GB2248108A

GB2248108A - Optical smoke detection system

Info

Publication number: GB2248108A
Application number: GB9020471A
Authority: GB
Inventors: David Theodore Nels Williamson
Original assignee: Individual
Current assignee: Individual
Priority date: 1990-09-19
Filing date: 1990-09-19
Publication date: 1992-03-25
Anticipated expiration: 2010-09-19
Also published as: GB9020471D0; GB2248108B

Abstract

The main problem with light scattering optical smoke detectors is to achieve a high sensitivity and discrimination, keeping stray light from the light source from affecting the photoelectric detector in the absence of smoke particles, whilst achieving a high light-collecting efficiency when smoke particles are present. This invention uses a laser or other highly collimated light source 6 to direct a beam through a tube 1 with the light- guiding properties of transparent solids such as glass or plastics, so that the light collecting efficiency can approach 100%. The tube channels the scattered light to a light detector 3. It is particularly adaptable to aspirated fire detection systems. <IMAGE>

Description

I MPROVEMENTS TO OPTICAL STOKE I)ET'CTIOi' EQUIPMENT Optical smoke detectors in which the light-scattering effect of smoke particles is used to signal the presence of smoke and hence fire are in widespread use. The main problem with their design is to achieve a high sensitivity and discrimination, keeping stray light from the light source from affecting the photoelectric detector in the absence of smoke particles, whilst achieving a high light-collecting efficiency when smoke particles are present.This invention shorts a method of doing this which uses a laser or other highly collimated light source in conjunction with the light-guiding properties of transrarent solids such as glass or plastics, such that the light collecting efficiency can approach lOtSos and which is particularly adaptable to aspirated fire detection systems.

A typical detector according to this invention is shorn by way of example in Fig.l. A tube 1 of methyl methacrylate (perspex, Elexiglass, etc.,) or other suitable transparent material with a refractive index such that it exhibits total internal reflection to light entering it at a suitable angle, is terminated at one end by a hollo conical light guide 2-- of similar material, which in turn is terminated by a suitable photoelectric detector 3.The junction between the tube and the conical light guide may be cemented using an optical cement of the same refractive index, or the surfaces may be polished and kept in contact by a retaining ring or clip (not shown), a smear of a substance with similar refractive index being aTrlied between the surfaces to ensure unimpeded light transmission.

A conical shaped light absorber 4 is retained inside the conical light guide so that a collimated beam of light shining /axially axially along the inside of the tube will be totally absorbed.

This may consist of a block of material such as amorphous carbon with a hole larger in diameter than the beam diameter the bottom of the hole being trepanned into a conical shape or faced at an angle so that any reflection is absorbed along the length of the hole and cannot emerge at an angle such that entry into the light guide through the inner surface of tube 1 is possible.

At the other end of tube I the end surface 5 is polished and placed in contact with a polished reflecting disc or washer which carries a suitable solid-state laser 6 at its centre.

Alternately the washer may have a thin transparent material such as a microscope slide cover-glass cemented to its face and the beam from an external laser or other highly collimated source shone through it axially along the tube and into the absorber. Entry and exit tubes 7 are provided at the ends of tube 1 through which air which may contain the smoke is circulated. The whole assembly may be held together by suitable end capes and long bolts (not shorn for clarity).

The outer surface of tube 1 may be roughened and coated with a reflecting layer to ensure that light entering the inner surface of the tube at angles which could be reflected back out again if the surface were smooth is broken up and reflected at angles ensuring capture in the guide. Light which is reflected out will cross the tube and is likely to be recaptured on the other side.

In use as a smoke detector, an air sample from the region where smoke could be present is passed continuously through tube 1 by means of a fan or trump, whilst the light beam is shone continuously or pulsed along its central axis from source 6 /and and absorbed at the far end in absorber 4. In practice a small amount of light will be scattered by dust particles in the air and will enter the light guide giving a threshold output signal from photocell 3. When smoke is present, however, the scattering effect will be greatly enhanced giving a clear increase in the hotocell outI-ut which may be used to give an alarm and/or measure the level of contamination.

The possible presence of abrasive dust in the atmosphere as sing through the tube could give rise to inner-surface abrasion which could lower its light-collecting efficiency.

The arrangement shown here would enable the tube 1 and its feed tubes 7 to be replaced easily as a low-cost component.

An alternative arrangement particularly suitable for use, for examffile, with a powerful gas laser is to use a "cover glass" arrangement at each end of the light guide tube so that the collimated light beam passes right through the axis of the tube and out of the other end. The conical light guide and absorber is replaced by a ring-shaped photocell which covers the end surface of the light guide tube and a disc of antireflection coated glass cer.ented to its rear surface in a similar way to that at the opposite entry end which carries the reflector ring. Several such detectors may be cascaded along the beam to give an economical multi-point detector.

If respired, reflecting mirrors or prisms may be used at each end of the detector tubes to fold the collimated light beam and give a more compact arrangement for a multipoint system as shorn in Fig.2. Alternatively, the collimated beam may be divided by means of partially reflecting mirrors of the semi-silvered or dichroic type, using the light-absorbent detector first described, as shoT in Fig,3, In either case the electrical outputs of the detectors may be processed separately or scanned in Sequence as desired. Normally a common fan or pump ill tray air through the detectors and their aspiration tubes.

Claims

CIAIIE

1. A smoke detector in which a highly collimated beam of light is surrounded by a tubular light collector and guide which collects the light scattered from the beam and channels it to a light detector element such as a photocell.

Air or gas which may contain the smoke is asked through the tube surrounding the light beam and if smoke articles are -present si.ill result in light scattering which enters the light guide and is channeled to the detector element.

2. A smoke detector as described in Claim 1 in which the light guide is a trans-arent tube the outer surface of which is embossed or etched with a pattern which breaks up light impinging on it radially which would otherwise be passed through the wall or reflected back and out of the tube.

3. A smoke detector as described in Claims 1 and 2 in which the outer surface of the light guide is coated with a light reflector such as an aluminium film to ensure total internal reflection irrespective of angle of incidence,

4. A smoke detector as described in Claims 1, 2 and 3 in which the collimated light beam is provided by a solid state laser mounted in an end cap which can provide a reflecting surface to seal the end of the light collector tube optimally and pneumatically.

5. A smoke detector as described in Claims 1, 2, 3 and 4 in which the end of the light collecting tube o,=osite to the light source is provided with an assembly consisting of a quasi-conical light guide which channels the ring of light emerging from the end surface of the tube into a photocell or other optical detector and which also contains a light absorber element which prevents or limits reflection of the collimated light beam which passes along the axis of the tube.

6. A smoke detector as described in Claims 1, 2, 3, 4 and 5 which the source/reflector, light collector tube and conical light guide and light absorber assembly are arranged as separate parts held together as an asse;,-.bly each section of which may be changed separately- to simlify iraintenance.

7. A smoke detector as described in Claims 1, 2 and 3 in which the ends of the bore of the light guide tube are fitted with transparent windows so that a collimated light beam may be passed right through the axis of the tube and, if required, other similar tubes may be placed along the same bear..

8. A multipoint smokedeteotlo:: system using a smoke detector as described in Claim 7 in which several detector tubes are cascaded using a common collimated light beam, folded if required by mirrors or prism.

9. A multipoint smoke detection system using a smoke detector as described in Claims 1 to 7 inclusive, in which a common collimated light source is divided into separate beams by partially reflecting surfaces.