US3502942A - Post-deflection-focus cathode-ray tube - Google Patents

Description

March 24, 1970 H N ETAL 3,502,942

POST -DEFLECTION-FOCUS CATHODE-RAY TUBE Filed Oct. 24, 1968 Inventors Ghulclm A. Khan Sam H. Kaplcm Attorney United States Patent US. Cl. 31531 5 Claims ABSTRACT OF THE DISCLOSURE A unitary color-selection and focus-electrode assembly for a postdefiection-focus color cathode-ray tube' utilizes the apertured shadow mask not only as the principal structural component of the assembly but also as a first lens electrode. The focusing system features two lens electrodes, the second electrode being formed of a mesh of electrical conductors, having interstitial dimensions that are small compared with the aperture dimensions of the shadow mask and spaced to the gun mount side of the shadow mask. Means, including an insulator, secure the conductors of the mesh to the side of the shadow mask, completing a focus-electrode assembly. Different operating potentials are applied to the lens electrodes to accomplish post-deflection-focusing.

Related applications The subject invention is closely related to that described and claimed in Patent 3,398,309, issued Aug. 20, 1968, in the name of Sam' H. Kaplan and in application Ser. No. 661,601, filed Aug. 18, 1967, now Patent No. 3,421,048, in the name of John A. Christensen, both of which are assigned to the assignee of the present invention.

Background of the invention The present invention is addressed to post-deflectionfocus type of cathode-ray tubes and finds special application to tubes for reproducing images in simulated natural color.

As a general proposition color cathode-ray tubes have an image screen which bears interlaced or interleaved series of phosphor materials arranged either as stripes which extend in one scanning direction or as dots. While the invention is useful in tubes employing either type of screen, it will be described, simply as a matter of convenience, in connection with a three-color tube in which the phosphor materials are deposited in the form of dots arranged to define a multiplicity of dot triads over the image screen. Each such triad will have a dot of green, a dot of blue and a dot of red phosphor as is well understood in the art.

Color selection in such a tube is accomplished by a color-selection electrode which has a pattern of apertures related to the pattern of dot triads on the screen so that if the gun mount of the tube develops and directs three electron beams through the color-selection electrode, each such beam is permitted to impact the phosphor deposits of only one of the three different phosphor materials. Again, this matter of color selection by the interposition of a color-selection electrode or shadow mask between the gun mount and the image screen is thoroughly understood in the art.

Tubes of this general nature are made in mass production and used quite successfully in the reception of television broadcasts. While providing acceptable image reproduction, they suffer from a brightness limitation imposed because the color-selection electrode has limited transmission for the electrons of the three electron beams. Indeed, the opaque portions of the shadow mask prevent all of the electrons from reaching the screen in achiev- 3,502,942 Patented Mar. 24, 1970 ing color selection but at a sacrifice in brightness. Many attempts have been made to improve color tubes in this regard by the introduction of additional electrodes "between the gun mount and screen to accomplish what is known as post-deflection-focusing. This is a technique of focusing the electron beams subsequent to scanning deflection in order to increase the number of beam electrons admitted by the color-selection electrode to the screen. As originally proposed, post-deflection-focus tubes required undesirably high focus voltages and suffered from adverse effects of secondary electrons which were generated within the focus electrode system and were able to make their way to the screen, in some structures even being accelerated to the screen. A much more attractive approach to post-deflection-focus is the subject of the above-identified Kaplan patent. It discloses a unitary structure serving both as the color-selection electrode and as aunipotential lens. Structurally, it comprises a metallic shadow mask both faces of which have a coating of insulation which, in turn, is provided with a coating of conductive material. This presents, in effect, three lens electrodes having apertures of essentially the same dimension and in complete alignment so that color selection is obtained in the usual way with an apertured color-selection electrode. If the outer pair of electrodes connect with the conductive wall coating of the tube envelope, while the inner electrode is established at a lower potential, this structure may also focusthe electron beams to enhance brightness by causing more electrons of the beams to impinge on the appropriate areas of the screen than is otherwise possible without post-defiection focusing. This arrangement is attractive both from the standpoint of the simplicity of its structure and the reduction in focus voltage requirement made possible by the unitary structure. Moreover, field-free spaces are established both in front of and after the focus-electrode assembly which is desirable, all as described in that patent.

The present invention is an extension of the concept of that patent, differing principally therefrom in a further simplification of the color-selection and focus-electrode assembly.

The color-selection and focus-electrode structure of the subject invention is also a further development of the counterpart structure of the copending Christensen application. The Christensen structure is like that of the Kaplan patent, featuring lens electrodes which are supported in common with the color-selection electrode and spaced on opposite sides therefrom. These electrodes in the Christensen structure are formed of metallic conductive meshes and, together with the color-selection electrode, constitute a unipotential lens. An advantage of the Christensen structure is that focusing may be accomplished with less focus voltage than required in the use of the Kaplan structure but it presents both structural and electrical problems. For example, it is difficult to construct a unipotential lens having mesh electrodes supported in close space relation with respect to a central electrode and it is difiicult, in view of the electric fields present and their tendency to bow the mesh electrodes, to maintain the desired space relation of the lens electrodes with respect to one another. The tendency to bowing is described in the Christensen application. The present invention avoids these difficulties and provides a thoroughly practicable and effective unitary color-selection and focus-electrode assembly for a color picture tube.

It is an object of the present invention, therefore, to provide an improved unitary color-selection and focuselectrode assembly for a cathode-ray tube.

It is another specific object of the invention to provide a color-selection and focus-electrode assembly for a color cathode-ray tube which is a simplified mechanical structure.

Summary of the invention In general, a post-defiection-focus color cathode-ray tube has an envelope supporting a phosphor coated screen on one end and an electron gun mount at the opposite end for developing and directing at least one electron beam along a predetermined path and through a beam deflection region toward the screen. An improved unitary colorselection and focus-electrode assembly, constructed in accordance with the invention for use in such a tube, comprises a first lens electrode constituting the principal structural component of the assembly and disposed across the beam path between the screen and the deflection region in substantially parallel relation to and in close proximity with the screen. This first lens electrode has a pattern of apertures for effecting color selection. by confining electrons of the beam to preselected portions of the image screen. There is at least one other lens electrode which is formed of a mesh of electrical conductors having interstitial dimensions small compared with the aperture dimensions of the first electrode. The mesh electrode, where only a single one is employed, is disposed in spaced parallel relation to and on one side of the first electrode. There are means, comprising an insulator, for aflixing the conductors of the mesh electrode to the contiguous side or surface of the first lens electrode to complete the focus electrode assembly. Also, there are means for establishing a predetermined potential difference between the lens electrodes of that assembly for beam focusing purposes.

Brief description of the drawings The features of the present invention which are believed to be novel are set forth with particularity in the appended claims. The invention, together with further objects and advantages thereof, may best be understood by reference to the following description taken in connection with the accompanying drawing, in the several figures of which like reference numerals identify like elements, and in which:

FIGURE 1 is a schematic representation in cross section of a shadow mask type of color cathode-ray tube having a unitary color-selection and focus-electrode assembly in accordance with the invention;

FIGURE 2 is a fragmentary view on an enlarged scale of a portion of the focus-electrode assembly of the tube of FIGURE 1; and

FIGURE 3 is a similar view of a modification of electrode assembly.

Description of the preferred embodiment The cathode-ray tube of FIGURE 1 is of the shadow mask variety for the production of images in simulated natural color. Its envelope 21 has a conical section, the large end of which terminates in a phosphor coated screen 22. The specifics of the screen are of no particular consequence but generally the screen has deposits of green, blue and red phosphors and, as indicated above, it will be assumed that the screen is of the mosaic type having phosphor deposits 23 of dot shape arranged to define a field of dot triads. As is typical, the screen is backed by a layer 24 of conductive and light-reflecting material which usually is aluminum. Of course, the backing layer is sufficiently thin to be pervious to electrons, which is necessary in order to excite the phosphors 23 of screen 22.

The opposite end of conical section 21 has the usual reduced diameter neck which encloses an electron gun mount 25. While tri-color tubes may employ a single electron gun, it is customary in commercial practice to employ three electron guns in mount 25 for individually developiug an electron beam directed along a path corresponding generally to the longitudinal axis of the tube and through a deflection region toward screen 22. The three guns are similar to one another and are of well known construction. Since they constitute no part of the present invention, they have not been shown in detail. The deflection region referred to is enclosed by a conventional deflection yoke 28 having line and field scanning coils to be energized by appropriate scanning signals so that the three beams are deflected to scan screen 22 in a series of fields of parallel lines in a manner well understood in the art.

Obviously, the three beams must be converged properly and this is usually accomplished by orienting the guns to achieve mechanical convergence in the center of the raster while the converged condition is maintained by a convergence yoke 28a energized by appropriately shaped signals at both line and field frequencies to accomplish dynamic convergence which is also well understood in the television art. In order to obtain color selection, there is an assembly 30, to be described more particularly hereafter, which has a pattern of apertures related to the pattern of triads on screen 22 so that each beam issued from gun mount 25 is, in' effect, confined to impact only the deposits of an assigned color phosphor of the screen. Suffice it to say at this juncture that assembly 30 is a shadow mask functioning in a totally conventional manner for the purpose of color selection.

As thus far described, the color picture tube of FIG- URE 1 is a well-known structure and operates in a fashion well understood in the art in response to luminance and chrominance information which control the three beams developed in gun mount 25. The modulation of these beams by such information, concurrently with their defiection under the influence of scanning yoke 28, results in the reproduction of an image in simulated natural color, assuming that the desired condition of beam convergence at color-selection electrode 30-is maintained by convergence yoke 28a throughout the scanning process. Particular attention will now be given to the feature of postdeflection-focusing of the electron beams originating in gun mount 25 for the purpose of increased brightness.

Post-deflection-focus is achieved by a unitary colorselection and focus-electrode assembly 30. It comprises a first lens electrode 31 disposed across the path of the three beams between image screen 22 and the deflection region in substantially parallel relation to and in close proximity with the screen. For a 23 inch rectangular tube the mean spacing of electrode 31 from screen 22 is 0.6

inch. It is the customary color-selection electrode or.

shadow mask formed of a conductive plate having a pattern of apertures properly related to the pattern of dot triads on screen 22 as required for color selection by confining electrons of the beams to preassigned or preselected elemental areas of the screen. This may be an apertured aluminum plate or it may be a plate of aluminum coated steel and is the member that gives mechanical strength and serves as the principal structural component of electrode assembly 30. In order to accomplish post-deflection-focus, it is necessary that there be additional electrodes to constitute a known form of electron lens. The lens may have three electrodes, being of the unipotential type as described in the Kaplan patent or it may be a two electrode lens as described, for example, in Patent 2,728,024, issued Dec. 20, 1955 to E. G. Ramberg. While either type of lens may be constructed in the assembly under consideration, greatest simplicity is achieved with the use of a two-element lens and accordingly, the structure as represented has only one other lens element 32 formed of a mesh of electrical conductors having interstitial dimensions that are small compared with the aperture dimensions of the first electrode 31. For example, the mean diameter of the shadow mask apertures is 22 mils and if the mesh electrode has conductors per inch, its average effective interstitial diameter is 1 mil. Lens electrode 32 is disposed in spaced parallel relation to and on the side of lens electrode 31 which is closer to gun mount 25. Electrode 32 may be a woven, knitted, expanded metal or elcctro formed mesh, the latter being represented in the drawing.

In order to achieve a unitary assembly of those elements, including shadow mask 31 and wire mesh 32, which cooperate in effecting color selection as well as post-deflection focusing, there are means comprising an insulator for affixing the conductors of the mesh to the appropriate side of the shadow mask. Various ways may be employed to complete the focus-electrode assembly; two different approaches are illustrated in the fragmentary views of FIGURES 2 and 3. In FIGURE 2, the insulator between lens electrodes 31 and 32 is a coating 33 appli d to or formed on the shadow mask. Where the shadow mask is made of aluminum or aluminum coated steel, insulating layer 33 is conveniently formed by anodizing which develops a coating of aluminum oxide. It will be observed that insulating layer 33 formed in this way coats both faces of shadow mask 31 as well as the wall surfaces of apertures 31a of the shadow mask. The mesh electrode 32 may be secured to shadow mask 31 by means of any inorganic adhesive, such as potassium silicate, which is compatible with the environmental requirements of the tube, that is to say, an adhesive the presence of which does not adversely influence tube operation. Solder glass, which is a low temperature fusible glass powder, may also be used to attach lens element 32 to shadow mask 31. Sauerlisen cement type No. 1 marketed by Sauerlisen Company is also suitable for this purpose. With the mesh electrode simply secured to coating 33 by the application of such an adhesive, electrons of the beams issuing from gun mount 25 and scanned across the electrode assembly 30 by fields originating from yoke 28, pass through the interstices of the wire mesh and directly impinge upon insulating layer 33. Their impingement results in an accumulation of a charge of positive polarity on the insulator. This charge accumulation, for the mask configuration illustrated in FIGURE 2, takes place not only on the face of the shadow mask to which mesh 32 is secured but also on the tapered wall surfaces of the shadow mask apertures 31a facing gun mount 25. The end result of such a charge accumulation would be a total distortion of the electron beam which is avoided by arranging for the charge to drain away, obviating charge accumulation and its destructive influence on spot configuration. More particularly, insulating layer 33 is, in turn, coated with a layer 34 of semiconductive material, such as chrome oxide which may be applied by vapor deposition. In any event, the semiconductor layer is applied to insulating layer 33 after which mesh electrode 32 is secured to the surface of shadow mask 31 facing gun mount 25.

Having thus constructed the unitary color-selection and focus-electrode assembly 30 for inclusion within the tube, it is necessary to consider appropriate means for establishing a predetermined potential difference between its lens electrodes 31 and 32 to achieve focusing. Aside from the structural elements 32-34, the shadow mask may be conventional, being provided with a frame to which the apertured electrode 31 is secured.

The mounting of this assembly within the tube may also be conventional and it occurs while envelope 21 has its conical section separated from the faceplate section as required for establishing the phosphor deposits 23 on the screen. Usually, the faceplate section has internally extending studs and the frame of shadow mask 31 is provided with three or more leaf springs suitably apertured to receive the mounting studs of the faceplate in order to removably position the shadow mask in proper orientation with respect to screen 22. As stated, this is all conventional but some departure in conventional tube processing is required in order to satisfy the potential conditions to be established on lens electrodes 31 and 32 for focusing the electron beams.

Instead of having a single coating of conductive material on the internal surface of the tube envelope, extending from its conical section to and in circuit connection with conductive layer 24 of screen 23, the tube under consideration has two such coatings that are separated from one another. One coating 21a is on the conical section of the tube envelope and extends generally from the location of electrode assembly 30 toward gun mount 25. The high voltage contact 210 usually provided in the envelope of the tube receives a connection from a high voltage source (not shown) so that coating 21a is maintained at a first predetermined operating potential. This potential corresponds in magnitude with the final anode voltage desired for the several guns of mount 25 and is extended to those guns by snubber springs 26 projecting from the several electron guns into engagement with conductive coating 21a.

A second coating 21b,of conductive material is deposited in the internal surface of the faceplate section of the envelope and is, therefore, spaced from and out of circuit with coating 21a. It not only coats the flanged portion of the faceplate but also extends to the conductive backing layer 24 of the screen. A second high voltage terminal 21d may be provided in the flange of the faceplate section to be energized from a high voltage source (not shown) to the end that coating 21b is maintained at a predetermined operating potential which is different in magnitude from the potential of coating 2111. In the specific structure under consideration, the potential applied to coating 21b is less than that applied to coating 21a for reasons to be made clear hereafter.

The described coatings are convenient vehicles for establishing a desired focusing potential difference between lens electrodes 31 and 32 of assembly 30. Specifically, mesh electrode 32 is conductively connected with coating 21a to be maintained at the same potential as that coating. This may be accomplished by a connector 32a extending from mesh electrode 32 and contacting wall coating 21a. As indicated in FIGURE 1, insulating layer 33 is interposed between mesh electrode 32 and all portions of electrode 31 since they are required to be insulated from one another. Electrode 31, in turn, is conductively connected with coating 21b so as to be maintained at the same potential as that coating. This connection is shown schematically at 31a although as a practical matter it may be provided simply through the conductive mounting springs of the frame to which electrode 31 is connected and the mounting studs of faceplate 22 so long as the conductive coating 21b extends to and is maintained in electrical contact with such studs.

When the described unitary color-selection and focuselectrode assembly 30 is finally installed within the faceplate section of the tube, after that section has been provided with the conductive backed screen 23, 24, the faceplate section is united with the remainder of the envelope by frit sealing. Upon the completion of the usual processing steps of such a tube, the final product is a shadow mask tube conventional in all respects except for assembly 30 which provides not only color selection but also post-deflection focusing of the electron beams from gun mount 25. Preferably, the operating potential of mesh electrode 32 is higher than that of electrode 31, representative values being 25 and 24.8 kv.,

respectively. The potential difference established between these electrodes introduces a lens or focusing effect at the apertures of electrode assembly 30 to reduce the diameter of the scanning beams at screen 22 to about half that of the beam diameter as it approaches assembly 30 with a resulting brightness gain approaching four times. The electron optics of such a two-element lens does not of itself constitute the inventive contribution. Such a lens is known to the art as described, for example, in the above-identified Ramberg patent. Rather, the inventive contribution is in the structural arrangement of the unitary assembly 30 with its attendant advantages.

It will be observed, for example, that there is a fieldfree space in the region of the conical section of the tube envelope up to lens electrode 32 and there is a similar field-free space in the region from lens electrode 31 to screen 22. The first-mentioned field, however, is at a higher potential than the other which gives distinct benefits with respect to secondary emission. Should secondary emission be experienced as the scanning of the beams fromgun mount 25 takes place, the electrons liberated will be attracted to electrode 32 which is at the higher potential. This collection of secondary electrons prevents their making their way to screen 22 which, otherwise, deteriorates the reproduced image. Moreover, the field-free space from electrode 31 to the screen avoids introducing changes in the path of the electrons of the scanning beams which is otherwise occasioned where the screen is at a high potential relative to the final electrode of the post-deflection-focus lens.

The representative operating potentials make clear that the focus power is reduced over structures of the type described in the Kaplan patent. This results from the fact that the interstitial dimensions of mesh 32 are small compared with the aperture dimensions of mask electrode 31 so that the potentials of the focus field extend across the apertures of mask 31 rather than being confined near the edge of the mask apertures as occurs with the structure of the Kaplan patent. The focus voltage required is, of course, determined by the spacing of assembly 30 from screen 22, the potential of electrode 32, and the spacing of the mesh electrode from shadow mask 31. Their separation is largely determined by the thickness of insulating layer 33 which, in turn, is a function of the anodizing voltage employed in forming the aluminum oxide. By control of the anodizing process, the thickness of layer 34 may be made approximately 1 mil or perhaps a little less. In an illustrative example, the mean spacing of mask electrode 31 from screen 23 is 0.6 inch and the desired operating voltage for mesh electrode 32 is 25 kv. For these parameters an operating voltage of 24.8 kv. for electrode 31 achieves a very acceptable condition of focus and a brightness gain of approximately two and one-half times. Since this result is attained with a simplified structure utilizing electrode 31 in conjunction with a single mesh electrode 32, this preferred embodiment has improved electron transmission not only over the usual shadow mask structures but even over the post-deflection-focus structure of the copending Christensen application. The electron transmission of the ordinary shadow mask assembly is of the order of 17% and with the two mesh aperture type unipotential postdefiection-focus structure of the Christensen application, it is increased to approximately 35%, whereas the described structure with its simplicity over that of Christensen has an enhanced transmission of approximately 44%. It has improved electron transmission because it employs but a single mesh in a two-electrode system.

It is preferred that mesh electrode 32 be disposed on the gun side of shadow mask electrode 31 because of the aforedescribed advantage of collecting secondary electrons. Of course, the relative positions of these electrodes may be reversed and mesh electrode 32 may be positioned on the screen side of assembly 30. Where this arrangement is adopted, the operating potentials of the lens electrodes must be reversed in that the electrode closer to the screen would be at a higher potential than the electrode that is closer to the gun assembly. Actually, the same approximate values of the lens voltages may be used as described with the arrangement of FIG- URE 1.

The modification of FIGURE 3 avoids the necessity of applying an insulating layer over the surfaces of shadow mask electrode 31. Instead, a coating 33a is applied to the surface portions of the metal conductors of mesh 32 which are contiguous to shadow mask electrode 31. This may be accomplished by vapor deposition of silicon dioxide or silicon nitride which, being confined to the surface of the conductors of the mesh that face electrode 31, is shielded from direct impingement by the electrons of the scanning beams. This avoids charge accumulation and obviates the need for semiconductor layer 34 of the embodiment of FIGURE 2. Of course, mesh 32 is again secured to mesh 31 by a suitable adhesive or the application of a glass frit with subsequent fusing or in any other conventional manner. The approach illustrated in FIG- URE 3 also avoids the need for coating the wall surfaces of mask apertures 31a.

By arranging the assembly 30 as a unitary structure, essentially of a laminated type in which electrode 31 is the principal support to which insulator 33 and mesh electrode 32 are attached, the arrangement is greatly simplified both with respect to its application and with respect to manipulation of the mask in the processing of the tube. As is well known, in the usual screening of a tri-color cathode-ray tube, it is necessary to insert and remove the mask assembly several times which is much more convenient with the described simplified structure than with other post-deflection-focus structures of the prior art. Moreover, since the focus voltage requirement is so much less than that of the prior art, as represented by the Ramberg patent, for example, it is less diflicult to provide shadow mask electrode 31 with a surface insulation layer having adequate voltage standofi capability to support and to insulate mesh electrode 32 from mask electrode 31. While particular embodiments of the invention have been shown and described, it will be obvious to those skilled in the art that changes and modifications may be made without departing from the invention in its broader aspects, and, therefore, the aim in the appended claims is to cover all such changes and modifications as fall within the true spirit and scope of the invention.

We claim: 1. In a post-deflection-focus color cathode-ray tube having an envelope supporting a phosphor coated screen at one end and an electron gun mount at the opposite end for developing and directing at least one electron beam along a predetermined path and through a beam deflection region toward said screen, an improved unitary colorselection and focus-electrode assembly which comprises:

a first lens electrode, constituting the principal structural component of said assembly, disposed across said path between said screen and said deflection region in substantially parallel relation to and in close proximity with said screen and having a pattern of apertures for efiecting color section by confining electrons of said beam to preselected portions of said screen; at least one other lens electrode formed of a mesh of electrical conductors, having interstitial dimensions small compared with the aperture dimensions of said first lens electrode, disposed in spaced parallel relation to and one one side of said first electrode;

means, comprising an insulator in the form of a coating applied on said one side of said first lens electrode, for afiixing the conductors of said mesh to said one side of said first electrode to complete a focuselectrode assembly, the surface of said insulator closer to said gun mount having a semi-conductive layer to avoid charge accumulation in response to impingement by the electrons of said beam; and

means for establishing a predetermined potential difference between said lens electrodes of said assembly.

2. A unitary color-selection and focus-electrode assembly in accordance with claim 1 in which said insulator coats not only said one side of said first lens electrode but also the wall surfaces of said apertures in said first lens electrode; and

in which said semi-conductive layer extends over the portions of said insulator which cover said wall surfaces of said apertures.

3. In a post-deflection-focus color cathode-ray tube having an envelope supporting a phosphor coated screen at one end and an electron gun mount at the opposite end for developing and directing at least one electron beam along a predetermined path and through a beam deflection region toward said screen, an improved unitary color-selection and focus-electrode assembly Which comprises:

a first lens electrode, constituting the principal structural component of said assembly, disposed across said path between said screen and said deflection region in substantially parallel relation to and in close proximity with said screen and having a pattern of apertures for effecting color selection by confining electrons of said beam to preselected portions of said screen;

at least one other lens electrode formed of a mesh of electrical conductors, having interstitial dimensions small compared with the aperture dimensions of said first lens electrode, disposed in spaced parallel relation to and on one side of said first electrode;

means, comprising an insulator applied essentially only to the surface portions of said conductors that face said one side of said first lens electrode and are substantially shielded against direct impingement by the electrons of said beam, for affixing the conductors of said mesh to said one side of said first electrode tive material spaced from said first coating and extending away from said gun mount and maintained at a second predetermined operating potential which is dilferent from said first potential,

in which said focus assembly includes said first and said other but no additional electrodes, and

in which said means for establishing a potential difference between said lens electrodes comprises means for connecting each of said lens electrodes to a different one of said wall coatings.

5. A unitary color-selection and focus-electrode assembly in accordance with claim 4 in which said other lens electrode is on the side of said first lens electrode facing said gun mount and is connected to said first coating on said envelope; and

in which said first coating on said envelope is maintained at a higher operating potential than said second envelope coating.

to complete a focus-electrode assembly; and 20 References Cited means for establishing a predetermined potential differ- UNITED STATES PATENTS ence between said lens electrodes of said assembly. 4. A unitary color-selection and focus-electrode as- 3,421,048 1/1969 Chnstensen 315*31 sembly in accordance with claim 1 in which said envelope 3,398,309 8/1968 Kaplan has a first coating of conductive material on the internal 25 2,728,024 12/1955 Ramberg 315-17 surface thereof which extends from approximately the location of said electrode assembly toward said gun mount and is maintained at first predetermined operating potential,

RODNEY D. BENNETT, 111., Primary Examiner J. G. BAXTER, Assistant Examiner US. Cl. X.R.

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