US2745035A - Color television tube target structure - Google Patents

Description

May 8, 1956 E. o. LAWRENCE COLOR TELEVISION TUBE TARGET STRUCTURE Filed Dec. 22, 1953 4 Sheets-Sheet INVENTOR. fin/:'57 waff/vc! #fray/Vix! May 8, 1956 E. o. LAWRENCE 2,745,035

COLOR TELEVISION TUBE TARGET STRUCTURE Filed DBC. 22, 1955 4 Sheets-Sheet 2 I N V EN TOR. [PA/[5r O, nuff/vc! WSW May 8, 1956 Filed Dec. 22,

E. O. LAWRENCE 2,745,035

COLOR TELEVISION TUBE TARGET STRUCTURE 1953 4 Sheets-Sheet 3 Fla. 6

1N VEN TOR. fax/fr 0, MMPI/vc;

BY MSM irma/wwf 4 Sheets-Sheet 4 l 4g, f

E. O. LAWRENCE COLOR TELEVISION TUBE TARGET STRUCTURE May s, 1956 Filed Dec. 22, 1953 nited States Patent CoLoR TELEvisIoN TUBE TARGET STRUCTURE Ernest 0. Lawrence, Berkeley, Calif., assigner to Chromatic Television Laboratories, Inc., New York, N. Y., a corporation of California Application December 22, 1953, Serial No. 399,754 Claims. (Cl. 315-14) This invention relates to target structures employed in cathode-ray tubes designed for the display of television images in substantially natural color. t constitutes a continuation-in-part of copending United States. patent application, Serial No. 265,366, led by the same inventor on January 8, 1952, for a Display Surface For Color Television Tubes, now U. S. Patent No. 2,669,675, granted February 16, 1954.

The invention relates to tubes of the type in which a plurality of dierent phosphors which are emissive of light, on electron impact, of diierent component colors additive to-produce white light, are distributed in a repetitive pattern which covers substantially the entire area of a display screen which forms one element of the target structure, The sub-areas of the screen occupied by the individual phosphors in tubes of this type are in at least one dimension of smaller size than the elemental areas or picture points of the images to be reproduced by the system and the display of individual colors is controlled by conlining the beam of cathode rays which traces the image to the particular phosphor or phosphors emissive of the color desired.

Since in the ordinary process of scanning a display surface the beam is caused to traverse successively substantially all portions of the display screen, means must be provided in tubes of this type for restricting that portion of the beam which actually impacts the screen to an area which is no larger than the sub-area within each picture element which is emissive of the desired colored light. Various tubes have been devised in which this restriction is accomplished by a mask which intercepts that portion of the beam which would otherwise strike the undesired phosphor or phosphors. ln the case of tricolor image reproduction (which is preferred as giving the best compromise between color iidelity and excessive bandwidth and apparatus complication) the use of such a mask presupposes that at least two-thirds of the energy available within the beam must be wasted.

, ln the type of tube to which this invention specifically pertains the restriction of the beam to a size which will limit it to an individual phosphor sub-area is accomplished by means of a multiplicity of electron lenses which converge the beam to a size smaller than that of the apertures through which it falls. It is the structure used so to converge the beam that forms the second element of the target. This may be a perforated screen or one or more grids comprised of linear conductors; however it may be constructed, whether of a single perforated plate, a single array of approximately parallel wires or narrow strips or tapes or a plurality of such sets of conductors, its function is the same, and for convenience it will generally be referred to hereinafter as the lensgrid or simply the grid."

The conductors of the lens-grid may occupy a very small proportion of its over-all area and the proportion of the beam intercepted by the structure is thus reduced from a minimum of two-thirds to something in therneighborhood of fteen percent or even less. Arrangements of 2,745,035 Patented May 8, 1956 One of the most satisfactory forms of electron lens l for accomplishing such post-deflection focusing, as is mentioned above, utilizes the display screen itself (which is made conducting) as one element of the multiplicity of electron lenses. Preferably conductivity is provided by depositing a thin lilm of metal, preferably aluminum, on the surface of the phosphor layer which covers the screen so that the metal film is faced toward the electron beam source. The grid structure, already described generally, is biased by a voltage which is substantially the same as that used to accelerate the beam as it issues from an electron gun (or guns) of substantially conventional type. The conducting lilm on the screen is made positive with respect to the grid, and by properly adjusting the ratio of the voltage applied to accelerate the beam to that between grid and screen al greater or less degree of convergence of the beam, after passing the grid, may be obtained. In addition to the advantage of simplicity offered by such a lens structure there is the Y additional advantage in the fact that a relatively low voltthus broadly indicated. Among the objects of the invention are to provide a target structure which will display the saine color over all parts of the screen under given conditions of color control applied to the tube, and thus give equal delity of color reproduction throughout the picture field; to provide means and methods for so positioning the phosphor areas with relation to the grid as to accomplish such color fidelity; to provide means and methods of the character described which are applicable to tubes of many types, including both those which use a plurality of electron guns and achieve their color control by virtue of the angle of incidence of the Various beams at the grid and those wherein the color control effected by means of micro-deflection at the grid; to

provide display screen conformations and methods of production of tubes which fail to meet the tolerances actually imposed.

Broadly consideredthe invention comprises a target structure including a substantially plane base on which phosphors emissive of a plurality of'component colors additive to produce white are deposited in a repetitive pattern of groups, each of which includes all of the phosphors employed, the dimensions of each group being, in one direction atleast, of the order of magnitude of one elemental area of the picture to be' reproduced. Means are provided for rendering the screen conducting and for applying an acceleration potential vthereto relative to a grid, which is mounted in a plane substantially parallel to the screen and is provided with apertures corresponding in number to the phosphor groups. The spacu ings .0f the phosphor groups .are So proportioned with ,respect to the spacings of the aperture centers that this ratio is greater than unity and is less than the ratio of the distance between the center of scanning deflection vof the cathode-ray beam and the screen to the distance .between that center of deiiection and the grid.

The nature of the invention will be better understood by reference to the following detailed description of a n number of specific embodiments thereof, taken in conjunction with the accompanying drawings wherein:

Fig. l is a schematic illustration of a cathode-ray tube of a type embodying the instant inYention, ,Operating-circuits for this tube being illustrated in block form;

Fig. 2 is an illustration of a portion ,of one type of display screen as used in the tube illustrated in Fig. 1, showing the pattern in which the phosphors are .disposed on the screen;

Fig. 3 is a similar view of a portion of a display screen f wherein the phosphors are disposed `in a linear or a strip pattern;

Fig. 4 comprises graphs illustrative of .the relationships,

Y in a tube using a single grid'of linear conductors, ofy

voltages used to accelerate the electron beam to the size of the spot on the display screen produced by beam im.

pact;

Fig. 5 is a diagrammatic illustration showing an elec-` tron trajectory between a single grid and the display screen;

Fig. 6 is a series of graphs illustrating the displace,- ment of the focal spot upon the screen, with respect to the perpendicular Idropped from the centers of the apertures in the grid, with varying anglesV of -incidence thereto and with different types of grids or degrees'of focusing;

Fig. 7 is an exaggerated illustration of the shape .of the pattern of phosphors upon adisplay screen with a target structure employing uniform spacing of apertures in the grid; and

Fig. 8 is a similarly exaggerated diagram illustrating the shape of the electrode structure .for use with a screen. having a rectilinear pattern of phosphors thereon.

One preferred form of the invention is embodied in a tube, basically of conventional form, which is indicated at 1 of Fig. l. Such a tube comprises the .usual evacuated envelope .3, which may be of al1 lglass construction or of metal and glass. lt has the usual viewing window 5 at its enlarged end and an electron gun 7 in the neck. Such a gun comprises an electron emitting cathode 9, a control grid 11, a rst anode 13 and 4a second anode 15. The tube, as shown for the purpose of illustrating one form of deflection control, is provided with pairs of deflecting plates 17 and 19 forl deflecting a beam ot cathode rays, produced by the gun, vertically .and 'horizontally respectively. lt is `to be understood, however, that, in the alternative, the more usual deflecting coils may be used where desired. However, electrostatic deection is practical in the tube of this type because of the relatively low initial velocity of the electron beam, and as will be shown hereinafter it possesses definite advantages with respect to color control.

Potentials for focusing and deflecting the beam are supplied by a radio receiver symbolized by the block 21.

No specific description of this receiver is believed necessary, since it is essentially conventional.

The display surface or screen and electron lens `system indicated generally by the reference character 23 will best be understood by reference to Fig. 2. Fig.- 2 shows the disposal of the color areas upon adisplay surface 25. Basically,as this ligure indicates, the three l phosphors which contribute luminescence in the primary colors of the additive systems are disposed upon the base 25 in strips which extend completely acrossthe display area in one dimension. Strips 25,1 ,are continuous, 'consisting entirely of a phosphor luminescent in a single primary color. These strips are substantially uniform in Width and are parallel, spaced apart by a distance `Substantially equal to their own width in this particular screen, although, as will be shown hereinafter, this is not a necessary condition. Intermediate strips 252 are discontinuous, comprising alternate blocks, 252 and 25"z, of the two remaining primary colors. The blocks 25'2, 252 as here shown are square, and the junctionsbetween the blocks are alined across the display surf-ace so that the blocks in any one row, transverse to the direction of the continuous strips, are all of one color.

There is no fixed rule as to which color of luminescence is provided by the continuous strips and which by the discontinuous ones; in a held-sequential system there is a slight advantage in making the continuous strips luminescent in green and the discontinuous ones alternately red and blue. This may also hold true for line-sequential systems. For certain dot-sequential system or for the constant luminance system, both of which may be considered as at least quasi-simultaneous systems, there may be some advantage in making the continuous strips luminescent in blue, but the advantage to be gained by any arrangement is not of suflicient importance to destroy the utility of a screen of this character for use with any presently known system, even if designed primarily for a different one.

The embodiment of the invention shown in Fig. lmay be considered, for the present purposes, as intended primarily for a field-sequential system and therefore strip 2,51 has been indicated, by the letter G, as luminescent in green, withl the block 252 luminescent in blue and blocks 25"'2 in red.

Spaced from the plane of the display surface 2S, by a distance short in comparison with the ytotal length of the path of the `electron beam, is a lens grid structure which is comprised of two sets of linear electrodes, those in each set being parallel and substantially uniformly spaced, although, as will hereinafter be shown, neither the width of the phosphor strip nor the spacing of the grid conductors is necessarily exactly uniform. The non-uniformitics are, however, although important, very small indeed, The electrodes are conveniently tine wires, although they may be narrow strips or tapes, mounted edge-on to `the beam path. In the particular tube shown the first set of these electrodes, designated alternately as 2,7 and 27", is mounted generally parallel to the phosphor strips -on the display surface. Electrodes 27 are connected to a common conductor 31; intermediate electrodes 27 are similarly connected to a common conductor 31. It may be noted that the mode of support for . electrodes 27 and 27 is not important to this specific invention and hence is not shown in detail. Various methods .of constructing such grids are shownin United States Letters Patents Nos. 2,653,263 and 2,695,372 of Ernest O. lLawrence, or in United States Patent No. 2,721,288, granted'on October 1S. 1955, to .laines T. Vale. VLeads 8, connecting to conductors 31 and 31', are provided for applying proper potentials to the electrodes of the structure, as by the horizontal coloroscillator 33.

A `second substantially similar set of linear conductors 35 and 35", similar to conductors 27 and 27 is mounted as'closely as conveniently possible to the rst set, between the latter and the electron gun. Leads 39 conneet to these electrodes and are also brought out of the tube so that the necessary potential relative to other elements can be applied thereto by a vertical color control oscillator 37. Owing to the difliculty of diagrammatic representation only two electrodes 35 and 35 of this second set are shown in Fig. l.

An electron permeable electrode, substantially coextensive with the display surface, is placed, with respect to the lens grid structure, so that when proper relative potentials are applied to the elements of the system electrons from the beam passing through the lens grid will be focused substantially on the display surface. Such a afnemen film is microscopic in thickness and serves the triple purpose of Vestablishing the lens forming the electric field with the lens grid structure, reecting luminescence from the screen outwardly through the window 5 and suppressing secondary emission of electrons from the display surface. This film is not shown in Fig. 2 but is indicated by the reference character referred in Fig. 1 to the surface of the base 25 and the connection 41 for applying the focusing potential.

The width of the phosphor strips on the display surface should not be greater than the dimension of the minimum elemental areas or picture points which the tube, and the system in which it is employed, are intended to resolve. Fig. 2 shows the relative positions of the phosphar strips and the electrodes at the center of the screen. Viewed from this aspect it will be seen that electrods 27 bisect the continuous strips 251, while the electrodes 27' similarly bisect the discontinuous strips 252. Similarly the transverse electrodes 35 bisect the blocks 252 in the transverse direction while electrodes 35 bisect the remaining blocks 252. In this portion of the structure a pencil of the electrons entering a mesh of the lens grid formed by an adjacent pair of electrodes of each of the two sets (if the potential of electrode 40 were the same as that on the lens grid) would be -distributed over an area of the display surface of which one-half would be a portion of continuous strip 251 and the other half which would be equally divided between portions of blocks 252 25"2; the resulting emission would be, in the case here considered, one-half green and one-quarter each red and blue.

As has been shown in the prior patents previously referred to, if a potential is applied between the lens grid structure and the electrodes 40 such that the latter is positive with respect to the lens grid, the electrons entering the mesh can be brought into focus in the plane of the display surface and electrode 40. In this case an electron following a path which is the average of all of the electron paths in the beam would strike the display surface at the junction of the three different color phosphors in the center of the quadrilateral area defined by the mesh.

If, now, proper potential differences are applied between the electrodes of the two sets, the focal point will be shifted to a degree depending upon the magnitude of the potentials applied. Undeected, the luminescence produced will be an unsaturated green. If the electrodes 35 be made negative with respects to electrodes 35', the beam will be deflected toward the latter and the resultant color will be yellow; if a reverse potential is applied a blue-green will result. Making electrode 27 negative with respect to electrode 27 results in mixed red and blue, or purple luminescence, whereas opposite deflection will give green. Electrodes 27 and 35 both made negative with respect to the other electrodes in their respective sets will give red luminescence while electrodes 27 and 35' nega tive will give blue. Increasing deliection in any of the directions mentioned will give increased saturation of the hues produced; up to the point where the entire focal spot falls on an area emissive of a single color. If sinusoidal voltages are applied between the electrodes in each set, and these voltages displaced in phase by 90 the focal spot will be spun in a circular path, the radius of which will depend upon the amplitude of the voltages applied, and if the frequencies of the potentials causing the spinning are sufliciently high so that the spot traverses all three colors within the period of persistance of vision, the psychological eect would be the same colors as produced by the undeflected image, in this case an unsaturated green, three-quarters of the energy being white and the remaining one-quarter green. Applying a fixed bias between conductor 27 and 27' the center of the circle about which the focal point is spun can be shifted so that the dwell of this spot on each of the three colors is equal, with a resultant pure white.

It will be seen, therefore, that each mesh of the lensgrid defines an area of the display surfaces in which all electrons entering that mesh may be brought into focus and that this area is so divided that one-half is luminescent in one primary color and one-quarter luminescent in each of the two others. In the center of the screen the area thus defined is very slightly larger in size than the lens grid mesh and lies directly behind the latter as viewed from the electron gun.

Because of the angles formed by the beam with the axis of the tube as its deflection increases, the correspondence in size between the meshes and the corresponding subareas of the screen (which may be referred to as phosphor groups or color cells) is not exact nor do the display screen areas lie perpendicularly behind the meshes which converge the electrons upon them except at the center of the screen. In spite of these facts the relative positions of any individual mesh and the center of the area of the display screen controlled thereby may be computed. The velocities of the electrons in the beam are proportional to the square roots of the potentials through which they have fallen. In order to scan any portion of the electron image the ratio between thetransverse and the longitudinal velocity of the electrons is known, this being the tangent of the angle of deflection. In the absence of deflecting potential between the electrodes of the grid itself, the electron following the mean path of those entering a mesh-i. e., one passing through the center of the mesh-will retain its same transverse velocity in its passage between the lensgrid and the display surface. The average longitudinal velocity of the electrons between the lens grid and the display surface is their velocity at the lens-grid plus one-half of the difference between that and their final velocity. The displacement of the point of impact of the mean-path electron from the base of the perpendicular dropped from the center of the mesh to the display surface will therefore be defined by an angle whose tangent is the transverse velocity over the mean velocity between the lens grid and the display surface; the distancev between the plane of the lens grid and the display surface being known, this defines the center of the area controlled by the individual mesh in question; the positions of the centers being defined and being also arranged in a regular quadrilateral array or repeating pattern, this establishes the areas themselves.

Stated somewhat more concretely, the velocity of electrons arriving at the lens grid is determined solely by the difference of potential between the latter and the cathode. The longitudinal component of this velocity is proportional to the cosine and the transverse component proportional to the sine of the angle between the path of the electron and the perpendicular to the lens-grid at the lensgrid itself. The velocity of the mean path electron as it reaches the display screen surface will again depend wholly upon the potential difference between the screen and the cathode, but the potential gradient between the lens grid and the screen will add velocity to the longitudinal component only; the transverse component will be unaffected.

If a tube of the character here described is to be operated to reproduce television images in substantially natural or true colors, with equal color fidelity throughout the screen, the centers of the color cells or phosphor groups must be electro-optically alined with the grid meshes which control them, and the electron beam as it passes through any individual mesh must be converged to such an extent that it may be so deflected as to fall on one phosphor only of the three comprising the group. These two requirements are interdependents. The degree of focusing desirable depends to some extent upon the system utilized for transmitting the color information. Where a line-sequential system or field-sequential system is used it may be desirable to make the degree of focusing or convergence just sufficient to insure that the beam may be confined to a single color phosphor when it is deected. With dot-sequential or quasi-simultaneous systems it is usually preferable to make the spot as small as possible,

but since the focusing varies with the degree of scanning 7 deflection of the beam it may be advisable to choose a compromise degree of focusing which brings the spot to minimum size at some portions of the screen other than the central area.

If any post deection focusing is to be used, however, the target structure must be designed so as to aline the grid apertures and their corresponding color cells for the degree of focusing desired. ln order to make such tubes commercially feasible the arrangements should be such that some degree of tolerance in spot size and focusing potentials is allowed for, even when it is desirable to use a focal spot of the maximum permissible size. What the factors are which control the positioning of the phosphor cells and the apertures will be considered in detail herein* after.

Since the principles involved apply generally to tubes using the post-detiecton focusing principle, however, and since the principles are more simply applied in connection with a tube wherein the lens-grid lies in a single plane, the factors involved will rst be considered in connection with a tube utilizing a screen whereon the phosphors are deposited in a pattern indicated generally in Fig.k3. Since the tube in which a screen of this character may be employed need differ from that shown in Fig. l only in the omission of one of the two lensgrids and its connections, e. g., the grid comprising conductors 35 and 35 and its connections with the vertical color oscillator 37, no separate diagram of the tube is believed necessary.

As in the case of Fig. 2 the illustration is of only a small portion at the center of the screen. In this case the phosphors are disposed in strips 45B, 45ex and 45B, indicating strips of red, green, and blue respectively arranged in the order red, green, blue, green, red, etc., two green strips being included for each one of the two other colors. The strips forming the screen in this case are parallel or very nearly so, as shown the strips are all of equal width, but this is not a necessary feature. Linear conductors, for example, the wires 47 and 47 extend across the screen in a direction generally parallel to the direction of the strips, the wires 47 being centered in front of the red strips 45a and the wires 47 similarly centered above the blue strips 45B at the center of the screen depicted in Fig. 3. this geometrical positioning no longer obtains, owing to the curvature of the electron trajectories as described above and to be considered more fully hereinafter. The apertures and the color cells are, however, effectively electro-optically alined throughout the target.

The electrodes 47 correspond in connection to electrodes 27 of Fig. l, while electrodes 47 correspond to electrodes 27. The similarly designated electrodes are electrically connected and deflecting potentials are applied from the horizontal color control oscillator 33, it being recognized that the electrodes may run either horizontally or vertically and that horizontal is in this case only specified as a matter of convenience, although there are certain advantages in having the color strips and electrodes vertical upon the screen and effecting the color control by horizontal deflection, and in the discussion it will be assumed that this is the case.

A consideration of the showing of Fig. 3 will make it at once evident that if the tube is to be able to display a substantially pure color the beam must be converged so that its width is no greater than one-half the pitch of the grid electrodes 47, 47', even if at maximum it is deflected so far as to center its spot of impact on the screen directly under the color-deocting electrodes. Under these conditions of minimum focusing the width of a color cell or phosphor group under the control of any one pair of electrodes is approximately one and one-half times the electrode-spacing or width of the aperture. Each red and each P llie color strip is shared by two adjacent color cells. As in apractical tube the spacing of thegrid wires will be something of the order of 30 mils, each individual color strip will, in the pattern shown, be one-half this, or

At points more remote from the screen centery l5 mils wide. A one mil error in relative positions of the electrodes and their` corresponding phosphor strips therefore amounts to a 6% percent error in spacing. For this reason, even where relatively large spot size is desired because of .the system of color transmission used, it is desirable .to make the beam converge to an extent greater than .the theoretical minimum so as to permit a reasonable degree of manufacturing tolerance in constructing the apparatus and to allow for variation in sensitivity to color deflection with varying angles of incidence at the grid.

Where the aperture is between linear conductors whose length is great in comparison with their separation the convergence of the beam can be expressed by the equation V1 :cathodegrid voltage Vz=gridscreen voltage Ay=convergence of beam yo=width of aperture between grid conductors =angle of incidence of beam at grid =oomponent of angle of incidence normal to grid conductors The two components., a and ,3 of the angle of incidence 0, respectively parallel and normal to the grid conductors, are related by the equation tan2 l9=tan2 +tan2 (2) It is convenient to call the ratio since this quantity will appear repeatedly in the discussion to follow. Using this notation, at the center of the screen where a=0, =0, Equation l reduces to Ay K n ya 1+\/1+A The width W of the spot is equal to 1.) M y 1+ ya y0 being a negative quantity. If W is negative the spot is overfocused, the focal point being between the grid and screen, the beam diverging after it passes through focus. If

W should theoretically be zero, and the spot a geometrical line without width.

'In practice a focus as line as this is not obtainable for several reasons. Aberrations are present in the electron lens, and the equation is strictly correct only for paraxial rays of the electron beam. The paths of the electrons within the beam are not truly parallel, although they are very nearly so. Scattering of electrons takes place in the metal film 40 which covers the phosphor layer and within the phosphor layer itself, and the light emitted by the phosphors is further scattered by the phosphor crystals.

In spite of these facts the equation gives a very fair guide to the degree offocusing which can be realized. Curve 49 of Fig. 4 isa graphical representation of Equation 3, and curve Slof the same figure shows the size of spot obtained as measured on an actual tube, with values of K ranging from zero (no post-deflection focusing) to eight. ln this curve the actual spacing between the wires of the grid was 231/3 mils. The upper scale of the ligure is given in terms of the quantity K; in the lower scale the figures represent the relative voltages of screen and grid with respect to cathode, which is K l.

l't will be noted that the experimental curve lies for the most part almost uniformly three mils above the theoretical curve, indicating that the various factors mentioned serve to increase the width of the spot by approximately this amount irrespective of the amount focusing used. Such widening of the spot occurs even where no post-deflection focusing is employed, and is in fact most pronounced at minimum focusing voltages, where the apparent width of the beam is greater than the apparent width of the aperture. This would appear to indicate that the scattering of the electrons and the light are the factors which are most important in causing the increase in spot size. Nonetheless the experimental curve brings out clearly the fact that the finest focus is obtained at the center of the screen when the factor K=3, as predicted by the equation.

It may be noted here that the equation given is that applicable to cylindrical lenses. lf a grid, still lying in a single plane, is to focus the beam in both dimensions, Equation l must be modified by dividing the right hand quantity by 2. This comes about because the lines of force which tend to converge the electrons are divided between those which cause convergence parallel to the a plane and those which cause convergence parallel to the plane. With this modification theoretically perfect focusing is obtained when K=8 in Equation 3. This is true irrespective of whether the aperture be square,

p hexagonal (as shown in Figs. ll and l2 of United States Letters Patent No. 2,692,532 referred to above) or circular. lf, however, one dimension is greater than the the other the lens approaches the cylindrical form, becoming highly astigmatic, and there is no focusing potential which will make it of minimum size in both dimensions.

Aberrations become more apparent as the grid apertures depart from the circular form, but the equation still offers a reasonable guide for design purposes. Thus with a grid having square apertures there is a certain amount of mutual shielding at the corners where the perpendicular conductors forming it join, which results in the field being weaker at the corners and stronger at the centers of the sides. The result is a pin cushion distortion, and since the intent is not to form a true image but merely to concentrate the beam, it may in some instances be desirable to overfocus those portions of the beam entering adjacent to the center of the sides of the apertures to such a degree that the corners of the pin cushion, although possibly still under-converged, come in to an equal radius with the expanded sides.

Furthermore, particularly when wide angles of scanning deection are used, it may be desirable to underfocus the beam somewhat at the center of the screen in order to cause less overfocusing at the edges, the smallest focus being at some point between the center and the edges of the field. What constitutes the best focus is therefore dependent, in some degree at least, on conditions having no direct relation to this invention, except as they determine the post-deflection acceleration to be applied between the grid and the screen.

Once the ratio of the voltage between cathode and grid to that between the grid and screen has been determined, however, the necessary relationship between the phosphor groups and the apertures through which electrons reach those groups becomes fixed within a relatively narrow range. lf the distances from the center of' beam defection to the grid and the screen are called Rq and Rs respectively, and the distances of a specific aperture and the phosphor group whereon electrons through that aperture impinge from the screen center are termed Sq and Ss, without post detiection focussing Rs T2? should equal Ss Sa The ratio of the spacing between the centers of any pair'l of apertures and their corresponding phosphor groups would then be equal in all parts of the field and measured in any direction; the positions of apertures and corresponding phosphor groups would follow the similar triangle law, where Sq=Rq tan 0 and Ss=Rs tan 0.

As soon as post-defiection focusing is employed, however, the simple, similar-triangle relationship no longer follows. Neither, however, does the relationship hold which has sometimes been assumed by writers who have treated post-deflection focusing, i. e., that the ratio between grid spacing and phosphor group spacing should be unity. The difference between these spacing-ratios is numerically small; in a practical tube wherein the maximum scanning-deflection angle 0 of the beam is 36 the difference between the two ratios suggested by the prior art is only 5%, using grid to screen spacings (RS-Rg) approaching the maximum values that have thus far been used in practice. Cumulatively, however, this difference in ratio amounts to several phosphor groups. As will next be shown, the proper value of spacing-ratio lies between unity and the similar-triangle value.

lf D is the distance between the screen and the grid, d is the displacement of the center of the phosphor group from the base of a perpendicular dropped from the center of the corresponding aperture, (the distance d bcing measured in the plane of the angle of scanning deilection), the relationship given verbally in the broad description of the invention can be expressed by the equation or, using the same notation as was used in discussing focussing,

= 2D tan 0 (5) 1+t/1+K S602 a) In the case of a screen such as is shown in Fig. 3, comprised of linear phosphor strips, each of a single color phosphor, displacement of the point of impact in the direction parallel to the strips and to the general direction of the grid conductors has no effect upon the color displayed. It is only the deflection traverse to the strips which is important. The displacement in this direction can be derived from the Equation 4 or 5 and written:

In a typical tube in which a screen of the type illustrated in Fig. 3 is used, the aspect ratio of the screen will be, under present standards, 4:3. With a 10%X14% screen, having a diagonal dimension of 18, the average width of the phosphor groups, measured from the center of a red strip to the center of the nearest blue strip, will be about 30 mils, and the maximum angle of deflection, on the diagonal, may be taken as 36. Under these circumstances the maximum value of the angle a, parallel to the grid wires will be approximately 231/2 while the maximum component normal to the grid wires, will be 30.2 approximately. The number of phosphor groups or color cells will be 479, formed between 480 grid wires.

Curve 53 of Fig. 6 is a plot of the quantity dp against tan at the axis of the screen, where a=0. Curve 55 is a similar plot at the upper or lower edge of the screen where a=23.5. These curves are plotted assuming a value of voltage-ratio K=3, which gives the most concentrated focal spot at the center of the field, and with a value of D=450 mils. For comparison there is also shown a graph 56 of the displacement db in accordance with the similar triangle law, where K=0; i. e., where no post-deflection focusing is employed. The quantity 11 tan is, of course, directly proportional to the lateral dimension of the screen. It is also proportional to the number of phosphor groups, counting from the center of the screen outward; exactly proportional if the strips comprising the groups are all of the same width and very closely proportioned if the strips are varied in width. Accordingly the lower scale of abscissas in Fig. 6 is in terms of the phosphor group numbers, counted from the center of the screen. n

Since these groups are, onthe average, 30 mils wide, it will be seen that with the assumed maximum values of the angie indicated on the graphs by the points 57* and 59, the displacement d is equal to the combined widths of several phosphor groups; 5.4 such groups on the horizontal axis of the screen and 5% such groups at the upper and lower edges. The displacement also differs from the displacement required under the similar triangle law by from 31/3 to 3.51 phosphor groups at axis and corners of the screen respectively.

The relationships between the point of impact of the beam passing through a particular aperture and the width of the phosphor groups may be more clearly seen from the diagram of Fig. 5, whereon is shown an electron trajectory between grid and screen, using the same voltage ratio K that was assumed in plotting the curves of Fig. 6 that have so far been discussed. In Fig. 5 the broken line G indicates the plane of the grid, and the solid line S that of the screen. The solid line 63 indicates the path of an electron passing through the center of an aperture at the edge of the screen (==approxi mately 30.2). The small circles 47, 47 indicate the conductors so numbered in Fig. 3. rl`he dash line 63 indicates the path which would be followed by the electron if there were no potential difference between the grid and the screen (K=0) and the broken line 65 is the perpendicular from the center of the aperture from which the displacement d is measured. The slope of line 66 is inversely proportional to the average velocity of the electrons between grid and screen. The curved portion of the solid line 63 indicates the parabolic trajectory of the electron between the grid and the screen. The short dashes across the line S indicate the edges yof the color cells. The drawing is approximately to scale and indicates clearly how far the electrons would miss the color cells upon which they were intended to impinge if the screen were so dimensioned that the respective color cells lay either directly behind the centers of the apertures or followed the similar-triangle law.

lt will be seen from Figs. 5 and 6 that the ratio of the spacings between the centers of the phosphor groups and the centers of the corresponding apertures should be always greater than unity and less than the ratio of the distances from the center of deflection of the beam to the screen and the grid respectively if the phosphor groups and the apertures are to be substantially electro-optically alined; i. e., if a converging beam through the aperture is to fall entirely within the limits of a single phosphor, emissive of the same color, when it is directed to any part of the screen.

As should be apparent from the general discussion of focusing requirements given above, such electro-optical alinement, although it must be substantially correct, need not be mathematically exact. The tolerance permitted depends both upon the screen pattern and the system of transmission, as well as upon the maximum angle of deliection employed. It will be seen from the curves of Fig. 6 that the curvature of the lines 53 and 55 is quite small. These curves can, therefore, be approximated by singie straight lines in cases where a considerable tolerance can be permitted. Increasing the gridtoscreen distance D decreases the tolerance, as does the use of dotsequential systems of transmission, where the beam is switched from color to color during the transmission `of picture information, as contrasted with lineor tieldsev of 316% of the phosphor group width.

12 quential systems where the switching occurs during the blanking period.

The present tendency is toward shorter tubes using wider angles of deliection. As is indicated by curves 53 and 55 the rate of curvature increases with increasing deflection, and therefore approximations which are per-k fectly valid with small maximum deflections become inadequate with greater ones. With deilections of no greater than 30 and grid-to-screen distances of approximately ten times the width of a phosphor group, the size of the screen being the same as that heretofore considered, the widths of the phosphor groups and the spacing of the grid wires can both be uniform without involving errors of more than 2 mils at the corners of the screens and with errors of less than one mil over the greater portion thereof if the proper average spacing-ratio is chosen. The errors which can be tolerated are, of course, dependent upon the maximum dimensions of the luminous spot upon the screen as compared to the dimensions of the sub-elemental areas of phosphor which it excites.

The curves of Fig. 6 are drawn for both greater maximum angles of scanning deilection and a greater gridto-scre'en spacing than are satisfactory for use with a constant ratio of grid wire pitch to phosphor group Y width. Either the wire pitch or the phosphor group spacing may be varied either continuously, in order to provide substantially perfect correction, or in zones to apply corrections which are completely adequate in a practical tube and which do not involve errors greater than those inevitable in Acommercial manufacture. At present the zone construction is preferred, since the changes in pitch as between adjacent apertures or phosphor groups are extremely minute-a fraction of a millionth of an inch on the average-and it is much more practical to make the corrections when they have become cumulatively of appreciable value.

As the curves of Fig. 6 show, the 14% inch sc reen in the tube described is only 324 mils wider than the corresponding grid on the central axis of the screen and only 310 mils wider at the top and bottom, a difference of only 14 mils as between center and edges. The total differ.- ence in screen and grid dimensions is only 2%%. Thus, taking average values, if the widthof'the color cells is 30 mils, the corresponding wire pitch is 29.325 mils, a difference of very slightly over 2/3 mil.

For substantially exact compensation the ratio of the spacing should be such that the phosphor groups are about 2.44 percent wider than the grid wire spacing` at the center and about 1% percent wider at the corners of the field.

Either curve 53 or 55 can be approximated by no more than three straight lines without involving an error of more than l mil at any part of the field, a maximum If errors of as much as 10% can be tolerated, the same spacing ratio may be used at the center and edges of the field. Where only this first order correction is employed the correct spacing ratio may be approximated by the use of straight grid wires and strictly parallel and straight phosphor strips. The displacements necessary for such an approximation may be derived from a curve drawn midway between curves 53 and 55. Preferably, however, a second order correction is applied. This involves making the ratio of phosphor width to grid pitch greater at the center of the eld than it is at the edges, to conform to the very slight barrel distortion of the rectilinear eld caused by the variation in refraction of the beam with focusing angle. This barrel effect is only about 14 mils in 14 inches or in the neighborhood of 1/10th of l per cent.

The first order correction of phosphor-width to gridwire-pitch ratio can be made in two Ways; the Width of the phosphor groups may be varied while the grid pitch is kept constant throughout the width of the screen or the pitch of the grid wires may be varied while the width of the phosphor groups is maintained constant. Each of gramas;

these arrangements has certain advantages of its own. Because, as disclosed in a 'copending United States patent application of the present inventor, Serial No. 399,753 the sensitivity to deflection of the spot varies with variation in the angle 0. In accordance with a law having much the same form as that relating to the size of the focal spot, and because the variation in grid pitch (if this is the method of correction employed) is in the right direction to compensate partially for the increase in sensitivity, there is some advantage of using this method of correction. Copending United States patent application of Robert Dressler, Serial No. 343,834, filed March 23, 1953, discloses a method of positioning the grid wires at a high degree of exactitude. Using, preferably, the curve 55, indicating the relative displacement of the grid apertures and the phosphor groups, the curve may be approximated by two or more straight lines; it is seldom necessary to use more than three. In accordance with the Dressler disclosure, the wires are positioned by bars of glass or like material which are accurately notched and are mounted at the edges of the screen, one on each side of the viewing area. The wires are stretched across these bars and are positioned by the notches therein. All of the notches can be made simultaneously with an array of accurately spaced cutters mounted on a single shaft, and since the accuracy may be built into the cutters the actual formation of the notches becomes a simple and inexpensive process. If, say, the 250 wires at the center of the screen are disposed with uniform pitch, the next 75 wires on each side of the central group are disposed with a slightly increased but uniform pitch and a final group, with a pitch again slightly increased extends to the edge of the screen, the error in the tube used for illustration, may be made less than one mil at any position along the upper and lower edges of the screen.

Alternatively, the screen may be laid out to a very large scale with the widths of the phosphor groups Varied in the inverse manner; the width of the phosphor groups will be greatest in the central portion of the screen with zones of successively decreasing width as the edges of the screen are approached. The design thus constructed may be then reduced photographically to proper size and silk screen stencils or other printing devices made therefrom. If the particular service in which the tube is to be employed will permit a compromise in which only the first order correction is necessary it is obvious that a curve intermediate curve S3 and 55 can be constructed in the same manner to determine grid pitch or phosphor group spacing as the case may be.

It is preferable, however, that the second order correction be made. The second order correction can also be applied in two different ways, irrespective of the method employed in the rst order of correction. One way which has proved very satisfactory in practice is to make a gelatin print of the screen pattern, mount this print in a frame, the sides of which may be bowed outwardly, and stretch the frame and the gelatin print by the necessary 14 mils (in the present case) to accomplish the second order correction. The gelatin print can then be rephotographed, while stretched, and the resulting negative used as a master from which any desired number of duplicates may be formed.

In the alternative, the second order correction can be applied to the grid, whether or not the first order correction was so applied. Copending United States patent application of Howard R. Patterson, Serial No. 364,778,

tiled June 29, 1953, discloses the use of damp rods of undulatory form which may be used to prevent vibration of the grid wires under the electro-static forces set up by the color-switching process. These damp rods may be preformed, as shown in the application mentioned. The undulations in the damp rods, passing under and over alternate grid wires, serve to position these wires laterally and by forming the damp rods with the correct pitches for Various zones, longitudinally of the Wires, the grid as a whole can be pulled into a slightly pin-cushion form which will give the desired correction.

The two general alternatives are illustrated in Figs. 7 and 8 respectively. Because it would be impossible, in a patent drawing, to show either the number of grid conductors or the number of phosphor strips actually used, both the relative widths of the phosphor groups and the magnitude of the correction is shown as applied in three zones, one central and two with equal and opposite cor- Vrections at the sides of the screen. The second order correction is shown as applied only at the two outer zones. Successive color cycles, red, green, blue green, are `shown cross-hatched in opposite directions and the junctions between the individual phosphors are indicated by dotted lines. In Fig. 7 the grid electrodes are shown as straight and uniformly spaced and the correction is applied to the phosphors; in Fig. 8 the reverse is true. The reference characters on the few parts shown correspond to those used in Fig. 3.

It will be recognized that the particular methods of varying the ratio between aperture spacing and phosphor group spacing that have been mentioned are only a few of those which may be employed to secure substantially the same results.

The factors leading to the grid-screen relationshipl here stated have been 4given thus at length in connection with the single-grid structure typiiied by Fig. 3 because of their relative simplicity and because precisely the same principles are involved as in the case of a double-grid structure of the type illustrated in Figs. l and 2. Whatever the structure or whatever the degree of focusing employed, the displacement d is proportional to the component of velocity imparted to the beam by the scanning deilection, measured in the direction of that component of velocity, multiplied by the electron transit time between the plane of the grid and the plane of the screen. In the case where two grids are used the focusing effect can be computed in accordance with the same general method employed in the case of a single grid, but this can only be done with a degree of accuracy which approximates that obtainable in a single grid if the two grids either lie so close together that they may be treated as if they lay in a single plane and Equation l modied by multiplying the right-hand terrn by 1/2 may be applied to give nearly correct results or if they are separated by a distance which is large in comparison to the separation of the grid wires, so that each grid, when viewed from the other, can be treated as if it were an equi-potential surface. At positions between the two mentioned the fields between the wires of the two grids are sutilciently warped to introduce inaccuracies which are greater than those obtained with the focusing formula given in Equation 1, supra.

With these reservations, however, the approximate convergence of the beam by the respective grids can be expressed by the equations:

For the rst grid:

tan

As used in these equations J and K are respectively the ratio of the intergrid voltage kand of the second-grid-toscreen voltage to the voltage between the cathode and the irst grid; C is the distance between grids and D' is the distance from the `second grid to the screen. The angle a, measured parallel to the electrodes of the first grid is of course transverse to those of the second.

As is the case of the single dimension focusing, the minimum-size spot is obtained whenthe quantity on the left of each of these equations becomes -lg when C becomes small as compared to the pitch of the grid electrodes the assumptionof uniform eld between the two grids, on which the focusing equations are based, is no longer valid. As the two grids approach the same plane, however, J approaches zero and K approaches 8 for a spot of minimum size at the center of the field. The displacement d under these circumstances is represented by curve 67 of Fig. 6.

ln the form of the invention illustrated in Figs. l and 2 it is usually desirable to separate the grids by a greater distance than would warrant their consideration as being in the same plane, since the latter arrangement accentuates thepinucnshion distortion of the spot. Itis also desirable, however, to space theA second grid from the screen by a distance fairly large in comparison to the pitch of the grid wires.

1n one such tube the spacing D' from second grid to screen is 360 mils and the inter-grid spacing 80 mils, the pitch of both grids being substantially equal and approximately 30 mils. With the proper focusing voltages this gives a substantially square spot.

Where the grids are so closely spaced as to give pin'- cushion distortion it may be reduced in eifect by overfocusing, increasing J slightly and K' materially.

As in the case of the single grid the convergence of the beam increases with the angle 0. There may therefore be a definite advantage in under-focusing at the center of the screen to avoid excessive over-focusing at the edges thereof, in order to obtain the best average focus.

The pattern of Fig. 2 possesses the unusual character- 1 istie that it permits the display of saturated colors even where the dimensions of the spot are equal to those of the aperture, provided the deflection of the beam is correct to centerthespot on the proper color phosphor. As a result it permits of a large latitude in the choice of the degree of post-deection focusing to be used,-par ticularly when employed with multi-gun tubes where the factor of rcolor-deiiection sensitivity does not enter; In

such multi-gun tubes it may be desirable to converge the beam only enough to allow `for a degree of manufacturing tolerance, perhaps 10% of the aperture width at the center of the screen. For tubes using grid color deflection acceptable results with either sequential or NTSC systems of color transmission are obtainable when the ratio between the voltage of thecathode and the mean of the two grids to that between cathode and screen is l :4, which leads to spot displacements and aperture to phosphor-group spacing ratiosy Vsubstantially as given by the single-grid formula above, provided the scanning deiiection angles employed are not too large.

In general, however, it is preferred to employ the more accurate formulas:

, more or less widely than those of the other.

As far as the amount of the displacement produced is concerned the only effect of either grid upon the other is its effect upon the component of the velocity normal to the screen. The wires of either grid may be separated In the case of the screen shown in Fig. 3, the continuous strips may run generally parallel to the wires of either grid, The barrel distortion of the field will occur no matter whether it be the rst or the second grid which causes it, although the amount of this distortion will differ as between the two grids. Since, as will be seen from curve 67 of Fig. 6, where K=8, the total displacement as between color cells and grid apertures is less when bidimensional focusing is used, because of the greater average velocity of electrons between the grids and the screen, the amount of barrel distortion will also be smaller, but it will still exist. It follows that the positions of the blocks transverse to the continuous strips will also be subject to the barrel distortion. l

It should be noted that although applying a pin-cushion correction to the grid causes electrons falling through it to be properly electro-optically alined with the corresponding phosphor group, it does not correct the small amount of barrel distortion which exists in the picture field. Even though the grid may be shaped to correct the pin-cushion distortion as far as the color displayed is concerned, the scanning deection will ordinarily be rectilinear and will not follow the form of the grid and hence the barrel distortion will still appear on the screen. As has been noted, however, the total amount of this distortion is very small and cannot ordinarily be noticed. Usually it is less than the distortion inherent in the seanning waveforms developed in the television receiver.

Any of the methods of compensation that have been discussed in connection with a single-grid type of target may be utilized with double-grid targets, as can any combination ofY these methods of correction. It therefore appears unnecessary to reconsider these methods.

One point which should be noted, however, -is that if electrical deflection is used, ias is shown in Fig. l, the centers of detiect-ion will be different with respect to the two dimensions of deflection. Where electro-magnetic deflection is used the -coils -in the deflecting yoke are normally -in the .same plane and the center of deflection will be the same for both directions of scanning deflection.

The degree of focusing desirable with double-grid tubes using a phosphor pattern of the type illustra-ted in Fig. 2 depends in some degree on the system by which color information is transmitted. 'Single-gun' tubes us-ing this type of pattern may be used with the NTSC system without breaking up the substantially :continuously Atransmit-V ted -color information into dot-sequential form. Using the .pattern of Fig. 2 .the transition from any color to any other .can be made direc-tly as it is unnecessary 'to traverse the green strip, for example, `in passing from red to blue and the electrodes .can be so biased that when no color deflection is applied Ithe resultant light is a pure white. The color displayed then depends on the direction of deflection and the saturation on its amplitude.

Under these circumstances a somewhat more pleasing effect is produced with a diffuse spo-t than with a sharply Having thus'set forth the invention, what is claimed is as follows:

f 1. In a cathode-ray tulbe for displaying television images in color including means for directing ya iiow of electrons against a target area across which they are adapted to be defiected` in two dimensions from -a center of defiection to trace a raster defining a field of view; Ia target insaid area comprising a display screen including a base, phosphors emissive on electron impact of light of different componen-t colors additive to produce white light disposed over substantially 4the entire area of said base in a repeating pattern composed of groups of all of said phosphors, the area covered by each group being in at least one dimension of the order of magnitude of one elemental area of the television 4image to be reproduced and an Aelectron permeable conducting layer covering said phosphors; an electrode structure mounted adjacen-t and substantially parallel to said screen, 'and terminals for applying different electrical potentials to vsaid conducting layer and said electrode structure, respectively, said electrode structure having :apertures defining the pupils of a multiplicity of electron lenses through which said electron flow can be focused on corresponding lgroups of phosphors, the centers of said groups of phosphors being substantially uniformly spaced and the spacing of said apertures being non-uniform and greater near the edges of said target/than at the center thereof.

2'. In a cathode-ray tube for displaying television images in color including means for directing a flow of electrons against a target area across which they are adapted to'be deflected in two dimensions from a center of deflection to trace la raster defining |a field of view; a target in said area comprising a display screen including a base,

phosphors emissive on electron impact of light of different component colors additive to produce white light disposed over substantially the entire area of said 'base in a repeating 'pattern of substantially parallel strips of the respective phosphors so arranged that any three successive strips comprises a group including all of said phosphors the width of which is of the order of magnitude of one elemental area of the television images to lbe re-.

produced, and an electron permeable conducting layer deposited over said phosphors; a grid of linear conduc.

tors mounted adjacent and substantially parallel yto said screen with the conductors .thereof crossing said screen in the same direction as said phosphorstrips and defining therebetween a multiplicity of apertures through which 4said electron flow may be directed to corresponding single groups of said strips, the ratio of the widths of the groups of three strips to the widths of the corre sponding apertures being greater at the longitudinal centers'of the groups of strips than lat the ends thereof.

3. The invention as defined in claim 2 wherein the centers of said lapertures are more widely spaced at the ends of said linear conductors than .at their centers.

4. The invention as defined in claim 2 wherein each adjacent pair of said linear conductors is uniformly spaced throughout its length and said groups of phosphor strips are wider attheir centers than at the ends thereof.

5. 'In a cathode-ray tube for displaying television images in color-including means for directing a ow of electrons against a target area across which lthey are adapted to be defiected in two dimensions from a center of deflection to trace a raster defining a field of view; a target in said area comprising a display screen including a base, phosphors emissive on electron impact of light of different component colors additive to produce white light disposed over substantially the entire area of said base in a repeating pattern composed of groups of all of said phosphors, the area covered by each group vbeing in at least one dimension of the order -of magnitude of one elemental area of the television image to be reproduced and an electron permeabley conducting layer covering said phosphors; an electrode structure mounted adjacent and substantially parallel to said screen, and terminals for applying different electrical potentials to said conducting' constant, and the ra-tio of aperture spacing to group spac-v ing increasing from yzone to Zone -outwardly from the center of said target.

6. In a cathode-ray tube for displaying television im-f ages in color including means for directing a flow of elec trons against a target area across which they are adapted y to be deflected in two dimensions from a center of deflection to trace a raster defining a field of view, a display screen disposed in said target area comprising a base,

phosphors emissive on electron impact of light of dif' ferent component colors additive to produce. white light disposed on said base, and an electron permeable conducting layer covering said phosphors, said phosphors being disposed over substantially the entirearea` of said base in a repeating pattern composed of groups of all of said phosphors, the area covered by each group being, in at least one dimension, of the order of magnitude of one elemental area of the television image to be r reproduced, an electrode structure mounted adjacentand substantially parallel to said screen, and terminals 'for applying different electrical potentials to said electrode structure and said conducting layer respectively, said electrode structure being apertured to define the pupils of a l multiplicity of electron lenses through each of which electrons of said iiow can be focused on a single one of said groups of phosphors, the ratio of'the spacings between the centers of said apertures and the centers of the respective groups of phosphors on which electrons of said iiow entering said apertures impinge being less than unity and greater than the ratio of the distance from said:

center of defiection to said electrode structure to the distance from said center of deliection to said screen.

7. In a cathode-ray tube for displaying television images in color including means for directing a flow of electrons against a target area across which they Vare adapted to be deflected in two dimensions from a center of deflection to trace a raster defining a eld of View, ai display screen disposed in said target area comprising a v base, phosphors emissive on electron impact of light of different component colors additive to produce white light disposed on said base, and an electron permeable conducting layer covering said phosphors, said phosphors `being disposed over substantially the entire area of said base in a repeating pattern composed of. groups of all of said phosphors, the area covered by-each :group bev ing in at least one dimension of the order of magnitude j of one elemental area of the television image to be j reproduced, electrode structure mounted adjacent and substantially parallel to said screen and terminals for ap-V v plying different electrical potentials to said electrode structure and said conducting layer respectively, said electrode structure being apertured to define the pupils of a multiplicity of electron lenses through each of ywhich electrons of said ow can be focused on a single one of said groups of phosphors, the ratio of the spacings between the centers of said apertures and the centers of the respective groups of phosphors on which electrons of said flow entering said apertures impinge being less than unity and greater than the ratioy of the distance from said center of deflection to said grid to the distance from said center of deliection to said screen, said ratio of spacings being greater adjacent to the edges of said screen than at the center thereof.

8. In a cathode-ray tube for displaying television images in color, including an electron gun for directing a beam of cathode rays against a target area across which yit is adapted to be deflected in two dimensions from aJ including phosphors emissive of all of said component colors being of the order-'of magnitude of one elemental area of the television images to be reproduced, a grid of elongated linear electrodes mounted in a plane substantially'parallel to said screen and adjacent thereto, and com prising two interleaved and mutually insulated sets, terminals for applying different electrical potentials to the electrodesy of said two sets and to said conductingV layer,

theV ratio of the spacings between thel electrodes of said j two sets tothe spacings of the centers of said. groups being less than unity and greater than the ratio of the distance between the center of deflection of said beam and said grid to the distance from said center ofA delico. tion to said screen.

9., In a. cathode-ray tube for displaying televisionA irn.V

agesiin; color, including an electron gunfor-directing a beam of cathode rays against a target area across which it is adapted to be deflected in two dimensions from a center` of deflection, to trace a raster denin'g a fieldl of View, a display screen disposed in said targetxarea comprising abase, strips of phosphors emissive on electron impact of light of different component colors additive to produce white deposited on said base in a cyclic order, and an electron permeable'conducting layer deposited over said phosphor strips, the width of each group of strips including phosphors emissive of all of said component colors being of the order of magnitude of one elemental area of the television images to be reproduced, and each group of strips being wider at the center than at'the ends thereof, a grid of elongated linear electrodes mounted in a plane substantially parallel to said screenV and adjacent thereto, and terminals for applying different electrical potentials to said conducting layer and said grid, the ratio of the spacing of the electrodes comprising said grid to the spacing between the centers of said groups being less'than unity and greater than the ratio of the distance from the center of deection of said beam to the plane of said grid to the distance from said center of deection to said screen.

10. In a cathode-ray tube for displaying television imagesin color including an electron gun for directing a beam of cathode-rays against a target area across which said beam is adapted to be deected in two dimensions from a center of deilection to vtrace a raster defining a field of view, a substantially plane display screen disposed in said area, said screen comprising a base, strips of phosphors emissive on electron impact of light of different component colors additive to produce white light disposed on said base in a cyclic order and an electron.- permeable conducting layervdeposited over said phosphorv strips, the width of each group of strips including phosphors emissive of all of saidY component colors being of the order of magnitude of one elemental area of the television images to be reproduced, a grid of elongated linearl electrodes mounted adjacent to said screen in a plane substantially parallel to the plane thereof, said electrodes being oriented with their length in directions generally parallel to the length of said strips, and terminals for applying different potentials to said conducting layer and to said grid, the ratio of the spacing of adjacent electrodes of said grid to the spacing of the centers 4of the adjacent groups of said strips nearest said adjacent electrodes being greater for pairs of adjacent electrodes at the edges of the screen to which said strips are parallel'than for pairs of electrodes passing over the center of the screen.

11. VIn a cathode-ray tube fordisplaying television images in color including an electron gun for directing a beam of cathode raysl against a target areaacrosgsgwhieh said beam is adapted to be detiected in two dimensions from a center of deflection to trace a raster defining a eld of view, a substantially plane display screen disposed-fili` said area, said screen comprising a base, strips of phonphors emissive-on electron impact of; light of. dierent component colors additive to produce` white light vdisposed on said base in a cyclic order and an electron-permeable, conducting layer deposited over said phosphor strips,xthe width of each group of strips including phosphors emissive of all of said component colors beingv ofY the order cimesnitude ofone elemental area of the television imagestoby reproduced, a grid of elongated linear electrodesJ mountedadjacent to said screen in a plane substantially parallel t0 the plane thereof, andterminals for applying diierent pov; tentials tosaid conducting layer and to said grid, said elec.- trodes being oriented in directions generally.y parallel to the length of said strips, the spacing of said electrodes being uniform at the midpointof the length of said elec! trodes and the spacing of the centers* of. said groups of' strips being uniform at the midpoint of the lengthof said strips, the ratio of said electrode spacings to saidgroup spacings being less thanunity and greater. than theratio of the distance between said center of deflection and theplane of said grid to the distance between said center of.

deflection and the plane of said screen. Y

l2. In a cathode-.ray tube for displaying television im.- ages in color including an electron gunforv directing a beam of cathode rays against a target area across, which said beam is adapted to be deflected in two dimensions from a center of deection to trace araster deninga leld of view, a substantially plane display screen disposed in said area, said screen comprising a base, stripsof phosphors emissive on electron impact of lightof different component colors additive to produce white light disposed on said base in a cyclic order andan electron-permeable conducting layer. depositedv over said phosphor strips, the width of each group of strips including phosphors emis.v sive of all lof said component colors being of the order of magnitude ofl one elemental area ofthe television imf ages to be reproduced, allof saidV groups having the same.

f width, al grid of uniformly spaced elongated linear-electrodes mounted adjacent to said screen in a plane substanf tially parallel to the plane of said screenand lterminals for applying different potentials to saidy conducting layer and to said grid, the ratio of the spacing of, theelectrodes4 ofi said. grid to the width of said groups being'less than unity and greater than the ratio of the distance between said centerfof deflection and the plane of said gridto the distance between said `center .of deiiection and" the plane of said' screen.

13. In. a cathode-ray tubeL for displaying television i111-,v ages in color including means for directing a owofekC- trons against a target area across which they are adaptedv to be deflected in two dimensions from a center'of' der ilectionv totrace a raster defining a ieldvof view, `a display screen .disposed in 'said target area comprising a bum phosphors emissive on` electron. impact of lighty of` differv ent component colors additive to produce white light dis: posed onsaid base, and an electron permeable conducting layer covering said phosphors, said phosphors being: disposed over substantiallyvthe entire areak of said base-in n repeating. pattern composed of groups of all of said phosf phors, the dimensions' of all of said groups being-equalv ,and of the order of'rnagnitude of one elemental area, of

the image to be reproduced, an electrode structure mount ed adjacent'and substantially parallel to said screen, and' salcl flow entering said apertures impinge being uniform throughout said target area and less than unity and greater than the ratio of the distance from said center of deflection to said electrode structure to the distance from said center of deection to said screen.

14. In a cathode-ray tube for displaying television images in color, including an electron gun for directing a beam of cathode rays against a target area across which it is adapted to be deected in two dimensions from a center of deection to trace a raster dening a field of View, a display screen disposed in said target area comprising a base, and strips of phosphors emissive on electron impact of light of diierent component colors additive to produce white deposited on said base in a cyclic order, the width of each group of strips including phosphors emissive of all of said component colors being of the order of magnitude of one elemental area of the television images to be reproduced, an electron permeable conducting layer deposited over said phosphor strips, a grid of elongated linear electrodes mounted in a plane substantially parallel to said screen and adjacent thereto, and terminals for applying diierent electrical potentials to said conducting layer and said grid, the ratio of the spacing of the electrodes comprising said grid to the spacing between the centers of said groups being less than unity and greater than the ratio of the distance from the center of deilection of said beam to the plane of said grid to the distance from said center of deflection to said screen and said ratio of spacing being greater at the portions of the screen at the ends of said phosphor strips than at the portions of said screen dened by the centers or said strips.

l5. In a cathode-ray tube for displaying television images in color, including means for directing a ow of electrons against a target area across which they are adapted to be deflected in two dimensions from a center of deection to trace a raster defining a substantially rectangular field of view, a display screen disposed in said target area comprising a base, phosphors emissive on electron impact of light of different component colors additive to produce white light disposed on said base in a repeating pattern composed of groups of all of said phosphors, the dimensions of each of said groups being in one dimension at least of the order of magnitude of one elemental area of the ltelevision image to be reproduced and said pattern covering substantially the entire area of said eld of View, an electrode structure mounted adjacent to said base and symmetrically disposed with respect thereto and equidistant from the corners of said raster and having apertures therein defining the pupils of a multiplicity of electron lenses through each of which said electron ow can be focused on a single one of said groups of phosphors, at least one electrode permeable to electrons between the phosphors on said base and said electrode structure, and connections for applying a diierence of potential between said electrode structure and said electron permeable electrode, the ratio of the spacings between the centers of said apertures and the centers of the respective groups of phosphors on which electrons of said flow entering said apertures impinge being less than unity and greater than the ratio of the distance from said center of deflection to said electrode structure to the distance from said center of deection to said screen.

References Cited inthe le of this patent UNITED STATES PATENTS 2,532,511 Okolicsanyi Dec. 5, 1950 2,631,259 Nicoll Mar. 10, 1953 FOREIGN PATENTS 866,065 France June 16, 1941

Images