US3894321A

US3894321A - Method for processing a color cathode ray tube having a thin foil mask sealed directly to the bulb

Info

Publication number: US3894321A
Application number: US436258A
Authority: US
Inventors: John H Moore
Original assignee: Zenith Radio Corp
Current assignee: Zenith Electronics LLC
Priority date: 1974-01-24
Filing date: 1974-01-24
Publication date: 1975-07-15
Anticipated expiration: 1992-07-15

Abstract

A method for processing color cathode ray tubes each having a color selection electrode consisting of a thin metal foil with a pattern of electron-transmissive apertures formed therein. The foil is stretched across the bulb of the tube and sealed directly thereto. According to this invention, such a tube may be exposed and screened without the necessity of resorting to a processing system which dictates the use of interchangeable masks.

Description

United States Patent [1 1 Moore July 15, 1975 METHOD FOR PROCESSING A COLOR CATHODE RAY TUBE HAVING A THIN FOIL MASK SEALED DIRECTLY TO THE BULB [75] Inventor: John H. Moore, Broadview, 111.

[73] Assignee: Zenith Radio Corporation, Chicago.

[22] Filed: Jan. 24, 1974 [21] Appl. No.: 436,258

[52] U.S. Cl. 29/25.l5; 29/25.13; 313/482; 313/402 [51] Int. C1. ..H01.l 9/18 [58] Field of Search 29/25.l3, 25.14, 25.15, 29/25.16; 316/17, 18, 19; 313/402, 403, 348,

[56] References Cited UNITED STATES PATENTS 2,842,696 7/1958 Fischer-Colbrie 29/25.15 X

3,600,778 8/1971 Martin 29/25.14

Primary E.\'aminerRoy Lake Assistant ExaminerJames W. Davie Attorney, Agent or Firm John H. Moore [57] ABSTRACT A method for processing color cathode ray tubes each having a color selection electrode consisting of a thin metal foil with a pattern of electron-transmissive apertures formed therein. The foil is stretched across the bulb of the tube and sealed directly thereto. Accord ing to this invention, such a tube may be exposed and screened without the necessity of resorting to a processing system which dictates the use of interchangeable masks.

] Claim, 15 Drawing Figures METHOD FOR PROCESSING A COLOR CATHODE RAY TUBE HAVING A THIN FOIL MASK SEALED DIRECTLY TO THE BULB CROSS REFERENCE TO RELATED APPLICATION This application is related to, but in no way dependent upon, a co-pending application Ser. No. 430,864 filed Jan. 4, 1974.

BACKGROUND OF THE INVENTION This invention is generally related to cathode ray tubes for use in color television receivers. It is specifically directed toward a method of producing a novel color cathode ray tube having an improved color selection electrode. Such a tube and its color selection electrode are disclosed and claimed iri the above mentioned co-pending application. However, for the sake of completeness and clarity, a brief background of that tube and its color selection electrode will be presented. In that way, the invention which is the subject of this application will be placed in a more understandable context. In addition, the Description of the Preferred Embodiment which follows will include a full disclosure of both this invention and that of the co-pending application in order to more readily point out their relationship.

During the past or years, color television has evolved from a laboratory novelty to an item of major commercial importance. This successful evolution is due in large part to advancement in the technology associated with the production of a color CRT (cathode ray tube), the most bulky and expensive component of a television receiver.

Over the years, a variety of color tubes have been designed to reproduce a colored image. However, each such tube has certain elements which are common to all other color tubes; namely, (1) one or more electron guns for generating a number of electron beams which are intensity modulated to correspond to the red, blue and green components of a received television signal, (2) a video display screen covered on one side with a periodic pattern of red-emissive, blue-emissive and green-emissive phosphor elements and, (3) in most tubes, a color selection electrode situated between the screen and the electron gun. The color selection electrode has a periodic array of apertures which permit a scanning electron beam to pass therethrough and impinge on only selected phosphor elements. A visual dis play is generated when selected phosphor elements are illuminated by a scanning electrode beam or beams in accordance with a received television signal.

Todays color CRT is vastly improved over its predecessor of only ten years ago, particularly with respect to recent improvements in the screen which greatly enhance the tubes brightness and contrast. The color selection electrode also has been greatly improved over the last few years. New manufacturing techniques have resulted in reducing the tolerances associated with the position of the electrode apertures. Improved forming techniques enable a manufacturer to produce such electrodes having very few non-uniformities. The use of so called graded color selection electrode has enabled color tubes to achieve a high degree of color purity, despite the degrouping effect at the outer edge of a CRT screen.

Despite the above mentioned improvements, particularly those in connection with the color selection electrode, a solution to a particularly troublesome electrode problem has, up until recently, remained unsolved. The nature of this problem depends on the type of electrode that is used in a particular tube. Therefore, to facilitate the explanation of this problem, all cathode ray tube color selection electrodes will be divided into two categories self-rigid and tensioned.

The typical self-rigid electrode is the popular spherically contoured shadow mask having a periodic pattern of dot or slot apertures. Its relatively large mass and its contour enable it to be self-rigid. It does, however, require a frame for attaching it to and supporting it within the CRT bulb.

An example of a tensioned electrode is the type formed from a series of vertically oriented, parallel spaced wires or narrow strips of metal. The wires are stretched across a mask frame which holds them under tension, thus forming a pattern of vertical slits through which an electron beam must pass to strike the screen.

The problem associated with both the self-rigid and the tensioned electrode is that both types require a frame for mounting them inside the bulb. This requirement leads to several important shortcomings. In the example given above of the tensioned electrode, a massive frame is required to withstand the force on it due to the tension in the electrode wires. Such a frame is not only heavy, but very expensive. Its use in color tubes with large screens is particularly disadvantageous because of the cost burden. Its use in small portable receivers is disadvantageous, not only because of its cost, but also because of the weight penalty.

In the case of the spherical self-rigid shadow mask, a less massive frame can be used but that frame must somehow be thermally compensated to insure that, as the tube heats up, the expansion of the frame and mask will not result in a loss of registration between the mask apertures and the phosphor elements on the screen. However, since frames can generally be only partially compensated for thermal expansion, there will be some non-compensating frame movement, particularly at the corners. This is why color purity is difficult to maintain in the corners of color picture tubes.

Another problem associated with the spherical masks is that it is difficult to etch accurately positioned apertures into it because of the mask thickness, generally about 6 mils.

A consideration of these problems points out the need for a drastic improvement in CRT color selection electrodes to eliminate the need for massive frames and, in some cases, the need for temperature compensation of the frames. Indeed, an ideal electrode might completely eliminate the need for any frame as a means of its support. Such an improved electrode must, of course, be capable of having its apertures etched at least as accurately as present electrodes. In addition any such improved electrode should be capable of having any desirable form of aperture such as slit, slot, dot or whatever. Such an improved color selection electrode, fully capable of satisfying the above mentioned deficiencies in prior art electrodes, is described in said co-pending application and below. One particularly desireable feature of it (as explained in the discussion to follow) is that it can make possible the manufacture of color CRTs having interchangeable color selection electrodes, each such electrode formed from a master and usable with any given CRT having a pattern of phosphor elements also formed from a corresponding master. Definite economic advantages can result from the use of interchangeable masks. In fact, a color selection electrode as described in said co-pending application was thought to be applicable only to an interchangeable mask system. But since most CRT manufacturers are geared to produce CRTs having noninterchangeable masks, it might be too much to expect a manufacturer to convert his factory to the production of CRTs having both an interchangeable mask system and color selection electrodes in accordance with said co-pending application. Attempting to incorporate two such drastic changes at once into a factory production line might be undesirable.

Accordingly, this invention is directed toward a method of processing the novel CRTs of said copending application so as to permit their use in a noninterchangeable mask system.

PRIOR ART US. Pat. Nos. 2,761,990, 2,842,696, 2,905,845, 2,916,644, 2,922,063, 3,489,966, 3,638,063, and 3,719,848 and Japanese Pat. No. 46-23941.

BRIEF DESCRIPTION OF THE DRAWINGS The features of the invention which are believed to be novel are set forth with particularly 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 conjunction with the accompanying drawings in which the renderings are highly schematic with certain dimensions exaggerated for clarity of illustration, and in which:

FIG. 1 is an exploded view of a color CRT having a thin foil mask;

FIG. 1A is a sectional view of the FIG. 1 CRT assembled;

FIG. 1B depicts another thin foil mask for use in a color CRT;

FIG. 1C is a sectional view of a CRT illustrating a method of and structure for aligning a thin foil mask with a CRT front panel;

FIG. 2 is a sectional view ofa CRT illustrating a novel front panel to which a thin foil mask is sealed;

FIGS. 2A and 2B depict alternate embodiments of the FIG. 2 front panel;

FIGS. 3, 4 and 5 are sectional views of color CRTs illustrating various means for attaching a thin foil mask to a CRT bulb;

FIG. 6 depicts a color CRT in which a thin foil mask may be usefully incorporated;

FIGS. 7 and 7A depict a thin foil mask for use in a post-deflection type CRT;

FIG. 8 is a partially exploded view of a lighthouse, a CRT mask and a CRT front panel useful in explaining a method of processing a CRT panel in accordance with this invention; and

FIG. 9 depicts a color CRT and an exposure light, useful in describing another method of processing color CRTs in accordance with this invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS As was pointed out above, this invention is directed toward a method for processing the novel color CRTs of said co-pending application so as to permit their use in a non-interchangeable mask system. Before proceeding with a detailed description of this invention, the

CRT structures of said copending application and a novel mask for use therewith will be described first. The mask which is the subject of this discussion takes the form ofa thin (0.5 mils) foil having a periodic array of openings through which an electron beam can pass. The foil mask is sealed directly to the CRT bulb in a way which places the mask in a position where it is essentially parallel to the CRT screen and spaced therefrom by the appropriate Q spacing, normally approximately 0.65 inch for a 25 inch tube.

To ensure that the foil mask is held in position under a tension sufficient to render the mask surface smooth and tight, the foil should have a co-efficient of thermal expansion which is sufficiently greater than that of the bulb itself so that, after having been sealed to the bulb at an elevated temperature, the mask can contract and be placed under tension in the bulb so as to maintain the mask in a fixed spaced relationship to the CRT screen throughout a normal range of tube operating temperatures. The contraction of the mask will also put the bulb in compression and tend to strengthen it, thereby making it more resistant to fracture.

Other characteristics of the foil mask will be set forth or be apparent in the discussion which follows.

Referring now to FIG. 1, there is shown a CRT faceplate or front panel 10, a foil mask 12 and a glass funnel section 14. Mask 12 has an outer sealing area 16 around its perimeter and an inner apertured area 18 through which one or more electron beams may pass.

Front panel 10 includes a wrap-around skirt 20 which terminates in a seal land 22. The inner surface of panel 10 is covered with a pattern of red-emissive, greenemissive and blue-emissive phosphor elements (not shown). Funnel section 14 also terminates in a matching seal land (not shown in FIG. 1) which mates with the panel seal land 22 for sealing the panel and funnel together by means of a fritcement.

The way in which the tube is preferably assembled is shown in FIG. 1A. Mask 12 is sealed directly to the bulb with its outer sealing area 16 sealed between the seal lands of front panel 10 and funnel 14. The foil mask may be large enough to extend beyond the panel joint as shown in the figure and may be cut away after the panel and funnel have been frit sealed together. The skirt 20 of front panel 10 maintains mask 12 at a predetermined Q distance from the phosphor coated screen 24 on the inner surface of front panel 10. This Q distance has been exaggerated in FIGS. 1 and 1A and in the figures to follow for purposes of clarity.

Although the masks and panels shown in FIGS. 1 and 1A and in the remaining figures are shown as being flat, no such limitation exists. Their structure is equally applicable to tubes with cylindrical panels and masks. Even so called flat panels do have some curvature in them which, for the sake of simplicity, has not been shown in these drawings. Because of the degrouping effect, both the flat and cylindrical panels are somewhat toroidal. This causes the screen to mask separation (Q distance) to decrease at the edges of the panel. Once again, for simplicity, the Q distance in these drawings has been shown as constant from mask center to the edges.

The preferred method of sealing the mask to the bulb as shown in FIG. 1A is to first heat the bulb and the foil to approximately 425C; the foil, stretched on a temporary frame, is placed over the bulb and sealed thereto by means of a devitrifying frit cement with the foil held firmly in place. The faceplate and funnel section are brought together in mating alignment, clamped together and then cooled. The mask is then placed under tension as a result of differential contraction between the mask and the bulb as the cooling proceeds.

In most cases the bulb and mask will cool down by about 300C. In view of this large cool-down range, the mask should have a material composition and a thickness which is such that the resulting mask tension maintains the mask in a fixed spaced relationship to the screen even under relatively high electron beam operating current levels without breaking the seal between the mask and the bulb. 1 1

In warming the foil and bulb as described above, their temperature should be raised slowly, preferably at a rate of from approximately 3 to 5 per minute, until the desired temperature is reached. With the front panel clamped firmly to the funnel section, the baking temperature of 425C should be maintained for approximately 1 hour, after which the temperature can be lowered slowly. A rate of temperature decrease of from 3 to 5C per minute has been found to be acceptable.

A material which was found to be satisfactory for use in a foil mask is cold-rolled steel having a coefficient of thermal expansion of approximately 116 X inches per inch per C. This was used with a picture glass having a coefficient of thermal expansion of approximately 95 X 10 inches per inch per C. Another material which may be used is nonmagnetic stainless steel.

For best results, however, one should avoid using materials such as copper which are relatively non-resiliant. Once stretched,'such materials may not spring back and retain their tension as will cold rolled or stainless steel. 2

Another important aspect of mask construction to consider is the thickness of the mask. In general the mask should be as thin as is practical to avoid putting undue stress on the seal land where it is attached to the bulb. A mask thickness of from 0.5 to 3 mils-has been found to be satisfactory for masks made of cold rolled steel. For materials having higher coefficients of thermal expansion, thinner masks should be used to reduce the stress on the bulb-to-mask joint.

One important benefit of using thin foil masks is that apertures can be etched therein in a much shorter time than that required for the etching of standard prior art masks. The longer the masks must be exposed to etchant, the less uniform their apertures .will be. By using a foil mask of from 0.5 to 3 mils thick, the reduced etching time required permits more accurate etching of the apertures and thus more standard masks. As a result of the greater accuracy and uniformity of mask apertures thus made possible, it becomes much easier to implement an interchangeable mask system wherein each CRT screen is exposed through amaster mask and each mask is also formed from a corresponding master. The pattern of apertures formed in the masks matches the pattern of phosphor elements screened onto the CRT screens so that any foil mask can be reliably mated with any screen. The details of a related interchangeable mask system are fully disclosed in US. Pat.

7 No. 3,676,914, issued July 18, 1972, and assigned to the assignee of this invention.

Another important advantage of this invention is that, with the foil mask firmly sealed to the glass bulb under tension, it cannot expand as it heats up under normal CRT beam current levels. As a result of the mask being unable to move, a tube having such a mask will retain its color puritywithout the need for any thermal compensation for the mask.

It should be pointed out that a thin steel foil mask as described, because of this thinness, deliberately does not introduce enough stress in the frit seal area tocrack or otherwise injure the seal. This is a; distinct advantage over some masks systems which embed relatively large diameter wires in the frit seal and which must,. therefore, take precautions to safeguard the integrity of the seal.

Referring now to FIG. 1B, there is shown another foil mask 15 which has an improved outer sealing area 16. In this mask, the sealing area 16 is perforated with a plurality of openings 25 through which the frit sealing material can flow, thereby facilitating an even distribution of frit cement and assisting in the formation of a strong bond between the bulbv and the outer sealing area of the mask. 1

FIG. 1B also. illustrates another advantage of using a thin foil mask. Note the unperforated area 26 which lies between the outer sealing area 16 and the inner apertured area 18 of the mask. This unperforated area 26 acts as an electron shield and eliminates the need for a separate electron shield, as customarily-found in prior art cathode ray tubes. I The mask shown in FIG. 1B may also have two or more alignment holes 19 located near the corners of the mask. These holes may mate with alignment nipples 21 in a panel to facilitate the accurte placement of a mask on a panel. See FIG. 1C. Nipples 21 pass through alignment holes 19 to fit intorecesses in the funnel.

Referring now to FIG. 2 there is shown another embodiment wherein the cathode ray tube frontpanelhas been modified by the addition of an inner ledge 28 near the periphery of thefaceplate section. The ledge has a top surface which is separated from the front panel screen 24 by the predetermined Q distance, with the foil mask 12 sealed to the top surface of the ledge. An electron gun 17 is positioned in the neck area of funnel 14. One advantage of this embodiment is that the front panel may have a skirt 20 of a standard size, that is, 2 to 3 inches long, rather than the very short skirt of the FIG. 1A embodiment (approximately 0.65 inches for a 25 inch tube). The use of the longer skirt shown in this embodiment may tend to provide a stronger bulb.

Another advantage which the FIG. 2 embodiment provides is that, with the mask 12 sealed to ledge 28 rather than in between the seal lands of the funnel and front panel, the funnel and front panel can be sealed together in their usual way, without the risk of weakening the seal by the addition of the mask. The constraints of the mask-to-ledge seal are then somewhat relaxed since that particular seal need not be air-tight, but need only be strong enough to support the mask 12 under tension. Furthermore, with the mask sealed within the bulb rather than at the outer edge of the bulb as in FIG. 1A, no electrical insulation need be placed around the panel-to-funnel joint to guard against the high voltage (25,000 volts, e.g.) applied to the mask,

In front panels with inner ledges, as exemplified by the FIG. 2 panel, the ledges may take on a variety of shapes, depending on the type of ledge used and its position within the front panel. FIGS. 2A and 2B are front views illustrating two examples of the type of ledges which may be used in the FIG. 2 type embodiments. In FIG. 2A the ledge 29 forms a continuous path around the panel. Such a ledge might be used where it is desireable to seal the entire periphery of the foil mask to a ledge, as in a dot-type mask where it is important that the mask be not allowed to shift in any direction, By sealing the entire periphery of the mask to a ledge, the mask remains under tension in every direction and is thus effectively prevented from shifting. I

FIG. 2B shows an alternate approach in which the panel 10 has two parallel ledges 31 to which opposite ends of a foil mask may be sealed. The use of this type of front panel may be appropriate where mask tensing is required on only one direction, for example, in the case of a slit or slot type mask. In that case, opposite ends of the mask would be sealed to ledges 31 with the slits or slots oriented in a direction perpendicular to the direction of the ledges 31.

An embodiment which permits the use of a standard front panel 10 is shown in cross section in FIG. 3. In this case, front panel 10 has a standard 2 to 3 inch skirt 20 with no modifications. The funnel section 14 has an inner ledge 28A near its periphery for supporting the mask as shown. The top surface of ledge 28A to which the mask is sealed is separated from the screen 24 by the pre-determined Q distance when the funnel and faceplate sections are sealed together. In this embodiment, ledge 28A may form a continuous path around the inner periphery of the funnel section similar to ledge 29 shown in FIG. 2A or it may be more similar to the FIG. 2B embodiment wherein there are two parallel ledges to which opposite ends of a foil mask may be sealed.

FIGS. 4 and 5 illustrate embodiments wherein the front panel is skirtless and substantially flat. In FIG. 4, the inner ledge 28B is located on the funnel section and can be similar to ledge 28A of the FIG. 3 embodiment. In FIG. 5 the front panel 10 has an inner ledge 28C near its periphery with the top surface of the ledge separated from the screen by the predetermined Q distance. 7

Although the figures above have shown a foil mask having a series of vertical slits which extend for substantially the entire vertical dimension of the mask, a foil mask as described herein and in said co-pending application is not limited to such an embodiment. Such a foil mask may have electron apertures consisting of vertical slits, vertical slots, small round holes, or any other suitable aperture shape. For example, see FIG. 6 which illustrates a color CRT whose novel mask 12 has a patternof canted rectangular apertures. The CRT screen 24 has an array of phosphor elements which are canted to correspond to the mask apertures. The mask and a cathode ray tube incorporating such a mask are described and claimed in another co-pending application Ser. No. 371,901, filed June 20, 1973. As described therein, mask 12 has a plurality of rows of elongated, equally spaced, electron transmissive apertures. Each aperture is canted relative to the direction of electron beam scan across the mask. Arrow 30 indicates the direction of electron beam scan. The major axes of the apertures in each row are parallel to one an-' other and of opposite polarity to the angle of canting of the aperture axes in adjoining rows. A mask having This disclosure has, up until now, depicted the thin foil mask in use in shadow mask type tubes. It is, of course, not limited to such applications, but is equally useful in television systems using PDF (post-deflection focus) masks. It is well known that higher mask transmissivity and higher picture brightness can be obtained if a mask is operated in the focus mode, that is, with the mask potential lower than the screen potential so that the electron beam is focused down, thus permitting larger mask apertures to be used. Physically, a thin foil mask which could be used in a PDF tube would be similar to the mask shown in FIG. 18 except that the size of the apertures would be greater.

This invention is also applicable to the high transmission type mask described in US. Pat. No. 3,452,242, issued to Miyaoka. Briefly, the mask described therein is a focus-type mask having a series of vertically oriented metal wires between which an electron beam is focused. The odd numbered wires may have an electrical potential somewhat greater than the screen potential while the even numbered wires may have an electrical potential somewhat less than the screen potential. The average mask potential is then equal to the screen potential. Refer now to FIG. 7 for an illustration of how a thin foil mask for such a system may be formed. As shown, the foil mask 32 lays over and extends beyond the ledges 27 of front panel 10. The excess foil which lies beyond the ledges 27 may then be cut away along dashed

lines

34 and 36, thus leaving a set of tabs on each side of the panel. See FIG. 7A which depicts the mask after the excess foil has been cut away. What is left are two electrically isolated arrays of electrically conductive strips intercalcated in a repetitive spaced sequence. A first electrical potential 39 is applied to the CRT screen 41 and second and third

electrical potentials

35 and 37 are applied to the two sets of strips at their tabs 33. The second and third electrical potentials are such as to cause electron beams having different angular orientations relative to the mask to impinge on different phosphor elements on the screen.

The idea of sealing a thin foil mask directly to the CRT envelope is also adaptable to present high brightness negative guardband color CRTs such as the CI-IROMACOLOR (trademark) tube manufactured by the assignee of this invention. In such a tube each phosphor element on the screen isseparated from adjacent phosphor elements by a light absorbing material deposited on the screen between the phosphor elements. Each of the mask apertures has a geometry which is such that its width in any direction in which misregistration between a beam landing and an impinged phosphor element can cause color errors is at least as great as the width of the phosphor elements which it exposes to the electron beam, and is wide enough to cause the electron beam landing to be wider than the phosphor element which it illuminates. This negative guardband principle, in combination with the conceptof including a light absorbing material between adjacent phosphor elements, is fully disclosed and claimed in U. S. Pat. No. 3,146,368, issued to Fiore et al and assigned to the assignee of this invention.

a pattern of apertures such as this is structurally strong of the electron beam through the mask.

In the discussion above, this disclosure has described a new and improved mask system for cathode ray tubes and has pointed out how its use could result in an improved cathode ray tube having an interchangeable mask system. The use of a thin foil mask makes interchangeability of masks more practical because mask 9 apertures can be etched in a' foil mask more uniformly and accurately than in .the relatively thick masks now inusef As'pointed out above, however, the thin foil mask and its corresponding CRT structures are not a part of 5 this invention. They have merely been described in order to place this invention in its proper context. The invention which is the subject of this application will now be addressed.

Despite the economic desirability of having interchangeable masks, one may desire to incorporate the above described thin foil masks into a CRT mask system which is not interchangeable. This would permit a manufacturer to gradually phase in the thin foil mask without jumping immediately to the use of both a thin foil mask which is sealed directly to the bulb envelope and an interchangeable mask system. Attempting both steps at once might unduly upset manufacturing processes and tooling requirements. This discussion to follow will be directed toward showing how, in accordance with this invention, the thin foil mask can be successfully incorporated into a noninterchangeable mask system wherein each CRT screen is exposed through its own individual mask and that mask is individually mated to its respective screen.

A problem with using a non-rigid mask in a noninterchangeable mask system is the difficulty, after having exposed the CRT screen through the mask and having then developed the screen, of re-aligning the panel with the mask accurately enough to maintain registration between the mask apertures and the associated phosphor elements. In the case where panels similar to the one shown in FIG. 5 are used, apparatus similar to that shown in FIG. 8 may be used in accordance with this invention, to expose the screen through the mask and re-align the panel with a mask as required.

The FIG. 8 apparatus is an exposure table or lighthouse. Its components will be first identified before proceeding to an explanation of the way in which it may be used.

The lighthouse 34 includes an outer housing 36 enclosing a light source 38 for generating exposure light. A frame 40, removable from housing 36, supports a mask 12 during screen exposure and includes alignment posts 42 which mate with alignment holes 41 in the mask. Three frame alignment posts 48, attached to housing 36, terminate in rounded heads 49 which support frame 40 in an aligned position. A pair of flat bars 54 terminate in end pieces 55 which are spring biased to permit bars 54 to be depressed under a weight. Clamping bars 46 have holes 44 which mate with alignment posts 42 on frame 40 and screws 47 which mate with holes in frame 40.

The use of lighthouse 34 is as follows: frame 40 is positioned on lighthouse 34 flush against rounded heads 49 of frame alignment posts 48. The lighthousse 34 is tilted, as shown, so that frame 40 will be held in posi tion by gravity. Mask 12 is then stretched across frame 40 and aligned with its alignment holes 41 mating with alignment posts 42. The mask is then clamped in place by screwing down clamping bars 46. The mask may be either mechanically stretched across frame 40 or it may be first heated and then placed in position on frame 40 so that, upon cooling, it will contract and be under tenslon.

With the mask now tensioned and held in accurate position on lighthouse 34, panel 50 is aligned on the lighthouse such that its outer edges are flush against panel alignment posts 58. This will place the mask sealing ledges 52 directly over bars 54. Under the weight of p anel'50, ba rs 54 will descend until the edges of panel 50 come to rest on shelves 56 of panel alignment posts 58. Thus positioned with its sealing ledges 52 facing downwardly and resting on mask 12, the screen of panel 50 is at its proper Q distance from the mask. The panel screen can now be exposed through the mask apertures in order to harden elemental areas of a photoresist coating (not shown) on the panel screen. The panel may then be removed from the lighthouse and developed by rinsing the screen with a solvent to remove unhardened phosphor areas.

The panel may then be repositioned upon the lighthouse in the manner described above with the panel resting on support ledges 56 and with its edges 60 flush against alignment post 58. This will place the panel and the mask in relative positions substantially identical to the relative positions they occupied during the previous exposure. The exposure and develop procedures may then be repeated until the panel has been completely screened.

All that then remains is to seal the panel to the mask and to a mating funnel to complete the processing of the front panel. To that end, a frit cement may be placed on the sealing ledges 52 of the panel. The panel is then repositioned on lighthouse 34 so that the relative positions of the panel and the mask are again substantially identical to their relative positions during the previous exposures.

Now, with the panel and mask in mating alignment on frame 40, the entire frame, mask and panel may be removed from lighthouse 34 and placed in an oven to bake the panel and mask in their aligned positions in order to cure the frit cement. After the cure is complet e, that part of mask 12 which extends beyond the edges of the panel may be cut away to free the mask from the frame.

Although this invention has been described in connection with a particular lighthouse, it is intended that this method of producing a color CRT with a foil mask not be limited to any particular apparatus. Various panel and funnel designs may call for obvious variations in the design of suitable lighthouse apparatus.

In the case where funnels such as those depicted in FIGS. 3 and 4 are used, wherein the mask is sealed directly to the funnel rather than to the panel, a noninterchangeable mask system using tensioned foil masks may be implemented in the following manner. See FIG. 9. A mask 12 is first sealed to funnel sealing ledges 62. A source of exposure lighting 64 is inserted through the neck of funnel 66. A panel 68, having a photoresist coating over its screen, is placed in mating alignment with funnel 66. Elemental areas of the photoresist coating are then exposed through mask 12. The panel may then be removed from the funnel and developed by rinsing it with a solvent to remove unhardened photoresist particles. This process is repeated for each of the red, blue and green phosphors.

Finally, a frit cement is placed at the joint between the panel and the funnel and panel 68 is positioned in mating alignment with its funnel 66 such that the relative positions of the panel and the mask are substantially identical to their relative positions during the exposure of the screen. The entire tube is then baked until the frit has been cured, thus completing the processing of the front panel and mask.

This disclosure has pointed out how, in accordance with the objectives set forth above, an improved CRT having a thin foil mask may be produced. Such a CRT has been shown to be adaptable to either an interchangeable or non-interchangeable mask system. While specific embodiments and means for practicing the invention have been described, it is evident that many alterations, modifications and variations will be apparent to those skilled in the art. Accordingly, it is intended to embrace all such alterations, modifications and variations which fall within the spirit and scope of the invention as defined by the appended claim.

I claim:

1. A method of producing a color cathode ray tube (CRT) having a funnel section, a thin foil color selection electrode having a pattern of apertures formed therein, and a front panel which has a screen on which phosphor elements are deposited and which has one or more electrode sealing ledges to which said foil electrode is attached for supporting said electrode in a position parallel to the screen and at a predetermined Q distance from it, said method comprising:

a. stretching said foil electrode across a supporting electrode frame; b. aligning the CRT panel, the frame and the electrode on an exposure table for exposing the panel screen to a source of actinic light directed thru the electrode apertures; g

c. positioning the CRT panelon the electrode such that its electrode sealing ledges are facing downwardly and rest upon said electrode, thus positioning the panel screen its proper Q distance fromlthe electrode; i' v d. exposing elemental areas of a first photoresist coating on the CRT screen through the electrode apertures;

e. removing the panel from the exposure table and developing the screen by rinsing it with a solvent to remove unhardened phosphor areas;

f. twice repeating steps (d) and (e) to expose and develop second and third photoresist coatings;

g. placing a first cement on the panels electrode sealing ledges;

h. repositioning the CRT panel on the electrode such that the relative positions of the panel and the electrode are substantially identical to their relative positions during exposure;

i. baking the foil electrode and panel while held in said relative positions to cure the frit cement; and

j. cutting away foil electrode material which extends beyond the edges of the CRT panel to free the.

mask from the frame.

Claims

1. A method of producing a color cathode ray tube (CRT) having a funnel section, a thin foil color selection electrode having a pattern of apertures formed therein, and a front panel which has a screen on which phosphor elements are deposited and which has one or more electrode sealing ledges to which said foil electrode is attached for supporting said electrode in a position parallel to the screen and at a predetermined ''''Q'''' distance from it, said method comprising: a. stretching said foil electrode across a supporting electrode frame; b. aligning the CRT panel, the frame and the electrode on an exposure table for exposing the panel screen to a source of actinic light directed thru the electrode apertures; c. positioning the CRT panel on the electrode such that its electrode sealing ledges are facing downwardly and rest upon said electrode, thus positioning the panel screen its proper Q distance from the electrode; d. exposing elemental areas of a first photoresist coating on the CRT screen through the electrode apertures; e. removing the panel from the exposure table and developing the screen by rinsing it with a solvent to remove unhardened phosphor areas; f. twice repeating steps (d) and (e) to expose and develop second and third photoresist coatings; g. placing a first cement on the panel''s electrode sealing ledges; h. repositioning the CRT panel on the electrode such that the relative positions of the panel and the electrode are substantially identical to their relative positions during exposure; i. baking the foil electrode and panel while held in said relative positions to cure the frit cement; and j. cutting away foil electrode material which extends beyond the edges of the CRT panel to free the mask from the frame.