Electro-Optic Smoke Detector

This invention relates generally to apparatus used in conjunction with fire detection and alarm systems. More particularly it is directed to an electro-optic device designed to detect and annunciate the presence of smoke in the air in or moving through the device. To increase the sensitivity of the device to small smoke particles the components of this device are arranged so that both light which is forward scattered and light which is back scattered are efficiently collected.

One of the limitations of optical smoke detectors has been that while they are extremely sensitive to the presence of large smoke particles they have been less sensitive to small smoke particles. By novel structural arrangements in the smoke chamber we have substantially improved the sensitivity of the optical smoke detector to small smoke particles. Small smoke particles are herein defined as those particles having a diameter (D) less than the wavelength of the light being used. The small smoke particles tend to be generated in flash fires or rapidly burning fires in contrast to smouldering fires which tend to result in large smoke particles to be sensed.

FIG. 1 is a diagrammatic representation of the improved smoke detector chamber showing the major components and their relationship.

FIG. 2 is a graphical representation of forward and backward light scatter in a smoke detector vs. smoke particle size.

Referring now to the drawings there is illustrated in FIG. 1 a cross section view of the smoke detector chamber. An exterior casing 10 has openings 11 and 12 in its lower and upper portions respectively for allowing air to flow through the sensing chamber 13. A source of radiant energy 14, in the visible, IR or UV spectrum, and hereafter called light, is transmitted through an imaging lens 15 into chamber 13 to be focused or directed towards a small mirror 16 which is coated on the center of a relatively large area detector 17. The source, lens and detector are all mounted within the casing 10, the mounting 20 for the lens 15 providing an opaque divider to allow light from the source 14 to reach the chamber 13 only by way of the imaging lens 15. In one embodiment the large area detector 17 is a photodiode model CLD31, by Clairex Corporation of New York, N.Y. This photodiode is designed to operate in the photovoltaic mode and has an active area of about 22 mm.sup.2. Its peak sensitivity is in the wavelength of 0.9-1.0 microns and is well suited for use with an infrared LED as the source 14. In this embodiment a gallium arsenide LED may be used, for example.

The transmitted light from source 14 is imaged by the imaging lens 15 onto the mirror 16 located in the middle of the detector 17. In the absence of smoke there is no light scatter and the transmitted light from the source does not fall on the detector 17 but falls entirely on the mirror and thus all of the light is reflected. When smoke is present the smoke particles cause a forward scatter 21 of the transmitted light which scattered light is collected by the detector 17. The arrangement shown allows high efficiency collection of forward scattered light partly because shallow angle scattering is collected by this novel chamber geometry.

The graphs of FIG. 2 show the scattered radiation pattern which exists due both to large (D>.lambda.) and small (D<.lambda.) smoke particles; and that for large smoke particles, e.g. the diameter D of the smoke particles is larger than the wavelength .lambda. of the light from source 14, the light scatter from the particles is predominantly forward scatter with very little backward scatter, and also that for small smoke particles, e.g. the diameter of smoke particles smaller than .lambda., the back scatter increases and is substantially equal to the forward scatter.

The use of the mirror 16 which reflects the light back generally towards the source makes is possible to also collect the back scattered light 22 of the reflected light just as effectively as forward scattered light of the transmitted light is collected. This efficient collection of the back scattered light is significant when smoke particles are small and provides an optical smoke detector with enhanced sensitivity for the detection of small particle smoke.

The electrical output of the detector 17 is electrically connected 23 to an alarm circuit 24. Detector 17 in response to light reflected from the smoke suspended within the chamber area 13 causes an electrical current to flow to alarm circuit 24. The alarm circuit may be an amplifier-relay combination, which when the signal from the detector reaches a predetermined magnitude, closes a circuit to activate an alarm device. In smoke detectors which are designed for easy flow of air through the smoke chamber, the ambient light surrounding the detector may not be completely excluded from the detector. In these detectors to eliminate any effect of ambient steady state light, the smoke detector apparatus generally includes a pulse light source and a pulse sensitive detection circuit which may, if desired, be synchronized to the pulse light source. This pulse responsive type smoke detector circuit is less sensitive to ambient light conditions.