US2793317A - Electron focusing structure - Google Patents

Description

y 1, 1957 E. o. LAWRENCE 2,793,317

ELECTRON FOCUSING STRUCTURE Filed Oct. 22, 1954 2 Sheets-Sheet l May 21, 1957 E. o. LAWRENCE ELECTRON FOCUSING STRUCTURE 2 Sheets-Sheet 2 Filed Oct. 22, 1954 INVENTOR. femur 0. Zia/25w:

vswri IITOiAIYJ 2,793,317 ELECTRON FocUsING STRUCTURE Ernest 0. Lawrence, Berkeley, Calif assignor to Chromatic Television Laboratories, Inc., New York, N. Y., a corporation of California Application October 22, 1954, Serial No. 464,141 9 Claims. (Cl. 315-2 1 This invention relates to post-deflection focusing structures for use in cathode-ray tubes intended for displaying television images in color, this application being a continuation-in-part of copending' application Serial No. 219,- 213, filed April 4, 1951, now Patent No. 2,692,532, and entitled Cathode Ray Focusing Apparatus.

The structures particularly to be considered in the present application are intended for use in determining the hue to be displayed on a television screen which comprises groups of a plurality of phosphors emissive on electron impact of light of different colors which are additive to produce white, arranged in a repeating pattern over the surface of the screen. Each group includes at least one phosphor of each of the different ones used, these usually being three, emissive, respectively, of red, green, and blue light, although this is not the only combination which may be used. The dimension of each group, in at least one direction, is of the order of magnitude of one elemental area or picture point of the image to be produced on the screen, so that each phosphor component of the group is sub-elemental in dimension. In, operation of the tube the screen is scanned bi-dirnensionally by an electron beam or beams so as to trace a raster thereon. If a plurality of beams are used they are converged so as to impact the screen at or nearly at the same, point, all of the beams falling, at any given instant, within an area of picture-point size.

The cross-section of the electron beam as it approaches the target area, within which the screen lies, is also of the order of magnitude of one picture element, so that if all of the electrons reached the screen following undeflected paths phosphors of all of the component colors would be exicited and the effect of white light would be produced.

In the application of which this is a continuation-inpart there are shown a large number of electrode structures which, mounted in front of the screen, form a multiplicity of electron lenses which converge the electrons forming a beam into a focal spot of sub-elemental size, whereby, at any instant, the electrons of a beam will impact only a phosphor emissive of light of the desired color. If a single beam is used means are provided for deflecting it, at will, onto the desired phosphor; if a plurality of beams are used, their angles of incidence at the screen are slightly different and they are sodirected as to fall, respectively, upon the different phosphors, the color displayed by the tube, at any instant, being determined by modulating the beams individually so that light of the proper color composition is emitted. V V

In one form of screen, to which the present specification is particularly directed, the phosphor pattern is comprised of narrow strips of the respective phosphors, and the apertures of the electron lenses are defined by elongated linear conductors, stretched substantially parallel to the phosphor strips. The positioning of the conductors with respect to the pattern on the screen is such that each aperture, formed between adjacent conductors, is electronoptically alined with a corresponding group of phosphor strips.

, 2,793,317 Patented lVlay 21, 1957 In order to constitute electron lenses, there must be associated with the structure thus described at least one additional electrode which is electron-permeable; i. e'., through which electrons can pass as they do through the apertures of the grid formed by the elongated conductors. When different potentials are applied to the grid and to the electron-permeable electrode or electrodes, electric fields are set up between the two which deflect the electrons of the beam, these fields constitutingin fact the electron lenses through which focusing is accomplished. The electron-permeable electrodes through which, in conjunction with the grid electrodes, these fields are established may take several forms; they may themselves'be of gridlilce structure, they may be formed of metal gauze, or they may be thin metallic films, the latter usually being the case where the electron-permeable electrode lies directly upon the display screen itself. The nature of the lens formed by the combination depends on both the positioning of all of the electrode elements (hereinafter, for convenience, sometimes referred to as lens elements, even though the fields themselves constitute the actual lens) and upon the ratio of the potentials of those elements with respect to the potential of the cathode from which the electrons are emitted and with respect to each other. By altering these potentials and the positioning of the electrodes the same structures may be either converging or diverging lenses, their focal lengths may be changed, and the points on the screens at which the various apertures focus may be altered.

Numerous electrode structures for forming cylindrical electron lenses of the character described' are shown and described in the application above referred to; broadly speaking these structures are generally equivalent, and the choice as to which structure is to be employed becomes largely a question of mechanical convenience. Under certain circumstances, however, certain arrangements have material advantages not possessed by the others. This specification refers broadly to an arrangement wherein there is mounted on at least one side of the grid which defines the lens apertures, an electron-permeable electrode, quite closely spaced in relation to the grid, and comprises conducting elements lying within the grid apertures as viewed along the electron path; preferably transverse to the grid conductors. With this arrangement there may or there may not .be also used an additional electron-permeable electrode in the form of a conductive film on the screen surface. In operation, electron-permeable electrode close to the grid is operated at a potential positive with respect to the grid, while the screen surface operates at a potential which is nearly the same as that of the lens element nearest it, but preferably slightly negativethereto;

Among the objects of the structure thus described are: to provide a focusing structure which will produce minimum distortion of the field' of view; to provide a focusing structure which is the analog of a thin lens, and wherein the electron paths from the lens apertures to the screen canbe made substantially rectilinear; to provide a lens grid structure wherein the spacings of the lens elements and the phosphor strips canbe made uniform" throughout the area of the viewing field; to provide focusing structure wherein relatively small potential differences can be used to achieve the required focusing effect; to provide a focusing structure which can be operated at potentials such as to minimize halation caused by spurious electrons falling on the screen; and to provide a lens grid structure which is particularly applicable to display tubes using the angle of incidence of electrons at the structure either from a plurality of electron guns or from a single gun whose beam is deflected to change the angle to control the color displayed.

In the accompanying drawings, to which reference will 'be made to explain the principles of operation of the present invention:

Fig. 1 is a schematic diagram of a cathode-ray dis- :play tube embodying the invention, the tube shown being Fig. 3 is a cross-sectional view-through a portion of the target structure of a tube, showing the relationship between the grid, electron-permeable electrodes and screen elements, as employed in one embodiment of the invention, this embodiment being one of those used to produce the analog of cylindrical optical lenses of the doubleconvex type;

Fig. 4 is a similar diagram illustrating another embodiment of a double convex lens;

Fig. 5 is a fragmentary sectional view, similar in type to Figs. 3 and 4, illustrating a structure which is the elecdicular wires, and illustrating diagrammatically the shape -of the electric fields which accomplish the focusing, the

plane of section being normal to the wires of the aperture grid; 7 7

Figs. 7A and 7B are cross-sectional diagrams through .the structure of Fig. 6, the plane of section in both cases being normal to that of Fig. 6. Fig. 7A illustrates the .relationships of the elements at the center of the screen while Fig. 7B is a showing of the same relationships as they appear at the edges of the screen;

Fig. 8 is a showing of a structure superficially somewhat similar to that of the present invention but which develops fields of a different type;

Fig. 9 shows the fields established by the same structure that is illustrated in Fig. 8 when the relative potentials of a grid and electron permeable elements are reversed; and

Fig/ 10 illustrates a structure of electrodes similar to that shown in'Figs. 8 and 9 but with the position of the 'conductors'of the grids shifted with respect to that of the two preceding figures, soas to accomplish a concentration of the electron beam which is not, however, a true focusing action, but which is an electron-optical analog of an aggregate of prisms.

With the exception of the target structure to which the present invention particularly relates, the elements of the tube, shown schematically in Fig. 1, are conventional. The tube comprises evacuated envelope 1, which is generally funnel-shaped and the body of which may be either of metal or glass. In the glass neck of the funnel three electron guns are mounted, each comprising a cathode 3, a grid 5, and one or more anodes 7. Individually the guns are conventional. Each produces a narrow beam of electrons converging to a common focal point 9. Electrons of the beam passing through this focal point are reconverged by a focusing coil included in a yoke 11 at a conjugate focus in the target area, generally defined by a window 13 which closes a larger end of the envelope. In accordance with conventional practice the yoke 11 also includes coils for deflecting the three beams concurrently and bidimensionally so as to trace a raster upon the display screen which occupies the target area. Except for the fact that the three electron guns are mounted side by side instead of clustered in a triangle, this construction is substantially conventional.

In the present instance, the fluorescent surface which comprises the viewing screen is deposited directly upon the window 13. The nature of this pattern is best illustrated in Fig. 2. It comprises repeating groups of strips of phosphors 15x, 15g and 15b, emissive upon electron impact of red, green, and blue light respectively. If the diameters of the electron beams as they enter the target area are in the neighborhood of 30 mils in diameter, the width of each group of strips should be no larger than this, and the individual strips will each be in the neighborhood of 10 mils wide, each strip extending across the entire viewing area. So far as the present invention is concerned it makes no difference whether the strips run vertically or horizontally as the tube is viewed when in operation.

Preferably, although not necessarily, a thin conductive film 17, usually aluminum, is deposited on the surface of the phosphor strips; The film may be connected to the envelope 1 if it is of metal, the conducting coating conventionally deposited upon the inner surface of the envelope if it be of glass, or a film lead 19 may be brought out through the envelope so that an external source of potential may be connected directly to the film and it may be operated at a potential different from that of the envelope itself.

The lens structure for focusing electrons so that they will impinge upon the specific phosphor which is emissive of the desired color is mounted adjacent to the screen and generally parallel thereto, the term generally parallel as used herein meaning that the corners of the structures so' designated are equidistant. It is to be understood that the screen, if deposited upon the window of the tube, will ordinarily be somewhat curved in one or both dimensions, but it is also to be understood that the screen may be deposited upon a'plane base, mounted entirely within the tube and occupying, generally, the target area. In

practice the spacing between the grid and screen will usually be in the range between 0.40 inch and 1 inch, but these are not limiting values.

In its preferred form, the electrode structure which establishes the fields for focusing the electrons comprises a grid 21 of tightly stretched, elongated, linear conductors, which may be either narrow metal tapes or wires. If tapes, they are mounted substantially edge-on with respect to the electron paths, so as to offer minimum obstruction to the passage of electrons between them. The tape formation may be preferred with tubes using 'microdeflection of a single beam for color control. With a three gun tube of the type here illustrated, however, the use of wires is ordinarily convenient for mechanical reasons; as far as the electron lens action is concerned, tapes and wires are equivalents. V

The positions of the conductors constituting the grid. with respect to the groups of phosphor strips, are illustrated in Fig. 2. As has already been indicated this figure shows the actual physical position of the grid conductors as viewed at the center of the screen. If it were possible to view the screen and the" grid along the paths of the electrons, it would represent the relative positions in all parts of the screen, but due to the refraction of the beam in passing through the electron lens structure, the relative positions might be different, although only slightly different, than if viewed optically from the center of deflection of the beams as'they are scanned over the target area. Electron-optically, therefore, the conductors 21 of the grid are each centered over a junction between successive groups of phosphors; in the case shown, a junction between a red and a blue phosphor stripe In the pattern illustrated in this figure, wherein there are an equal number of strips of each phosphor in the over-all pattern, the junc tion could justas well be between a red and a green strip, or a green and a blue strip.

Positioned closely adjacent to the grid 21, generally parallel. thereto and preferably equally spaced on each side thereof, are electron- permeable electrodes 23 and 25. These electrodes can be either of a fine metal gauze, as illustratedin Figs. 1 and 3, or they may comprise tightly stretched, fine wires. Y

The essential feature of these electrodes, if they are to exercise the function which gives this type of focusing structure its particular advantages, is that they must inelude conductive elements lying within the apertures defined by the grid, and preferably centered within the apertimes as viewed along the electron paths entering them. Ideally, the two electrodes 23 and 25 should approach, as nearly as possible, equi-potential planes, so that the lines of force which terminate upon the grid conductors will terminate on elecrodes 23 and 25 in a substantially equal distribution thereacross. Practically, it would be difficult, if not impossible, to meet this ideal situation in a structure which could be physically realized. If the electrodes 23 and 25 are to be self-supporting, the conducting elements which comprise them must be of finite size. This implies that these electrodes themselves must be apertured and their physical embodiments must include conductors upon which the lines of force of the electric field will concentrate. Such a concentration of field means, in turn, that each aperture formed between the conductors becomes the aperture of an electron lens, which will have a diverging eifect upon the electron beam when the field at the grid apertures has a converging effect. The smaller the aperture the fewer will be the lines of force from the conductors defining each one and hence the less will be the effect of the fields upon the electron beam. The smaller the apertures, however, the greater will be the number of electrodes required to form them, and hence the greater will be the proportion of the beam electrons which are intercepted by the conductors, and therefore the less efiicient will be the electrode structure. One of the features of the present invention, in its preferred form, is that it permits the use of electronpermeab'le electrodes of extremely open character without introducing diverging effects which are noticeably deleterious with respect to the image produced by the tube.

The electron lens structure here illustrated is operated withelectrodes 23 and 25 at the same or nearly the same potential, and with the grid 21 negative with respect to both. With this arrangement, as will be shown hereinafter, effect of the fields between the various elements of the electron lens structure is to deflect the electrons in a direction normal to the conductors of the various elements. Electrons of a beam passing through electrodes 23 and 25 are diverged or defocused by the fields set up thereby along the phosphor strips, whereas electrons passing between the grid electrodes tend to converge onto a single strip. This latter is the desired effect, and by malting the apertures in electrodes 23 and 25 much smaller than those between the grid conductors, the converging effect completely dominates the divergence and the lens exercises its desired function. A fine meshed, metal gauze, therefore forms a very satisfactory lens of the type herein considered. If it be assumed, however, that such a gauze is formed of a square mesh, with certain of the conductors running parallel to those of the grid and others running transverse thereto, it will be recognized that the former introduce a partially neutralizing effect, increasing the focal length of the desired lens, this defocusing efiect becoming less and less as the number of conductors running parallel to the grid conductors is increased. The transverse conductors have no effect on the desired line focus. Neither does divergence in this dimension change the color displayed by the tube, since the electrons of the .beam are merely shifted along a phosphor strip emissive of the same color. Furthermore, as will be shown, with a suitable disposition of the two electrodes 23 and 25, the eifect of the deflections produced by the transverse elements can be very largely compensated. The longitudinal elements of the mesh can therefore be eliminated entirely, while those extending transversely of the grid apertures can be reduced in number.

A structure embodying such construction leads to a disposition of the conductors of electrodes 23 and 25 such as is illustrated in Fig. 2. The spacing of these conductors, which are numbered in the figures to correp nd h fe nce h rac er a l d *9 he spective electrodes as a whgle, does notnecessarily bear any direct relationship to that of the grid electrodes, although preferably it should be closer to reduce the size of the diverging apertures. It will be noted, that as viewed along the electron paths conductors 23 are spaced midway between conductors 25. It will further be noted that they are materially smaller in diameter. in a practical tube the grid conductors 21 may be 4 to 6 mils in diameter, whereas the conductors 23 and 25 may be as small as 1 mil or even less. This is possible because conductors 23 and 25 are operated at a potential positive to the grid and therefore much heavier field concentrations may be permitted without danger of breakdown due to field emission of electrons from the smaller conductors.

fnFig. 6 there is shown a fragmentary cross-sectional view of the conductors and screen, with the plane of section. normal to the grid conductors. In this figure the curved lines, bearing arrows, represent roughly the shape of the electric fields between the grid and the electrodes 23 and 25. The dotted lines 31 indicate electron paths through one of the apertures of the grid. The relative potentials applied to the electrodes of the structure are also indicated.

Consider an electron entering near the left edge of the aperture between a pair of the conductors 21. It will be seen that it will out nearly all of the lines of force from conductor 21 to electrode 23 as it passes through the upper half of the structure. The result is a retardation or deceleration in this region, plus a lateral acceleration which gives it a component of velocity away from the conductor 21 and transverse to its original path. Passing on, beyond the conductor 21 it will be reaccelerated in the original direction to its original total velocity but with an additional transverse component in the same direction as that received in passing through the first region. An electron passing through the structure near the right of the aperture and the same distance from a conductor 21 will be subjected to equal increments and decrements of. velocity in the longitudinal direction but its transverse acceleration will be in the opposite direction, away from the nearer conductor 21. Electrons passin through the exact center of the aperture will, however, receive no lateral acceleration. The beam will be therefore converged in passing through the lens structure. The potentials applied to conductors 23 and 25 are here assumed to be equal, and if they are equally spaced from the grid, the electrons passing through the center of the aperture will emerge from the structure with their original velocity and traveling in the same direc tion as they were when they entered it, although slightly refracted away from the tube axis.

What happens after the electrons emergefrom electrode 25 depends to a large degree upon the relative potential of the latter electrode relative to the surface of the screen. If the latter consists of phosphors uncoated with a film 17, secondary electrons will be emitted from the phosphor surface, depending in number ,upon the velocity with which the beams .strike the screen, and the screen will acquire an equilibrium potential which is within a relatively few volts of that ;of the electrodes 25. :If, as is preferable, the film ;17 is present, the number of secondary electrons emitted-will begreatly reduced, .those which return to the screen will have lower velocities so that they do not penetrate the film, and, moreover, the relative potential of the surface can be established at will. With no potential difference applied electrons will follow rectilinear paths between :the lens structure and the ,screen. If there is a potential difference between electrode 25 and the screen, any electrons which are approaching the screen at an angle will follow parabolic paths. As such tubes are operated, however, the velocity in the lens-screen region ishighand the deviation from rectilinearity -is very slight unless the potential- V 7 difierence is quite large in comparison to the total accelerating voltage between'the cathodes and the screen. It is preferable, therefore, to operate with the film 17 at approximately the same potential as the screen. By varying the potentials of the electrodes of the screen slightly, slight errors in grid-screen alinement can be compensated.

In the above discussion the deviation of the electrons from their straight-line paths as they pass through the focusing structure itself has been neglected. The closer electrodes 23 and 25 are together, the less such deviation will be. Furthermore, the closer they are together, the less will be the potential difference required between them and the grid 21 in order to establish a given num ber of lines of force and achieve a given degree of focus- On the other hand, the more distant electrodes 23 and 25 are from the grid, the more uniform will be the distribution of field along the lengths of the transverse conductors and therefore the more nearly will the deflection of the electrons be proportional to their distance from the center of the aperture. A very satisfactory result is obtained, however, if the separation of the electron-permeable electrodes from the grid is equal to the spacing of the grid conductors, although some slight improvement in this respect can be obtained by greater spacing.

Figs. 7A and 7B illustrate the defocusing eflect of the transverse conductors, the plane of section in these figures being normal to that of Fig. 6. Fig. 7A shows the relative position of the conductors at the center of the screen, while Fig. 78 illustrates their relative position at the edge. 'It will be seen that the arrangement is such that an electron entering near one side of an aperture in the elect-rode 23 will pass near the opposite side of an aperture in electrode 25, and that the acceleration normal to these electrodes, and therefore parallel to the phosphor strips, is in opposite direction and therefore tends to cancel out. Such cancellation is not complete, however. An electron passing through electrode 23 closely adjacent to the center of the aperture-will be subject to little or no deviation, whereas it will pass close to the edge of an aperture in electrode 25 and be subjected to maximum deviation. The maximum deviation along any electron path, however, will only be approximately one-half of what it would 'be if the conductors were alined parallel to the path of the rays. Both electrodes have a diverging effect, but the effect of electrode 25 will be the lesser of the two owing to the fact that it is closer to the screen. With the electrode conductors staggered as shown the refraction will be more nearly analogous to that of a pair of prisms than to that of a lens, the beam being split into two.

If conductors 23 and 25 were spaced equally to the grid conductors and were alined, the spreading of the beam in the direction of the phosphor strips would be equal to their degree of concentration transversely of the strips. If they were still equal in number but staggered, as shown, the amount of dispersion would be cut roughly in half. The amount of dispersion is, however, directly proportional to the size of the apertures, and by combining closer spacing with the staggered relationship the amount of divergence can be made to have negligible effect. Some slight divergence is not at all disadvantageous. 'It is frequently desirable to damp possible vibration of the conductors comprising the grid 21, and a very effective way of doing this is to thread damp rods, comprising glass rods of a diameter approaching that of the conductors of the grid, transversely across them. Slight divergence of the beam serves to eliminate the shadows of such damp rods, as well as to eliminate any shadows resulting from the electrodes themselves.

As canbe seen from the examination of Fig. 7B, where thefjelectrons of the beam enter the electron lens structure at nan angle,.the.fields to whichthe electrons are subjected are in the same direction as in the center of the screen, where the electron paths are normal to'the lens structure. In order to accomplish this, the spacing of the conductors 25 must be greater than that of conductors 23. The spacings of the two sets of conductors should be substantially proportional to the distances of the respective electrodes from the center of deflection of the electron beam.

It has elsewhere been shown that such a linear relationship does not hold with electron lenses formed between a grid of conductors and a conductive film such as 17 deposited on the screen, with the film operated at a potential positive to the grid conductors. Using such a two-- element lens system, all electrons are deviated toward the axis of the tube (as defined by the perpendicular from the center of deflection to the screen), and if the grid electrodes are uniformly spaced the strips of phosphor must be slightly narrower at the edges of the screen than at the center. It has also been shown that this correction for refraction is directlyproportional to the distance between the lens-f'orming elements. The change in refraction of the principle ray, passing through the center of each aperture, varies so little as between adjacent apertures that it becomes important only cumulatively. Usable tubes can be constructed applying :a uniform correction throughout the screen where the maximum angle of deflection of the beam is small; i. e., less than about 30". If the correction is to be made mechanically, however, tubes employing deflections of larger value can be corrected in zones, using phosphor strips of equal width for some fixed distance outward from the center of the screen, and strips of very slightly less width in one or more successive zones. In a tube wherein the lens-screen distance is from half to three-quarters inch (500 to 750 mils) the total correction in a tube wherein the maximum angle of deflection is 35 or 36 degrees, may be of the order of l or 2 phosphor groups, the screen being that much narrower than one wherein the electrons were not refracted by post-deflection focusing. A similar refraction effect takes place within the lens element here described. It is usually unnecessary that it be compen sated for,- as far as the conductors 23 and 25 areconcerned, for two reasons. One is that the total thickness of the lens structure is ordinarily only 10% or less of the distance between the grid and screen of a twoelement lens of the type mentioned and therefore the effects to be corrected for are only about 5% as great, as the second half of the lens partially corrects the refraction produced by the first half; the second is that the positioning of the conductors 23 and 25 is not nearly as critical as the positioning of phosphors. A linear correction, with the spaces between the conductors 23 and 25 respectively proportional to distance from the center of deflection, as mentioned, is therefore entirely adequate.

The same factors which have been mentioned in connection with the spacing of the conductors 23 and 25 renders the correction of the screen pattern for refraction simpler and more accurate than the correction by Zones, in the other type of lens mentioned. When the screen is operated within even a few hundred volts of that of the electrode 25, substantially all of the refraction wiil take place within the. lens structure itself, the paths of the electrons when they leave the lens structure will be substantially rectilinear, and therefore a single uniform correction can be applied to the screen, with a re-' sult even more accurate than is given with a zonal correction for a two-element lens, using a single grid and a positive electrode on the screen surface. Furthermore, by operating the conductors 25 at a somewhat higher potcntial than conductors 23, thereby making the lens fields slightly asymmetrical, the refraction of the principal ray of the beam within the lens can be almost completely neutralized, so that the spacing of the phosphor groups may be in the same proportion to the spacing of the grid conductors as -their respective distances from the center of the screen.

While all of the figures showing field conformations have shown the grid conductors as wires, it will be recognized that the same principles apply when the slat or tape formation illustrated in Fig. 3 is employed. With as wide strips as are shown in Fig. 3 the field conformations will be slightly different, and electrodes 23 and 25 can be slightly nearer the edges of the tapes as in the case where wires are used, but the direction of the fields and their focusing etfects are identical. The focusing fields are, however, confined to the portion of the grid elements adjacent to their edges and therefore it will be seen that the tapes may degenerate into wires without changing the over-all effect as far as the focusing function of the grid is concerned.

Fig. 4 shows another form of three-element electron lens and grid structure having much the same effect but somewhat more difiicult to construct. In this case, conductors 33 and 35 comprise the elements which are mounted parallel to the strips of phosphor, whereas the element 37 is a gauze or transverse conductors such as have been described. As far as focusing fields are concerned, this structure also gives the analog of a double convex lens, and the elements 35 will be operated at the same potential or nearly the same potential as the screen. Electrons, therefore, still follow rectilinear paths from the lens structure to the screen, and since the structure is symmetrical as far as the focusing fields at the edges of conductors 33 and 35 are concerned, the path of the ray entering the center of the aperture is a continuation of or parallel to the path of the parting electrons. The structure in the preceding figures is preferred, however, because it does not require exact alinement between conductors 33 and 35. Mechanically, therefore, the structures previously described offer fewer complications, although the effects produced may be made very nearly if not quite the same. Both this and the preceding structures have been referred to as analogous to double-convex lenses, but it would perhaps be more strictly accurate to consider the structures first described as analogous to two lano-convex optical lenses with their convex surfaces face-to-face, and the structure of Fig. 4 as analogus to a double convex lens, or two plane-convex lenses, back to back. The focusing effect is substantially the same in either case. It will be noted that in this case, as in the others, the gauze or transverse conductors are operated at a potential positive with respect to the other conductors of the structure.

A somewhat less desirable but still completely operative structure is shown in Fig. 5. In this figure only the two elements are used to establish the fields which constitute the lens and the latter is therefore equivalent to a planoconvex optical lens, and can be considered as constituting one-half of either the lens of Fig. 3 or that of Fig. 4. If the screen is allowed to seek its own potential, and therefore operates at substantially the potential of the apertureforrning electrodes 39, the paths of the electrons between the. grid and the screen will be substantially rectilinear as before.

Where permeable electrode 41 comprises either gauze or parallel transverse wires, the structure does not possess the, flexibility of operation which the double-convex structures do, but is nonetheless capable of giving satisfactory resuits. Being assymetrical, the emergent beam is not parallel to the incident beam, and the correction of the screenpattern for refraction will be substantially the same as with single-grid and film lenses if the screen, and the electrodev nearest it are operated at the same potential. This can be verylargely compensated, however, by applying a moderate potential difference between these two elements to introduce an opposite refraction. If the aperture-forming grid is nearer the screen the latter should be made: the more positive element, but if the electron-permeable electrode is the nearer, the. screen shouldv be negative to effect compensation of refraction.

In contrast to the structures. thus far described are those shown in Figs. 8, 9., and 1.0,. which are structures which have been suggested for the. over-all purpose of those above discussed. In Figs. 8 and 9 the structures are the same, each comprising three grids composed of electron optically alined conductors 43, 45 and 47. The conductors of the respective grids are alined parallel to the electron paths, and the two figures dilfer in that in Fig. 8, the central grid is operated at a negative potential with respect to the other two Whereas in Fig. 9 the reverse is the case. In either of these arrangements focusing of the electrons occurs. It will be seen from the lines indicating the field directions, the electrons passing near one edge of the aperture will be subjected to equal transversely accelerating forces in both directions. In Fig. 8 the electrons are traveling most rapidly as they pass between the conductors 43 and are therefore subjected to the diverging component for a relatively short time. They are retarded as they approach electrodes 45 and are subjected to a converging component and the latter therefore has more time to act. so that the transverse velocity component is reversed in direction. As they pass electrodes 47, they are again subjected to a diverging field, but again they are traveling at a greater velocity and this field'has less effect. The net effect is a convergence. The converging eifect of such a lens as this, however, is a second order effect. The potentials required to produce a given convergence, in a practical tube, must be an order of magnitude greater than those necessary in lenses of the type to which this specification is directed. In Fig. 9 the same general effects occur. Here the first and last fields traversed are converging and are effective when the electrons are traveling at their minimum velocity, while the electrons are traveling most rapidly when subjected to the diverging fields traversed as they pass near the middle set of electrodes 45. The same remarks as apply to Fig. 8, with respect to necessary focusing potentials, also apply to Fig. 9. In addition to the necessity for higher focusing potential the structures require exact alinement between the three grids.

In contrast to the structures in Figs. 8 and 9 is that of Fig. 10 which is similar to the two previous figures with the exception that the electrodes 43' and 47' are shifted by one-half of the width of the apertures formed between the conductors 45', these being the apertures which determine the alinement with respect to the phosphors on the screen. Strictly speaking this is not a lens structure at all, because any electron entering it must cut all of the lines of force existing between three sets of electrodes and therefore will be deflected to an equal degree in one direction or the other. Electrons entering to left of conductors 45 and to the right of conductors 43 will be deflected to the left, while those entering the other side of the aperture between conductors 43 will be deflected to the right. If the potentials are correctly chosen a width of the beam as it strikes the screen may be reduced by one-half, the structure being the analog of a series of parallel prisms. For some purposes this may be a desirable structure, and it will be seen that it conforms to the broad description of the present invention in that there are conducting elements lying within the apertures of the grid, as viewed along the electron paths, while the structure of Figs. 8 and 9' do not. These latter structures are operative with relatively low voltage tubes but where high electron velocities are employed the outer electrode voltages required to produce a given focal length are so high as to introduce danger" of breakdown by field emission from the electrode elements. Additional conductors, parallel to electrodes 43" and 47' and in the same plane as conductors 45' would make the arrangement conform more nearly to a true lens, the optical analog being a multi-sided prism whose surfaces approximated the surface of a cylinder. The disadvantage of such a structure, in intercepting a relatively large proportion of the beam as it passes through the ions, have already been discussed.

Where a single conductor parallel to each pair of grid" 1t the exact alinement between them does not have the same importance, but if their alinement is not carefully calculated a further interception of electrons may occur.

With respect to the types of lenses to which this specification is particularly concerned, the electrodes of the bi-convex types have been shown as uniformly spaced and the potential differences between the grids and the electron permeable-electrodes have been indicated as equal. Neither of these is a necessary condition. Just as bi-convex optical lenses may have different curvatures of their two faces, and still have the same focal length as a symmetrical lens of the same general type, so may an electron lens. A given voltage between the lens-forming elements will produce the same total displacement of the beam, irrespective of the separation of the elements of the lens structure, but the electron-optical coefficient of refraction and hence the thickness of the lens required to produce such a displacement of the beam depend also on the separation of the elements to which the voltage differences are applied.

' Therefore, just as the aberrations of optical lenses can be largely compensated by combining lenses of different focal length with different coefficients of refraction, so can errors of refraction be corrected with electron lenses of more than two elements.

It is this fact which gives the structures here described their flexibility. With a lens of the type shown in Figs. 2, 3, and 6, the same focal length can be had, for a given velocity of incident electrons, if the arithmetic sum of the voltages between the grid 21 and electrodes 23 and 25 respectively is a constant, but the refraction, which determines the positions onv the screen at which the focused beams will impact, depends on how the voltages contributing to the sum are divided between the two. The voltage relationship between the final element of the lensgrid structure gives an additional degree of freedom of design. Therefore such structures as are here described not only require smaller corrections but they permit larger manufacturing tolerances, since adjustments in the computed corrections can be made after a tube embodying the structures is completed.

structurally, numerous ways of constructing grids of the general type here considered have been proposed and used. This invention, however, does not relate to the mechanical structure but to the disposition of the conductors relative to each other, and particularly to the principles which lead to such disposition. Such details as have been shown and described are therefore not to be considered as limiting the scope of the invention, all

intended limitations being expressed in the following claims.

, I claim:

1. In a cathode-ray tube for displaying television images in color, including means for projecting electrons in modulated beam conformation against a target area across which said electrons are adapted to be deflected to trace a raster, target structure Within said area comprising a screen having a display surface of phosphors emissive on electron impact of light of different colors additive to produce white light and arranged in a repeating pattern of groups of generally parallel strips, each group including at least one strip emissive of each of said colors, a grid of elongated linear conductors mounted adjacent to said screen with said conductors substantially parallel to said strips and with each pair of adjacent conductors defining an aperture which is electron-optically alined with a corresponding group of strips, a pair of electronpermeable electrodes mounted respectively on each side of said grid and substantially coextensive with said grid and each including conducting elements within the area defined by each of said apertures as viewed along the paths of electrons projected therethrough, and connections for applying diiferent potentials to said grid and said electrodes respectively.

2. The combination as defined in claim 1 wherein each tion for applying an electrical potential thereto.

5. In a cathode-ray tube for displaying television images in color, including a plurality of electron guns positioned to project a like number of electron beams converging on a target area across which said beams are adapted to be concurrently deflected to trace a raster, target structure within said area comprising a screen having a display surface of phosphors emissive on electron impact of light of different colors additive to produce white light and arranged in a repeating pattern of groups of generally parallel strips, each group including at least one strip emissive of each of said colors, a grid of elongated linear conductors mounted adjacent to said screen with said conductors substantially parallel to said strips and with each pair of adjacent conductors defining an aperture which is electron-optically alined with a corresponding group of strips, a pair of electron-permeable electrodes mounted respectively on each side of said grid and substantially coextensive with said grid and each including conducting elements within the area defined by each of said apertures as viewed along the paths of electrons projected therethrough, and connections for applying different potentials to said grid and said electrodes respectively.

6. In a cathode-ray tube for the display of television images in color which includes means for projecting electrons in modulated beam conformation against a display screen having a surface of phosphors, emissive on electron impact of different color light arranged in a repeating pattern of groups of substantially parallel strips, each group including at least one strip emissive of each color and the width of each group being of the order of magnitude of one elemental area of the images to be displayed, a focusing structure substantially coextensive in area with said screen mounted adjacent thereto and comprising a grid of tightly stretched linear conductors mounted substantially parallel to said phosphor strips to define an aperture between each pair of adjacent conductors which is electron-optically alined with a corresponding group of phosphor strips and an electron-permeable electrode comprising conductors extending in a direction transverse to said grid conductors mounted generally parallel to said grid, said grid being materially closer to said electrode than to said screen, and connections for applying different potentials to said grid and said electrode.

7. Ina cathode-ray tube for the display of television images in color which includes means for projecting electrons in modulated beam conformation against a display screen having a surface of phosphors, emissive on electron impact of different color light arranged in a repeating pattern of groups of substantially parallel strips, each group including at least one strip emissive of each color and the width of each group being of the order of magnitude of one elemental area of the images to be displayed, a focusing structure substantially coextensive in area with said screen mounted adjacent thereto and comprising three generally parallel electrodes, at least one of said electrodes comprising a grid of tightly stretched linear conductors extending in a direction substantially parallel to said phosphor strips, each adjacent pair of said conductorsdefining an aperture electron-optically alined with a corresponding one of said strips, a second of said electrodes comprising conductors extending transversely of 'said apertures'and the third of said electrodes comprising conductors extending in a direction substantially parallel to the conductors of one of the other two of said electrodes, and connections for applying different potentials to each of said electrodes, the spacing between said electrodes being materially less than the spacing between said grid and said screen and the conductors of the center electrode extending in a direction transverse to the conductors of the two outer electrodes of the structure.

8. In a cathode-ray tube for the display of television images in cOlor which includes means for projecting electrons in modulated beam conformation against a display screen having a surface of phosphors, emissive on electron impact of different color light arranged in a repeating pattern of groups of substantially parallel strips, each group including at least one strip emissive of each color and the width of each group being of the order of magnitude of one elemental area of the images to be displayed, a focusing structure substantially coextensive in area with said screen mounted adjacent thereto and comprising three generally parallel electrodes, at least one of said electrodes comprising a grid of tightly stretched linear conductors extending in a direction substantially parallel to said phosphor strips, each adjacent pair of said conductors defining an aperture electron-optically alined with a corresponding one of said strips, a second of said electrodes comprising conductors extending transversely of said apertures and being more closely spaced than said grid conductors, and the third of said electrodes comprising conductors extending in a direction substantially parallel to the conductors of one of the other two of said electrodes, and connections for applying diiferent potentials to each of said electrodes, the spacing between said electrodes being materially less than the spacing between said grid and said screen and the conductors of the center electrode extending in a direction transverse to the conductors of the two outer electrodes of the struc-' ture.

9. In a cathode-ray tube for the display of television images in color which includes means for projecting electrons in modulated beam conformation against a display screen having a surface of phosphors, emissive on electron impact of different color light, arranged in a repeating pattern of groups of substantially parallel strips, each group including at least one strip emissive of each color and the width of each group being of the order of magnitude of one elemental area of the images to be displayed, a focusing structure substantially coextensive in area with said screen mounted adjacent thereto and comprising a grid of tightly stretched linear conductors mounted substantially parallel to said phosphor strips to define an aperture between each pair of adjacent conductors which is electron-optically alined with a corresponding group of phosphor strips, a pair of electron-permeable electrodes positioned respectively on each side of said grid and comprising a grid of elongated linear conductors extending transversely of said apertures and more closely spaced than the conductors of said first-mentioned grid the conductors of said electron-permeable electrodes being positioned in staggered relationship as viewed along the electron paths through said apertures.

References Cited in the file of this patent UNITED STATES PATENTS Re. 23,672 Okolicsanyi June 23, 1953 2,172,845 Holzer Sept. 12, 1939 2,213,547 Iams Sept. 3, 1940 2,228,978 Schade Jan. 14, 1941 2,669,675 Lawrence Feb. 16, 1954 2,692,532 Lawrence Oct. 26, 1954

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