PROCEEDINGS OF THE I.R.E.

on top of the nitrocellulose film. Trhe phosphor plate isrebaked in air at 425°C for a short time to remove thenitrocellulose film. The phosphor plate is then readyfor operation in a color kinescope.

ACCCUR1ACY (ONSIDERATIONS

The accuracy necessary for the printing of the colorphosphor has been achieved without unduly compli-cated equipment. On the printing table, the wayVs of thecross feed are machined to a high precision. Four ad-justable hardened and ground steel dowel pins and fourhardened and ground steel bushing inserts, handlappedtogether in sets, maintain accurate positioninig of thestencil frame. Dial gauges are used to position the tableandl the glass plate to an accuracv of better than 0.001inch. Accurate temperature control and constant hu-midity in an air-conditioned area provide for constantcondlitions of gelatin exposure and gelatin drying. Like-wise, temperature control keeps the dimensions of glassplotographic plates and the parts of the printing equip-ment constant and reproducible. Careful comparison ofthe printed pattern with a glass-plate photographicnegative in contact with the printed pattern, by meansof a conventional 30-power microscope, gives an accu-

rate determinationi of any errors which may occur instencil making or printing. It is a conservative estinmatethat the accuracy maintainecl for the printing techniquesdescribed is better than + 0.001 inch over printed pat-terns of the order of a foot in size.

ACKNOWLEDGMENT

The successful development of the processes describedin this paper has been due to the co-operation and workof many. A sincere acknowledgment is due to G. W;\olfe,E. V. Space, A. E. Chettle, W. J. Bachman, D. Pearson,E. J. Smith, S. Kozar, and many other members of thetube development shop at the Harrison, N. J. plant ofthe RCA Tube Department.

Special acknowledgment is imiade to H. Rosenthal ofRCA Laboratories Division for, his work on dot moire,and to L. J. Caprarola, at Harrison, who originated thevastly inmproved "noncompensated" printing technique.

Mention should also be made of the help given byi\Ir. Masi of the Francis A. Masi Company, Newark,N. J., during the early experimental stages. It was in hisshop that one of the authors received indoctriination inthe art of silk screening, and it was there that gelatinstenicils for some of the earlv tubes were made.

Three-Beam Guns for Color Kinescopes *H. C. MOODEYt, SENIOR MEMBER, IRE AND D. D. VAN ORMERt, ASSOCIATE, IRE

Summary-The three-beam gun assembly for the aperture-masktri-color kinescope consists of three guns located so that their axesare mutually parallel, equidistant from, and spaced 120 degreesabout the axis of the assembly. The No. 4 grids open into a largecommon cup. The three electron beams are converged to a point onthe aperture mask by an electrostatic lens formed between the largecup and the conductive neck coating. The focus of the individualbeams is controlled mainly by the voltage applied to grid No. 3. Thepo,tentials of grids Nos. 3 and 4 may be varied in synchronism withthe scanning frequencies so as to maintain beam focus and con-vergence over the entire screen area.

The individual guns, of glass-beaded construction, are held inposition by support spacers welded to eyelets around the gun cylin-ders. Many standard-type gun parts and manufacturing proceduresare used. The beam spacing and gun placement are related to thedesign dimensions of the screen assembly. The guns are positioned

INTRODUCTIONHE DESIGN and construction of several types ofthree-beam gun assemblies for color kinescopes,particularly those for use with the shadow-mask

phosphor-dot screen, are discussed in this paper. Other

*Decimal classificatioii: R583.6X535.6. Original malnuscript re-ceived by the Instituite, August 15, 1951.

t Tube Department, RCA Victor Division, Radio Corporationof America, Lancaster, Pa.

with accurately machined, three-fingered fixtures which use the gunapertures as reference positions.

Separate leads for cathodes and grids Nos. 1 and 2 of the threeguns permit adjustment of grid drive charactexistics so the ratiosof three beam currents are constant with grid drive. Thus, correctcolor is maintained, independent of changes in brightness.

A variation of the parallel-beam type of gun uses a single set ofcylinders in which triple-aperture disks are placed in appropriatepositions. The final cylinder, which contains no aperture disk,forms an electrostatic converging lens with the neck coating. Anarrangement of mechanically converged guns, used with magneticdynamic convergence, is also described. Another type of gun, de-signed for a minimum neck diameter, employs electrostatic diver-gence of the beams before convergence. Still another type of con-struction is the "coincident-crossover" gun in which the beams maybe focused and converged by a common lens system.

papers in this series1' 2 describe the operation and use ofthese gun assemblies, as well as single-beam guns, incolor kinescopes of the shadow-mask type. The salientfeature of the three-beam gun assembly is the availabil-ity of a separate and independenitly controlled beam ofelectrons for excitation of each of the phosphors in the

1 R. R. Law, "A one-gun shadow-mask color kinescope," PROC.I.R.E., pp. 1194-1201; this issue.

2 H. B. Law, "A three-gun shadow-mask color kinescope," PROC.I.R.E., pp. 1186-1194; this issue.

1 236 October

Moodey and Van Ormer: Three-Beam Guns for Color Kinescopes

screen. Thus, the intensity of each of the three primarycolors may be varied independently of the other two.

Initial experimental tubes2 used mechanically con-verged guns; i.e., the assembly was constructed with theguns tilted at an appropriate angle toward the tube axis.In pilot-plant production, however, it was consideredadvisable for reasons discussed in this paper to use theparallel-gun type of construction with an electrostaticbeam-converging lens.

DESIGN FEATURES

The three-beam gun assembly (Fig. 1), which wasused in color kinescopes for the 1950 demonstrations ofthe RCA color television system,3'4 consists of threesimilar guns located so that their axes are mutuallyparallel, equidistant from the axis of the assembly, andspaced 120 degrees about the latter. This constructionpermits accurate alignment of the three guns by meansof a parallel-membered, three-fingered jig; it is a compactarrangement mechanically, requiring a tube neck diam-eter of only 2 inches. The design of the guns is a modi-fication of a conventional cathode-ray tube gun de-sign, and consists of an indirectly heated cathode, con-trol grid, cup-shaped grid No. 2, a grid No. 3 containinga beam-masking aperture, and a grid No. 4. Beyond thispoint, there is somewhat less resemblance to conven-tional guns. The small-diameter grid-No. 4 cylinders ofthe three guns open into a common cup of large diam-eter, and are connected to it electrically. The grids No. 3for the three guns are connected to each other inter-nally; the cathodes, control grids, and grids No. 2 ofthe individual guns use separate leads. The three heat-ers are connected internally in parallel. Connections toall gun electrodes are brought out through a 14-leadstem and a base filled with plastic to provide insulationbetween leads for high-voltage operation of the tube. Aconductive coating on the inner surface of the neck isconnected internally to the metal envelope and thescreen assembly.When the tube is in operation, an electrostatic elec-

tron lens is formed between the common grid-No. 4cup and the neck coating by the potential difference be-tween these electrodes. This lens serves to converge thethree originally parallel electron beams to a point onthe aperture mask of the screen assembly.2 In perform-ing this function, the converging lens also tends to focusthe electrons within each of the three beams. The majorbeam-focusing action, however, which permits sharpfocus of the individual beams at the point of beam con-vergence on the mask, is supplied by a separate lens for

I T. R. Kennedy, Jr., "RCA shows all-electronic tube as key tocolor television," N. Y. Times; March 29, 1950. "New color tele-vision tube seen bringing color programs to the home," Radio Age,vol. 9, pp. 3-5; April, 1950.

4 "Improved RCA color TV," Tele-Tech, vol. 10, pp. 31, 59;January, 1951. "RCA improves color in latest showing," Broad-casting, pp. 73, 77; December 11, 1950. See also J. Gould, "RCAexhibits improved color; complicating of the TV battle seen," N. Y.Times; December 6, 1950.

each beam formed between grids Nos. 3 and 4. With thisarrangement it is possible to control beam focus andbeam convergence separately by adjusting the voltagesapplied to grids Nos. 3 and 4.

Fig. 1-Photograph of three-beam gun usedin RCA color kinescopes.

Because the aperture mask and screen are flat, the dis-tance that the beams travel between the gun and screenvaries during the scanning cycle. If the converging lenspotentials, which produce beam convergence at thescreen center, were constant, they would tend to over-converge the beams at the edges of the screen. For thisreason a dynamic convergence potential of proper waveshape, in synchronism with the scanning frequencies,may be applied to grid No. 4 in order to obtain bestconvergence at all points on the screen.' It may also bedesirable to vary the grid-No. 3 voltage dynamically in asimilar fashion to obtain best beam focus over the entirescreen area.The converging lens shape and, hence, its action are

affected by the depth of the grid-No. 4 cup if the depthis appreciably less than the diameter of the cup. A shal-low cup would permit the lens field to extend into thesmall-diameter portions of the electrode, and would dis-tort the beams astigmatically. However, by having the

' A. W. Friend, "Deflection and convergence in color kinescopes,"PRoc. I.R.E., pp. 1249-1263; this issue.

1951 1237

PROCEEDINGS OF THE I.R.E.

depth of the cup nearly equal to or greater than itsdiameter, this effect is avoided because the converginglens field barely reaches the bottom of the cup, and is notappreciably affected by the three separate openings inits lower surface.

Since the proper functioning of the aperture-mask-screen assembly dep)ends upon the accuracy with whichthe three beams pass through the "color centers" inthe deflection plane,6 it is important that the separa-tion between the three gun axes, the distance from gun todeflection plane, and the distance from deflection planeto screen be properly set (Fig. 2). The latter distance isdetermined by the size of screen and maximum deflec-tion angle desired for the tube. The distance betweengun and deflection plane is adjusted so that, wvhen thedeflecting yoke is in position, there is sufficient space be-tween it and the gun to avoid interaction between theyoke field and the converging lens field. Considerationsgoverning the separation of the beams in the deflectionplane (i.e., the location of the color centers) are coveredin another paper in this series.2

PRINCIPAL PLANE OFCONVERGING LENS

Fig. 2-Diagram illustrating three-beam convergence.

With these parameters set, then, the necessary sepa-

ration between gun axes could be determined if the loca-tion of the principal plane of the converging lens were

known. Actually, a reasonable value for gun separationwas chosen for a first approximation and the principalplane position computed from operational measure-

ments. A second approximation, based on these resultsand consisting of adjustment of either the gun separa-

tion or the distance from the gun to the deflection plane,then gave sufficiently accurate results. For the diametersand voltage ratios used for the converging lens, theprincipal plane of this lens lies about midway down thegrid-No. 3 cylinders, some three inches below the topof the gunl. Fig. 3 shows how the position of the prin-cipal plane is determined. Small errors in beam position-ing may be corrected by adjustment of three small ex-

ternal magnets placed near the three guns.5

6 D. D. Van Ormer and D. C. Ballard, "Effects of screen toleranceson operating characteristics of aperture-mask tri-color kinescopes,"PROC. I.R.E., pp. 1245-1249; this issue,

BEAM l APERTUREMASK

TUBE AXIS i S-

PRINCIPAL PLANE PHOSPHOR-OF CONVERGING CONVERGING DEFLECTION DOT

LENS LENS PLANE PLATE

Fig. 3-Diagram of beam convergence, showing method of determin-ing location of principal plane of converging lens.

FABRICATION OF THE THREE-GUN ASSEMBLY

The design and method of fabrication of the three-gun assembly are determined for the most part by thespace considerations of the completed tube, particu-larly those of over-all length and neck diameter. To per-mit the minimum neck diameter for the desired beamspacing at the deflection plane, the beams are mademutually parallel up to the converging lens. Because ofits ready availability, standard-size tubing, one-halfinch in diameter, is used for the gun elements. Sincethe standard method of gun-element support, usingeither two supports 180 degrees apart or three sup-ports 120 degrees apart, does not permit the desiredbeam-to-axis spacing before convergence of 0.415 inch,three standard-type guns could not be used for the as-sembly without certain changes.The position of each gun axis is determined by means

of an accurately machined, three-fingered fixture whichuses the apertures rather than the outer parts of theelectrodes for reference. This fixture permits the use ofnonprecision, production-type, support elements andspacers. Thus "floating" on this positioning fixture, thecRylinders are secured by glass beading or welding. Thefirst method of intergun support considered was an all-beaded construction in which all the separate elements

GLASS/ / B~~~~EAD

STUD

CYLINDER

Fig. 4-Three-gun structure having all-beaded support.

of the complete gunI assembly (with the exception ofthe common grid-No. 4 cup) are placed on the position-ing fixture and beaded 120 degrees apart (Fig. 4). Thegrid-No. 4 cup is then positioned by aniother fixture andwelded to the grid-No. 4 cylinders. This type of con-struction has the advantage that all elements of theassembly can be isolated electrically.

October1238

Moodey and Van Ormer: Three-Beam Guns for Color Kinescopes

The miethod adopted later, which offers greatermechanical stability, consists of beading the indi-vidual guns with two beads each, 120 degrees apart, assubassemblies. The subassemblies are then positionedon the positioning fixture and the floating positioningsystem again used. With the subassemblv guns posi-tioned parallel to each other, their apertures serving asreferences, the guns are secured in place by welding anevelet collar, fitted around the cylinder of a supportinggun element, to a loose-fitting support spacer. After theplane of support has been determined by welding oneeyelet to its supporting gun element and welding all ofthe eyelets to the support spacer, the two remaining eye-lets are welded to their respective gun elements. Thisprocedure permits accurate positioning without the useof precision-made cylinders and spacer parts. 'In theearly trials of this method it was found that the inter-gun supports placed between the control grids provideda loop which absorbed too much of the radio-frequencypower intended for degassing the cathode-grid as-semblies on exhaust. To isolate the control grids elec-trically and to prevent overheating due to this absorp-tion of power, the position of the intergrid supports waschanged from the No. 1 grids to the lower part of theNo. 3 grids. The bottom of the grid-No. 4 cup serves asthe second support spacer; the positioning proceduredescribed above is used in placing and securing bothspacers.The heater-cathode-control-grid assembly is the

standard structure used on all RCA beaded-type elec-tron guns. Grid No. 2 is a drawn cup with its edgerounded and electropolished. The rolled edge is omittedto provide liberal electrical separation of these cups afterassembly. Since this part is positioned by its apertureon the beading fixture, the lower beading stud is placedas close as possible to the bottom of the cup and madeslightly longer than the upper stud in order to minimizethe torque encountered when the gun is beaded. Theends of the cylinders for grids Nos. 3 and 4, which formthe focusing lens for individual beams, have rollededges of small radius. The top edge of the grid-No. 4cup is rounded and electropolished, rather than rolled,to allow use of a cup of maximum diameter which wouldprovide adequate breakdown separation from the neckcoating.

In the color kinescope both the gun alignment andthe distance between the gun and screen are controlledin production so that in operation the beams will passthrough the deflection plane in such a manner6 that bothpure color and proper beam convergence are readily ob-tainable. For these controls, seals for both the neck tub-ing and gun are made with the help of fixtures whichalign the neck and gun properly with respect to thecolor-screen mounting posts.7

7 B. E. Barnes and R. D. Faulkner, "Mechanical design of aper-ture-mask tri-color kinescopes," PROC. I.R.E., pp. 1241-1245; thisissue.

LIGHT OUTPUT AND COLOR CONTROL

In the design of the gun assembly the operating char-acteristics of each phosphor determine the requireddrive characteristics of the corresponding guns. Thedrive characteristics of each gun are co-ntrolled so thatcorrect color is maintained independent of changes inbrightness. When a black-and-wlhite picture is repro-duced by the color kinescope, the color temperature ofthe whites and grays, from the high liglhts down to thedeepest shadows, is essentially constant. Because thephosphors used in tri-color kinescopes show negligiblecurrent saturation in the range of current densitiesused, the achievement of constant color temperature re-quires only that the ratios of the three beam currentsbe held constant over the desired brightness range. Thedrive characteristics of eaclh gun are controlled byadjustment of the potential differences between thecathodes, control grids, and grids No. 2, all of which areprovided with separate base-pin connections. With thisflexibility it is not even necessary to maintain closemeclhanical tolerances to produce the control character-istics required of each gun.

Since the drive characteristics depend directly uponcertain phosphor characteristics, a consideration of thecharacteristics of the phosphors used in the tri-colorkinescope is of interest. Table I shows the relative effi-ciencies of the phosphors and the relative luminositiesof the three colors needed to produce white.

TABLE I

RelativeRelative Luminosity

Phosphor Lu ninous to Pro luceEfficiency 7,300(K

White

Red (zinc phosphate: manganese)8 25.3 82.5Green (zinlc sili,;ate: mang-inese) 100.0 100.0Blue(zinc sulfide:silver and calcium-magnesium silicate: titanium) 26.6 40.0

It would appear from the above table that currentequalization to produce white could be accomplishedby adjusting the phosphor efficiencies. Lowering the effi-ciencies of the bltue- and green-emitting materials mightappear, at first glance, to be a simple solution. Such re-duction of phosphor efficiencies would, however, requirea compensating increase of total beam current to main-tain light output. Even though the requisite video drivemight be available for this purpose, there is a furtherlimitation, namely, aperture-mask expansion due to theincreased heating.7When the phosphor efficiencies are not adjusted, the

difference between the "red-gun" and the "green-gun"

8 A. L. Smith, "Luminescence of three forms of zinc orthophos-phate: Mn," Jour. Electrochem. Soc., vol. 98, pp. 361-368; September,1951.

1951 1 239

PROC)cAI)ING(S OF 711TH fRK.R.Oct

currents needed to produce white will result in slightly(lifferent optimunm focusing conditions at high currentlevels. However, over the range of currents used in thetri-color kinescope, independent focus control of eachguIn (for example, by means of separate grid-No. 3leads) or gun modifications are niot required for satis-factory results.

DEVELOPMENTAi THREE-BE.Am GUN TYPES

During the course of the development of the three-beam gun, several other types of gun construction andoperating principles were investigated.A variation of the parallel-beam tyvpe of gun coInsists

of a single set of electrodes in which each aperture diskcontains three apertures, spacecl 120 degrees about thecenter. Tlhree cathodes are enclose(d in a comnmon con-trol-grid cylinder having an aperture located above eachcathode; a triple-aperture disk serves as grid No. 2, andtriple-aperture disks are likewise used in the grid-No. 3cylinder, both at the lower end and near the upper end.Above grid No. 3 is a grid No. 4, consisting of a simplering; the conductive neck, coating, metal shell, andscreen assembly are connected internally. A compoundfield produced by the potentials on grid No. 4 and theneck coating penetrates the apertures near the top ofgricd No. 3, providing bearn-focusing action. As thebeams pass into the grid-No. 4 region, they meet acommon convergence field produced by the penetrationinto grid-No. 4 cylinder of the field produced by theneck-coating potential. Raising the grid-No. 4 voltageincreases the beam-focusing action and reduces theconverging action; lowering this voltage, of course, pro-duces the opposite effects. There is, therefore, a pointat some distance in froont of the gun at which beamfocus and beam convergence occur simultaneously; thecenter of the aperture mask is located at this point. ThisguIn is especially suitable wNhen the three beams must beso closlx spaced that thie use of a separate set ofcylinders for each beam is not feasible. The gun re-quires a relatively small dynamic-convergence voltage;this voltage produces little defocusing of the beams.Fabrication of this gun presents mechanical problems,however, if separate grid-Nos. 1 and 2 controls are re-quired for the three beams. Furthermore, the aperturemethod of focusing subjects the beams to sphericalaberration of considerable magnitude. Masking of thebeams and focusing action must be accomplished by aseparate set of triple apertures in order to avoid theentrance of secondary electrons (from the edges of themasking apertures) into the convergence field.The parallel-gun structure described at the begin-

ning of this paper (Fig. 1) allows a considerable degreeof flexibility in tube and screen dimensions, and has cer-tain practical mechanical advantages in assembly. How-ever, development of a special jig has recently demon-strated a practical method for accurate assembly of the

guns in a tilted position with respect to the axis of thestructure. The angle of tilt is adjusted so that when thegun is sealed into the tube neck at the proper distancefrom the deflection plane the three beamiis pass throtughithe respective color centers. Minor (leviations from thedesired alignment may be compensated in the samie wayas deviations in the parallel-gun type.6 The mechanicallyconverged type requires no electrostatic converginglens, so that the three grid-No. 4 cylinders mav be coIn-nected directly to the conductive neck coating (withoutuse of a common cup), and thus operate at the voltageof the final electrode. Beam focusing is accomplishedentirely by the lenses between grids Nos. 3 and 4. Theadvantages of this gunl assembly as compared with theparallel-gun type are an increase in grid-No. 3 potential,with consequent improvement in gun efficiency, elim-ination of a converging lens, and redluction in the maxi-mum potential on the leads brouight out through thebase. This gun structure, however, has a slightly in-creased maximum cliameter. Dynamic convergencemust, of course, be supplied electromagnetically.Another type of gun, designed for a minimum neck

diameter, employs electrostatic clivergence of the beamsbefore convergence. This feature permits electrical ad-justment of the beam-convergence aingle atnd of thebeam spacing at the deflection plane in the finishedtube. Electrostatic divergence is accomplished by a lensformed bv the combination of a flat aperture disk anda cylinder. The close spacing of disk and cylinder flat-tens the normally converging portion of the lens so es-sentially only the diverging portion affects the beanms.

Still another type of construLction being developed isthe "coincident-crossover" type gun. Its operation andconstruction are based on the principle that if the beamsappear to originate from the same source, i.e., havecoincident crossover points either real or virtual, acommlon electron-optical system can be used to focusand converge all beams. An additional feature of thegun is that the divergence of the beams from the cross-over point can be controlled electrically, thus making itpossible to adjust in the finished ttube the positions ofthe beams in the deflection plane.

ACKNOWLEDGM ENTS

Many of our associates have contributed to the (le-velopment described in this paper. For guidance and forideas useful in three-beam gun design, the authors areparticularly indebted to L. B. Headrick and L. E. Swed-lund; for indispensable supporting work in means forfabricating the gun, to N. L. Graham; and for develop-ment of a practical method for assembling mechanicallxyconverged guns, to V. M. Hutchison. Much of our workwould have been ineffective without the developmentof adequate testing equipment, for which credit is due toP. A. Richards, R. WV. Hagmanni, P. NI. Kelly, and D. J.Ransom.

1 24() Ocstobfer