GB2335976A - Replaceable optical cell for scattered light smoke detection apparatus - Google Patents

Abstract

In alight scattering smoke detector, alight beam is passed through a sample of air from an area being protected and into a light trap. If smoke is present, light will be scattered onto a carefully positioned pickup. High sensitivity systems are aspirated and therefore require significant air flow, and also use mirrors and/or lightguides to collect and focus the maximum amount of scattered light onto the pickup. Unfortunately the collector and pickup are exposed to the air flow and will gradually become covered with smoke or dust particles, reducing the accuracy and efficiency of the system. To avoid this, a replaceable optical chamber protects the pickup and acts as a collector thus allowing servicing without need for disassembly. Because the principal optical elements are retained within the main detector assembly, removing the optical chamber does not disturb the optical alignment and avoids the need for realignment or recalibration.

Description

1 2335976 1 Improvements to optical smoke detection equipment.

When small particles are dispersed in an aerosol and illuminated, the particles scatter light. This forms the basis of all optical smoke detectors.

A light beam is passed through an air sample from the area which is being protected, and into a light trap. If smoke is present, scattered fight will fall on a carefully positioned pickup. The smoke level determines the amount of scattered light and therefore the signal from the pickup.

Because light scattedng is a low level effect, ambient light must be excluded from inside the detector and stray light [from reflections from the main beam, off the detector chamber walls, light trap etc] must be rigorously excluded.

There are three main requirements for such detectors:

to collect as much light scattered by the smoke as possible to prevent stray light flooding the system to protect the collectorlpickup from degradation by accumulation of dirt overtime For high sensitivity systems, mirrors andfor lightguides are typically used to collect and focus the maximum amount of scattered light onto the pickup.

Stray light may be controlled by precisely aligning the light source with the fight trap, in order to prevent the edges of the beam being reflected from the edges of the light trap entrance back into the chamber.

Typically high sensitivity systems are aspirated, and therefore require significant air flow. Unfortunately the collector and pickup are exposed to the air flow and Will gradually become covered with smoke or dust earticles overtime, and therefore reduce the accuracy and efficiency of the system. This can be reversed by cleaning the mirror and pickup, but in current systems such an operation requires a return to factory to strip, clean, reassemble, realign and recalibrate the detector.

This invention proposes a replaceable optical chamber, which protects the pickup and acts as a collector thus allowing for simple field servicing without need for disassembly. Because the principle optical elements [light source, pickup & light dump] are retained within the main detector assembly, removing the optical chamber does not disturb the optical alignment thus doing away with the need for realignment or recalibration. Although the replaceable chamber could be cleaned and reused, it is envisaged it will in made of inexpensive materials and therefore disposable.

2 Fig 1 shows typical replaceable optical chamber according to this invention. The replaceable optical chamber 1 is a transparent cylinder covered on its outer surface with a mirror coating.

The chamber is placed within the smoke detector as shown in Fig 2. The detector assembly 2, consists of a light source 3, which emits collimated or focused light, on one side of a bore 4 through the assembly. This bore is designed to hold the replaceable chamber, Which can be slid in and out. The light beam passes through 4 and fails into a light trap 5 opposite the light source. The hollow space has entry 6 and exit 7 holes, through which the air sample is drawn. The light detecting element 8 is located off the optical axis of the light beam, so that it is not directly illuminated and should only receive scattered light.

The replaceable optical chamber fits inside the bore 3 within the main detector body and is inserted/removed through the entry or exit holes. The coating has been removed where the cylinder is adjacent to the light source, the light dump and the light detector element.

The light detector element is located outside the chamber and abuts up to the replaceable chamber, which protects it from contamination by the air stream, which passes through the middle of the chamber. If required an index matching material may be used between the pickup and the outside wall of the chamber.

A facet, slot or keyway in the chamber mates with its equivalent in the detector to ensure accurate alignment after replacement.

When smoke enters the chamber, it is illuminated by the beam and scatters light in all directions. Some light fails directly on the pickup, some is reflected by the mirror coating onto the pickup and some, by reason of total internal reflection within the chamber walls is guided into the pickup. The latter two processes multiply significantly the amount of light received by the pickup.

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Claims (1)

1. Claims

   Claim 1 A replaceable hollow optical chamber, within the main assembly of an optical smoke detector, wherein air borne particles drawn from the outside environment are illuminated with light, that does not require disassembly of the said main assembly for replacement of the said chamber or recalibration of the said smoke detector after replacement of the said chamber; since the said chamber does not contain andlor locate any of the principle optical elements of the detector, these being the light source, the light trap and the light detecting element.

   Claim 2 A removable optical chamber as described in claim 1, which covers the light detecting element of the smoke detector and so protects it from contamination by particles conveyed in the air steam, the said chamber being removed for cleaning or replacement.

   Claim 3 A removable optical chamber as described in claim 1, which is composed of a transparent material, whereby the shape of the said chamber and refractive index differences between the main detector, the said chamber and the air flow cause scattered light from the smoke, which would otherwise be lost, to be reflected, refracted andlor guided toward the light detecting element.

   Claim 4 A removable optical chamber as described in claims 1 & 3, having its exterior covered with a mirror coating to reflect and to guide light scattered by the smoke into the light detecting element.

   Claim 5 A removable optical chamber as described in claim 1, having its interior covered with a mirror coating to reflect and to guide light scattered by the smoke into the light detecting element.

   Claim 6 A removable optical chamber as described in claims 1, 3 & 4 or claims 1 & 5 but with the mirror coating removed or reduced in front of the light source, the beam dump and the pickup in order to maximise the amount of light which may be scattered, and to minimise the amount of unscattered light which may be reflected into the light detector element.

   Claim 7 A removable optical chamber as described in claims 1 & 3 but with holes made in the chamber walls in front of the light source and light trap, to allow for the passage of light from the light source to the light trap without passing through the chamber surfaces or body which may scatter light from surface contamination, andlor imperfections in surface finish or material, giving rise to an excessive background light level.

   Claim 8 A removable optical chamber as described in claims 1 & 3, which is composed of a transparent material, whereby the shape of the said chamber and refractive index differences within the material of the said chamber scattered light from the smoke, which would otherwise be lost, to be reflected, refracted andlor guided toward a light detecting element