US2451640A

US2451640A - Control system

Info

Publication number: US2451640A
Application number: US593153A
Authority: US
Inventors: Robert R Thalner
Original assignee: RCA Corp
Current assignee: RCA Corp
Priority date: 1945-05-11
Filing date: 1945-05-11
Publication date: 1948-10-19
Anticipated expiration: 1965-10-19

Description

Oat.` 19 1948. R. R. THALNER 2,451,540

CONTROL SYSTEM Fiieduay 11, 1945 '5 sheets-sheet 1 Oct. 19 1948. RJR. THALNER 2,451,640

CONTROL SYSTEM Filed May 1l, 1945 5 Sheets-Sheet 2 VIDE 0 SIGNA/ AMI? OUTPU VIDEO AMR INVENTOR ROBE/er E. THM/VH? ATTORNEY Oct. 19 1948. R, R, THALNER f 2,451,640

CONTROL SYSTEM Filed May 11, 1945 5 sheets-sheet s SIGNAL OUTPUT wot-0 A MP VIDEO A11/IA mvENToR @035er 1Q. HAL/vfe svygw ATTORNEYV Oct. 19 1948. w R. R. THALNER CONTROL SYSTEM 5 Sheets-Sheet 4 Filed May 11, 1945 INVENTOR- af 1Q. THALNER NNN Q5 u ATTORNEY Oct. 19 R R'THALNER 2,451,640

CONTROL SYSTEM Filed May 11, 1945 5 sheets-sheet '5` SIGNA/ OUTPUT V/PEO AMP ATTORNEY igatented ct.

rfisgili CONTROL SYSTEM Robert R. Thalner, Princeton, N. J., assigner to f America, a corporation Radio Corporation o of Delaware Application May 11, 1945, Serial No. 593,153

14 Claims.

The present invention relates to a method and means for controlling the scanning beam intensity ina television camera tube.

In particular, the invention will be foundto be applicable to camera tubes of the so-called low-velocity scanning beam types. A tube known as the Orthicon is thus .one of the types with which lthis invention finds particular use. The Orthicon type camera tube is known, per se, in the art,V and details concerning its construction and general operation may be had by reference to the article entitled The Orthicon, a television pick-up tube, by Messrs. Rose and Iams which was published in the RCA Review for October, 1939, and which appeared, With certain bibliography, in pages 186 to 199 inclusive.

The storage-type pickup tube known as the lOrthicon diiers from other camera tubes used in television transmitting systems in the use of a low-velocity beam of electrons for scanning the mosaic. In an Iconoscopa for example, the cathode ray scanning beam impacts the mosaic element at high velocity, and has the effect of exciting secondary emission from the mosaic surface. Variations in the number of these secondary electrons Which are collected constitute the video signal output, as is already understood in the art.

While both of the above types of tube include a mosaic element on which there is set up an image in the .form of stored electrostatic charges, the V.principle of operation of the Orthicon is such that the electrons in the low-velocity scanningfbeamv may be collected from the beam as 4it passes over the elemental areas of the mosaic surface and employed to .neutralize the positive charges existing .on these areas. The number of scanning kbeam electrons collected at any instant will depend on the magnitude of the negative charge .deficiency possessed by the particular v surface area being instantaneously scanned by the cathode ray scanning beam, and this negative charge deiiciency, in turn, in proportional to the brilliance of the homologously associated point on the optical image to be transmitted.

`Thus, the positive charges on the mosaic, which represent an electrostatic image corresponding point-for-point. to the optical image, areneutralized by the scanning beam electrons. Due tothe differences in magnitude of the positive charges acquired by the various elemental areas of the mosaic surface, the total number of electrons present in the scanning beam is not always required to neutralize a particular mosaic area. "The excess beam electrons or, in

' tube.

other Words, those not collected by the mosaic, are returned toward the electron gun end of the camera tube. Y

In one species of the proposed Orthicon camera tube, the video signal output is derived directlyfrom the mosaic as a result of variations in the number of scanning beam electrons which are collected thereby. In another species, those electrons remaining in the beam after the electrons therein required for yneutralization have been collected by the mosaic are returned to an electron-collecting electrode adjacent the electron gun and there collected to produce the signal current. In the latter arrangement, the signal output current of the 4camera tube is equal to the original beam current as produced by the electron gun less the number of electrons lost by the beam to neutralize the positive charges as it scans each point of the mosaic so that the quantity of electrons lost ,in this manner is proportional to the density of the electron image on that particular mosaic portion and hence the brilliance of the corresponding point on the optical image. Accordingly, the electron beam from the gunof the camera tube is modulated by the mosaic potentials in accordance with the Characteristics of the optical image to be televised, and the modulated electron beam utilized as the video signal output of the tube.

'The number of .electrons instantaneously co1- lected from the vscanning beam is controlled by the potential of the mosaic at the point of instantaneous scanning of the mosaic by the scanning cathode ray beam or, as above stated, on the negative charge deficiency of that elemental mosaic area. If such charge deficiency in zero, as `a result of zero illumination being received from the corresponding point on the optical image, then .approximately all of the electrons present in the scanning beam Yat that instant will be vreturned toward the electron gun. The vnumber of electrons so returned Will be approximately .equal to fthe number originally emitted, or in other Words vthe returned electrons will approximately equal in value the beam current of the Returned electrons approximately equal in number to those in the emitted scanning beam Vwhen collected constitute one limit of video signalcurrent.

At the other extreme is the condition when substantially none of the electrons emitted from the gun is returned from the moisaic. This occurs when substantially all of the electrons present in the scanning beam are required to make up the negative charge deficiency on a particular 3 mosaic area due to the extreme brightness of the corresponding or homologous point on the optical image. The Zero current thus produced in the output circuit constitutes the other video signal limit.

It will be clear from the above :description that under the ideal conditions set forth lthe emitted scanning beam is modulated between the limits of zero -and'100%. It will be further appreciated that to produce 100% modulation substantially all of the scanning electrons must be collected by the mos-alc and none lall-owed 'to return toward the electron gun end of the camera itube. In- -asmuch as the number of electrons in the scanning beam at any instant is norm-ally substaniti'ally constant, it is obvious that `there is only one value of negative charge deficiency .at which any particular elemental mosaic area will collect substantially all of the electrons present in the scanning beam and be completely neutralized thereby. This value of nega-tive charge deiiciency at which substantially all of .the electrons in the scanning beam are collected to produce complete neutralization represents the maximum value of brightness permissible at any one pointJ on `the optical image without introducing distortion into the output of the ycamera tube.

The above statement will be appreciated when it is considered that an increase in brightness beyond the maximum value mentioned will increase the negative charge deciency on the Inos-a'ic element under consideration to a point where there 4are insufficient electrons present in the scanning beam to make up the deficiency. Distortion of the televised image accordingly results.

Since undermodulati-on of the scanning `beam of a low scanning beam velocity camera .tube is undesirable, the beam current is usually set so ythat 100% modulation will result at the maximum brightness level anticipated for the particular object to be televised. It often happens, however, that for various reasons |the maximum brightness of the Iobject exceeds this anticipated level, necessitating manual adjustment of the va'lue oi the beam current. On the other hand, should .the maximum brightness of the object be below the anticipated level, the noise-to-signal ratio in the output is increased unless manual adjustment is made. Y

In my copending U. S. patent application Serial No.572,009, filed January 9, 1945, I have disclosed a method and mean-s'for maintaining the beam current in a low scanning beam velocity television camera tube of the type having a separate photocathodefully'rnodulated regardless of variations in the illumination, thereby producing a uniformly clean and powerful signal with a minimum of shading and, in addition, eliminating the noise due to an unnecessarily 'high beam current=under low light conditions. As shown in this copending application, the above is accomplished by utilizing the secondary electrons emitted when the electron image produced by lthe photocathode of the tube impinges on the mosaic. Since the number of'secondary electrons so emitted is proportional to the average streng-th of the'overall illumination on the photocathode, these secondary electrons are collected and .caused to produce a voltage which is applied to the control electrode of the camera tube gun so as to vary the'beam Y means for utilizing a por-tion of the video signal current of the tube in such a manner that the Y beam `current is of a value just suflicient to discharge the mosaic under the light conditions-then prevailing. Y

The present invention features a method and output of the camera tube to accomplish the same objectives, thus eliminating lthe necessity of ernploying 'a separate circuit for lcollecting and storing the secondary electr-ons emitted from the mosaic by the impingem'ent lthereon of the elec-tron image produced by the photocathode. Furthermore, sin-ce the present invention ldoes not require the presence of secondary electrons to crea-te a compensating control variation, the system disclosed herein is not restricted to a camera tube of the type having la separate photocath-ode. Instead, a camera tube may :be employed in which an object to be televised is focused directly on a so-called -double'mosaic which, if desired, may be of the type disclosed by L. E. Flory in U. S. Patent No. 2,045,984, gran-ted June 30, Y1936, or, if the mosaic is semi-transparent, of the type shown in Paten-t No. 2,150,980, issued March 21, 1939, .to H. G. Lubszynski et al.

One object of the present invention, therefore, is to provide a method .and means for automatically controlling the grid bias in a television camera tube.

Another object of the invention is to provide a method and means for maintaining a substantially modulated beam current for a lowvelocity scanning beam type television camera 'tube regardless of variations in the average overall illumination received from the object being televised.

A further object ofthe invention is -to provide a method and. means for utilizing a portion of the video signal output of a low scanning beam velocity `type television camera tube so as to control the beam intensity of theytube.

Other obje-cts and advantages will be apparent from the following description of preferred forms of the invention and from the drawing, in which:

Fig. 1 illustrates schematically a preferred form of circuit incorporating the present invention;

Fig. 2 is a curve which is referred to in eX- plaining the operation of the circuit of Fig. 1; and

ing alternative larrangements in accordance with the present invention.

-Referring iirst to Fig. 1, there is shown a television pickup or camera tube I 0. TheV elements of tube I0 for producing an electrostatic image are of the Ycharacter referred to in my abovementioned Ycopending U. S. patent'application, Serial No. 572,009. These elements hence are not herein to be described in detail, butthey will be understood to include a photocathode I2 on which an image of an object I4 is focused by means of a lens I6. Photocathode I2 is connected to the negative terminal of a battery I8 or other source of potential. Illumination falling on photocathode I2 causes an emission of electrons from the inner surface thereof, such emission, as is well known in the art, being in the form of an electron image each point of which corresponds in density to the strength of the illumination on the corresponding point of photocathode I2.

The velocity of the electronsV thus emitted from the surface of photocathode I2 is increased by an accelerating electrode 20 which is shown as an annular band of metal onthe Ywall of tube I0 but which may be of .any other suitable type, and which is connected toV an intermediate pointl on battery I8, toward a mosaic electrode 22 which is connected to the positive terminal of battery I8. f While mosaic 22 is preferably composed of semi-conducting material as set forth in a co- Figs. 3, 4, 5, and 6 are circuit diagrams showpending U. s. Vpatent application of Albert Rose, filed September 2.0,' 1940 as Serial No. 357,543, now abandoned, it may, if desired, be of the so- Qalled d011b1m0Sa0 type as disclosed by Flory Patent, No. 2,045,984, above mentioned. The photocathodestructure I2 may readily be formed as disclosed by Patent No. 2,248,977, granted to Flory et al. on July l5., 1941. A suitable electron lens (not shown) which may, for example, `be of the type disclosed in the mentioned Flory et al. Patent No. 2,248,977, or in Patent No. 2,189,319, issued February 6, 1940, to G. A. Morton, is employed` to QGIlS on the mosaic electrode 22 the electrons emitted from the surface of photocathode l2.

Electrons impacting the mosaic 22., in turn, cause secondary electrons to be released therefrom, these secondary electrons being collected by a screen 24. The release of secondaryfelectrons by a particular element or area of mosaic 22 leaves such element with a positive charge or, in other words, with a negative charge deficiency. The amount of such actual deciency is dependent uponthe density of the electron image at that particular point.

The positively charged mosaic 22 is then scanned by means. of an electron beam produced by an electron gun at the opposite end of tube to., this electron gun being of any suitable type which includes a cathode 26, a grid 23 andan accelerating anode (not shown). The beam deflecting means of tube l0 is conventional and be magnetic, electrostatic or a combination. One preferred form may be considered as being of theform U. S. patenty application of Albert Rose, Serial No. 357,543. The deflecting electrode system is consequently omittedr from the drawing for the sake. of clarity and simplicity of illustration.

As the scanning beam travels across the surface ot Inosaic22,A electrons; fromv the beam neutralize the positively lcharged mosaic elements. The beam normally supplies sufficient electrons., to make.v up the negative charge deficiency of each image point or element. If a particular-element isnot positively charged, or if such positive charge issmall enough so that allV of the electrons. available inthe scanning beam during the instant of passage are not required to make up. the negative charge deciency on that element, then the remainingelectrons in the beam or, in` other words,

those not. employed to neutralize. the electrostatic..

charge.: representing each image point or. element, are caused to return along a path substantially parallel with the scanning beam toward the end of,..tube, [0. from which they are emitted. Upon arriving atz the end of tube l0 containing the.

llltron gun, these returnedA electrons, are co1- lected by a signal plate 30 forming a part of. the tubev output circuit. Signal plate 30 may be of any suitable design, such for example, as, a circular disl; having a central aperture through which the scanning beam electrons emitted from cathodel 25 may pass.

The signal output of tube I0 is amplified, if desirable or necessary, by two videor amplifiers 3 4gandA 36. In accordance with the present invention, a portion of the signal outputenergy of tube I is utilizedVv to provide a control variation for the electron scanning beam of the camera. tube ill@ manner now to be described.

In, the cir-cuit shown in Fig. 1, the top plate (in. the, drawing) of a condenser 38 is connected between the two video` amplifiers 34. and 36. The.

other plateof condenser 38 is connectedto ground through a. resistor 40. Shunted across resistor disclosed by the aforementioned apart.

mis adiode 42 having its cathode connectedV to ground. A, second diode 44 has its cathode connected to the anode of diode 42. The anode of diode 44 is connected to ground through a resistor 4.6 in parallel with aV condenser 48.

Diodes

42 and 44 may, if desired, comprise the two sections of a tube of the general type known as the 6H6.

In the waveform 49 of Fig. 2 which is the waveform of a signal such as might be present in the output. of amplifier 34, the video signal variations are of such polarity as to extend in a negative direction to a certain maximum amplitude during each line-scanning interval, this maximum amplitude being indicated by the reference character 50. Extending' in the opposite, or positive, direction from the A.-C. axis 52 of the signal, are blankng pulses 54 formed by the application of a seriesl of blanking pulses 56 (Fig. l) from a suit,- able source (not shown) to screen 24 of tube l0 over conductor 5.8.

A port-ion of the signal output of video amplifier 34 such,l for example, as represented by the Wave.- torm-4$i, is applied. to the

diodes

42 and 44 through condenser 38. When a positive portion of wave 49., such asI one of the blankin-g pulses, 54, reaches

diodes

42 and 44, the former will be rendered conductive to. charge. condenser 38. by an amount determined by the height of the top of the` posi.- tive pulse 54 above the A.C. axis 52.

Duringv the conduction of diode 4.2, both point 56 vand the cathode of diode 44 are at D.C/.l ground potential. When now a negative. portion of wave. 49v reaches

diodes

42 and 44, diode 44 willbe rendered conductive to develop across the resistorcondenser combination 46, 4.8 in. the anode circuit of diode 44 a voltage which isequal to the amplitude of the mostv negative point 5i!` ofv wave 49. taken .withrespect to D..C` ground. Since this D.C. grou-nd level, as described above,A has. been set at the top oi blanking pulse 54, the voltage` across. condenser 48 will be that developed between point 59 and the top of. pulse' 54 or, in other words, will be the peak-to-peak voltage of the signal wave 49.

' When the.- illumination received by photocathode I2 from any point on object I4 increases, the corresponding element of mosaic 22 becomes more positive-a. This results in a decrease in the num.- ber of electrons collected by the signal plate 34 asthe electron beam. scans that particular mosaicelement, and as a result the most positive and negative points on the signal output curve, as,` for example, the point 50 and the top of pulse 54Y on the illustrative waveworm 49, move. further Y This increase. in peak-to-peak signal. voltage increases proportionally the charge on condenser 48..

The time constant of resistor 4S-and condenser 4.& is chosen to give a satisfactory D.C. variation with respect to changes in illumination, be-l 'ingpreferably in ther order of the time required tosscan several frames. In any event, suchA time: constant should not be less than one line-scan.- ning interval, as otherwise instantaneous' A.C. variations will result which are objectionable-for reasons. which will later become apparent.

The` voltage-developed on condenser 48 is ap-y plied tothe control grid 5810i a tube 60 which lss designed to act as a1D..-C. amplifier. inasmuch-Y as-the voltage developed on condenser 4B is negatire' with respect. to. groundy an increase in the:

voltage oncondenserv 4% in response to arl-,increaseinthe peak-to-peak signal voltagel output',

of camera tube. HJ'v places a more' negative bias?. onsthe. control grid 58 ottube-lil). This reduces;

the plate current of tube 60 and causes the plate end of load resistor 62 to become more positive. This decreased potential drop across resistor 62 is applied, by means to be 'hereinafter described, as a decrease in negative bias on the control grid 28 of camera tube i6 to increase the intensity of the beam current of tube l@ to a point where it is just suiiicient to neutralize the charges on mosaic 22 under the increased illumination conditions and thus prevent overmodulation of the scanning beam.

Since tube t) is designed for substantially linear operation, variations in the negative charge on condenser 48 will produce substantially proportional variations in the positive voltage appearing on the plate end of load resistor 62.

To obtain a control voltage for the grid 28 of camera tube l Which is of correct polarity, the positive voltage appearing on the plate end of load resistor 62 is bucked out with a negative voltage from a source of potential (not shown) connected to the terminal 54, this latter source being of such value as to produce a negative voltage at point 68 Which varies in substantially inverse proportion to the changes inY positive potential appearing on the plate end of load resistor 62.

4'Ihe circuit of Fig. 3 is essentially the same as that of Fig. 1 with the exception that two video amplifiers t8 and TQ have been added and the D.C. amplifier tube 6i] and its associated elements omitted. Whereas in Fig. 1 the resistorcondenser combination 45, 48 was connected in the anode circuit of diode 44 to produce a negative charge on condenser 48, in Fig. 3 the same resistance-condenser combination is connected in the cathode circuit of diode 42 to produce a positive charge on condenser 48. The varying .positive charge produced on condenser 48 during operation of the system causes variations in the negative D.C. bias placed ongrid 28 of camera tube lll by a battery or other source of potential 'I2 through resistor 46.

While the systems of Figs. 1 and 3 are satisfactory for many operations, they do not have the same stability as the circuit of Fig. 4 (later to Vbe described), clue to the fact that they are regenerative. If the bias on the camera tube IG is accidentally set too positive, the scanning beam current is caused to reach an unnecessarily highV level; The excessively large number of electrons present in the scanning beam tends to produce a condition known as shading, which has the same eiect, insofar as the output of the camera tube is concerned, as an increase in the amount of illumination falling upon the photocathode. Consequently the output of the camera tube increases, which in turn causes a still further increase in the scanning beam current.

In Fig. fi is shown a system whereby the regenerative operation of the circuits of Figs. 1 and 3 is overcome by the utilization of a limiter tube 'I4 connected across the condenser 8. Whenever the positive voltage on condens-erw increases to a point where it exceeds a predetermined Ymaximum value as determined by the potential of battery T6, diode 'I4 is rendered conductive.l The voltage developed on condenser 48 isthus prevented from reaching a point suliciently high to cause objectionable regeneration.

In Fig. 5 is shown means whereby an automatic video gainV control may be added to the system of Fig. 4. This is accomplished by utilizing a Vparallel resistor-

condenser combination

18, 80 in the anode circuit of diode 44. A negative voltage on condenser sfwill-be developed during operation of the system of Fig. 5 in the same manner that a negative voltage is developed on condenser 48 in the system of Fig. 1.

If the signal fromrcamera tube l0 increases, forY example, there isY need for more scanning beam current and less video gain. In Fig. 5 the Voltage on condenser 48 Will become more positive due to the increase in peak-to-peak signal voltage as hereinabove brought out, and this increase in voltage on condenser 48 is applied to grid 28 of camera tube Ill, as in Fig. 4, to increase the scanning beam current. The voltage on condenser et, however, will become more negative, and this increase in negative voltage on condenser e@ is applied over a conductor 82 to reduce the gain of the video amplifier 36in any suitable manner, such as by increasing its negative bias.

The system of Fig. 6 is similar to that of Fig. 5, except that the gain control voltage is appliedto the signal circuit not only over conductor 22Y after a portion of the signal has been detected by diodes 4.2 and 44, but also over a con-` Vductor 8e before the signal is detected. This tends t0 stabilize the system because it eiectively reduces the gain of the rectifier circuit.l

If the voltage on condenser d8 exceeds a predetermined maximum value, diode T4 is rendered conductive, thereby placing all ofthe voltage that would appear on. condenser 48 across condenser il. This increased negative charge on condenser 3Q is applied over conductors 82 and 8d to reduce the amplitude of the signal and restore the normal operating status of the system. If desired, the limiter tube 'M and battery 'l5 may be omitted from the circuit of Fig. 6. Such an omission, hovvever,` tends for increased instability and possibly excessive regeneration.

Other modications, of course, will be obvious to those skilled in the art to which the inven-l tion is directed. Y

Having now described the invention, what is claimed and desired to be secured by Letters Patent is the following: f 1. In a television system, a camera tube of the type in Which a mosaic electrode Within said tube is adapted tobe scanned by a low-velocity scanning beam to thereby develop an output rectified energy toV control the degree of amplication of said signal in inverse proportion thereto.V l Y 2. In a television system, Va camera tube in which a cathode ray scanning beam is developed to scan a mosaic electrode on which electrostatic charges are developed in proportion to the brillance of an optical image, the signal output of saidtube having a peak-to-peak value which is a measure of the brilliance of said optical image, a peak-to-peak rectifier, means for applying the signal output of said camera tube to said peak-to-peak rectifier, an energy storagel circuit, means for applying the output of said peak-to-peak rectifier to said energy storage circuit so as to develop a relatively smooth D.C`.

' having an' amplitude approximately equal to the peak-to-peak value of the signal output of said camera, tube, and means for applying the output of said energy storage circuit to said camera 9 tube to control the intensity of said scanning beam in direct proportion to said peak-to-peak value of the signal output.

3. A television system in accordance with claim 2, in which the said energy-storage circuit comprises a resistance-condenser combination having a time constant equal at least to the time required for the said scanning beam to make one complete scansion of the said mosaic electrode.

4. In a television system, a camera tube of the type in which a mosaic electrode within said tube is adapted to be scanned by a low-velocity scanning beam to thereby develop output signals, a pair of video amplifiers in the output circuit of said camera tube, a pair of diodes, means for applying a portion of the signal output of said camera tube appearing between said video amplifiers to the anode of one of said diodes and to the cathode of the other of said diodes, means connecting the cathode of said one diode to ground, a parallel resistor-condenser combination, means connecting the anode of said other diode to ground through said parallel resistorcondenser combination, and means for applying the charge developed on said condenser during Aoperation of the system to control the intensity of said scanning beam.

5. The combination of claim 4 in which said last-mentioned means includes a D.C. ampli- Iier, the charge developed on said condenser being applied to control the Vvoperation of said D.-C. amplifier.

6. The combination of claim 2, further comprising means for limiting approximately a predetermined value the energy stored by said energy-storage circuit.

7. The combination of claim 2, further comprising means for limiting to approximately a predetermined value the energy stored by said energy-storage circuit, said limiting means comprising a diode and a source of potential con-.

nected in series across said energy-storage circuit, the positive terminal of said potential source being connected to the cathode of said diode and the anode of said diode being connected to the positive output terminal of said energy-storage circuit.

8. In a television system, a camera tube of the type having a mosaic electrode, and wherein a low-velocity scanning beam is developed and deeoted to Yscan said mosaic electrode to thereby produce output signals, means for amplifying th'e signal output of said camera tube, a pair of diodes, means for applying a portion of the signal output of said camera tube to the anode of one of said diodes and to the cathode of the other of said diodes, a first energy-storage circuit connected between the cathode of said one diode and ground, a vsecond energy-storage circuit connected between the anode of said other diode and ground,

means for applying the output of .said first energy-storage circuit to control the intensity of said scanning beam, and means for applying the output of said second energy-storage circuit to control said amplifying means.

9. In a television system, a camera tube of the type wherein a cathode-ray scanning lbeam is developed and then deflected to scan a mosaic electrode in said tube to produce output signals, a first amplifier and a second ampliiier connected in cascade, means for applying th'e signal output of said camera tube to said first amplifier, a pair of diodes, means for applying a portion of the signal output of said first amplier to the anode of one of said diodes and to the cathode of the other of said diodes, a rst energy-storage circuit connected between the cathode of said one diode and ground, a second energy-storage circuit connected between the anode of said other diode and ground, means for applying the output of said first energy-storage circuit to control the intensity of said scanning beam, and means for applying the output of said second energy-storage circuit to control the gain of both said first and second ampliers.

10. In a television system having a camera tube of the low-Velocity scanning-beam type, the method for automatically adjusting the electron beam current to light intensities which comprises, deriving from said tube a picture signal representing a succession of instantaneous values of light received by said tube, rectifying a portion of said picture signal and deriving therefrom a unidirectional signal proportional to the peak-to-l peak amplitude of said picture signal, averaging said unidirectional signal over a plurality of scanned elds, and using said averaged unidirectional signal to control the electron beam current in direct proportion to said averaged unidirectional signal.

11. The method according to claim 10 including the steps of amplifying said picture signal and using said averaged unidirectional signal to control the amplication in inverse proportion to said averaged unidirectional signal.

12. The method according to claim 10 including the steps of amplifying said picture signal both' before and after performing said rectifying step, and using said averaged unidirectional signal to control both amplirlcations in inverse proportion to said averagedundirectional signal.

13. In a television system, a camera tube of the low-velocity scanning-beam type, said tube containing means forcontrolling the electron current of the beam and means providing a picturesignal output, means for rectifying a portion of said output and deriving therefrom a unidirectional potential proportional to the peak-to-peak amplitude of said output, means for averaging said unidirectional potential over a plurality of scanned fields, and means for applying said averaged unidirectional potential to said beam control means in a direction to vary the electron current in direct proportion to said peak-to-peak amplitude.

14. The combination dened by claim 13 wherein said applying means comprises means for placing Aan upper limit upon the unidirectional potential applied to said beam control means.

ROBERT R. THALNER.

REFERENCES CITED The following references are of record in the le of this patent:

UNITED STATES PATENTS Number Name Date 2,084,700 Ogloblinsky June 22, 1937 2,182,578 Blumlein Dec. 5, 1939 2,222,759 Burnside Nov. 26, 1940 2,292,817 Bedford Aug. 11, 1942 2,307,375 Blumlein Jan.v 5, 1943 2,345,282 Morton Mar. 28, 1944 2,404,098 Schade July 16, 1946 2,404,173 Hansen July 16, 1946