US3136918A - Cathode ray tube and method of operation - Google Patents

Description

June 9, 1964 R. T. WATSON 3,136,918

CATHODE RAY TUBE AND METHOD OF OPERATION Filed Dec. 16, 1960 mmvron. Fazszr 7. MUM

This invention relates to improvements in cathode ray tubes of the type employing post-deflection acceleration of the electron beam and to methods of operation thereof.

One type of cathode ray tube includes an envelope having a faceplate, a neck, and an interconnecting funnel section. An electron gun is disposed in the neck and projects an electron beam toward the faceplate. Means is provided either as a part of the electron gun or as a separate means external of the envelope for bidirectionally deflecting the electron beam as it passes through a primary deflection zone located in the region where the neck is I joined to the funnel.

It has recently been proposed to provide such tubes with post-deflection acceleration arrangements wherein a mesh electrode is mounted across the envelope in the electron beam path just beyond the primary deflection zone. In such an arrangement, the practice has been to operate the mesh electrode at a potential equal or approximately equal to the potential on the last electrode of the gun and to operate a conductive coating electrode on the funnel and on the screen at a considerably higher potential. Thus, the electrons of the beam are projected at a relatively low velocity through the primary deflection zone, and then, after passing through the mesh electrode, are greatly accelerated for high velocity impingement on the phosphor screen.

In such tubes according to the prior art, the mesh electrode has either been mounted as a part of the electron .gun'or as a separate electrode mounted in the funnel section of the envelope. Although gun-mounting of the mesh electrode is desired for simplicity of tube fabrication, funnel mounting arrangements are, on the other hand, preferred for beam deflection reasons and for wide deflection angle tubes. Mounting of the mesh as a part of -the gun limits the transverse dimension of the mesh elecnode to one which can be inserted through the neck of the tube. Thus, deflection in the primary deflection zone is physically limited not by the diameter of the neck opening, but rather by the diameter of some frame structure of the mesh electrode.

On the other hand, when the mesh electrode is mounted in the funnel the beam can be deflected a greater. amount in the primary deflection zone permitting extremely wide-angle tubes to be de signed In extremely wide-angle deflection tubes of the type described, it is'desirable that maximum deflection in the primary deflection zone be obtained. This desirability requires that the mesh electrode thus. be mounted in the funnel of the envelope which results in a relatively extensive spacing of the mesh from the electron gun and an extremely close spacingof the mesh from the phosphor screen. As a result, secondary electrons emitted by the -mesh are attracted to the screen rather than to a gun electrode and result in decreased contrast of the produced image.

In cathode ray tubes of the type described, it is also known to correct certain raster distortions such as pin- United States Patent U M 3,136,918 Patented June 9, 1964 cushioning by selectively shaping the mesh electrode, e.g., into various domed contours. However, for purposes of economy, it is preferred to avoid complex mesh electrode contours. I

It is therefore an object of my invention to provide a new and improved cathode ray tube of the type described and a method of operation thereof which provides good scan sensitivity, improved contrast of the image, and economical correction of common raster distortions.

Briefly, according to my invention, a cathode ray tube of the type described employs a novel combination of post-deflection acceleration and post-deflection deceleration of the electron beam. Three separate conductive surfaces are provided on the inner wall of the envelope such as by coatings of conductive material. A first coating extends from the mesh electrode back into the neck of the tube. A second coating is disposed on the funnel and spaced from both the mesh electrode and the phosphor to each of the conductive coatings.

In the operation of the cathode ray tube, according to my invention, a given voltage is applied tothe first coating; a substantially highervoltage is applied to the second coating; and a voltage having a value between that applied to the first and second coatings is applied to the third coating. Accordingly, a post-acceleration system is provided wherein the electron beam is deflected at a relatively low velocity to provide scan sensitivity, and then upon passing through the mesh electrode is rapidly accelerated. Following this, the electron beam is decelerated before impinging upon the phosphor screen. By virtue of the decelerating field wherein the phosphor screen voltage is lower than the voltage on the funnel, secondary electrons emitted from. the mesh electrode are attracted to the funnel, thus improving image contrast.

According to a preferred embodiment of my invention, the first and second conductive coatings on the neck and funnel, respectively, are separated by a specially-shaped high resistance coating to provide a desired control of linearity and pincushion correction. An annular conductive plate is mounted at one of its edges to adjacent one edge of the high resistance coating and extends out over the high resistance coating. This plate prevents electrons from impinging on the high resistance coating and thereby prevents an objectionable charge build-up of electrons on the coating.

In the drawings:

FIG. 1 is an elevation view with parts broken away and partly in axial section of a preferredembodiment of a cathode ray tube according to my invention; and

FIG. 2 is a transverse section view through the funnel of a cathode ray tube according to my invention looking toward the neck, and wherein' the mesh electrode and electron gun are removed so as to more clearly illustrate the shape of the high resistance coating.

Structure In FIG. 1 a cathode ray tube 10, according to my invention, is shown to comprise a glass envelope 12 having a faceplate 14, a neck 16, and an interconnecting funnel section 18. A phosphor screen 20 is provided on the inner surface of the faceplate 14. The phosphor screen 20 may comprise any suitable phosphor material appliedby any of the well-known techniques such as -settling,-slurry-ing, or evaporating. a

" An electron gun 22 is mounted in the neck 16 and is.

adapted to project an electron beam 23 toward the phosphor screen 20. The electron gun 22 m'ay'be of any suitable type as is also well known in the art. Suitable electrical potentials are applied to the electrodes of the gun 22 through terminal prongs 24 which form part of a stem base structure 26. v

Suitable means, such as'magnetic deflection 0011s 28, is

provided for bidirectionally deflecting the electron beam 23 in a primary deflection zone 32. As is shown, the primary deflection zone 32 comprises the region just beyond the electron gun 22 where the neck 16 joints the funnel section 18. Y

A dome-shaped mesh electrode'34 is disposed transversely in the path of thebeam 23 adjacent the primary deflection zone 32 and between the zone 32 and the phosphor. screen 20. The mesh electrode 34 comprises a mounting ring 36 to which a dome-shaped mesh member is attached. The mounting ring 36 is in turn fixed to the funnel section 18, such asby cementing thereto or by sealing it in or throughthe glass wall of the funnel 18.

Although a domed shaping of the mesh electrode 34is preferred for reason hereinafter described, other shapes,-

such as flat, may be provided.

In accordance with myinvention, the

- 1 ward the corners of the'rectangular phospho'r'screen 20. As will be 'hereinatfer described, suchf lobed shape of the I resistive coating 50 serves to desirably shapethelectrostatic fields which 'afiect the resulting shape of the scanned 1 inner surface of i r the envelope 12 is provided withthree separated conductive surfaces. In the case of the all-glass envelope 1 2, as I illustrated, the conductive surfaces are provided as conductive coatings on the glass envelope. G e

A first conductive coating -38 is provided on a first portion of the envelope extending from and in contact withthe mounting ring 36 toward and into the neck 16.

. The final electrode 40 of, the electron gun 22 may also be .44 may be specially shaped along its edge adjacent the first conductive coating 38. Both the first conductive coating 38 and the second conductivecoating 44 may comprise any known, suitable material, such as, a carbon I cussed hereinafter in more detail. n t With respect to the above-mentioned feature 'ofpre screen coatings, respectively; 'However, it

oxide. Provision of the high resistance coating 50 results in a uniformfpotential drop fromthe neck coating 7 38 to the funnel coating 44, prevents the' area between V the neck and funnel coatings 38 and 44 fromretaining the chargefof any electrons which might impinge there on, and also prevents the corners of the scanned raster .from expanding toorapidlythereby causing pincushioning andnonlinearity. This latter feature will be dis- .vention of pincushiom'ng, theQresistive coating 5.0is

[preferably specially shaped as shown in FIG. 2. As there 'shown inthe case of a tube 10 having ajgenerally rec? tangular phosphor screen20, the resistive coating 50 is shaped to comprise'four lobes 52 extending generally toraster on'the phosphor screen 20.

An annular metallic shield 53 is provided ini'conjunce '7 A ,tion'with the high resistance coating 50. The shield 53 is mounted at-itsinner edge'to therim36 of the mesh electrode'ancl extendsout over the resistance coating 50; 4 The shield 53 prevents excessive impingement of electrons upon the resistance coating 50 whenithe beam 23 is 'scanned closely thefreto j v a V The three conductive'coatings 38, 44,'and 46-are electrically separated from each other-so that they maybe composition. Such a material Widely used in the industry 1 is known as Aquadag.

A third conductive coating is provided over the phosphor screen20. .The third coating 46 may comprise a suitable, known material, such as aluminum, evaporated upon the phosphor layer 20.

the phosphor layer 20, an aluminum layer. be evaporated upon the organic layer, andthen the organiclayer be' volatilized by baking and thereby removed. .According to such known techniques, a smooth conductive thin aluj minum coating 46 is provided on the phosphor layer 20.

The third conductive coating 46 is coextensive with the phosphor layer 20 and is spaced from the second conductive coating 44 by an annular area 48. The annular area 48 may be coated with a high resistance material or, as shown, may comprise only the bare surface of the envelope 12. Inorder tomount the mesh electrode 34 within the envelope 1 2, it may bede'sired to provide' access for mountingby providing a separatefaceplate 14 and funnel 18, which are then frit sealed together in the region of the annular area 48; Accordingly, a special high resistance frithaving a conductivity greater than ordinary glass can be'used so as to provide the same -etfect'as would be achieved by coating the annular area 48 with a high resistance material, such as iron oxide.

For purposes of simplicityof reference,1the first coat- 'ingf38, second coating 44, and third coatingi46 will hereinafter be referred to simplyas the neck, funnel, and

According to well-known techniques, an organic material may first be appliedto operated at' three different electrical 1 potentials, Ac-

cordingly, separatederminal means 54, .56, and 58 are providedformaking electrical connections,respectively, 1 to the'neck, funnel, and screen coatings 38, 44,and 46.

As shown inFIG. 1, the terminal means'54, 15,6,'and 58 may comprise lead-ins sealed through the 'glass envelope 12 and contacting their'associated conductive coatings.

2 Operation; 2 I

. Thetube 10 is operated withanovel combination {of post-deflection acceleration and post-deflection deceleration of the electron beam23. Post deflection acceleration is obtained by applying a relatively low potential .to

' the'neck coating'38' and a relativelymuch higher potential to the funnel' coating 44. These potentials may for example, be respectively ina ratio ofapproximately 1 to 2. 'In accordance with such operation, the" electron A 1 "beam 23 is projected at relatively low velocity through the primarydefiection zone 32' and is thus'given an excep tionally large deflection by a comparatively low' power magnetic deflection yoke 28. After passing through the mesh electrode 34, the relatively high voltage foni the funnel coating 44 causes theelec'tron bea'm 23 to be substantially accelerated.' Thus, ,both good scan sensitivity and go'o'd'lightoutput are obtained. a

Deflection sensitivity is enhanced not only bythe low ivelocity beam deflection in the primary. deflection zone 32, but also by virtue .of-the electrostatic field established between theme'sh electrode 34 and the funnel coating 44.

' This field is generally illustrated by the equipotential lines A, B, C, and D. When the electron beam 23 is projected through the accelerating field ABCD in a peripheral re-" gion as indicated in FIG; 1, the accelerating field ABCD, by'virtueof its shape;v tends'to defiectthe beam 23 fur- -ther:out toward the edge 'of-the phosphor screen 20' to provide further scan enhancement: I

Prew'ously proposed dome mesh post-accelerating tubes' have comprised a single conductive coating 'over boththe funnel and the phosphor screen so that these portionsof the tube were necessarily operated at the same electrical 1 potential. V In such "tubes, secondary electrons emitted will be apprei "Iciated that the neck coating'38 may ac'tually extend a part way onto the funnel18 as shown. The neckand 1 funnel coatings 38 and 44 are separated by a highre sistance' coating 50 of ajsuitable material, suchlas iron from the mesh electrode caused a reduction in contrast in two different ways: (1) secondary electrons emitted from the front of the mesh electrode, i.e., facing the phosphor screen, were attracted directly to the phosphor screen, and resulted in diffused luminescence thereof; (2) secondary electrons emitted from the back of the mesh electrode, i.e., facing the electron gun, were focused through the mesh electrode openings and attracted onto the phosphor screen to give a halo effect around the beam.

This halo results from the fact that the secondary electrons are lower in velocity than the primary beam electrons and hence respond to the supplementary deflection field ABCD to a greater extent than do the primaries.

In tubes of this type wherein the funnel coating 44 is separate from the screen coating 46, such reduced contrast is substantially prevented by operating the screen coating 46 at a potential somewhat below the potential on the funnel coating 44. I have found that a screen potential approximately halfway between the potentials on the neck and funnel coatings 38 and 44 is suitable.

By applying a lower potential to the screen coating 46 than exists on the funnel coating 44, a decelerating electrostatic field is established. This field is represented by the equipotential lines E, F, G, and H. In the case of an electron beam 23 projected off the central axis of the tube 10, such as is illustrated in FIG. 1, the decelerating field EFGI-I will serve to supplement the deflection of the electron beam 23. Even more important though, secondary electrons emitted from the mesh electrode 34 will now be attracted to the higher potential funnel coating 44 rather than to the phosphor screen 20.

It will be appreciated that the novel combined post acceleration and post deceleration operation does not necessarily require that the high resistance coating 50 or the annular shield 53 be included in the tube 10. The resistive coating 50 and the annular shield 53 constitute preferred embodiment features. Even in the absence of these features, the three-coating, three potential, post-acceleration-deceleration operation provides significant improvement over prior art tubes.

Although it has been proposed by the prior art to correct pincushioning by a special shaping of the dome mesh 34, it is preferred that any complex fabrication of the mesh electrode 34 be avoided, if possible. According to my invention, the mesh electrode 34 can be provided with a simple spherical curvature. Pincushion correction can then be obtained by a special shaping of the resistive coating 50. FIG. 2 illustrates this special shaping. As shown therein, the resistive coating 50 when used with a generally rectangular phosphor screen 20, comprises four lobes 52, one each extending generally toward the corner of the rectangular phosphor screen 20. The effect of the lobes 52 is to weaken the concentration of the equipotential lines A, B, C, and D in the region where they curve in toward the edge of the mesh electrode 34 and the funnel coating 44. Such weakening of the field results in the electron beam 23 being subjected to less of a supplementary deflection as it emerges from the mesh electrode 34. Since this weakening of the electrostatic field ABCD is provided generally in line with the corners of the rectangular raster scanned on the phosphor screen 20, the effect is that the corners of the raster are pulled in, thus correcting pincushioning.

As previously stated, the annular shield 53 serves to prevent electrons from striking the resistive coating 50 and thus cause an electron charge to be built up thereon. Such charge build-up would ordinarily occur when the beam 23 is scanned very close to the resistive coating 59 during that time when the corners of the raster are being scanned. Thus, such charge build-up would occur generally in four localities, i.e., in the region of each of the lobes 52. The result of such charge build-up is that the beam in passing adjacent the charged areas is deflected away therefrom. Thus, a clipping of the corners of the raster results. Such corner clipping would not constitute a correction of pincushioning since it would in fact amount to merely clipping the corners rather than shrinking the overall deflection in the directions toward the corners. The presence of the annular shield 53 exerts some effect on the shaping of the electrostatic field ABCD. Should it be desired to utilize to the fullest advantage a specialized shaping of the domed mesh electrode 34, then it may be desired to eliminate the annular shield 53.

According to a preferred embodiment and operation of the tube 10, the resistive coating 50 was shaped as shown in FIG. 2 and the annular metal shield 53 was mounted to the support ring 36 of the mesh electrode 34. The phosphor screen 20 conformed to that of industry standard 21 inch tube types. The mesh electrode 34 was provided with a spherical curvature of approximately 1.9 inches radius and the diameter of the mounting rim 36 was about 4 inches. Voltages of 10 kv., 22 kv., and 17 kv., were applied respectively to the neck, funnel, and screen conductive coatings 38, 44, and 46. The final electrode 40 of the electron gun 22 was connected to the neck coating 38 by contacts 42 as illustrated. A magnetic deflection yoke of the type generally employed for providing a maximum angle of deflection, approximately of that produced in the tube 10, was used and energized for such 80% deflection. With the tube 10 operated as herein stated, a full raster was obtained which has good linearity, good pincushion correction, and good contrast.

What is claimed is:

l. A cathode ray tube comprising an envelope, including a faceplate, a neck, and an interconnecting funnel, a

phosphor screen on said faceplate, an electron gun in said neck for projecting an electron beam upon said screen, a mesh electrode mounted across said envelope in said beam path adjacent the neck end of said funnel, a first conductive coating on said envelope extending from said mesh toward said gun, a second conductive coating on said envelope between and spaced from both said screen and said mesh, a third conductive coating on said screen, a high resistance coating on said envelope separating said first and second conductive coatings, an annular conductive shield supported at one edge thereof from adjacent an edge of said high resistance coating and extending out over said high resistance coating, and separate external terminal means connected to each of said conductive coatings for supplying different electrical potentials thereto.

2. A cathode ray tube comprising an envelope including a faceplate, a neck, and an interconnecting funnel, a phosphor screen on said faceplate, an electron gun in said neck for projecting an electron beam upon said screen, a mesh electrode mounted across said envelope in said beam path adjacent the neck end of said funnel, a first conductive surface on said envelope extending from said mesh toward said gun, a second conductive surface on said funnel between and spaced from both said screen and said mesh, a third conductive surface on said screen, and means including a voltage source for applying a first electrical potential to said first conductive surface, a second electrical potential substantially higher than said first electrical potential to said second conductive surface, and a third electrical potential having a value between said first and second electrical potentials to said third conductive surface.

3. The cathode ray tube according to claim 2 and wherein said second potential is appproximately twice said first potential and said third potential is approximately halfway between said first and second potentials.

4. The cathode ray tube according to claim 3 and wherein said first potential is about 10 kv., said second potential is about 22 kv., and said third potential is about 17 kv.

5. A cathode ray tube comprising an envelope including a generally rectangular faceplate, a neck, and an interconnecting funnel, a generally rectangular phosphor screen on said faceplate, an electron gun in said neck for projecting an electron beam upon said screen, a mesh electrodemounted'across said envelope in said beam path 'adjacent to the neck end of said'funnel, a first conductive coating on said envelope extending from said mesh toward said gun, a second conductive coating on said er'ivelope between'and spaced from both said screen and said mesh a third conductive coating on said screen, a

high resistance coating on said envelope separating said "first and second conductivecoatings and having a shape comprising four lobes extending generally toward the four corners of said rectangular screen, and separate external terminal nieans connected to each of said conductivecoatings for supplying different electrical potentials thereto.

References Cited in the file of this patent UNITED STATES PATENTS Burdick etl; Apr. 25, 1961

Claims (1)

1. 5. A CATHODE RAY TUBE COMPRISING AN ENVELOPE INCLUDING A GENERALLY RECTANGULAR FACEPLATE, A NECK, AND AN INTERCONNECTING FUNNEL, A GENERALLY RECTANGULAR PHOSPHOR SCREEN ON SAID FACEPLATE, AN ELECTRON GUN IN SAID NECK FOR PROJECTING AN ELECTRON BEAM UPON SAID SCREEN, A MESH ELECTRODE MOUNTED ACROSS SAID ENVELOPE IN SAID BEAM PATH ADJACENT TO THE NECK END OF SAID FUNNEL, A FIRST CONDUCTIVE COATING ON SAID ENVELOPE EXTENDING FROM SAID MESH TOWARD SAID GUN, A SECOND CONDUCTIVE COATING ON SAID ENVELOPE BETWEEN AND SPACED FROM BOTH SAID SCREEN AND SAID MESH, A THIRD CONDUCTIVE COATING ON SAID SCREEN, A HIGH RESISTANCE COATING ON SAID ENVELOPE SEPARATING SAID FIRST AND SECOND CONDUCTIVE COATINGS AND HAVING A SHAPE COMPRISING FOUR LOBES EXTENDING GENERALLY TOWARD THE FOUR CORNERS OF SAID RECTANGULAR SCREEN, AND SEPARATE EXTERNAL TERMINAL MEANS CONNECTED TO EACH OF SAID CONDUCTIVE COATINGS FOR SUPPLYING DIFFERENT ELECTRICAL POTENTIALS THERETO.

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