EP1408469B1 - Fire detection method and fire detector for its implementation - Google Patents

Abstract

Detecting fire according to scattered light principle includes emitting pulsed radiation of first and second wavelengths respectively along first and second radiation axes into a measuring volume, and separately measuring radiation scattered on particles in the measuring volume under a forward scattering angle of more than 90[deg] and a backward scattering angle of less than 90[deg]. Detecting fire according to scattered light principle includes emitting pulsed radiation of first wavelength along a first radiation axis into a measuring volume, emitting pulsed radiation of second wavelength shorter than the first wavelength along a second radiation axis into the measuring volume, and measuring radiation scattered on particles located in the measuring volume under a forward scattering angle of more than 90[deg] and a backward scattering angle of less than 90[deg]. The forward and backward scattered radiation of first and second wavelengths are measured separately from each other. An independent claim is also included for a scattered-light fire detector comprising a measuring chamber which communicates with the ambient air and delimits a measuring volume, a first light emitting diode (LED) (1.1a) that emits infrared radiation into the measuring volume, a second LED (1.1b) that emits blue light into the measuring volume from a different direction than the first LED, first and second photodetectors (1.2a, 1.2b) situated opposite of each other on a common main axis with respect to each other to measure the radiation scattered by particles situated in the measuring volume. Radiation axes of the first and second LEDs enclose an acute angle of less than 90[deg] with the main axis and intersect in a point situated on the main axis and in the center of the measuring volume.

Description

The invention relates to a method for detecting fire according to the scattered-light principle by pulsed irradiation of a radiation of a first wavelength along a first radiation axis and a radiation of a second, shorter wavelength along a second radiation axis into a measurement volume and measurement of the particles in the measurement volume scattered radiation below a forward spread angle of more than 90 ° and a backward spread angle of less than 90 °. In the method, the forward and backward scattered radiation of the first and second wavelengths are measured and evaluated separately.

The invention further relates to a scattered light fire detector for carrying out the method.

From the WO 00/07161 is a scattered light smoke detector, which operates according to the aforementioned method known. To be able to make a statement about the type of smoke, the difference of the wavelengths of the incident light is as large as possible. Forward and backward scattering of each wavelength are detected by separate photosensors.

From the WO 01/59737 is known in particular for installation in ventilation and air conditioning ducts scattered light detector, which operates according to the aforementioned method and irradiate in the measuring chamber, a first LED infrared light and a second LED blue light. The LEDs are pulsed alternately. The radiation generated by the "infrared" LED allows the detection of large particles typical of a smoldering fire. The scattered radiation generated by the "blue" LED allows detection of small particles typical of open flame fires. This is explained by Rayleigh's law that the intensity of the scattered light for particles smaller than the wavelength decreases with the fourth power of the wavelength. The latter is indeed correct, but does not do justice to the actual conditions in the fire detection according to the scattered light principle. The known fire detector comprises only a photoreceiver, which provides only two information about the scattered radiation intensities, depending on the embodiment, either the intensity of the forward scattered radiation in the infrared and blue wavelengths or the corresponding intensities of the backscattered radiation or the intensity of the forward scattered radiation in the infrared wavelength range and the backscattered radiation in the blue wavelength range. However, the respective arrangement geometries lead to the fact that the measurement volumes from which the respective scattered radiation originates are not identical.

From the DE 199 02 319 For example, a fire detection method is known in which the alarm decision is made depending on the ratio of the intensity of the IR forward scattered radiation to the intensity of the IR backscattered radiation. The corresponding fire detector works either with two infrared LEDs and a photoreceiver or vice versa with an infrared LED and two photoreceivers. The angle at which the forward scattered radiation is measured is preferably 140 ° and the angle at which the backscattered radiation is measured is preferably 70 °. The formation of the ratio of the intensities of the forward and the backward scattering radiation makes it possible to distinguish lighter from dark smokes because bright smoke provides a high forward scatter signal and a comparatively small backward scatter signal, while, conversely, dark smoke provides a lower forward scatter signal but a relatively higher reverse scatter signal. The processing of the absolute intensities or signal levels taking into account the fundamentally lower intensities in the backscatter area in relation to the intensities and the concurrent intensities generated by the same particles in the same concentration in the forward scatter area Processing the ratios or quotients of these signal levels also makes it possible to distinguish certain types of deception from smoke. For example, high concentration steam produces a high forward scatter signal, which in the prior art leads to the triggering of an alarm, but in this case a false alarm. However, the formation of the quotient of the forward scattering intensity and the backward scattering intensity gives a value characteristic of water vapor, which is largely concentration-independent. By determining this quotient and taking it into account in the further signal processing, the otherwise resulting false alarm can thus be suppressed. However, the known method and the detector operating thereafter have the disadvantage of insufficient sensitivity for small and very small particles in common with all other known constructions of infrared light scattered light detectors. This makes it especially difficult to detect open fires in time, in particular wood fires whose smoke is characterized by a very small particle size. In the case of a corresponding risk situation, therefore, the ionization band detectors which respond very well to small particles and which work with a weakly radioactive preparation must still be used. Because of this radioactive preparation, the production of Ionisationsbrandmeldern is complex and their use unpopular and in some countries even generally prohibited.

The invention has for its object to provide a method that considerably improves the sensitivity of scattered light fire detectors for small particles and thus the usability of such detectors for the detection of hot and very hot fires with little additional effort, without this at the cost of increased False alarm frequency goes.

In the method of the initially cited class, this object is achieved in that the forward scattered radiation and the backward scattered radiation of the first and the second wavelength are measured and evaluated separately from one another.

In each measurement cycle, four measured values can be obtained in this way, which can be processed individually as well as combined with each other in order to be able to make a reliable alarm decision after comparison with assigned reference values.

The stray radiation of the first and second wavelengths are measured on opposite sides of the measuring chamber on the same major axis.

Alternatively or additionally, the radiations of the first and the second wavelength are irradiated from opposite sides of longitudinally coincident radiation axes into the measurement volume. The resulting point symmetry to the center of the measurement volume ensures that the measured scattered radiation intensities originate from identical measurement volumes, which facilitates their comparability.

Preferably, therefore, of the signal levels which correspond to the four measured intensities of the scattered radiation, the corresponding quiescent level multiplied by a factor ≦ 1 is subtracted, the result values weighted and the weighted values calculated in an evaluation logic, compared with stored values, the comparison results linked and rated; Depending on the result, at least one alarm signal is generated (claim 4). Depending on the intelligence implemented in the detector, depending on the result, e.g. a pre-alarm signal, a smoke identification signal, a main alarm signal, etc. are generated.

In particular, the ratio between the weighted values of the forward scattered radiation intensity and the backward radiation intensity of the first wavelength and the ratio between the weighted values of the forward scattered radiation intensity and the reverse scattered radiation intensity of the second wavelength can be formed and calculated in an evaluation logic, compared with stored values, the comparison results linked and evaluated, and results dependent at least one alarm signal are generated (claim 5).

Further, the ratio of the weighted values of the forward scattered radiation intensity formed the first and the second wavelength and the ratio of the weighted values of the backward radiation intensity of the first and the second wavelength and the calculated ratio values are calculated in a Auswertelogik, compared with stored values, the comparison results linked and evaluated and results dependent at least one alarm signal are generated (claim 6 ).

In addition, the determined ratio values can themselves be set in relation and the result compared with stored values and the comparison result taken into account in the further processing (claim 7).

Favorable geometric relationships are obtained when the forward scattered radiation of the first and second wavelengths at the same forward scattering angle and the backward scattered radiation of the first and second wavelengths are measured at the same backward spread angle (claim 8), which on the one hand reduces the cost of optoelectrical components to two LEDs and two photoreceptors, such as photodiodes, limited and on the other hand, a similar principle, similar electrical processing of all four measured values allowed.

Suitably, the first wavelength and the second wavelength are chosen so that they are not in an integer ratio to each other (claim 9). Namely, if the first wavelength and the second wavelength are e.g. in the ratio of 1: 2, particles at the first wavelength, e.g. generate a particularly large forward scatter signal even when illuminated with the second wavelength to produce an over-peaked in the manner of a secondary maximum signal. On the other hand, particles with a circumference equal to the longer wavelength, which then reflect particularly well, would strongly absorb at half the wavelength, thus producing almost no stray light.

In the current state of technology of manufacturing LEDs, it is advisable to choose the first wavelength in the range of the infrared radiation and the second wavelength in the range of the blue light or the ultraviolet radiation (claim 10).

Preferably, the first wavelength is in the range of 880 nm and the second wavelength in the range of 475 nm, alternatively 370 nm (claim 11).

The pulse / pause ratio of the radiation of the first and the second wavelength is expediently greater than 1: 10,000, and preferably in the range of 1: 20000 (claim 12), because high radiation intensities are necessary to achieve sufficient sensitivity. The electrical power required for this not only loads the power supply of the detector, but also leads to a considerable heating of the radiation-generating chips of the LEDs, so that after each pulse a sufficiently long cooling time is required to prevent overheating.

For carrying out the method according to the invention and thus for solving the underlying problem is suitable a scattered light fire detector with a communicating with the ambient air measuring chamber which limits a measurement volume in which radiate an infrared radiating and a blue emitting LED from different directions and in which the particles scattered in the measuring volume radiation is photoelectrically measured and evaluated, said detector According to the invention comprises two photoreceiver, which, with respect to the measuring volume opposite, have a common main axis with which the radiation axes of the two LEDs include an acute angle of less than 90 ° and intersect in a lying on the major axis point in the center the measuring volume is (claim 13).

The LEDs can be arranged on the same side of the main axis (claim 14). The one photoreceiver then measures the forward scattered radiation of the infrared radiating LED and the backward scattered radiation of the blue emitting LED, while the other photoreceiver conversely measures the forward scattered radiation of the blue emitting LED and the backward scattering of the infrared radiating LED.

Alternatively, the LEDs may be arranged symmetrically to the major axis (claim 15) so that the one photoreceptor measures both forward scattered radiation and the other photoreceiver measures both reverse scattered radiation.

Preferably, however, the LEDs are arranged point-symmetrically to the center of the measuring volume, so that their radiation axes coincide (claim 16). Consequently, both the LEDs and the photoreceivers are in pairs exactly opposite. This has the advantage that the measured four scattered radiation intensities emanate from an identical measurement volume. Moreover, this symmetrical arrangement also facilitates the largely reflection-free design of the measuring chamber, allows a substantially symmetrical structure of the board, sit on the LEDs and the photoreceptor and leads to a rotationally symmetric and thus at least largely independent of the air inlet direction sensitivity of the detector.

Preferably, the radiation axes of the LEDs with the major axis each include an acute angle of about 60 ° (claim 17). The respective backscatter radiation is then measured at this angle, while the corresponding forward scattered radiation is measured at the complement angle of 120 °. It has been shown that this is a favorable compromise between the value of 70 °, which is more favorable for the measurement of the backscatter radiation, and the diameter of the measuring chamber, which decisively influences the outside diameter of the detector.

In order to protect the photoreceiver from direct illumination by the LEDs and from illumination by reflected radiation on the walls of the measuring chamber and to keep the illumination of the measuring volume low by reflected radiation, each LED and each photoreceiver expediently sits in its own tube; In addition, outside the measuring volume, between the LEDs and the photoreceptors, diaphragms and radiation traps are arranged (claim 18).

Fig. 1

eine in Höhe der optischen Achsen geschnittene Aufsicht auf die die Messkammer tragende Grundplatte des Brandmelders in einer ersten Ausführungsform

Fig. 2

die entsprechende Ansicht einer zweiten Ausführungsform und

Fig. 3

die entsprechende Ansicht einer dritten Ausführungsform.

The method according to the invention is explained below with reference to the drawing, which illustrates three embodiments of a corresponding stray light fire detector in three embodiments. It shows:

Fig. 1

a view in elevation of the optical axes of a plan view of the measuring chamber bearing base plate of the fire detector in a first embodiment

Fig. 2

the corresponding view of a second embodiment and

Fig. 3

the corresponding view of a third embodiment.

The method according to the invention is based on the following:

Depending on the nature of the burning material, a wide range of combustion products is produced, which for the sake of simplicity are referred to below as aerosols or also as particles. Hot fires produce large quantities of small diameter aerosols. For example, an aerosol formation or cluster comprising 100 molecules of CO _{2 has} a diameter of approximately 2.5 nm. Fires with low energy conversion per unit time, ie in particular so-called smoldering fires, generate aerosols with a diameter of up to 100 μm and sometimes also macroscopic suspended matter. eg ash particles. A flare fire detector suitable for detecting all types of fires would therefore have to recognize aerosols with diameters of 2.5 nm to 100 μm, ie be able to cover a range of five orders of magnitude.

Because of their high efficiency are used as radiation sources in scattered light fire detectors in practice only infrared-emitting GaAs LEDs, which produce a wavelength λ of 880 nm. The intensity of the scattered radiation caused by a particle depends primarily on the ratio of the diameter of the particle assumed to be a sphere for simplicity to the wavelength of the incident radiation. In addition, although the shape and the absorption coefficient of the particle also play a role, these parameters are naturally not influenced in the present context. For a particle diameter below 0.1 λ, the so-called Rayleigh scattering decreases in proportion to λ ⁴ . It follows that fire detectors working with infrared emitting LEDs for particle diameters of less than approx. 90 nm have a steeply decreasing sensitivity. In addition, the Rayleigh scattering is not omnidirectional but has pronounced maxima at 0 ° and 180 ° and pronounced minima at 90 ° and at 270 °. For particles with diameters of 0.1 λ to 3 λ, in the case of an infrared-emitting LED thus of approx. 90 nm to approx. 2.5 microns, on the other hand, the Mie scattering is decisive, which is even more direction-dependent than the Rayleigh scattering and also shows destructive and constructive interference effects by interaction of the irradiated with the reflected radiation on the particle. Above 3 λ, the scattering intensity is largely wavelength-independent and depends primarily on the type and shape of the particle.

It follows that the low sensitivity of stray light fire detectors for hot fires, e.g. open wood fire, which is due to the large wavelength of the infrared radiation in relation to the diameter of the particles to be detected. This can be countered neither by increasing the amplification of the signal supplied by the photoreceptors nor by increasing the intensity of the irradiated radiation, because in both cases the sensitivity of the detector to large and macroscopic particles, e.g. Dusts, industrial process vapors and cigarette smoke become too large.

By alternately irradiating the measurement volume with infrared radiation and blue light and separate processing of the received scattered radiation signals proportional can indeed, as mentioned in the introduction WO 01/59737 basically known, the sensitivity of the detector for small diameter particles, especially those for which the Rayleigh scattering is decisive, be considerably increased. It can easily be shown by calculation that the sensitivity is around the factor 10 and more increased. Increasing the sensitivity of the small diameter particle detector, however, is not sufficient in itself to obtain a safe alarm decision, ie to avoid false or false alarms. In particular, it is contrary to in the WO 01/59737 the assumption that the irradiation of the measurement volume with blue light for large particles and for small particles gives scattered radiation of approximately equal intensity. Rather, this investigation has shown that even small particles in the infrared range and in blue light give scattered radiation of very similar intensity, both in the forward and - at lower level - in the reverse radiation range. As has also been shown, only the addition of the angular dependence of the intensity of the scattered radiation makes it possible to obtain reliable criteria which make it possible to differentiate between deceptive quantities and follow-on products largely independently of the type of burned material.

According to the invention, four scattered radiation intensities are therefore measured in each measurement cycle, namely the forward scattered radiation and the backward scattered radiation in the infrared range and the same values in the range of blue light. From the signal levels which are proportional to the measured intensities, in order to increase the measurement dynamics and to simplify further processing, the corresponding quiescent level, preferably with a safety discount (corresponding to a multiplication of the quiescent level by a factor <1), is subtracted. The result values obtained in this way are then compared in an evaluation logic with stored values, in particular threshold values. Additional information is obtained by forming the quotients of the result values and re-comparing with stored reference values. The results of these operations, in turn, can be reconciled, for example, with the environment in which the detector is used. linked and evaluated. In this way, a series of meaningful intermediate results, eg for different pre-alarms, and finally also alarm signals can be obtained.

In Fig. 1 a first, preferred embodiment of a detector suitable for carrying out this method is shown. On a base plate 1.7 a schematically indicated with a thin circle, spherical measuring volume is defined with a center 1.5. In this measurement volume sends an infrared emitting LED 1.1a along a first radiation axis. Directly opposite is a blue-emitting LED 1.1b, which sends into the measuring volume along a second radiation axis. The first and second radiation axes coincide. At an angle of α = 120 ° to this common radiation axis, a major axis also passes through the center 1.5 of the measurement volume. Opposite one another, a first photodiode 1.2a and 1.2b are arranged on this main axis. Thus, the main axis, on which the respective reception axes of the two photodiodes lie, includes an acute angle β = 60 ° with the first radiation axis of the "infrared" LED 1.1a. The same acute angle accordingly includes the major axis with the (second) radiation axis of the "blue" LED 1.1b. Thus, the photodiode 1.2a measures the infrared forward scattered radiation generated by the "infrared" LED 1.1a on particles in the measurement volume at an angle of 120 ° and the blue scattered radiation generated at the "blue" LED 1.1b at a backward dispersion angle of 60 ° , Conversely, the photodiode 1.2b measures the blue forward scattered radiation generated by the "blue" LED 1.1b at the angle α of 120 ° and the infrared backscatter radiation generated by the "infrared" LED 1.1a at a backward angle of 60 °. To avoid spurious reflections, the LEDs and photodiodes are in tubes such as 1.6. For the same reason are arranged between the LEDs and the photodiodes suitably shaped apertures such as 1.3a, 1.3b and 1.4a and 1.4b.

On the base plate 1.7 are further sensors, e.g. at 1.8 a temperature sensor and at 1.9 a gas sensor arranged.

As usual, located below the base plate 1.7 is a circuit board for generating the current pulses for the LEDs 1.1a and 1.1b and for processing of the photodiodes 1.2a and 1.2b supplied electrical signals. As is also usual, the base plate 1.7 is housed in a detector housing (not shown) which allows for an exchange between the ambient air and the air in the measuring chamber, but keeps extraneous light away from the measuring chamber.

FIG. 2 shows a second embodiment of the detector, with the same components as in FIG. 1 but in a different geometric arrangement. To clarify this, the first digit of the respective reference numerals instead of "1" is here "2".

In contrast to FIG. 1 only the radiation axes passing through the measuring center 2.5 of the infra-red emitting LED 2.1a and the blue-emitting LED 2.1b coincide. With the radiation axis of the former, the reception axis of the photodiode 2.2a encloses an angle α1 = 120 ° and with the radiation axis of the blue-emitting LED 2.1b an angle β2 = 60 °. The receiving axis of the photodiode 2.2b closes inversely with the radiation axis of the infrared-radiating LED 2.1a an angle α1 = 60 ° and with the radiation axis of the blue-emitting LED 2.1b an angle α2 = 120 °. Accordingly, the first photodiode 2.2a measures the forward scattered radiation of the "infared" LED 2.1a and the backward scattered radiation of the "blue" LED 2.1b. The second photodiode 2.2b conversely measures the forward scattered radiation, the is generated by the "blue" LED 2.1b and the backscatter radiation generated by the "infared" LED 2.1a.

The photodiodes 2.2a and 2.2b can exchange their position with the LEDs 2.1a and 2.1b, respectively, so that then the two photodiodes with respect to the measuring center 2.5 exactly opposite each other.

This geometric arrangement of the four components, ie the two LEDs and the two photodiodes, is less favorable than that according to FIG FIG. 1 because only 75% of the four measured scattered radiation originate from the same measuring volume. This is illustrated by the sectional areas between the radiation beams, which are shown in a highly simplified manner, ignoring the angular dependence of the intensity of the transmitted radiation as well as the sensitivity of the photodiodes and the diffraction effects occurring at the unavoidable edges. For detectors that - as in the embodiment - include other sensors such as 2.8 and 2.9, it is added that the measuring center 2.5 is strongly eccentric to the center of the base plate 2.7. As a result, the sensitivity of the detector is not, as in the case of the first embodiment, omnidirectional, but dependent on the direction from which the after-fire products enter the detector and its measurement volume.

FIG. 3 shows a third embodiment of the detector, with the same components as in Fig. 2 but in a different geometric arrangement. To illustrate this, the first digit of the respective reference numeral instead of "2" is here "3".

In contrast to Fig. 1 only the receiving axes of the photodiodes 3.2a and. passing through the measuring center 3.5 fall 3.2b together. These receive axes form the main axis. With the latter, the "infrared" LED 3.1a includes an acute angle β1 = 60 ° and an obtuse angle α1 = 120 °. The "infrared" LED 3.1a with respect to the main axis is opposite the "blue" LED 3.1b, which accordingly includes with the main axis the acute angle β2 = 60 ° and the obtuse angle α2 = 120 °. Thus, the photodiode 3.2a receives both the forward infrared scattered radiation and the blue forward scattered radiation while the photodiode 3.2b receives both the infrared backscatter and the blue backward scattered radiation.

Unlike in the case of Fig. 2 In this case, the two LEDs and the two photodiodes can not be arranged in a position-reversed manner, because in this case the two photodiodes would simultaneously measure the forward scattered radiation of one LED and then the backward scattered radiation of the other LED, ie deliver four measured values, of which two, however in pairs at least approximately equal.

As in the case of Fig. 2 are also in the embodiment according to Fig. 3 only 75% of the four measured stray radiation from the same measurement volume. Cheaper than in the case of Fig. 2 is that the measuring volume, even if the detector contains other sensors such as 3.8 and 3.9, closer to the center of the base plate 3.7 is located so that the sensitivity of the detector is less dependent on the direction from which follow the fire products in the detector , Also cheaper than compared to Fig. 2 is according to the geometry Fig. 3 the arrangement of all diaphragms 3.3a, 3.3b and 3.4a, 3.4b near the measurement volume and substantially symmetrically around it. However, under otherwise identical conditions, the positioning of the "blue" LED 3.1b requires a comparison to Fig. 1 larger diameter of the base plate 3.7.

Although it applies to all embodiments that the scattered radiation at angles of 120 ° or 60 ° are measured. However, compliance with these angles is not a necessary condition for carrying out the method proposed by the invention. It is only important that the angles are chosen so that on the one hand sufficiently high intensities can be measured in the forward scattering direction and in the backward scattering direction, and on the other hand sufficiently different intensities in the forward scattering area and in the backward scattering range of the respective particles can be measured for as many different fire following products.

Claims (19)

1. A method for recognizing fires according to the light-scattering principle by pulsed irradiation of a radiation of a first wavelength along a first radiation axis and a radiation of a second wavelength which is shorter than the former along a second radiation axis into a measuring volume of a measuring chamber delimiting the measuring volume and measurement of the radiations scattered on particles disposed in the measuring volume under a forward scattering angle of more than 90° and under a backward scattering angle of less than 90°, with the forward scattering radiations and the backward scattering radiations of the first and second wavelength being measured and evaluated separately from one another, characterized in that the scattering radiations of the first and second wavelength are measured on opposite sides of the measuring chamber on the same principal axis.
2. A method according to claim 1, characterized in that the radiations of the first and second wavelengths of opposite sides are radiated into the measuring volume along coincident radiation axes.
3. A method for recognizing fires according to the light-scattering principle by pulsed irradiation of a radiation of a first wavelength along a first radiation axis and a radiation of a second wavelength which is shorter than the former along a second radiation axis into a measuring volume of a measuring chamber delimiting the measuring volume and measurement of the radiations scattered on particles disposed in the measuring volume under a forward scattering angle of more than 90° and under a backward scattering angle of less than 90°, with the forward scattering radiations and the backward scattering radiations of the first and second wavelength being measured and evaluated separately from one another, characterized in that the radiations of the first and second wavelength of opposite sides are injected into the measuring volume along coincident radiation axes.
4. A method according to one of the claims 1 to 3, characterized in that the corresponding quiescent value levels multiplied with a factor ≤ 1 are subtracted from the signal levels which correspond to the four measured intensities of the scattering radiations, the resulting values are weighted, and the weighted values are processed in an evaluation logic, compared with the stored values, the results of the comparison are combined and evaluated, and at least one alarm signal is generated depending on the result.
5. A method according to claim 4, characterized in that the ratio between the weighted values of the forward scattering radiation intensity and the backward scattering radiation intensity of the first wavelength and the ratio between the weighted values of the forward scattering radiation intensity and the backward scattering radiation intensity of the second wavelength are formed, and the determined ratios are processed in an evaluation logic, compared with stored values, the results of the comparison are combined and evaluated, and at least one alarm signal is generated depending on the result.
6. A method according to one of the claims 4 or 5, characterized in that the ratio is formed of the weighted values of the forward scattering radiation intensities of the first and second wavelength with respect to one another and the ratio of the weighted values of the backward scattering radiation intensities of the first and second wavelengths with respect to one another, and the determined ratios are processed in an evaluation logic, compared with stored values, the results of the comparison are combined and evaluated, and at least one alarm signal is generated depending on the result.
7. A method according to claim 5 or 6, characterized in that the determined ratios are put in relation to each other on their part and the result is compared with stored values and the result of the comparison is considered in further processing.
8. A method according to one of the claims 1 to 7, characterized in that the forward scattering radiations of the first and second wavelength are measured under the same forward scattering angle and the backward scattering radiations of the first and second wavelength under the same backward scattering angle.
9. A method according to one of the claims 1 to 8, characterized in that the first wavelength and the second wavelength are chosen in such a way that they are not in an integral ratio with respect to one another.
10. A method according to one of the claims 1 to 9, characterized in that the first wavelength lies in the range of infrared radiation and the second wavelength lies in the range of blue light or ultraviolet light.
11. A method according to one of the claims 1 to 10, characterized in that the first wavelength lies in the range of 880 nm and the second wavelength lies in the range of 475 nm, alternatively 370 nm.
12. A method according to one of the claims 1 to 11, characterized in that the pulse/pause ratio of the radiation of the first and second wavelength is chosen larger than 1:10000 and preferably in the range of 1:20000.
13. A scattered-light fire detector, comprising a measuring chamber which communicates with the ambient air and delimits a measuring volume in which an infrared-radiating and blue-radiating LED radiates from different directions and the radiation scattered on particles disposed in the measuring volume is photoelectrically measured and evaluated, characterized in that the detector comprises two photodetectors, the two photodetectors are situated opposite one another with respect to the measuring volume on a common principal axis, and the radiation axes of two LEDS enclose an acute angle of less than 90° with said principal axis and intersect in a point disposed on the main axis, which point is situated in the center of the measuring volume.
14. A detector according to claim 13, characterized in that the LEDs are arranged on the same side of the principal axis.
15. A detector according to claim 13, characterized in that the LEDs are arranged symmetrically to the principal axis.
16. A detector according to claim 13 or 15, characterized in that the LEDS are arranged in a point-symmetrical manner relative to the center of the measuring volume, so that their radiation axes coincide.
17. A detector according to one of the claims 13 to 16, characterized in that the radiation axes of the LEDs each enclose an acute angle of approximately 60° with the principal axis.
18. A detector according to one of the claims 13 to 17, characterized in that each LED and each photodetector sits in an own tube, and diaphragms and radiation traps are arranged in the measuring chamber, outside of the measuring volume, between the LEDs and the photodetectors.
19. A detector according to one of the claims 13 or 15 to 18, characterized in that the first photodetector receives the forward scattering radiation of the infrared-radiating LED and the backward scattering radiation of the blue-radiating LED and the second photodetector receives the backward scattering radiation of the infrared-radiating LED and the forward scattering radiation of the blue-radiating LED.

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