US2483457A

US2483457A - Cathode-ray tube

Info

Publication number: US2483457A
Application number: US586740A
Authority: US
Inventors: Feldt Rudolf; Lempert Irving Edmund
Original assignee: Allen B du Mont Laboratories Inc
Current assignee: Allen B du Mont Laboratories Inc
Priority date: 1945-04-05
Filing date: 1945-04-05
Publication date: 1949-10-04
Anticipated expiration: 1966-10-04
Also published as: GB615563A; FR926000A

Description

@ci 4 i949- R. FELD? ETAL y 2,433,457

'cATHoDE-RAY TUBE Filed April 5, 1945 #5w-'J7 MWI/WENTORJ' BY y HTTONEY Patented Oct. 4, 1**949 UNITED STATES ENT OFFI CE.

vCATHODE-RAY TUBE Application April 5, 1945, Serial N0. 585,740

3 Claims.

This invention relates to a cathode-ray tube that .is provided with a plurality of 'spaced 4coaxial conducting rings to which successively higher `potentials are to be applied in steps :increasing toward the outer end of the tube. The portion 'of the tube .to which the rings are applied is cylindrical'ly 'shaped and has lan 'approximately flat outer circular end.

This tube is of the sort in which a cathode-ray or :beam is produced by an electron gun having a control electrode. The beam is focussed electrostatcally by a focussing yelectrode and is :deflected electrostaticallyby deflectione'lectrodes which deect the beam in 'a known =way.. The beam strikes a luminescent screen and produces a spot or trace of light of which the brilliance varies in accordance with .the screen material, the beam accelerating potential and the beam current.

It is known that with cathode-ray tubes of this character -high accelerating voltages `are needed in order to obtain a highly `bright spot or trace v:on the screen material. Such spots or traces :on 'the screen of a cathode-ray tube face 'are desirable 'or even necessary:

(a) When high speed transients are to be observed or recorded.

(b) When an optical projection of an 'image is to be made upon a screen as in -oscillog-raphy or with television projection.

Bright spots or traces can be ,produced on 'the screen by using high .current or high voltage, or both, for the beam. However, when high current is used the beam diameter 'or size of this spot. or trace on the screen is increased,causingdecrease of resolution of the picture. lOn the contrary,

high voltage decreases the beam diameter so that the size of the spot or trace is decreased.

Troubles arise with high voltages, such as insulation diiculties, low sensitivity of deection of the beam, distortion of the pattern on the screen due to high ratio oi intensifier potential to second anode potential, and spot distortion or astigmatism due to unsymmetrical field distribution.

It has also been found that with cathode-ray tubes so shaped that their diameters increase gradually towards their screens, serious distortion of the spot on the screen and serious distortion of the patterns occur When ratios of more than about two to one are used.

It is well known that the amount of deection of the beam decreases with increase of accelerating potential when the accelerating voltage is applied to the beam of electrons before the beam is deected. However, by applying some of the accelerating potential after deflecting the beam,

the acceleration of the beam can be increased without decreasing the deflection sensitivity proportionally. But heretofore when accelerating potentials have been applied to cathode-ray beams after deection thereof, distortion of the pattern traced by the beam on the 'screen and decrease of the amount of screen area that vwas utilized have resulted inthe cases VWhere the inftensiiier potential was considerably more than twice that of the second lanode potential.

Troubles which have heretofore limited the ratio of intensifier vpotential to second anode potential in cathode-ray tubes, but are overcome by the vpresent invention, are: voltage breakdown between electrodes is avoided; haphazard electrical 'charges on the glass walls of such tubes are prevented, thus avoiding axial dissy-mmetry and permitting operation at higher potentials; nonuniformity of screen potential and potentials at other points on 'the inside of the tube bulb caused by tube mountings and other objects that come into contact with the outside tube which have occurred when high post-accelerating potentials were applied to the beams with deection plates at ground potential are improved. Also, distortion of the pattern on the screen is avoided or greatly improved and much better vfocussing of the beam is obtained.

By the Ypresent invention accelerating potential can be applied to a cathode-ray beam without serious distortion after the beam has been deected even when such potential is up to about ten times that of the second wanode potential.

The invention may be understood from the description in connection with the accompanying drawing which is 'a somewhat diagrammatic side View of an illustrative embodiment of the invention.

In the drawing, reference character I indicates a glass cathode-ray tube which comprises a neck 2 and body portion 3 which are cylindrical. The cathode-ray beam generating, accelerating and focussing electrodes of a known sort are diagrammatically indicated by reference character 4. The horizontal and vertical deflection plates are indicated at 5 and 6. The way to generate and deflect the beam so that the end thereof will traverse the inner face of the tube is old and well known.

A plurality of annular surfaces or rings 8, 9, l0 and Il of conducting material such as platinum or graphite with spaces between adjacent rings is applied to the inside wall of the cylindrical body portion 3 of the tube. The ring 8 may be wide enough t0 extend some distance into the neck 2 of the tube. The cylindrical shape of this portion 3 enables the beam to be kept at a maximum distance from the edges of electrostatic fields where distortions of the cathode-ray beam would otherwise occur. These rings 8, 9, I and I! are maintained at successively higher potentials in any desirable way. For example, these rings may be connected by terminals I2, I3, I4 and I5 to successively higher potentials from sources, not shown, which provide the lowest potential at terminal I2 and the highest at terminal I5, ranging by equal steps from say 10,000 volts to 30,000 volts. The large cylindrical portion 3 of the tube I may be coated on the inside with a thin layer I6 of material of high resistance indicated by dots, with potentials applied only to terminals I2 and I5, the potential applied at terminal I being suiliciently higher than that applied to terminal I2 to provide the desired difference of potential.

When such a resistive coating is used, electrical charges are prevented from accumulating on the wall of the tube between conducting areas 8, 9, I0 and II and no arcing occurs between these areas even when a Very high dilerence of potential is applied between areas 8 and Il. Also, no external connections are necessary for the areas 9 and I0 when the resistance coating is used, and stray emission of electrical Charges does not take place or is greatly reduced.

A screen or layer or" luminescent material 20 is applied to the inside surface of the large end of the tube I. An electrically conducting coating or layer 2l of material such as a coating of chromium thin enough to be substantially transparent may be applied to the inside surface of the face of the tube I before the luminescent material 20 is applied. This coating together with the coating I6 and the conducting rings 8, 9, IB and I I, form a complete internal electric shield ior the tube, thus avoiding interference with the functioning of the tube by external electric elds, and definitely fixing the potential distribution which determines the tube performance.

With this invention a 10 to 1 ratio of postaccelerating potential of the cathode-ray beam after the electrons leave the high potential anode indicated at 4' may be used without substantially reducing the useful screen area and with little, if any, distortion of the luminous spot which the beam makes on the screen 20. Also, with a 10 to 1 ratio there is only slight reduction of useful screen area, and this useful screen area remains circular instead of assuming a barrel-like shape.

Instead of the rings 8, 9, I 0 and II shown on the drawings being equally spaced, the spacing of these rings may be changed so that, for example, the spacing between the ring 8 and the ring 9 4 would be much larger, and the other spacings would be smaller. Such distribution of the rings results in further reduction of pattern and spot distortion.

What is claimed is:

1. A cathode-ray tube having a neck and a cylindrical body portion of larger diameter than said neck, a series of spaced bands of conducting material on the inside of said cylindrical portion, a luminescent screen at the end of said cylindrical portion said screen being applied over a transparent conductive coating and a high resistance coating connecting said transparent coating to one of said bands of conducting material.

2. A cathode-ray tube having a neck and a cylindrical body portion of larger diameter than said neck, a series of spaced bands of conducting material on the inside of said cylindrical portion, and a luminescent screen at the end of said cylindrical portion electrically connected to said band closest to it by a high resistance coating and a transparent conductive coating over which said luminescent screen has been applied.

3. A cathode-ray tube having a neck and a cylindrical body portion of larger diameter than said neck, a series of spaced bands of conducting material on the inside of said cylindrical portion and a luminescent screen at the end of said cylindrical portion connected to the band of conducing material closest to it by a high resistance metallic coating and a transparent conductive coating over which said screen has been applied.

RUDOLF FELDT. IRVING EDlVIUND LEMPERT.

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

UNITED STATES PATENTS Number Name Date 2,123,636 Schwartz July 12, 1938 2,181,563 Knoll Sept. 27, 1938 2,266,773 Law Dec. 23, 1941 2,269,115 Koch Jan. 6, 1942 2,278,478 Oliver Apr. '7, 1942 2,297,752 Du Mont Oct. 6, 1942 2,404,046 Flory et al. July 16, 1946 2,409,514 Pratt Oct. 15, 1946 2,413,276 Woli Dec. 24, 1946 FOREIGN PATENTS Number Country Date 441,693 Great Britain Jan. 20, 1936 549,278 Great Britain Nov. 13, 1942