This invention relates to an optical smoke detector of extremely high sensitivity. A similar detector is disclosed in my European Patent Application EP-A-140 502 with the same priority date as the present application.
The present invention is particularly adapted for use with an axial-light absorber as described in my European Application EP-A-135 361, also having the same priority date, and can utilise a sampling chamber as disclosed in my European Application EP-A-140 502.
An associated smoke detector is disclosed in my European Application EP―A―132969, published after the priority date of the present application.
With optical smoke detectors it is necessary to provide a light source of low capacity to irradiate any smoke particles that are drawn into the sampling chamber.
Reduction in energy input lengthens the operational life of the light tube and serves to decrease current drain from a standby battery required to maintain operation in the event of mains failure. Reduction in current drain either increases the life of the battery or reduces the capacity requirement and therefore the cost of the standby battery.
The present invention is directed at allowing the use of low energy consumption light sources to provide the aforementioned advantage. However, such light sources; such as Xenon flash tubes, do not produce light from a point source but rather over an elongate length. The instant invention has as an object the provision of a smoke detector which is able to use such elongate light sources, by including suitable reflecting means to usefully focus the light energy produced.
U.S. Patent No. 2,812,686 discloses a pollution detector. It, however, relies on a substantially point-source light source, and uses lenses rather than a specially configured reflector as in the instant invention.
French Patent No. FR-A-2348502 discloses various reflectors for light sources. It does not teach the application of these light sources to pollution detection apparatus. It further does not reach the use in a pollution detector of an elongate light source, nor a reflector having a corresponding configuration to an elongate light source.
The present invention provides smoke detecting apparatus comprising:
Conveniently, the reflector means is supported by means comprising a mounting base and peripheral flanges for mounting said reflector means in sealed relation on said sampling chamber.
The window in the sampling chamber may be an open window to enable circulation of the atmosphere in the sampling chamber within the reflector body when in operation.
The invention will be described in greater detail having reference to the accompanying drawings in which:
The reflector body 60 includes a concave U-shaped reflector element 61 designed to focus light impinging thereon from each infinitesimal cross-sectional element of a U-shaped Xenon flash tube into a central region 72 of air sampling chamber 70. The body 60 is attached to a flat surface of the chamber 70 by sealing flanges 60a and 60b. The flat surface 71 of chamber 70 enables simplified sealing and allows the use of an open window 74 for transmission of light into the chamber. The open window allows for circulation of air around the flash tube 62 thereby preventing the build up of potentially damaging ozone in the device.
Alternatively, the light window 74 may be sealed by clear glass or plastic (not shown) to seal the reflector chamber, whereby the reflector chamber 60 can be filled with an inert gas such as nitrogen. Further, alternatively, a flash tube of quartz glass which prevents the formation of potentially corrosive ozone, can be used. However, the use of an inert gas is costly whereas the latter alternative prevents detection of scattered ultra-violet light, thereby altering the calibration of the sampling tube in respect of certain products of combustion.
The reflector 60 is provided with a mounting base 63 which is preferably in the form of a printed circuit board serving as a mounting for the electrode leads of the flash tube 62. Sealing of the circuit board base 63 to the reflector body 60 by sealing flange 60c and sealing of the flanges 60a and 60b to the side of the sampling tube 71 is preferably achieved by a silicone rubber glue. This allows operation of the chamber at other than atmospheric pressure.
The size of the window aperture 74 and the spacing between the light tube 62 and the window together with the focal distance of the concave curved reflector are each optimized to maximize light intensity within the sampling chamber without unduly increasing spurious or stray incident light reflected off the internal walls of the sampling chamber.
The curvature of the reflecting element is developed to follow the "U" shape of the flash tube such that the light output from the whole length of the tube is focused through the flash window into the centre of the sampling chamber.
Accordingly, the use of the present invention directly results in a reduction of energy consumption of the lamp by a factor of 2 and without any loss of sensitivity in the detector.
A commensurate extension in lamp life is achieved.