US2801355A - Target structure for color television display tubes - Google Patents

Description

y 1957 V c. s. NUNAN 2,801,355

TARGET STRUCTURE FOR coma TELEVISION DISPLAY TUBES Filed May 10, 1954 4 Sheets-Sheet l Arrow 5Y5 y 0, 1957 c. s. NUNAN 2,801,355

TARGET STRUCTURE FOR COLOR TELEVISION DISPLAY TUBES Filed May 10. 1954 4 Sheets-Sheet 2 IN VEN TOR. (24/6 5. A u/m/v July so, 1957 v c. s. NUNAN 2,80 5

TARGET STRUCTURE FOR COLOR TELEVISION DISPLAY TUBES Filed May 10, 1954 v 4 Sheets-Sheet 3 IN V EN TOR. (04/6 5. A/uA/A/v gamma lrropmi/i C. S. NUNAN July 30, 1957 TARGET STRUCTURE FOR COLOR TELEVISION DISPLAY TUBES Filed May 10. 1954 4 Sheets-Sheet 4 United TARGET STRUCTURE FORYCGLOR TELEVISION DISPLAY TUBES Application May 10, 1954, Serial No. 428,622

8 Claims. (Cl. 313-48) This invention relates to target and color control structures for cathode-ray tubes designed for display of television images in substantially natural color. Specifically, it relates to such tubes of the type wherein the cathode-ray beam which traces the image is refocused in the'space adjacent to the display screen so that a beam, which is of the order of magnitude in cross-sectional area of one elemental area or image point of the picture displayed, is concentrated on a sub-area of smaller size. The sub-area on which the beam is focused is occupied by -a phosphor emissive of light of one component color of a combination additive to produce White. in the region of the display screen has been termed and will be referred to herein as post deflection-focusing. Where post deflection focusing is used a supplemental electrode structure or grid, apertured to permit the passage of electrons of the cathode-ray beam, is mounted close to Such refocusing of the beam Patent and generally parallel to the display screenand there is I also provided an auxiliary, electron-permeable electrode in the same general region. The structure is such-that, when a suitable potential difference is applied between the apertured grid and the electron permeable auxiliary electrode, there is formed a multiplicity of electron lenses distributed over the target area and each having an aperture of the order of magnitude of one picture element. The screen is provided with sub-areas of phosphors grouped in a repeating pattern across the screen, each group comprising phosphors .emissive of all of the.component colors of the additive system employed and'each group electro-optically alined with a corresponding aperture of the grid. The particular "color displayed at any instant isdetermined either by the angle of incidence of the beam at the grid or by microdeflection of the beam ac complished in the plane of the grid-itself. In the latter case the grid is formed out of two setsof'interleaved electrodes. Potential differences between the electrodes of the two sets establish fields which deflect the electrons passing through, the mean potential of the two sets being maintained, with respect to the auxiliaryclectrode, such as to effect the desired lens action and bring the beam'to a refocus upon the proper color.

In tubes of the post-deflection focusing type the most convenient form to manufacture is one wherein the auxiliary electron permeable electrode is a thin conducting film, usually of aluminum, deposited onthesurfa-ceof the display screen itself. This not only has the advantage of simplicity of construction but reduces the power required to effect the scanning deflection of the beam when producing an image of a given brilliance, since the same potential difference which does the post-deflection focusing also;accelerates the electronsand imparts greater energy to them as they arrive at the-screen.

When the system of focusing thus described isemployed there are three factors which must betaken intopaccount if satisfactory pictures are, to be produced. The=acceleration imparted. to the electrons between the-grid? andthe display screen deflects themfrom their straight'linettravel,

Patented July 30, 19157 the amount of deviation from, the straight path becoming greater as the scanning angle is increased; accordingly, the phosphor areas must be alined with their corresponding grid' apertures. electro-optically rather than structurally, to take account of the curvature of their paths in the grid s'creen region. The. principles underlying such electro-optical alinement are set forth in the copending application of Ernest 0. Lawrence, Serial No. 399,754, filedDecember 22, 1953. The focusing effect of the gridto-scree'n field also varies with the angle of deflection, the focusingeffect. being. stronger with greater angles offincidence of'the beam at the grid. Hence if the beam is brought to a sharp focus at the center of the screen it will be overfocused and produce a larger focal spot as it "is deflected toward the edges of the screen; conversely, sharp focus at the screen edges willrcsult in less than complete convergence at the center of the screen and hence a. compromise value of focusing voltage is frequently used in order to obtain a minimum average spot size. Finally, where micro-deflection of the beam is used to accomplish color switching, the sensitivity of the beam to a given deflecting voltage also increases with the angle of incidence of the beam at the grid. This'latter effect can, be 'compensateito a large degree, by varying the shape of the phosphor areas as is disclosed in thecopending application of Ernest 0. Lawrence,- Serial-No. 399,753, also filed December 22, 1953.

The varying sensitivity to micro-deflection at the grid can also be compensated by imparting a curvature to the screen-or the. grid, ,so that electrons travelling in theinterspace between screen and grid at the center of the screen have alonger distance to travel after the micro-deflecting impulse has been applied than those traversing the grid structure at the edge of the screen. By proper. computation of the grid or screen curvature the displacement of the point ofimpact by a given applieddefleeting impulse becomes. substantially uniform. The problem of electrooptical alinement of the grid apertures and phosphor groups can be solved in this latter case by applying the same principles as where uniformgrid-to-screen spacing is. employed. The expedient has, however, no effect on the defocusingofthe beam by varying angles of incidence.

Thebroadpurpose of the present invention is to provide a targetstructure, including display screen and electron lenssystem, which permits simultaneous solution of the-three problems of electro-optical alinement, uniform focusing-throughout the display area and uniform-microdeflection sensitivity. Other objects and advantages accomplished by the invention are the provision of a structure which permits a maximum duty cycle to be utilized in the display of either color or monochrome television images, to provide a target structure which will give uniform spot size and definition, both as to color and monochrome, in all parts of the screen; to provide a structure wherein the display screen may be formed upon the viewing window of the tube itself, instead of upon an auxiliary base positioned within the tube and behind thewindow, the screen having sulficient strengthto withstand atmospheric pressure even in large sized tubes, and, by virtue of theimposition of the screen upon the window, avoiding the transmission losses through multiple glass surfaces at which reflection losses can occur; to provide a type of target' structure which may, in an approximate, slightly modified form, he cheaply constructed and readily .computed and still offer to a high degree the advantages of nearly uniform focusing and micro-deflection sensitivity; and to provide a target structure which, in either its substantially exact or more approximate form, is fully capable of commercial production at reasonable cost.

In the type of electron lens or aggregate of a multiplicity' of electron lenses formed between an apertured,

grid and the substantially equi-potential plane formed by a conducting film on the screen or other electron permeable electrode, the position of the focal point or maximum convergence of the electron beam is determined wholly by the ratio of the accelerating voltages between the beam source and the grid to that between the grid and the equi-potential plane, the shape of the grid apertures, and the angle of incidence of the beam at the grid, and is substantially independentof the distance between grid and the equi-potential surface. In accordance with the present invention an additional electrode structure is utilized, this structure comprising a I second grid mounted between the first grid and the screen.

face thus described lying wholly on the side of the second I the second grid is also preferably comprised of wires tightly stretched across the display screen in a direction normal to that of the electrodes comprising the first grid.

When so arranged the wires of the second grid act to form slightly diverging lenses, increasing in some degree the size of the focal spot in a dimension parallel to the phosphor strips, but have. no defocusing effect in the direction of width of the strips. With other types of focusing and display screen, the effect of the second .grid

is to increase slightly the focal length of each of the component electron lenses of the aggregate. Moreover, when the invention is used in tubes of the type using linear grid electrodes and strip phosphors a relatively close approximation to the desired uniform focusing and uniform deflection sensitivity can be attained by using a screen which is curved in only one dimension; i. e., is a section of a right cylinder whose axis lies in a plane at right angles to the planes of the grid electrodes. .When used in this modified form in a tube wherein the maximum scanning deflection is 72 the deflection sensitivity may be held constant to within :2.2% over the entire screen area and the maximum spot size will be only 2.2% of the aperture width greater than the minimum;

in practice this means the width of the spot will vary between 3 mils and 3% mils; the corresponding variations in a like tube using a planar screen and grid are 11%. In the case of a tube using a 90 scanning deflection, the other structural parameters being the same, the variation in spot width will be between a minimum of approximately 3 mils and a maximum of approximately 4 and the micro-deflection sensitivity can be held constant to within i-3% throughout the screen. Using a flat grid and screen the variation in deflection sensitivity is about i20% and the variation in spot sizeis over 100%, i. e., between a minimum of 3 mils and a maxi- -mum of about 7 mils.

It will thus be seen that very large advantages accrue from even the modified approximate form of the invention.

Referring to the drawings: e

Fig. l is a diagrammatic view, in cross-section, of a cathode-ray color-television display tube embodying the present invention;

Fig. 2 is a fragmentary cross-sectional view of a disicate the order of this arrangement.

lying the phosphor coating, there is a thin conducting 'and therefore the device is fully operative even though equivalent thereto.

play screen adapted for use in a tube of the type diagrammed in Fig. 1;

Fig. 3 is an exaggerated perspective view of a viewing screen, shaped in accordance with the present invention, and adapted to give complete focusing and deflection sensitivity correction over the entire screen;

Fig. 4 is a similar view of a cylindrical screen, adapted to give approximately uniform focusing and deflection sensitivity throughout the screen area; and

Fig. 5 is a family of curves illustrating, in terms of the spacing between the grids of the tubes of this invention, the screen curvature in various planes parallel to the conductors of a linear-element deflecting grid.

With the exception of the target structure, comprising the display screen and the grids associated therewith, the tubes in which the present invention is incorporated may be conventional in structure, comprising an evacuated envelope of generally funnel-shaped form, which may be either of glass or of glass and steel construction. In the present instance the latter type of construction is shown, the tube comprising a cylindrical glass neck 1 wherein is mounted an electron gun having an electron emitting cathode 3, a control grid 5, a first anode 7 and a second anode 9, with the usual connections for applying suitable electrical potentials to each of these elements. The second anode is preferably connected either to the funnelshaped metal shell 11 which constitutes the body of the tube, or, if the tube be of all glass construction, to a conductive coating deposited upon its inner surface. The large end of the funnel-shaped body of the tube is closed by a viewing window 13, which, in the case of the tube illustrated in Fig. 1 constitutes the base upon which the light emitting phosphors of the display screen are deposited. It will be noted that the window is illustrated as curved, i. e., convex outwardly. The exact form of this curvature will be discussed hereinafter.

Fig. 2 shows, upon an enlarged scale, a small crosssection of the window 13 and the phosphor screen thereon. Deposited upon the base 13 are strips of phosphors which are emissive, upon electron impact, of light of different component colors additive to form white. Each group of strips extends across substantially the entire face of the screen in one dimension, and its width, which is uniform or very nearly uniform throughout its length, is of the order of magnitude of one element or picture 'point of the television images to be displayed thereon.

In the type of tube illustrated the strips are deposited in a repeating pattern in the order red, green, blue, green, red, etc., and in the diagram of Fig. 2 the various strips are designated as the initial letters R, G, and B, to indi- Preferably, overfilm 15, which is permeable to electrons. Such films are well known in the art and they are usually of aluminum. They serve the triple purpose of establishing a definite potential for the screen, of reflecting back, out through the window, light which would otherwise be radiated back into the body of the tube and lost, and of suppressing, to a considerable degree, the secondary emission of electrons. This conducting film is not, however, an absolute essential in the operation of the tube, since under the bombardment of the cathode rays the screen will emit secondary electrons until it reaches an equilibrium potential which is only a few volts negative to certain portions of the electrode structure of the target the film 15 may be omitted.

Stretched across the viewing window or screen and in a direction substantially normal to the phosphor strips is a grid 17 of fine wires or other linear conductors The spacing of the conductors of the grid 17 (hereinafter referred to as the second grid, even though it is the first to be described) is not critical, although preferably it is of the same order of magnitude as the spacing of the centers of successive green strips tin thewscreen; Asawill be shown hereinafter it isaconvenient that the conductors of the grid l7contactz the edges of the screen, although .this,.;too, isnot essentialto the operation of the tube as will be set forth in detail hereinafter. It will be obvious from a-description of the construction that:the grid "17 is substantially planar when considered as a :whole.

Mounted in a plane parallel to that-iofj the grid 17 is a second grid oflinear electrodes, 19 and 19', whichextend across the surface of the screen in a direction substantially normal to the extension of the conductors of grid 17 and therefore substantially parallel to theaphosphor strips. The: conductors 19 and 19 alternate. The set of conductors 19 is connected to a common lead 21 which is brought out through the envelope. Similarly conductors 19' are connected "to a common lead 21 also brought out through the wall of the envelope. The two sets of conductors are mutually insulated so that apotential difference can be established'between the two sets ofconductorsto effect micro-deflection of'the beam.

The conductors 19 and.19' are so positioned that the center of the aperture'formed between each adjacent pair of electrodes is electro-optically centered in front of the center of'a corresponding one of the green emitting strips G; that is, they are so positioned that whenthe electronfor concentrating the beam from the electron gun and for scanning it over the surface of the target area, are employed. Since such focusing and deflecting coils are conventional and are not a part of the present invention they are omitted for simplicity in the drawing and description.

As is usual in the operation of cathode-ray display tubes for television use, the cathode 3 is operated at the lowest 'potential'in the system, and anodes 7 and 9 are progressively more positive. The two sets of electrodes 19.and 19, comprising the first grid, are preferably operated at a potential of between 200 and 400 volts negative to that of the second electrode and the tube shell, assuming that "the latter is in the neighborhood of 5000 volts to 8000 volts positive with respect to the cathode. This, of course, refers to the mean potential of the two sets of electrodes; in operation an oscillating voltage of perhaps 400 volts peak to peak is applied between the two sets of electrodes of-the grid. The second grid 17 is operated at a potential considerably higher than the mean potential of the first grid, and the conducting film 15 is connected to and therefore operated at the same potential as the grid17.

The cathode-ray beam is deflected bidimensionally over the area of the viewing screen to trace a raster thereon in accordance with .usual television practice, vertically at substantially 60 cycles per second and horizontally at substantially 15,750 cycles per second in accordance with present standards in transmission in the United States, or in accordance with whatever other standards may be employed in the service for which the tube is to be used. Preferably the conductors 19 and 19' extend across the screen in the direction of the higher frequency or line deflection.

As a result of the voltages applied to the various electrodes of the tube the cathode-ray beam developed by the electron gun travels in a substantially straight line from a center of deflection, the exact position of which depends on the position of the deflecting coils, to reach the plane of the first grid 19-19 at varying angles of incidence. As the beam passes through the plane of the considered-overall, and through the major portionof-the distancebetwen the-two ,grids it is substantially uniform and-in ardirection normal to the plane of bothgrids; thereforeit-bends, :the beam so thatits angle of incidence to-the-;second grid is less than that of its angle of incidence to-the first grid. In the immediate neighborhood of the grids thefield is concentrated on the conductors-comprising them. Inzthe plane of the first grid this .field is directed away-from the wires, giving the electrons constituting the-beam an impulse which tends to converge them. This impulse is in the direction normal to the wires and its effect is-to make the beam converge into a fine-line parallel to-the grid conductors. In a practical tube the. distance between the grids will be of the order of 10to 15 times .the-distance between thegrid conductors, andwith these proportions no material error is involvedin, considering that the entire impulsewhich causesthe. convergence of the beam is applied to theplane of the grid itself. The same-holds true of the shape of thefields .at,the,second grid .17, but here the direction of the field issuchas to cause a divergence of the beam. Thisdivergence, however, doesnot affect the linear character of the focus imparted by the first grid, since there,

e the screen is materially shorter than that betwen the grids, and-therefore the electrons have a shorter time in which to diverge, even at the center of thetube where the distance between the second grid and screen is greatest.

When there isa potential diiference between the two sets of electrodes of the first grid there issuperposed upon the two effects mentioned an additional impulse which deflects-the entire beam toward the more positive electrode of the pairforming the aperture through which the beam enters, and if this-impulse is of the proper magnitude the beam will be deflected from the green strip, upon which it was originally centered, to fall upon either the red or blue strip as-the case may be.

It will be seen from the above that in the region between, the first grid and the screen, the electrons of the beam are subjected to three different accelerations which divert them from their straight line paths. The first of these is an acceleration directly toward the second grid and screen, resulting in a refraction of the beam as a wholeand causing its center to fall on a point of the screen closer to the axis of the tube than it would if the field were not applied. The second is the converging or focusing action of the second grid. The third is the micro-deflection of the beam which controls the color displayed by the tube and which will be referred to hereinafter as the color deflection to distinguish-it from the scanning deflection.

In the copending applications of E. 0. Lawrence, Serial Nos. 399,753, and 399,754, cited above, equations are given showing the magnitude of each of these deflections, at various pointsof the screen, in tubes which are substantially similar to the one here described except for the fact that the screen is planar and the accelerating field is applied directly between the screen surface and the color deflecting grid. It should be ob vious that on the axis of the tube this accelerating field 'has no refracting efi'ect, since the general direction of acceleration coincides with the line along which the beam final velocity in this direction.

over the entire screen. "the screen as to accomplish all of these purposes will next the screen is lower and it is therefore subjected to the accelerating field for a longer time and because the component of velocity added in the grid-to-screen' region, normal to the screen, is a greater proportion of its total The sensitivity'to focusing is also greater at greater angles of incidence, both because of the longer time taken by the electrons in passing through the field where focusing deflection takes place and because there is a longer time during which the electrons are traveling between grid and screen and the velocity components causing the convergence is effective. Finally, the sensitivity to the color deflection increases with increased angle of incidence for the same reason, and substantially to the same extent as does the focusing sensitivity.

As long as planar grids and screens are used, therefore, certain approximations must be made to obtain the best over-all focus and over-all deflection sensitivity throughout the screen. As has been shown in the copending applications above mentioned, the focusing effeet is substantially independent of the distance between grid and screen. If it were possible, in a practical tube, to give the screen and grid both a curvature concentric about the center of scanning deflection, the angle of incidence of the beam would be a constant and uniformity of both focusing and color deflection would be achieved. With screens of the size now demanded for television viewing, however, such concentricity would involve either an unduly long tube or a screen so convex as to cause an apparent distortion of the image when viewed. Either the screen or the grid can be curved to give substantially uniform deflection sensitivity, but in other than the concentric relationship this does not help defocusing.

In accordance with the present invention, however, using two substantially planar grids, and applying focusing potential between the two grids so that the beam is not over-focused at its maximum angle of incidence, the screen can be given a curvature and the relationship between the pitch of the grid wires and the width of *the phosphor strips can be so adjusted that not only 'are the groups of phosphor strips electro-optically alined with the corresponding apertures in the color control grid, but the screen lies in the focal surfaces of the various electron lenses forming the color grid and because of the similarity of the conditions for focusing and for color deflection sensitivity, the latter is substantially uniform The conditions for so forming be discussed in detail.

Refraction is proportional to the square root of the potentials of these various elements with respect to the cathode. The

potential of the first grid is so nearly that of the second anode of the electron gun that the space between the gun and the first grid may be considered, without appreciable error, as an equi-potential space, and all lines of force originating on one grid may be considered as terminating upon the other, since the space between the second grid and the screen'is substantially equi-potential.

In this analysis, as'well as those which follow with respect to focusing and other deflection sensitivity, the following notation will be used:

Potential difierences between cathode and first grid=V1;

Component of angle of incidence of beam at first grid in plane parallel to grid electrodes=a;

Component of angle of incidence in plane normal to wires of first grid=;3; t 1

Total angle of incidence of beam at first grid=6 (tan 0= tan a+tan i3);

Displacement of beam electrons in the )3 plane (i. e., in direction normal to wire of first grid) by fields efi'ective in the grid-screen region=y; subscripts indicate the nature of the displacements considered.

Because of the fact that displacement of the point of beam impact in the direction parallel to the electrodes of the first grid is so small as to cause no visible distortion of the picture and, furthermore, because deflection in the latter direction causes no change in color displayed on the screen, components of deflection in the latter direction need not be considered in the design of equipment in accordance with the present invention.

Using the same type of derivation as that given in the copending Lawrence applications above identified, it can be shown that the displacement y of the center of the beam under the influence of the accelerating field alone is I 2D tan 6 F tan [3 1+ /1+ksec. 0 1+kseo. 0

The quantity y is the distance in the direction normal to the electrodes of the first grid, between the foot of a perpendicular dropped from the center of one of the electrodes 19 or 19' and the center of a corresponding red or blue emitting phosphor strip, as the case may be, at a given point of incidence of the beam corresponding to a particular value of 0; or, stated otherwise, it is the distance measured in the same direction between the foot of a perpendicular dropped from the center of the aperture between two electrodes 19 and 19 and the center of the corresponding strip of green emitting phosphor. As in the case of planar grids and screens, the quantity y varies with both a and 5 components of the angle 0. It results in screen dimensions which are very slightly larger than the dimensions of the active portion of the grid, but are smaller than the ratio of the distance between the center of deflection and the first grid to the distance between the center of deflection and the screen. When the beam is deflected over a rectangular raster at the first grid the refraction causes a very slight barrel distortion of that raster as it appears on the screen, but this distortion is only of the order of one-tenth of one percent of the screen dimension and is not perceptible at all in ordinary viewing. In order to meet the requirement of electro-optical alinement, each group of strips must be located at a distance from the axis of the screen which exceeds by the amount y-, the

distance of the corresponding aperture of the grid from the grid axis. Since varies from point to point over the grid, the ratio of pitch of the grid wires to the phosphor spacings varies from the center of the sceen outwardly toward its edges, as is the case with the single planar grid and planar screen, and correction for the distortion and pitch change can be accomplished in the same manner as is disclosed in the Lawrence application, Serial No. 399,754, identified above. As is the case with the single grid and planar screen, the change in ratio of spacing can be accomplished either by varying the widths of the phosphor strips or by varying the pitch of the grid electrodes, either continuously or in zones, from the center of the screen out toward the edges. For the purpose of this invention it is of little practical importance whether the change of ratio is accomplished by varying the width of the groups of phosphor strips or by I varying .the pitchof the. grid conductors. Theoretically the former offers a slight advantage -in mantaining: constant color-deflection sensitivity, but the differences are so small that in practice they'may be ignored.

Focusing,

Although. Equation .1 is given interms .ofithe. component angle ofincidence B, .the actual point .of :impact of any. electron upon the screen is related .to 'i'ts point :of entry at the grid. by its component of velocity parallel to the grid plane, times the. time interval between the instant of passing through'the color control gridand the impact upon the-screen; The forcesto which an elec- -tron is subjected in passing through the grid do not change itsi absolute. velocity but do change the-=rela'tionship between the components of that velocity parallel and normal respectively to the plane of the grid. In effect, therefore, the focusing and deflecting forces at-the grid change: the point of impact of any individual electron in the same manneras a change inthe-component'angleiof incidence :5. What the, changeinfiwill be-for agiven focusingoncolor deflecting impulse can be. computed as will be shown hereinafter. The efie'ct of such changes on the position of the point of impact is, however, proportional to andtaking the derivitive of Equation 1 with respect to fi-gives Y 2 W i 1+ /1+Ksec. 9 w l-t-Ksecfifi F tan a It has been shown in the Lawrence applications above referred to-that the convergence of the beam dueto the focusing-;fielcl,;fora grid of linear conductors, is equal to the right handquantity of Equation '2 multiplied by In the above-equation Ay represents the displacement .of the point of impact of electrons entering the aperture, and ye represents the distance, normal to the grid electrodes, between the point of entry of the electrons into the aperture and the aperture center. It follows that when from the value of 1, multiplied by the width of the aperture, plus this minimum width of=practicalfocal newt-teammates verywlosel the actual width otnhe foca1.=spdt as':observedand measured.

In accordance with .the present invention awsuitable valueiswhosenfonthe separation -D betweenthe -first and second: grids, and-the screen is soiformed that over its surface -ther-valuevofi F, theseparation between second grid. and i'screen, .isasu'ch ithat throughout the target area the quantity isnsubstantially equal, to -;l. Since the valueof Dis a matter of choice, the quantity which is really of interest is the ratioofF to D.-,- Therefore, setting For manufacturing reasons it is convenient to make :the-second grid contact the edges of the screen, so that at this :portion of -.the structure F=0. This is not a neces- -sary:; condition for the practice of the invention, but it :does combine maximum post-deflection acceleration, .maximum:sensitivity to both scanning and color control deflections, andminimum bulk of the target structure, and is therefore to be preferred. Actually, however, the

screen can be formedito conform with the requirements of Equation 4 so long as the quantity F at its edges equal to or greater than 0.

In Fig. 5 there is given a series of curves in which the value of as computed from Equation 4, is: plotted for various values 'of the angle 0 and its component [3. These curves are. plotted for tubes of two different designs. In this figure, curves 25, 27, 29 and 31 are plotted with respect to .ahtubeof. the deflection type, while curves 3'1, 33 and 35 refer'to 72 tubes; i. e., with regard to the first set of curves the maximum value of 0, when the beam isdefiected to the corners of the display screen, is taken at 45, while in the latter set of curves the maximum angle of 0 is'36.

In plotting all of these curves it has been assumed that the e1ectrodes-19, 19' extend in a direction parallel to the longer dimension of'the picture field and that the latter has the standard aspect ratio of 3 units vertical to 4 horizontal. Intheseplots, with the exception of curve 30, the'absci'ssas are given in terms of tan a and the ordihatesin-terms of the ratio hasin each case been chosen substantially to bring the minimum value of. F to 0. For the 90 tube this value of K'is 1 .892, While for the 72 tube K=2.210.

' Curve 25 gives the values of F on the horizontal axis of the screen, where tan [3:0. Curve 27 is plotted for a 'value of tan 9:0.361. and curve29 for a'zvalue ofitan 8:0.600, i. e., for the upper and loweredges .of the field where the angle )8 is a maximum. .1

The three curves for the 72 deflection tube have a generally similar significance. Curve 31 is drawn for tan 18:0; curve 33 for tan fi=0.308 and curve 35 for the upper or lower edges of the field in a 72 .tube where tan 5:0.436.

The particular significance of curves 27 and 33 will be discussed hereinafter in connection with a modified form ofthe invention; for the present it is sufficient to note that these curves lie approximately equidistant from those representing the maximum and minimum concavity of the screen, along its axis and at its edges respectively.

Deflection sensitivity When a color changing potential is applied between the conductors 19, 19"the effective change, AB in the angle )3 can be expressed by-the equation:

where Va is the voltage applied between conductors 19, 19', s is the spacing of the wires or other electrodes of the grid and w is to the diameter of the grid wires, the other symbols used having the same significance as before. It may be noted in passing that the expression given is actually for the change in sine 3 but because of the small values of deflection angle used in practice, the angle and its sine are effectively equal. Since Equation 2 gives the change in y for small changes in the value of 19, the relative displacement of the spot,

Itwill be seen that Equation 7 is similar in form to Equation 3, and except for coeflicientswhichare constant (or can be made constant) for any given tube, the equations are substantially identical. The ratio .K is a constant and is the same for both equations. The deflecting voltage Va, is, at any instant, constant throughout the tube. The only quantity which may vary .with' angle and which is not identical in the two equations is the denominator in the first term at the right of the equality in Equation 7. If the refraction correction is accomplished by varying the width of the groups of phosphors, rather than the pitch of the grids, this term is also a constant over the surface of the screen. It follows that if the concavity of the screen is made such that the focus is constant throughout, the deflection sensitivity will also be constant. If the refraction correction is accomplished by varying the pitch of the grid wires, the widths of the phosphor groups being maintained a constant, there will be a slight difference in the sensitivity to deflection due to the changes in the quantity In one tube of approximately the characteristics here described the quantity s, the spacing of the grid wires, averaged approximately 30 mils and the diameter of the grid wires w is 6 mils. The total difference in spacing of grid wires requisite to accomplish the necessary screen, was approximately one percent. The corresponding change in the quantity Zn iii) was about 0.6%. The variation in sensitivity over the surface of the screen would therefore be in this same ratio, and the difference or departure from uniformity is sosmall in comparison with the ordinary manufacturing tolerances that it can usually be neglected, irrespective of whether the ratio of change in pitch to phosphor spacing is accomplished by varying pitch or phosphor spacing. It will always be less than the errors involved by making the spacing ratio changes in zones instead of continuously if this is the method adopted. As has been shown in the previously mentioned Lawrence applications, displacements of up to approximately one mil can easily be tolerated.

If the refraction correction, applied to maintain the apertures and phosphor groups in electro-optical alinement, is accomplished by varying the width of the phosphor groups, there is some theoretical advantage in maintaining the width of the central strips (green in the example here given) constant and taking up the width variation in the strips which are electro-optically centered under the grid wires, i. e., the red and blue emitting strips. Because the total variation is so small, however, this advantage is more theoretical than practical. The over-all difference in phosphor group width as between center and edge of the screen is, in the case mentioned, only about one percent, and if the difference is applied equally to the red, green and blue strips, it is only about one-tenth of a mil. Since this is of a lower order of magnitude than the mechanical precision which at present seems desirable to maintain in commercial manufacture, any errors due to the method in which the correction for refraction are applied can usually be neglected.

Actually, errors much greater than thosewhich have been discussed immediately above can be tolerated in many cases. An examination of the curves of Fig. 5 makes it readily apparent that the curvature of the screen about an axis parallel to the wires of the color-control grid is much less than that about an axis at right angles thereto. Curve 30 shows the variation of the ratio F/D along the vertical axis of the screen of a 90 deflection tube, where the curvature is greatest. The substantial coincidence of curves 25, 27 and 29 at the edge of the screen, where tau oc=0.8 indicates that there is substantially no vertical curvature of the focal surface in this portion of the field. The maximum degree of curvature in the vertical plane, for exact focus, is only about one-fourth as great as that in the horizontal plane.

Many of the advantages of the invention can therefore be obtained by the use of a screen which is, in form, a right cylinder. In a 90 tube, the contour of such a cylinder conforms in curvature to substantially the value shown in curve 27 or, in a 72 tube, with that indicated by curve 33. These curves are drawn for values of the angle 5 where the amount of defocusing is substantially equal at the top and bottom and at the center of the vertical axis of the screen, the beam being underfocused at the center and overfocused at the upper and lower edges. For a 90 deflection tube this condition is substantially met where tan fi=0.40, for a 72 tube where tan 5:0.308.

Such a screen is shown in somewhat exaggerated perspective in Fig, 4. In a 90 tube the screen surface will coincide exactly with the focal surface only for deflection angles where tan fi=0.40, 18:21.89. For other angles of scanning deflection both the deflection sensitivity and the size of the focal spot will differ from the optimum. With this arrangement there will be two horizontal scanning lines, corresponding to vertical deflections of i21.8 from.the horizontal axis of thescreen, where change in pitch ratio, from the center to the edge of the the focus is practically perfect. It will be practically 2,801,5iii

perfect, also, at the left and right-hand edges of the screen. In the zones outside of thetwohorizontal lines of exact focusing,.thebeam passingthrough a:color.-grid aperture will pass through-the cross-overipoint which represents exact focus before reaching the screen, and be slightly larger-than its minimum size by the; time it reaches the screen, whereas in the central zone between the two lines 'ofperfect focus in1will..:not quite reach its minimum size where it impacts thescreen. Similarly in the outer horizontal zone the deflection sensitivity will be slightly greater than the average whereas in the central horizontalfzone the deflection sensitivity will be under theaverage; The errorsmentioned will, of course, be greatest-*alongthe line markingthe vertical axis of the display screen. Actually the maximum increase in spot'width, using apertures in the first grid of approximately 30mi'ls wide, will be about 1 mil and the difference: in deflection: sensitivity about :3.6%. These figures comparewith increases in spot size of over 4 mils (133%) and insensitivity of over 40% for a 90 tube using a plane screen and a single grid. The variations in a 72 tube will be much less. It will therefore be obvious that :even::the approximately corrected form of the invention oflfers great practical advantages.

The use of a cylindrical' screen, giving anapproximate correction, hasthe advantage that it is somewhat easier to 'print the screen accurately upon asurface which is curved in only-one dimension than it is to do so upon a screen curved Withrespect to both its axes. From the manufacturing point of view it is more ditficult to incorporate a cylindrical screen as a portion of the envelope of the tube itself than it is to do this with a screen that is curved in two dimensions, although the cylindrical screenmay bebowed at its ends, outside of the screen area, and molded so that it' can be joined or sealed to theenvelope and willwithstand the atmospheric pressure. Cylindrical screens may also, if desired, be mounted inside ofthe envelope in the same manner as planar screens described in the previously identified Lawrence applications.

In this connection it may be noted that although the curves of Fig. 5show the relative values of the distance F'between the second grid and th'e screen, the abscissas and the ordinates are on greatly different scales and therefore the curves do not give any visual picture or" the actual degree of curvature employed. In a tube using an eighteen inch display screen, i. e., a tube wherein the surface on which the raster displayed measures eighteen inches on the diagonal, the length of the display surface is approximately 14% inches and its height approximately, inches. In a 90 tube having a display surface of these dimensions the distance D may be 0.450 inch. In the case of a completely corrected display surface in a tube using these parameters, the center of the screen, as viewed from the electron gun side, will be 0.255 inch behind the plane of the second grid, amounting to just a little over a quarter-inch bulge in the screen in. its span of approximately 14 inches, whichis obviously very slight. With a 90 deflection tube the departure. of the fully-corrected screen from the cylindrical form .is only a little over inch in a span of 10% inches. If, in a 90 deflection tube, the approximate cylindrical correction is adopted. the bulge along the horizontal axis of the screen is 0.23 inch in the 14% inch spanand the departure. from the true. focal surface-is at maximum only 0.025 inch. In a'72 tube using the same inter-grid spacing D, the bulge of the display screen for a fully corrected tube is only 0.153 inch maximum, and. that of the compromise cylindrical screen only 0.137 inch. As viewed from the outside of the tube, therefore, the screen has enough of a bulge to withstand atmospheric pressure but not enough so that its curvature is'obtrusive or to cause an apparent distortion of the television image because it is displayed on a curved surface.

The fact that the actual curvature-of the. field is slight, is important forone other reason; The equations given in-this-specificationare basedon animplicit assumption that the displacementof the points "of electron impact on the screenzis the same as it would be if the small portion of the screen in the immediate area wherein the deflection takes place were a flat surface, parallel to thetwo grids. As far as the focusing deflection is concerned, this would involve substantially no error, since the theoretical focus is a geometrical line and the actual focal spot departs so:llittle from the theoretical value as to be completely negligible. In the case of the color deflection, however, the curvature could make a difference. Actually it does ,not,since the deflection is in a direction normal to .theqgrids major curvature and therefore the distance between second grid and screen does not change appreciably-within the area through which thedeflection occurs.

The fact that the ,secondgrid: has a diverging or defocusing effect in the direction normal to the focusing effect of the first grid has been mentioned. In a tube wherein the electrodes of the first grid run parallel to the scanning line, as is. here described, the effect of the defo'cusingaction-is to-lengthenthe focal line or spot in the direction ofthe line scanning. If the primary focus of the beam, as accomplished by the electron gun and the usual focusing; coil is poor, resulting in an initially over-large spot, this may be important, but under ordinary.circumstances it is not. The amount of the defocusing action :which occurs is directly proportional to the distance F,- since the space between the second grid and the screen is substantially a unipotential space as the second gridand screen are directly connected. Where the electrodes-of the second grid approach the screen very closely, some of the lines of force which the theory of operation; of'the invention assumes terminate on the second gridwill actually terminate on the screen itself; i. e., in the narrow zone where the distance between second grid, and soreeniis small in comparison with the separation of the second grid wires. In this area, however, the amount of convergence in the second gridscreen region, required to bring the beam to the minimum focal size, is so small that it is unimportant. In the. zone where this is true the second grid causes substantially no divergence in the plane normal to the second-grid wires. In the portion of the target area Where the-distance F is a maximum the increase in the length of the focal line is less than 20% of the diameter of the beam as it enters the focusing structure, and it is to be remembered that this can be as small as is normally the case in monochrome tubes. Actually the principal eifect of the divergence of the beam is to make any shadow which the second grid wires might otherwise cast upon the screen less prominent.

It should be evident that there is a rather wide range of structures which may be used in accomplishing the purposes of this invention. Others have shown methods of manufacturing grid structures of various types; this specification is not concerned with the method of constructing the grids but rather with their positioning, and any satisfactory structure can be used. Mention has already been made of the fact that the screen may be mounted inside of the. tube window in the same manner as planar screens since the curvature of the screen is so slight. If the approximate type of correction is used, with a cylindrical screen curved in only one dimension, the screen may be formed of plate, heated and bent in known manner. If the screen is curved bidimensionally, as in the preferred form, it may be molded into the required shape, and later sealed to the body of the envelope. If this latter is the procedure chosen, it is convenient to curve the longer edges of the approximately rectangular glass window downward, outside of the target area, so thatthe edge which is to be sealedto the envelope lies substantially in a single plane. Although, for tubes which are to be used with the NTSC system of standards of television transmission it is preferred that the color control grid electrodes run parallel to the direction of line scanning rather than normal thereto, the same principles can be used with the color control grid electrodes running normal to the scanning lines. The details of construction mentioned are therefore not intended to limit the scope of the invention herein presented, all intended limitations being expressed in the following claims.

Having now described the invention, what is claimed 1. In a cathode-ray tube for the display of television images in color which comprises an evacuated envelope including a window area through which the images may be viewed and an electron gun within said envelope for developing a beam of cathode ray directed toward said window area and adapted to be bidimensionally deflected thereacross to trace a raster; a target and color control structure comprising a substantially planar grid positioned adjacent to said window area and approximately equal thereto in size, said grid having a multiplicity of apertures therein closely and substantially uniformly spaced over substantially the entire area of said grid, a second grid of like character positioned between said first grid and said window area in a plane substantially parallel to said first grid, terminals external to said envelope connecting respectively to said grids for applying different electrical potentials thereto, and a display screen mounted in said window area and comprising a transparent base, a coating on said base comprising phosphors emissive on electron impact of light of different colors, said phosphors being disposed in groups electro-optically alined with corresponding apertures of said first grid and forming a repeating pattern covering substantially the entire area of said screen, each group being in at least one dimension of the order of magnitude of one elemental area of the images to be reproduced and including all of said phosphors, a conducting film disposed over said coating and means for connecting said second grid to said conducting film, said base being concavely curved toward said electron gun.

2. In a cathode-ray tube for the display of television images in color which comprises an evacuated envelope including a window area through which the images may be viewed and an electron gun within said envelope for developing a beam of cathode rays directed toward said window area and adapted to be bidimensionally deflected thereacross to trace a raster; a target and color control structure comprising a substantially planar grid positioned adjacent to said window area and approximately equal thereto in size, said grid comprising a plurality of linear electrodes closely spaced to form a multiplicity of apertures therebetween substantially uniformly spaced over substantially the entire area of said grid, a. second grid of like character positioned between said first grid and said window area in a plane substantially parallel to said first grid, and with its linear electrodes running in a direction substantially normal to that of the electrodes of said first grid, terminals external to said envelope connecting respectively to said grids for applying different electrical potentials thereto, and a display screen mounted in said window area and comprising a transparent base, a coating on said base comprising strips of phosphors emissive on electron impact of light of difierent colors, said phosphor strips being disposed substantially parallel to the electrodes of said first grid in groups forming a repeating pattern covering substantially the entire area of said screen, the width of each group being of the order of magnitude of one elemental area of the images to be reproduced and including all of said phosphors and each group being electro-optically alined with a corresponding aperture of said first grid, a conducting film disposed over said coating and means for connecting said second grid to said conducting film, said base being concavely curved toward said electron gun.

3. In a cathode-ray tube for the display of television images in color which comprises an evacuated envelope including a window area through which the images may be viewed and an electron gun within said envelope for developing a beam of cathode rays directed toward said window area and adapted to be bidimensionally deflected thereacross to trace a raster; a target and color control structure comprising a substantially planar grid positioned adjacent'to said window area and approximately equal thereto in size, said grid comprising a plurality of linear electrodes closely spaced to form a multiplicity of apertures therebetween substantially uniformly spaced over substantially the entire area of said grid, a second grid of like character positioned between said first grid and said window area in a plane substantially parallel to said first grid and with its linear electrodes running in a direction substantially normal to that of the electrodes of said first grid, terminals external to said envelope connecting respectively to said grids for applying different electrical potentials thereto, and a display screen mounted in said window area and comprising a transparent base, a coating on said base comprising strips of phosphors emissive on electron impact of light of different colors, said phosphor strips being disposed substantially parallel to the electrodes of said first grid in groups forming a repeating pattern covering substantially the entire area of said screen, the width of each group being of the order of magnitude of one elemental area of the images to be reproduced and including all of said phosphors, and each group being substantially electro-optically .alined with a corresponding aperture of said first grid, a conducting film disposed over said coating and means for connecting said second grid to said conducting film, said base being concavely curved toward said electron gun, the depth of the concavity and the curvature thereof being such that for at least one component angle of deflection in the direction normal to the electrodes of said first grid, application of a potential difierence between said grids which will bring electrons entering said apertures to a focus at the surface of said screen at one component angle of deflection in the direction parallel to the electrodes of said first grid will bring electrons substantially to a focus at the surface of said screen at any other component angle of deflection in said last mentioned direction.

4. In a cathode-ray tube for the display of television images in color which comprises an evacuated envelope including a window area through which the images may be viewed and an electron gun within said envelope for developing a beam of cathode raysdirected toward said window area and adapted to be bidimensionally deflected thereacross to trace a raster; a target and color control structure comprising a substantially planar grid positioned adjacent to said window area and approximately equal thereto in size, said grid comprising two interleaved and mutually insulated sets of elongated linear conductors, adjacent conductors being substantially uniformly spaced over substantially the entire area of said grid to form a multiplicity of .apertures therebetween, a second grid positioned between said first grid and said window area in a plane substantially parallel to said first grid, and comprising elongated linear conductors disposed in a direction substantially normal to that of the conductors of said first grid, terminals external to said envelope connecting respectively the sets of conductors of said first grid and to said second grid, for applying dilferent electrical potentials thereto, and a display screen mounted in said window area and comprising a transparent base, a coating on said base comprising strips of phosphors emissive on electron impact of light of diflerent colors, said phosphor strips being disposed in groups forming a repeating pattern covering substantially the entire area of said screen, the width of each group being of the order of magnitude of one elemental area of the images to be reproduced and including all of said phosphors, and each group being substantially electro-optically alined with a corresponding aperture of said first grid, a conducting film disposed over said coating and means for connecting said second grid to said conducting film, said base being concavely curved toward said electron gun, the curvature and concavity of said screen being such that with respect to at least one aperture of said first grid the application of potentials to said grid and screen which will bring electrons entering said one aperture at any one angle of incidence to a focus at said screen will also bring electrons entering said aperture at any other angle of incidence to a focus at said screen.

5. In a cathode-ray tube for the display of television images in color which comprises an evacuated envelope including a window area through which the images may be viewed and an electron gun within said envelope for developing a beam of cathode rays directed toward said window area and .adapted to be bidimensionally deflected thereacross to trace a raster; a target and color control structure comprising a substantially planar grid positioned adjacent to said window area and approximately equal thereto in size, said grid having a multiplicity of apertures therein closely and substantially uniformly spaced over substantially the entire area of said grid a second grid of like character positioned between said first grid and said window area in a plane substantially parallel to said first grid, terminals external to said envelope connecting respectively to said grids for applying different electrical potentials thereto, and a display screen mounted in said window area and comprising a transparent base, a coating on said base comprising phosphors emissive on electron impact of light of difierent colors, said phosphors being disposed in groups electro-optieally alined with corresponding apertures of said first grid and forming a repeating pattern covering substantially the entire area of said screen, each group being in at least one dimension of the order of magnitude of one elemental area of the images to be reproduced and including all of said phosphors, a conducting film disposed over said coating and means for connecting said second grid to said conducting film, said base being concavely curved toward said electron gun and its concavity and curvature being such that the coated surface of said base substantially coincides with a surface defined by the face of the electron lenses formed by said grids when a potential difiference is applied thereto such as to bring the focus of any of said electron lenses upon said surface.

6. In combination with a cathode-ray tube comprising an evacuated envelop and an electron gun adapted to direct a beam of electrons against a target area within said envelop, said beam being deflectable to fall or any portion thereof, a target structure positioned within said target area and comprising a cylindrically curved display screen, a coating comprising a repeating pattern of strips of phosphors emissive upon electron impact of light of different colors additively producing white deposited on said screen in a direction normal to planes including the cylindrical axis of said screen, a grid of parallel conductors tautly supported across chords of said cylindrical screen 6 and secured closely adjacent to the edges thereof, and a second grid of parallel conductors tautly supported in a plane substantially parallel to the plane of said first mentioned grid, the conductors of said second grid being parallel to and electro-optically alined with strips of said coating.

'7. In combination with a cathode-ray tube comprising an evacuated envelop and an electron gun adapted to direct a beam of electrons against a target area within said envelop, said beam being deflectable to fall on any portion thereof, a. target structure positioned within said target area and comprising an electron lens structure comprising a pair of spaced parallel grids, each grid comprising a multiplicity of parallel linear conductors, the conductors of the two grids extending in mutually perpendicular directions, connections for establishing different electrical potentials on said grids, a curved display screen mounted closely adjacent to the one of said grids more distant from said electron gun and a coating on said screen of light emissive phosphors, the curvature of said screen being substantially such that said phosphor coating lies on a surface defined by the foci of the electron lenses formed by said grids with respect to electrons from said gun when directed to the various areas of said electron lens structure.

8. In combination with a cathode-ray tube having an electron gun, including a cathode and an electron accelerating anode, directed toward a target area within an evacuated envelop, a target structure mounted within said area comprising a first grid and a second grid mounted in substantially parallel planes separated by a distance D, each of said grids comprising a multiplicity of substantially parallel linear conductors and the conductors of said second grid extending in a direction substantially normal to that of the conductors of said first grid, connections for applying diiferent voltages between said cathode and said first grid and said first and second grids, a curved display screen mounted closely adjacent to said second grid, and a coating of phosphors emissive of light on electron impact deposited on said display screen, the curvature of said display screen being such that its distance F from the plane of said second grid bears to the distance D the ratio 2 /I+K see? 0 1+tan a tan 5 K sec. 6 1+K sec. 0

FID=

References Cited in the file of this patent UNITED STATES PATENTS Re. 23,672 Okolicsanyi June 23, 1953 2,606,246 Sziklai Aug. 5, 1952 2,669,675

Lawrence Feb. 16, 1954

Images