US3788848A

US3788848A - Methods of manufacture of color picture tubes

Info

Publication number: US3788848A
Application number: US00217884A
Authority: US
Inventors: A Morrell
Original assignee: RCA Corp
Current assignee: RCA Licensing Corp
Priority date: 1972-01-14
Filing date: 1972-01-14
Publication date: 1974-01-29
Anticipated expiration: 1991-01-29
Also published as: JPS4882768A; NL7217275A; DE2301555A1; FR2168032A1; BR7300206D0; CA982390A; IT973240B; FR2168032B1; GB1416752A; BE793998A; ES410394A1

Abstract

TWO OR MORE SELECTED PORTIONS OR ZONES OF EACH COLOR PATTERN OF THE MOSIAC SCREEN OF A SHADOW MASK COLOR PIC TURE TUBE ARE SUBSTANTIALLY SEPARATELY PRINTED IN SEPARATE PHOTOGRAPHIC EXPOSURES USING DIFFERENT LOCATIONS FOR THE LIGHT SOURCE IN THE LIGHTHOUSE, TO PROVIDE DIFFERENT CORRECTIONS FOR MISREGISTER IN DIFFERENT ZONES. THE DIFFERENT LOCATIONS MAY INVOLVE DIFFERENT LATERAL SPACING OF THE LIGHT SOURCE FROM THE CENTRAL AXIS OF THE ELECTRON GUN STRUCTURE, OR DIFFERENT DISTANCE BETWEEN THE LIGHT SOURCE AND THE MASK, OR BOTH.

Description

Jan. 29, 1974 M MORRELL 3,788,848

METHODS OF MANUFACTURE OF COLOR PICTURE TUBES Filed Jan. l4, 1972 2 Sheets-Sheet 1 Jan. 29, 1974 8 A. M. MORRELL 3,788,848

RELATIVE BRIGHTNESS OF LIGHT FIELD /O METHODS OF MANUFACTURE OF COLOR PICTURE TUBES Filed Jan. 14, L972 2 Sheets-Sheet 2 mbmoos OOQOO o 2 31.4 6 7 DISTANCE FRO M CENTER OF United States harem: O

3,788,848 METHODS OF MANUFACTURE OF COLOR PICTURE TUBES Albert Maxwell Morrell, Lancaster, Pa., assignor to RCA Corporation Filed Jan. 14, 1972, Ser. No. 217,884 Int. Cl. G03c 5/00 US. Cl. 9636.1 11 Claims ABSTRACT OF THE DISCLOSURE 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 locations for the light source in the lighthouse, to provide different corrections for misregister in different zones. The different locations may involve different lateral spacing of light source from the central axis of the electron gun structure, or different distance between the light Source and the mask, or both.

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 different one of the three primary colors, red, green and blue, and with each mask aperture associated with a triangular group 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 in trios of three dots of different 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 of the 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 olf 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 different 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 on the 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 a 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 3,788,848 Patented Jan. 29, 1974 colloidal graphite in water which is then dried and processed to chemically 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 convergence (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 screen-printing light rays are not affected. Thus, unless compensation is made for the differences between the beam paths and the light ray paths, serious misregister of the beam spots with the phosphor dots will result, i.e., the spot and dot centers will not coincide.

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 include astigmatic misregister, force-shortening misregister, and the effects of the earths magnetic field and distortion of the faceplate when the tube is evacuated.

Radial misregister may be avoided by incorporating an axially-symmetric radial-correction light retracting element 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 of this radial lens is to move the effective location of the light source axailly 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 effective center of deflection of the corresponding electron beam.

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 involved and the central longitudinal axis of the tube, desinged to correct for radial misregister and partially correct for degrouping misregister. This lens moves the effective location of the light source both axially (toward the screen) and outwardly from the central axis. The two Epstein et al. patents relate to screen printing with the light source positioned at a point corresponding to the center of deflection of the beam of the color being printed, called the first order color center. The distance between the center of deflection of a beam and the central longitudinal axis of the tube (in the S-plane) is called the S-value. In first order printing, the light printing ray and the electron beam portion for a particular dot on the screen pass through the same mask aperture.

Morrell et a1. Pat. 3,282,691 teaches the printing of color tube screens with the light source positioned substanitally at a second order color center, which may be located in the same S-plane as the first order center but on the opposite side of the central axis and at a 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 as fore-shortening and yoke astigmatism, by means of a suitably designed correction lens.

Herzfeld et al. 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 entire screen area for all causes of misregister. However, even with this improved 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 8;; is determined by the fromula 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 phos phor dots in the central region, but badly misregistered at the edges.

-In my 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, respectively, 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 is y, the change in S required for complete correction at the edge is where p is L-q, and L, p and q are measured along the beam path at the particular maximum deflection angle involved, and the new S value is The values of q at the center and the edge to print equally spaced dots (equal size trios) is determined from This printing method, sometimes called a compromise S and q method, reduces the degrouping misregister by one-half at the edge, introduces an equal amount of 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 or more different exposures, using different locations for the light source in the lighthouse in different 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 at least one zone, or in each zone. The difierent locations may involve either the lateral spacing S of the light source from the central axis of the electron gun structure, or the distance p between the light source and the shadow mask, or both. Either first or second order printing may be used.

In one example of the invention, the compromise S and q method of my Pat. 2,855,529 (supra) is modified by dividing the screen and mask into a central zone and an outer zone, modifying the mask contuor (changing the q-spac ng), and substantially separately exposing the two zones with two different S values. Preferably, each exposure should include at least a radial correction lens. The two S values may be chosen so as to produce substantially perfect register of the spots and dots, either at the center or at the edge, or at both center and edge. The small offsetting of the dots in the region where the two zones merge, produced by the two spearate exposures, can be tolerated.

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 diflerent 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 thed light fields transmitted by two different light filters; an

FIG. 6 is a sketch used in explaining the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIGS. 1 and 2 show, as an example, a three-beam tricolor shadow mask color picture tube 1 comprising an evacuated glass envelope 3 made up of a faceplate panel 5, a funnel 7 and a neck 9. For example, the tube 1 may be a 25V rectangular color tube having a maximum viewing dimension of 25 inches and a maximum half-defiec' tion angle of 55 (diagonal). The panel 5 comprises a viewing faceplate 11 and a peripheral flange or side wall 13 which is sealed to the large end of the funnel 7. A 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 produced on the inner surface 11' of the faceplate 11 by one of the methods described hereinafter. 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 means, 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 of the phosphor screen 19. Each aperture 15a 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 P- P, 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 PP', 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 screen 19 were printed with each light source at the center C and without a radial correction lens. The three centers of deflection 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 methods of forming a pattern of elemental areas corresponding to each 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) in such manner as to obtain better correction for various forms of misregister, in at least one of the zones.

For example, in FIG. 3, which shows the open end of the 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, namely, an inner circular zone 29 and an outer zone 31 constituting all of the area surrounding the inner zone 29. The inner 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 inner zone 29 is exposed; and the outer zone 31 may be exposed predominantly by projecting light from the same or 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 US. 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 haying 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 this embodiment of the present invention, the S values are different for the two exposures, to obtain better misregister correction, in at least one 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 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 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, along the S-axis. A light reflector 55 is positioned behind the lamp 53. A lens assembly 56 is mounted on a support ring 57 and standoff 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 diiferent lighthouses) using two ditferent 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 the central 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 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. Preferably, the collimator 47 used for the outer zone exposure is larger than that used for the inner 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 embodiment of the invention, which is an improvement over the compromise S and q method of my Patent 2,855,529, the mask contour (determined by the distance q of the mask 15 from the fixed contour faceplate surface) and the S value of the light source 47 may be chosen for each exposure such that degrouping misregister will be substantially completely eliminated: either (1) at the center; (2) at the edges or corners (ends of diagonals); or (3) at both the center and the edges or corners.

An example wherein full correction is made for degrouping at the corners only, as shown in FIG. 3, will be described. In this case, the conditions at the center may be determined by the conventional comprise q and S method to produce equal size trios of tangent dots 27 (with the appropriate aperture size) and some grouping of the beam spots relative to the dots, as also shown in FIG. 3. FIG. 6 shows the geometry involved, with zero and maximum deflection beam paths 21 shown in dashed lines from the center of deflection C. Assume the following initial conditions: L,,=10.5 (at center), q,,==.500", p =10.0, 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"), tangent dots of a trio having a trio size (average distance of the centers of the three dots from the center of the trio) of .010 (10 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. Assume that the total degrouping error y, due to dynamic convergence, is .002" (2 mils) at the corners of the tube, e.g., at 55 deflection.

Also, the mask contour is changed to make Under these conditions the central dots will be tangent,

with a dot trio size of mils, and the beam spots will ge grouped by 1 mil to a spot trio size of 9 mils, due to the change in q.

In the exposure of the outer zone 31, instead of printing the screen with the light source at the center of deflection of each beam, which would result in maximum degrouping error, or with the light source spacing S increased by /2 AS, as in comprise S and a, the spacing S is increased (from S =.200") as by AS=.O40", to provide full correction for degrouping at the corners. Also, the mask contour is changed at the corners to make La q 5 L at 0:55 is determined from the tube geometry to be 15.01" (approx). Therefore, q=.596" and p=14.4l4". Under the above conditions, the mask will have radii of curvature of 39.7" at the center and 38.1" at the corners. The dots printed at the corners of the screen in the outer zone exposure will be tangent and have a dot trio size of 10 mils (except for foreshortening errors) which should register substantially with the beam spots in the operation of the tube.

In the narrow annular region between the center and the edge where the

zones

29 and 31 merge, and where the exposure of the screen coating is substantially equal for the two exposures, each dot will be somewhat elongated due to the offset exposures from the two diflerent light source positions. However, the reduced exposure of the non-overlapping portions of the elongated dots reduces the hardening and adherence of these portions, and hence, the elongation of the dots will be very small and can be tolerated. If S and q are chosen so as to obtain full correction for degrouping errors at both the center and corners, in the two exposures, the amount of offsetting of the printed dots in the intermediate annular region will be doubled. However, due to the compensating effect described, the actual elongation of the dots will 'be within acceptable tolerance limitations. In the finished tube, the beam spots will be substantially registered with the centers of the elongated dots in the intermediate region.

The method just described could be carried out in a lighthouse with no correction lens 61, to compensate only for part or all of 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 61 by any known method. This correction lens 61 can then be used in printing screens in two exposures with different S and q values in accordance with the present invention.

In the example described above, the S-value of the light source was chosen equal to S -I-AS in the outer zone exposure, in order to compensate for the degrouping error caused by the outward shift. AS of the deflection center of the beam with increased deflection angle 0 The present invention can also be used to substantially separately print two or more zones of the screen surface in separate exposures, in which the light source is located closer to the mask center (smaller p and p) in the outer .455" (approx) zone exposure than in the central zone exposure, in order to compensate for part or all of the radial misregister caused by the axial shift AP of the deflection center of the beam with increased deflection angle 0. In this case, the change in P0 and p may be accompanied by suitable change in mask contour (with the same screen contour) to satisfy the formula for producing equal size dot trios on the screen. However, since the amount of the change in p (and p8) would 'be very small compared to p and L, the change in mask contour would be negligible, and unnecessary.

It will be understood that any number of different portions or zones of the screen may be printed in the same number of exposures, with different locations for the light source in different zones, Moreover, instead of making a series of separate exposures, each exposing substantially only a predetermined zone, a continuous exposure could be used, by sweeping a zone over the photosensitive coating while moving the light source to produce the best correction for misregister at each instantaneous position of the zone. For example, an annular zone could be swept over the coating by means of suitable adjustable irises or shutters interposed between the light source and the screen support.

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 color 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, 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 in a first zone of said array, including predominantly exposing said said first zone from a light source positioned at a first location relative to said support; and a (b) photographically printing on said support the pattern of said array of phosphor elements in a second zone of said array including predominantly exposing said second zone from a light source positioned at a second location relative to said support;

said first and second locations being different positions from where light striking said support for exposing elements respectively in said first and second zones will be in substantial register with electron beams of an operational tube when the beams strike those same elements.

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 different 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 to said screen to produce beam spots thereon; 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 predominantly a first zone of said coating through said mask apertures to light from a first relatively small light source positioned at a first location relative to said mask support;

(0) exposing predominantly a second zone of said coating through said mask apertures to light from a second relatively small light source positioned at a second location relative to said mask and support; and

(d) 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; said first and second locations being different positions from where light passing through an aperture of said mask onto said coating for exposing said elements, respectively, in said first and second zones, will be in substantial register with said electron beam spots, whereby misregister of said beam spots with said element in subsequent operation of said tube will be substantially eliminated in a portion of at least one of said zones.

3. The method of claim 2, wherein the distance between said mask and said support is such that the spacing between adjacent portions of said coating is substantially constant oversaid screen support.

4. The method of claim 2, wherein said first zone is a circular central part of said coating, and said second zone is a part of said coating surrounding said circular first zone.

5. The method of claim 4, wherein said screen support has a longitudinal axis perpendicular thereto at its center, and said second light source has a greater spacing from said axis than said first light source.

6. The method of claim 5, wherein said spacing and the distance between said mask and said support are chosen such that degrouping misregister will be substantially eliminated near the outer edge of said second zone.

7. The method of claim 6, wherein said spacing and the distance between said mask and said support are chosen such that degrouping misregister will be substantially zero at the center of said first zone.

8. The method of claim 2, wherein, in each of said exposure steps, a light-retracting correction element is interposed between said light source and said mask to compensate for residual misregister causes.

9. The method of claim 8, wherein said correction element is an axially-symmetric element designated to minimize radial misregister.

10. 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.

11. 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 phosphor elements.

References Cited UNITED STATES PATENTS 3,677,758 7/ 1972 Kaplan 963 6. 1 3,685,994 8/1972 Frey 96--36.1 2,733,366 1/1956 Grimm et al 9636.1 3 ,222,172 12/ 1965 Giufirida 963 6.1 3,672,893 6/ 1972 Robinder et al 9636.1 3,476,025 11/1969 Herzfeld -1 I. TRAVIS BROWN, Primary Examiner E. C. KIMLIN, Assistant Examiner