A LIGHT-SCATTERING-TYPE SMOKE DETECTOR

Abstract: A light-scattering-type smoke detector having a project­ ing element and a light receiving element surrounded by a labyrinth is disclosed wherein the labyrinth comprises an array of a plurality of light shielding columns arranged in a circle each having a substantially J-shaped cross section, and the head portion of each column is inclined relative to the bisector of an angle formed between the lines connecting a first reflecting point with the projecting and light receiving elements, the light shielding columns having darkly colored light reflective surfaces.

Description: BACKGROUND OF THE INVENTION The present invention relates to a smoke detector, and more particularly to a light-scattering-type smoke detector. In a light-scattering-type smoke detector, when the smoke enters into its smoke-detecting chamber the light issued from a projecting element is adapted to be scattered and then be received by a light receiving element. Since the smoke-detecting chamber is formed so that it allows the free entry of ambient air, but not entry of outside light it is surrounded by a so-called labyrinth. The labyrinth known hitherto was formed by a plurality of light shielding columns each having a T-shaped cross section and arranged in a circle, the columns being colored black and delustred. However, in conventional light-scattering-type smoke detectors, in order to improve the light shielding characteristics the heads of the T-shaped light shielding columns constituting the labyrinth were arranged to be in surface contact with the outer periphery of the labyrinth, the smoke entrance area formed at the outer periphery of the labyrinth then being very narrow. Therefore, in order to ensure the quantity of smoke necessary for detection the outer diameter of the labyrinth has to be made large, this necessarily making the size of the smoke detector large. Further, since all of the light issuing from the projecting element to impinge upon the inner wall of the labyrinth is not absorbed there, the scattered light incidents upon the light receiving element so that the noise light output N of the light reflected from the inner wall of the labyrinth becomes large. Therefore, the ratio of the signal light output S of the light scattered by the smoke to the noise light output N, i.e. S/N decreases, lowering the performance characteristics of this type of smoke detector. DE-2828190 discloses a smoke detector provided with a labyrinth formed from a number of columns of complex cross-section, having head-portions aligned to form a generally circular outer wall punctuated by relatively narrow entrance slits. Light reflections from the labyrinth walls within the detector are controlled by providing a sharp edge on the leg portion of the J and which faces into a measuring space surrounded by the labyrinth. FR-1438305 discloses a smoke detector in which there is no attempt to control reflections from interior walls, by their geometry or positioning and which includes a labyrinth formed from columns of J-shaped cross-section, the heads of the J's being aligned to form a generally circular inner wall, again punctuated by relatively narrow entrance slits. SUMMARY OF THE INVENTION It is a primary object of the present invention to provide a light-scattering-type smoke detector which has small dimensions. It is another object of the present invention to provide a light-scattering-type smoke detector which has a larger S/N ratio than that of a conventional smoke detector of this type. It is a further object of the present invention to provide a light-scattering-type smoke detector in which the total area of the openings for allowing the outside atmosphere into the labyrinth is made large relative to the area of the peripheral wall of the labyrinth. According to the present invention a light-scattering-type smoke detector comprises a chamber containing a light projecting element and a light receiving element the chamber communicating with a labyrinth effective to prevent the passage of light from the exterior of the detector into the interior of the chamber whilst allowing the passage of air currents and smoke therethrough, the labyrinth being formed from a plurality of light shielding columns each having a substantially J-shaped cross-section, characterised in that the head of each J-shape in the field of view of the light receiving element is disposed towards the interior of the detector and forms an interior surface inclined relative to the bisector of the angle between any two lines connecting any point on said surface to the light projecting element and to the light receiving element respectively, and the leg of substantially each J-shape is disposed within the concave space surrounded by the head and leg portions of the neighbouring J-shape, such that the labyrinth presents free passages of large cross-sectional area both at the interior and the exterior ends thereof, whilst ensuring that the reflection of light from said interior surfaces onto the light receiving element is minimal. Thus, according to the present invention, in operation, upon impinging of the light issued directly from the projecting element upon any reflecting point on the head portion of the light shielding column, a part of the light is absorbed by the head portion, the remaining part being reflected as a first reflected light, but in this case, since the head portion is inclined relative to the bisector of the angle formed between the lines connecting the first reflecting point with the projecting and the light receiving element this first reflecting light cannot impinge upon the light receiving element. However, the smoke is guided by the leg portions of the light shielding columns. BRIEF DESCRIPTION OF THE DRAWINGS These and other features of the present invention are defined in the claims. The following description of an exemplary embodiment of the invention is made with reference to the accompanying drawings, in which: Fig. 1 is an elevational view of a light-scattering-type smoke detector provided by the present invention; and Fig. 2 is a sectional view of an emodiment of the present invention taken along the lines II-II of Fig. 1. DESCRIPTION OF THE PREFERRED EMBODIMENT As shown in Fig. 1, disposed within the cover 2 of a light-scattering-type smoke detector 1 is a labyrinth 4 surrounded by an insect repellent net 3. As shown in Fig. 2 the labyrinth 4 comprises an array of a plurality of light shielding columns 5 each having a substantially J-shaped cross section, arranged in a circle, their upper and lower ends being closed by bottom plates 6 and 7 (see Fig. 1). The surfaces of the columns 5 and the inner surfaces of the bottom plates 6, 7 are darkly colored and have light reflectivity. As shown in Fig. 2 for a typical one the head portion 8 of the column 5 is inclined at its first light reflecting point P relative to the bisector O of the angle 2 ϑ formed between the lines connecting the point P with the projecting element 9 and the light receiving element 10 by an angle α, which does not include 9O°. The forward ends 8a of the head portion 8 lie on an inner periphery 4a of the labyrinth 4, the rearward ends 8b being disposed within the concave spaces 14 each formed by the head portion 12 and the leg portion 13 of the light shielding column 11 adjoining the light shielding column 5. The bottoms 13a, 15a of the leg portions 13, 15 of the light shielding columns 5, 11 lie on the outer periphery 4b of the labyrinth 4 so as to be in point contact therewith. Thus the first reflected light reflected at the first reflecting point P of the head portion 8 is reflected by the head portion 17 of another light shielding column 16, and in this case the head portion 17a of the light shielding column 16 inclines relative to the bisector R of the angle 2 γ formed between the lines connecting the second reflecting point Q of the head portion 17 of the light shielding column 16 with the first light reflecting point P of the head portion 8 of the light shielding column 5 and the light receiving element 10 by an angle β, which does not include 9O°. The forward end 17a of the head portion 17 of the light shielding column 16 lie on the inner periphery 4a of the labyrinth 4, the rearward end 17b of the head portion 17 being disposed within the concave space 21 formed,by the head portion 19 and the leg portion 20 of the light shielding column 18 as is typically indicated in Fig. 2 for the light shielding column 16. The ends of the bottom portions 22a, 20a of the leg portions of the light shielding columns 16, 18 are in point contact with the outer periphery 4b of the labyrinth 4. Further, in Figs. 1 and 2 the reference numeral 23 indicates a lens mounted in front of the light receiving element 10, 24 a printed circuit board, and 26 a light shield member. Thus the optical axes of the projecting and light receiving elements 9, 10 are substantially in parallel with the bottom walls 6, 7 and intersect each other near the center axis of the labyrinth 4, the light beam issued from the projecting element 9 and being directed in the direction indicated by the arrow A5, impinges upon the head portion 8 of the light shielding column 5 a part of the light being absorbed thereby, the remaining light being reflected at the first reflecting point P to form the first reflected light. In this case, since the head portion 8 of the light shield column 5 is inclined relative to the bisector 0 of the angle 2 ϑ formed between the lines connecting the first reflecting point P on the head portion 8 with the projecting element 9 and the light receiving element 10 this first reflecting light does not travel along the line connecting the first reflecting point P and the light receiving element 10, but instead proceeds towards the head portion 17 of the light shielding element 16. The light impinging upon the head portion 17 is partially absorbed thereof, the remaining light being reflected at the second reflecting point Q to form the second reflected light beam. In this case, since the head portion 17 is inclined relative to the bisector R of the angle 2 γ formed between the lines connecting the second reflecting point Q with the first reflecting point P on the head portion 8 of the light shielding column 5 and the light receiving element 10 the second reflected light does not travel along the line connecting the second reflecting point Q to the light receiving element 10, but impinges upon the head portion 19 of the adjoining light shielding column 18. By this repeated absorption and reflection of the light beam issued from the projecting element 9 the travel distance of the light is made longer, thus the light from the projecting element 9 is remarkably attenuated. On the other hand, upon occurrence of a fire, the smoke enters the labyrinth 4 through the gaps between the light shielding columns 5, 11, 16, 18 etc. In this case, since the bottoms 13a, 15a, 20a, 22a of their leg portions are in point contact with the outer periphery of the labyrinth 4 the area of the opening 25 relative to the outer peripheral wall surface of the labyrinth 4 is made large, thus allowing flow of greater amounts of smoke. The smoke flowing through the openings 25 is guided by the leg portions 13, 15, 20, 22 and enters the labyrinth 4 at a slower speed while impinging upon the head portions 8, 12, 17, 19. It will be appreciated that according to the present invention the light beam issued from the projecting element is partly absorbed and the remaining light is reflected forwards by the head portions of the light shielding columns, but the reflected light does not impinge upon the light receiving element. In other words, since the head portions are inclined relative to the bisector of the angle formed between the lines connecting the first reflecting point with the projecting and light receiving element the reflected light does not travel along the line connecting the first reflecting point to the light receiving element. Therefore, since the component of the noise light output N becomes small the S/N ratio is increased, improving the performance of the smoke detector. Further, by the use of light shielding columns each having a substantially J-shaped cross section the total area of the openings relative to the area of the peripheral wall of the labyrinth is made larger than that obtained with the conventional shielding columns having a T-shaped cross section. Consequently, since the total opening available is large in proportion to the diameter of the labyrinth, thus the smoke detector can be made smaller. Moreover, since the head portion of the other light shield column is inclined relative to the bisector of the angle formed between the lines connecting the second reflecting portion with the first reflecting point and the light receiving element the second reflected light does not travel along the line connecting the second reflecting point to the light receiving element, so no reflecting light impinges upon the light receiving element. With the increase in the travel distance of the light issued from the projecting element by its repeated absorbtion and reflection, the noise light is enormously attenuated. One experiment conducted proved that the S/N ratio was about 10, whereas the S/N ratio in a conventional smoke detector of comparable capacity indicated a value of 2 to 3. It is to be understood that although a single embodiment of the present invention has been illustrated and described, the present invention is not to be limited thereto except insofar as such limitations are included in the following claims:

Claims: 1. A light-scattering-type smoke detector (1) comprising a chamber containing a light projecting element (9) and a light receiving element (10, 23) the chamber communicating with a labyrinth (4) effective to prevent the passage of light from the exterior of the detector into the interior of the chamber, whilst allowing the passage of air currents and smoke therethrough, the labyrinth being formed from a plurality of light shielding columns (5, 11, 16, 18) each having a substantially J-shaped cross-section, characterised in that the head (8, 12, 17, 19) of each J-shape in the field of view of the light receiving element is disposed towards the interior of the detector and forms an interior surface inclined relative to the bisector (O) of the angle (2 ϑ) between two lines connecting any point (P) on said surface to the light projecting element and to the light receiving element respectively, and the leg (13, 15, 20, 22) of substantially each J-shape is disposed within the concave space surrounded by the head and leg portions of the neighbouring J-shape, such that the labyrinth presents free passages of large cross-sectional area both at the interior and the exterior ends thereof, whilst ensuring that the reflection of light from said interior surfaces onto the light receiving element is minimal. 2. A light-scattering-type smoke detector as claimed in claim 1 further characterised in that said labyrinth is closed at the upper and lower end of said columns by substantially planar top and bottom walls, (6, 7) respectively. 3. A light-scattering-type smoke detector as claimed in claim 2 wherein said top and bottom walls have inner surfaces of low reflectivity. 4. A light-scattering-type smoke detector as claimed in claim 2 or 3 further characterised in that the light projected from said light projecting element travels substantially parallel to the top and bottom walls and the light receiving element is adapted to receive light travelling substantially parallel to the top and bottom walls. 5. A light-scattering-type smoke detector as claimed in any preceding claim further characterised in that the labyrinth surrounds the periphery of the chamber. 6. A light-scattering-type smoke detector as claimed in any preceding claim further characterised in that the interior surfaces are inclined relative to the bisector (R) of an angle (2 γ) formed between a line connecting any point (Q) on said surfaces to said light receiving element, and a second line connecting the point (Q) to a second interior surface (8), said second line forming an angle with a third line connecting the second interior surface with the light projecting element, which angle is bisected by a line extending perpendicularly to the second interior surface.

Inventors: MOCHIZUKI, MIKIO NOHMI BOSAI KOGYO CO. LTD.
Applicants: NOHMI BOSAI LTD
Application year: 1986
Main classification: G08B17
IPC codes: G08B17, G01N21