US2131892A

US2131892A - Electron discharge device

Info

Publication number: US2131892A
Application number: US103487A
Authority: US
Inventors: Harley A Iams
Original assignee: RCA Corp
Current assignee: RCA Corp
Priority date: 1936-10-01
Filing date: 1936-10-01
Publication date: 1938-10-04
Anticipated expiration: 1955-10-04

Description

Oct. 4, 1938. H. A. IAMS ELECTRON DISCHARGE DEVICE Filed Oct. 1, 1936 Hall INVENTOR HARLEY A. IAMS ATTORNEY Patented Oct. 4, 1938 PATENT OFFICE ELECTRON DISCHARGE DEVICE Harley A. Iams, Berkeley Heights, N. J.,.assignor, by mesne assignments, to Radio Corporation of America, New York, N. Y., a corporation of Delaware Application October 1, 1936, Serial No. 103,487

9 Claims.

on the other surface, the front or illuminated surface being scanned by a cathode ray beam from an electron gun having at least two anodes to produce currents which flow through an output resistor connected between the signal plate of themosaic electrode and ground to generate the picture signals.

Now it has been found that parasitic signals are developed superimposed upon the picture signals taken from the signal plate. These parasitic signals may take the form of base line wander at the line scanning or picture scanning frequency or at both, or they may also develop an unnatural shading of the re-created picture. Thus if the true picture signals are regarded as consisting of variations; corresponding to the brightness of the mosaic elements scanned; in an upward direction from a base line corresponding to picture black, the parasitic signals may have the effect of causing the base line to assume a saw-tooth form. The base line may fall relatively suddenly to a minimum value at the commencement of the scanning of a line of the image and may rise 35 steadily as the scanning of the line proceeds. In

addition to this parasitic signal occurring at line frequency, there may also be found a parasitic signal of similar general form occurring at the frame or picture frequency. The reason for this is believed to be as follows:

In the operation of the cathode ray transmitting tube, the average potential of the elements of the mosaic screen is always in the neighborhood of the potential of the second anode 5 'of the electron gun. These photo-electrically sensitive elements emit secondary electrons freely when struck by the scanning beam, and. the second anode collects both the secondary electrons and the photo-electrons emitted by the 9 mosaic screen. when an element is scanned by the cathode ray beam, the secondary electrons emitted by the scanned element flow partly to the second anode of the gun and partly to neighboring elements on the mosaic'screen. and

Q the scanned element becomes charged to a potential slightly positive with respect to the second anode. Thus if the potential of the second anode is 1000 volts positive with respect to the cathode, the scanned element may be at 1001 volts positive as the scanning beam passes from it to 6 adjacent elements, and it begins to collect secondary electrons emitted from the adjacent elements then being scanned, so that its potential gradually falls. As soon as its potential falls below that of the second anode (1000 volts) it emits photo- 10 electrons under the influence of light from the object falling on the photo-sensitive surface of the elements, and these photo-electrons are collected by the second anode. This emission of photo-electrons tends to make the potential of the element rise, but the higher its potential, the more secondary electrons emitted from other parts of the screen does it tend to collect, to drive its potential more negative again. The final potential of the element before again being scanned is that potential it acquired by the previous scansion, decreased by the secondary electrons acquired by it from the secondary electrons emitted from other parts of the screen and increased by the photo-electrons lost by the element under the influence of the incident light. This final potential may be of the order of 998 volts for an element illuminated with average brightness. When the illuminated element is again scanned its potential is instantly restored to 1001 volts,- and the resulting discharge of the condenser constituted by the element and the signal plate causes in the circuit associated with the signal plate an impulse which is amplified and transmitted. 36

Assume now that the cathode ray beam scans the screen in a series of approximately parallel straight lines from left to right across the screen, and starts a complete scan of the screen in the top left-hand corner. Now when, in scanning any 0 line, the beam is on the left-hand end of a line most of the secondary electrons emitted will be collected by the second anode, but when the beam is on the right-hand part of the line, there will be a number of positively charged elements to the left of the scanning beam, which collect secondary electrons emitted from the elements on the part of the line then being scanned. Thus there will be set up a field at the surface of the screen which is more positive at the left-hand 0 side of the screen than at other points on its surface, and tend to attract electrons to the lefthand side of the screen. The effect of this field is to cause the base line wander of the picture signals at line frequency.

of the electron gun..

A similar effect will be produced as the beam moves down the screen from line to line, as the upper part of the screen will tend to attract electrons emitted from the lower partand thus give a base line wander of the picture signals at frame frequency.

A further source of parasitic signals in a cathode raytransmitting tube such as that considered above is in unequal secondary emission from all parts of the mosaic screen. This unequalemission results from the combination of the finite size of the mosaic particles, the non-uniform distribution of charges over the surface of the mosaic screen, variable charges on the glass wall of the tube, and the non-uniform electrostatic field produced by the second anode.

It is the object of the present invention to provide means for removing or reducing these and other parasitic signals.

According to the present invention a. cathode ray tube having a target or mosaic electrode comprising a mosaic screen of mutually-insulated elements capacitively associated with a common signal plate, an electron gun for directing a cathode ray onto the screen, and deflecting means for causing the ray to scan the screen, is provided with a compensating electrode system disposed between said gun and said screen and out of the path of the scanning beam and biased to produce transverse to the scanned surface of said screen a potential gradient which serves to minimize para,-

sltic signals generated in said signal electrode.

The biasing potentials necessaryfor this purpose are usually small compared with the potential difference between the cathode and secondanode The invention will now be described with reference to the accompanying diagrammatic drawing tional type electron gun and a bulbous section enclosing a parallelogrammic and usually rectangular mosaic electrode of a well-known type. The mosaic electrode is so mounted in the envelope that an optical image may be projected on its front surface by a lens system i 9, and the illuminated surface scanned by the scanning beam from the electron gun in the neck of the envelope.

The electron gun, of the conventional type, is shown as comprising a cathode 3, a control electrode 5 connected to the usual biasing battery, and first anode l associated with a second or beam focusing anode i3, preferably a conducting coating on the inner surface of the envelope 5 near the neck, which acts as a collector electrode for electrons from the mosaic electrode. These anodes are maintained positive with respect to the cathode 3 by a battery 9 and the electron stream leaving the first anode l is accelerated and concentrated into an electron scanning beam being connected to the positive terminal of the battery i5, usually through a ground.

In accordance with my invention I provide for the mosaic screen a compensating electrode which is adjacent the mosaic electrode but out of the path of the optical image focused on the mosaic screen and also out of the path of the cathode ray scanning beam, and which at suitable bias potential will suppress or eliminate parasitic signals which otherwise occur. Preferably the compensating electrode is close to the edge of the mosaic electrode and is insulated from it. I have found that a compensating electrode at one side and another at one end of the rectangular mosaic electrode will suppress'much of the parasitic signals at both line frequency and at picture frequency, but for more complete suppression I prefer to use a compensating electrode system of four electrodes, 23, 2d, 25, and 26, two of which are mounted along the two parallel sides of the screen, and the other two along the top and bottom edges. Incompensatingsfor parasitic signals resulting from unequal secondary emission from the mosaic particles, I prefer to sub-divide the electrodes on two or more edges of the mosaic screen. Thus two of the electrodes such as 23 and 26 may be divided as shown by the dotted lines resulting in individual electrodes 23a, 23b,

23c, and 26a, 26b, and 250. Electrodes 24? and 25 may be left continuous or these may likewise be sub-divided in similar manner.

Each of the auxiliary electrodes is connected to a voltage divider 27, which serves as a source for the bias potential for the electrodes, through a lead and also through a resistor 28 which reduces the capacitance of the mosaic electrode to ground. The compensating electrodes may be biased preferably a few volts positive or negative with respect to the second anode it in compensating for parasitic signals arising from unequal secondary emission. The electrodes along any side of the mosaic screen may be further biased positively or negatively with respect to each other to give the desired compensating or suppressing field at the mosaic screen surface.

With the tube as illustrated, scanning preferably takes place in a series of lines from side to side of the screen beginning at the left-hand top corner. The base line wander at line frequency may be corrected by biasing to a few volts positive with reference to the second anode the compensating electrode 2d which is near the side of the screen on which the line scanning ends, and at frame frequency by similarly biasing the compensating electrode 23, near the bottom of the target. In this way there is produced along the surface of the mosaic screen a field which compensates for the field due to the scanned elements, and minimizes the parasitic signals. The positive electrode 2d at the righthand 'end of each line of elements traversed by the scanning beam during line scanning attracts the secondary electrons emitted by each scanned element and counteracts the eifect of the posi- @ill tively scanned elements in the line to the left 1 5 of the scanning beam. The positive electrode 23 at the bottom of the screen likewise counteracts the efiect of the positively scanned elements above the line of scanning. I have obtained good results with a battery 9 of 250 volts, a battery ii of 750 volts, and a bias on the compensating electrode of about 5 volts positive with reference to the second anode I3. I have also found that resistors 28 of a value of about 100,000 ohms, are suitable.

For suppressing parasitic signals resulting from non-uniform secondary emission from the mosaic screen I have found it desirable to bias the auxiliary electrodes either positively or negatively in such a manner that the transmitted picture when "'re-created'is of uniform shading? It has been plies either positive or negative potential sources as desired for the suppression of parasitic signals due to inequalities of secondary emission over the mosaic surface.

The auxiliary or compensating electrodes may be used also to generate corrective signals which maybe mixed with the signals from the signal plate either directly or after amplification. For example the auxiliary electrodes, fed from a suitable bias potential source through a high resistance may be connected through a condenser to the mosaic electrode conductive coating, or to a tapon the impedance which is connected to the signal plate and across which the picture signals are developed, or to a suitable point in the picture signal amplifier.

It is obvious that the

compensating electrodes

23, 24, 25,'and 26 of the figure may be replaced by compensating electrodes attached to the walls of the bulbous portion of the tube and arranged between the cathode ray gun in the neck portion and the mosaic electrode assembly, that the electrodes-may be in the form of wires let in through the glass walls, and that they may be perpendicular to the glass, or may be arranged parallel to the sides of the screen, as long as they are in proper relation to the screen and are given suitable positive or negative biasing potentials.

What I have indicated the preferred embodiment of my invention of which I am now aware and have also indicated only one specific application for which my invention may be employed, it will be apparent that my invention is by no means limited to the exact forms illustrated or the use indicated, but that many variations may be made in the particular structure used and the purpose for which it is employed without departing from the scope of my invention as set forth in the appended claims.

Having now particularly described and ascertained the nature of said invention, and in what manner the same is to be performed, I declare that. what I claim is:

1. A cathode ray transmitting tube comprising a mosaic electrode including a mosaic screen of mutually insulated photo-electrically sensitive elements and a common signal plate back of said screen and capacitively associated with said elements, scanning means including an electron gun for directing a ,scanning beam to said screen anda collector electrode in front of said screen, a compensating electrode mounted out of the path of the scanning beam and adjacent one end of the paths followed by the scanning beam in scanning aseries of said elements maintaining said compensating electrode at'a potential with respect to said screen for producing in conjunction with said collector electrode along the surface of said screen an electrostatic field by which the effects of the field due to the scanned elements of the series are minimized.

2. A cathode ray transmitting tube comprising a planar mosaic electrode including a mosaic screen of mutually insulated photo-electrically sensitive elements each capable of holding an electrostatic charge and a common signal plate back of said screen and capacitively associated with said elements, scanning means including an electron gun for directing a scanning beam to said screen and a collector electrode in front of said screen, a plurality of compensating electrodes'adjacent and distributed along the edge of said screen and out of the path of the scanning beam and means to maintain each of said compensating electrodes at a potential which will equalize the efiect of the charges on said photoelectrically sensitive elements produced by scanning said screen.

3. A cathode ray transmitting tube comprising a parallelogrammic mosaic electrode including a mosaicscreen of mutually insulated photo-electrically sensitive elements each capable of holding an electrostatic charge and a common signal plate back of said screen and capacitively associated with said elements, scanning means including an electron gun for directing a scanning beam to said screen to liberate secondary electrons from said elements and a collector electrode in front of said screen, compensating electrodes adjacent one side and one end of said screen and means to maintain each of said compensating electrodes at a potential which will produce an electrostatic field which in combination with the electrostatic fields produced by the collector electrode and the charged elements of the screen will insure the uniform distribution over the surface of the screen of secondary electrons produced by scanning said screen.

4. A cathode ray transmitting tube as claimed in claim 3 having a plurality of compensating electrodes along said side and said end of said mosaic screen.

5. A cathode ray transmitting tube comprising a parallelogrammic mosaic electrode including a mosaic screen of mutually insulated photo-electrically sensitive elements and a common signal plate back of said screen and capacitively associated with said elements, scanning means including an electron gun for directing a scanning beam to said screen, a plurality of compensating electrodes adjacent and distributed along the sides and edges of said screen and means to maintain each of said compensating electrodes at a potential with respect to said screen for producing in conjunction with said collector electrode along the surface of said screen an electrostatic field by which the efiects of the field due to the scanned elements of the said screen are minimized and the distribution of secondary electrons over the surface of the screen made more uniform.

6; A cathode ray transmitting tube comprising a target electrode adapted to receive an optical image, a. collector electrode in front of said target electrode, and scanning means including an electron gun for directing a scanning beam over F predetermined potential with respect to said target electrode for producing in conjunction with said collector electrode along the surface of said screen an electrostatic field by which the effects of the field due to scanning the target electrode in one direction are minimized.

7. A cathode ray transmitting tube comprising a parallelogrammic target electrode adapted to receive an electron image, scanning means including an electron gun for directing a scanning beam to said target electrode to liberate secondary electrons from said target electrode and a collector electrode in front of said target elec trode, compensating electrodes adjacent one side and one end of said target electrode, and means to maintain each of said compensating electrodes at a predetermined potential with respect to said target electrode for producing in conjunction with said collector electrode along the surface of said target electrode an electrostatic field by which the effects of the field due to the scanning of said target electrode are minimized and the distribution of secondary electrons produced by the scanning beam on said target electrode over the surface thereof made more uniform. V

8. A cathode ray transmitting tube comprising a target electrode adapted to receive an optical image, a collector electrode in front of said target electrode, and scanning means including an electron gun for directing a scanning beam over said target in a series of parallel paths, a compensating electrode mounted out of the path of the arsaeea scanning beam and adjacent one end of the paths followed by the scanning beam, and means to maintain said compensating electrode at a positive potential of approximately five volts with respect to said target electrode for producing in conjunction with said collector electrode along the surface of said screen an electrostatic field by which the effects of the field due to scanning the target electrode in one direction are minimized.

9. A cathode ray transmitting tube comprising a parallelogrammic target electrode adapted to receive an electron image, scanning means ineluding an electron gun for directing a scanning beam to said target electrode to liberate secondary electrons from said target electrode and a collector electrode in front of said target electrode, compensating electrodes adjacent one side and one end of said target electrode, and means to maintain each of said compensating electrodes