US2465667A - Method of and means for controlling the beam current in television camera tubes - Google Patents

Description

March 29, 1949. R. R. THALNER 2,465,657

METHOD OF AND MEANS FOR CONTROLLING THE BEAM CURRENT IN TELEVISION CAMERA TUBES Filed Jan. 9, 1945 CTRO/V MUL 77 PL 152 86 VIDEO OUTPUT R 5a a & Q 8 a W E g Q \g I in so? IN VEN TOR zeoaaer k. THAL/VEK.

A TTORN EV Patented Mar. 29, 1949 UNITED STATES PATENT OFFICE METHOD OF AND MEANS FOR CONTROL- LING THE BEAM CURRENT IN TELEVISION CAMERA TUBES Robert R. Thalner, Princeton, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application January 9, 1945, Serial No. 572,009

pickup tube known as the Orthicon tube that distinguishes it from the iconoscope is the use of a beam of low-velocity electrons for scanning the mosaic. In one species of Orthicon tube, the image of the object to be televised is focused on a translucent mosaic and there sets up an image as a stored electrostatic charge. The beam of scanning electrons, impinging on the reverse side of the mosaic, returns the charge distribution to equilibrium. Since the scanning beam strikes at a given instant only a particular point of the mosaic, if that point has previously lost a negative charge then a scanning electron will be collected from the beam. The number of electrons so collected at that point will depend on the magnitude of the electrostatic charge which has been developed and which, in other words, is proportional to the brilliance of image illumination at that point. In this manner each point in the mosaic is restored to equilibrium according to its needs, and the variations in mosaic potential constitute the signal output.

In the second series of Orthicon tube, known as the image multiplier Orthicon tube, the image of the object to be televised is focused by a lens system on a photosensitive transparent glass plate known as the photocathode. Light passing through the plate excites photoelectric emission of electrons from the sensitive surface, these electrons being emitted in the form of an electron image. This electron image is drawn down the length of the tube to a mosaic surface which is capable of emitting secondary electrons, and since the density of the electrons at any point in the electron image corresponds to the brightness of the optical image at that point, the number of secondary electrons emitted by the mosaic surface will vary in accordance with the brightness of such optical image as projected over the entire surface of the photo-cathode.

Another feature of the image multiplier Orthicon tube is that, while the number of scanning electrons collected at each point on the mosaic depends upon its negative charge deficiency as determined by the density of the electron image at that point, these scanning electrons collected by the mosaic do not enter the signal circuit. Instead, the remaining electrons in the scanning beam or, in other words, those which are not collected by the mosaic, return toward the end of the tube containing the electron gun. These returned electrons then pass through an electron multiplier structure, and the output of this electron multiplier constitutes the signal current.

In the above arrangement it is seen that the signal output current from the scanning 0r camera tube is the original beam current as produced by the electron gun less the number of electrons subtracted from the beam as it scans each point of the mosaic, the number subtracted in this manner being proportional to the density of the electron image on that particular mosaic portion and hence the strength of the illumination on the corresponding point of the photocathode.

Accordingly, the electron beam from the gun of the image multiplier Orthicon 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 released by the scanning beam is controlled by the potential of the mosaic just under the beam or, as above stated, on the negative charge deficiency of the mosaic at that point. If the charge deficiency of any mosaic portion is zero as a result of zero illumination being received by the corresponding point on the photocathode. then all of the electrons present in the scanning beam at that instant will be returned to the electron. multiplier end of the scanning tube. The number of electrons so returned will be approximately equal to the number emitted by the electron gun or, in other words, the returned electrons will approximately equal in value the beam current of the tube. The voltage drop attained by passage through an external resistor of returned electrons equal in number to those in the emitted scanning beam constitutes the black level of video signal output.

At the other extreme is the condition when none of the electrons emitted from the gun is returned from the -mosaic. This occurs when all of the electrons in the scanning beam are required to make up the negative charge deficiency on a particular mosaic portion due to the extreme brightness of the corresponding point on the photocathode. The zero voltage thus developed across an external resistor may be considered to constitute the White level of video signal output.

It will be clear from the above description that under the ideal conditions set forth, the emitted scanning beam is modulated from zero (black level) to 100% (white level). It will be further appreciated that to produce 100% modulation for the white portions of the image being televised, all of the scanning electrons must be collected by the mosaic and none allowed to return toward the electron gun end of the camera tube. Inasmuch as the number of electrons in the scanning beam at any instant is substantially constant, it is obvious that there is only one value of negative charge deficiency at which any particular point on the mosaic will collect all of the electrons present in the beam and be completely neutralized thereby. This value of negative charge deiiciency of a mosaic portion at which all of the electrons in the scanning beam are collected to produce complete neutralization represents the maximum value of brightness permissible on any one point on the photocathode without introducing distortion into the output of the camera or scanning tube.

The above statement will be understood when it is considered that an increase in brightness eyond the maximum value mentioned will increase the negative charge deficiency on the mosaic to a point where there are insuflicient electrons present in the scanning beam to make up the deficiency. Distortion of the televised image will accordingly result. Since undermodulation of the scanning beam of an image Orthicon tube is undesirable, the beam current is usually set so that 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 object exceeds this anticipated level, necessitating manual ad ustment of the value of the beam current. On the other hand, should the max-mum brightness of the object be below the anticipated level. the noiseto-signal ratio in the output is increased unless a manual adjustment is made. i

The present invention thus has as one of its features the provision of a method and means for maintaining the beam current in a television camera tube of the Orthicon type fully modulated 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 Ligh beam current under low light conditions. Broadly, this is accomplished by utilizing the secondary electrons emitted when the electron image produced by the photocathode impinges on the mosaic. These secondary electrons emitted from the mosaic bear a direct relation to the overall density of the electron image, which, in turn, is proportional to the average strength of the overall illumination on the photocathode. Consequently an increase in illumination from the object being televised increases the number of secondary electrons emitted from the mosaic, and these secondary electrons are collected and caused to produce a voltage dependent upon the strength of the illumination. The Voltage thus produced is applied to the grid of the electron gun of the Orthicon tube so as to vary the beam current of the tube in such a manner that the beam current is of a value just sufficient to discharge the mosaic under the light conditions then prevailing.

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 of the Orthicon type.

A further object of the invention is to provide a method and means for maintaining a modulated beam current for an image multiplier Orthicon tube regardless of variations in the illumination received on the photocathode thereof.

A still further object. of the invention is to provide. a method and means for collecting the secondary electrons emitted when the electron image produced by the photocathode of an image multiplier Orthicon tube impinges on the mosaic, and for utilizing these collected secondary electrons to control the beam current of the tube.

Other objects and advantages will be apparent from the following description of a preferred form of the invention and from the drawing, the single figure of which illustrates schematically a preferred form of circuit incorporating the present invention.

In the drawing is shown a television image pickup or camera tube It of the Orthicon type. A tube of the general type to be described is referred to in detail and claimed in Patent No. 2,433,941 granted January 6, 1948 to P. K. We'lmer. Accordingly, a detailed description will not herein be included, but the camera tube will be understood to include a photocathode [2 on which an image of an object i4 is focused by means of a lens l6. Photocathode I2 is connected to the negative terminal of a battery I! or other source of potential. Illumination falling on photocathode l2 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 l2.

Electrons thus emitted from the surface of photocathode B2 are accelerated by an accelerating electrode which is shown as an annular band of metal Ha on the Wall of tube it but which may be of any other suitable type, and which is connected to the positive terminal of the battery ll, toward a mosaic electrode element 18 which is maintained at positive potential with respect to photocathode I2. While mosaic element I8 is preferably composed of semi-conducting material as set forth by the mentioned Weimer patent, it may if desired be of the so-called double mosaic type as disclosed by W. Hickok in U. S. Patent No. 2,047,369, granted July 14, 1936. The photocathode structure IZ may readily be formed as disclosed by Patent No. 2,213,547, granted to H. A. Iams on September 3, 1940. A suitable electron lens (not shown) which may, for example, be of the type disclosed in the mentioned Iams patent, is employed to focus on the mosaic electrode I8 the electrons emitted from the surface of photocathode l2.

Electrons impacting the mosaic E8 in turn cause secondary electrons to be released therefrom, the number of secondary electrons released by each arriving primary electron being governed by the velocity of the primary electrons at the moment of impact. Such velocity is determined principally by the voltage of battery I! as applied to accelerating electrode Ma, and this voltage is preferably chosen so that more than one secondary electron is released from. the surface of mosaic element [8 for each impacting primary electron, or in other words the secondary emission ratio is greater than unity.

The secondary electrons released from mosaic I8 are collected by a screen 20. The number of secondaryelectrcns collected by screen-20 is sub-- stantially proportional to the overall density of the electron image. Since the latter is a function of the average brilliance of the overall illumination received by photocathode 12, it will be appreciated that variations in this average brilliance will produce corresponding variations in the number of electrons collected by screen 20.

Therelease of secondary electrons by a particular image point or element of mosaic [8 leaves such element with a positive charge or, in other words, with a negative charge deficiency. The amount of such actual deficiency is dependent upon the density of the electron image at that point.

The positively charged mosaic I8 is then scanned by means of an electron beam produced by an electron gun at the opposite end of tube l0, this electron gun being of any suitable type which includes a cathode 22 and a grid 24. The beam deflecting means of tube I0 is conventional, and is disclosed in a magnetic form by the aforesaid Weimer patent and in a combination of electrostatic and electromagnetic form in Iams and Rose Patent No. 2,213,175, granted August 27, 1940. It is consequently omitted from the drawing for the sake of clarity.

As the scanning beam travels across the surface of mosaic l8, electrons from the beam neutralize the positively charged picture elements. The beam normally supplies sufficient electrons to make up the negative charge deficiency of each image point or element. If a particular element is not positively charged, or if such positive charge is small enough so that all of the electrons available in the scanning beam during the instant of passage are not required to make up the negative charge deficiency on that element, then the remaining electrons 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 H3 from which they were emitted. Upon arriving at the end of tube l0 containing the electron gun, the returned electrons pass through an electron multiplier 26 (as disclosed by Weimer, supra, for instance) the output of which, as shown, constitutes the video signal.

In accordance with the present invention, the secondary electrons collected by screen 213 flow to ground through two resistors 28 and 393, and then return to photocathode E2 to complete the circuit. This flow of electrons causes a charge to be built up on a condenser 32 which is connected in parallel with resistor 23!], the charge being of such polarity that the upper plate of the condenser (as viewed in the drawing) is negative. Since, as above brought out, the number of secondary electrons collected by screen M is proportional to the average overall illumination on photocathode I2, it follows that the charge built up on condenser 32 will also be proportional to this average illumination.

The time constant of resistor 38 and condenser 32 is chosen to give a satisfactory D.-C. variation with respect to changes in illumination, being preferably in the order of the time required to scan several frames. In any event, such time constant should not be less than one line scanning interval, as otherwise instantaneous A.-C. variations will result which are objectionable for reasons which will later become apparent.

The voltage developed on condenser 32 is applied to the grid 34 of an electron discharge tube 36, grid 34 being connected to the negative terminal of a low voltage source (not shown) through a resistor 3?. Tube 36 is designed for substantially linear operation, so that an increase, for example, in the negative bias on grid 34 produces a proportional decrease in current through load resistor 38. Such a decrease in the plate current of tube 36 causes the positive potential at point 40 to increase in proportion to the increase in charge on condenser 32.

The voltage at point 40 is applied to the grid 24 of the camera tube It! through an additional resistor 42, and a still further resistor 44 is connected between the upper end of resistor 42 and the negative terminal of a source of high voltage (not shown), the lower end of resistor 33 being connected to the positive terminal of the same high voltage source.

The values of resistors 38, 42 and 44 are preferably so chosen that point 46 is of negative polarity regardless of the operating status of tube 36. In other words, resistors 33 and 42 together are greater in value than resistor M by such an amount that point ill (and thus grid 24 of the camera tube ill) remains negative even when point- 45 is at its maliirnum positive potential, as occurs when minimum current is passing through load resistor 38.

From the above it will be appreciated that when the average overall illumination on photocathode 62 increases for any reason, there will be an increased number of secondary electrons collected by screen 29 from mosaic l8, an increased electron flow through resistor 39, and an increase in the charge on condenser 32. This will cause a more negative voltage on 3 1. As grid 34 becomes more negative, the plate current through resistor 38 will decrease, thereby causing the voltage at point 2-8 to become more positive. This increase in positive potential at point 4!) causes point 46, and consequently the camera tube grid 24, to become more positive, thus increasing the current in the scanning beam. By a proper selection of circuit constants, the controlled scanning beam current can be made just sulficient to discharge the mosaic it under the increased illumination conditions. Since discharge tube 36 is designed for linear operation, any decrease in average overall illumination on photocathode l2 will bring about a decrease in the beam current in the same manner as for the increase taken above as an example.

It is desirable in camera tubes of the type herein described that the video output be at black level during retrace. This may be accomplished by adding negative blanking pulses at point 48, these negative blanking pulses causing all of the electrons in the scanning beam to be repelled from the mosaic and returned toward the multiplier end of tube Ill. Accordingly, it will be appreciated that it readily becomes possible now to reinsert the D.-C. component representative of the average image brilliance at any point in the system (prior to transmission) where such component has become lost due to amplification in the socalled A.-C. amplifiers, or the passage of the resulting video signal output through transformers or the like.

While the invention discloses a preferred form of circuit to accomplish the above-stated aims and objectives, it will be appreciated that various modifications may be made without departing from the spirit or scope of what is herein disclosed and claimed.

Iclaim:

1. In a television system having a camera tube of the low-velocity scanning-beam type, the method for automatically adjusting the beam current to varying light intensity which comprises, developing in said tube secondary electrons sub stantially proportional in number to the overall light intensity received in said tube, collecting and storing said secondary electrons, deriving a proportional voltage therefrom, and using said voltage to adjust the beam current in direct proportion to said seccndaryelectrons.

2. The method according to claim 1 wherein the secondary electrons are stored over a period of not less than one line scanning interval, and up to the time required. to scanseveral frames of said beam.

3. In a television system, a camera tube of the low-velocity scanning-beam type, said tube containing means to control the beam current and a mosaic electrode which releases secondary electrons substantially proportional in number to the overall light intensity received in said tube, means for storing said secondary electrons and developing a proportional voltage therefrom, and means for applying said voltage to said beam control means in a direction to vary the beam current in direct proportion to said secondary electrons.

4. The combination defined by claim 3 wherein said storing means comprise a condenser, and said voltage is the charge in said condenser.

5. In a television system, a camera tube having an image section and a scanning section separated by a mosaic electrode, means in said image section including a photocathode for emitting an electron image repersentative of an optical image illuminating said photocathode, means for continuously accelerating the electron image toward said mosaic electrode, an energy storing device, means responsive to the efiect of the electron image on said mosaic electrode for feeding to said energy storing device an amount of energy substantially proportional to the overall illumination on said photocathode, and means for appl ing the energy thus stored to said scanning section in a direction to vary the beam current in direct proportion to the overall illumination on said photocathode.

6.. The-combination defined by claim 5 wlierein said last. mentioned means comprise an electronic amplifier having an input circuit connected to said energy storing device, and whereinsaidseanning section contains a control grid connected to. the output of said amplifier.

7. A system according to laim 5, in which the saidzenergystoring device includes a resistancecondenser combination having a time constant equal at least to the time required for the said scanning "beam to make one scansion of the said mosaic electrode.

S In a-television system, a camera tube, of the type in which an electron image is caused to beprojected upon a mosaic surface to thereby release fromqsaid surface secondary electrons in "an. amount dependent upon the average overall density of said electron image, and in which Scanning means operates on the mosaic to produce video signal output voltages, a time constant circuit, means for applying the secondary electrons released from the mosaic surfaceto said time constant circuit so as to develop a, voltage the value of which varies in proportion to the average overall density of said electron image, and means for applying said voltage to said scaning means in a direction to make the intensity thereof directly proportional to said voltage.

ROBERT R. THALNER.

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

UNITED STATES PATENTS Number Name Date 2,084,700 Ogloblinksy June 22, 1937 2,215,186 McGee et al Sept. 17, 1940 2,250,293 Bunger July 22, 1941 2,288,402 Iam June 30, 1942 2,345,282 Morton Mar, 28, 1944 2,404,098 Schade July 16, 1944 FOREIGN PATENTS Number Country Date 490,391 Great Britain Aug. 15, 1938

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