US2728021A

US2728021A - Post-deflected cathode ray tube

Info

Publication number: US2728021A
Application number: US343081A
Authority: US
Inventors: Heary S Blanks
Original assignee: RCA Corp
Current assignee: RCA Corp
Priority date: 1953-03-18
Filing date: 1953-03-18
Publication date: 1955-12-20
Anticipated expiration: 1972-12-20

Description

Dec. 20, 1955 H, 5, BLANKs 2,728,021

' POST-DEFLECTED CATHODE RAY TUBE Filed March 18, 1953 wmmm%wmm INVENTOR.

v HENRY S. BLHNKS deflected variety.

' wherein the particularscreen-area' to which the cathodecb-plariar grill mounted in the grates 2,728,622 Patented Dec. 20, W55

POST-DEFLECTED CATHODE RAY TUBE Henry S. Blanks, Princeton, N. 1., assignor to Radio Cor- This invention relates to improvements in color-kinescopes and other cathode-ray (CR) tubes of the post- (A post-deflected CR tube is one ray beam is directed at. any given instant is determined by the deflecting or switching ,yoltages applied to a bi-part 'pathof the beamadjacent to the target side of the screen.)

Post-deflected line-screen color-kinescopes (such, for

sent the same or corresponding parts in all figures, l designates a transparent or translucent glass screen plate which is provided on its rear or target surface with a multiplicity of groups of parallel phosphor strips or lines G (green), R .(red), B (blue) of diiferent color-response characteristics. As is conventional, the phosphor covered" surface of the screen has an electron-permeable conductive coating 3 thereon which isconnected, as by a lead 5 to a voltage source exemplified bythe voltage divider 7. Appropriate operating voltages are marked on the drawing. A switching grill, mounted adjacent to the metalized target surface 3 of the line screen, is made up of alternate and intermediate wires 9 and 11, respectively, which are connected, at the sides of the screen (see Fig. 2)

to external leads l0 and 12 respectively. Here the alternate wires 9 are connected through their common lead 10 to a point on the voltage divider 7 several thousand volts negative with respect to the metalized screen 3. A source of switching current, exemplified by the squarewaveoscillator 13, has one terminal connected to the external lead 12 for the intermediate grill wires 11 and its other terminal 14 to substantially the same point on the voltage divider 7 to which the lead 10 for the alternate example, as the one shown in U. S. patent Schroeder 2,446,791) operate most efiiciently in television systems of the field-sequential variety wherein the switching frequency is of the order of, say, 144 fields per second. If such tubes are-to be used in displaying NTSC (National Television System Committee) signals using switching and keying frequencies of the order of from 3 to 4 megacycles per second, one can use either sine-wave switching or squarewave-switching. Sine-wave switching results in a noticeable decrease in picture-brightness (at least if colorpurity is maintained by biasing the beam off during transit of the beam from one color to the next), .and square-wave switching necessitates the use of a costly high-power oscillator (to overcome the high inter-wire and wire-to-screen capacitive effects at the indicated high switching-frequency). I V

Accordingly, the principal object ofthe present invention is to provide a post-deflected cathode-ray tube which canbe keyed and modulated at any desired speed without loss of brightness and without excessive power.

' Another and related object of the present invention is to provide a method of and means for minimizing the effective capacitance in color-screen units of the postdefiected variety, and thus to permit rapid switching with a minimum of power.

The foregoing and otherobjects are achieved in accordance with the method .of the invention by limiting the application of the deflecting or switching voltages to the particular elements of the switching grill that'lie within the immediate path of the beam. Thus only a small fraction of the total capacitance of the screen-unit is charged at any given instant and, as a consequence, less charging current is. required from the oscillator or other source of switching energy.

of the present invention;

Fig. 2 is a partial rear-elevation ot the color-screen unit shown in Fig. l; and

Fig. 3 is an enlarged viewof one of the metalized glass intermediate" wires used in the bi-part co-planar switching grill of the color-screen unit shown in Figs. 1 and 2.

In the drawing wherein like reference characters reprewires 9 is connected. I

In accordance with the invention the wires of the intermediate group 11, have a core 15 (see Fig. 3) constituted of glass or other insulating material and are coated with a conductive metal layer 17 except for small areas 19, 19 near each end of said wires. These otherwise bare glass portions 19, 19' of the intermediate wires 11 are provided preferably only at their leading end-portions 19 with a filling or bridge 21 consisting either of a photoconductive material (such, for example, as cadmium sulphide), or of a B. I. C. (bombardment induced conductor) material (such, as for example magnesium fiuoride) which -when energized, in a manner later described by the scanning beam 23 (Fig. l), operates to connect that particular wire to the source 13 of switching voltage.

The photoconductive bridge 21 between the metalized parts of each of the glass wires 11 is rendered conductive by light picked-up from the screen when it is energized by the electron-beam 23 from a single electron-gun (not shown). The light that renders the photoconductive bridge material 21 conductive may be derived either from the color-phosphor lines (G, R or B) or from an auxiliary of -scan of the beam. The decay time of the photoconductive material should be not less than the time required for the scanning of one line and as little longer than this time as possible.

The direction-of-scan is along the color-phosphor lines G, R and B on the screen. The scanning beam is preferably keyed on" at the beginning of each line-of-scan so that irrespective of the amplitude of the video signal at the sides of the picture, the intensity of the beam as it starts its excursion will 'be sufiicient to render the bridge conductive. When the bridge or filling 21 on each metalized wire 11 comprises a bombardment induced conducting material (instead of a photoconductive material) the auxiliary phosphor strips 25 on the screenplate may be omitted.

It-will be observed upon close inspection of Fig. 1 that since the screen 1-3 is maintained at a potential several thousand volts positive with respect to the bi-part coplanar switching grill 9ll, a cylindrical-lens field is established in the grill-screen space, as taught in Flechsig's French Patent 866,065. As a consequence the beam electrons that pass between the grill wires are focused sharply upon the particular color line'to which they are directed by the instantaneous relative voltage of the alternate and intermediate wires within the immediate path of the scanning beam.

Cadmium sulphide, cadmium selenide, selenium, thallous sulphide, lead sulphide, zinc oxide and copper oxide are typical of the pho'toconductors which may be used as the bridging material. All of these photoconductors may also be classified as bombardment induced conductors since they are rendered conductive by beam impact, as are magnesium fluoride, silica, germanium and numerous other B. I. C. materials which may be employed in the practice of the invention.

What is claimed is:

1. Method of operating a post-deflected cathode-ray tube of the kind having a beam-deflecting grill made-up of spaced-apart elements between which electrons from a scanning-beam pass in their transit to pre-selected elemental areas on a nearby screen; said method comprising applying beam-deflecting voltages only to those elements of said grill that lie in the immediate path of scan of said beam.

2. In a cathode-ray tube having an electron-gun and a screen mounted in a position to be scanned by a beam of electrons from said gun, a beam-deflecting grill disposed in the path of said beam and having a multiplicity of discrete beam-deflecting elements extending across said screen, an external lead for supplying beam-deflecting voltages to said grill elements, and means responsive to the instantaneous position of said beam for electrically connecting to said external lead only those beam-deflecting elements that lie in the immediate path-of-scan of said beam.

3. In a cathode-ray tube having an electron-gun and screen, an external lead for supplying beam-deflecting voltages to said grill elements, and means responsive to the instantaneous position of said beam upon said screen for electrically connecting to said external lead only those beam-deflecting elements that lie adjacent to said instantaneous position of said beam upon said screen.

4. The invention as set forth in claim 3 and wherein said lastmentioned means comprises a photoconductive bridge connecting said beam-deflecting elements to said external lead.

5. The invention as set forth in claim 4 and wherein said photoconductive bridge comprises cadmium sulphide.

6. In a cathode-ray tube having an electron-gun and a screen mounted in a position to be scanned by a beam of electrons from said gun, a beam-deflecting grill 'disposed in the path of said beam and having a multiplicity of discrete beam-deflecting elements extending across said screen, an external lead for supplying beam-deflecting voltages to said grill elements, and means responsive to the instantaneous position of said beam upon said beamdefiecting elements for connecting only the beam-impacted ones of said elements to said external lead.

7. The invention as set forth in claim 6 and wherein said last-mentioned means comprises a bridge constituted of a bombardment induced conducting material connecting said beam-deflecting elements to said external lead.

8. The invention as set forth in claim 7 and wherein the material of which said bridge is formed comprises magnesium fluoride.

References Cited in the file of this patent v UNITED STATES PATENTS Hansen Sept. 18, 1951