The present invention broadly relates to a photoelectric smoke detector and, more specifically, pertains to a new and improved construction of a small-size or miniature photoelectric smoke detector.
In its more particular aspects, the present invention specifically relates to a new and improved construction of a photoelectric smoke detector which is equipped with a light emitter for radiating light into a measuring chamber and a light receiver for detecting scattered light caused by the entry or presence of smoke in the measuring space.
Dark chambers of conventional photoelectric smoke detectors have a complicated labyrinth construction or double covers and/or additional light shielding plates to prevent the environmental light from entering the dark chamber but to facilitate entry of smoke into the same, see, for example, US-A-3,708,675.
However, such photoelectric smoke detectors as mentioned above, being large in size, are not suitable for use as photoelectric smoke detectors to be built-in into equipment such as electronic computers or to be installed in lavatories in aircraft. These photoelectric smoke detectors also have the disadvantage that miniaturization thereof by merely reducing the size of their dark chambers result in lowering the SN ratio.
To improve the sensitivity of conventional photoelectric smoke detectors it has been suggested to reflect the light beam of a light emitter more than once by a couple of reflecting plates and to arrange a light receiver outside the path of the reflected light, see, for example JP-A-57,42,842. However, while this construction is suitable to improve the sensitivity, simultaneously the noise caused by light scattered from the reflecting plates is enhanced.
Therefore, with the foregoing in mind, it is a primary object of the present invention to provide a new and improved construction of a photoelectric smoke detector which does not exhibit the aforementioned drawbacks and shortcomings of the prior art constructions.
An important and more specific object of the present invention is directed to the provision of a new and improved construction of a photoelectric smoke detector which, although being a small-sized construction, possesses a favourable signal-to-noise ratio for smoke detection.
Yet a further significant object of the present invention aims at providing a new and improved construction of a photoelectric smoke detector which is relatively simple in construction and design, extremely economical to manufacture, highly reliable in operation, not readily subject to breakdown or malfunction and requires a minimum of maintenance and servicing.
Now in order to implement these and still further objects of the invention, which will become more readily apparent as the description proceeds, the photoelectric smoke detector of the present development is manifested by the feature that, a dark chamber forming the measuring space comprises at least one wall comprising planar or curved surfaces arranged substantially in front of the light emitter for reflecting the light radiated from the light emitter at a predetermined angle to the optical axis of the light emitter. The dark chamber also possesses side walls arranged substantially parallel to the optical axis defined by the light emitter.
The photoeletric smoke detector according to the present invention is equipped with a dark chamber of the aforesaid construction such that the dark chamber itself is capable of performing the light trap function. More specifically, the dark chamber is designed in such a way that the light beam radiated by the light emitter is reflected from the wall surfaces of the dark chamber several times without allowing the initially reflected light to reach the light receiving element of the light receiver. The reflected light, if any, reaching the light receiver or receiving element is attenuated to a very weak intensity. Therefore it is not necessary to specially provide a light trap means. Thus a small-sized photoelectric smoke detector with a narrow measuring space is obtained.
The invention will be better understood and objects other than those set forth above, will become apparent when consideration is given to the following detailed description thereof. Such description makes reference to the annexed drawings wherein throughout the various figures of the drawings there have been generally used the same reference characters to denote the same or analogous components and wherein:
The following describes embodiments of the photoelectric smoke detector according to the present invention in reference to Figures 1 through 3. In these figures, (a) is a cross section and (b) is a transverse section of the dark chamber according to the present invention, wherein the components are indicated by numerals as follows: 1 - a light emitter which is equipped with a light source such as light emitting diode and a lens, and radiates a convergent light beam; 2-a light receiver comprising a light receiving element such as solar cell and a lens; 3 - a light shielding plate provided between the light emitter 1 and the light receiver 2; 4, 5 - side walls arranged nearly in parallel with the optical axis of the light beam radiated from the light emitter 1; 6 - a front wall with a plane or curved surface which is arranged in front of the light emitter 1 to reflect the light at a predetermined angle to the optical axis; 7 - a rear wall with plane or curved surface which is arranged in the rear of the light emitter 1 to reflect the reflected light from side walls 4, 5 at a predetermined angle so that the reflected light does not directly irradiate the light receiving element of the light receiver 2; 8, 9 - a bottom wall and a top wall with apertures through which smoke can flow in (not shown on the drawing) and which are provided in the center of the bottom wall 8 and near both ends of the top wall 9. The wall surfaces are of black color and finished up like a mirror. Further in the figures, s - a starting point of the light beam; arrow - a point where the light beam reaches; black dots - reflection points on the side wall 4; white dots - reflection points on the side wall 5; triangle - a reflection point on the top wall 9.
Accoring to preferred embodiment of the invention the walls of the dark chamber have mirror-like surfaces with good absorbance. In this case, for example, the convergent light beam radiated from the light emitter 1 impinges on the left side surface of the front wall 6 and is absorbed by the black surface. Unabsorbed light is reflected from the mirror-like surface nearly as it is, i.e. in the form of convergent light beam without being diffused, and from the left wall 4, the right and left walls 5, 4, further from the top wall 9, the right wall 5, the left wall 4, the left side of the rear wall 7, and then impinges on the outer wall of the light receiver 2. As can be seen from the above description, the dark chamber acts as light trap, where the light radiated from the light emitter 1 is absorbed by the black surfaces and attenuated while being reflected several times, and eventually becomes a very weak light.
Further describing the idea in reference to Fig. 2, the light radiated from the light emitter 1 is reflected from the left side of the front wall 6, the left wall 4, the right, left, right walls, the top wall 9, the left wall 4, the right wall 5, and the right side of the rear wall 7 and then impinges on the outer wall of the light receiver 2. In Fig. 3, the light radiated from the light emitter 1 is reflected from the left side of the front wall 6, the left wall 4, the top wall 9, the right, left, right, left walls and the left of the rear wall 7, and then impinges on the outer wall of the light emitter 1, thus being attenuated to a very weak light.
On the other hand, if a fire breaks out and smoke enters the dark chamber, scattered light irradiates the light receiving element of the light receiver 2, where an output to give a fire alarm develops. Fig. 4 shows characteristics of the dark chamber, with S representing output of the light receiving element when a predetermined density of smoke for decision on fire entered the dark chamber, N representing output of the light receiving element in normal condition, and 6 representing angles of the front and rear walls 6, 7. According to this Figure, a sufficient SN ratio for practical use is obtained at 6 of not more than 140° or not less than 210°.
The photoelectric smoke detector according to the present invention is equipped with a dark chamber of the above composition whereby the dark chamber itself is capable of performing a light trap function. More specifically, the dark chamber is designed in such a way that the light beam radiated by the light emitter is reflected several times from the side walls via the front and rear walls, without allowing the initially reflected light to reach the light receiving element of the light receiver, and that reflected light reaching the light receiving element if any is attenuated to a very weak one. Therefore, this idea has such an effect that a small-sized photoelectric smoke detector which has a narrow measuring space and does not require special provision of a light trap means is obtained.