US3788847A - Methods of manufacture of color picture tubes - Google Patents

Abstract

TWO OR MORE SELECTED PORTIONS OR ZONES OF EACH COLOR PATTERN OF THE MOSAIC SCREEN OF A SHADOW MASK COLOR PICTURE TUBE ARE SUBSTANTIALLY SEPARATELY PRINTED IN SEPARATE PHOTOGRAPHIC EXPOSURES USING DIFFERENT OPTICAL SYSTEMS IN THE LIGHTHOUSE, TO PROVIDE DIFFERENT CORRECTIONS FOR MISREGISTER IN DIFFERENT ZONES. THE DIFFERENT EXPOSURES INVOLVE FIRST ORDER COLOR CENTER PRINTING IN ONE ZONE AND SECOND ORDER COLOR CENTER PRINTING IN ANOTHER ZONE, WITH DIFFERENT LIGHT REFRACTING CORRECTION ELEMENTS IN THE TWO EXPOSURES, EACH ELEMENT BEING DESIGNED TO CORRECT FOR MISREGISTER IN THE RESPECTIVE ZONE.

Description

Jan. 29, 1974 I R ,H GODFREY 3,788,847

METHODS OF MANUFACTURE OF COLOR PICTURE TUBES Filed Jan. 14, 1972 I 5 Sheets-Sheet 1 '1974 R. H. GODFREY METHODS OF MANUFACTURE OF COLOR PICTURE TUBES Fil'ed Jan. 14, 1972 3 Sheets-Sheet 2 Fig. 3;

6' l o {1, "DISTANCE FROM CENTER OF PANEL-INCHES 4 mmmzEoEm MEEfiE Jan. 29, 1974 R GQDFREY 3,788,847

METHODS OF MANUFACTURE OF COLOR PICTURE TUBES Filed Jan. 14, L972 3 Sheets-Sheet :5

United States Patent 3,788,847 METHODS OF MANUFACTURE OF COLOR PICTURE TUBES Richard Hugh Godfrey, Lancaster, Pa., assignor to RCA Corporation Filed Jan. 14, 1972, Ser. No. 217,785

Int. Cl. G03c 5/00 US. Cl. 96-361 9 Claims ABSTRACT OF THE DISCLOSURE BACKGROUND OF THE INVENTION This invention relates to the manufacture of shadow mask type color picture tubes comprising a viewing faceplate on which is deposited a mosaic screen of systematically-arranged color phosphor elements, such as dots or lines, a multiapertured shadow mask mounted near the screen, and means for projecting a plurality of electron beams through the mask to the screen.

In a conventional dot-screen color tube, three electron beams in a triangular or delta array are projected from a delta gun through a mask having a hexagonal array of circular apertures to a screen comprising three arrays of circular color phosphor dots, with each array adapted to emit light of a diflerent one of the three primary colors, red, greenand blue, and with each mask aperture associated with 'a triangular group or triad of three different color dots. The screen may include a matrix layer of light absorbing material, such as graphite, having a multiplicity of holes in which the color phosphor dots are deposited, for improving the contrast of the screen in ambient light.

The phosphor dots of the screen of a dot-screen color tube are usually laid down intrios of three dots of diifer-' ent color-emitting phosphors, e.g., red, green and blue, by a direct photographic printing process wherein a photosensitive coating on the faceplate is exposed through the apertures ofthe mask to light from a small light source located at a predetermined position relative to the mask and screen, and the exposed coating is developed, as by washing off the unhardened unexposed portions'of the coating, leaving the desired pattern of exposed hardened dot portions of the coating, for one color. This process is repeated for each color, with the light source at a ditferent position for each color. The mask may be detachably mounted on the faceplate panel so that it can be easily removed and replaced in exactly'the same position for each exposure. In a non-matrix tube, phosphor powder may be mixed directly with the photosensitive material in the coating, or applied to the dot portions of the coating after the latter has been exposed, to produce the desired pattern of phosphor dots onthe screen. The screen of a matrix color tube may be made in the following manner, as described in Mayaud Pat. No. 3,558,310. The dot, portions of the photosensitive faceplate coating are exposed and hardened in three separate exposures, one for each color array, after which the unexposed portions are removed, and the resulting dot pattern is then overcoated with a light-absorbing coating of colloidal graphite in water which is then dried and processed chemically to remove the dot portions of the photosensitive coating and leave the faceplate coated with a graphite layer having the desired holes for the color phosphor dots. The three color dot arrays are then photographically printed on the screen in separate lighthouse exposures, as in a non-matrix tube, to produce the phosphor dots in and slightly overlapping the matrix holes.

In the operation of the tube after manufacture, the electron beams are subjected to forces such as scanning (i.e., horizontal and vertical deflection) and dynamic con vergence (to maintain convergence of the beams near the screen at various angles of deflection) which affect the electron beam paths (and hence, the landing points or spots of the beams on the screen) in ways that the screenprinting light rays are not affected. Thus, unless compensation is made for the diflerences between the beam paths and the light ray paths, serious misregister of the beam spots with the phosphor dots will result, i.e., the corresponding spot and dot centers will not coincide. V

Misregister of the type wherein a trio of beam spots is shifted as a unit radially outward from the center of the screen relative to the associated dot trio, caused by an axial shift of the deflection centers of the beams toward the screen with increasing angles of deflection, is termed radial misregister. Misregister of the type wherein the individual spots of a spot trio are all three moved outwardly from each other, caused primarily by dynamic convergence forces applied to the beams resulting in lateral shifts of the deflection centers, is termed degrouping misregister. Other types of misregister are produced by the astigmatic characteristics of beam deflecting yokes, the foreshortening effect of the curved screen, the beam path curvature produced by the ambient magnetic fields, and the azimuthally variable distortion of the panel-maskscreen system when the tube is evacuated.

' Radial misregister may be avoided by incorporating an axially-symmetric radial-correction light retracting ele-, ment or lens in the light paths from the light source to the photosensitive screen coating as taught by Epstein et a1. Pat. No. 2,817,276, dated Dec. 24, 1957. The effect ofthis radial lens is to move the effective location of the light source axially toward the screen so that at each angle to the central axis the ray of light appears to originate at a virtual source located at the axially-shifted or Ieffective center of deflection of the corresponding electron eam.

Epstein et al. Pat. 2,885,935, dated May 12, 1959,

teaches the use of an aspheric axially-asymmetric lens having a single line of symmetry in the S-plane which,

passes through the center of deflection of the beam in- 'volved and the central longitudinal axis of the tube, de-

In first order printing, the light printing ray and the electron beam portion for a through the same mask aperture.

Morrell and Godfrey Pat. 3,282,691 teaches the printing of color tube screens with the light source positioned substantially at a second order color center, preferably located in the same S-plane as the first order center but on the opposite side of the central axis and at a particular dot on the screen pass,

distance 25 from the axis. In this case, the light printing ray and the electron beam portion for a particular dot pass through adjacent mask apertures. This makes it possible not only to correct for most of the degrouping error by suitably adjusting the q-spacing at each deflection angle, but also to provide better correction for other causes of. misregister, such a fore-shortening and yoke astigmatism, by means of a suitably designed correction lens. v Herzfeld et a1. Pat. 3,476,025, dated Nov. 4, 1969, teaches the design and use of a completely asymmetric correction lens to provide acceptable compensation at each of a multiplicity of points distributed over the entirescreen area for all causes of misregister. However, even with thisimproved lens, it is not possible to obtain completely perfect register at every point on the screen, particularly in printing screens for wide-angle (e.g., 110) color tubes, due to the necessity for blending the required elemental slopes on the lens into a smooth continuous lens surface. Usually, in photoprinting the screen of a shadow mask tube, the photosensitive coating is deposited on the faceplate of the panel, the shadow mask is then mounted in the panel at a given distance q (which may be variable with radial distance from the center) from the faceplate, and the panel-mask assembly is placed on a lighthouse housing containing a small light source positioned at or near the center of deflection of the electron beam (for the particular color being printed) of the color tube in which the panel is to be used. This center of deflection is usually at the mid-plane of the deflection yoke, and is spaced a distance S along the S-axis from the central longitudinal axis of the electron gun structure, mask and screen. The distance S is determined by the electron gun, and is related to the other parameters by the formula where q is the spacing between the mask and the screen at the central axis, L is the distance between the deflection plane and the screen at the central axis, and a is the spacing between aperture centers on the mask, to produce equally-spaced beam spots (and phosphor dots) at the center of the screen. If a screen were printed under these conditions, with no correction lens, and incorporated in a color tube, which was then operated with normal scan and dynamic convergence applied, the beam spots would be registered (centered) with the phosphor dots in the central region, but badly misregistered at the edges.

'In Morrell Pat. 2,855,529, the degrouping portion of the misregister at the edges is reduced by printing the screen in a single exposure, with first order printing, with different S and q values, for the position of the light source and the mask-screen spacing respectively, designed to decrease the degrouping at the outer edges, produce exact register at an intermediate region, and introduce some grouping misregister at the center. For example, if the measured degrouping misregister at the edge of y, the change is S required for complete correction at the edge is The values of q at the center and the edge to print equally spaced dots (equal size triads) is determined from & q 3S! This printing method, sometimes called a compromise S and q method, reduces the degrouping misregister by v 4 one-half at the edge, introduces an equal amount 0 grouping of the spots relative to the dots in'the center, and eliminates degrouping misregister at a region midway between the center and the edge. A disadvantage of this method is that it leaves the total amount of degrouping misregister between center and edge the same.

SUMMARY OF THE INVENTION A pattern of elemental areas corresponding to each color array of the mosaic screen of a shadow mask color tube is photographically printed in at least two stages, involving two different exposures, using different optical systems in the lighthouse in the two exposures, and predominantly exposing only a particular zone of the screen coating in each exposure, in order to produce better correction for misregister errors in each zone. The different exposures involve first order color center printing in one zone, and second order color printing in another zone, with diflerent light retracting correction elements in the two exposures. Each element is designed to correct for misregister in the respective zone.

The invention may be used to print either matrix or non-matrix screens, dot or line screens, and/or screens involving other than three colors.

The difierent exposures may be made with different lighthouses or with a single lighthouse in which the light source, location, size, or shape, etc. can be changed.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a side elevation view, partly in longitudinal section of a shadow mask type color picture tube in which the mosaic phosphor screen is photographically printed in accordance with the present invention;

FIG. 2 is an enlarged fragmentary rear elevation view of the mask and screen of FIG. 1;

FIG. 3 is a plan view of the open end of the faceplate panel of FIG. 1 prior to screening;

FIG. 4 is a partially broken away side elevation view of a lighthouse on which the exposure steps of the invention may be practiced;

FIG. 5 is a graph showing the relative brightness across the light fields transmitted by two different light filters; and

FIGS. 6 and 7 are sketches used in explaining the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS multiapertured color-selection or shadow mask 15, in

spaced substantially parallel relation to the faceplate 11, is detachably mounted on the side wall 13 by conventional means 17. A dot-type mosaic color phosphor screen 19 is formed on the inner surface 11 of the faceplate 11. A conventional electron gun structure 20 is mounted in the neck 9 for generating and directing three electron beams 21 (the paths of which are shown in dashed lines) toward the mask 15. The tube is adapted to be used with conventional beam-deflecting beans, such as a magnetic yoke 23, to cause the three beams to scan the beams 21 in a raster over the mask '15 and screen 19, and conventional means 25 for applying dynamic convergence forces to the beams, in synchronism with the beam scanning forces, to cause the beams to converge near the screen at all deflection angles.

FIG. 2 shows the relation between the apertures 15a of the mask 15 and the color dots 27 off the phosphor screen 19. Each aperture a is associated with a triad of three dots 27, e.g. red, green and blue, as shown.

In the operation of the tube 1, at zero deflection, the three beams 21 pass through centers of deflection C in the plane of deflection PP, and converge near the screen 19. As the angle of deflection increases, the effective plane of deflection containing the effective centers of deflection C moves forward (toward the screen 19) to plane P which moves all of the beam spots on the screen radially outward (from the center of the screen). This would cause radial misregister if the dots 27 of the screen 19 were printed with each light source at the center C and without a radial correction lens. The three centers of defiection C also move outwardly, relative to the points C, as a result of dynamic convergence, which causes degrouping of the beam spots in each trio of spots associated with the same aperture 15a of the mask 15. Ideally, all of the beam spots should be exactly centered or registered with the corresponding dots.

The present invention relates to a method of forming a pattern of elemental areas corresponding to at least one color array of the mosaic color screen 19 of red, green and blue dots 27, on the inner surface 11' of the faceplate 11, by substantially separate exposures of two or more predetermined portions or zones of the screen surface (instead of the usual single exposure of the entire surface) using first order color center printing in one zone and second order color center printing in another zone, to obtain better correction for various forms of misregister, in each of the zones.

For example, in FIG. 3, which shows the open end of the rectangular faceplate panel 5 of FIG. 1 prior to screening and sealing to the funnel 7, the faceplate area is arbitrarily divided into two contiguous zones, bounded by circular arcs 28, namely, a middle zone 29 and an outer zone 31 constituting all of the area on each side of the middle zone 29. Preferably, the middle zone 29 extends from three-fourths to four-fifths of the radial distance from the center to the edge of the faceplate. The middle zone 29 may be exposed predominantly by projecting light from a small light source through a neutral density filter having a radially variable density such that substantially only the middle zone 29 is exposed; and the outer zone 31 may be exposed predominantly by projecting light from the same or a different size light source through a different filter having a density that varies in such manner that substantially only the outer zone 31 is exposed, as disclosed in a copending application of Harry R. Frey, Ser. No. 140,345, filed May 5, 1971, now U.S. Pat. 3,685,994 issued on Aug. 22., 1972, entitled Photographic Method for Printing a Screen Structure for a Cathode Ray Tube. In that application, the purpose of the two separate exposures was to facilitate printing relatively large edge dots through a mask having apertures graded from large diameter in the center to small diameter at the edge, without printing the dots too large in the center. The S-value of the light source was conventional, and the same for both exposures, in the Frey application. In the present invention, the methods of exposure are different for the two exposures, to obtain better misregister correction, in each of the zones.

The lighthouse 34, shown, for example, in FIG. 4, comprises a light box 35 and a panel support '36 held in position by bolts (not shown) with respect to one another on a base 37 which in turn is supported at a desired angle by lugs 38. The light box 35 is a cylindrical cup-shaped casting closed at one end by an integral end wall 39. The other end of the light box 35 is closed by a plate 41 which fits in a circular recess 43 in the light box '35. The plate 41 has a central hole therein through which a light pipe 45, referred to as a collimator in the tube-making art, in the form of a tapered glass rod, extends. The small end 47 of the collimator 45 extends slightly beyond the plate 41 and constitutes the small light source of the lighthouse. The

larger end 49 of the collimator 45 is held in position by a bracket 51 opposite an ultraviolet lamp 53. The S-value of the light source (end 47 of collimator 45) may be adjusted by moving either the faceplate panel 5 or the collimator 45 relative to the other. A light reflector 55 is positioned behind the lamp 53. A lens assembly 56 is mounted on a support ring 57 and stand-off spacers 58 by bolts 59. The support ring 57 is clamped in position between base 37 and the panel support 36. The lens assembly 56 preferably includes a correction lens 61 and a transparent filter support plate 63 held and spaced from each other by a separate ring 65, an upper clamp 67 and a lower clamp 69. The upper surface of the plate 63 has thereon a variable density light intensity correction filter 71. The filter 71 may be formed of very small preformed carbon particles in gelatin or other clear colorless binder, as disclosed in the Frey application. The filter has essentially a neutral gray transmittance varying only in the intensity of grayness.

In printing the phosphor screen 19 in the two zones 29 and 31 of FIG. 3, the faceplate surface 11' is coated with a photosensitive coating 72 and then successively exposed in the lighthouse 34 (or in two different lighthouses) using two different filters 71 in the two exposures. One filter 71 is designed to have a radially variable density producing a light field having a brightness such as that shown by curve 73 in FIG. 5, for predominantly exposing only the middle zone 29; and the other filter 71 is designed to produce a light field having a brightness such as that shown by the curve 75 in FIG. 5, for predominantly exposing only the outer zone 31. The total exposure at each radial distance is the sum of the two curves 73 and 75, as shown by the dashed curve 77. The two filters should be designed so that the curves 73 and 75 cross each other at or near the arcs 28 in FIG. 3. Preferably, the collimator tip 47 used for the outer zone exposure is larger than that used for the middle zone exposure, to facilitate producing a greater exposure at the outer edge where the mask apertures are smaller as in the Frey application.

In this example of the invention, the dot pattern for each color in the outer zone 31 is printed predominantly only in a first exposure with first order color center printing, that is, with the light source located in the lighthouse at the first order color center, which corresponds to the center of deflection of the electron beam associated with the particular color dot pattern being printed. On the other hand, the dot pattern for each color in the middle zone 29 is printed predominantly only in a second exposure (either before or after the first exposure) with the light source located at a second order color center, preferably the one located in the same S-plane as the center of deflection of the tube but on the opposite side of the central longitudinal axis of the gun and tube, at a distance 28 from that axis, as described in the Morrell and Godfrey patent referred to above- The relationships between beam paths and second order center light paths are shown in FIG. 3 of that patent. Each exposure should be made through a light refracting correction element or lens designed to produce the best possible corrections for misregister in the respective zone.

In printing the outer zone by first order printing, the compromise S and q method of Morrell Pat. 2,855,529 may be modified by choosing a mask contour (determined by the variation in q) in the outer zone and an S-value of the light source in the lighthouse such that degrouping misregister will be substantially completely eliminated at some arbitrary point in the outer zone, e.g., at the outer edge or corner, and designing a correction element to correct for all other misregister causes throughout that zone, in one exposure.

An example wherein full correction is made for degrouping at the corners of the screen, e.g., at point 79 in FIG. 3, will be described. FIG. 6 shows the geometry involved, with zero and maximum deflection beam paths 21 shown in dashed lines from the center of deflection C of one of the beams. Assume the following initial conditions: L =10.5" (at center), q =.500, p =l0.0", S =.200"', R =40.7" (screen radius) and a=.0287". If each color pattern of the screen were printed with the light source positioned at the center of deflection C of the beam (S S =.200"), dots in trios having a trio size (average distance of the centers of the three dots from the center of the trio) of .010" mils) would be formed, and these dots would register with the beam spots in the operation of the tube at the center of the screen. However, since the effective center of deflection moves forwardly and outwardly, to point C, as the beams are deflected to maximum deflection with dynamic convergence applied, the beams would be badly misregistered with the phosphor dots at the endge of the screen due to degrouping, axial misregister, etc.

Assume that the total degrouping error y, due to dynamic convergence, is .002" (2 mils) at the corners of the screen, e.g., at 55 deflection. In the exposure of the outer zone 31, instead of printing each color pattern of the screen with the light source at the center of deflection C of the beam involved, the spacing S between the light source and the central axis AA is increased (from S =.2O0") y to S'=.240". Thus, the light source is placed at point C" in plane P-P in FIG. 6, to provide full correction for degrouping at the corners. Also the mask contour at the corners is changed to make the distance L at 0=55 is determined from the tube geometry to be 15.01" approX.). Therefore, q=.596" and p'=l4.414", at the corners. Under these conditions, the mask will have a radii of curvature of 38.1" at the corners. The dots 27 printed at the corners in the outer zone exposure will have a dot trio size of 10 mils (except for foreshortening distortion) and should register substantially with the beam spots in the operation of the tube.

The method just described could be carried out in a lighthouse with no correction lens 61', to compensate only for the degrouping error due to dynamic convergence. However, for best results, the final screen should be printed in a lighthouse incorporating at least a radial correction lens, and preferably a continuous correction lens designed to compensate for all errors not corrected by the modified compromise S and q method just described. For example, a screen can be printed as described above, without a lens, placed in a color tube, which is operated to measure the residual misregister errors; and the results of these measurements can be used to design a continuous correction lens 61a by any known method. This correction lens'61a can then be used in printing screens in the outer zone exposure with the modified S and q values in accordance with the present invention.

The outer zone exposure is made through a variable density light filter 71a having a brightness variation such as curve 75 of FIG. 5, to limit the exposure substantially to the outer zone 31.

FIG. 7 shows the geometry for printing the inner zone 29 of each color dot pattern with second order printing. Two paths 21 of one beam at zero and an intermediate deflection angle 6 are shown in dashed lines, with centers of deflections C and C. The light source is positioned at point C", a second order color center on the opposite side of the central axis AA from the center of deflection or first order color center C, and at a distance 28 from that axis, as shown. The exposure is made through a variable density light filter 71b having a brightness variation such as curve 73 in FIG. 3, to limit the exposure substantially to the inner zone 29, and a light refracting correction element 6112 designed to correct for all misregister causes. This lens 61b may be designed and used in the manner described in Morrell and Godfrey Pat. 3,282,691. The value of q, at the center of the mask and screen is determined from the formula in as,

to produce equal size beam spot trios at the center (undeflected beams).

Two outstanding advantages of second order printing are: (1) most, if not all, of the correction for degrouping misregister can be accomplished merely by adjusting the mask-screen spacing q, since the spot trio and dot trio size can be changed in opposite directions by a given change in q; and (2) the shapes of the dot trios printed are similar to the shapes of the astigmatically-distorted beam spot trios which result when a conventional magnetic deflection yoke is used to scan the beam over the screen and dynamic convergence is applied thereto. As a result, second order printing produces better meshing of adjacent dot trios in the central portion of the screen. However, I have found that a second order printing has the disadvantage of being highly sensitive to both qchanges (from bogie) and changes in the aperture spacing a caused by unequal mask stretch during the mask forming operation. Both of these changes are greatest at the outer edge of the mask. On the other hand, q changes and mask stretch present very little problems in first order printing. For these reasons, I have used second order printing in the inner zone, to obtain better spot-dot register and good meshing, and first order printing in the outer zone, where second order printing has disadvantages. The inner zone also includes the critical minor axis where first order correction lenses cannot cope with the beam spot trio distortions brought about by yoke astigmatism, particularly in a dual-lens-source system.

Instead of limiting the second order printing to the inner zone 29, the entire screen may be printed with second order printing in one exposure without the filter 71a, with the outer zone printed with first order printing in a separate exposure with the filter 71b to provide a fill, thus distorting the finished phosphor dots in such a way that full beam spot landing on the dots is achieved in spite of the dot location errors produced by mask stretch and/or q errors in the outer zone.

Although the invention has been described in connection with a non-matrix dot screen color tube, it will be understood that the invention can also be used to print each color pattern of a matrix dot screen tube, or a linescreen tube of the shadow mask type, with or without opaque guard bands between the phosphor lines.

I claim:

1. In the manufacture of a shadow mask picture tube having a mosaic color phosphor screen including an array of discrete phosphor elements disposed on a support, all of which are adapted to emit light of a given color, a multi-apertured shadow mask, and means for projecting an electron beam through said mask to said screen; the method of laying down said array on said support, comprising the steps of:

(a) photographically printing on said support the pattern of said array of phosphor elements predominantly in a first zone of said array, said first zone including a middle portion of said array, including exposing said first zone from a first light source, positioned substantially at a second order color center for said array, and refracting the exposure light through a first continuous light refracting correction element; and

(b) photographically printing on said support the pattern of said array of phosphor elements predominantly in a second zone of said array said second zone including outer portions of said array including exposing said second zone through apertures of said shadow mask from a second light source, positioned substantially at the first order color center for said array, and retracting the exposure light through a second continuous light retracting correction element; the surface contours of said two correction elements being diiferent, each of said correction elements being designed to minimize misregister between the beam spots and the phosphor elements in the operation of said tube. 2. In the manufacture of a color picture tube having a mosaic color phosphor screen comprising a plurality of arrays of discrete phosphor elements, the elements of each array being adapted to emit light of a diiferent color, an apertured shadow mask spaced from said screen, and means for projecting a plurality of electron beams, one for each array, through the apertures of said mask and onto said screen; the method of laying down one of said arrays of elements on a screen support by a direct photographic process, said method comprising the steps of:

(a) applying to said screen support a coating comprising a photosensitive binder; (b) exposing at least a first zone of said coating through said mask apertures to light from a first relatively small light source positioned substantially at a second order color center for said one array and retracting the exposure light through a first continuous light retracting correction element interposed between said light source and said mask; exposing predominantly only a second zone of said coating through said mask said first zone including a middle portion of said coating; apertures to light from a second relatively small light source positioned near the first order color center for said one array and retracting the exposure light through a second continuous light refracting correction element interposed between said second light source and said mask said second zone including an outer portion of said coating; and ((1) then developing said exposed coating to produce a pattern of exposed portions of said coating corresponding to said one array of elements on said screen support; the surface contours of said two correction elements being different, each of said correction elements being designed to minimize misregister between the beam spots and the phosphor elements in the operation of said tube.

3. The method of claim 2, wherein the entire surface of said coating is exposed in step (b).

4. The method of claim 2, wherein predominantly only said first zone is exposed in step (b).

5. The method of claim 4, wherein said first zone is a middle portion of said coating, and said second zone is made up of the two outer portions of said coating on each side of said middle zone.

6. The method of claim 5, wherein the boundaries be tween said zones are located at three-fourths to four-fifths of the distance between the center and the outer edge of said screen support.

7. The method of claim 1, wherein said screen support has a longitudinal axis perpendicular thereto at its center, and said second order color center is the one that is on the opposite side of said axis from said first order color center.

8. The method of claim 2, wherein said mosaic screen comprises three hexagonal arrays of color phosphor dots, and said means is adapted to project three electron beams in a A-array onto said screen.

9. The method of claim 2, wherein said coating includes a color phosphor material, and said exposed portions of said coating constitutes said one array of elements.

References Cited UNITED STATES PATENTS 3,685,994 8/1972 Frey 9636.1 3,476,025 11/1969 Herzfeld 1 3,672,893 6/1972 Robinder et a1 9636.1 3,222,172 12/1965 Giuffrida 96-36.], 2,733,366 1/1956 Grimm et al. 96-36.1

NORMAN G. TO'R'CHIN, Primary Examiner -E. C. KIMLIN, Assistant Examiner US. Cl. X.R. 951

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