US2731582A - Grid structure for color television tube - Google Patents

Description

Jan. 17, 1956 J. coo 2,731,582

GRID STRUCTURE FOR COLOR TELEVISION TUBE Filed March 23, 1955 3 Sheets-Sheet 1 Uwwm/wi 111mm: w/

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GRID STRUCTURE FOR COLOR TELEVISION TUBE Filed March 23, 1953 3 Sheets-Sheet 2 CONDUCT/V5 COAT/N6 24 0M MODE 8.4!?

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INVENTOR. Les/ie J Cook ,4 T Tom in" Jan. 17, 1956 J. COOK 2,731,582

GRID STRUCTURE FOR COLOR TELEVISION TUBE Filed March 23, 1953 3 SheetsSheet 3 N0 8M5 0A) MODE am? 1 Smoow REG/0H -26 51.567704! SIIHDOW SCAM/W416 PnrrERu u/m/ SHORT 41005541? (Pas/7W5 8M5) y M A smmuma PflTTER/U WITH TALL 4/005 519/? /P05/7'/V ems) INVENTOR. Les/1'6 J Cook GRID STRUCTURE FOR COLOR TELEVISION TUBE Leslie J. Cook, Lafayette, Calif., assignor to Chromatic Television Laboratories, Inc., New York, N. L, a corporation of California Application March 23, 1953, Serial No. 343,887 9 Claims. (Cl. 315-14) The present invention relates to cathode-ray tubes of the type adapted to effect thereconstitution of polychrome images. More particularly the invention relates to cathode-ray tubes having a grid of coplanar parallel wires positioned adjacent to a striped phosphor screen, or target electrode, and to means for enhancing the color fidelity of the image reproduced on this screen by inhibiting any tendency the wires of the grid may have to vibrate as a result of the cyclicapplication of electrical potentials thereto.

Cathode-ray tubes constructed with a grid of parallel wires located adjacent to a striped phosphor screen are already known in the art and serveto focus the beam electrons into a pattern of thin parallel lines registered with the phosphor strips of the screen. The PDF (post-defiection-focusing) type of cathode-ray tube operation has been set forth by Ernest 0. Lawrence, in various of his copending United States patent applications, such as Serial No. 219,213, filed April 4, 1951, issued October 26, 1954, as Patent No. 2,692,532, and Serial No. 234,190, filed June 29, 1951, issued June 21, 1955, as Patent No. 2,711,493.

In order to facilitate an understanding of the principles of the present invention, a brief description of one such form of single-gun PDF tube will now be given. This description should be construed as exemplary rather than limiting, since it will be seen that the invention is obviously applicable to tubes constructed along diiferent lines. In general, however, the tube may incorporate a .screen, or target electrode, made up of a relatively large number of very narrow phosphor strips laid down in a predetermined sequence to develop, when imparted by a scanning cathode-ray beam light in a selected chromatic sequence, such as red, green, blue, green, red, green, etc. The phosphors are then aluminized, or the screen in some other manner is provided with an electrically conductive coating. For convenience of reference the phosphor strips will be herein identified by a reference to the color of light developed, such as the red phosphor. Similarly the grid wires will be identified by the fictitious reference to the color of light which is developed when the grid wire is in its most positive state relative to the electron source, such, for instance, as ared grid wire when the grid Wire potential is such that the electrons from the source impact a phosphor strip to develop red light.

A grid assembly is located adjacent to such phosphor screen. The grid may be made up of parallel coplanar wires, and so related to the. phosphor strips that, in an electron-optical sense, there is a wire aligned with each blue strip, and similarly a wire aligned with each red strip. The red wires are connected to a common terminal, while the blue wires are similarly joined together electrically.

Between the plane of the wire grid assembly and the conductive coating on the phosphor screen, there may be established a difierencepfpotential ofsuch magnitude and polarity as to create a series of converging cylindrical lenses for the electrons in the scanning beam. In other words, the beam electrons entering between any pair of grid wires in passing to the target are brought to a line focus at the plane of the target or screen, this line structure having no necessary direct geometrical relationship to the path covered by the scanning beam in tracing the lines of the image raster.

It will now be appreciated that, as the beam electrons travel from the electron gun they may be focused by the above-described lens structure into a series of lines parallel to the phosphor strips. if there is a zero potential difference between the red and fblue terminals of the wire grid, then these lines formed by the beam electrons may be caused to lie within the boundaries of the green strips. lf'the wires associated with the red strips are made positive relative to the wires electronoptically related to the blue strips, the beam electrons will be deflected, and the thin lines will now lie within the boundaries of the red strips. Similarly, the electrons will strike the blue strips when the wires associated with such strips are relatively positive with respect to the red? wires. Difierent component colors are thus displayed according to the potential difference (if any) existing between the two sections of the grid wire assembly.

Accordingly,.color control in a cathode-ray tube having a grid assembly of the above nature (whether used for PDF or not) is brought about by a cyclic change in the potentials applied to selected wires of the grid, with the electrostatic charges on the wires exerting attractive forces the magnitude of which varies as a function of this potential change. If the forces thus exerted vary at a frequency close to the natural resonant frequency of the wires, the latter will vibrate, resulting in an oscillation of the line pattern on the phosphor screen. If the magnitude of this oscillation is sufficiently great, color contamination and/or electrical shorting can occur. Even with relatively little wire vibration the electron beam may be defocused, and image reproduction seriously impaired.

The above problem has been recognized. One solution is proposed in a copending United States patent applica tion of James T. Vale, Serial No. 252,664, filed October 23, 1951, and assigned to the same assignee as the present application. In the Vale disclosure, the vibration of such a wire grid is damped, and one preferred arrange ment for carrying out this objective includes an insulating cord or strand (which may be a ceramic thread) wound, or passed in-and-out, between the parallel conductors in a region near the wire frame or support.

The above-mentioned Vale method of reducing the amplitude of grid wire vibration by bringing into contact with the wires a mechanically lossy material is efiective for many purposes where some residual vibrationcan be tolerated. However, friction between the wires and the damping member necessitates the use of materials able to resist this abrasive action.

An alternative method of substantially eliminating grid wire vibration is disclosed in the present application. This embodies the principle that the frequency of vibration of a supported wire is a function of the distance between its supports. Hence, if a wire of given length (having a certain natural resonant frequency) is rigidly held at one or more points intermediate its ends,

' then the wire segments between the constraining points will have higher resonant frequencies corresponding to their shorter lengths. If these increased resonant frequencies are sufficiently higher than the driving frequency (i. e., the rate of change of potentials applied to the wires) then the tendency of the wire segments to vibrate will be materially decreased.

One object of the present invention, therefore, is to provide an improved form of cathode-ray tube suitable for the reconstitution of polychrome images.

A further object of the invention is to substantially completely overcome, in a polychrome cathode-ray tube having a color-control grid structure of coplanar parallel wires, any tendency toward vibration the wires of the grid structure may possess when color-changing potentials are cyclically applied thereto.

-An additional object of the invention is to provide, in a polychrome cathode-ray tube having a color-control grid structure of coplanar parallel wires, 2. method and means for reducing, or substantially eliminating, any electron shadow which might otherwise result from the employment of one or more vibration-reducing elements in conjunction with the wire grid.

Other objects and advantages will be apparent from the following description of a preferred form of the invention and from the drawings, in which:

Figure 1 is a diagrammatic representation of one form of cathode-ray tube in which the present invention may be incorporated;

Figure 2 is a perspective view of a number of the grid wires and phosphor strips of Figure 1, showing one pos sible relationship therebetween;

Figure 3 is a perspective view of a preferred type of vibration-reducing member constructed in accordance with the present invention;

Figure 4(a) is a cross-sectional view of the vibrationreducing member of Figure 3, showing in addition the electron shadow region which would normally appear on the target electrode in the absence of the conductive coating of the present invention;

Figure 4(b) is a representation of a scanning raster such as would be produced when utilizing the structure of Figure 4(a);

Figures 5(a) and 6(a) show the result of adding a conductive coating to the vibration-reducing member of Figure 4(a) so as substantially to eliminate the electron shadow region of the latter figure; and

Figures 5 (b) and 6(1)) are representations of scanning tasters such as would be produced when utilizing the arrangements of Figures 5(a) and 6(a), respectively.

In Figure 1 of the drawings there is generally identified by the reference numeral 10 one type of cathode-ray tube in which the present invention may be incorporated. Thistube 10 includes the usual components for developing a beam of electrons and for deflecting this electron beam in substantially mutually perpendicular directions so as to trace an image raster on the tube target electrode. Since these components are well known in the television art, no detailed description of their operation is believed necessary.

Adjacent to the end wall of the tube 19, and positioned to be impinged by the electron beam 12, is a target electrode 14. A grid 16 of parallel wires lies in a plane adjacent to the plane of such target electrode.

The target electrode 14 may assume a number of forms, one of which consists of a plurality of phosphor strips 15 deposited side-by-side on a thin plate of glass or other transparent material. These phosphor strips 15 have the property of fluorescing in different colors, and, as an illustration, the target electrode may include strips respectively fluorescing in the three primary colors, red, green and blue. Any selected colors, however, may obviously be represented by the strips according to the particular phosphor compositions employed.

As best shown in Figure 2, the phosphor strips are laid down in a predetermined chromatic sequence, with a green strip between each red and blue strip. The order in which the strips appear, however, forms no part of the present invention. Moreover, a discussion of phosphor screen constructions folrns part of copending application Serial No. 234,190 referred to above, and hence no additional details are believed necessary in the present dethe grid wires, diameter of the grid wire frame, etc.

scription except to indicate that the screen is given a thin coating of aluminum, or other electrically conductive electron-permeable material, which may be deposited or placed upon the phosphors in any suitable manner.

As above stated, the grid 16 consists of a plurality of parallel wires lying in a plane adjacent to the plane of the phosphor strip 15. One possible relationship of the grid wires and phosphor strips is illustrated in perspective in Figure 2. It will be noted that the relative dimensions and spacings of the components in this figure are intentionally distorted. However, each pair of wires, in an electron-optical sense, subtends strip areas constituting one color cycle.

In a preferred form of tube design, a potential is applied to the conductive coating on the phosphor screen which is different from the average, or D.-C., potential of the wires of the grid. This gives rise to a plurality of cylindrical electrostatic lenses, which serve to focus the electrons in the scanning beam into a series of fine lines registered with the phosphor strips. Thus, in effect, the structure 16 performs the dual function of a lens-grid and a color-control component. However, the invention is obviously applicable to cases where the grid 16 serves as a color-changing device alone, as will appear below.

It has been stated above that one of the principal features of the present invention consists in substantially completely overcoming any tendency toward vibration the wires of a color-control grid assembly of the class described may possess by constraining such wires at one or more points between their ends, and thus increasing their natural resonant frequency so that it is distinct from the rate at which the color-control potentials are varied.

Fundamentally, the above objective is achieved by utilizing any suitable type of solid constraining means. One method comprises casting a single filament of cementitious material (such for example as Sauereisen cement) on the center line of the grid structure and at right angles to the wires, thus embedding the latter in a rigid matrix. In practice, this method is not completely satisfactory as the cement tends to shrink while curing, thus displacing the wires. On the other hand, if the latter are held with a restraining fixture, the cement is prone to crack.

Another approach consists in alternately interleaving a pair of glass threads in-and-out between the grid wires and along the center line of the grid structure. The thread is then impregnated with some such material as potassium silicate to make a rigid assembly. This expedient may be acceptable for tube designs where an extremely precise grid unit is not required, but in a majority of cases the resulting wire displacement precludes its use.

The method which has proven to be the most satisfactory in practice is based upon the use of one or more rigid strips, or bars, of glass or other ceramic insulating material. One form is illustrated in Figure 3, where such a bar, designated by the reference numeral 18, is provided on one edge with a series of V-shaped notches 20 on the same centers as the wires of the grid 16. The sides of the bar 18, as shown in the drawing, are fiat, and the bar should preferably be relatively wide with respect to its thickness /2" width and .032 thickness is satisfactory in many cases) although its exact dimensions are not critical and will vary in accordance with the length of The plane of the bar 18 should be oriented to include the center of deflection of the electron beam. It is important in the construction of a tube of the character described that the likelihood of a shadow on the target be reduced to a minimal possibility. To this end the node bars, when positioned relative to the grid Wires, should be secured and anchored in such a manner that the sides are alined with and become parallel to the electron trajectory toward the target.

The bar 18 is scout-that the wires 16 (when in position as shown in Figure 3) lie at the bottom of each V-shaped notch. In practice, these notches 20 are each filled with a cementitious binder 22 (such as Sauereisen cement), and the bar is placed against the grid Wires so that each wire is effectively imbedded in a ball of binding material, which, after drying, locks the wires inplace in the same relative position in each notch. The ball of binding material is shown in Figure 3 as extending slightly over the edge of the bar 18. For the stated reasons it will be appreciated that the bar 18 may appropriately be termed a node bar, since it creates a node, or point of zero vibration, for each of the grid wires 16. Although some shrinkage of the cementitious binder 22 may occur as it dries, there is no appreciable displacement of the grid wires, since the discontinuity of the binding areas causes the resultant force developed in any one direction to be negligibly small. In other words, the single filament of cementitious material formerly employed is effectively broken down into a plurality of small bits, the shrinkage of which can be disregarded.

With the node bars located as described and with the cementitious binders serving to secure the node bars and the grid wires to each other it will be apparent that the grid wires thus form the support within the tube for the node bars. In the manufacturing process the precise location of each node bar with respect to the grid wires is determined in accordance with the geometry of the tube type being constructed, it being apparent that with a long tube, where the electron path between the electron gun and the target is long as compared to any target dimension, there will be but small angular change in the support position of one node bar relative to the other. However, for shorter tubes where the electron path from the electron gun to the target is a considerably smaller'multiple of any target dimension the angular change in the plane of each node bar with respect to each other node bar becomes more pronounced in order to maintain surface parallelism with respect to the electron beam trajectory. in either instance, it should be borne 'in mind that it is desirable that as few of the beam electrons as possible should be intercepted by the node bars in order that the already mentioned shadow effect be overcome.

Since the node bar 18 lies between the electron gun of tube and the target electrode 14, it possesses two properties which are common to all structures positioned so as to intercept a scanning beam. Firstly, when the structure is a dielectric, it will acquire an electro-static charge if there is an unbalance between the number of impinging beam electrons and the number of electrons lost through secondary emission. Secondly, being in the beam path, the structure will tend to cast an electron shadow on the target electrode.

The first of the above problems is solved by rendering at least a portion of the surface of the node bar electrically conductive, and then connecting this conductive surface to a point of fixed potential. One method of doing this is to paint at least a portion of the bar surface nearest the electron gun with a thin film of material such as aquadag. The conductive film 24 thus created is electrically connected to any constant potential point which is positive in polarity, and of the proper magnitude, with respect to the average, or D.-C. potential of the grid wires 16. The establishment of such relative potentials between the conductive coating 24 of the bar 18 and the grid wires 16 is more or less schematically illustrated in Figure 3 as comprising a battery 25, although it will be appreciated that such an element would not actually be used in practice. The establishment of an optimum potential for the node bar as will later be pointed out in more detail, brings about a solution of the second of the two problems previously set forth-namely, that of an electron shadow on the target electrode.

Let us first assume that the conductive coating 24 is omitted, and the bar 18 employed without this feature. Reference to Figure 4 (a) shows that the bar 18 inter- 6 cepts certain of the scanning beam electrons (having the trajectories 24) and thus creates a shadow region 26 under the bar. An actual raster which might be produced (the scanning lines being at an angle ofapproximately both to the grid wires 16 and to the node bar 18) is illustrated in Figure 4 (b). The shadow region is obviously quite pronounced, and would prove definitely objectionable to an observer.

In Figure 5 (a) is shown the effect of providing the node bar 18 with the conductive coating 24 described above, and then connecting this coating 24 to any suitable fixed potential point which is relatively positive with respect to the average potential of the grid wires 16. From the configuration of the electric field lines 28 surrounding the coating 24, it will be seen that many scanning beam electrons (which tend to cross these field lines at right angles thereto) will be bent inwardly near the lower portion of the bar 18 and thus tend to fill up the region 26 formerly in shadow. In other words, a majority of the electrons which pass in the immediate neighborhood 7 of the bar, but which do not impinge the conductive coating 24, will be deflected intothe region 26. Ohviously, these electrons thus deflected would otherwise have impinged the target electrode 14 outside of the shadow region, so that actually the latter is filled at the expense of the electron flux immediately adjacent to it. A certain amount of image distortion may result, but this can be minimized in a manner now to be brought out.

The extent of the region from which the filling electrons come is dependent upon the vertical height of the bar 18 (its width as previously considered in connec tion with Figure 3). If this vertical dimension is considerable, as in Figure 5 (a) for example, then the shadow region 26 is filled in with electrons from a substantial distance on each side of the bar 18. The resulting distortion is fairly smooth and flowing, as illustrated in Figure 5 (b).

When the node bar 18 is shorter, as shown in Figure 6 (a), the electric lines of force differ considerably in configuration from those shown in Figure 5 (a). The electrons filling up the shadow region 26 now come only from immediately adjacent the bar, and there is an abrupt stretching of the resulting image (as shown in Figure 6 (11)) which may be objectionable to an observer. Accordingly, the height of the bar 18 should be optimized (for each tube design in which such a bar is employed) so as to produce minimum apparent video distortion. Experiments seem to show that best results are obtained when the width of the shadow is reduced to a very small value (such as .005", for'example) rather than completely eliminated. However, this is a matter of individual preference.

In one design of a cathode-ray tube with which the node bar of the present invention may be employed, the wire grid 16 is biased negatively with respect to the metal shell of the tube. Accordingly, in such a case it is not normally necessary to provide a separate electrical lead for the node bar, and it may be connected within the tube directly to this metal shell. Since the potential difference between the conductive coating on the node bar, on one hand, and the average potential of the grid wires, on the other, is what constitutes the operating condition of these elements, the latter may be adjusted by varying the relatively negative grid voltage while the node bar potential remains constant.

It should be understood that use in the present application of the expression adjacent to in describing the spatial relationship of the wire grid and target electrode is to be interpreted as covering a condition wherein the wire grid lies in a plane slightly spaced apart from the plane of the target electrode, and is not to be confused with a case where these two members are contiguous, or actually in physical contact with one another.

Having thus described the invention, what is claimed is:

1. In a cathode-ray tube designed for the reconstitution of polychrome images, a color-control grid structure of parallel wife's-lying in a plane adjacent to the surface of the target electrede scanned by the electron bear'n'of said cathode-ray tube, and means for'inaterially reducing any tendency the parallel grid wires of said color-control structure may have to vibrate when ditferent color-control potentials are cyclically applied thereto, said vribationreducing means including insulating means for effectively dividing each grid wire into a plurality of sections each of which has a higher natural resonant frequency than that possessed by the wire prior to its etiective division.

2. A node bar for a cathode-ray tube designed for the reconstitution of polychrome images and having a colorcontrol grid structure of parallel wires lying in a plane adjacent to the surface of the target electrode scanned by the electron beam of said cathode-ray tube, said node bar comprising a strip of insulating material lying substantially in the plane of said grid wires and extending transversely thereto, and binding means rigidly securing said grid Wires to said insulating strip at substantially equally-spaced points ther'ealong, thereby substantially to preclude any tendency the said grid wires would otherwise possess to vibrate when different color-control potentials are cyclically applied thereto.

3. A node bar according to claim 2, in which said strip of insulating material is provided with a series of equallyspaced notches along one surface thereof, said grid wires being respectively receivable in said notches.

4. A node bar according to claim 3, in which each of the said equally-spaced notches in said insulating strip is of substantially V-shape and in which each wire is positioned generally at the apex of the V and wherein the said binding means comprises a mass of cementitious material surrounding the wire at its contact with the notch and contacting the node bar surface so that said grid wires are respectively embedded in a cementitious mass and secured to the node bar thereby.

5. A node bar for a cathode-ray tube designed for the reconstitution of polychrome images and having a colorcontrol grid structure of parallel wires lying in a plane adjacent to the surface of the target electrode scanned by the electron beam of said cathode-ray tube, said node bar comprising a rigid strip of insulating material extending substantially transversely to the direction of said grid wires, means for rigidly securing said grid wires to said insulating strip at such points along the latter as to maintain the parallel relationship of said grid Wires, and means for rendering at least a portion of the surface of said insulating strip electrically conductive.

6. A node bar according to claim 5, further comprising means-for 'connecting the conductive surface portion of said insulating strip to a point of fixed potential.

7. The combination according to claim 6 in which the said grid wires have an average operating potential, further comprising means for maintaining a selected difference between the fixed potential of the conducting surface portion of the insulating strip', on one hand, and the average operating potential of the said grid wires, on the other, the former being relatively positive in polarity with respect to the latter.

8. A node bar for a cathode-ray tube designed for the reconstitution of polychrome images and having a colorcontrol grid structure of parallel wires lying in a plane adjacent to the surface of the target electrode scanned by the electron beam of said cathode-ray tube, said node bar comprising an elongated member of insulating material disposed generally transversely to the direction of the parallel grid wires, means for securing said grid wires to said elongated member at such points therealong as to maintain the parallel relationship of said grid wires, and further means for substantially completely eliminating the electron shadow which would normally be formed on the surface of the said target electrode due to the presence of said elongated member in the path of the electron scanning beam of said cathode-ray tube.

9. The combination of claim 8 in which said grid wires have an average operating potential, and in which said means for substantially completely eliminating said electron shadow includes means for rendering at least a portion of the surface of said elongated member electrically conductive, together with means for establishing a predetermined value and polarity of bias between such conductive surface portion of said elongated member and the average operating potential of said grid wires, thereby to deflectelectrons from said scanning beam onto that portion of the surface of said target electrode where said electron shadow would normally appear due to the presence of said elongated member.

References Cited in the file of this patent UNITED STATES PATENTS 2,416,056 Kallmann Feb. 18, 1947 2,446,791 Schroeder Aug. 10, 1948 2,461,515 Bronwell Feb. 15, 1949 2,568,448 Hansen Sept. 18, 1951 2,590,764 Forgue Mar. 25, 1952

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