EP0588232B1 - Optic smoke detector - Google Patents

Abstract

An improved optical smoke detector is obtained if at least one planar-optical element (5) is arranged in the beam path between radiation source (1) and radiation receiver (2). This element can be either a diffractive element, preferably a holographic-optical element (HOE) or a micro-Fresnel reflector (MFR). <IMAGE>

Description

The invention relates to an optical smoke detector according to the preamble of claim 1. Smoke detectors of this type are generally known. They are used in particular as automatic fire detectors for the early detection of fires.

Smoke detectors occupy a special position among the multitude of types of automatic fire detectors on the market, since they are best suited to detect fires at such an early stage that countermeasures can still be successfully initiated.

There are two main types of smoke detectors: ionization smoke detectors and optical smoke detectors. In ionization smoke detectors, the accumulation of air ions on smoke particles is used; The second type of smoke detector uses the optical properties of aerosols to detect smoke. Here, one uses either the weakening of a light beam by smoke ("extinction detector") or the scattering of light on smoke particles ("scattered light detector"). Since the extinction by smoke is relatively low, the measuring section must be quite long in order to enable reliable detection of smoke; or complex constructive and / or electronic measures must be taken to enable safe detection of damage fires. The last-mentioned scattered light detectors are therefore the most widespread, since the measuring path can be so short that they can be designed as so-called "point detectors".

DE-A-28 22 547 (Hochiki; 07.12.78) describes a line extinction detector in which a transmitter emits light. Part of the emitted light falls on a radiation receiver after it has passed a measuring section. In the presence of smoke in the measuring section, the output signal of the radiation receiver is reduced depending on the smoke density and the output signal is fed to a threshold and comparison circuit, an alarm signal being generated in a downstream evaluation circuit if the output signal falls below a predetermined value, the alarm threshold . Lenses are arranged both in front of the radiation source and in front of the radiation receiver in order to bundle the light beam which traverses the measurement path. The bundling systems are structurally very complex.

Most older optical smoke detectors, which work according to the light scattering principle, use forward scattering. Large smoke aerosol particles cause a strong effect, while small smoke particles cause little scattered light. Smoke detectors, which take advantage of the backward scatter, have a more uniform sensitivity, what enables a more universal use. The weaker scattered light intensity, however, requires more electronic effort. In addition, the risk of stray light being reflected from the walls of the housing onto the receiver is particularly great, so that a complicated optical labyrinth (for example a large number of diaphragms, as in EP-A-0 031 096) is required to cause reflections keep away from the radiation receiver inside the detector.

The diaphragms in the measuring chamber of the smoke detector according to EP-A-0 031 096 also serve in combination with optical converging lenses in front of the light source and the receiver to direct the light beam directed onto the measuring chamber or the radiation scattered from the measuring chamber in order to focus the Reduce the overall length of the smoke detector.

The reduction in the size of smoke detectors is not only desirable for aesthetic reasons, but is also sought for reasons of simplified mass production of the smoke detectors. For example, a smoke detector has been proposed in DE-A-37 43 737 (Hochiki; 07.07.88), in which a special design has made it possible to reduce the detector dimensions. The economic mass production is complicated, for example, by the fact that a printed circuit board has to be equipped with a wired photodiode in a separate operation, possibly by hand.

However, it should not be forgotten that disturbing stray radiation (which may be caused, for example, by contamination) from the measuring chamber can strike the photosensitive receiver. In DE-A-38 31 654 (Beyersdorf; 22.03.90) it was proposed to measure the contamination of the measuring chamber by a second photodiode and, if necessary, to prevent an alarm if the contamination exceeds a predetermined value.

In order to reduce the dimensions and the number of components, a stray radiation smoke detector was proposed in GB-A-2 236 390 (Matsushita; April 3, 1991), which uses a wired IRED as a radiation source and as a receiver on an integrated circuit placed on a printed circuit board Has a photodiode lying flat on the print; a prism with an integrated lens serves as a deflection and focusing element for concentrating the scattered radiation from the measurement volume onto the photodiode. This prism with its integrated lens is relatively expensive; in addition, the exact placement of the lens required is quite complicated.

In the unpublished CH-A-682 428 and in EP-A-0 462 642 (Ajax de Boer; December 27, 1991) scattered-light smoke detectors are described in which the polarization of the scattered light to detect the smoke concentration, the particle size and partly the Shape of the particles is exploited. This allows the smoke detector to respond more evenly different types of fire can be achieved. The patent documents give no indication of a more compact design or a simplified construction of optical smoke detectors.

Based on this prior art, the invention has for its object to provide an optical smoke detector that does not have the disadvantages of the known optical smoke detectors and, in particular, to create an optical smoke detector that can be used for a compact design and a reduced number of components inexpensive mass production.

Another object of the invention is to improve the manufacturing technology in such a way, in particular to reduce the manufacturing tolerances to such an extent that the adjustment work, which is still required manually in some cases, is eliminated or at least reduced to a minimum.

This object is achieved in an optical smoke detector of the type mentioned by the characterizing features of claim 1. Preferred embodiments of the invention and refinements are defined in the dependent claims.

A particular advantage of a preferred embodiment of the smoke detector according to the scattered radiation principle is the expansion of the degrees of freedom of the optical smoke detectors of the prior art by the planar-optical elements (POE), such as holographic-optical elements (HOE), microfresh elements (MFE), such as e.g. Microfresh reflectors (MFR) and phase-matched microfresh reflectors (PMFR) and in the improvement of the detection of different fires given by the evaluation of the polarization of the scattered radiation.

The fact that, in the preferred embodiment of the optical smoke detector according to the invention based on the scattered radiation principle, the radiation emanating from the radiation source and the radiation scattered from the smoke particles to the radiation receiver are guided practically parallel to the ceiling of the room enables a very flat construction of the detector. This radiation guidance is achieved through the use of planar-optical elements (POE) as focusing optical deflection elements [holographic-optical elements (HOE), microsfresnel elements (MFE), e.g. Microfresh Reflectors (MFR) and Phase Adjusted Microfresh Reflectors (PMFR)].

In a further preferred embodiment of the smoke detector according to the invention based on the extinction principle, the use of planar-optical elements (POE) as focussing optical elements [holographic-optical elements (HOE), microsfresnel elements (MFE), such as, for example, microfresh reflectors (MFR) and phase-matched microfresh reflectors (PMFR)] a simplified design of the smoke detector, which enables cheap mass production.

Microfresh elements (MFE) are diffractive Fresnel lens structures in microscopic dimensions, as they are mentioned as transmissive elements in US-A-4,936,666 (3M-Company; June 22, 90). The production of such microfresh lenses for transmission and reflection in an on-axis configuration is described, for example, by T. Shiono et al. in Optics Letters, Vol. 15, No. 1, 84 (01/01/90). The phase-adapted microfresh reflectors (PMFR) that can be used according to the invention are a planar arrangement of inclined and curved microfaces, which consist of sections of ellipsoids. They are used as surface mirrors and are therefore covered with a reflective layer. The micro surfaces are phase-matched, that is, the optical path from one focal point to the other over each of the micro surfaces always differs by an integral multiple of the light wavelength.

The design of the optical smoke detectors with the microfresh elements has the advantage over that with the holographic-optical elements that the smoke detectors are less sensitive to chromatic aberration and are better suited for mass production. The phase-matched microfresh elements (MFE) and the holographic-optical elements (HOE) are flat optical elements that can be automatically equipped and precisely placed. Both are simply constructed and can therefore be manufactured very inexpensively.

Another advantage of the optical smoke detector according to the invention is that a photodiode and control electronics of an infrared light-emitting diode (IRED) can be integrated into an integrated circuit (IC) of the receiving electronics. Only a few switching elements remain, e.g. the charging capacitor, voltage stabilization and protective elements for the communication lines that cannot be integrated into an IC. This significantly reduces the number and space requirements of electronic components.

With the integration of the photodiode and the receiving electronics in an IC, the connecting wires, which otherwise act as antennas, between the photodiode and the first stage for current / voltage conversion become very short. This makes the optical smoke detector significantly less sensitive to interference, which makes it possible to achieve detection reliability equivalent to the previous optical smoke detectors with a smaller, cheaper photodiode area and thus a lower signal level.

The microfresh elements (MFE) and the holographic-optical elements (HOE) allow a larger optical aperture than conventional lenses. It is thus possible to collect more scattered radiation and raise the signals to a higher detection level with the advantage of better immunity to electrical interference.

Furthermore, the microfresh elements (MFE) allow a design with two (or more) focal points. A stray light detector of this type maps the stray volume onto two (or more) separate radiation receivers, which can be covered with crossed polarizers. In the absence of smoke, both photodiodes receive radiation from an identical background (assuming that radiation from the background only falls on the photodiodes after several reflections on the labyrinth and thus unpolarized). The so-called basic pulses for each of the two photodiodes therefore remain the same even when the scattered light detector becomes increasingly dirty. The scattered light detector can thus be easily expanded into a detector using polarization filters without further optical elements.

Figur 1

einen Vertikalschnitt durch einen erfindungsgemäßen Rauchmelder mit zwei planaren optischen Elementen (POE),

Figur 2

einen Horizontalschnitt durch eine andere Ausführungsform eines erfindungsgemäßen Rauchmelders mit einer Strahlungsquelle ohne optisches Element und einer Photodiode mit einem planaren optischen Element als Umlenkelement darüber,

Figur 3

einen Vertikalschnitt durch den Rauchmelder gemäss Figur 2 entlang der Linie A - B (Photodiodenkompartment und Meßkammer),

Figur 4

eine weitere Ausführungsform eines Streulichtrauchmelders gemäß Figur 1 mit Strahlungsquelle auf dem Print und planarem optischen Element über dem Strahlungsempfänger,

Figur 5

eine weitere Ausführungsform eines Streulichtrauchmelders gemäß Figur 4 mit zusätzlichem ebenem oder gekrümmtem Spiegel,

Figur 6

die Draufsicht auf die Struktur eines phasenangepaßten Mikrofresnelreflektors (PMFR),

Figur 7

einen Querschnitt durch einen phasenangepaßten Mikrofresnelreflektor (PMFR) gemäß Figur 6, bei dem die Mikrostruktur auf der Vorderseite des Substrates liegt,

Figur 8

einen Querschnitt durch einen phasenangepaßten Mikrofresnelreflektor (PMFR) gemäß Figur 6, bei dem die Mikrostruktur auf der Rückseite des Substrates liegt,

Figur 9

eine weitere Ausführungsform eines optischen Rauchmelders mit einem planaren optischen Element, welches die Strahlung auf zwei Strahlungsempfänger konzentriert, und mit Polarisatoren unterschiedlicher Polarisationsebene in jedem der Strahlengänge,

Figur 10

eine weitere Ausführungsform eines optischen Rauchmelders mit einem planaren optischen Element, dem ein Gitter überlagert ist, welches die Strahlung auf mehrere Strahlungsempfänger konzentriert, und mit Polarisatoren unterschiedlicher Polarisationsebene in jedem der Strahlengänge,

Figur 11

die Draufsicht auf einen phasenangepaßten Mikrofresnelreflektor (PMFR) mit aufgeprägtem linearem Gitter,

Figur 12

einen Querschnitt durch einen Extinktionsrauchmelder mit transmissiven planar-optischen Elementen,

Figur 13

einen Querschnitt durch einen Extinktionsrauchmelder mit reflektiven planar-optischen Elementen; und

Figur 14

einen Vertikalschnitt durch einen Streulichtrauchmelder mit Ellipsoidspiegel.

The invention is explained in more detail below with reference to the preferred embodiments shown in the drawings. Show it

Figure 1

a vertical section through a smoke detector according to the invention with two planar optical elements (POE),

Figure 2

3 shows a horizontal section through another embodiment of a smoke detector according to the invention with a radiation source without an optical element and a photodiode with a planar optical element as a deflecting element above it,

Figure 3

3 shows a vertical section through the smoke detector according to FIG. 2 along the line A - B (photodiode compartment and measuring chamber),

Figure 4

1 shows another embodiment of a scattered-light smoke detector according to FIG. 1 with a radiation source on the print and a planar optical element above the radiation receiver,

Figure 5

4 a further embodiment of a scattered light smoke detector according to FIG. 4 with an additional flat or curved mirror,

Figure 6

the top view of the structure of a phase-adjusted microfresh reflector (PMFR),

Figure 7

6 shows a cross section through a phase-adapted microfresh reflector (PMFR) according to FIG. 6, in which the microstructure lies on the front of the substrate,

Figure 8

6 shows a cross section through a phase-adapted microfresh reflector (PMFR) according to FIG. 6, in which the microstructure lies on the back of the substrate,

Figure 9

a further embodiment of an optical smoke detector with a planar optical element which concentrates the radiation on two radiation receivers and with polarizers of different polarization levels in each of the beam paths,

Figure 10

another embodiment of an optical smoke detector with a planar optical element, on which a grating is superimposed, which concentrates the radiation onto several radiation receivers, and with polarizers of different polarization levels in each of the beam paths,

Figure 11

the top view of a phase-adapted microfresh reflector (PMFR) with an embossed linear grating,

Figure 12

2 shows a cross section through an extinction smoke detector with transmissive planar-optical elements,

Figure 13

a cross section through an extinction smoke detector with reflective planar-optical elements; and

Figure 14

a vertical section through a scattered light smoke detector with an ellipsoidal mirror.

FIG. 1 shows an optical smoke detector according to the invention, namely a scattered-light smoke detector with two planar optical elements (POE). There are an infrared light emitting diode 1 in surface mounting technology (SMD-IRED) and a photodiode 2 in surface mounting technology (SMD photodiode) mounted on a printed circuit board 9. A planar-optical element (POE) 5 is arranged above the radiation source (SMD-IRED) 1 or above the radiation receiver (SMD photodiode) 2 in order to deflect the radiation emitted or scattered on aerosol particles. This requires two deflecting and focusing optical elements, for example two holographic optical elements (HOE) 5 or two microsfresnel elements (MFE) 5.

With the currently available holographic-optical elements (HOE) and microsfresnel elements (MFE), certain difficulties can arise when implementing a scattered light smoke detector according to the invention which is equivalent to conventional scattered light detectors, since diffraction-optical elements can only be produced with an efficiency that is well below 100% . As a result, the surface of the diffraction-optical element acts as a diffuse scattered light source, as a result of which a considerable part of the radiation emitted by the radiation source 1 floods the measurement volume 8 as diffuse radiation. This scattered radiation can be a multiple of the light that is scattered on fire aerosol particles. A reduction of the interference radiation requires much more complex mechanical diaphragms than were previously common.

FIGS. 2 and 3 show an embodiment of a scattered light smoke detector which is improved compared to the scattered light detector according to FIG. 1 and which has a wired diode 1 which emits infrared light without an optical element, a photodiode 2 on the printed circuit board 9 and a holographic optical element (HOE) 5 or has a phase-adapted microfresh reflector (PMFR) 5 as a deflecting element. The photodiode serving as the radiation receiver 2 is located in a blackened compartment 16, which is only connected to the interior of the detector by an aperture 4. As a result, the interference radiation emanating from the surface of the planar-optical element (HOE or PMFR) as diffuse scattered radiation can largely be eliminated.

According to a particularly preferred embodiment (see FIG. 3), the aperture 4 is covered with a radiation-permeable film or a polarization filter in order to keep any dust that may enter the detector from the radiation receiver. A scattering angle of 70 to 110 ° is often used for scattered-light smoke detectors. In such detectors, the use of a polarization filter with an oscillation plane which is perpendicular to the scattering plane causes an adjustment of the sensitivities of the detectors for the detection of open fires which produce aerosols with small particles and of detectors for the detection of smoldering fires which Generate aerosols (smoke) with large particles.

According to a further preferred embodiment of the scattered light detector described above, two different colored light sources (e.g. red and infrared) lying close together are used. In this case, two radiation receivers (photodiodes) are used, which are attached at the points at which the radiation is focused by the phase-matched microfresh reflector (PMFR). Due to the achromasia of the phase-adjusted microfresh reflectors (PMFR), no chromatic aberrations are to be expected as a result of the relatively broad spectral distribution of IRED and LED radiation.

FIG. 4 shows a further embodiment of the scattered light smoke detector; however, there is no planar-optical element (POE) above the radiation source 1. The radiation source 1, an infrared radiation emitting diode (IRED), is mounted on the printed circuit board 9. The radiation beam 6 of the radiation source 1 is kept narrow by diaphragms 4, and the radiation which is not scattered on smoke particles 12 in the direction of the planar-optical element 5 attached above the radiation receiver 2 disappears in the light sump (labyrinth) 3.

FIG. 5 shows a further embodiment of the scattered light detector according to FIG. 4, in which a flat or curved mirror 13 is mounted above the radiation source 1, through which the light from the radiation bundle 6, which is not scattered by smoke particles 12 in the direction of the radiation receiver 2, moves sideways is deflected into a labyrinth 3 and absorbed there. This makes it possible to mount the labyrinth 3 in a place where it takes up more space and can therefore be made more effective.

FIG. 6 shows the structure of a phase-adapted microfresh reflector (PMFR), as can be used in a scattered light smoke detector, seen from above. Figures 7 and 8 show sections through the phase-matched microfresh reflector (PMFR). The PMFR are called "phase-adjusted" because the optical path [li + l'i], or [(li + k) + (l'i + k)] from the radiation source 1 to the radiation receiver over each of the ellipsoid micro-surfaces is always different differs by a multiple of the light wavelength.

The structure can be on the front or on the back of the substrate. The latter version is the least dust and corrosion sensitive, since the mirrored structure can be provided with a protective lacquer. The phase-adjusted microfresh reflector (PMFR) can be manufactured in such a way that the structure is written in photoresist using a laser writing system. A nickel embossing stamp is made and reproduced. By embossing in plastic substrates such as polymethyl methacrylate (PMMA), polyvinyl chloride (PVC) or polycarbonate (PC), the phase-adjusted microfresh reflectors (PMFR) can now be produced inexpensively in large quantities.

The phase-adjusted microfresh reflectors (PMFR) are optimized for a wavelength of 880 nm (infrared) and have a profile depth of up to approx. 3 µm that varies over the active area of 17 x 12 mm ² , for example (FIGS. 7 and 8). The phase-adjusted microfresh reflectors are located on the transition zone between diffractive and purely reflective or refractive elements. Reflection or transmission takes place on the micro-surfaces and diffraction appears at the transition edges between the micro-surfaces with superimposed superposition of the refracted light component in the second focal point. As already mentioned, the phase-matched microfresh reflectors (PMFR) also have the advantage that they are less sensitive to chromatic aberration than the holographic optical elements (HOE).

FIG. 9 shows a further preferred embodiment of a scattered light detector according to the invention. This scattered light detector has a planar-optical element (POE), which has a structure consisting of (concentric) areas A, B, .., which is arranged and designed such that the radiation emitted by the radiation source 1 is directed onto two different radiation receivers 21 , 22 falls. For example, the radiation is deflected by the concentric zones A onto the photodiode 21 and through the zones B onto the photodiode 22; the area ratio of the sum of zones A and the sum of zones B can be chosen freely.

Polarization filters 14, 15, preferably those with mutually perpendicular polarization planes, can be arranged above the two radiation receivers 21, 22, which makes it possible to detect the scattered radiation after its polarization; this enables the advantages described above with regard to the adjustment of the sensitivity of the detectors for the detection of open fires and smoldering fires to be achieved. With the previously known optics, two elements would be required for this, which would also represent two different areas (with different background radiation) of the measurement volume. In contrast, the planar-optical element (POE) described here forms one and the same area from the measurement volume. By using two radiation sources, four focal points can be obtained, which makes it possible to analyze the scattered radiation according to color and polarization.

The scattered radiation deflected by the planar-optical element (POE) can be divided into a plurality of radiation receivers, for example, with a planar-optical element, as shown in FIG. 11. The deflection of the scattered radiation takes place here by means of a phase-adapted microfresnel reflector (PMFR), as shown in FIG. 6, and the distribution of the scattered radiation among the different radiation receivers is carried out by diffraction on a phase-matched microfresnel reflector (PMFR), the grating structure being the grating structure is adapted to the main wavelength of the radiation source.

Depending on the structure of the superimposed grating, one, two or more diffraction orders (= focal points) can be obtained. The energy distribution within the different diffraction orders can also be selected by a suitable choice of the lattice structure, e.g. a sine grating has the diffraction orders -1, 0, +1, whereby the energy in the orders -1 and / or +1 can be made large by suitable selection of the structure depth or by suitable "blazing". In contrast, a rectangular grid has many orders. Going further still, a lattice structure of suitable shape can always be found for a freely selectable number of focal points and a freely selectable energy distribution in the focal points.

FIG. 10 shows an embodiment of an optical smoke detector according to the invention, in which a planar-optical element (POE) 5 in the manner of a deflecting mirror is used. The planar-optical element (POE) is shown in FIG. The deflection of the scattered radiation takes place here through the elliptically arranged, phase-adapted micro surfaces, which alternately belong to ellipsoids with different focal points, and the distribution of the scattered radiation among the different radiation receivers 21, 22, 23, 24, 25 takes place by diffraction at a phase-adapted microfresh reflector ( PMFR) superimposed, linear grating, the grating structure is adapted to the main wavelength of the radiation source.

The radiation source 1 consists of a radiation in the near infrared emitting diode (IRED) and a red light emitting diode (LED), which are arranged in a common housing. The structure of the linear grating of the mirror 5 is selected so that the radiation is deflected to five different focal points, in which radiation receivers 21, 22, 23, 24, 25 are located. According to a preferred embodiment, polarization filters 14 with parallel polarization planes are arranged in front of two of the radiation receivers 21, 22, while polarization filters 15, whose polarization planes are perpendicular to the polarization planes of the first two polarization filters 14, are arranged in front of two other radiation receivers 24, 25. There is none in front of one of the radiation receivers 23 Polarization filter, so that this radiation receiver 23 receives light of all wavelengths and all levels of polarization.

The following radiation can then strike the radiation receiver, for example: First radiation receiver 21: infrared light, polarized perpendicularly (to the scattering plane); second radiation receiver 22: red light, vertically polarized; third radiation receiver 23: infrared light and red light, not polarized; fourth radiation receiver 24: red light polarized in parallel; fifth radiation receiver 25: infrared light, polarized in parallel. With an appropriate design of the evaluation circuit, this makes it possible to determine whether the radiation scattered in the measurement volume 8 originates from large or small smoke particles. This also enables the smoke detectors to respond more evenly to different fires (open fires - small smoke particles or smoldering fires - large smoke particles).

Figure 12 shows a cross section through a smoke detector based on the extinction principle. A planar-optical element (POE) 5 is arranged in front of a radiation source 1, by means of which the radiation from the radiation source 1 is combined to form an approximately parallel radiation beam 6. A second planar-optical element 23 is arranged in front of a radiation receiver 2, by means of which the radiation which has passed through the measurement volume 8 is focused on the radiation receiver 2. Instead of the transmissive planar-optical elements 5, 23, reflective, planar-optical elements can also be used, which are arranged at an angle of, for example, 45 ° to the radiation in the measurement volume 8 (cf. FIG. 13).

FIG. 14 shows a further embodiment of a scattered light smoke detector which has a wired diode 1 which emits infrared light and has no optical element, a photodiode 2 on the printed circuit board 9 and an ellipsoid mirror 24 as a deflecting element. The photodiode serving as the radiation receiver 2 is located in a blackened compartment 16, which is only connected to the interior of the detector by an aperture 4.

Claims (18)

1. Optical smoke alarm with at least one radiation source (1), at least one radiation detector (2) and a measurement volume (8) located between radiation source (1) and radiation detector (2) and open to the outer atmosphere, wherein the radiation detector (2) detects the changes in radiation which are caused by smoke particles (12) present in the measurement volume (8) and emits an electric output signal as a function of the incident radiation, and with an electronic evaluation circuit which evaluates the electric signal emitted by the radiation detector (2) and emits an alarm signal when the output signal assumes a predetermined signature, characterised in that at least one planar-optical element (5) is arranged in the optical path between radiation source (1) and radiation detector (2).
2. Optical smoke alarm according to claim 1 for smoke detection according to the stray radiation principle, with a measurement chamber containing the measurement volume (8) and with a labyrinth (3) for the absorption of interfering radiation which has penetrated the measurement chamber or has formed in the latter, characterised in that the at least one planar-optical element (5) is arranged inside the measurement chamber and is preferably formed by a holographic-optical element or by a microfresnel reflector.
3. Optical smoke alarm according to claim 2, characterised in that the at least one planar-optical element (5) is formed by a reflective element provided with a micro-area structure on its front side.
4. Optical smoke alarm according to claim 2, characterised in that the at least one planar-optical element (5) is formed by a reflective element provided with a micro-area structure on its rear side.
5. Optical smoke alarm according to claim 2, in which the at least one planar-optical element (5) is formed by a microfresnel reflector, characterised in that its surface structure is so arranged and adjusted that the path difference across the various micro-areas of rays which are reflected by the microfresnel reflector comes to a whole multiple of the central light wavelength lambda, preferably the wavelength of 880 µm.
6. Optical smoke alarm according to one of claims 2 to 5, characterised in that the radiation source (1) and the radiation detector (2) are arranged on a print plate (9), and that a single planar-optical element (5) assigned to the radiation source or to the radiation detector is provided.
7. Optical smoke alarm according to one of claims 2 to 5, characterised in that the radiation source (1) and the radiation detector (2) are arranged on a print plate (9), and that two planar-optical elements (5) are provided, one of which is assigned to the radiation source and the other to the radiation detector.
8. Optical smoke alarm according to claim 6 or 7, characterised in that a plane or curved mirror (13) is arranged above the radiation source (1) in such a way that the light not scattered on smoke particles (12) is directed into the labyrinth (3).
9. Optical smoke alarm according to one of claims 6 to 8, characterised in that the radiation source (1) is formed by a diode emitting an infrared radiation or a laser diode.
10. Optical smoke alarm according to one of claims 2 to 9, characterised in that there is arranged in front of the radiation detector (2) a polarizing filter (15), preferably one with a plane of polarization perpendicular to the scatter plane defined by the direction of propagation of the radiation from the radiation source (1) and the radiation detector.
11. Optical smoke alarm according to claim 2, characterised in that two differently coloured radiation sources (1) and at least two radiation detectors (2) separate in space are provided.
12. Optical smoke alarm according to one of claims 2 to 11, characterised in that two radiation detectors (21, 22) separate from one another in space and polarizing filters (14, 15) arranged in front of the latter are provided with different planes of polarization preferably perpendicular to one another, and that the at least one planar-optical element (5) comprises on its surface deflecting the incident radiation two regions (A, B) which comprise focal points varying from place to place and are so arranged and adjusted that the stray light falling upon the planar-optical element out of the measurement volume (8) falls upon the two radiation detectors (21, 22).
13. Optical smoke alarm according to one of claims 2 to 11, characterised in that two differently coloured radiation sources (1) separate from one another in space and at least five radiation detectors (21, 22, 23, 24, 25) separate from one another in space are provided, that the at least one planar-optical element (5) comprises on its surface deflecting the incident radiation two regions (A, B) with focal points varying from place to place and bears a superimposed structure, preferably a lattice structure, wherein the focal points are so arranged and adjusted that the stray light falling upon the planar-optical element out of the test volume (8) is focussed onto the five radiation detectors by diffraction, and that polarizing filters (14, 15) with different planes of polarization preferably perpendicular to one another in pairs are arranged in front of four of the radiation detectors.
14. Optical smoke alarm according to one of claims 2 to 12, characterised in that the at least one planar-optical element (5) is laminated onto the labyrinth (3).
15. Optical smoke alarm according to one of claims 2 to 12, characterised in that the at least one planar-optical element is applied directly to the labyrinth (3) by the injection moulding method.
16. Optical smoke alarm according to claim 2, characterised in that the planar-optical element is formed by a shallow deviation mirror (24).
17. Optical smoke alarm according to claim 16, characterised in that the shallow deviation mirror (24) is formed [by] a dip in a plastics casing.
18. Optical smoke alarm according to claim 1, for smoke detection according to the extinction principle, with an open measurement volume (8) and with radiation source (1) arranged at the side of the latter and radiation detector (2), characterised in that the at least one planar-optical element (5, 23) is arranged in the region of the radiation source and/or of the radiation detector and is preferably formed by a holographic-optical element or by a micro-fresnel reflector.

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