WO1982003487A1

WO1982003487A1 - Optical fire detector

Info

Publication number: WO1982003487A1
Application number: PCT/SE1982/000089
Authority: WO
Inventors: Telefon Ab L M Ericsson; Erik Gustaf Lindgren
Original assignee: Telefon Ab L M Ericsson
Priority date: 1981-03-25
Filing date: 1982-03-25
Publication date: 1982-10-14
Also published as: GB2111278A; SE432029B; SE8101921L; GB2111278B; CH657929A5

Abstract

An optical fire detector in which a radiation source (4) generates a ray beam, which, after having passed through the air in a room, is received by a detector (5) for determining if the characteristic properties (intensity, modulation) of the received beam have been affected by combustion, generating aerosols or convection in the air. The fire detector includes a Fresnel lens (1) with two focal points, on one side of the room, the radiation source (4) being disposed at one focal point and the beam detector (5) at the other. On the other side of the room there is arranged a retroreflector (6), which by total internal reflection re-emits the beam sent from the Fresnel lens (1). An indicating circuit (11) connected to the radiation detector (5) indicates that the change in the characteristic values of the emitted and received beam exceed threshold value.

Description

OPTICAL FIRE DETECTOR

TECHNICAL FIELD

The present invention relates to optical fire detectors in which a radiation source generates a pencil of rays or beam which, after passing through the air in a room, is received in a detector for determining if the characteristic properties (intensity, modulation) of the received beam have been effected by combustion aerosols or convection in the air¬

BACKGROUND ART

Apparatus of the type mentioned necessitate a separate radiation source on one side, and a detector on the other side of the room which is to be monitored. The optical axes of the ray source and detector must coincide, and the adjustment thereof will therefore be troublesome. Furthermore, the ray source and detector must be connected by an electric cable.

If the ray source and detector are placed on the same side, and a flat mirror on the other side of the room, there are also difficulties with adjustment, since all three parts must be adjusted and the apparatus will be very sensitive for changes in the position of the mirror.

If the mirror is replaced by a reflector, it will be easier to direct the rays towards the detector, since the apparatus will not be sensitive to changes in the position of the reflector, but the reflector reflects the rays with some spread, so that the detector only gets a portion of the radiation.

DISCLOSURE OF THE INVENTION

The object of the invention is to provide an optical fire detector which is not sensitive to positional changes, and where practically the whole of the radiation is reflected to the ray detector simultaneously as the rays are prevented from striking the radiation source.

The fire detector in accordance with the invention is characterized as will be seen from the claims.

OMPI DESCRIPTION OF FIGURES

The invention will now be described below while referring to the appended drawing, on which Figure 1 illustrates an apparatus in accordance with the invention, including a Fresnel lens and associated retroreflector, and Figure 2 schematically illustrates a detector circuit for sensing the changes in the monitored room.

PREFERRED EMBODIMENT

Figure 1 illustrates a Fresnel lens conventionally made up from annular parts 2 and 3. The surfaces of rings 2 and 3 have different angles of inclination, and are calculated such that the lens has two focal points f₁ and f₂. Thus, if a radiation source, e g a light-emitting diode 4 (LED) is placed at one focal point f₂, parallel rays are emitted, and after reflection these can be collected at the focal point f₁, where a photodetector 5 is arranged. Opposite the lens there is arranged a retroreflector 6, i e a reflector of the type from which the light is redirected by total internal reflection into its incident direction, and which can be constructed from prisms, for example. The absorption will be minimum in this way, and the rays will be re-emitted to the Fresnel lens. By disposing the ray source at one focal point of the lens and the photodetector at the second one, it is achieved that the rays passing through the room can be received in the photodetector with minimum loss, and the space requirement of the fire detector will be the least possible.

Figure 2 illustrates an example of a fire detector utilizing the inventive concept. The Fresnel lens and detector have the same denotations as in Figure 1. A modulator, denoted by 7, modulates the radiation intensity, so that 20 kHz pulses are obtained. Radiation from the LED 4 is sent in parallel rays from the Fresnel lens 1 and after reflection in the retroreflector 6 the rays will arrive at the photodetector at the focal point f₁. The photodetector 5 generates an electric signal which is fed via an amplifier 8 to a band-pass filter 9 adjusted to 20 kHz. The filter output signal is fed through a rectifier 10 and an integrating circuit 11 to a comparator 12. The comparator 12 is also given a reference value constituting the signal stored in a memory 13, e g the one that was received some hours earlier. If the value of the received signal deviates from the reference signal, this means that the absorption of the radiation in the monitored room has been altered by smoke generation, and the comparator gives a signal to a threshold detection means 20 which gives an alarm when a given predetermined value is reached.

The output from the filter 9 is also connected to a low-pass filter 14, which only passes frequencies under 150 Hz. Since the refractive index of the air varies with its temperature, as soon as convection in the air increases as a result of a fire, the light is modulated at these low frequencies in a known manner. The photodetector 5 is then modulated with these low frequencies, and the electric signal generated can pass through the filter 14, an amplifier 15 and a rectifier 16 to a threshold detection means 21. In this way there is obtained an indication for smoke particles and for heat generation.

It is obvious that the invention is not limited to the described embodiment, and any optional indicating means may be used which makes a comparison between passages of radiation through a gaseous medium in different conditions.

Claims

C L A I M

An optical fire detector in which a radiation source generates a pencil of rays or beam, which after passing through the air in a room and being re-emitted by a retroreflector on the other side of the room, is received by a ray detector for determining whether the characteristic properties (intensity, modulation) of the received ray have been affected by combustion, generating aerosols or convection in the air, characterized in that the fire detector includes a Fresnel lens with two focal points, where the ray source is disposed at one focal point and the radiation detector at the other, and in that the detector is connected to an indication circuit giving an alarm if the difference between instant characteristic values of the reflected beam and corresponding previously stored reference values exceed a threshold value.