Patent Publication Number: US-2017370835-A1

Title: Optical detector of a value of an atmospheric physical quantity representative of a danger

Description:
TECHNICAL FIELD OF THE INVENTION 
     The present invention relates to an optical detector of a value of an atmospheric physical quantity representative of a danger, and an alarm device comprising same. It applies, in particular, to the detection of flammable gases, smoke, aerosols or dust in residential, industrial, commercial or recreational public or private works structures and buildings. 
     STATE OF THE ART 
     “ATEX” zones are zones in which there is a risk of an explosive atmosphere. These zones are the subject of a regulation known as “ATEX”. The purpose of this regulation is to control the risks relating to explosion in these atmospheres. There are several sub-divisions in the classifications of these zones: “0”, “1” and “2” for gases, “20”, “21” and “22” for dust. For each of these zones, regulations impose the use of specific materials to eliminate the risks of explosion. 
     In general, an ATEX product must be designed to avoid heating and sparks in contact with the explosive atmosphere. 
     Smoke detectors use several physical principles, and mainly the ionization of gases and optical scattering or absorption. In such detectors, the atmosphere must be allowed to enter inside a measurement chamber, since it is the particles present in this atmosphere that are to be detected. This constraint makes ATEX smoke detectors very difficult to design. 
     In the case of ionic smoke detectors, for example, the measurement chambers comprise electrodes designed to create an electric field for driving ions. These electrodes and the associated electrical circuits are therefore in direct contact with the atmosphere and therefore with the flammable gases that may possibly be there. 
     Optical smoke detectors are separated into scattering-based detectors and absorption-based detectors. 
     The principle of an optical scattering-based smoke detector is based on utilizing, firstly, an emitter of light rays and, secondly, a receiver of light signals scattered by the ambient air, the receiver being outside the field lit by the emitter. When there is no smoke in the air entering the detector, the receiver only receives a very small amount of scattered light. In contrast, when there is smoke in the air entering the detector, this smoke scatters the light originating from the emitter and thus lights the receiver. 
     In these optical detectors, light-emitting diode or laser diode emission circuits, for example, are associated with phototransistor or photodiode types of light receivers. Here again, electrical and electronic circuits are in contact with the atmosphere and therefore with any flammable gases that may possibly be there. 
     In optical absorption-based smoke detectors, an emitter of light rays and a receiver are used, both elements being arranged such that the receiver can receive the rays emitted by the emitter, either directly or after being reflected onto at least one reflector. The presence of smoke on the path of the beam has the effect of reducing the light signal received by the receiver. Here again, electrical and electronic circuits are in contact with the atmosphere and therefore with any flammable gases that may possibly be there. 
     SUBJECT OF THE INVENTION 
     The aim of the present invention is to enable the production of smoke, aerosol, dust or gas detectors that can operate in an explosive atmosphere. 
     To this end, according to a first aspect, the present invention envisages an optical detector of a value of an atmospheric physical quantity representative of a danger, the detector comprising:
         at least one measurement chamber accessible to the atmosphere; and   at least one electronics compartment for receiving an electronic device for detecting the value of the atmospheric physical quantity representative of a danger, the electronic detection device comprising at least
           one light emitter;   one light receiver that is sensitive to at least one portion of the wavelengths of the light rays emitted by the emitter; and   electronics for processing detection signals;
 
the detector also comprising:
   
           a first light guide facing the emitter in order to direct the light emitted by the emitter from the electronics compartment comprising said emitter to a detecting zone in a measurement chamber; and   a second light guide facing the light receiver of said electronics compartment, in order to direct light originating from said detecting zone to the electronics compartment in order to be received by the receiver, the amount of light received by the receiver being representative of the value of the physical-quantity representative of a danger;
 
wherein the electronics compartment is separated from the measurement chamber and isolates all the electronic elements it contains from the atmosphere; and the light guides are arranged to enter the electronics compartment in a way that is seal-tight to the atmosphere.
       

     The physical-quantity representative of a danger can be measured by optical means. For example, this physical quantity is a level of particles in the air or a presence of gas detectable by spectroscopy. 
     Therefore, it is possible to benefit from optical smoke detection in the measurement chamber, this chamber being open to the atmosphere but not containing any electrical circuit likely to present a risk with regard to the ATEX regulations and having the electrical and electronic portion of the detector completely isolated from the atmosphere. 
     In some embodiments, for at least one electronic device, the first and second light guides consist of a single part comprising a link resistant to the passage of the light from the first light guide to the second light guide. 
     In some embodiments, the link resistant to the passage of the light bears a centering stud. 
     Thanks to these provisions, the positioning of the single part comprising the prisms is reproducible and precise. Therefore, positioning the light emitter and receiver components is performed at the same time. It is therefore simplified and the reproducibility of the electronic smoke detection circuit&#39;s sensitivity is improved. Reproducibility of the emission/reception angles of the light rays is also improved. The production of light reflectors is made easier since they can be molded at the same time as the link connecting them. In effect, the inventor has determined that one problem with optical scattering-based detectors concerns the detection of faults and in particular the absence of emission by the light emitter component and/or where there is a loss of sensitivity in a light receiver component. However, these faults are critical since they limit, even prevent, the detection of smoke. 
     In some embodiments, said link comprises an optical guide designed to carry a portion of the light emitted by the emitter to the receiver, the electronic smoke detection unit being designed to detect the absence of reception, by said receiver, of said portion of the light emitted by the emitter, and to emit a signal representative of this absence of reception. 
     Thanks to these provisions, a very small portion of the light emitted by the emitter arrives continuously at the receiver. When it is detected that the receiver is no longer emitting a signal representative of this portion or is emitting an attenuated signal, the electronic unit signals a detector fault or malfunction. The portion of the light that arrives continuously is calibrated to always be lower than the level of light required for detecting smoke so that this permanent portion does not disrupt the detection of smoke. 
     In some embodiments, the link resistant to the passage of the light is a split optical guide, a portion of the split optical guide emerging at the exterior of the optical detector in a way that is seal-tight to the atmosphere. 
     Thanks to these provisions, it is possible to:
         check the operation of the emitter component by positioning an external receiver component, for example in a movable casing that can be positioned opposite the place where said optical guide emerges;   especially in the case where the emitter component is likely to emit in the visible spectrum, communicate at least one item of information to the outside such as, for example, to signal a detection of smoke or a failure of the smoke detector circuit; and/or   communicate with the smoke detection circuit through the emission, for example via a remote control, of a light signal to the place where said optical guide emerges.       

     In some embodiments, the link resistant to the passage of the light is an optical guide forming a chicane, one optical guide comprising a zone absorbing light in the wavelengths of the light rays emitted by the emitter and/or one optical guide comprising a zone reflecting light in the wavelengths of the light rays emitted by the emitter. 
     Thanks to these provisions, the risks of parasitic lighting of the receiver via the optical fiber are reduced. 
     In some embodiments, the electronics compartment isolates all the electronic elements from the atmosphere by a seal-tight casing. 
     In some embodiments, for at least one light guide, there is at least one O-ring to connect said light guide to the electronics compartment at the point where said light guide enters the electronics compartment. 
     In this way, seal-tightness is ensured around the light guides when they enter the electronics compartment. 
     In some embodiments, the electronics compartment isolates all the electronic elements from the atmosphere by coating all the electronic elements it contains with an electrically insulating resin. 
     In some embodiments, seal-tightness is ensured by directly molding light guides in a resin for encapsulating the electronic elements and metal parts of the electronics compartment. 
     In some embodiments, the end of at least one of the light guides is buried in the resin. 
     In some embodiments, at least one light guide is surrounded by a sleeve whose refractive index is lower than the refractive index of said light guide. 
     In this way, a light ray that enters the light guide at a suitable angle undergoes multiple internal reflections. Thus, if the material forming the core of the light guide is not too absorbent for the light transmitted, the light that enters at one end of the light guide is almost entirely retrieved at the other end of the light guide. 
     In some embodiments, at least one light guide is surrounded by a light-reflecting layer. 
     In some embodiments, at least one emitter emits light rays with wavelengths in the infrared spectrum, and each light guide comprises at least one portion made of silica or polycarbonate. 
     Each light guide can be comprised of a material that absorbs little of the wavelengths transmitted. For example, in an embodiment the light emitter is arranged to emit light rays with wavelengths in the infrared spectrum, and each light guide contains at least silica. 
     In another embodiment, each light guide is made of a plastic material, easy to mold or inject. It is advantageous for this material to have two specific properties: good transmission in the infrared and limited retraction during demolding of molded parts. Polycarbonate is one of these materials. 
     Because of its sensitivity in the infrared, the receiver is not very sensitive to the ambient light, in the visible spectrum, which reduces the risks of a false alarm and the transmission of light rays is facilitated in each waveguide. 
     In some embodiments, said value of the physical-quantity in the detecting zone corresponds to the scattering of the light emitted by the emitter to the receiver. 
     In some embodiments, said value of the physical-quantity in the detecting zone corresponds to the reduction, by absorption, of the light directed from the emitter to the receiver. 
     In some embodiments, the detector comprises at least one reflector for reflecting the light originating from the first light guide, in the measurement chamber, to the second light guide. 
     In some embodiments, the first light reflector comprises a convergent reflector arranged to focus the light in the detecting zone, and/or the second light reflector comprises a convergent reflector arranged to focus the light originating from the detecting zone onto the receiver. 
     Thanks to these provisions, the detecting zone, where the light rays pass through any smoke to be detected, is smaller, which reduces the risks of parasitic reflection and the noise level. 
     In some embodiments, the detector also comprises:
         a casing for housing the measurement chamber and arranged to allow air to pass while minimizing the introduction of parasitic light into said measurement chamber; and   an intermediate mount placed in the casing, equipped with an optical wall arranged to prevent the light emitted by the light guide located on the emitter side from reaching the light guide located on the receiver side.       

     In some embodiments, at least one of the light guides has one end placed in the measurement chamber and comprising a cylindrical portion extending up to and including the traversal of the seal-tight casing. 
     In some embodiments, the end positioned in the measurement chamber has the shape of a lens enabling the light beam that comes from it to be shaped. 
     This lens makes it possible to shape the light beam that comes from the measurement chamber. 
     In some embodiments, the end positioned in the measurement chamber has the shape of an optical prism. 
     In some embodiments, the detector also comprises a reflector for the light waves emitted by a first light guide, this reflector being positioned to send said light beam back to a second light guide. 
     In some embodiments, the measurement chamber consists of the premises to be monitored, the optical detector being placed in the vicinity of a first extremity of the premises, the light beam emitted through the first light guide traversing said premises, the reflector being placed in the vicinity of the opposite extremity of said premises, and being positioned to send said light beam back to the second light guide. 
     According to a second aspect, the present invention envisages an alarm device comprising at least one optical detector that is the subject of the present invention, and an emitter of alarm signals, where the emission of alarm signals is representative of the detection by an optical detector of a value of the physical quantity representative of a danger. 
     As the features, advantages and aims of this alarm device are similar to those of the detector that is the subject of the present invention, they are not repeated here. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
       Other advantages, aims and characteristics of the present invention will become apparent from the description that will follow, made as an example that is in no way limiting, with reference to the drawings included in an appendix, in which: 
         FIG. 1  represents, schematically and in cross-section, an optical detector according to a first embodiment of the present invention; 
         FIG. 2  represents, schematically and in cross-section, an optical detector according to a second embodiment of the present invention; 
         FIG. 3  represents, schematically and in cross-section, an optical detector according to a third embodiment of the present invention; 
         FIG. 4  represents, schematically and in cross-section, an alarm system according to an embodiment of the invention; and 
         FIGS. 5 to 10  represent schematically, fully or partially, embodiments of an optical detector that is the subject of the present invention. 
     
    
    
     DESCRIPTION OF EXAMPLES OF REALIZATION OF THE INVENTION 
     For reasons of clarity, the figures are not to scale. 
     An optical detector of a value of an atmospheric physical quantity representative of a danger according to a first embodiment is represented schematically in  FIG. 1 . In this embodiment, the optical detector is configured to detect the presence of smoke in the atmosphere around the detector, the value of the physical quantity representative of a danger being the amount or level of smoke particles or aerosols, associated to a fire, in the atmosphere. 
       FIG. 1  shows an optical detector  100  according to the first embodiment comprising, in a casing  105 , a measurement chamber  110  accessible to the atmosphere, and an electronics compartment  120  for receiving an electronic smoke detection unit  130 . The electronic smoke detection unit  130  comprises a light emitter  131 , a light receiver  132  that is sensitive to at least one portion of the wavelengths of the light rays emitted by the emitter  131 , and electronics for processing detection signals  133 . 
     In a particular embodiment, the casing  105  has openings in chicanes to allow the atmosphere in the measurement chamber  110  to pass through a detecting zone D while minimizing the penetration of ambient light into the detecting zone D. The internal walls of the casing  105  can be arranged to reflect the light rays as little as possible. 
     The optical smoke detector  100  also comprises:
         a first light guide  141  facing the emitter  131  in order to direct the light emitted by the emitter  131  from the electronics compartment  120  to the detecting zone D in the measurement chamber  110 ; and   a second light guide  142  facing the receiver  132  in order to direct light originating from said detecting zone D to the electronics compartment  120  in order to be received by the receiver  132 .       

     In this embodiment, the amount of light received by the receiver  132  is representative of the presence/absence of smoke particles in the detecting zone D. 
     Each light guide  141  and  142  can consist of a material that absorbs little of the wavelengths transmitted. For example, in this embodiment, the light emitter  131  is arranged to emit light rays with wavelengths in the infrared spectrum, and each light guide  141  and  142  consists at least of silica. Other materials may be suitable, for example plastic materials, easy to mold or inject, such as polycarbonate. Such a material, such as many amorphous polymers, exhibits limited retraction during demolding of molded parts. 
     The light emitter component  131  is, for example, a light-emitting diode operating in the infrared spectrum. The light receiver component  132  is, for example, a photodiode or phototransistor operating in the infrared spectrum. 
     The electronics compartment  120  comprises a seal-tight casing  125  configured to isolate all the electronic elements of the electronic smoke detection unit  130  from the atmosphere. In some embodiments, seal-tightness can be ensured in other ways. For example, in another embodiment, this set of electronic elements is coated with a resin to isolate these electronic elements from the atmosphere. In an example of realization, the thickness of the resin on the component that extends farthest from the electronic circuit is at least 30 mm. 
     The first light guide  141  and the second light guide  142  are arranged to enter the electronics compartment  120  in a way that is seal-tight to the atmosphere. In this way, the exposure of electronic elements of the electronic smoke detection unit  130  to the atmosphere is avoided. Seal-tightness around the light guides when they enter the electronics compartment can be ensured in various ways. For example, in this first embodiment, the optical detector  100  comprises O-rings  151  and  152  to ensure seal-tightness between the first and second light guides,  141  and  142 , and the electronics compartment  120 , respectively where the first and second light guides,  141  and  142 , enter the seal-tight casing  125 . 
     In some embodiments, seal-tightness is ensured by directly molding light guides  141 ,  142  in a resin for encapsulating the electronic elements and, preferably, metal parts of the electronics compartment  120 . In a particular embodiment of the invention, one end of each light guide  141  and  142  is buried in the resin. 
     In the first embodiment shown in  FIG. 1 , the first light guide  141  and the second light guide  142  are each surrounded by a sleeve whose refractive index is lower than the refractive index of the light guide. In this way, a light ray that enters a light guide at a suitable angle undergoes multiple internal reflections. Thus, if the material forming the core of the light guide is not too absorbent for the light transmitted, the light that enters at one end of the light guide  141 ,  142  is almost entirely retrieved at the other end of the light guide  141 ,  142 . In some embodiments, at least one of the light guides  141 ,  142  is surrounded by a light-reflecting layer. 
     The electronic detection unit  130  comprises supply and signal processing components for, firstly, supplying electricity to the emitter  131  and the receiver  132  and, secondly, processing the electrical signals output from the light receiver component  132  to determine whether smoke is traversing the detecting zone D. These components and their connection are known to the person skilled in the art of smoke detectors and thus they are not described any further here. 
     The light guides  141  and  142  are oriented relative to each other in such a way that, if there is no smoke in the detecting zone D, the light emitted by the emitter  131  does not reach the receiver  132 . When smoke particles enter the measurement chamber  110  and reach the detecting zone D, light is directed by smoke particles to the light guide  142 , which directs it to the receiver  132 . When the electronics for processing detection signals  133  detects that the amount of light received by the receiver  132  exceeds a predefined limit value, an alarm signal is triggered in an alarm module to indicate the presence of smoke. 
     In a second embodiment of the invention shown in  FIG. 2 , the optical detector  200  also comprises a reflector  260  to direct light emitted by the emitter  131  to the receiver  132  if there is no smoke in the detecting zone D. When particles, for example smoke particles, enter a measurement chamber  210  and reach the detecting zone D, light is scattered or absorbed by these particles such that the amount of light directed by the first light guide  241  to the second light guide  242  is decreased. When the electronics for processing detection signals  133  detects that the amount of light received by the receiver  132  is below a predefined limit value, an alarm signal is triggered in an alarm module to indicate the presence of smoke. 
     An optical detector of an atmospheric physical quantity value representative of a danger according to a third embodiment is represented schematically in  FIG. 3 . In this embodiment, the optical detector  300  is configured to detect the presence of smoke in the atmosphere, the value of the physical quantity representative of a danger being the amount or level of smoke particles or aerosols, associated to a fire, in the atmosphere.  FIG. 3  shows an optical detector  300  comprising an electronics compartment  120 , similar to the electronics compartment  120  of the first embodiment, for receiving an electronic smoke detection unit  130 . The electronic smoke detection unit  130  comprises a light emitter  131 , a light receiver  132  that is sensitive to at least one portion of the wavelengths of the light rays emitted by the emitter  131 , and electronics for processing detection signals  133 . 
     In this third embodiment of the invention, the measurement chamber consists of a zone  310  of the premises to be monitored, in which the optical detector  300  is installed. A reflector  360  is installed in the premises to direct, by reflection, light emitted by the emitter  131  to the receiver  132  if there is no smoke in a detecting zone DD of the zone  310  of the premises. 
     When smoke particles enter the zone  310  of the premises and reach the detecting zone DD, light is scattered by the smoke particles such that the amount of light directed by the first light guide  341  to the second light guide  342  is decreased. When the electronics for processing detection signals  133  detects that the amount of light received by the receiver  132  is below a predefined limit value, an alarm signal is triggered in an alarm module to indicate the presence of smoke. 
     An alarm system  1055  according to an embodiment of the invention is shown in  FIG. 10 . The alarm system  1055  comprises an alarm device  1020  and optical detectors  1005  according to one of the embodiments of the invention. Each optical detector  1005  is connected to the alarm device by a link  1045 , wired or not, for transmitting detection signals to the alarm device. The alarm device  1020  comprises an emitter of alarm signals  1050 , where the emission of alarm signals is representative of the detection by at least one of the optical detectors  1005  of an atmospheric physical quantity value representative of a danger. The alarm device  1020  comprises, for example, a loudspeaker  1050 . 
     In a fourth embodiment shown in  FIG. 4 , the first light guide and the second light guide have light reflectors on the measurement zone side, formed here of the surfaces  465  of two prisms, respectively  461  and  462 . 
     The prism  461  facing the emitter  431  is arranged to direct the light LE emitted by the emitter  431  to a detecting zone D. The prism  462  facing the receiver  432  is arranged to direct, in the presence of smoke in the detecting zone D, the scattered light LR originating from said detecting zone D to the receiver  432 . The intersection of the light cone emitted from the first light guide  461  and the usable reception cone of the second light guide  462  defines the detecting zone D. 
     The light rays LE and LR, usable for detecting smoke, and the detecting zone D are shown by dashed lines in  FIG. 4 . Each of prisms  461  and  462  has a flat lower surface  463 , an oblique flat side surface  460  and a curved surface  465  forming a convergent mirror. 
     Prisms  461  and  462  are, for example, made of polycarbonate. This material has the advantage of being, at least partially, transparent in part of the infrared spectrum. Thus, the receiver is not sensitive to ambient light, which reduces the risks of a false alarm. The transmission of light rays is facilitated both in the prism on the light emitter side and in the prism on the light receiver side. In addition, this material exhibits limited retraction during demolding of molded parts. 
     As shown in  FIG. 4 , the shape of the curved surface  465  of prisms  461  and  462  and the angle of incidence of the light rays LE and LR on this curved surface  465  make it a convergent mirror whose focal length is substantially equal to the distance traveled by the central light rays emitted by the light emitter  431  before reaching the curved surface  465 , multiplied by the optical index of the material the prism is made of. In this way, the light rays output from the prism facing the light emitter component  431  are practically parallel. 
     For reasons of symmetry, the light rays from the detecting zone D converge, thanks to the curved surface  465  of the prism facing the light receiver component  432 , on the sensitive portion of this receiver  432 . 
     The electronic detection unit including the emitter and the receiver is housed in an electronics compartment seal-tight to the atmosphere. 
     The first light guide  441  and the second light guide  442  are arranged to enter the electronics compartment  420  that houses the electronic detection unit in a way that is seal-tight to the atmosphere to avoid the exposure of electronic elements  430  to the atmosphere. Seal-tightness around the light guides when they enter the electronics compartment can be ensured in various ways. For example, in this fourth embodiment, the optical detector  400  comprises O-rings  451  and  452  to respectively connect the first and second light guides,  441  and  442 , to the electronics compartment  420 , where the first and second light guides,  441  and  442 , enter the seal-tight casing  425 . 
     In some embodiments, seal-tightness is ensured by directly molding light guides  441 ,  442  in a resin for encapsulating the electronic elements and, preferably, metal parts of the electronics compartment. In a particular embodiment of the invention, one end of each light guide  441  and  442  is buried in the resin. 
     As shown in  FIG. 5  and  FIG. 6 , the prisms  461  and  462  form, in a particular embodiment, a single mechanical part  440 , with a link  445  connecting prisms  461  and  462  within this single part  440 . 
     In some particular embodiments, this link comprises an optical guide that carries a portion of the light emitted by the emitter  431  to the receiver  432 , the electronic smoke detection unit  430  being designed to detect the absence of reception, by said receiver  432 , of said portion of the light emitted by the emitter  431 , and to emit a signal representative of this absence of reception. Thus, a very small portion of the light emitted by the emitter  431  arrives continuously at the receiver  432 . When it is detected that the receiver  432  is no longer emitting a signal representative of this portion or is emitting an attenuated signal, the electronic detection unit  430  signals a fault or malfunction of the optical detector  400 . The portion of the light that arrives continuously is calibrated (by means of the geometry of the mechanical part) to always be less than the level of light required for detecting smoke, so that this permanent portion does not disrupt the detection of smoke, and to generate a signal greater than the thermal noise, at the output from the receiver  432 . 
     Measuring the amount of light arriving continuously at the receiver  432  thus allows the aging, or fault, of the emitter  431  and/or the receiver  432  to be measured. The aging or fault preferably causes a signal to be emitted, light, sound or to a central system, representative of the problem and of the need to carry out repair or maintenance operations on the smoke detector. 
     In some embodiments, the mechanical link  440  forms a split optical guide  480 , as shown in  FIG. 6 , to prevent the parasitic light from the emitter component  431  reaching, by means of it, the receiver component  432 . In some embodiments, the first light reflector formed by a surface  465  of prism  461  is connected to the second light reflector, formed by a surface  465  of prism  462 , by means of a link resistant to the passage of parasitic light, to form a single mechanical part  445 . For example, the material forming the single part is not, between the prisms, transparent to the wavelengths emitted by the light emitter. 
     In some embodiments, the link between prisms  461  and  462  is formed of any other means arranged so as to prevent parasitic light passing from the emitter to the receiver. For example, the link can be formed by an optical guide comprising a central zone in the form of a chicane, a zone absorbing light in the wavelengths of the light rays emitted by the emitter and/or a zone reflecting light in the wavelengths of the light rays emitted by the emitter. 
     The mechanical part  445  can, advantageously, be obtained by injecting polymer, eg polycarbonate, into a mold by positioning in the injection tool the hole through which the molten material enters the mold in the area corresponding to the central zone of the split optical guide  480 . This makes it possible to use the injection sprue, formed by the material having filled the feed channel between the nose of the injection cylinder and the mold inlet, to produce the optical guide and thus to save material and avoid an additional operation, i.e. extracting the sprue, when the injected parts are retrieved. These injection techniques are known to the person skilled in the art of working polymers and thus they are not described any further here. 
     As shown in  FIG. 7 , in this embodiment of the invention a smoke detector  400  comprises a casing  405  comprising two separate zones: the measurement chamber  410 , which contains the detecting zone D accessible to smoke particles, and the electronics compartment  420 , which houses the electronic smoke detection unit  430 . The casing  405  has openings in chicanes to allow air to pass through the detecting zone D while minimizing the penetration of ambient light into the detecting zone D. The internal walls of the casing are arranged to reflect the light rays as little as possible. As shown in  FIG. 8 , the surface of the seal-tight casing  425  of the electronics compartment  420  on the measurement chamber  410  side is equipped with an optical wall  820  which, if there is no smoke in the detecting zone D, prevents the light emitted by the emitter  431  to the detecting zone D via prism  461  from reaching the receiver  432  via prism  462 . The optical wall  820  has two opposite surfaces  821  that are crenelated to reflect the light rays as little as possible. 
     The prism  461  facing the emitter  431  is positioned on one of the sides of the wall  820  and the prism  462  facing the receiver  432  is placed on the other side of the wall  820  such that the light rays cannot circulate directly from one prism to the other. The prism  461  facing the emitter  431  focuses the light in the detecting zone D. When smoke particles are present in the detecting zone D, the light is scattered towards the prism  462  facing the receiver  432 , which collects it and sends it to the receiver  432 . 
     In the embodiment shown in  FIGS. 6 to 8 , the mechanical part  445  comprising the two prisms  461 ,  462  is mounted on a printed circuit  411  such that the configuration of prism  461  in relation to prism  462  is fixed. The mechanical part  445  is equipped with a device for precise positioning relative to an intermediate mount  810 , fixed to the surface of the casing  425 , namely centering studs (not shown) designed to cooperate with holes located on the intermediate mount  810 , holes  471 ,  472 ,  473  and  474  designed to cooperate with pins located on the intermediate mount  810 , or clips located on the intermediate mount  810 . The positioning of the prisms is thus easily reproducible. This makes it possible to reproduce the emission/reception angles of the light rays. 
     Two openings  825 ,  826  in the surface of the mount  810  allow the two prisms  461 ,  462  to be positioned in the measurement chamber, the split optical guide  480  between the two prisms being positioned on the other side of the mount. The mount  810  is arranged to prevent the parasitic light passing to the scattering zone, except through the prisms  461 ,  462 . 
     On the surface located on the printed circuit side, it can be advantageous to equip prisms  461  and  462  with two flat undercuts so as to allow the emitter  431  and receiver  432  to be positioned inside these undercuts, so that the prisms can come into contact with the printed circuit. 
     In a variant, the prisms and/or the mechanical part come into contact with the printed circuit by avoiding the above undercuts, through the provision of stops  443 ,  444 ,  438  and  439 . 
     In the embodiment shown in  FIG. 9 , the printed circuit  411  is fixed to the intermediate mount  810  by clips located, for example, on the periphery of this intermediate mount  810 . In this way the optical prisms are precisely positioned with regard to the intermediate mount  810 , which is itself precisely positioned with regard to the printed circuit  411 . It is thus easy to reproduce the positioning of the prisms over a series of printed circuits. 
     In the embodiment shown in  FIG. 5 , the link  445  takes the form of an optical fiber that emerges on the outside of the electronic smoke detection circuit. 
     In this way, it is possible to:
         check the operation of the emitter component by positioning an external receiver component, for example in a movable casing  1025  opposite the place where the optical fiber  1045  emerges;   especially in the case where the emitter component is likely to emit in the visible spectrum, communicate at least one item of information to the outside such as, for example, signal a detection of smoke or a failure of the smoke detector circuit visually or by means of a movable casing  1025 ; and/or   communicate with the smoke detection circuit by emitting, for example with a remote control  1025  shown in  FIG. 10 , a light signal to the place where the optical fiber emerges.