US3334258A

US3334258A - Apparatus for effectively reducing overscan in a cathode ray tube

Info

Publication number: US3334258A
Application number: US394563A
Authority: US
Inventors: Sam H Kaplan; Theodore S Noskowicz
Original assignee: Rauland Borg Corp
Current assignee: Rauland Borg Corp
Priority date: 1964-09-04
Filing date: 1964-09-04
Publication date: 1967-08-01
Anticipated expiration: 1984-08-01

Description

Aug.1, 1967 'SH KAPLAN Em 3,334,258

APPARATUS FOR EFFECTIVELY REDUCING OVERSCAN IN A CATHODE RAY TUBE Filed Sept. 4. 1964 INVENTORS Sam H. Kaplan Theodore S. Noskowicz AT RNEY United States Patent O 3,334,258 APPARATUS FOR EFFECTIVELY REDUCING OVERSCAN IN A CATHODE RAY TUBE Sam H. Kaplan and Theodore S. Noskowicz, Chicago, Ill., assignors to The Ranland Corporation, Chicago, Ill., a corporation of Illinois Filed Sept. 4, 1964, Ser. No. 394,563 8 Claims. (Cl. 313-75) This invention is directed in general to cathode ray tubes and, more specifically, concerns increasing the contrast of such tubes.

In cathode ray tube operation, especially in the case of those used for television receivers, it is common practice to overscan the screen area in displaying an image on the tube face. The practice of overscanning has been adopted in order to compensate for both variations in line voltage and for the conliguration of the tube envelope itself. It is known, for example, that variations in line voltage aiect picture size and loverscanning assures an image covering the entire screen area even in the face of the minimum voltage expected. With respect to envelope conguration overscanning at the corners of the screen is unavoidable if the scanning pattern is to till the whole screen area.

A disadvantage of overscanning is loss of contrast since electrons of the scanning beam tend to be reflected off the neck and sides of the tube producing secondary electrons and both the primary and secondary electrons rain onto the screen in a random pattern.

Electron shields having a maximum diameter less than the diameter of said neck have heretofore been suggested to intercept the electrons and avoid loss of contrast, but they have had many disadvantages. For example, some shields have been positioned adjacent the screen area but this type of shield is relatively inetlicient since most stray electrons which are generated in the funnel close to the neck of the tube are not intercepted by such a shield. It has also been suggested that a shield having a rectangularly shaped aperture be placed in the funnel adjacent to the neck. This requires, however, that the tube have a removable faceplate to allow for the placement of the shield which is larger than the tube neck. The use of such a shield would be prohibitive for black and white television because of the attendant cost of the two-piece envelope construction. Finally, electron shields featuring a rectangular opening require that the angular orientation of the shields be absolutely precise. Again, this requirement would significantly increase the production cost of the tube.

It is, therefore, a major object of this invention to provide a cathode ray tube with improved contrast capability.

It is another object of this invention to prevent the impingement of stray electrons upon the screen of a cathode ray tube.

It is still another object of the invention to accomplish the above in a simple and economical manner.

The invention is practiced in a cathode ray tube having a beam deection eld with predetermined center of deflection. In accordance with the invention a cathode ray tube comprises an evacuated envelope including a neck, a funnel and a substantially rectangular-shaped faceplate having an image screen thereon. An electron gun is supported within the neck portion for projecting an electron beam along a predetermined path, through the deflection lield, toward the screen. An electron shield having a maximum diameter less than the diameter of the tube neck 3,334,258 Patented Aug. 1, 1967 ICC and having a circular aperture is supported by the gun with the shield on the screen side of the center of deflection and with the aperture thereof disposed across the path of the beam and substantially co-axial with the envelope funnel. The position of the shield and the diameter of the shield aperture are such that the deflected electron beam is effectively shadowed from the initially ared portion of the envelope funnel and is confined to scanning on the screen a pattern having a maximum dimension equal to the diagonal measure of the rectangular faceplate.

The features of the present invention which are believed to be novel are set forth with particularity in the appended claims. The organization and manner of operation of the invention, together with further objects and advantages thereof, may best be understood by reference to the following description taken in connection with the accompanying drawing, in the -several figures of which like reference numerals identify like elements, and in which:

FIGURE 1 is a partial schematic diagram showing a cross-sectional view of a cathode ray tube and associated yoke assembly including the subject invention;

FIGURE 2 is a partial cross-sectional view taken along lines 2 2 of FIGURE 1; and

FIGURE 3 is a perspective view partially cut away of a portion of the gun structure of the cathode ray tube of FIGURE 1.

Referring now to FIGURE l, the cathode raye tube 10 there shown comprises an evacuated envelope having a neck portion 11, a ared conical or funnel portion 12 and a substantially rectangular-shaped faceplate 13 opposite neck 11, which bears a rectangular viewing or image screen. A phosphor layer 15 is coated on the inside of faceplate 13 and an aluminum layer 16 backs phosphor layer 15 to increase the brightness of the television picture in a manner well known in the art. A gun assembly 17, which in this case consists of a single electron gun, is located in the neck of the tube and is oriented to project a beam of electrons along a predetermined path, through a deflection field presently to be discussed, toward viewing screen 13. As is usually the case, the beam path corresponds with the longitudinal axis or center line of the tube indicated by dashed line 14. Gun assembly 17 includes a cathode 18,

control grids

19 and 20 and accelerating-focusing

electrodes

21, 22 and 23. All of the individual elements of the gun are retained by supporting posts 25 and the unitary structure is centered in neck 11 by means of a pair of snubber springs 26 which are welded to the flange end of nal electrode 23. The springs 26 also establish a conductive connection between electrode 23 and screen 15 by contacting an internal conductive coating 27 of colloidal graphite which is painted on the interior of funnel 12.

The detiection system for the tube includes a yoke 28 which encircles neck 11 and a portion of funnel 12. The yoke has a pair of horizontal deflection coils 29 and a pair of vertical deflection coils 30, illustrated in FIGURE 2. A cylindrical iron or ferrite core 32 surrounds both sets of deection coils and the entire assembly is maintained in position by a retaining ring 51. The yoke 28 also includes a non-magnetic support frame 33 which separates vertical coil 30 from horizontal coil 29. As thus far described the cathode ray tube is of standard construction and operation.

Numeral 31 indicates the yoke reference line (Y.R.L.)

31 which is a standard reference for a cathode ray tube for determining the location of the gun assembly.

The position lon axis 14 at which lateral deflection of the electron beam is initiated is represented at point 24. The electron beam is of course curved since it passes through -a deflection field of lfinite length. The maximum deflection is represented by dashed lines 36 extending from point 24 to` the edges -of screen 13.

In laccordance with the invention, improvement in contrast is obtained through the use 'of an electrically conductive shield 34 (see FIGURE 3) having a circular, centrally located aperture 34a through which the beam passes on its way to screen 13. Shield 34 is supported by gun assembly 17 with the shield positioned on the screen side `of reference point 24 yand with the aperture disposed across the beam path. More particularly, shield 34 is attached to final electrode 23 of the gun by means of a pair of supports 35 which extend from the flanged portion of the final electrode. The location of shield 34 and the diameter of its aperture 34u determine the scanning pattern traced by the electron beam 4on screen 13 in response to the deflection field produced by vyoke 28. For example, as the shield is moved toward the screen past deflection point 24 its aperture must increase in size to accommodate the normal sweep range of the electron beam. Conversely, as the shiel-d is moved back toward reference point 24 shield aperture 34a may be reduced in size to accommodate the same scanning excursion of the beam. In practicing the invention, the position of the shield and the diameter of aperture 34a are such that the electron .beam of the tube, in response to the deflection eld of yoke 28, scans on screen 13 a pattern having a maximum dimension equal to the diagonal measure of the rectangular faceplate. An electron beam which is overscanned loutside of this limit is of course intercepted by the shield as indicated by lines 52. Shield 34 is discontinuous, including gap 38 shown in FIGURES 2 and 3 to prevent the establishment of eddy currents. However, an offset portion 40 overlaps the gap and simulates a continuous intercepting surface for the shield.

A getter ring 37 is attached to the side of shield 34 which faces the screen area.

Dual centering means are pnovided for centering the electron beam in aperture 34a of shield 34 and in yadldition, for centering the scanning raster on screen 13. The first centering means includes a pair of

permanent ring magnets

41, 41 which encircle neck 11 and are received in support frame 33t. This is a well-known arrangement for centering the electron beam in Iaperture 34a. An additional pair of

ring magnets

42, 42 are `afiixed to yoke 28 and encircle funnel section 12 of the tube to center the raster `on the screen. The ring magnets themselves are well known in the art, each ring being polarized with a north and south pole at diametrically opposed positions on the ring. When the rings of any pair are rotated relative to one another to have their respective north and south poles adjacent, essentially zero deflection is imparted to the electron beam; conversely, when `all poles are adjacent a maximum deflecting force is produced.

In operating the described tube structure, after the tube has been energized its electron beam is centered in aperture 34a by manipulation of rings 4l1, 41. Thereafter, the scanning r-aster, which results from the influence of the deflection fields of yoke 28 upon the beam, is centered ron screen 13 by manipulation of the remaining pair of centering magnets 42, 4t2. Of course, these latter centering magnets may be replaced by a controlled D.C. centering current supplied to the deflection yoke. Having thus adjusted the scanning, the tube is operated in conventional manner but it will be found that there is a significant improvement in contrast. The aperture 34a of shield 34 causes the scanning pattern on screen 13 to` cover the rectangular image area of the faceplate and to have a maximum dimension equal to the diagonal dimension of the faceplate. As a consequence, overscanning, especially at the corners of the raster, is avoided since electrons of the beam that would accomplish overscanning such as those represented by lines 52 are, in fact, intercepted by shield 34. By thus confining the sc-ansion of the beam, particularly at the corners of the faceplate, the stray electrons, both primary and secondary electrons, which heretofore have diluted contrast, are greatly minimized because the beam does not now impinge against portions of the envelope which give rise to such stray electrons.

In addition to improving contrast, the described structure is free of the difficulties attendant the use of shields having rectangular-shaped apertures. The described structure, featuring a shield with a circular aperture, greatly facilitates the production problems and minimizes the cost of the tube. Moreover, since the shield may be mechanically supported from the gun assembly and may have overall dimensions that permit its being positioned within the neck yof the tube, the invention is applicable to monochrome tubes and does not require that the faceplate be severable from the funnel section of the envelope.

While a particular embodiment of the invention has been shown and described, it will be obvious to tho-se skilled in the yart that changes and modifications m-ay be made without departing from the invention in its broader aspects, and, therefore, the aim in the appended claims is to cover all such changes .and modifications las fall within the true spirit and scope of the invention.

We claim:

1. A cathode ray tube having a beam deflection eld with a predetermined point at which beam deflection is initiated, `said tube comprising:

an evacuated envelope including la neck, a funnel,

and a substantially rectangular-shaped faceplate having an image screen thereon;

an electron gun within said neck f-or projecting an electron beam along a predetermined path, through said deflection field, toward said screen;

and an electron shield having a maximum ldiameter less than the diameter of said neck and having a circular aperture, supported by said gun with said shield on the screen side of said predetermined point and with the aperture thereof disposed across said path and substantially cO-axial with said envelope funnel, the position of said shield and the diameter of said Iaperture being such that the deflected electron beam is effectively shadowed from the initially flared portion of said funnel and is confined to scanning on said screen a pattern having a maximum dimension equal to the diagonal measure of said rectangular faceplate.

2. A cathode ray tube having a beam deflection field with a predetermined point at which beam deflection is initiated, said tube comprising:

an evacuated envelope including a neck, a f-unnel, and

a substantially rectangular-shaped faceplate having an image screen thereon;

an electron gun within said neck for projecting an electron beam `along a predetermined path, through said deflection field, toward said screen;

`an electron shield having a maximum diameter less than the diameter of said neck and having a circular aperture, supported by said gun with said shield on the screen side of said predetermined point and with the aperture thereof disposed across said path and substantially co-axial with said envelope funnel, the position of `said shield and the diameter of said aperture being such that the deflected electron beam is effectively shadowed from the initially flared portion of said funnel and is confined to scanning on said screen a pattern Ihaving a maximum `dimension equal to the `diagonal measure lof said rectangular faceplate;

and a pair of independently adjustable centering means for centering said beam relative to the aperture of said shield and for centering said scanning pattern relative to said screen.

3,334,253 6 3i. A cathode nay tube having a beam deflection field to stimulate a continuous barrier for electrons exwith a predetermined point at which beam deflection is inicept for the aperture thereof. tiated, said tube comprising: 6. A cathode ray tu-be having Ia beam deflection field an evacuated envelope including a neck, a funnel, and with a predetermined point at which beam deflection is a substantially rectangular-shaped faceplate having an image screen thereon; y

an electron gun within said neck for projecting an electron beam along a predetermined path, through said deection field, toward said screen;

initiated, said tube comprising:

an evacuated envelope including a neck, a funnel, and a substantially rectangular-shaped faceplate having an image screen thereon;

an electron Igun within `said neck for projecting an electron beam along a predetermined path, through said deflection eld, toward said screen;

and a conductive electron shield having a maximum diameter less than the diameter of said neck Iand having a circular aperture and a slot extending from said aperture to the outer periphery of said shield, supported by said gun with said shield on the screen side of said predetermined point and with the aperture thereof disposed across said path and substantially co-axial with said envelope funnel, the position of said shield and the diameter of said aperture being such that the deflected electron beam is effectively shadowed from the initially flared portion of said funnel and is confined to scanning on said screen a pattern having a maximum dimension equal to the a conductive electron shield having a maximum diameter less than the diameter of said neck and having a circular aperture and a slot extending from said aperture to the outer periphery of said shield, supported by said gun with said shield on the screen side 5 initiated, said tube comprising:

'an evacuated'envelope including a neck, a funnel, and a substantially rectangular-shaped faceplate having an image screen thereon;

an electron gun within said neck for projecting an elecan electron shield having a maximum diameter less than lo tron beam along a predetermined path, through said the diameter of said neck and having a circular apdeection field, toward said screen; erture, SuPPorted 'hy Said gun With Said Shield 0n a conductive electron shield having -a maximum diamthe Sereen Side of Said predetermined Point and With eter less than the diameter of said neck and having the aperture thereof diSPoSed aeroSS Said Path and a circular aperture and a slot extending from said s-ubstantially co-axial with said envelope funnel, the aperture to the outer periphery of said shield, supposition of Said Shield and the diameter of Said ported by said gun withsaid shield on the screen side aperture being such that the deflected electron beam 0f Said predetermined point and with the aperture iS etfeetiVelY shadowed from the initially ilared Por' thereof disposed across said path and substantially tion of Said funnel and iS ooniined to Seanning on co-axial with said envelope funnel, the position of Said Sereen a Pattern haVing a rnaXirnunl dimension said shield and the diameter of said aperture being equal t0 the diagonal measure -of Said rectangular such that the deiiected electron bea-m is effectively faceplate; shadowed from the initially dared portion of said and a first centering device positioned on the gun side of funne1 and is confined t0 Scanning on said screen a Said predetermined Point for eentering Said hearn pattern having a maximum dimension equal to the relative to the aperture 0f Said Shield and a Second 25 diagonal measure of said rectangular faceplate; centering device positioned on the screen side of said a member overlapping Said slot to cause said shield Shield for centering Said Soanning Pattern relatiVe to simulate a continuous barrier for electrons except t0 Said SCreenfor the aperture thereof;

4- A Cathode 'ray tube having 'a beam deflection 'field :and a getter ring having an internal diameter substanwith a predetermined point at which beam dellection is tially larger than the diameter of said aperture, supported by said gun in coaxial alignment with and on the screen side of said shield. 7. A cathode ray tube having a beam deilection field with a predetermined point at which beam deilection is initiated, said tube comprising:

an evacuated envelope including a neck, a funnel, and a substantially rectangular-shaped faceplate having :an image screen thereon;

an electron gun within said neck for projecting an electron beam along a predetermined path, through said deflection field, toward said screen;

an electron shield having a circular aperture having a maximum diameter less than the diameter of said neck and, supported in said neck of said envelope by said gun with said shield on the screen side of said predetermined point and with the aperture thereof disposed across said path and substantially co-axial with said envelope funnel, the position of said shield and the diameter of said aperture being such that the deflected electron beam is eiectively shadowed from diagonal measure of saidrectangular faceplate. the initially flared portion of said funnel and is con- .5 A cathode lay tube, llavmg `a beam dellectloll lleld fined to scanning on said screen a pattern having a with a predetermined point at which beam deflection is maximum dimension equal to the diagonal measure initiated, said tube comprising: of Said rectangular faceplate;

all evacuated. envelope lllcllldlllg a neck a funnel dnd 55 and a pair of independently adjustable centering means a s ubstalltlally lectallgular'shaped faceplate havmg for centering said beam relative to the aperture of :anlmage Screen.th.eleol.li said shield and for centering said scanning pattern an electron gun Within said neck for projecting an elecrelative to said Screen.

troll bam along a Pledelelmllled Path through Sad 8. A cathode ray tube having a beam deflection field dellectlon eld lowald Sall Screen; with a predetermined point at which beam deflection is initiated, said tube comprising:

an electron gun within said neck for projecting an 0f Sad predetermined Poldi and With the aperture electron beam along a predetermined path, through thereof disposed across said path and substantially Said deflection eld toward said screen; eo-aXial With Said envelope funnel, the Position of an electron shield having a circular aperture having a Said Shield and the diameter 0f Said aperture being maximum diameter less than the diameter of said such that the deflected electron beam is effectively neck and, supported in said neck 0f said envelope shadowed from the initially dared portion of said by said gun with said shield on the screen side of funnel and is confined to scanning on said screen a said predetermined point and with the aperture therepattern having a maximum dimension equal to the of disposed across said path land substantially co-axial diagonal measure of said rectangular faceplate; with said envelope funnel, the position of said shield and a member overlapping said slot to cause said shield and the diameter of said aperture being such that the deected electron beam is effectively shadowed from the initially flared portion of said funnel and is confined to scanning on said screen a pattern having a maximum dimension equal to the vdiagonal measure of said rectangular faceplate;

and a rst pair of centering magnet rings positioned on said neck on the gun side of said predetermined point for centering said beam relative to the aperture of said shield and a second pair of centering `magnet rings positioned on said funnel of said neck on the screen side of said shield for centering said scanning pattern relative to said screen.

References Cited UNITED STATES PATENTS JAMES W. LAWRENCE, Primary Examiner.

10 v. LAFRANCHI, Assistant Examiner.

Claims

1. A CATHODE RAY TUBE HAVING A BEAM DEFLECTION FIELD WITH A PREDETERMINED POINT AT WHICH BEAM DEFLECTION IS INITIATED, SAID TUBE COMPRISING: AN EVACUATED ENVELOPE INCLUDING A NECK, A FUNNEL, AND A SUBSTANTIALLY RECTANGULAR-SHAPED FACEPLATE HAVING AN IMAGE SCREEN THEREON; AN ELECTRON GUN WITHIN SAID NECK FOR PROJECTING AN ELECTRON BEAM ALONG A PREDETERMINED PATH, THROUGH SAID DEFLECTION FIELD, TOWARD SAID SCREEN;