US2971117A - Color-kinescopes, etc. - Google Patents

Description

Feb. 7, 1961 H, B, LAW 2,971,117

COLOR-KINESCOPES, "E10.

Filed March 1, 1956 F'.]. I a? fill L-Jd .300 1 IN VEN TOR.

A E E 1 United Tttes Patent A COLOR-KINESCOPES, ETC.

Harold B. Law, Princeton, NJ., assignor to Radio Corporation of America, a corporation of Delaware Filed Mar. 1, 1956, Ser. No. 568,862

' 5 Claims. (31. 315-13 2,971,117 Patented Feb. 7, 1961 the latter face is presented, across an intervening space, to the metallized surface of the screen (to permit the establishment of a focusing field therebetween) and the insulating material faces the gun so that, under electronbombardment, the insulating material acquires an electrical surface-charge of a sign and magnitude capable of preventing secondary-electrons from entering the lensfield through the mask-apertures. To render the surface charge on the gun-side of the composite mask of the invention more nearly uniform the insulating material may be metallized. This metallized surface may be conthan masked-target tubes operating on the Crookesshadow principle. This is so because the concentrating effect of the lens field or fields upon the electron-beam or beams in a focus-mask tube permits the use of larger mask (or grill)' apertures than can be used in the mask of a Crookesshadow or shadow-mask tube. Thus, other factors being equal, there are more electrons ('an d hence more light) available at the screen of a focus-mask tube than in a shadow-mask tube. (As to this see French Patent 866,065 of 1941. See also Epstein U.S. Patent 2,315,367 and Law copending appln. Ser. No. 525,326, now abandoned, wherein the'lenses are of the spherical instead of cylindrical variety).

When a color-tube of the shadow-mask variety is converted to a focus-mask tube, by operating the mask at a lower potential than the screen, picture contrast suffers and, indeed, may become very poor, largely because a great many low-velocity secondary-electrons, liberated at the mask, are accelerated to the phosphor screen-and produce spurious light. This problem has long been recognized and it has previously been proposed to provide the screen-unit of such tubes with an appropriately energized auxiliary mask having apertures which are either much smaller or considerably larger than those of the focusing mask. The trouble with an auxiliary mask having very small holes (see Fig. 10 of Ramberg 2,728,024) is that it reduces light output. One trouble with an auxiliary mask having larges holes (see Epstein 2,659,026) is that the holes in both masks must be substantially perfectly aligned and this adds considerably to the cost of manufacture. Furthermore, since the focusing mask is still accessible (through the large holes in the auxiliary mask) to the high-velocity primary electrons, a great many secondary-electrons are released and enter the lens-field.

Accordingly, the principal object of the present invention is to obviate the foregoing disadvantages of present day focus-mask tubes and to provide a tube of the subjectvariety which shall be characterized (a) by its ability to produce pictures of high contrast and brightness and '(b) by the simplicity and economy of its parts.

Stated generally, the foregoing and related objects are achieved in accordance with the invention by'the provision of anapertured mask comprising a core, layer, film or base formed of insulating material and having at least one conductive face. When the composite mask of the invention has one insulating face and one conductive face nected, when necessary or desirable, to an external lead to permit the application of a suitable biasing potential thereto, or it may remain unconnected and permitted to assume an equilibrium potential sufiicient to prevent the passage of secondary-electrons through the mask-apertures, yet not so high as to permit electrical break-down across the insulating material. In the latter case the potential which the mask will assume is determined (a) by the average beam-current and (b) the electrical resistance of the insulating material between the conductive faces of the mask.

The inventionis described in greater detail in connection with the accompanying single sheet of drawings, wherein: Fig. 1 is an elevational view, partly .in section of a 3-gun tri-color kinescope of the focus-mask dot-screen varietylconstructed and operated in accordance with the present invention; A p Fig.2 is a fragmentary plan view, taken from the gunsideofthe screen-unit of the tube of Fig. 1, showing 'a hexagonal mosaic pattern of mask-apertures and color: phosphor screen-areas;

Fig. 3 is an enlarged sectional view of the focusing mask of Fig. 1; v

Figs. 4-8 are fragmentary sectional views of various embodiments of composite masks constructed in accordance with and embodying the invention.

In Figs. 1 and 2 the invention is shown as applied to a 3-gun tri-color kinescope 1 of the so-called focus-mask dot-screen variety wherein the red (R) blue (B) and green (G) phosphor dots (see Fig. 2) are arranged in a hexagonal mosaic pattern on the rear or target surface of aglass screen-plate 3 and the three electron-guns 5r, 5b, 5g are arranged delta (A) fashion in the glass neck 7 of the tube in a position to scan said different colorf: phosphor. The color-screen plate 3 may be of any desired shape (e.g. circular or rectangular) and curvature (e.g. spherical, cylindrical or fiat). In the instant case it is in the form of a circular section of a spherical shell and comprises the front-end or window of the cylindrical cap 9 at the closed end of the metal body portion. or cone 11 of the kinescope. An electron-transparent, light-reflecting, metal film 13 covers the entire target surface of the phosphor screen and renders it electrically conductive. As in copending application Serial No. 525,326, now abandoned, of the same inventor the screen is connected, 'as through an extension 1311 of its conductive surface 13 to the metal cone 11 through the metal cap portion 9 of the tube. The cone 11 comprises an accelerating electrode through the hollow interior of which the separate electron-beams (r, b and g) pass in their transit to the color-screen. A single lead 15 serves toconnect the screen and hence the cone 11 to the ultor potential of, say, 18,000 volts, which is derived from an appropriate source of voltage exemplified by a voltage divider 17.

The composite mask 19 of the invention, as illustrated in Figs. land 3, comprises two conductive faces 19a, 19c, electrically isolated from each other by an inter; posed insulating layer 1%. The mask is appropriately curved to conform, generally, to the contour of the concave surface of the screen 3. As is conventional, the (circular) apertures 1912 in the mask are arranged in the same systematic (hexagonal) pattern as the pattern of phosphor-dots on the mosaic screen, there being one mask aperture for each group (RBG) of phosphor dots. The mask 19 is supported in spaced-apart relationship with respect to the concave surface of the screen as upon a cylindrical metal frame 21 which is secured to the inner surface of the metal cap portion 9 of the envelope as by means of three studs or pins 23. The supporting studs 23 are constituted of a ceramic or other suitable insulating material capable of electrically isolating the mask 19 from the metallized surface 13 of the screen-plate 3, and from the metal cone 11 and cap portion 9 of the tube. Electrical lead 25 for the outer convex face 19a of mask 19 and the electrical lead 27 for the inner concave face 190 extend through an insulating bushing 29 in the metal cap 9 of the tube and are adjustably connected, as indicated by the arrows, to points on the direct current source 17 which are several thousand volts lower than the ultor point to which the screen and cone are connected. With the metal surface 13 of the screen and the cone (or accelerating electrode) 11 maintained at a potential of several thousand volts positive with respect to the convex face 19a of the mask the field (not shown) about each mask-aperture comprise a spherical-lens which operates to accelerate and to focus the bundles or jets of electrons which pass therethrough upon the particular color-phosphor dots which lie in their path. On the other or concave side 19c of the mask 19 there is a somewhat weaker field tending to accelerate electrons towards the cone 11. Electrons coming from the guns 5, therefore, decelerate to the mask and those getting through the mask holes 19h then accelerate to the phosphor screen.

Although the holes 1911 in the mask 19 may be considerably larger than the apertures in a conventional shadow-mask a significant fraction of each electron beam strikes the target surface of the mask and, as a consequence, a large number of low velocity secondary electrons are released from said surface. If these low velocity secondary-electrons are permitted to pass through the mask holes 19h, they would be accelerated to the phosphor screen by the focusing field about each aperture and would thus produce spurious light. To prevent the generation of spurious light resulting from the impact of secondary electrons upon the screen, it is only necessary to prevent the low velocity secondary-electrons from going through the apertures in the mask and this is accomplished by applying the proper positive voltage to the bias mask face, 190, relative to the other or focusing face 19a of the mask. The resulting relatively weak field on the concave side 190 of the mask has substantially no effect upon the high velocity primary electrons but operates to prevent the low-velocity secondary-electrons from going through the apertures 1911 to the phosphor screen. Instead they are accelerated toward the cone 11 where they are elfectively lost.

One practical way of making a two-sided or double mask is to start with a sheet of aluminum clad on both sides with copper (see Fig. 3) then photographically laying down the pattern of apertures on both pieces of copper with the aid of suitable resist, and next etching holes in the copper on both sides. An alkali is then used to etch the holes in the aluminum layer until only sulficient aluminum remains to support the copper. Thereafter the remaining aluminum is anodized completely to form the insulating support (1%) for the oppositely located copper faces 19a, 190. The two metal faces 19a, 19c may be considered as separate masks, insulated one from another and having aligned holes which may be of the same diameter or of different'diameters. Preferably the mask surface 190 which faces the electronguns has holes smaller than those on the surface. 19a

4 facing the screen so that the latter face (19a) cannot be struck by the high velocity primary electrons.

Another method of forming a two-sided or double mask involves the use of glass (e.g. Cornings Fotoform glass) which can be fabricated, by photographic techniques, to provide holes tapered in size from one surface to the other, as shown in Fig. 4. A conductive metallic coating ISM-19c applied to each surface by evaporation or printing completes the desired doublemask structure.

In a tube containing a two-sided or double mask of the construction shown in Figs. 1-4 it was found that essentially all the low-velocity secondary electrons were prevented from reaching the phosphor screen by operating of the metal faces (i.e. the bias face at +50 volts with respect to the other or focus face 19a).

In each of the mask structures thus far described, the secondary electrons are prevented from reaching the phosphor screen by applying the proper voltage to the bias face, of the mask. Another mask-structure, shown in Fig. 5, eliminates the need for supplying a separate bias voltage. The structure consists of a metal base 1% covered on its. target surface with an insulating material 19b. The. insulator 19b must have a secondary-emission ratio greater than one, at the potential at which the conductive part 19a of the mask is to run. This potential will ordinarily be between 3 and 6 kv. The insulating coating 19b prevents the secondary electrons from going through the mask holes because, under elec' tron bombardment, it charges positive with respect to the other or focusiug face 19a. The insulating material 19b thus acts as the bias face described above.

Willemite has been found to operate satisfactorily as the insulating coating, but other phosphors or insulating powders or enamels also work well, providing their first secondary-emission cross-over lies above. the potential of the focusing face 19a by at least 50 volts. The willemite has been applied by settling from a liquid suspension onto the mask. Thicknesses of 6 mg./crn. and more have been used.

In applying the willemite or other insulating material it is important that all portions of the mask that are to be exposed to the electron beam(s) be covered with the insulating material. Thus, referring to Fig. 5, in applying the insulating material 19b to a mask, whose holes have been etched from both sides of the metal and, consequently, contain knife-edges 19k, it is important that the. insulating material cover the edges within the holes. Where the mask holes taper outwardly, in the direction of the screen, as indicated at 19t in Fig. 6, the insulating material need not extend into the mask holes when said tapered surfaces 19t are not accessible to the electron beam or beams. Where the taper is in the opposite direction, as it is in Fig. 7, the tapered surfaces 19t' must be coated with the insulating material 1%.

In the embodiment of the invention shown in Fig. 8 the insulating layer 1% is provided on its target or beam side with a conductive coating 19c which charges to a positive value which is the same over the whole surface and is dependent upon the average beam current. Also influencing the amount of charge is the leakage between the conductive layer or bias face 190 and the focus face 19a. It is quite easy to fix this leakage at a desired value during manufacture by a judicious selection of the quantity and kind of insulating material employed. The leakage should be such that the bias face 19c will assume a potential of at least 50 volts positive with respect to its focus face 19a, but not more than a value that would cause break-down or arc-over between said faces. If it becomes necessary or desirable to hold the potential of the conductive surface 190 at a potential that cannot be easily gotten by simple charging of that surface the potential of said surface may be established by applying a potential thereto through a suitable lead, asin Fig. 1. In using a mask of the structure shown in 'Fig. 8, a potential difference of 175 volts between the bias face 19c and the focus face 19a proved satisfactory.

What is claimed is:

1. A cathode-ray tube comprising a mosaic screenelectrode, a mask having electrically separate major faces mounted adjacent to said screen-electrode and containing a pattern of apertures which is systematically related to the elemental areas of which said mosaic is comprised, electron-gun means mounted in a position to. scan said mosaic through the apertures in the major faces of said mask, a hollow accelerating electrode intermediate said electron-gun means and said mask through which primary electrons from said gun-means travel in their transit to said screen-electrode, means electrically connecting said screen-electrode to said hollow accelerating electrode for applying a common operating potential to said electrodes, and separate electrode leads individual to the separate major faces of said mask for establishing:

(a) an electron-lens field for said primary electrons about.

the apertures in said mask and (b) a retarding field for secondary-electrons released by impact of said primaryelectrons.

2. The invention as set forth in claim 1 and wherein said apertured mask comprises an insulating body portion coated on its said major faces with a conductive substance. I

3. A cathode-ray tube containing an electrically conductive light-emissive viewing screen of the mosaic variety, a mask comprising an insulating base having electrically separate oppositely located major faces disposed in space relationship with respect to said screen and containing a pattern of apertures which is systematically related to the elemental areas of which the mosaic pattern of said screen is comprised, a source of primaryelectrons mounted in a position to scan said screen through the apertures in said mask, the major face of the mask which is presented to said screen being electrically conductive to permit the establishment of an electron-focusing electric field therebetween, and the major face of said mask which is presented to said primary electrons being constituted essentially of a material capable of being maintained at a potential which is electrically positive With respect to its said oppositely located major face to provide a retarding field therebetween for secondary-electrons released by impact of said primary electrons upon said material.

4. The invention as set forth in claim 3 and wherein the major mask-face which is presented to said primaryelectrons is constituted of an insulating material having a secondary-to-primary electron-emissive ratio greater than 1:1. 7

5. The invention as set forth in claim 3 and wherein both major faces of said mask are constituted entirely of metal.

References Cited in the file of this patent UNITED STATES PATENTS 2,889,483 Kerstetter' June 2,

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