US2108097A

US2108097A - Television transmitter

Info

Publication number: US2108097A
Application number: US742200A
Authority: US
Inventors: Alda V Bedford; Ray D Kell
Original assignee: RCA Corp
Current assignee: RCA Corp
Priority date: 1934-08-31
Filing date: 1934-08-31
Publication date: 1938-02-15
Anticipated expiration: 1955-02-15
Also published as: GB461646A

Description

Feb. 15, 1938. A. v. BEDFORD ET AL TELEVISION TRANSMITTER Filed Aug. 3l, 1934 5 Sheets-Sheet 1 INVENTORS Alda 'I/ Bedford Ray D. Kel/L BY www@ ATTORNEY Feb. 15, 1938. A. v4 BEDFORD ET AL TELEVISION TRANSMITTER Filed Aug. 31, 1934 3 Sheets-Sheet 2 Y md f5 D r. I ,N T @Fb Pu Nf Q @me T MBKW vb ay 5 m m m Mm 7 W w w A z L il; L n a o C v w, mw 5 9 M n 9 m m Z :u W0 0 CT o o Hm a 1 w. Z 6 D 1 2 l wm m LU ll., ll I Liwl.. m w w 4N 0 0 0 M5522 @EN on. nmzt Hotham;

wsorimu St @s Feb. l5, 1938. A V, BEDFORD ET AL 108,097

TELEVISION TRANSMITTER Filed Aug. 31, 1934 3 Shee'bS-Sheel'l 3 /MPULSE 2% Y 10 o /g. 'L I- 7 I BZ PICTURE w o 0^ L 125 scan 90;?"1 ,q l ''ME O o -H 20g o 'r/ME MPULSE '7'0 SPHHY INVENTQHS Alda Vedfrd Ray Keil Patented Feb. 15, 1938 UNITED STATES amaca: y TELEVISION TRANSMITTER Alda. V. Bedford, Collingswood, arid Ray D. Kell,

Haddonfield, N. J., assignors, by mesne assignments, to Radio Corporation of America,

New York, N. Y., 'a corporation of Delaware Application August 31,

18 Claims.

Our invention relates to television transmitters and particularly to transmitters of the type employing cathode-ray tubes. j

A preferred form of device for converting the elements of a picture into electrical impulses, whereby the picture may be transmitted, comprises a cathode-ray tube containing a mosaic of light-sensitive elements so positioned that it may be scanned by the cathode ray. The mosaic is 10 also so positioned that an optical image of the picture to be transmitted may be formed thereon. A transmitter tube of this type is described in detail in an article by V. K. Zworykin published in the Proceedings of the Institute of Radio Engineers for January 1934.

One of the. main objects of our invention is to improve the sensitivity of a cathode-ray tube of the above-mentioned type.

A further object of our invention is to p'rovide 20 a television transmitting system which does not require an excessive amount of illumination on the subject being televised.

Other objects, features and advantages of our invention will appear from the following description taken in connection with the accompanying drawings, in which Figure 1 is a schematic diagram of a preferred embodiment of our invention,

Fig. 2 is a circuit diagram which is referred to 30 in explaining the operation of the transmitter tube shown in Fig. 1,

Figs. 3, 4 and 5 are curves which are referred to in explaining the operation of the apparatus shown in Fig. 1,

Fig. 6 is a schematic diagram of another embodiment of our invention, and

Figs. '7 and 8 are curves which are referred to in explaining the operation of the apparatus shown in Fig. 6.

A preferred embodiment of our invention is illustrated in Fig. 1 where a cathode-ray transmitter tube is indicated at. I. The tube comprises a highly evacuated envelope having an electron gun which consists of an indirectly 45 heated cathode 3, a control electrode 5, and a first anode 1.

A second anode 9, which may consist of a metallic coating on the inner surface of the envelope, is provided both for accelerating the electrons 50 -and for aiding in the focusing of the electron beam as well as for functioning as a photoelectric cell anode as will be explained hereinafter.

The electrodes of the cathode-ray tube I may be maintained at the proper potentials by means of any suitable voltage source such as batteries.

1934, Serial No. 742,200l

As illustrated, the control electrode is maintained negative with respect tothe cathode 3 by means of a battery II having a potentiometer I3 shunted thereacross. The first anode 1 is maintained positive with respect to the cathode 3 by .means of a battery I5 while the second anode 9 is maintained more highly positive with respect to the cathode 3 by means of the battery I5 and a second battery I1 connected in series therewith. The second anode 9 is connected to the positive terminal of the battery I1 through a conductor I9, a resistor 2| and ground.

A mosaic 23 of light-sensitive elements is positioned inside the transmitter tube envelope in such position that it may be scanned by the electron beam produced by the electron gun. The mosaic 23 is also so positioned that an optical image of the object to be televised may be formed thereon, such an object being indicated by the arrow 25. An image of the object 25 is formed upon the mosaic 23 by means of an optical system indicated at 21, this optical system being mounted in the wall of a light-tight box, indicated by the dotted lines 29, inwhich the transmitter tube I is located.

As stated in the above-mentioned article, the mosaic 23 may be constructed in various ways, a preferred construction consisting of a sheet of mica 3| having a metallic coating 33 on its back surface to form what will be referred to as the signal plate, and having a mosaic of light-sensitive elements 35 on its front surface. The light-sensitive elements 35 preferably are insulated from each other and are insulated from the signal plate 33 by means of the mica sheet 3l.

In accordance with one embodiment of our invention, an auxiliary cathode 31 is provided so positioned with respect to the mosaic 23 that electrons may be sprayed thereon. In order .to control the flow of electrons from the auxiliary cathode 31 to the mosaic, a control electrode 39 is provided.

The cathode-ray tube I may be provided with horizontal and vertical deflecting coils 4I and 43, respectively. The electrical impulses for producing a flow of saw-tooth current through the deecting coils for scanning purposes may bel generated by means of a disc 45 driven by a synchronous motor 41 which is connected to a 60- cycle power'line. The disc 45 has a large number of small evenly spaced openings 49 near its circumference for generating the horizontal synchronizing impulses and a single opening 5I which is comparatively large for generating the framing or vertical synchronizing impulses. The appa- 55 ratus for generating these impulses includes a light source 53 and an optical system 55 for directing light through the openings in the disc 45 upon a photoelectric' cell 51 which is included in the input circuit of an amplifier 59.

'I'he framing impulses are impressed upon a vertical deiiecting circuit 6| which is connected to the vertical deflecting coils 43 for causing sawtooth current to pass therethrough. The current which flows through the coils 43 is represented by the curve 60 in Fig. 3.

The horizontal deecting impulses are separated from the framing impulses in a separating circuit B3 and impressed upon a horizontal deflecting circuit 65 which is connected to the horizontal deflecting coils 4| for causing the higher frequency saw-tooth current to pass therethrough.

As the electron beam is caused to scan the mosaic 23, charges are successively released from elements of the mosaic whereby a ow of picture current is produced which may be amplied by a suitable amplifier 61. As is well known, the scanning beam brings the mosaic to a certain equilibrium potential each time it scans the mosaic due to the emission of secondary electrons. Thus, at the end of a scanning period, the mosaic is in condition to repeat the cycle of operation. The iiow of picture currentl may be traced from the signal plate 33 of the mosaic, through a conductor 69, a resistor 1l, a battery 13, and potentiometer 15 to ground, and through ground, the resistor 2| and the conductor |9 to the second anode 9. The resistor 1| and the battery 13 are connected in series in the 'input circuit of the amplier 61, the battery 13 with its shunting potentiometer 'l5 being so connected that either a positive or negative potential of any value Within certain limits may be applied to the signal plate 33 for improving the operation of the transmitter under certain conditions.

'I'he action of the transmitter tube in generating a picture current will be more clearly understood by referring to the circuit diagram in Fig. 2.

In this diagram two light-sensitive elements of the mosaic are indicated at 11, these lightsensitive elements being electrically connected to the signal plate 33 only through the capacity between a light-sensitive element and the signal plate represented by the condensers 19. 'When an optical image is formed upon the mosaic, electrons iow from each light-sensitive element or photoelectric cell cathode 11 to the second anode 9, the number of electrons owing from a lightsensitive element depending upon the intensity of light striking it. Therefore, the condenser 19 which the light-sensitive element forms with the signal plate 33 is charged to a value depending upon the light intensity at the particular point under consideration.

When the electron beam strikes the lightsensitive element in scanning the mosaic, the condenser 19 is discharged through the resistor, this discharge current being the picture current. It will be apparent from Fig. 2 that since there is no conductive connection between the lightsensitive elements 11 and the second anode 9 it is impossible to obtain maximum sensitivity of the photoelectric cell circuit by including a battery in the circuit for making the second anode positive with respect to the light-sensitive elements 11. While the battery 13 is shown in the circuit, it is included for a different purpose, which will be described later. Y

In accordance with one feature of our invention, we give the second anode 9 a certain positive potential with respect to the light-sensitive elements of the mosaic 23 by impressing a squaretop wave of voltage across the resistor 2| in the second anode circuit. 'I'his square-top voltage wave may be generated by means of a disc 8| which is driven by a synchronous motor 83 connected to the 60-cycle power line. 85 is provided in the disc 8| near its circumference to permit light from a suitable source 81 to pass therethrough and strike a photoelectric cell 89 once for every rotation of the disc. The photoelectric cell 89 is connected in the input circuit of a suitable ampliiler 9|, and the output circuit of the amplifier 9| is connected through a coupling condenser 93 across the resistor 2 I.

The resulting voltage which appears across the resistor 2| is shown by the curve 95 in Fig. 4. It will be seen that the voltage impulse is applied to the second anode 9 in a negative direction whereby a smaller positive voltage is applied to the second anode between voltage impulses because of the fact that the current supplied to the resistor 2| must ow through the coupling condenser 93. Consequently, the zero axis of the curve is so located that the area of the curve above the axis is equal to the area of the curve below the axis. By comparing curve 95 with curve 60 in Fig. 3 it will be apparent that the positive impulse applied to the deflecting coils 43 and the negative impulse applied to the second anode 9 begin at the same time.

It will be apparent that at the end of the negative impulse the second anode 9 has a deilnite positive voltage with respect to the light-sensitive elements 35 whereby photoelectrons released by the optical image are more readily drawn away from the mosaic, thus rendering the transmitter tube more sensitive.

Although a picture may be projected onto the mosaic 23 at the same time that the mosaic is being scanned by the electron beam, it is preferred to project the picture onto the mosaic for a short period and then scan the mosaic. In order to obtain the preferred operation we provide a light shutter 91 which is driven by a synchronous motor 99 connected to the 60-cycle power line. An opening is provided in the shutter disc such that light from a suitable source |03 may be directed upon the object 25 being televised for a certain period, for example, 8% of the total time between picture frames. In the drawings a mirror |03 is shown reecting the light towards the object 25 merely for the purpose of simplifying the lay-out of the drawings.

1 Because of the fact that the mosaic 23 loses a large number oi electrons at the time the photoelectrons are drawn away from it by the second anode 9, it is desirable to supply the mosaic with additional electrons from some source just before the optical image is projected thereon.

The circuit and apparatus for supplying the mosaic with electrons during the return period of the electron beam and just prior to the time the picture is projected onto the mosaic will now be described. The auxiliary cathode 31 is connected through a resistor to ground while the control electrode 39 is connected through a resistor |01 and through a biasing battery |09 to ground. It is desired to apply a negative voltage impulse to the cathode 31 and to the control electrode 39 at the same time the negative voltage impulse is applied to the second anode 9, whereby the mosaic 23 will be made positive with respect to the auxiliary cathode 31 and electrons will be released and sprayed uniformly over the mosaic.

An opening The negative impulses for the auxiliary cathode 31 are generated bymeans ofthe disc 8| which has a second opening I therein diametrically opposed to the opening 85 and spaced farther from the circumference of the disc than the opening 85. A photoelectric cell H3 is so positioned that it receives light through-the opening from a suitable light source H5 at the same time that the other photocell 89 receives light through the opening 85. The photoelectric cell 3 is connected to the input circuit of a suitable ampliiier H1, and the output circuit of the amplifier is connected through a coupling condenser H9 to the auxiliary cathode 31. The output circuit of the amplifier H1 is also connected through a potentiometer |2| and through a. coupling condenser |23 to the control electrode 39 whereby the control electrode receives a negative impulse at the same time as the auxiliary cathode, but an impulse of a lower value. For the purpose of explanation it will be assumed that a negative voltage impulse of 100 volts is applied to the second anode 9, that a negative voltage impulse of the same value is applied to the auxiliary cathode3?, and that the adjustment of the potentiometer |2| is such that a negative impulse of 80 volts is applied to the control electrode 39. It will also be assumed that the biasing` battery |09 applies a negative voltage of 20 volts to the control electrode 39. Hence it is apparent that the cathode is swung 20 voltsfurther negative than the control electrode. This is equivalent to a 20 volt positive swing on the control electrode with respect to the cathode. This is enough to overcome the effect of the biasing battery |09 and permit ilow of electrons from the cathode 'during this interval.

Referring to Figs. 4 and 5, a cycle of operation is illustrated in which the negative impulse applied to the second anode 9 lasts for 2% of the total time from the beginning of one framing impulse to the beginning of the next framing impulse. At the same time, a negative voltage impulse of the same duration is applied to the auxiliary cathode 31 and to the control electrode 39 whereby electrons are sprayed upon the mosaic 23. The impulse applied to the cathode 31 is represented by the curve |20 in Fig. 5. By applying a negative voltage to the second anode 9 at the same time that the negative voltage is applied to the auxiliary cathode 31, electrons are prevented from moving over to the second anode instead of to the mosaic as would be the case if the second anode were positive with respect to the auxiliary cathode.

As indicated in Fig. 4, at the end of the negative impulse, the second anode becomes positive with respect to the mosaic, whereby the photo-electric cell portion of the transmitter tube is rendered more sensitive than it is when operated in the usual manner. Immediately following the negative impulse, an object of the image 25 being televi ed is projected upon the mosaic for a period which, in the example being described, is 8% of the total time between framing impulses. At the end of this time, the light is cut oil? from the object 25 by means of the shutter 95 and the dark mosaic is then scanned by the cathode-ray beam for the remaining 90% of the time between framing impulses.

At the end of the scanning period, the above described ,cycle of operation is repeated, elec- -trons being sprayed upon the mosaic to replace the photo-electrons which were drawn over to the second anode during the period the optical image was on the mosaic.

The battery 13 which is connected between the signal plate and ground is not essential vto the operation of our transmitter but it has been found that, under certain conditions, the operation will be improved it the signal plate is made negative with respect to ground, while under other conditions, the operation will be improved if it is made positive with respect to ground.

In Fig. 6 there is shown a modiiication of the .invention as shown in Fig. 1. In the two figures,

like parts are indicated by the same reference numerals. The transmitter tubes are identical ln construction except that the tube shown in Fig. 6, the metallic coating which forms the second anode 9 is extended to form an anode for the auxiliary cathode 31. In this construction, the extended section of the metallic coating 9, the auxiliary cathode 31 and the control electrode 39 form a gun for spraying electrons upon the mosaic 23. It will be noted that in the tube shown in Fig. 1 the electrons from the auxiliary cathode 31 are not shot upon the mosaic, but instead, are pulled over to it by the potential on the signal plate 33.

In operating the transmitter tube shown in Fig. 6, a positive potential impulse which may have a duration of 10% of the time between framing impulses is impressed upon the second anode 9. This impulse is represented by the curve |25 in Fig. 7. At the same time, a negative impulse |21 (Fig. 8), which may have a duration of 2% of the time between framing impulses, is impressed upon the auxiliary cathode and the control electrode to release the electrons.

The amplitude of the positive impulse applied to the second anode 9 is made such that when added to the negative potential applied to the auxiliary cathode 31, the potential difference between the auxiliary cathode and the second anode is suilcient to` project electrons from the cathode 31 upon' the mosaic 23. It will be apparent that during the period the positive impulse is imypressed upon the second anode 9, the photo-electric cell circuit of the tube is in a sensitive condition. Therefore, at the end of the negative impulse which is applied to the auxiliary cathode 31, an image of the object being televised is projected upon the mosaic 23 and then cut 01T at the end of the positive impulse, as indicated by the legend in Fig. '1. The mosaic 23 is then scanned by the cathode-ray beam for the remaining 90% of the time between framing impulses.

From an inspection of Fig. 7 it will be evident that, at the end of the positive impulse, the second anode goes negative with respect to the mosaic whereby the tube is rendered insensitive so that there would be no advantage in having an image of the picture projected upon the mosaic continuously. Therefore, as described above, the studio light is interrupted by the shutter 91, shown in Fig. 1, with no loss in eiliciency of operation and with a great reduction in the glare and heat of studio lights.

It will be understood from the foregoing description that various modifications may be made in our invention and we desire, therefore, that only such limitations shall be placed thereon as are necessitated by the prior art and set forth in the appended claims.

We claim as our invention:

1. 'I'he method of operating a television transmitter tube ofthe type comprising an evacuated container having a mosaic of light-sensitive elements therein, said elements being mounted adjacent to a non-foraminous signal plate whereby they are capacitively connected thereto, and having an electrode positioned to collect photoelectrons from said mosaic, which comprises the steps of periodically sweeping said mosaic with an electron beam whereby said mosaic is brought to an equilibrium potential at the end of each sweeping period due to the emission of secondary electrons from said mosaic and impressing a voltage impulse across said signal plate and said electrode at the end of each sweeping period.

2. The method of operating a television transmitter tube of the type comprising an evacuated container having a mosaic of light-sensitive elements therein, said elements being mounted adjacent to a signal plate whereby they are capacitively connected thereto, and having an electrode positioned to collect photoelectrons from said mosaic, which comprises periodically impressing a voltage impulse across said signal plate and said electrode, spraying electrons upon said mosaic each time said voltage impulse occurs to replace electrons drawn over to said electrode and then sweeping said mosaic with an electron beam, whereby said mosaic is brought to an equilibrium potential at the end of each sweeping period due to the emission of secondary electrons from said mosaic.

3. In combination, a cathode-ray tube comprising an evacuated container, a mosaic of lightsensitive elements therein, said elements being mounted adjacent to and insulated from a nonforaminous conducting plate, an electrode common to said elements and positioned to collect photoelectrons therefrom, a circuit connecting said plate and said electrode, means for supplying voltage impulses to said circuit whereby said electrode periodically becomes positive with respect to said elements, and means for sweeping said mosaic by an electron beam at the end of each of said voltage impulses whereby said mosaic is brought to an equilibrium potential at the end of each sweeping period due to the emission of secondary electrons from said mosaic.

4. In a picture transmitting system, a cathoderay tube comprising an evacuated container, a mosaic of light-sensitive elements therein, a nonforaminous conducting plate, said elements being mounted adjacent to said plates whereby they are capacitively connected thereto, an electrode common to said elements and positioned to collect photoelectrons therefrom, means for generating an electron beam in said container, means for scanning said mosaic -periodically with said electron beam whereby said mosaic is brought to an equilibrium potential at the end of each scanning period due to the emission of secondary electrons from said mosaic, and means for irnpressing a voltage impulse between said conducting plate and said electrode at the end of each scanning period.

5. A system according to claim 4 characterized in that additional means is provided for spraying said mosaic with electrons at the end of each scanning period for the purpose of replacing electrons drawn over to said electrode.

6. In a picture transmitting system, a cathoderay tube comprising an evacuated container, a mosaic of light-sensitive elements therein, a conducting plate, said elements being mounted adjacent to said plate whereby they are capacitively connected thereto, an electrode common to said elements and positioned to collect photoelectrons therefrom, means for generating an electron beam in said container, means for scanning said mosaic periodically with said electron beam whereby said mosaic is brought to an equilibrium potential at the end of each scanning period due to the emission of secondary electrons from said mosaic, and means for impressing a negative voltage impulse between said conducting plate and said electrode at the end of each scanning period, said impulse being fed through a condenser whereby said electrode is made positive with respect to said plate at the end of said impulse. 1

'7. In a picture transmitting system, a cathode-ray tube comprising an evacuated container, a mosaic of light-sensitive elements therein, a conducting plate, said elements being mounted adjacent to said plate whereby they are capacitively connected thereto, an electrode common to said elements and positioned to collect photoelectrons therefrom, means for generating an electron beam in said container, means for sweeping said mosaic periodically with said electron beam, a circuit including a resistor connecting said conducting plate to said electrode, a circuit including a `condenser in series therewith connected across said resistor, and means for applying a negative voltage impulse to said electrode through said condenser at the end of each sweeping period.

8. Apparatus according to claim 6 characterized in that means is provided for projecting an image of the object being televised upon said msaic during the period between the end of the negative impulse and the beginning of the sweep period following said impulse.

9. In a picture transmitting system, a cathoderay tube comprising an evacuated container, a mosaic of light-sensitive elements therein, a conducting plate, said elements being mounted adjacent to said plate whereby they are capacitively connected thereto, an electrode common to said elements and positioned to collect photoelectrons therefrom, an impedance unit connected between said electrode and said plate, means for generating an electron beam in said container, means for sweeping said mosaic periodically with said electron beam whereby said mosaic is brought to an equilibrium potential at the end of each scanning period due to the emission of secondary electrons from said mosaic, means for making said electrode periodically rst negative and then positive with respect to said mosaic, and means for projecting the image of an object to be tele- Vised upon said mosaic during at least a portion of the period said electrode is made positive, and means for taking picture signals from said impedance unit.

10. Apparatus according to claim 9 characterized in that means is provided for spraying electrons upon said mosaic during the period said electrode is negative for the purpose of replacing electrons drawn to said electrode.

11. In a picture transmitting system, a cathode-ray tube comprisingan evacuated container, a mosaic of light-sensitive elements therein, a conducting plate, said elements being mounted adjacent to said plate whereby they are capacitively connected thereto, an electrode common to said elements and positioned to collect photoelectrons therefrom, an impedance unit connected between said electrode and said plate, means for generating an electron beam in said container, means for making said electrode periodically rst negative and then positive with respect to said mosaic, means for projecting the image of an object to be televised upon said mosaic during the first part of said positive period, means for scanning said mosaic with said electron beam during the last part of said positive period whereby said mosaic is brought to an equilibrium potential at the end-of each scanning period due to the emission of secondary electrons from said mosaic, and means for taking picture signals from said impedance unit.

12. Apparatus according to claim 11 characterized in that means is provided for spraying electrons upon said mosaic during the period said electrode is negative for the purpose of replacing electrons drawn to said electrode.

13. In a picture transmitting system, a cathode-ray tube comprising an evacuated container,

a mosaic of light-sensitive elements therein, av

conductingA plate, said elements being mounted adjacent to said plate whereby they are capacitively connected thereto, an electrode common to said elements and positioned tocollect photoelectrons therefrom, an impedance unit connected between said electrode and said plate, means for generating an electron beam in said container, means for sweeping said mosaic periodically with said electron beam whereby said mosaic is brought to an'equilibrium potential at the end of each scanning period due to the emission of secondary electrons from said mosaic, means for impressing a short positive voltage impulse and a longer negative impulse alternately upon said electrode for making it positive for a certain period with respect to said mosaic, means for projecting an image oi the object to be televised upon said mosaic during the period said electrode is positive, and means for taking picture signal from said 'impedance unit.

14. Apparatus according to claim 13 characterized in that means including an auxiliary electron gun is provided for spraying electrons upon said mosaic during a portion of the period said electrode is positive. l

15. In a picture transmitting system, a cathi ode-ray tube comprising an evacuated container, a mosaic of light-sensitive elements therein, a non-foraminous conducting plate, said elements being mounted adjacent to said plate vwhereby they are capacitively connected thereto, an electrode common to said elements and positioned to collect photoelectrons therefrom, means for generating an electron beam in said container, means for scanning said mosaic periodically with said electron beam whereby said mosaic is brought to an equilibrium potential at the end of each scanning period due to the emission oi secondary electrons from said mosaic, means for increasing the photoelectric sensitivity of said tube between scanning periods, and means for projecting an image of the subject to be televised upon said mosaic during, and only during, the period of said increased sensitivity.

16. Apparatus according to claim 15 characterized in that said last means includes a device for projecting light upon said subject during, and only during, said period of increased sensitivity.

17. In a picture transmitting system, a cathode-ray tube comprising an evacuated container, a mosaic of light-sensitive elements therein, a conducting plate, said elements being mounted adjacent to said plate whereby they are capacitively connected thereto, an electrode common to said elements and positioned to collect electrons therefrom, means for generating an electron beam in said container, means including a cathode and a control grid for spraying said mosaic with electrons when said cathode and control grid are at the proper relative potential, means for simultaneously driving said electrode and said cathode and control grid negative with respect to said plate for a certain period and for driving said cathode and control grid to said proper relative potential during said period, and means for scanning said mosaic by said electron beam at the end of said period.

18. In a picture transmitting system, a cathode-ray tube comprising an evacuated container, a mosaic of light-sensitive elements therein, a conducting plate, said elements being mounted adjacent to said plate whereby they are capacitively connected thereto, means for generating an electron beam in said container, an electrode common to said elements and positioned to collect electrons therefrom and also positioned out of the path of said electron beam, means including a cathode, a control grid and an anode for spraying said mosaic with electrons when said cathode, control grid and anode are at the proper relative potentials, a connection between said anoderand said electrode for maintaing them at the same potential, means for simultaneously driving said electrode positive with respect to said plate and said cathode and control grid negative 4with respect to said plate for a certain period,

and for driving said cathode, grid and anode to said proper relative potential during a portion of said period,A means for projecting an optical image of a picture upon said mosaic for another portion of said period, and means for scanning said mosaic with said electron beam at the end of said period.

. ALDA V. BEDFORD.

RAY D. KELL.