US 3739365 A
Fire detecting apparatus includes two photoelectric devices, each having different spectral sensitivities. A difference signal corresponding to the difference between the output signals of the photoelectric devices is generated and an alarm signal is developed when the difference signal deviates by a predetermined amount from a predetermined value or range of values, depending upon application. A preferred embodiment also includes a delay means for delaying generation of the alarm signal for a predetermined time.
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United States Patent 1191 Miiller APPARATUS FOR DETECTION OF A FIRE OR 0F FLAMES Peter Miiller, Oetwil, Switzerland Cerberus AG., Mannedorf, Switzerland Filed: Dec. 1, 1970 Appl. No.: 94,113
Foreign Application Priority Data Dec. 3, 1969 US. Cl. IMO/228.2, 250/83.3 H, 250/83.3 UV,
' 250/220, 356/222 Int. Cl. G08b 21/00  Field of Search 340/228 R, 228 S,
340/228.1, 228.2, 171; 250/83.3 UV, 83.3 R,
References Cited UNITED STATES PATENTS 6/1965 Vasec et a1. 340/228 R 2,897,485 7/1959 Johnson 340/228 R Switzerland 18107/69 Primary Examiner-Donald J. Yusko Assistant Examiner-William M. Wannisky Attorney-Flynn & Frishauf  ABSTRACT Fire detecting apparatus includes two photoelectric devices, each having different spectral sensitivities. A difference signal corresponding to the difference between the output signals of the photoelectric devices is generated and an alarm signal is developed when the difference signal deviates by a predetermined amount from a predetermined value or range of values, depending upon application. A preferred embodiment also includes a delay means for delaying generation of the alarm signal for a predetermined time.
23 Claims, 12 Drawing Figures 2 Sheets-Sheet 1 Fig. 6
Patented June 12, 1973 3,739,365
2 Sheets-Sheet 2 APPARATUS FOR DETECTION OF A FIRE OR OF FLAMES FIELD OF INVENTION The present invention relates to apparatus for thedetection of a fire or of flames by means of emitted rays.
Apparatus of this type is preferentially utilized as a fire alarm or as a control unit for combustion installatrons.
BACKGROUND OF THE INVENTION It is already known that the presence of a flame may.
A known'device utilizes the typical flickering of flames, i.e. the variation of intensity of the light radiation of the flame in a very low-frequency oscillation zone, as the distinguishing feature of a flame vis-a-vis disturbance light radiation. In this known device the radiation strikes a photoelectric element whose output signal is conducted to a frequency-selective amplifier whose band-pass lies in the order of magnitude between 5 and 25 Hz. The amplifier then feeds the amplified signal to a switching network. Even if the. frequency band-pass of the amplifier optimally corresponds to the rate of flickering of flames, disturbances and false alarms are relatively common occurrences. If accidental variations of intensity in the ambient light radiation lie in the same frequency zone, for example through shadings or false flashes due to vibrating or slowly moving objects, false flashes of sunlight on water surfaces, flickering or wavering light sources, etc., the a false alarm could be generated.
It has been attempted to eliminate the disturbance effect of external light radiation sources by utilization of an infra-red sensitive photo cell or by connecting of an infra-red filter which is especially translucent for flame radiation in front of a photo cell. However, this works only if the infra-red radiation of the disturbance light source is very small. With this construction a strong disturbance light source could cause a false alarm.
Another known device utilizes the fact that aflame possesses a relatively large proportion of long-wave radiation (infra-red, for example) and only a small pro portion of short-wave radiation (blue, for example). This known device utilizes two different photoelectric devices, for example photo resistances with different an alarm signal when the voltage at the junction point has a certain value. In a device of this type a false alarm can' be generated by constant infra-red light radiation sources, such as heaters or heating ovens. On the other hand, with known device, an alarm signal can not be generated if a high intensity disturbance radiation in the short-wave zone ispresent. This apparatus is thus only conditionally utilizable, with the result that a DC. amplifier must be used whose operating point must be kept stable. This leads to complicated circuitry and additional expenditures.
A simple combination of the best features of the two known devices described above, namely the detection of the typically wavering intensity of light radiation of flames as well as of the relation between long-wave and short-wave radiation, is unfortunately not possible. For example, the utilization of an AC. amplifier with the known device using two photo resistances in series having different spectral sensitivity would produce an inoperable system. This is because in a flame, the red portion of the light radiation exhibits almost the same intensity fluctuations in time as the blue portion, and the resulting red-blue fluctuation relation remains almost constant. Thus, at the junction point of the two photo resistances an almost constant A.C. potential without marked A.C. variation occurs.
It is therefore the main object of the present invention to provide a reliable apparatus for flamedetection that will be substantially. uninfluenced by external disturbance light radiation sources.
SUMMARY OF THE INVENTION In accordance with the present invention, two photoelectric devices, each having different spectral sensitivities are provided. A difference signal corresponding to the difference between the output signals of the photoelectric devices is generated, and an alarm signal generator is responsive to the difference signal for generating an alarm signal when the difference signal deviates by a predetermined amount from apredetermined value.
A particularly effective embodiment of, the present invention comprises at least one pair of photo elements having differentspectral sensitivities connected in series or in parallel with opposite polarities. The-output of the photo element arrangement, which is the difference between the output signals of the two photo elements, is coupled to theinput of an analyzerthat is sensitive over a limited low-frequency AQC. range.- The analyzer includes a discriminator circuit which emits an alarm signal when theoutput signal of the analyzer deviates by a predetermined amount from a predetermined value.
In a particularly suitable embodiment, the spectral sensitivities of the two photo elements are varied in such a waythat for predetermined disturbance light radiation, the difference .of the output signals of the two photo elements is smaller (i.e., by at least a factor of 10) than theindividual signals, that is, the difference signal is essentially zero.
DRAWINGS FIG. 1 is aschematic representation of a device in accordance with the present invention;
FIG. 2 illustrated a circuit for passive photo elements with two oppositelyconnected switching networks;
FIG. 3 illustrates a circuit for passive photoelements with two oppositely connected switching networks and a common direct-current supply;
FIG. 4 illustrates a circuit for active photoelements;
FIG. 5 illustrates a circuit for active photoelements with a common tuning potentiometer;
FIG. 6 illustrates a circuit for currentemitting photoelements;
FIG. 7 illustrates a dual photoelement;
FIG. 8 illustrates two photoelements with sensitivities that are variable by means of a common screen or light shield;
FIG. 9 illustrates a device comprising two photoelements with reflection filters;
FIG. 10 illustrates a device comprising two photoelements with a common dichroic filter;
FIG. 11 illustrates a circuit for the evaluation of the signals of the photoelectric; and
FIG. 12 illustrates a device with more than two photoelements.
DESCRIPTION OF PREFERRED EMBODIMENTS FIG. 1 represents schematically a fire detection device-in accordance with the present invention. The red light rays r and blue light rays b emitted from flame 1 simultaneously strike photoelectric cells 2 and 3, respectively. The term photoelectric cells is understood to mean any device which under the action of light radiation changes its electrical characteristics. Examples of active photoelements are selenium cells, silicon cells, solar cells, etc. Examples of passive photoelements are gas-filled or vacuum photo cells, photo diodes, photo resistances, etc. Photoelectric cells 2 and .3 have a different spectral sensitivity. Cell 2 is responsive to red light and cell 3 to blue light. This can for example be effected either in that the photosensitive layers of the cells consist of different materials, in that filters of different spectral permeability r or b are placed in the path of the light rays, or in that reflection filters with different spectral reflection are used for the two cells. The two photoelectric devices emit coherent electric signals, for example, voltages or currents of differing intensity depending upon impinging light. The intensity of the two signals of cells 2 and 3 can be tuned (or varied) in different ways independent of each other, such as by mounting a mechanical screen in front of the photoelements, by using various ballast resistors connected to the photoelements, or by using additional amplifiers and other circuit devices.
The output signals of the photoelectric devices are conducted to a device 4, which generates an output signal which is a function of the difference between the two signals from cells 2 and 3.'
This difference signal is conducted to a band pass filter 5, which passes only the A.C. portion of the difference signal which lies in a specified frequency range. The frequency range between 2 and 50 Hz has been found to be particularly suitable in practice. If still better selectivity of the flame radiation as against disturbance light radiation is desired, this frequency range (i.e. the band pass of filter 5) may be even more narrowly restricted, for example, to the range between 5 and 25 Hz.
The out put of band pass filter 5 is conducted to an amplifier 6, the output of which is conducted to a discriminator 7. Discriminator 7 generates an alarm signal which is fed to an alarm or control device 8 if the incoming signal thereto exceeds or falls short of certain predetermined values. Discriminator 7 is preferably a circuit which emits an output alarm signal if the input signal thereto deviates positively or negatively by a certain amount from a fixed value, or in either direction by a certain amount from zero. Alternatively, an alarm signal can be fed to alarm device 8 if the effective value or another appropriate mean value of the output signal from amplifier 6 exceeds a given threshold value.
The sensitivities of the photoelectric devices (i.e., cells 2 and 3) can be tuned (or varied) as described above, in such a way that for a disturbance light radiation that occurs particularly often, i.e. for sunlight or for especially strong light sources in the vicinity of the monitoring apparatus, the output-signals of the photo electric devices for a radiation of this spectral composition would be equal. Thus, the difference signal becomes zero, and the discriminator circuit 7 will in this case under the action of such a disturbance radiation emit no alarm or control signal. With all other light radiations having different spectral compositions the difference of the electrical output signals of photoelectric devices 2 and 3 will not be zero, but will deviate from zero in one direction or the other.-The discriminator circuit will in this case emit an alarm signal to be fed to claim device 8. In this manner the screening-out of certain known disturbance light radiations which would normally cause a false alarm is easily effected without great-expense.
The discriminator circuit 7 can also be designed such that an alarm signal is emitted only when the difference signal from circuit 4 deviates from zero with a certain predetermined polarity. Thus, a disturbance light radiation of quite specific spectral composition which would normally cause a false alarm may be easily and completely screened out, and that beyond this, an alarm can be set off only if, in the light radiation striking the photocells, the longwave portion is preponderant. In special cases, the system can be such that an alarm will be generated when the shortwave portion is preponderant, for example with fire alarms which react only to the ultraviolet light radiation of a flame.
In addition, the discriminatorcircuit 7 can also contain an integrator (or other appropriate delay means) so that an alarm will not be set off immediately upon receiving short voltage impulses, butonly when the ex- .ceeding of the predetermined values exists forv a specified length of time. In this manner short duration disturbances, through voltage impulses of short duration, will not cause generation of a false alarm.
Further, the discriminator circuit 7 may include a locking or latching circuit such that upon the setting off of an alarm, the alarm automatically holds in its on" condition, and can be re-set from a central station. This can be. easily effected by connecting a latching relay, bistable multivibrator, or the like, to the output of the discriminator as is well known in the art. The actuating of the alarm device can also be indicated by an optical indicator device 9 such as a light, which is installed either in the flame detection unit itself or in the alarm central control station and can serve for the localization (i.e., identification) of an actuated alarm unit. Optical indicator 9 can be connected to alarm device 8 (as shown) or to discriminator 7.
' The sequence of the steps of generation of the difference signal, frequency filtering and amplification can be changed at will. Naturally the different circuits can be comprised single elements, as shown, or by combined special devices. For example, the generation of the difference signal can be accomplished by corresponding coupling of the photoelectric devices. The band pass filter amplifier and discriminator can be comprised in a single analyzer unit, which may also include a circuit for generating the difference signal (or its equivalent).
For the connection of the device of the present invention with a central alarm station, known circuits for fire alarm systems may be utilized. For example, a device for function-monitoring of the system may also be provided whereby a signal is emitted from the central station to produce alarm-simulating conditions in the device so that the device is enabled to emit an alarm signal that can be registered in the central station. Such function-monitoring can be carried out in a known fashion through digital analysis, through logical circuits or through additional A.C. signals. Throughout the drawings, the same reference numerals are used to designate the same or similar elements.
FIG. 2 illustrates anarrangement-in which two photoelectric devices 11 and 12 are connected together in a differential circuit, which is connected to an analyzer 10. The photoelectric devices 11 and 12 each comprise a passive photoelement 13, for example a photo resistance or a photo diode, a ballast resistor 14 and a battery 15. Any suitable D.C. source can be used in place of batteries 15. In front of the photoelement 13 is a filter 16 having a certain spectral permeability. The pho' toelectric device 12 differs from device 11 only in that the filters 16 possess different spectral permeabilities. Device 11 is made sensitive to red light and device 12 is made sensitive to blue light, as is indicated in FIG. 2. The potential drop at ballast resistor 14 serves as an output signal of the photoelectric mechanism. The two devices 11 and 1 2 are now connected with each other at either end of the ballast resistors 14 in such a way that the respective potential drops at the two ballast resistors have opposite polarities. The resistors 14 are connected with the analyzer via leads 17 and 18. The signal AU appearing across leads 17 and 18 is the difference AU of the voltage at the two ballast resistors 14, and thus the difference of the output signals of the two photoelectric devices 11 and 12. The analyzer 10 then combines the functions of circuits 5, 6 and 7 of FIG. 1.
active photoelements needs no voltage source and the photoelectric devices 11 and 12 can, in its simplest form, consist only of a photoelement 13 and a ballast resistor 14. The photoelements are again connected with opposite polarity so that at leads l7 and 18 from the ballast resistors, once more the difference of the output signals exists and can be conducted to analyzer 10.
Additionally, on one of the ballast resistors 14, a reference potential may be tapped off and fed to analyzer 10 via lead 19. The reference potential can be used in the analyzer 10 to detect the sense (or polarity) of the difference of the output signals of the photoelectric de- 'vices. In this manner, not only a specified disturbance radiation can be prevented from generating false alarms, but additionally the alarm is set off only when the radiation becomes preponderant in a specified portion of the light spectrum.
In FIG. 5 two networks with active photoelements 13 with differing spectral sensitivity as in FIG. 4 are connected together. However, the ballast resistors for the two photoelements are made up of a single common potentiometer 20 with a variable tap. One portion of the resistance of potentiometer 20 serves as the ballast for circuit 11, the remaining portion serving as a ballast resistor for circuit 12. By means of the variable tap, the
FIG. 3 illustrates a circuit in which two photoelectric 14 once more have opposite polarity, so that again the.
signal AU representing the difference of the output signals of devices 11 and 12, is conducted to analyzer 10. With this arrangement, the greater part of the output signals of both photoelectric devices can be modulated independently of each other and optimal tuning of response characteristics, to remove sensitivity to a specified disturbance radiation may be obtained.
FIG. 4 illustrates two photoelectric devices 11 and 12,- which comprise active photoelements 13. To this end selenium, silicon, or solar cells may be used, or any other type of photoelectric cell which gives a voltage or a current in response to light. The coupling network for relation of the two resistance portions and also the sensitivity-relation of the two devices 11 and 12, may be varied and tuned to eliminate sensitivity to a particular disturbance radiation. Instead of voltage, current can also serve as output signal of the photoelectric mechanism.
In FIG. 6 there are two active current-sending photoelements 11 and 12, for example selenium or silicon cells with differing spectral sensitivty, connected in parallel with each other and in parallel with analyzer 10. The current difference A1 of the two photoelements 11 and 12 then flows into the conductors and analyzer 10. In this case, analyzer 10 must be modified to have a small input resistance relative to the internal resistance of photoelements l0 and 11, to sense the current AI.
FIG. 7 illustrates an arrangement of two photoelectric elements in the form of photo-resistances, on a common base material 21.This constitutes a dual element whose two halves 22 and 23 have the same characteristics. The two photoelectric elements 22 and 23 are covered by optical filters .24 and 25, which, however, pass different frequency ranges of the light radiation spectrum. Such an arrangement has the advantage that in use, both photoelements are impinged by very nearly an equal light radiation intensity. Alternatively dual cells with layers 22 and 23 of different spectral sensitivity can be used. The two layers 22 and 23 can be arranged on top of each other, whereby the top layer is permeable for the radiation for which the lower layer is sensitive.
FIG. 8 shows an example of a mechanism for adjustment of the effective characteristics of two photoelements 27 and 28. A mechanically movable screening device 26, such as a diaphragm, damping filter or other material for either blocking, reducing or otherwise changing light transmission characteristics, can be moved in such a way that one or both of the photoelectric devices 27 and 28 may be partially screened.
FIG. 9 illustrates an arrangement in which the incoming light radiation is impinged on two reflection filters 29 and 30 with different spectral reflection characteristics, the light being then conducted onto photoelements 27 and 28. This is equivalent to various other arrangements described above. In FIG. 10 a dichroic fil-- ter 31 is mounted in the path of incoming light radiation. Filter 31 reflects only the portion of the radiation having a specified spectral composition on to a photoelement 27. Filter 31 passes another portion of the radiation with differing spectral composition therethrough onto photoelement 28. With this arrangement, exactly equal radiation for both photoelements 27 and 28 is obtained. As a dichroic filter 31 may be used very tyin metal layers, for example gold and copper or transparent optical layers whose thickness lies in the order of magnitude of the light-wave lengths, as well as combinations of such layers with different refractive index (which have recently become known as cold-light mirrors, warm-light mirrors or interference filters).
FIG. 11 illustrates the circuitry of an analyzer 10. Input leads 17 and 18 are the leads shown, for example, in FIGS. 2 to 6. The difference signal'of two photoelectric devices is conducted to analyzer over leads 17 and 18. Thus, in this embodiment, a circuit (such as circuit 4 of FIG. 1) for generating the differential signal in this case is not necessary. The difference signal, conducted over terminals 17 and 18 to the analyzer 10, is fed through aninput capacitor 32 to a first transistorized amplifier stage 33. Capacitor 34 on the output of amplifier stage 33 serves to limit the high frequencies. The output 35 of the first amplifier stage 33 is coupled to further amplifier stages (not shown) and then to the discriminator circuit. The discriminator includes the two rectifiers 36 and 37 which serve for rectification and signal doubling; and capacitor 38, its charging resistance 39 and its bleeder resistor 40, which serve as an integration stage with a specified time constant, i.e. for the time lag of the discriminator. Once the charge on capacitor 38 reaches a specified value, break-down (Zener) diode 41 which is coupled to the output-of the integration stage becomes conductive and turns on controlled rectifier (i.e., SCR) 42. This causes a signal to be fed to alarm-lead 43, thereby actuating an alarm device 44 which can give off an acoustic or an optical signal, or which controls an appropriate switching operation. In this embodiment, analyzer 10 includes also a control or alarm device 8 of FIG. 1. The controlled rectifier 42 is connected such that once turned on, it remains turned on, even when the input signal falls below the actuating threshold value as determined by break-down diode 41. Controlled rectifier 42 can be turned off by means of circuit breaker 45 which effectively opens lead 43. An optical indicator device 46 is connected into the switching network of the controlled rectifier 42; this permits visual recognition of the actuating state of the controlled rectifier 42 and of the alarm device.
Naturally for a flameor fire-detection device of the type described above any and all other circuits known in the art may be utilized, as long as they serve the same function to carry out the present inventive concepts. Instead of working with transistors and semiconductors the circuit can also be fabricated with vacuum tubes and instead of a controlled rectifier, an ionical relay, for example a cold cathode valve, can be utilized, which can simultaneously serve as a visual indicator for the state of the circuit instead of using a-separate indicator device.
Likewise, other known discriminator circuits can be used, for example, those that generate the effective value of the signal or those that generate an alarm sig-- nal when the instantaneous value of the AC. signal is exceeded in a predetermined direction. Also, digital discriminators may be used which, for example, when a predetermined limit is exceeded, generate an impulse and only give out an alarm signal if a specified number of impulses have been generated within a predetermined period of time.
The input amplifier stages of analyzer 10 must have input impedances which are compatible with the impedances of the photoelectric devices.
Furthermore, it is not necessary that only two photoelectric devices be used in the present invention. To provide a greater input signal to the analyzer, a greater number of photoelectric devices can be coupled together such that the outputs of all devices having one spectral sensitivity are additively combined, and the outputs of all devices having the other spectral sensitivity are additively combined. Also, devices of equal spectral sensitivity can be grouped in a unit, or also, devices of differing sensitivity can be connected alternatively in series. Further, pairs of oppositely connected photoelectric devices of differing sensitivity can be connected in series in such a manner that each pair is sensitive for radiation coming from a specified direction. In this manner, fireor flame-alarm device with good peripheral sensitivity may be constructed.
FIG. 12 represents such a peripherally sensitive device with four pairs of photoelectric devices. Each pair contains two active photoelements 47, in front of which a red filter 48 or a blue filter 49 is mounted as shown in FIG. 12. The pairs can naturally also be set up as dual-photoelements. For such pairs are connected in series such that each pair is sensitive only to radiation from a given direction. The ends of the series connection of pairs of elements are conducted over leads 17 and 18 to an analyzer, which is similar to analyzer l0 discussed hereinabove. The operation of FIG. 12 should be apparent.
I claim: 1 1. Apparatus for detecting fire or flames in the presence of disturbing radiation of a predetermined spectral composition resulting from ambient light which should not give an alarm comprising:
first and second photoelectric devices, each having different spectral sensitivities with respect to different spectral ranges with maximum response, each in the different spectral ranges and producing respective output signals; electrical circuit means including said photoelectric devices for generating a difference signal corresponding to the difference between the output signals of said first photoelectric devices and the output signals of said second photoelectric devices,
said difference signal being essentially zero for the disturbing radiation and having an a-c component in a low-frequency range of about between 2-50 Hz differing from zero'in the presence of, and due to the flicker of flames;
means sensing the a-c component in said lowfrequency range only, of said difference signal; and alarm generating means responsive to the sensed a-c component of the difference signal in said lowfrequency range only for generating an alarm signal when the a-c component of said difference signal in said low-frequency range exceeds a predetermined value.
2. Apparatus according to claim 1, wherein the low frequency range of the a-c component to which the alarm signal generating means is responsive lies between and 25 Hz.
3. Apparatus according to claim 1 wherein said alarm signal generating means includes:
filter means for passing only signals corresponding to said difference signal within a predetermined frequency range; and
a discriminator means coupled to the output of said filter means for generating said alarm signal when said difference signal deviates by a predetermined amount from said predetermined value.
4. Apparatus according to claim 1 wherein said alarm signal generating means generates said alarm signal when the magnitude of said difference signal deviates by a predetermined amount from zero.
5. Apparatus according to claim 1 wherein said alarm signal generating means generates said alarm signal when said difference signal deviates by a predetermined amount with a predetermined polarity from zero.
6. Apparatus according to claim 1 wherein said alarm signal generating means includes means for delaying for a predetermined period of time generation of said alarm signal when said difference signal exceeds the predetermined value by.
7. Apparatus according to claim 1 comprising means for changing at least one of the spectral sensitivity and the amplification of at least one of said photoelectric devices such that the difference of the output signals of said photoelectric devices for the disturbance light radiation with a pre-specified spectral composition is much smaller than the output signals of each individual photoelectric'device.
8. Apparatus according to claim 7 wherein said difference signals for said disturbance light radiation with said prespecified spectral composition is smaller by at least a factor of 10 than said output signals of each individual photoelectric device.
9. Apparatus according to claim 1 wherein said photoelectric devices are coupled together in series opposing relation and said difference signal generating means comprises means coupling the free terminal of said devices to said alarm signal generating means.
10. Apparatus according to claim 1 wherein said difference signal generating means comprises means coupling said photoelectric devices together in parallel opposing relation and means coupling said parallel connected devices to said alarm signal generating means.
11. Apparatus according to claim 1 wherein each of said photoelectric devices comprise at least one active photoelectric element and a ballast resistance coupled thereto, the output signal of the device being the potential drop at said ballast resistance or part thereof.
12. Apparatus according to claim 1, wherein each of said photoelectric devices comprise at least one passive photoelement, a ballast resistance coupled thereto, and a direct-current supply coupled thereto, the output signal of the device being the potential drop at said ballast resistance or part thereof.
13. Apparatus according to claim 12 wherein said photoelectric devices are opposingly connected together and to a common direct-current supply.
14. Apparatus according to claim 13 comprising a potentiometer coupled as a common ballast resistor to said photo-electric devices such that one portion of said potentiometer is a ballast resistor of one of said photoelectric device and that another portion of said potentiometer is a ballast resistor of the other photoelectric device.
15. Apparatus according to claim 1 wherein said photoelectric devices include respective photoelements having different-type photosensitive layers with differing spectral sensitivities.
16. Apparatus according to claim 1 wherein said photoelectric devices include respective photoelements having photosensitive layers of the same type and respective filters having differing spectral permeability or reflection characteristics mounted in the path of the light impinging on said photoelements.
17. Apparatus according to claim 1 comprising means for varying the light impinging on at least one of said photoelectric devices.
18. Apparatus according to claim 1 wherein said photoelectric devices include respective photoelements oil" a common base-material, and means for causing said photoelements to exhibit a differing spectral sensitivity.
19. Apparatus according to claim 1 comprising a dichroic filter connected in front of both photoelectric devices which are arranged such that the portion of the radiation passed by said dichroic filter strikes one pho toelectric element, while the portion of the radiation reflected by said dichroic filter strikes the other photoelectric device.
20. Apparatus according to claim 1 comprising a plurality of pairs of oppositely connected photoelectric devices, each of said pairs being connected in series relative to one another and arranged such that each pair receives light radiation from a different direction.
21. Apparatus according to claim 1, comprising means for maintaining said alarm signal in its on condition when an alarm signal has been generated.
22. Apparatus according to claim 3 wherein said discriminator means comprises means for automatically maintaining said alarm signal in its on condition when an alarm signal has been generated.
23. Apparatus according to claim 1 wherein said alarm signal includes an optical signal for visually indicating said alarm signal.
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