US3008053A

US3008053A - Light detectors

Info

Publication number: US3008053A
Application number: US399870A
Authority: US
Inventors: Hendee Charles Franklin
Original assignee: US Philips Corp
Current assignee: US Philips Corp; North American Philips Co Inc
Priority date: 1953-12-23
Filing date: 1953-12-23
Publication date: 1961-11-07
Anticipated expiration: 1978-11-07

Description

Nov. 7, 1961 c. F. HENDEE LIGHT DETECTORS Filed Dec. 23, 1953 3 Sheets-Sheet 1 Bxl/@WWW AGENT Nov. 7, 1961 c. F. HENDEE LIGHT DETECTORS Filed Dec. 25, 1953 3 Sheets-Sheet 2 11W/nvm;

Iimmllv' @ARLES Nov. 7, 1961 C. F. HENDEE LIGHT DETECTORS 3 Sheets-Sheet 3 States are Filed Dec. 23, 1953, Ser. No. 399,870' 7 Claims. (Cl. Z50-207) This invention relates -to a method of and apparatus for detecting light radiation and more particularly to irnproved photomultiplier -tube circuits for determining the intensity of an incident flash of light which has an intensity far below the light level detectable by normal direct voltage operation of photomultiplier tubes.

The conventional photomultiplier tube is a high vacuum device in which the photo-current produced at a lightsensitive cathode is multiplied many times by secondary emission occurring at successive intermediate electrodes or dynodes prior to reaching the collector or anode of the tube. --For normal operation of a photomultiplier tube, a rated operative value of constant direct voltage is irnpressed on the successive tube electrodes to direct electron ow in a direction from the light-sensitive cathode to the anode via each of the dynodes. The resultant output current is alinear function of the incident illumination on the cathode.

However, photomultiplierV tubes are notoriously noisy and the noise is particularly undesirable when the tubes are used to detect low light levels. A recognized'source of such noise is the thermal emission of electrons from the photocathode and dynodes of the tube. Thermal emission of electrons is a sporadic emission of electrons from the tube electrodes which occurs at room temperature but which may increase or decrease depending upon the increase or decrease, respectively, of the electrode temperature. Ordinarily when the tubes are used at very low light levels, it is customary to cool them, for example, with Dry Ice, to low temperatures in order to reduce the noise caused by the thermal electrons and thereby to enhance the precision of measurement by a more favorable signal-to-noise ratio. But such cooling is relatively expensive and cumbersome and, in many cases, inconvenient.

In many circuits which utilize the output from photomultiplier tubes, operating signals stronger than those yielded at the output of a conventional direct-voltage operated photomultiplier tube are required. The desired signal strength may sometimes be obtained by feeding the output of the photomultiplier tube to an amplifying circuit or, more conveniently, by merely increasing the value of the direct voltage at the tube electrodes. However, when the value of the direct voltage at the electrodes is elevated beyond the rated value, gas-generated noise is likely to occur in the tube. This noise is introduced as a result of excessive bombardment of the dynodes and anode by high velocity electrons which release positive particles, or ions, from the bombarded electrodes. These ions ow toward the photocathode and strike intermediate dynodes which release electrons therefrom to produce an unwanted` signal in the output of the tube. The production of gas-generated noise is usually the limiting factor with respect to the degree of gain which can be realized from conventional photornultiplier tube circuits employing a direct voltage on the tube electrodes.

Circuits employing a photomultiplier tube have been used to analyze the intensity of a ash of light as a function of time. This may be accomplished by observing the output of a photo-tube or photomultiplier on the screen of an oscilloscope, if sulicient gain is provided between the photo-emitting surface and the oscilloscope and, if in addition, the light intensity at the photo-emitting surface is greatenough to provide a relatively large number of photo-electrons during any interval of the flash. If either of these two conditions is not met, this method is not feasible.

Y When the intensity of a light ash is low, a photomultiplier tube operating at its rated direct voltage may not be able to determine the intensity or even the presence of the light due to the masking elect of the thermal noise in the tube.

The principal object of the invention is to provide a method and apparatus to determine not only Whether a weak light ash is present but, when it is present, also to determine its intensity and, furthermore, todetermine how its intensity varies with time. Moreover, this invention seeks to provide means by which it is possible to determine the intensity of repetitive light ashes even Where the intensity is so low that, for example, only one photo-electron is emitted for every ashes.

It is also an object of this invention to provide apparatus for the detection of light which because of its lo-w intensity has been heretofore undetectable.

It is a further object4 of this invention to provide an improved photcmultiplier tube circuit wherein thephotomultiplier tube is operated at voltages many times the ratedvdirect voltage without producing gas-generated noise therein. v

It is another object of this vinvention to provide high gain, e.g., of the order of 1000 times higher than with normal direct voltage operation, in a-photomul'tiplie'r tube for a short duration of time. K

It is still another object of this invention to observe the light intensity change during any particular time interval of a weak ash of light.

Yet another object of the invention is to pulse-code communication system.

A further object of the invention is to provide a lig'ht` pulse-object detection system.

It is still another object of the invention to provide a light pulse friend or foe detection system.

Briey stated, in a circuit according to the invention, the electrodes of the photomultiplier tube are normally cut-off from all operating voltage, thereby rendering the tube normally inoperative, and only during a short interval of time corresponding to the interval in which the incoming light intensity is to be observed, are voltages, which may be several times greater than the rated direct voltage of the tube, impressed on the electrodes. By restricting the interval of time during which the photomultiplier tube is on, to one of sutliciently short duration, e.g., one micro-second or less, gas-generated noise current is unable to develop. This effect is obtained even at operating voltages many times greater than the rated direct voltage such that gain is appreciably stepped up while noise is maintained at substantially zero level. The mass of the positive ions is much greater than that of the electrons. Therefore, during the interval in which the voltage is on the tube, these positive ions do not complete one transit between the dynodes and, consequently, cannot generate secondary electrons. l

The output of the photomultiplier tube is applied to an integrating circuit and then it may be indicated, fr example, by means of an electronic micro-ammeter. Consequently, with this pulsing technique considerably higher gains for a short duration of time are obtained in photomultiplier tubes and very few thermalelectrons are received in the integrating circuit; The thermal noise in the tube is reduced by a factor equal to the ratio of the time the voltage is on to the time it is otf.

In accordance with another aspect of this invention, the intensity of a light pulse, or any part thereof, as a function of time, is determined by observing the output of a photornultiplier tube pulsed in accordance with this inprovide a light vention as the tube pulsing operation is displaced in phase throughout the interval of the light pulse, or the desired part thereof. l

Still further objects and advantages of the invention will be apparent from the following description and appended claims considered ingV drawing, in which:

FIG. 1 is a schematic diagram of a Simplified embodi-r mentV of a pulsed photornultiplier tube circuit in accordance with the invention;

i rate is not critical andv may be, for example, at'a rate Vwithin the range from about l to V100,000 pulsesV per,

together with the accompany- FIG, 2 illustrates one form of a pulse generator which may be used in theY circuit in accordance with the invention; Y

FIG. 3 is a'more detailed diagram of another embodiment of a pulsed photomultiplier tube circuitin accordance with the invention; v

FIG. 4 illustrates a light-beam communication system in accordance with this invention;

FIG. 5 illustrates a light-beam ranging system in ac- Y lcordance with this invention; and Y Y n FIG. 6 illustrates a light-beam identification and/or signaling system.

VReferring to the drawings and more particularly to FIG.Y

1, in an arrangement according to the invention there'is i provided a light source 10, for example, a light emitting hydrogen thyratron' which may be controlled toproduce light pulses at a predetermined rate.

Means to activate'-V the light source, such as 'f1-variable trigger rate generator 11 (e.g., an astable multivibrator), is coupled tothe light source 10 through conductor 12 to control the recurrence rate of the light pulses. In this manner, which is known in the art, a light ilash having a duration of less than 1 microsecond may be produced. Y

Y The light-sensitive cathode 13a of a photomultipler tube 13 is Vdisposed to receive'light from the light source 10. The photomultiplier tube 13, illustrated in FIG. l, has nine dynodes, each designated by reference character 13b, disposed between the cathode 13a and an anode 13e and it will be readily apparent that other types of photomultiplier tubes may be substituted.V A voltage divider 14 having a plurality of similar resistors 15, each having a low value of the order of, for example, 100 ohms, is connected betweenthe'cathode 13a and a point of ground potential with intermediate connections or taps thereof connected to the dynodes 13b.

A pulse generator 16, which produces photomultiplier tube operating pulses, is connected to the cathode 13a Vof the photomultiplier tube 13, through a coupling capacitor 17 and,'furthermore, to the dynodes 13b through equally spaced taps on the voltage divider 14. 'I'he pulse generator 16 is preferably provided with means for controlling the pulse amplitude and theV pulse width.

The output voltage from the trigger rate generator 11, in addition to being applied to the light source l10, is also applied'as al synchronizing impulse to the pulse generator variable delay device 18 which provides meter or a recording potentiometer whenever-,Ja permanent record of the output signal is desired.

' 3,008,053 Y j i second.

During the time when the light ash is receivedrat the cathode of photomultiplier tube 13, a voltage pulse hav-Y Y ing an amplitude which may be manytimes the normaldirect voltage operating value'of the photomultiplier tube .13 is applied to the electrodes of tube 13 to direct the flow of electrons from the photocathode 13a to Vthe anode 13e via the dynodes 13b. The voltage pulse-which is derived Vfromthe pulse Ygenerator 16 may have a short duration of the orderV of one-hundredth of a microsecond, but the Vduration may not be shorter than the'transit L from the cathode to the anode of time fofV` the electrons the tube.

The. variable delay'device 18 may be adjusted so thatk the tube 13 is conducting only during a short interval of time corresponding to the interval :in which the incoming light intensity is -to be'observed.` The tubeV conducting period should'not exceed the time it takes to develop gas-generated noise in the tube 13,A which varies with the voltage applied Vtothe tube electrodes but which is usually Vabout Ione microsecond. The routput voltage from the photomultiplier tube 13 Vwhich is applied to the indicating device 21 through the integrating circuit 22, Vis

substantially free fromv gas-generated and thermal noise. Although a' lightsourcel controlled by thenvariable trigger rate generatorll isillust'ratedinfthe drawing,

itis 4understoodthat the photomultiplier'tube circuitin accordance with this invention may be used to detecty the intensity Yof any light which either-originates within-or withoutV the circuit, whether it befconstant, variable or pulsatory light, by proper adjustments in the Vvariable trigger rate generator 11 and the variable delay device 18 to effect coincidence between the Vactivation of the photomultiplier tube 13 andthe desiredY aspect of the incident light. f

` generator which may be used to-supplyivoltage pulses to the photomultiplier tube 13 shoWn'in'FIG. Vl. This pulse,

generator comprises a conventional power supp-ly 23 which is capable of providing direct voltages of, say, from 0 to5,000 volts, a predetermined length of coaxial-cable 24,

" a-resistor 25 connected between the cable 24 and ground,

In the operation of the circuit illlustrated1nFIG. L y

it will be' seen that a light Hash from light source 10 is emitted, under the control of the trigger rate generator 11, and intercepted by the light-sensitive cathode ofjthe photomultiplier tube 13. vThe rate of the voltage pulses from the pulse generator 16 is preferably equal to the y' ate of ,the light pulses .fromtherlight source 10. This only a brief description.

a charging reactor 26 connected between the cable 24 and the power supply 23 and a gaseous triode tube 27 connected to ground and the cable 24 during conduction thereof.V The voltage at the control grid of thegaseous Y triode tube 27 controls the repetition rate of the output voltage pulses developed across resistor 25. The duration of one of these'pulses is determinedY by the length of the coaxial cable `24'which is chargedV by the voltageV from the power supply 23 With'the aid of charging reactor 26.

The voltage from the power supply 23 is applied to cable 24 so that each time when the gaseoustriodeltube 27 discharges, the shield of the coaxial cable 24 becomesnegative and a negative pulse is developedV across the resistor 25. lThis negative pulse can then be transmitted'to the electrodes of the photomultiplier tube 13`through coupling capacitor 17Vtok the voltage divider 14 show n in FIG. 1. YReferring now to FIG. 3, whichis a more detailed'dia- Vgram of another embodiment of a pulsed photomultiplier tube circuit in accordance with this invention, a variable frequency astable :multivibrator 30 having a twin-triode tube 31 supplies `a triggering voltage to the light source` the operacircuit 2S and to the light detector circuit 29, tion of which is similar to the above-described operation ofthe circuit illustrated in FIG., l

Considering iist the light source circuit 28, it is seen that the triggering voltage from the multivibrator 3,0 is applied to the control grid of the amplifier tube section 32.

The amplilied triggering voltage is then applied to rthe Y control grid of a cathode-follower ampliiier tube'section 33 in order toV impedance match the integrating circuit FIG. 2 illustrates diagrammaticrally one form of a pulse and therefore needs These principles have been previously used in light-beam ranging devices. However, an objectionable feature of type of system is that the intensities of reflected light are usually very low and, hence, heretofore have been undetectable.

In a light-beam ranging system, a fast rising light pulse must be transmitted to the reflecting object to obtain satisfactory resolution. This light pulse may be derived satisfactorily from a known light source comprising a pulsed tube, for example, a thyratron, capable of producing a light pulse with a rise time of less than 0.1 microsecond, which may be even as low as 0.01 microsecond. If 0.01 microsecond pulses are used in a ranging device, the resolution is about plus or minus 1.5 meters.

In accordance with this invention, the reflected light is intercepted by a phototube which is pulsed on for approximately a 0.1 microsecond interval of time at a predetermined time after the light pulse has been transmitted to the reeeting object. The output of the pulsed phototube is fed to a sensitive current indicating device through an integrating circuit. The voltage pulses which pulse on the phototube are phased to produce the maximum current at the output of the phototube, i.e., a voltage pulse is applied to the phototube at a time interval corresponding to the time in which a reflected light pulse is received at the phototube. The time interval between the instant at which the light pulse is transmitted from the light source and the reflected pulse is received in the phototube, is a measure of the distance of the reflecting object, i.e.,

w-here d is the distance, c is the velocity of light, and l is the time interval.

The light pulse and the phototube may be pulsed repetitively at rates up to at least 20,000 pulses per second. By using this pulse technique and current integration, it has been found that ten photoelectrons per second can be readily detected. This means that only one effective reected photon out of every 2,000 pulses is required to detect the delay time of the reflected light-beam.

FIG. illustrates a light-beam ranging system in accordance with invention. The trigger rate generator 11 connected to the light pulse source 78 controls the transmitted light pulse repetition rate. The transmitted pulses 79 are intercepted by the photocathode 13a of photomultiplier tube 13 after they lare reflected by an object 80.

The trigger rate generator 11 is also connected to the photomultiplier tube 13 through a variable delay device 18 and a serially connected adjustable pulse generator 16 in the manner described hereinbefore with reference to FIG. 1. The variable delay device 18 is adjusted to a position at which the reflected light pulses and the voltage pulses from the pulse 'generator 16 coincide in time at the photocathode 13a. The output of the photomultiplier tube 13 is then fed from the anode 13e to an integrating circuit 83, to which is connected an indicating device 84. The variable delay device 1S may be calibrated to give direct readings of distance between the photomultiplier tube 13 and the reflecting object 80.

In light-beam radar systems utilizing photomultiplier tubes operating on direct voltage, light from the light pulse source which is scattered by the atmosphere and objects in the immediate vicinity of the transmitter, produce a strong undesirable signal in the photomultiplier tube which tends to overload the tube for an interval of time immediately following the transmission of each light pulse. The recovery time of the photomultiplier tube after being overloaded is greater than the duration of the back-reflected light. A reflected light pulse received at the tube from the reflecting object during the recovery time is undetectable due to the large and noisy output of the photornultiplier tube caused by the back-scattered light. Since in the present invention the photomultiplier tube is not pulsed on until the light pulse from the re flecting object is received, back-scattered light cannot overload the tube. Therefore, the present invention eliminates the :need for an opaque partition between the light pulse source and the receiver, which is ordinarily necessary to minimize the effect of back-reflected light when operating a photomultiplier tube in `a light-beam radar system on direct voltage.

Yet another embodiment of this invention relates to a receiver in a light-beam identification and/or signaling system.

In a military operation i-t is very often desirable to identify an object as friend or enemy. This can be accomplished by equipping all friendly objects with a light source that can be pulsed on in any preset series of pulses whenever a proper signal is received. This signal may be received from a radio or light-wave receiver that is tuned to a proper frequency for receiving a signal from a central interrogating transmitter. Y

At the interrogating transmitter, or in line of sight o the interrogated object, a light receiver is disposed to receive the light pulses from any object that is friendly. Enemy objects, not having the proper system equipment, do not transmit the preset signal when interrogated and are thereby identified.

In accordance with this invention the light receiver includes a photornultiplier tube having operating voltage pulses applied thereto only during the time that a light pulse is anticipated. Consequently, the sequence of the operating voltage pulses corresponds with the preset coded sequence of light pulses from the friendly light transmitter.

This system has the following advantages over a system using a detector which is continuously sensitive: (1) great gain is obtained by pulse operation; (2) a predetermined code can be used, thereby providing greater security against enemy duplication; (3) the possibility of enemy jamming is almost completely eliminated; and (4) interference from other light flash sources, such as exploding shells, searchlights, etc., is eliminated. Furthermore, greater secrecy may be obtained by transmitting infra-red or ultra-violet light instead of ordinary artificial or natural light.

In FIG. 6 there is illustrated `a light beam identification system in accordance with this invention. A synchronizing device 85, for example, a radio transmitter, either broadcast or directional, located preferably at station #1, which may be -a central interrogating station, transmits a signal which is received in receiver 86 of station #2. at the interrogated object. A pulse generator 87, connected to the output of receiver 86, produces voltages pulses when a signal is lreceived by the receiver 86. The voltage pulses from the generator i87 are passed through a coder 88 and applied to the light pulse source 89 in a coded sequence. A preset coded sequence of light pulses is then transmitted from the light pulse source 89 to station #l wherein the light pulses are intercepted by the photocathode 13a of the photomultiplier tube 13.

The signal from the synchronizing device is also picked up by the receiver 90 at station #l and applied to the pulse generator 91 which sends voltage pulses through a variable delay device 92 to coder 93. This coder 93 applies a preset coded sequence of operating voltage pulses, corresponding to the sequence of light pulses, to the photocathode-13a and dynodes 13b of the photomultiplier tube 13, which is constructed and operated as described hereinbefore with reference to FIG. 1. The variable delay device 92 is Vadjusted to a position at which the light pulse sequence and the voltage pulse sequence are in phase at the photocathode 13a of the photomultiplier tube 13. It can be readily seen that the variable delay device 92 may be inserted in other parts of the system to produce the desired phasing between the light pulses and the voltage pulses. The identification of friend or foe may be made, for example, by means of au indicating device 94 34, 35 which is connected across resistor 36; The rheo- Y stat 34, which is part of the integrating circuit 34, 3.5,. is used as a continuous variable delay device for the positive voltage which is derived from the resistor 36. When the positive voltage across capacitor 35 of the integrating cir-

V cuit

34, 35 reaches a predetermined'value, the gaseous Y. triodc or thyratron 37 ofthe pulse shaper r55is caused to fire. During the firing or conduction of the thyratron 37, a positive signal is taken from across'resistor 38 toptovide a positive'pulse to operate the blocking oscillator 39, which includes the Y triode section 40. The outp-ut of the blocking oscillator39 is taken yfrom the tertiary` winding 41 of the oscillator 39 and applied to thev control grid of another cathode-follower amplifier tube section 42-which provides a low impedance trigger to the light-emitting hydrogen thyratron 43. The filter-'circuit 44' is inserted be: tween the thyratron 43 and Ythe blocking oscillator 39't'o prevent transients from vreilecting back to the blocking oscillator 39 when'the thyratron 43 tires. The pulse-'forming vnetwork 45 whichis coupledjtothe Vthyratron 43, controls theV duration of the .light pulses emitted Y:from thyranature of the modulating signal are transmitted. An example of a known delta modulation system wherein intelligence is conveyed via electrical Ypulses is fully described in the Phi-lips Research Reports, volume .7," pp. 442- 466,-December 1952, andfPhilips Technical Review, volume13,'pp.237-268March11952. y

Iii-accordance with, ,theV present invention, the delta modulated light pulses `are radiated and intercepted'to establish a communication In this light-beam sysiteni the` receiverl is la light-,sensitive device, namely a Y photonuiltiplier tube, .which` produces an electrical pulse each time a transmitted light pulse is received; V Thedetectoris made sensitiveto lightonly during the interval of time during which a;ligl:tt'pulse expected. Ait other times, tllatY is',in' theinter'pulse time,the light-sensu tive device is made inoperative and, therefore'. noise can'- n not enter the system during this time. 1

tron 43. The ariode'voltage for the thyratron 43 is fed Y through the hold-off' diode. 46,1 which Vprevents inverse voltage from getting back into the power supply circuit, not shown, which is connectedat point 47.` The pulseforming network 45 and thehold-oft' diode 46and associated circuitry are conventionally used ywith thyratrons and, therefore, further explanation is deemedunnecessary.

Now, considering the light detectorcir'cuit29, itis seen that the triggering voltage `from the multivibrator j is also applied to the control grid ofthe 'ampliiier tube section 48, The amplified triggering voltage is then applied to the vcontrol grid of a ,cathode-follower'amplifier tube section 49 which provides a low impedance trigger through Vthe step delay line S0 to the thyratron 51. 'f l'he step delay `line` 5l) may have a plurality; of `step,pos`itions, eg., three step positions, as shown in FLG. 3. Position a may represent no delay; position bA one microsecond delay; and position 'c two microseconds delay. Thelength of coaxial cable 'S2 determines ,theV width or the output pulserrfrom The receiver in communication system,which is illustrated in FIG. 4, is kconstructed.andoperated-similarlyto the circuit described iii-FIG. lfand, therefore', only Y 'abrief description is deemed` necessary. Y

"FIG, 4shows a transmitter57 traiismittingflightpulses 58 to a receiver 59 which is synchronized with the trans'- mitter by `a'signal from the synchronizing device 60 which may be, tor example, Va radio transmitterl or a-` transmissionV Line. The block diagramof transmitter 57Vinclu`des a pulse generator v61l u'rhichfsuppliesjav series'of identical equidist-ant pulses 62 to the junctionpoint 63T through a r pruls'e modulator 64 with either unchanged -oropposite polarity'.V VThe action ofV the pulse *modulatorV 64 is gov'- erned bythe polarity ofthe difference voltage obtained ,y from the difference meter-65 by comparison of the modthyratron 51 in a manner which Vis well-known in the art. The positive pulse from Athyratron-Sl is then amplified in the amplifying tube 53 where itis inverted and applied as a negative pulse of high amplitude to the'photomultiplier tube 13 through capacitor 54. The photomultip-lier tube 13 and its output circuit, which includes the integratiug'circuit ZZ'and the electron microarnineter 56, may

be similar in design and operation to the photomultiplier tube' and its output circuit illustrated in FiG. l,

Although pulsed photomultiplier tubes are known in the art, this invention provides an improved photornu'l'ti'plier tube' circuit `which can determine the intensity Iof light which has a lower intensity than any intensity heretofore detected and, furthermore, to determine the intensity of this light asa function of time.VV

From the foregoing description it` can be seen that the present'invention also provides an improved photomultiplier tube circuit in which the photomultiplier tube is s-ubstantially free from gas-generated and' thermal noise even Y when-,operated at voltages havinga Vmagnitude many timesthatof its rateddirect voltage.

inasmuch as the invention makes it' possible to detect light pulses of extremely small intensity values, it will be evident that' the invention has'particular utility in connection with communication systems making use oflig'ht pulses whose periodicity, amplitudeduration or phase 'position' is modulated'as a function of an intelligence or where fis the reduction factor, PWA is the voltage pulseY Vulating Vsignal y66 and the lstep-like approximating signal 67ffonm'ed ironrtheseries of pulsesV 68by integratiorrvin the integrating network 69. The suppressor 70 suppressesV the negative pulses'and passes the signal .'71`to the light K v pulsesoua'cez'TZ which: produces light pulses581 corr'e-VV Y spondingL to the electrical signal 71..

The iight pulses ss are intercepted-by the pimp-catliode 13a of the photomultiplier tube V1 3 in Vthereceiver 59. The trigger rate :generator 1v1,- thevariablev delay device 18 and -the adjustable pulse'vgene'rator 16 aretinred andadjusted to apply a pulse voltage to the photocathode 13a and dynodes 13b through the voltage divider Y14 each time a light pulse isexpected atthe photocathodea. When theV signal pulses at theV anode of the tube 13 are applied to theintegnating circuit 22, asignal 7'3 is pro:-

duced thereacrosswhich is a close approximation of the originalsignal 66. The signal appearing across theintegratin-gecircuit ZZ is supplied to a utilizing device 76, for Vexample,a loudspeaker, via a low-pass filter 74y and amplifier 75-suppressing` inter alia the pulse. recurrence Y frequency. Other suitable circuits,` such las those de#` scribed in the above-'mentioned publications, may be coupled-to the outputgofith'e tube to reproduce the original signal. Y y Y advantages. of this Vlight-beam communicatiousystem are:Y (l) no gas-generated noise; `(2) thermal noise is reduced by the factor i width, and Pr is the voltage pulse repetition rate; and (3) v extremely high gain is possible.V v Ain ,important advantage of this communication system over other types of light-beam systems is its ability to Y operate when the light signal received is so weak that only one photoelectron isY emitted for each pulse received. c

Other systems, eg., amplitude modulation and pulse fre-Y quency modulation, levels.VV

A still Yfurther embodiment of this' inventicnjrelatesto a light-beam ranging system` forideteimining the distance to areeoting object in accordanceV with radar principles."

require; considerably higher light such as an electron microammetenwhich is connected to the output of the photomultiplier tube 13 through the integrating circuit 95.

It is to be understood that the above-described embodiments are illustrative of the application ot the principles of this invention. Numerous other embodiments may be devised by thoseY skilled inthe art to which this invention pertains without departing from the scope thereof.

Having thus described my invention, what I claim and desire to secure by Letters Patent is:

1. In -a light detector, the combination comprising a photonlultiplier tube including a plurality of dynodes, said tube having the characteristic of producing gas-generated noise signals when voltages lhaving relatively high values are applied to said dynodes for more than a relatively short period of time, resistance means connecting said dynodes to va point of reference potential to render said tube normally inoperative, a source of electrical pulses having a period of time shorter than the time in which said gas-generated noise is normally developed in said tube, and means connected to apply said pulses to said dynodes.

2. In a light detector, the combination comprising a photomultiplier tube including a plurality of dynodes and a cathode, said tube having predetermined operating voltage ratings for said dynodes and said cathode, said tube having the characteristic of producing gas-generated noise signals when voltages having relatively higher Values than said voltage ratings are applied to said cathode and dynodes for more than a relatively short period of time, resistance means connecting said dynodes and cathode to a point of reference potential to bias said cathode and dynodes at a value to render said tube normally inoperative, means providing direct voltage operating pulses for said dynodes and said cathode having values substantially in excess of said voltage ratings for a period of time shorter than the time in which said gas-generated noise is developed in said tube, and means connected to apply said pulses to said dynodes and said cathode, respectively.

3. A light detector comprising a photomultiplier tube having electrodes including a photocathode, a plurality of dynodes and an anode, said ftube having the characteristic of producing gas-generated noise signals when voltages having relatively high values are applied to said dynodes for more than a relatively short period of time, a. plurality of series-connected resistors having one end connected to said photocathode, the other end connected to ground and the junctions therebetween connected respectively to said dynodes, an indicating device, an integrating circuit coupling said device to said anode, and a source of unidirectional pulse voltages having a predetermined low value and a predetermined high value coupled to the electrodes of said photomultiplier tube through said series-connected resistors for directing the ow of electrons from said photocathode via said dynodes to said anode, the duration of each of said unidirectional pulse voltages being less than the time in which said gas-generated noise is developed in said photomultiplier tube, said low value being of a magnitude producing substantially cut-oi conditions in said tube.

4. A light detector, `as set forth in claim 3, including voltage-determining means connected to control said values of pulse voltage whereby said predetermined low value is zero and said predetermined high value is subswantally greater than the rated direct voltage for the electrodes of said photomultiplier tube, and including means connected to control the duration of said pulse voltages whereby the duration of each of said pulse voltages is not greater than one microsecond.

5. A light detector comprising a photomultiplier tube having a photocathode, a plurality of dynodes `and an anode, said tube having the characteristic of producing gas-generated noise signals when voltages having relatively high values are applied to said dynodes for more than a relatively short period of time, a voltage divider having a plurality of equally spaced tapsconnected between said photocathode and ground, said taps being respectively connected to said dynodes, said voltage dividerv being adapted to bias said cathode and dynodes at a value to render said tube normally inoperative, a current-sensitive device, an integrating circuit coupling said device to said anode, a pulse generator coupled to said photomultipl-ier tube through said voltage divider for applying to said photocathode and said dynodes voltage pulses having a magnitude substantially greater than the magnitude of the rated direct voltage of said photomultiplier tube for directing the ow of electrons from said photocathode to said anode, the duration of each of said pulse voltages being less than the time in which said gas-generated noise is developed and being not greater than one microsecond, and a variable trigger rate generator coupled to said pulse generator to control the pulse repetition rate thereof.

6. A light detection system comprising a photomultiplier tube having a photocathode, a plurality of dynodes and an anode, said tube having lthe characteristic of producing gas-generated noise signals when voltages having relatively high values are applied to said dynodes for more than a relatively short period of time, a plurality of series-connected resistors having one end connected to said photocathode, the other end connected to ground 'and the junctions therebetween connected respectively to said dynodes, the series resistor connection between said dynodes and cathode and ground providing a bias on said dynodes and cathode that renders said tube normally inoperative, an indicating device, an integrating circuit coupling said device to` said anode, a pulse ygenerator coupled to said series-connected resistors for applying to said photocathode and said dynodes voltage pulses having a magnitude many times the magnitude of the rated operative direct-current voltage of said photomultiplier tube, the duration of each of said pulse voltages being less than the time in which said gas-generated noise is developed and being not greater than one microsecond, a variable trigger rate generator coupled to said pulse generator to control the pulse repetition rate of the voltage pulses, a light source for producing pulsatory light and disposed to transmit said light to said photocathode, said variable trigger rate generator being coupled to said source to control the repetition rate of said pulsatory light, and a variable delay device interposed between said trigger rate and said pulse generators for phasing the voltage pulses from said pulse generator with respect to the light pulses from said light source.

7. A light detection system comprising Ia variable trigger rate generator producing voltage pulses, a first pulse amplifier coupled to the output of said generator, a rst pulse Shaper, a continuous variable delay device coupling said amplifier lto said shaper, =a blocking oscillator coupled to the output of said Shaper, alight source comprising a gas discharge tube, a lter circuit coupling the output of said blocking oscillator to said light source for preventing transients from retlecting back to said blocking oscillator, a second pulse amplifier coupled to the output of said variable trigger rate generator, a second pulse shaper providing shaped pulses each of which has a duration less than one microsecond, a step delay device coupling said second amplier to said second shaper, a photomultiplier tube having a photocathode, an anode and a plurality of dynodes, said tube having the characteristic of producing gas-generated noise signals when voltages having relatively high values are applied to said dynodes for more than a relatively short period of time, said photo cathode being disposed to receive light from said light source, a plurality of series-connected resistors having one end connected to said photocathode, the other end connected to ground and the junctions therebetween connected respectively to said dynodes, the series resistor connection between said photocathode, dynodes and ground providing a bias on said dynodes that renders Yamplifying and applying said shaped pulses Withga nega- Y said. tube nQrmally inoperative, a ,thi'd pulse amplifier,

tive polarity to said photocarthode and throughY said re-V sistor to said dynodestqrender said Vtube operativeY Vin a pulsatory manner, said shaped pulses having durations less than the .time in `which said gas-'generated rnoise Ais developed, a' current-sensitive device, and ian integrating circuit coupling said current-sensitive device to said anode.

References Cited in the lile of patent `UNITED STATES PATENTS 2,234,329 `Wolff Mar. 11, 1941 v2,524,807 Y2,538,062 n ,1 2,594,703 Y Handen Nov. 5, 1957