DE1573207A1 - Backscatter mist meter - Google Patents

Description

Hoffman Electronics Corporation, 4501 N Arden Drive, El Monte, California, / U. S.A.

concerning:

"Backscatter mist meter"

The invention relates to a device for Measurement of the concentration of fog or the concentration of others that impair the atmospheric view Substances and relates in particular to a device that combines a gallium arsenide diode with a Photocell used, which work together according to the backscatter method.

A number of methods have heretofore been used to improve visibility in fog or haze conditions to measure in the atmosphere. Earlier methods used direct observation of target points with different Distances were built up. When the theory of light weakening and light contrasts has been developed, visibility was measured by measuring the attenuation of light. It was based on the knowledge that the scattering and absorption of light by fog reduced the width to the same extent as the proportion of light

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which reached a receiver at a certain distance from a source. This principle is used in transmissiometer systems that have a constant intensity and a light source Have light receiving element at a fixed distance therefrom, measuring the amount of light reaching the receiving element. The ratio of the amount of light, that is received under IT fog conditions to the amount of light that is received under a clear view is a Measure of the ability of the lever to weaken the light, and this can be mathematically interpreted as four den that one obtains the distance at which one can expect to recognize a given object. The attenuation coefficient is measured and the visibility is calculated. Usually the meteorological range is used, this being the distance below which the contrast is at two Percent of its original value has dropped.

So that the transmissiometer systems an accurate measurement result can deliver, the baseline must be comparable to the meteorological range that is being measured. Accordingly, if it is intended to measure meteorological ranges on the order of 2 to 10 miles, then it must the base line should be at least 1 km. Furthermore, for reasons of accuracy, the receiving field of view must not must be larger than the light source, otherwise the forward scattering would be added to the direct radiation. It can be seen that a number of problems arise. First, a long baseline can't always be easy to set up. Second, the optical alignment of the system is difficult to achieve and maintain, because the field of view of the receiver is small. Third, an intense radiation source is required. Fourth, long cables are required.

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It has already been established that other measuring methods could be used for the light attenuation caused by fog. In particular, it was found that: if the light scattered out of a ray would be scattered in a fixed amount, the measurement of the light, that is scattered in a certain direction, accordingly would be a measure of the oscillation coefficient under which The prerequisite is that the scattering is the main cause of the weakening of the light. The most desirable kind of one Such an instrument is the one in which the light is measured, which through the lever in the direction of the light source scattered back wiiM. Dien is commonly used as a backscatter system designated. The obvious advantage of such a system is that the instrument is compact because the light source and the receiver can be arranged laterally to one another. Fin many other significant advantages is that it is basically well suited for measuring large meteorological ranges. Furthermore, the optical alignment not particularly difficult to set up or maintain. However, up to now it has been disadvantageous that either a very intense light source was required or complicated signal conditioning techniques were used had to be in order to obtain a usable reception signal. Previous systems used very intense flashing lights, in order to achieve that the low proportion of the backscattered light to the receiver compared to the background lighting could still be measured.

The system of the device according to the present invention makes use of the fact that a gallium arsenide diode emitted 'light can easily be modulated at high frequencies, so that the static background light can be switched off, noise, which is introduced by the photonic character of the light, is still suppressed by a synchronized receiving device.

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Accordingly, a particularly intense light source is not required. It is also used advantageously made that the diode emits radiation in a relatively narrow frequency band, and that a light-sensitive A semiconductor photocell, such as a silicon photocell when used as a detector, a Has peak light sensitivity that is close to the wavelength the diode lies. The sensitivity of a silicon photocell falls on both sides of this wavelength, so that it acts as an optical filter, further reducing the effects of unwanted ambient light will. Since soxTohl is the source as well as the recipient are solid components, the entire system can be built solidly, with the advantage of low voltages, high reliability and low power consumption.

In order to achieve high long-term accuracy, a second photocell is used to supply a reference signal that depends directly on the light source. This reference signal is weakened and with the signal from the Receiving photocell mixed, but phase shifted by 180 °. Then the attenuation is adjusted so that the both signals cancel each other out at a certain density of fog. This compensation or the zero display is considered to be a measure that the mist has reached this predetermined density. Through this The display of this predetermined value is omitted regardless of the source intensity or the gain factors. The attenuation of the reference signal can also be used can be controlled by a control loop, so that the display of the loop gain is a measure of represents any fog density, regardless of the source intensity or of the gain factors, where only is required that a sufficient signal is obtained to operate the system.

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The equalization or zero display can be further processed in various ways. You can e.g. activate a fog horn or other warning device. To prevent an instantaneous interruption the light path activates such a warning device, the system is equipped with a first timer circuit, which is set to a certain desired time lapse, e.g. one minute. When the backscattered If radiation falls below the critical value again within this minute, the timer circuit is reset immediately. the Display or warning device is not activated. To prevent the warning device from being put out of operation prematurely is, a second timer circuit is provided, which is also to be set to a desired time interval. This timer circuit can be set, for example, so that the mist is below a predetermined value during 15 minutes before the warning device is put out of operation. This second circle of timers arises also back quickly.

It is accordingly the subject of the present invention * an improved device for backscatter measurement of the density of fog or other atmospheric conditions.

The subject of the present invention is also such a device using gallium arsenide diodes and silicon photocells.

Another object of the present invention is a fog meter that is simple, compact, durable and that requires low operating power.

Another object of the present invention is a backscatter mist meter with high accuracy, the is insensitive to changes in background light intensity.

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An embodiment of the invention will now be described with reference to the accompanying drawings those

Fig. 1 is a side view, partly in section, of the device according to the present invention,

Fig. 2 is a block diagram of the device according to the present invention,

Figure 3 is a schematic diagram of the light source of the device according to the present invention shows,

Fig. 4 is a schematic diagram of the signal mixer;

Figure 5 is a schematic diagram of the timer circuit and

Fig. 6 shows a partial block diagram of a modification of the apparatus according to the present invention.

In Fig. 1, the mechanical and optical arrangement of the device is shown. A transmitter or light source and a receiver 11 are attached to a stand or other suitable support 12 by means of brackets or the like. A control unit 13 is also mounted on the stand 12. The transmitter 10 has a tube 14 in which a radiation source 15 is fastened, which has a gallium arsenide diode fastened at the focal point of a parabolic reflector or mirror 16. The reflector 16 projects the light through a window 17 in a beam 18 of approximately 1 ° solid angle. The window 17 is shaded by a screen 19. A reference signal device 20 is positioned within the tubes 14 to receive light directly from the source 15.

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The receiver 11 is similar in many respects to the transmitter 10 and includes a tube 24 with a receiving device 25, which is preferably by means of setting means for the maximum setting of the return signal in the focal point a parabolic reflector or mirror 26 is located. The receiver 11 is also provided with a window 27 and a screen 28 fitted. The optical axis of the receiver field of view 29 is adjusted to be the optical axis of the beam 18 of the transmitter at any given distance, e.g. 10 m, interspersed. Light reflected from the fog volume or the like, which both the transmitter beam 18 and the Receiving field 29 belongs to, is focused by the reflector 26 on a photocell, which is in the receiving device 25 is appropriate. The tested room volume is within a one-degree beam from about 2 m to infinity, however the backscattered signal is generally insignificant after 25 m, if the axes of the transmitter and receiver for a The penetration is set to be about 10 m in length and the tubes are several inches from each other ("several inches apart "). Light that is reflected from within a cone 30, which is determined by the angle B, is effective over the entire mirror surface. Outside this cone, light that reaches the mirror can no longer die Reach receiving facility.

In Pig. Figure 2 is a block diagram of the electrical circuitry of the apparatus of the present invention. An oscillator 35 of any conventional type feeds a monostable multivibrator 36, the output of which is fed to a gallium arsenide diode 38 via a driver amplifier 37. The gallium arsenide diode 38 can be modulated, for example, with a frequency of 8 kHz. The output of the oscillator 35 also feeds a phase shift network 39, which in turn feeds a reference signal amplifier 40. The amplifier HQ generates an output signal which is fed to the first input of a phase-sensitive detector 4l.

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The signal fed to the diode 38 generates radiation which is directed to a reference detector 42 and a measurement detector 43 impinges. Each of the detectors has a silicon photocell, which is a photosensitive Semiconductor element is. The electrical output of the reference detector 42 is via a variable damping arrangement 44 fed to the input of a preamplifier 45. The two Inputs of the preamplifier 45 are compared, and a The error signal is fed to an amplifier 46. The exit of amplifier 46 becomes the other input of the phase sensitive Detector 4l supplied.

The phase-sensitive detector 4l is used to compare the phases of the signal from the amplifier is generated with the phase of the signal that the gallium arsenide diode 38 modulates. The parts that are in phase generate a DC output voltage, while noise, that has a random phase relationship, balances out to zero. The output of the phase-sensitive detector 4l feeds one Zero detector circuit 47, the output of which is fed to the timer circuits which control the operation of a relay 49. The relay 49 controls the operation of any alarm device 50, such as a foghorn.

In operating the circuit just described, the gallium arsenide diode 38 is connected to the output of the multivibrator 36 controlled so that it generates light with the frequency of the oscillator 35 £, for example 8 kHz. That generated

0 Light is emitted with a wavelength of about 9000 A with a very narrow bandwidth, whereby the infrared range is included. The emitted light is picked up by the reference photocell 42, which has a peak efficiency at about 9000 A and in turn produces an electrical output proportional to the light received. This output is fed to the attenuator 44 which is variable so that the level of the reference signal for each

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desired value can be selected. The photocell detector 43 receives a portion of light from the diode 38, which is backscattered by the atmosphere, and is produced a proportional electrical output signal. This output signal is sent together with the output of the Attenuator 44 fed to a preamplifier 45, where they are switched against each other, so that the output of the preamplifier 45 is an error signal which is the difference between the preset or reference value and the actual value. The photocell detector 43

has a peak efficiency of 900 A and works accordingly, as an optical pilter since its narrow band is substantially identical to that of the diode 38. That diminishes the noise from stray light without the need for expensive optical filtering devices.

The phase of this error signal is then compared with the phase of the signal which is used for modulating the diode 38, ie with the output of the oscillator 35. The output of the phase sensitive detector 4l is fed to a zero detector circuit 47 which generates an output pulse when the error signal passes through zero, ie when the actual value exceeds the preset or reference value. The output of the zero detector circuit is fed to timer circuits 48. As explained in detail later herein, the timer circuits 48 serve to keep a signal from the relay 49 unless the actual value continually exceeds the preset value for more than a predetermined time, for example one minute. This prevents the alarm device from being activated as a result of an accidental or temporary condition. The timer circuits 48 also serve to prevent the alarm device 50 from being switched off until a predetermined time interval has passed after the reference value has again exceeded the actual value. This ensures that the alarm device 50 is not put out of operation prematurely while dangerous atmospheric conditions are still present. ■ BATHROOM

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In Pig. 3 shows the circuit details of the transmitter. The mode of operation and the basic construction are conventional and well known, so there is no need for a detailed explanation thereof. The circuit will powered by any suitable voltage source, e.g. a battery or a stabilized voltage source, which are connected to connections 55 and 56. the Applied voltage is smoothed by a pilter circuit 57 and fed to a transistor oscillator 35, which has an output signal at any desired frequency, e.g. 8 kHz. The output of the oscillator 35 feeds a trigger 59 which feeds a monostable multivibrator 36 once for each oscillation of the oscillator 35 switches through. The multivibrator 36 and The voltage supplied to the remaining parts of the transmitter is filtered by an LC circuit 60 and a capacitor 61. Of the The output of the multivibrator 36 is fed to a driver amplifier 37 fed via an emitter follower stage 62. The emitter follower 62 is desirable as it is preferably the foregoing Parts of the transmitter are accommodated in the control unit 13 and only the amplifier 37 and the diode 38 are in the transmitter arrangement 10 are located.

The output of the driver amplifier 37 is fed to the gallium arsenide diode 38 by means of a transformer 63 coupled. A limiter circuit made up of a zener diode 64 and an oppositely polarized conventional diode 65 is across the primary winding of the transformer 63 switched in order to keep excessively high reverse voltages away from the diode 38. The output of the oscillator 35 is also about a line 65 is fed to the phase shift network 39 shown in FIG. The construction of the phase shift network 39 and reference signal amplifier 40 are fully known and need not be shown or described in detail do be.

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Pig. 4 illustrates how the outputs of the reference photocell 42 and the measuring detector photocell 43 are switched against each other in order to generate an error signal. The exit the measuring photo cell 43 is connected to a coordinated circuit, which also includes the secondary winding 69 of a transformer and a pair of capacitors connected in parallel 71 and 72, the capacitor 71 being variable for the purpose of tuning. The matched circle acts as a filter for unwanted signal components and supports the natural signal selection made by the previous optical filtering was achieved by the photocell 43. The through the variable Attenuator 44 reduced output of the reference photocell 42 is inductively coupled into the tuned circuit by means of the coil 73, which serves as the primary winding of the transformer 70. It can be seen that the attenuated output value of the Reference photocell is abandoned with a 180 ° phase shift, so that only the difference between the outputs of the Reference photocell 42 and the measuring photocell 43 the preamplifier is fed. The construction of the preamplifier 45 and of amplifier 46 are conventional, with one or both preferably having a sufficient number of tuned stages to maintain the desired narrow frequency band.

The construction of the phase-sensitive detector 4l is also conventional and not new to the expert » As noted above, the phase detector 4l compares the signal from the amplifier 46 with the output of the Oscillator 35, so that the mean Rauscliausgang of the Phase detector becomes zero. The phase shift network is provided so that the output of the oscillator 35 »the is fed to the amplifier 40, can be brought into phase with the main component of the pulses received from the Multivibrator 36 are generated. That happens that the Output of the oscillator 35 by about half a pulse width is moved.

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The one in Pig. The zero detector circuit 47 shown in FIG. 5 has a voltage divider consisting of the resistors 75 and 76 which are connected via a direct voltage source, preferably the same direct voltage source which is connected to the connections 55 and 56 in FIG. The zero detector circuit 47 also has a transistor 78 which receives the output of the phase detector 4l, a diode 79 and a resistor 80. The diode 79 is selected so that it has the same forward voltage drop as the base-emitter voltage of the transistor 78 is. The anode of the diode 79 is connected to a line 81 and thereby connected to the connection point of the resistors 75 and J6 . As a result, the line 8l has a higher potential than the ground, and the diode 79 is conductive via a path which contains the resistor 80. As a result, the emitter of transistor 78 is at a voltage which is lower than the voltage on line 81 by an amount which is equal to the forward voltage drop of diode 79.

When the attenuated output value of the reference photocell 42 is greater than the output value of the measurement photocell 43., the signal applied to the base of transistor 78 is negative, i.e. the voltage at the base is lower than the voltage on line 8l. When the output value of the measuring photocell increases, the voltage that the base goes down of transistor 78 is fed through zero and then positive. When the input voltage reaches zero i.e. when the voltage at the base is equal to the voltage on the line 81, the transistor 78 is immediately conductive, as the voltage drop has brought the emitter of the transistor 78 via the diode 79 to a voltage value which is lower than that which is now appears at the base, and since the voltage drop across the diode is equal to the base-emitter voltage of the transistor. The diode 79 accordingly serves to compensate for the Basls-emitter voltage shift of transistor 78.

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The output value of the zero detector circuit 47 becomes the base of a switching transistor through an emitter follower 84 85 supplied. The transistor 85 together with a resistor 86, a capacitor 87 and a second transistor 88 form part of a first timer circuit, i.e. a circuit, which prevents the alarm device from responding unless the preset condition lasts longer than a predetermined time interval persists. This first timer circuit works in the following way:

When transistor 78 is non-conductive, the transistors that form emitter follower 84 conduct with the The result is that the base of the transistor 85 is held at a positive potential determined by the values of the resistors 89 and 90 is determined. Since the transistor 85 is conductive, the capacitor 87 is not charged, and the base of the Transistor 88 is held at a voltage approximately equal to the voltage on line 81. The emitter of the Transistor 88 is connected to the wiper of a potentiometer 92, which together with a resistor 93 a Forms voltage divider, which is connected between the positive line of the voltage source and line 81. As a result, the emitter of transistor 88 is biased to a voltage that is greater than the voltage at the base of transistor 88 is so that this transistor is not made conductive. However, if transistor 85 opens, the capacitor 87 begins to charge through the resistor 86. When the voltage across the capacitor 87 reaches a value which is higher than the voltage at the emitter of the transistor 88, this transistor becomes conductive. It can be seen that the time at which this line begins is determined by the setting of the potentiometer 92 is determined.

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The output of transistor 88 is connected via a resistor ⁹¹ to the base of a PNP transistor 95, the emitter of which is connected to the positive terminal of the voltage source via a diode 96 and whose collector is connected to ground via resistors 97 and 98. An NPN transistor 100 has its base connected to the junction between resistors 97 and 98 and its emitter connected to line 81. The collector of the transistor 100 is connected via a resistor 101 to the wiper of a potentiometer 102, one end of which is connected via a resistor 103 to the positive terminal of the voltage source. The other end of potentiometer 102 is connected to line 81 through diodes 104, 105 and 106.

The junction of the collector of translator 100 and resistor 101 is connected to the base of the first of a pair of cascaded MPN transistors 108 and 109. Accordingly, it is connected to the base of the transistor 108, the emitter and collector of which are connected to the base and the collector of the transistor 109, respectively. The emitter of transistor 108 is connected to the junction between diodes 105 and 106 through a resistor 110, and the emitter of transistor 109 is connected directly to that point. The collectors of both transistors 108 and 109 are connected to the base of an emitter follower transistor 111 and via a resistor 112 to the positive terminal of the voltage source. The collector of transistor 111 is directly connected to the positive terminal. The emitter of the transistor 111 is connected to ground via a resistor 11M, and a capacitor 115 is connected from this emitter to the base of the transistor 108 . The emitter of the transistor 111 is also connected via a resistor 116 to the base of the transistor 117 of a Schmltt-Trlggerkreises, which has a transistor 118 in addition to the transistor. The transistors 108, 109 and 111, the capacitor 115 and the associated circuit means form a circuit which is similar to a Miller integrator

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With transistors 85 on and 88 off, transistor 95 is kept off as its Base is held at a voltage which corresponds essentially to the voltage of the positive terminal of the voltage source, When transistor 95 turns off, transistor 100 is also turned off because its base is essentially at ground potential and not sufficiently positive with respect to its Emitter is to switch it through. The base of the transistor

108 is at a voltage that is determined by the setting of the wiper arm of the potentiometer 102, since the potentiometer 102, the resistor 103 and the diodes 104 to 106 serve as a voltage divider between the positive terminal of the voltage source and the line 81. The emitter resistance 110 is large in relation to potentiometer 102 and resistor 103, and the base of transistor 108 is now positive enough with respect to the emitter that this transistor is on. The transistors 108 and 109 cooperate and as a result the transistor is

109 also switched through. The emitter voltage of the transistor 111 is essentially followed by the collector spamiiing Transistors 108 and 109. Accordingly, the emitter voltage is of transistor 111 is essentially equal to the voltage of line 81. Capacitor 115 is now essentially connected between the voltage of the line 81 and a voltage value between the desi positive terminal of the voltage source and the Le'itung8l, which is determined by the setting of the wiper of the potentiometer 102, and the capacitor 115 charges up. The emitter voltage of the Transistor 117 is above the voltage on line 81 since transistor 118 is conductive via resistor 120. Accordingly, the turned on transistor 118 holds a transistor 117 switched off.

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When transistor 88 turns on, transistors 95 and 100 turn on too. When the transistor 100 turns on, the transistors 108 and 109 turn off, since the base of the transistor 108 is not sufficiently positive in with respect to its emitter voltage is to stay switched off. The emitter of transistor 111 follows the collector voltage of transistor 108 and 109. The charge on capacitor 115 is now changing rapidly, or the capacitor is discharging, whereby the transistor 117 is turned on. As long as transistors 95 and 100 are turned on and the transistors 108 and 109 remain off, the transistor remains on.

If transistor 95 turns off, turning off transistor 100 as well, the transistors will try 108 and 109 through again. However, the charge on the capacitor must change enough before the transistors 108 and 109 can turn on. The charge on capacitor 115 changes slowly when the voltage increases at the collectors of the transistors 108 and 109, until finally the transistor 117 switches off again a suitable time interval, such as 15 min. Resistors 101 and 11Ί are typically large, such as one MOhm each, with a two hundred and fifty micro-farad capacitor 115, with a substantial time delay is provided which can be varied by changing the setting of the potentiometer 102.

The emitters of the trigger circuit transistors

117 and 118 are connected together and connected to ground through resistor 120. The collectors of the transistors 117 and 118 are connected to the positive terminal of the voltage source via resistors 121 and 122, respectively. The collector of the Transistor 117 is connected to the base of transistor 118 via a resistor 123, and also flows via one Resistor 12 * 1 is connected to ground. The collector of the transistor

118 is connected through a resistor 126 to the base of a transistor 127, the emitter of which is via a diode 128

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is applied to the positive terminal of the voltage source. The collector of transistor 127 is connected through a resistor 129 to the base of transistor 130 in a relay driver circuit which consists of this latter transistor and a transistor 131.

The base of transistor 130 is connected to ground via resistor 132. The emitter of the transistor 130 is connected to ground via a diode 133, and its collector is connected to the base of the transistor 131 and connected to the positive terminal of a relay voltage source ^{via a resistor 13 1 J.} The emitter of the transistor 131 is connected to ground via a diode 136 and the diode 133, and its collector is connected to the positive terminal of the relay voltage source via a winding 137 of the relay 49. A diode 138 connected across the winding 137 keeps interference voltages away from the transistor 131. The winding 137 actuates a switch 139 of the relay 49, which can be used to switch on an alarm device 50 or the like. As will be explained later, winding 137 is normally not energized to hold switch 139 open and when winding 137 is energized, switch 139 closes and turns on the alarm.

When transistor .117 is off and transistor 118 is on, as discussed above the transistor 127 is also turned on. When transistor 127 is on, transistor 130 is is turned on and keeps transistor 131 off. When the transistor 131 is not conductive, the winding becomes 137 is not energized and holds switch 139 open as shown above.

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When transistor 117 turns on and transistor 118 is off, the transistor switches 127 also off and thus also turns off transistor 130. When transistor 130 turns off, turns on the transistor 131 through. With this switching through of the transistor 131, the winding 137 is excited and allowed accordingly switch 139. to close. As soon as the switch 139 closes, the alarm device becomes 50 or the like, whereby the presence of hazardous atmospheric conditions is indicated.

Instead of activating an alarm device, a qualitative or continuous reading can also be provided to record the level of hazardous atmospheric conditions. An error signal can be generated and used in a control loop for controlling the variable attenuator 44 in order to obtain the output signal of the Reference detector 42 and the measuring detector 43 to be kept the same. As shown in Fig. 6, the phase sensitive detector 41 can be switched on and in the control loop for the control of the attenuator 44 are used. A control element 150, such as a servomotor, can be connected to the outputs of the phase detector 4l are switched on in order to create a mechanical output value for the change in the attenuator 44. A An error signal at the output of the phase detector 4l causes the control element 150 to adjust the attenuator 44 until the inputs on the phase detector have become the same.

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Claims (17)

Claims 1) Device for the measurement of backscattered radiation, characterized by a radiation source with a gallium arsenide diode, the radiation of which is directed into the atmosphere to be examined, and by a first radiation detector with a photo semiconductor for receiving the radiation emitted from the atmosphere contained components is reflected. 2) Device according to claim 1, characterized by a modulation device for the radiation of the Gallium Arsenide Diode. 3) Device according to claim 2, characterized by first circuit means in operative connection with the modulation device and the radiation detector for generating an error signal. 4) Device according to claim 3j characterized by the second Switching means for setting a predetermined Value of the error signal, preferably by exceeding the zero value. 5) Device according to claim 4, characterized by time switching means for generating an output signal a predetermined deviation of the predetermined error signal value over a predetermined time interval. 6) Apparatus according to claim 1, characterized in that the photo semiconductor is a silicon photo cell. 7) Device according to claim 1, characterized by a second radiation detector irradiated directly from the source, preferably also with silicon photocell, for the output of a reference signal. - IO -009821/0584 ^SAD o ^R1GlNAL 8) Device according to claims 3 and 7 » characterized in that the second radiation detector is in operative connection with the first circuit means. S) Device according to one or more of the preceding claims, characterized in that the received Radiation essentially from the volume of the atmosphere originates, which interspersed with predetermined opening angles of the radiation source and the radiation detector will. 10) Device according to one or more of the preceding claims, characterized in that the first radiation detector and the radiation source and its opening angle are spatially arranged so that essentially the backscattered radiation hits the detector. 11) Device according to claim 10, characterized by reflectors for emitted and received radiation .. 12) Apparatus according to claim 5, characterized by delay means for maintaining the output signal over a predetermined time interval when the predetermined deviation is no longer present. 13) Device according to one or more of the preceding Claims, characterized by a phase-sensitive detector, at whose inputs the modulation frequency or the error signal are applied and the output of which is connected to a signal height detector which is connected to the timer connected. 1 * 0 device according to claim 13> characterized in that the signal level detector after the signal has been exceeded Zero at its input, an output signal to the timer gives away. 15) Device according to one or more of the preceding Claims, characterized by a controlled by the time switching device Eir.schaltvorrichtung füi? an alaria facility. 00S821 / 0B64 BATHROOM ORIGINAL - 2 «- -m - 16) Device according to one of claims 1 to 3j, characterized by an attenuator for the output signal of the radiation detector vn & dureh a control device for the attenuator, the setpoint of which the Pshler signal is zero, 17) Device according to claim 7 and one or more of the preceding claims ₃ marked by a preferably adjustable attenuator for the reference signal and by circuit means for comparing the attenuated 3esugsslgnals with the output signal of the first radiation detector. 18} Device according to claim 17, characterized by a control device for adjusting the value zero at the output of the comparison circuit means by adjustment the attenuator, 009821/0584