EP0117162B1 - Method for the detection of a heat source particularly of a forest fire in a controlled zone, and system for the application of this method - Google Patents

Abstract

The invention relates to a system for detecting sources of heat, comprising a certain number of watching stations distributed within the area to be watched, comprising an infrared radiation detector located at a level above the area to be watched and connected to a motor for imparting to the detector a periodic angular, step-by-step motion, and a device for logic processing of the information received by the detector and for transmission through the telephone link, a central station being equipped with a data processing device including a memory in which the known sources of heat of the area to be watched are recorded or entered.

Description

The subject of the present invention is a method for detecting a heat source which may appear in a predetermined area or space, if necessary of a large extent, in particular a fire in a forest, and a system for putting it implementing this process.

• Lorsque les conditions atmosphériques sont propices à la naissance ou au développement d'incendies, la zone forestière concernée est placée sous surveillance. Un certain nombre de tours ou postes d'observation d'une hauteur relativement importante est réparti dans la zone. Au sommet de chaque tour un sapeur pompier scrute l'horizon afin de détecter visuellement une fumée non répertoriée sur la liste en possession du guetteur, signalant l'existence d'un incendie. S'il constate un tel incendie, il avertit par liaison téléphonique le centre des sapeurs pompiers forestiers. Dans ce centre, sur une carte d'état major, le direction identifiée est repérée et, après réception d'un appel d'une tour voisine, on procède à la localisation par triangulation de l'incendie repéré. Pour des raisons de précision de visée, l'intervention d'une troisième tour est nécessaire ou au moins souhaitable.
• On constate que ce procédé et dispositif de surveillance présentent notamment les inconvénients majeurs suivants :
  • - surveillance limitée dans le temps (diurne et selon les conditions atmosphériques - brouillard) ;
  • - difficulté d'observation systématique régulière sur tout l'horizon, due au facteur humain ;
  • - problème de précision sur la visée ;
  • - difficulté d'observation de l'importance et de la nature du feu ;
  • - nécessité de sélection humaine entre fumée autorisée et incendie ;
  • - absence d'indication sur le sens de propagation de l'incendie ;
  • - nécessité d'attendre au moins une seconde information en provenance d'une autre tour ;
  • - rotation d'un personnel important.

The method and system currently used for this purpose, in particular for detecting a fire in a forest, can be summarized as follows:

• When atmospheric conditions are favorable for the initiation or development of fires, the forest area concerned is placed under surveillance. A number of relatively high towers or observation posts are spread across the area. At the top of each tower, a firefighter scans the horizon to visually detect smoke not listed on the list in the possession of the watchman, signaling the existence of a fire. If he finds such a fire, he warns by telephone the center of the forest firefighters. In this center, on a staff card, the identified direction is located and, after receiving a call from a nearby tower, the location is carried out by triangulation of the identified fire. For reasons of aim accuracy, the intervention of a third round is necessary or at least desirable.
• It can be seen that this monitoring method and device have the following major drawbacks in particular:
  • - monitoring limited in time (daytime and depending on weather conditions - fog);
  • - difficulty of regular systematic observation over the whole horizon, due to the human factor;
  • - problem of precision on aiming;
  • - difficulty in observing the size and nature of the fire;
  • - need for human selection between authorized smoke and fire;
  • - absence of indication on the direction of propagation of the fire;
  • - need to wait for at least a second piece of information from another tower;
  • - rotation of important staff.

French patent application No. 2 224 818 discloses a method and device for detecting fires by which the region to be monitored is angularly scanned at constant rate using at least one detector sensitive to the infrared radiation characteristic of a fire ; temporarily stores the successive intensities measured by the detector for the corresponding successive orientations; a spatial correlation is carried out between the intensity measured for each of the measurement orientations and an average intensity over all or part of the same lap, and / or a temporal correlation relating to the intensities measured during successive laps; and an alert is triggered in the event of a lack of spatial or temporal correlation, the information coming from different towers being able to be transmitted to a regional center to locate the suspect zone exactly.

However, the known method and device do not allow discrimination between authorized smoke and a fire. Indeed, insofar as a spatial correlation is carried out between the intensity measured for each of the measurement orientations and an average intensity under all or part of the same turn, and / or a temporal correlation relating to the intensities measured during successive turns, the appearance of authorized smoke will necessarily be interpreted as a fire.

The object of the present invention is to propose a method and a system for the detection of a heat source, in particular of a fire, which do not have the abovementioned drawbacks inherent in known methods and systems.

The invention therefore relates to a method for the detection of heat sources, in particular forest fires, in a zone or space in particular of large extent, of the type consisting in monitoring said zone from at least one monitoring station. , transmit information relating to the detected heat sources to an information processing device and locate the heat source on the basis of information from the various monitoring stations, the area being monitored using a radiation detector infrared, characteristic of a heat source, at each monitoring station required to perform, periodically, and preferably permanently, angular sweeping movements of the area to be monitored; and characterized in that it consists in transmitting to a central station, via a transmission link, such as a telephone link, the information relating to all the heat sources detected by the infrared detectors and originating treatment devices; previously memorize at the central station the information relating to heat sources authorized to be disregarded; automatically compare the information from all infrared detectors to the central station with the information previously stored at the central station; determine at least one unauthorized newly appeared heat source from the comparison results and issue an alarm signal as soon as the newly appeared heat source is determined.

According to an advantageous characteristic of the invention, the detector is moved in a step-by-step manner and the information that the detector has received during each corresponding stop phase is transmitted to the central station.

According to another characteristic, the detector is caused to perform a scanning movement vertical during each stop phase, around a horizontal axis, the scanning advantageously being carried out at a high frequency.

According to the method proposed by the invention, the information relating to the intensity of infrared radiation received during the stopping of each step or of several steps is transmitted to the aforementioned center in digital form, stores this information in the center and returns it to the detector device in which the information received from the center is compared with the information initially transmitted to it and indicates to the center with the following information, where appropriate by a bit of value "0 or" 1 s' there was equivalence between the information compared and invalidates the information stored in the center in the event of a difference.

According to another advantageous characteristic of the invention, data is transmitted with each information relating to the intensity of infrared radiation such as a synchronization bit which is used in the center to synchronize the allocation to said information received relating to the radiation, information relating to the corresponding angular position of the detector.

The system for implementing the method according to the present invention is characterized in that it comprises a central station equipped with a computer device, such as a computer device, connected to the information processing devices via a telephone line, said computer device comprising a memory in which the information relating to authorized heat sources of an area to be monitored and not to be taken into account is previously stored, which device is adapted to automatically compare each item of information the infrared detectors with the information previously stored in the memory and determining at least one source of newly appeared unauthorized heat from the results of the comparison.

According to an advantageous characteristic of the system, the detector is provided with means allowing vertical scanning during each angular position corresponding to a step of the detector.

According to yet another advantageous characteristic, the vertical scanning means is formed by a mirror vibrating at a high frequency.

• - la figure 1 montre sous forme d'un schéma bloc le système de détection de sources de chaleur suivant la présente invention ;
• - la figure 2 montre de façon schématique et à plus grande échelle l'ensemble détecteur représenté à la figure 1 ;
• - la figure 3 est une vue de dessus sur le détecteur représenté sur la figure 2 ;
• - la figure 4 montre de façon schématique et à plus grande échelle un autre mode de réalisation de l'ensemble détecteur représenté à la figure 1 ;
• - les figures 5 et 6 montrent respectivement le signal de sortie produit par le détecteur avant et après une opération de mise en forme, en fonction de la position angulaire du détecteur ;
• - la figure 7 illustre le principe de structure et de fonctionnement du système suivant la présente invention, sous forme d'un schéma bloc ; et
• - les figures 8 et 9 illustrent schématiquement la structure des octets transmis au poste central.

The invention will be better understood and other objects, characteristics, details and advantages thereof will appear more clearly during the explanatory description which follows, made with reference to the appended schematic drawings given solely by way of example illustrating a mode of the invention and in which:

• - Figure 1 shows in the form of a block diagram the heat source detection system according to the present invention;
• - Figure 2 shows schematically and on a larger scale the detector assembly shown in Figure 1;
• - Figure 3 is a top view on the detector shown in Figure 2;
• - Figure 4 shows schematically and on a larger scale another embodiment of the detector assembly shown in Figure 1;
• - Figures 5 and 6 respectively show the output signal produced by the detector before and after a shaping operation, depending on the angular position of the detector;
• - Figure 7 illustrates the principle of structure and operation of the system according to the present invention, in the form of a block diagram; and
• - Figures 8 and 9 schematically illustrate the structure of the bytes transmitted to the central station.

The method and system according to the present invention are particularly suitable for detecting heat sources such as fires in large forests. It is therefore advantageous to describe the invention by taking as an application example a forest monitoring system as shown in FIG. 1. It should be noted that the invention is however in no way limited to such a application and can be used in any case where it is a question of detecting and locating in a predetermined area the appearance of an object or a phenomenon, mobile or immobile, which emits infrared radiation.

The embodiment of the invention, represented in FIG. 1, for the detection of forest fires, comprises a certain number of surveillance towers of which only one is represented. These towers are of sufficient height and are appropriately placed so that their top is located at a level above any fire sources to be detected in the monitored forest area.

At the top of these towers 1 is installed an equipment item comprising in particular an infrared radiation detector-detector assembly 2, rotatable around a substantially vertical axis in order to be able to perform a horizontal scanning movement, an optical encoder 3 associated with this assembly and intended for determine the angular positions of the latter, a device 4 for processing the signals produced by the detector and representative of the intensity of the infrared radiation received, as well as a modem 5 connected to a telephone line 6 and intended to adapt the electrical signals to the properties of the telephone line.

Telephone lines such as line 6 connect the various monitoring stations at the top of the towers 1 to a central station 7, advantageously a computer, via modems 8 and eight-channel multiplexers 9, each associated with eight modems. The reference numeral 10 designates a storage and archiving device associated with the computer 7.

It should be noted that the telephone line link from the central station to the various surveillance stations could be replaced by any other means of communication. Of course, the type of link should be chosen according to the infrastructure already in place or which can be installed, easily and economically.

Referring to Figures 2 and 3 we will describe below after in more detail the sensor-detector assembly of infrared radiation 2.

This assembly 2 comprises an optical radiation collecting device, provided with an infrared filter 12, a spherical collecting mirror 13 and the detector proper 14 which is provided with a window 15 of rectangular shape according to FIG. 3. assembly surrounded in Figure 2 by a line in broken lines is rotatable about a vertical axis centered on the center of the slot 15 and on the detector located below and fixed. This assembly is driven in rotation by a stepping motor 16 according to the direction of rotation indicated at 17 so that the horizontal scanning movement can be carried out. The slot 15 which is part of the rotary assembly is shown in dotted lines in several angular positions above the fixed sensitive surface 18 of the detector 14.

To allow scanning of the vertical field, a vibrating mirror 20 is placed in the rotary assembly in the focal zone of the collecting optic 13 so as to move the image of the detector in the image focal plane of this optic or, in other words , so as to form on the fixed detector 14 the image of part of the vertical field. This mirror vibrates at a relatively high frequency for reasons which will be explained later.

In FIG. 3, the angular optical encoder intended to define the angular position of the rotary assembly can be recognized during the scanning of the horizontal field.

Before explaining the operation of the system and the various operations of the process for detecting a fire, including a specific processing of the signals produced by the detector, some considerations will be given below which make it possible to understand according to which criteria the detector and the optical and mechanical device as well as the signal processing device could be advantageously chosen. The objective of the system according to the present invention, given by way of example only, is to detect forest fires up to distances of up to 20 km. It is important that the information, that is to say the radiation emitted by a fire, is transmitted to the detector with the minimum of absorption. By taking into account the absorption spectrum of the atmosphere of infrared radiation, we note the existence of a certain number of spectral bands or windows which are particularly transparent to this radiation. In the context of the invention, the band of wavelengths ranging from 3 to 5.5 _"" m is used as windows. In this band the atmospheric transmission is good and the stray radiation such as for example the solar radiation is limited. This window proved to be advantageous for optimal monitoring 24 hours a day, so even in broad daylight. Detectors which are effective in this wavelength range are for example PbSe detectors, cooled to -45 ° C. It should however be noted that the choice of infrared radiation detector depends on the conditions and specific criteria of each application case.

In the context of the invention, it is sought for example to obtain a spatial resolution which is of the order of 15 meters to 20 km away. It is therefore advisable to be able to operate within 15 m to 20 km of a fire. However, at 20 km a segment of 15 m is seen at an angle 2a _h = 15/2 x 10 ⁴ rds = 7.7 x 10- radians. The number of elementary sectors resolved for each turn will therefore be Za / 2ah = 8192 sectors = 2 ¹³ .

The optical encoder provided for defining the angular positions of the rotary optical system must therefore be able to differentiate 2 ¹³ different directions per revolution.

For reasons related to the maximum rate of transmission of usual telephone lines, and to the number of useful information to be transmitted, the fastest speed of analysis of the horizon will advantageously correspond to one revolution in 40 seconds. The analysis time of a sector is therefore approximately 5 ms corresponding to a frequency of 200 Hz. To have good spatial resolution it is advantageous for the detector to have a bandwidth approximately 10 times greater, that is to say about 2 kHz.

Regarding the vertical observation field and in particular the frequency of vertical scanning, the following should be taken into account: if the observation towers have an altitude of 40 m and if the upper limit of the vertical observation field is the horizontal direction, which avoids being in direct observation of the sun, except in the morning and in the evening, and taking as a lower limit a shadow zone of 200 m around the foot of the tower, we obtain a vertical field angle equal to 15 x 10- ² rds. It should be noted that this gray area is relative, because any fire which would be born there will be immediately detected because of the fumes which would cross the observation area, at a distance very close to the detection system. Due to the disproportion between the vertical and horizontal fields and the resulting requirements for the dimensions of the detector, it is advantageous to associate the detector with the vibrating mirror 20. As has been said above, this mirror advantageously vibrates at a sufficiently high frequency that the detector seems to see the entire vertical angular field at once. It is then advantageous to choose a vibration frequency of the order of 20 kHz, that is to say 100 times higher than the analysis frequency of a horizontal sector and 10 times higher than the upper limit of the detection system bandwidth (2 kHz).

It should however be noted that the use of a vibrating mirror to perform a vertical scan is not compulsory and could be eliminated if the vertical field is sufficiently weak or the sensitive surface of the detector is sufficiently large. Any other appropriate means could also be used to remedy the problems of disproportion mentioned above.

FIG. 4 represents another embodiment of the sensor-detector assembly for infrared radiation 2. This includes a device _; . sitive optical radiation collector, provided with the infrared filter 12, a device 13a forming an objective and the detector 14 provided with the window 15. As for the other embodiment of FIGS. 2 and 3, the assembly surrounded in FIG. 4 and by a dashed line is rotatable about a vertical axis centered on the center of the slot 15 and on the detector 14 located below and fixed. This assembly is rotated in the same way as that of Figure 2 so that the drive means 16 has been omitted in Figure 4. To allow scanning of the vertical field, a rotating mirror 20a in the direction of the needles d a watch is placed in the rotary assembly in the focal zone of the objective 13a so as to form on the fixed detector 14 the image of part of the vertical field. The mirror 20a has several reflecting surfaces 20,1-20, ₈ arranged for example in an octagon and focuses the infrared radiation on the detector 14 via the converging lens 13b located above the detector 14 and the slot 15. The mirror rotates around of the axis passing through the center 0 of the octagon and perpendicular to the axis in phantom through the centers of the lens 13b, of the slot 15, and of the detector 14. The angular speed of rotation of the mirror 20a must be chosen so as to be high enough so that the detector seems to see the entire vertical angular field at once. This speed can be chosen in accordance with the vibration frequency of the mirror 20 defined above.

The method and the operation of the system according to the invention, which has just been described, appears from the description of the operation which will be given below with reference to FIGS. 5 to 9. The detector 2 emits an electrical signal directly proportional to the intensity of infrared radiation received. This signal is transmitted to the processing device 4 in which it is amplified at 22, shaped at 23, converted to 24 in digital form by an analog-digital converter and processed in a logic processing and transmission control circuit 25, before reaching modem 5, as shown in figure 8.

By way of example, FIG. 5 shows the electrical output signal from the detector 2 as a function of the angular position OEH of the rotary optical assembly of the detector, during the scanning of the horizontal field. This signal has peaks in a, b and c which are representative respectively of a light from 5 m to 20 km, from 5 m to 15 km and from 5 m to 5 km. The sudden rise in d of the output signal level is caused by the sun. The distinction between sources of heat to be identified, such as fire sources a, b and c, heat source d and others, will be made in the central station as described below. By considering the peaks representative of a fire, in FIG. 5, it can be seen that these peaks are characterized less by their amplitude relative to the parasitic background - amplitude which can be relatively small as in the case of the peak a - but rather by the shape of these ridges which is distinguished by very steep front and rear flanks, that is to say very short rise times.

By taking advantage of this feature, we obtain at the output of the shaping circuit 23 the pulses a ', b', and c 'and d' (Figure 6), which correspond to the peaks a, b, c and d. The analog-digital converter 24 converts each pulse which contains the information of the intensity of an infrared radiation emitted by the heat source into an 8-bit digital signal.

The logic processing and transmission control module 25 receives for each heat source the digital signal relating to the intensity of the radiation and the corresponding information relating to the angular position of the rotary optical assembly, which has been generated by the optical encoder 3 in the form of a 13-bit digital signal; if each angular position corresponds to an angular segment of 2a _h = 7.7 x 10-4 radians as in this example. The module 25 first orders the signals so that they can be transmitted by the modem 5. Since the modem only accepts 8-bit signals in series, the module 25 combines the two pieces of information respectively of 13 parallel bits and 8 parallel bits into 8 bit serial information. Figures 8 and 9 illustrate the structure of the information bytes transmitted by the modem 5 to the central station 7 (Figure 1). Therefore, the module 25 should perform information compression. This consists first of all of not transmitting the 13 bits of angular position. In the 8-bit train sent to modem 5, only one bit is used for position tracking. This is a synchronization bit which is always in the low logic state "0 •, except when switching to the digital angular position 0000000000000 where the synchronization signal is at the high logic level" 1 This bit allows the reconstruction of information relating to the angular position in the central station. The latter comprises for this purpose a 13-bit counter which is reset to zero by the synchronization bit and which is incremented by one step with each new transmission of a byte. With regard to the radiation intensity level data, the precision of 8 bits is not necessary and one is satisfied with an accuracy of 3 bits. It is therefore possible to send, using a byte, the information relating to two angular positions of the detector sensor, formed by the rotary optical assembly. The 13-bit counter of the central station, that is to say of the computer 7, is therefore incremented by two steps with each new transmission of a byte. The remaining bit of the byte is used for an indication of the proper functioning of all the systems installed on tower 2. Figures 8 and 9 show two bytes transmitted successively. The configuration of each byte shown is characterized by a synchronization bit D _o , a D bit, error, 3 bits D ₂ to D ₄ of radiation intensity data relative to an angular position n (Figure 8) or n + 2 (figure 9) and 3 bits D5 to D ₇ of respective intensity data at the following angular position n + 1 (figure 8) or n + 3 (figure 9).

Each byte thus formed is transmitted to the modem 5 which sends it to the modem 8 by the permanent telephone line 6. At the output of the modem 8, the byte is stored in the computer of the central station 7. It is then re-injected by the computer in the modem 8 which retransmits it to the modem 5 to arrive at the processing module 25. The latter then compares the start byte and the return byte. If the two bytes are equivalent, the transmission was successful and the computer memorized correct data. If the two bytes are different, there has been a transmission error and the computer has stored false data. Once the comparison of the two bytes is complete, the transmission control logic processing module 25 acts on the control of the stepping motor 16 for driving the sensor formed by the rotary optical assembly of the detector 2. If the sensor was in the comparison in position n + 1, the motor places the sensor now in position n + 2. The sensor produces a new output signal which will be processed by the shaping circuit 23, after amplification at 22, and applied to the analog-digital converter 24. The module 25 then initiates the analog-digital conversion and stores the radiation intensity byte relative to the position n + 2. Then, the module 25 acts again on the rotation command of the motor 16, to place the sensor in the position n + 3. It triggers a new analog-digital conversion, then from the new data recorded, sends to the modem 5 the byte n + 2, n + 3 according to FIG. 9 This byte contains in bit D, the information relating to the comparison of the previously transmitted byte containing the information relating to the angular positions n and n + 1 (FIG. 8). If the previous comparison operation has found a transmission error, the error bit of the new byte is in the high logic state "1" which causes the byte (n, n + 1) to be invalidated previously. recorded by the computer.

The operations which have just been described allow a permanent verification of the proper functioning of the system. An important operation will be described below which makes it possible to detect a fire from other heat sources, which must not give rise to the triggering of an alarm. Indeed, other sources of heat could cause the production of signals similar to the peaks indicative of a fire, which have been indicated in FIG. 5. For example, the sudden change in signal level d, which is generated by the sun (sunrise or sunset) and indicated in module 25 in the form of an impulse of is transmitted to the central station or the computer 7. Likewise, habitat fumes and in certain cases motor cars, trains, the planes etc .... usually located or crossing the zone monitored by the detector 2 would be brought to the attention of the computer 7. To eliminate an unjustified triggering of an alarm, the sources 7 were listed in the central station parasitic heat which should not be taken into account. To this end, the central station computer is provided with a memory in which information relating to sources has been recorded. To find out if a heat source identified by detector 2 is a fire, the computer performs, following the reception of each byte, after having associated with the information received the information relating to the angular position of the detector sensor , using its 13-bit counter and the synchronization bit D _o contained in the byte received, a comparison with the content of its memory.

It is thus easy to distinguish an immobile parasitic heat source fire. To also allow the distinction of a fire from a parasitic but mobile or transient heat source, such as a car or a train, the fact that this source moves can serve as a criterion for distinction. Appropriate programming of the computer thus allows the computer to detect a fire, even vis-à-vis mobile stray heat sources.

It appears from the description of the system according to the invention, which has just been made, that the central station 7 or more exactly the computer permanently receives a flow of information from the various monitoring stations each equipped with an infrared radiation detector. If a heat source turns out to be a fire, it can be easily located from the start and action can be taken immediately to extinguish the start of the fire.

Of course, the invention is in no way limited to the detection of a fire. In general, the invention can be used to detect the appearance or penetration into a monitored area of any object or phenomenon giving rise to the emission of infrared radiation. The invention could thus be used to monitor, for example, a border.

It should also be added that the information relating to the nature of the detector, to the mode of verification of the proper functioning of the system and to the configuration of the messages in digital form could be different without departing from the scope of the invention.

Provision could also be made for the radiation sensor to perform a vertical scanning step-by-step movement and transmit information relating to the position in the vertical field to the central station. In the station we could thus locate the source of the radiation from the data relating to the positions in the horizontal and vertical fields.

Claims (17)

1. A method for detecting sources of heat, more particularly forest fires in an area or a space particularly of great extent, according to which the said area is watched from at least one watching station, information relating to the detected sources of heat is transmitted to a data-processing device, and the location of the source of heat is determined in accordance with the information received from the different watching stations, the area being watched by means of an infrared radiation detector, which radiation is characteristic of a source of heat, at each watching station, the said detector being caused to accomplish, periodically and preferably permanently, angular movement for scanning the area to be watched, characterized in that it consists in transmitting to a central station 7 through a transmission link, such as a telephone link, the information relating to all the sources of heat detected by the infrared radiation detectors and coming from the data-processing devices ; in storing previously at the central station the information relating to authorized sources of heat not to be taken into account; in automatically comparing at the central station each information from all the infrared radiation detectors to the information previously stored at the central station ; in determining at least one newly born non authorized source of heat from the results of the comparison and in transmitting an alarm signal as soon as the newly born source of heat is determined.

2. A method according to claim 1, characterized in that the detector 2 is displaced step by step and transmits to the central station the information which the detector received during each corresponding period of stoppage.

3. A method according to one of claims 1 or 2, characterized in that the detector is caused to perform a horizontal and/or vertical scanning motion.

4. A method according to one of the foregoing claims, characterized in that there is transmitted in digital form to the central station 7 the information relating to the intensity of the infrared radiation received during the stoppage following each step or several steps, this information is stored at the central station 7 and is returned to the detector device 2 in which the information received from the central station 7 is compared to the information initially transmitted to the latter, and there is indicated in the following information to be transmitted, if appropriate by a bit of 0 or 1 value, whether there was an equivalence between the information compared, and the information stored at the central station is invalidated in the case of a difference.

5. A method according to claim 4, characterized in that there are transmitted with each information relating to the intensity of the infrared radiation, data such as a synchronizing bit, and this bit is used at the central station to ensure the attribution to the said received information relating to the infrared radiation, of the information relating to the corresponding angular position of the detector.

6. A method according to one of the foregoing claims, characterized in that the information relating to the intensity of the infrared radiation received is transmitted in the form of an octet containing a checking bit Do and a synchronizing bit D,.

7. A method according to claim 6, characterized in that each octet comprises the information relating to the infrared radiation received corresponding to two angular positions of the detector.

8. A method according to one of the foregoing claims, characterized in that the nature of a source of heat is detected taking into account the properties of such source, such as its evolution or displacement within the watched area.

9. A system for detecting sources of heat, for carrying out the method according to one of claims 1 to 8, comprising a certain number of watching stations distributed within the area to be watched, comprising an infrared radiation detector, which radiation is characteristic of a source of heat, at each watching station located at a level above the area to be watched and connected to a motor for imparting to the detector a periodic angular, step-by-step motion, and a device for logic processing of the information received by the detector, characterized in that it comprises a central station (7) equipped with a data-processing device, such as a computer device, connected to the data-processing devices through the medium of a telephone link, the said data-processing device comprising a memory in which is previously stored information relating to authorized sources of heat not to be taken into account of an area to be watched and being adapted to automatically compare each information from all the infrared radiation detectors to the information previously stored in the memory and to determine at least one newly born non authorized source of heat from the results of the comparison.

10. A system according to claim 9, characterized in that there is associated with the detector 2 an optomechanical device provided with an optical coding device 3 which delivers a digital signal defining the angular position of the detector, the coding device 3 being connected to the information processing device 4.

11. A system according to one of claims 9 or 10, characterized in that the detector 2 is provided with a means 20, 20a allowing a vertical scanning in each angular position of the detector of a horizontal scanning.

12. A system according to claim 11, characterized in that the vertical scanning means 20 consists of a mirror vibrating at a high frequency.

13. A system according to claim 12, characterized in that the detector is provided with a rectangular aperture 15 and that an optical device is mounted upstream of said aperture, which comprises a mirror 13 collecting the received infrared radiation, in the focal region of which is arranged the vertical-scanning vibrating mirror 20, and in that the aperture 15, the vibrating mirror 20 and the collecting mirror 13 constitute a rotatable assembly, a fixed detecting member 14 being placed below the aperture 15.

14. A system according to claim 11, characterized in that the vertical scanning means 20a consists of a mirror rotating at a high angular speed.

15. A system according to claim 14, characterized in that the detector is provided with a rectangular aperture 15 and in that an optical device is mounted upstream of the said aperture, which comprises an objective member 13a collecting the infrared radiation received, in the focal region of which is arranged the rotary mirror 20a, and a convergent lens 13b receiving the infrared radiation from the rotary mirror and focusing this radiation onto a fixed detector 14, and in that the aperture 15, the rotary mirror 20a and the objective 13a constitute a rotatable assembly, the fixed detector 14 being placed below the aperture 15.

16. A system according to claim 13 or 15, characterized in that the rectangular aperture 15 is in the form of a slit, the smaller side of the aperture being parallel to the axis of rotation of the vibrating mirror 20.

17. A system according to one of claims 9 to 16, characterized in that the information processing device 4, which comprises a logic processing and transmission module 25, is connected to the motor 16 driving the detector 2 to cause the angular displacement of the latter according to a predetermined program.

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