of the preceding claims wherein the screening member is a mirror.

4. Apparatus to either of claims 2 or 3 wherein all surfaces within the enclosure, other than the light reflective surface, are arranged for absorbing substantially all light incident thereon.

5. Apparatus according to any one of claims 2-4 wherein the reflective surface comprises a concave mirror.

6. Apparatus as substantially hereinbefore described with reference to the accompanying drawings.

7. A method for the optical detection of combustion products which comprises: introducing light through an opening into an enclosure of optically nontransmissive material; focussing light from the opening to a focus in the absence of a combustion product within the enclosure; screening the light at the focus from a detector; and detecting the light, by means of the detector, which is deviated by refraction to fall outside the focus by a combustion product acting as a local phase object within the enclosure.

8. A method according to claim 7 wherein focussing of the light is achieved by a reflective surface reflecting light from the opening to the focus.

9. A method according to any claim 7 or 8 wherein the screening is achieved by incident light rendering material non-transmissive of the focus.