US2861208A

US2861208A - Combination focusing and converging lens for multiple beam tubes

Info

Publication number: US2861208A
Application number: US491468A
Authority: US
Inventors: Robert E Benway
Original assignee: RCA Corp
Current assignee: RCA Corp
Priority date: 1955-03-01
Filing date: 1955-03-01
Publication date: 1958-11-18
Anticipated expiration: 1975-11-18

Description

Nov. 18, 1958 R. E. BENWAY 2,861,208

COMBINATION FocusING AND coNvERGING LENS FOR MULTIPLE BEAM TUBES Filed March 1, 1955 2 Sheets-Sheet 1 Maxam/viz INVENTOR. Rasen-r EBENWBY Maw Nov. 18, 1958l R, E. BENWAY COMBINATION FOCUSING AND CONVERGING LENS 2 sheets-sheet 2 FOR MULTIPLE BEAM TUBES Filed March l, 1955 INVENTOR. Rossm- E. BENWBY BY Robert E. Benway, Lancaster, Pa., assigner to Radio Corporation of America, a corporation of Delaware Application March 1, 1955, Serial No. 491,468

4 Claims. (Cl. 313-70) "fhis invention relates to means for focusing and converging a plurality of electron beams, and more particularly, to a combination focusing and converging lens for multiple beam tubes.

This invention is especially useful in cathode ray `viewingtubes for color television receivers. In such tubes, threeelectron beams are modulated with three-color signals, respectively, and directed thru apertures in an aperture mask to a` three-color phosphor dot screen. The three electron beams, theapertures in the mask and the phosphor dots of the screen are so oriented that electrous of each beam strike only the phosphor dots of one color. it is necessary for the three electro-n beams to be focused to small spots in the plane of the mask and screen target. It is additionally necessary to converge the three beams so that they merge at the same'small spot in the `plane of the mask and screen target.

It is au object of this invention to provide an improved and simplified electrode structure for both focusing and converging a plurality of electron beams.

It is another object to provide an improved combina- States arent tion electrostatic focusing and converging lens which l is relatively insensitive to power supply voltage Variations. It is a further object to provide an improved multiple beam gun requiring one less direct current biasing potential than has previously been required.

it is a still further object to avoid distortion and arcing caused by charges collecting on the glass mount beading by which the gun electrodes are supported.

ln one aspect, the invention comprises a multiple beam gun for use in a color kinesco-pe and including a cupshaped electrode mounted coaxially in the neck of the tube. The end of the cup-shaped electrode is provided with symmetrically arranged apertures thru which the electron beams pass on their way to theV target.` The inner-surface of the tube neckis coated with a conductivesubstance which is biased with a considerably higher positive potential than is applied to the cup-shaped electrode. The dimensions of the cup-shaped electrode are so chosenthatfthe resulting electric eld configuration causes a focusing of theindividual beams as they pass thru the respectiveapertures in the end of the cup-shaped member, and also causes the beams toV converge to the same point at the target.

These and other objects and aspects of the invention will be apparent to those skilled in the art from the following more detailed description taken in conjunction with the appended drawings, wherein: Y

Figure l is a broken sectional view of a color kinescope `constructed according to the teachings of thisinvention; f.

Figure 2 is a sectional View taken on Figure 1; p

Figure is an enlarged sectional View showingrthe combination focusing and `converging lens. forming a part ofthe color `kinescope of Figure 1; and p Figure 4' a. chart `which will be referred to in detheVY une 2 2 of neck 10.

scribing how the relative dimensions of the cup-shaped and consists of phosphor dots of three different` colors.

An aperture mask 14 is positioned in spaced relationshipV Y with the phosphor dot screen, each aperture in the mask 14 being in alignment with a cluster of `three phosphor dots of three diiferent colors on the screen 1 The neck end of the envelope is provided with a base` equilateral triangle, a first grid assembly 17, a pre-g,

focusing second grid assembly 18, and a third grid assemf bly 19. A conductive coating 20 on the inside of the` neck 10 of the tube constitutes a fourth or accelerating electrode. The usual deflection yoke 9 surrounds the The third grid assembly 19 includes a cylindrical member 21, an apertured end plate 22 thru which the beams enter from the left as diverging beams, an apertured plate 23 positioned at an intermediate point in the cylinder 21,

and an apertured cup-shaped electrode 25. The apertures in the plate 23 are slightly smaller than the size of to be equally spaced and equidistant from the longitudinal axis 28 of the tube. The respective apertures in all the grid assemblies and the respective cathode 16' t are in alignment. The grid assemblies are mechanically mounted in insulated relationship by means of glass mountbeadings 24.

Referring to Figure 3, showing an enlarged portion of Figure l, the cup-shaped electrode 25 consists of a circular end Wall or plate 26 and a cylindrical wall 27. The cylindrical wall 27 has an axial length L and a diameter D. The apertures in the end plate 26 have centers spaced a distance g from the longitudinal axis 2S of the tube. p The apertures are round and have a diameter a. erating electrode 20 consisting of a conductive coating ori the inner surface of the neck 10 has a diameter d.

rthe accelerating electrode 20 may, for example, be biased to a po-tential of 27,000 volts with respect to the p first grid 17, andthe third grid assembly 19 including cup-shaped electrode Z5 may be biased to a potential of 3,000 volts relative to the first grid 17. Because of the potential difference between the accelerating electrode 20 and the cup-shaped electrode 25, and because of the physical geometry of the electrodes, a complex electric field configuration is` created which may-Ibo illustrated by the dashed equi-potential lines in Figure 3. The electric force exerted upon electrons passingn thru the field is exerted at right angles with the equil potential lines.

It rwill be noted that some of the equi-potential lines in Figure 3bulge into and thru the apertures in thetcirthe tube. In order to prevent focusing distortion, it is necessary that the equi-potential lines in the apertures be Patented Nov. 18, 1958 l A phosphor dot screen The gun includes.l

The accel- V.portions vof end`26vnear the apertures therein.

perfectly symmetrical; Ifithe-lines are to some extent lopsided, thebeam becomes distorted as it goes towards the target so that it ceases tov have a circular cross-section and does not focus to a small'spotvat the target. Symmetry -of' the equi-ipotentialllines inthe apertures is accomplished by proportioning the cup-shaped electrodel 25fs'oV that the equi-potential linesvk adjacent to thel end plate 26 inlthe region ofy the apertures Aare substantially paralleli tothe endv plate 26. This is accomplished by making length L of thecylindrical'wall 27 small comparedn withthe diameter D. Thev lengthv L should be less than one-halfthe diameter D.

large relativeltothe'diameter Diso that the equi-potential linesfhave;suicientlcurvatureto cause the three beamsv to converge at the same point at the target.

It is thus far apparent that the cup-shaped electrode 25 must -haveacylindrical'wall 27 of length Lsuiciently small relative to the diameter D -to'provide undistorted focusing, andlthe length-L must-be sutiiciently large rela-1 tiveV to the diameter D to. provide i sufficient convergence. Byproperly proportioningthecup-shaped electrode 25, bothfocus`` and convergence can be obtained at the target. Asanlexample ofa tube actually constructed and successfllyoperatedgthefcup-shaped electrode 25 had f Vthe dimensions shown onFigure 3 of the drawings.v

The focusingl'enses-formed in the apertures in the end plate 26` should be? shielded fromV electric fields which may be set-*up by otherrelectrodesin the-tube. This shielding is performed bythe cylindrical member 21 forming apartof the third gridassembly 19. All partsV of the third gridv assembly 19, including the cup-shaped electrode 25, are in` electrical contact with each other and arevbiased to the same potential.- rPhe cylindrical member 21 provides a field-free path for thebeamsso that no disturbingl electric'iield can' 'exist on. theA left- Yhand 'side of the end plate-26'and symmetricallenses are established inthe apertures intendA 26.'--

rlhef-proper length Lof-"the cylindrical wall 27 relativeto'-tl1`e1diameter D is influenced by the-fdiametera andi thef spacing g1 of the' apertures in' the end plate 26.

The* larger? thendiairneterl av and* spacing g; the' shorter should'be the length L and/ orthelargershould be they diameter D', inl-order to avoid focus distortion due to lack of-parallelisrn between equi-potential lines and the vFocus distortionl can alsol result from insufficient Vspacing be tween'1v thezapertures.) The'ratio-ot the aperture spacing gto `thezaperture diarrieter a should be greater than 0.9

inorder tof prevent interaction between the three focus-- inglens'es -in'thefapertures in end`f'265i If theV apertures v areftoo`c1os`e together, the field in one Yaperture distorts theeldsinthe other apertures. 'Y

'Achart'of Figure-4"illustrates how the proper length L.for"the cylindrical'wall 27y was experimentally determined; Starting with a cup-shaped' electrodehaving a trom-Figurewi"that'whenf'the` lengtlrl: of` thel cylindrica static tubes.- It has beenfoundthat a yone, percent powe f charged and'thereby-` distorting the focusing andconvrg khaving a cylindrical wall' and Van 'endvwallipr'ovidedw 4- v the degree of convergence was noted. It will be seen i wall 27 was greater than 0.63`inch, thel threebeams were. over-converged so that they crossed before reaching the i A target. Similarly, when the length L was less than 0.63 the beams were under-converged. With a length L Tof* Y 0.63 inch both focus and'V convergence was achieved. 3 In practicing the invention, the apertures in the end, plate 26v shouldbe designed soY that' the ratio of g to' is greater than 0.9, where g is the distance ofthe aper ture centers fromthe axis 2S, and ais thezape'rture dian eter. In this way, interaction between` the focusing lenses in the apertures is avoided. With givenA values of g and a, the dimensions L and D of the cup-shaped elec trode 25 must be Selected'so that the equi-potential liner adjacent to the end plate 26 in the region of the apertures are substantially parallel to the end plate 26. Under this condition, the equi-potential lines extending into the apertures are. Asubstantiallysymmetrical; with. the; aper turesso that there is no focust distortion. This ,is-accomg plished, by making L relatively small lcomparedw-itli; D, and more speciiically, by making L less'thamone half .D.- f It willbe apparentrto those skilled in the art that objects of the invention can be achieved with structuren` having differentdimensions than those shownin Figur 3. For example,xif the vspacing gofthe apertures i greater than .that shownit will be necessary to increasesY the diameter ID. and/ or reduce the length Lof the.cfu shaped electrode. 25 in order tomaintain parallelism betweenthe equi-potential linesadjacent to the-end plat 26 in the region of the apertures. Similarly, if the spa ing g of the-apertures is less Vthan that shown.inve changes should be made inthe dimensions Dfand L. A the same time, the diameter D must be kept Ismall enough; and/ or the lengthL must be' kept large enough tofslimulr taneously provide kkthe necessary degree of convergence According to this invention,.a singlegridassembly Y1.9 in combination with the accelerating electrodej20serv to. both focus and converge the beams. Heretofore, ,i has been necessary to provide one gridvassemblyvfor focusing and a separate Vgrid assembly forco'nverging. It has beenznecessary to apply dierent vbiasing potentiay to the two grid assemblies. Accordingto this invention a .single grid. assembly is employed forbothz'purposes and. only one source of, biasing jpgotentialisv used: co pared with the two previously necessary. rsafresujl there are-savings in the power. supply as.l well as savings in the` number, of electrodes inthetube andrareducti in.k the number of, bias potentials whichY must. beiV appli thru.the'prongs;inthebase-.of the.tube.- t y -In .ahtube constructed accordingvto thistinvention, the convergence is muchlessaiected lby power supply yvolt-.xV age variations than hasbeen the case with prior electro supply voltage change results in. onlynarfour mil:(0.0 inch) separation of eachI spot from the axis 2 8-'at target of the tube. Thisconvergence sensitivity isab'o one-half the Abest which,rhastuheretofore, been obtaineg; with an,y electrostatic converging `lens.. "l

Aifu'rther advantage of the gun. structure sof` this ini/en tion is that thev glass beading 24 used to. mountthe c1652- trodesoftthe. gun in' insulated Yrelaticinshipfis s o arran thatthere. is.Y noY possibility vof the beading becom ing fields. j

According to Y.the teachingsbf this, invention there provided an improved multiple beam cathode, ray'. Y

including a simplified third grid assembly which sim taneously focusesand converges the beams. 1

, Whatis claimed is: v l

1. In a triple, beam. gun., means to,Y focus andconver the: electron, beams -comprisingya cup-shaped electr' three circularY apertures equally."spaced.-'fromeach `o and from the center of said end wall, a coaxial cylindrical accelerating electrode of larger diameter than said cupshaped electrode and surrounding at least the open end of said cup-shaped electrode, and means spacing and insulating said cup-shaped electrode from said cylindrical accelerating electrode whereby said accelerating electrode may be maintained at a higher positive potential than said cup-shaped electrode, the ratio of the axial length of said cup-shaped electrode to the diameter thereof being less than 0.5 to provide undistorted focusing and to provide convergence.

2. -In a multiple beam gun, means to focus and converge the electron beams comprising, a cylindrical accelerating electrode, a cup-shaped electrode mounted coaxially with said accelerating electrode, said cup-shaped electrode having a circular end wall provided with circular apertures for said beams and a cylindrical wall with the open edge thereof extending at least partially within said accelerating electrode, and means spacing and insulating said cup-shaped electrode from said cylindrical accelerating electrode whereby a. potential difference may be maintained between said electrodes, the axial length of said cylindrical wall being less than one-half the diameter thereof.

3. In a multiple beam cathode ray tube having a tubular envelope, a plurality of beam-forming electrodes in one end of said envelope, a target `at the other end of said envelope, and an accelerating electrode yon the inner surface of said envelope, a combination focusing and converging electrode positioned between said beam-forming electrodes and said target, said focusing and converging electrode comprising a cylindrical member and a coaxial cup-shaped member fixed thereto and having an end wall closing said cylindrical member, said end wall of said cup-shaped member having a plurality rof beam apertures, the open edge of said cup-shaped member being coaxially positioned within and spaced from said accelerating electrode, the axial length of the cylindrical walls of said cup-shaped member being less than one-half the diameter thereof.

4. In a multiple beam cathode ray tube having a tubular envelope, a plurality of beam-forming electrodes in one end of said envelope, a target at the `other end of said envelope, and an accelerating electrode on the inner surface of said envelope, a combination focusing and converging electrode positioned between said beam-forming electrodes and said target, said focusing and converging electrode comprising a cylindrical member and a coaxial cup-shaped member xed thereto having an end wall closing said cylindrical member, said end wall having a plurality of beam apertures symmetrically arranged around the longitudinal axis of said focusing and converging electrode, the open edge Iof said cup-shaped member being coaxially positioned within #and spaced from said accelerating electrode, the axial length of the cylindrical walls of said cup-shaped member being less than one-half the diameter thereof, the spacing of the centers of said `apertures from the longitudinal axis of the tube divided by the diameter of the apertures being greater than 0.9.

References Cited in the file of this patent UNITED STATES PATENTS 2,545,120 Swedlund Mar. 13, 1951 2,661,436 Van iOrmer Dec. 1, 1953 2,663,821 Law Dec. 22, 1953 2,690,517 Nicoll etal Sept. 28, 1954 2,717,323 Clay Sept. 6, 1955 2,729,759 Kratz et al. Ian. 3, 1956 2,761,990 Amdursky et al. Sept. 4, 1956