US3841875A

US3841875A - Method for grading the screen of a color tube

Info

Publication number: US3841875A
Application number: US00366397A
Authority: US
Inventors: Y Park; R Pekosh
Original assignee: Zenith Radio Corp
Current assignee: Zenith Electronics LLC
Priority date: 1972-04-28
Filing date: 1973-06-04
Publication date: 1974-10-15
Anticipated expiration: 1991-10-15

Abstract

In the formation of a screen for a color cathode ray tube, a method for exposing a photosensitive coating on a cathode ray tube faceplate comprises illuminating the coating with radiation actinic to said coating emanating from an effective point source through a shadow mask disposed adjacent to said coating having a predetermined pattern of apertures formed therein so as to impress on the coating a corresponding pattern of latent aperture images. The method also includes altering the energy distribution pattern impressed on the coating through the mask by imposing thereon a two-dimensional, time-varying shading function.

Description

United States Patent [1 1 Park et a]. Oct. 15, 1974 [54] METHOD FOR GRADING THE SCREEN 0F 1259,0324 v/lwm Burdick cl alt 4. 05/! R A COLOR TUBE 3.6745388 7/1972 Dodd cl aI....I.. 96/44 3,698,903 lO/l972 D )dd t 1!. 96/44 [75] Inventors: Yong S. Park, Hanover Park; L I

: y mend Pekosh Nlles both of Primary Examiner-David Klein Assistant Examiner-Edward C. Kimlin [73] Assignee: Zenith Radio Corporation, Chicago Attorney, Agent, or Firm-John H. Coult Ill.

[22] Filed: June 4, I973 [57] ABSTRACT [21] Appl. No; 366,397 In the formation of a screen for a color cathode ray Related U S A cation Data tube, a method for exposing a photosensitive coating 6 pp on a cathode ray tube faceplate comprises illuminating 2] g 1972' the coating with radiation actinic to said coating emanating from an effective point source through a shadow mask disposed adjacent to said coating having [52] US. Cl. 96/36.l, 96/27 E, 3555/51/51, a predetermined pattern of apertures formed therein so as to impress on the coating a corresponding patg tern of latent aperture images. The method also inle 0 can 95/6 355/71 eludes altering the energy distribution pattern impressed on the coatin throu h the mask by imposin h d l h d' f g t ereon a twoimensiona time-varying s a mg unc- [5 6] References Cited tiom UNITED STATES PATENTS 3,146,368 8/1964 Fiore 96/36.l 12 Clam, 3 Dmwmg Flgures METHOD FOR GRADING THE SCREEN OF A COLOR TUBE RELATED APPLICATIONS This application is a division of application Ser. No. 248,751, filed Apr. 28, 1972, now U.S. Pat. No. 3,762,289, and assigned to the assignee of this application. This application is related to an application entitled A RECTANGULAR GRADE BLACK SUR- ROUND SCREEN in the name of Leonard Dietch, filed Apr. 4, I972, Ser. No. 240,93l, now U.S. Pat. No. 3,790,839, and assigned to the assignee of the present invention.

BACKGROUND OF THE INVENTION The present invention concerns the grade pattern of viewable image elements or phosphor deposits on the faceplate of a color television picture tube. The image elements in a conventional tri-gun shadow mask color cathode ray tube comprise deposits of light emitting phosphors having different color responses. The term viewable" indicates the maximum area of lightemitting surface, which in a conventional positive guardband tube is the area of the electron beam impinging thereon.

During screening of the color tube faceplate, successive exposures through the shadow mask, are made in a lighthouse. The exposures are taken with the faceplate and exposure source occupying relatively different positions, corresponding to the different electron gun positions in the finished tube, with the faceplate coated with a photosensitive slurry bearing a phosphor of an appropriate primary color. The size of the ultimate phosphor deposits on the faceplate is in part determined by the length and intensity of exposure, and under normal conditions, the phosphor deposits are larger than the projected areas of the corresponding apertures in the shadow mask.

Recently, color television picture tubes having elemental phosphor deposits separated from one another over the faceplate, with the spaces between phosphor deposits filled with a light absorptive material, have found widespread commercial acceptance. This type of color television picture tube, commonly referred to as a black surround tube, is fully described in U.S. Pat. No. 3,146,368 issued Aug. 25, 1964, in the name of .loseph P. Fiore et al, and assigned to the assignee of the present invention. In one method of manufacturing a black surround tube, a matrix of opaque, non-reflective material is initially deposited on the inner surface of the faceplate. The matrix defines a plurality of phosphor receiving holes arranged in triads, which holes determined the viewable area of the latter-applied phosphor deposits, since the black surround is opaque. Thus, the maximum light emitting area is restricted by the black surround hole area even though the cross section area of the electron beam impinging on the phosphor deposit may be greater than that of the hole. In such tubes, the purity tolerance or guard band may be reversed with numerous advantages in brightness and contrast characteristics as fully set out in the abovementioned Fiore et al, patent. Thus, a negative tolerance of guard band means that the electron beam area is larger than the effective phosphor dot area. The negative tolerance may be photographically achieved as set out in the Fiore et al patent or may be attained by enlarging the shadow mask apertures after screening has been performed.

As indicated in the aforementioned co-pending Dietch application, a black surround with holes having areas graded in accordance with a rectangular pattern affords desirable benefits in terms of brightness and guard band. The areas of the holes in the black surround may be controlled by appropriately shading the faceplate with its photosensitized coating during exposure since the sizes of the images projected through the shadow mask apertures are a function of exposure intensity and duration. Particularly, a shader" plate comprising a glass plate or other transparent substrate having a deposited metal of micro thickness for attenuating the exposure rays may be employed in the lighthouse between the exposure source and shadow mask. The intensity distribution of the exposure energy can thus be controlled to achieve the desired hole area grading pattern.

Accordingly, it is an object of the invention to provide an improved method of screening a color cathode ray tube of the black surround type.

It is a specific object of the invention to provide an improved method of screening color cathode ray tube which enhances purity.

Another object of this invention is to provide a novel method of producing a color tube screen having viewable image elements diminishing in area from the center to the edge and characterized in that image elements of similar area are disposed along a substantially rectangular locus centered with respect to the faceplate.

BRIEF DESCRIPTION OF THE DRAWINGS The present invention is defined with particularity in the appended claims. The invention, together with further objects and advantages thereof, may best be understood by reference to the following description taken in connection with the accompanying drawing in the several figures of which like reference numerals identify like elements, and in which:

FIG. 1 is a cross sectional view of a lighthouse incorporating shading mechanism executing the method of the invention;

FIG. 2 is a view of the shading mechanism taken along the lines 22 of FIG. 1; and

FIG. 3 is an exploded perspective view of a portion of the shading mechanism.

DESCRIPTION OF THE PREFERRED METHOD The drawings and ensuing text describe apparatus which performs a preferred execution of the inventive method. Referring to FIG. 1, a lighthouse 10 including a housing 11, a light assembly 12 and an exposure surface 16, is shown. The faceplate or panel 20 of a conventional color picture tube is shown mounted in position'on exposure surface 16. A conventional aperture type shadow mask2l is mounted closely adjacent the inner surface of panel 20. The inner surface of faceplate 20 is conventionally coated with a photosensitive material such as polyvinyl alcohol (pva) sensitized with ammonium dichromate, whose solubility in water is changed in response to ultraviolet light. Light assembly 12 includes an ultraviolet light source 13, a collimator l4 and a lens 15. Collimator l4 emits the ultraviolet rays at its tip and is generally designed to simulate a point source. Lens 15 is indicated in block form and may comprise any of a well known number of configurations for displacing the light rays falling on faceplate 20 to compensate for the differences in trajectories be tween lightrays and electron beams in a color tube. Lighthouse may be conventional in all respects with the exception of shadingmechanism 50 located between light source 13 and the exposure surface 16.

Conventional photosensitive coatings such as pva sensitized with ammonium dichromate respond to ultraviolet light in a manner determined by both the exposure duration and intensity. Generally speaking, a longer exposure or a higher intensity exposure results in a larger area of insolubilized pva. Consequently, if the shadow mask apertures are circular, the latent images of those apertures formed in the coating will be circular, but of a size determined by the intensity and duration of exposure.

While the tip of the collimator simulates a point source of light and radiates substantially equally in all directions throughout the deflection angle of the color tube, a peaking of intensity in the central area of the faceplate occurs because the tip is not located at the center of the radius of curvature of the faceplate. Thus, the intensity distributionof the light energy falling on the inner surface of faceplate decreases from center to edge. In a rectangular panel, the extremities of the diagonals are farthest from the light source and, therefore, the intensity is lowest. This type of energy distribution will not result in the desirable rectangular grade pattern for the exposed areas of the coating.

The shading mechanism for obtaining the desired light distribution will best be understood by referring to FIGS. 1, 2 and 3 together. As shown in FIG. 1, a support plate 17 is mounted to the lower portion of lighthouse 11 and supports a bearing 18. A cam plate support 29 is mounted for rotational movement on bearing 18. A movable cam plate 30 is either attached to cam 5 plate support 29 or maybe formed integrally therewith.

A stationary drive plate 40 is affixed to housing 11 in overlying relationship with cam plate 30. A pair of vertical shading plates 61 are mounted in drive plate 40 for rectilinear movement toward and away from each other along a path parallel to the major axis of faceplate 20. Four rack supports 56 are fastened about the circumference of drive plate 40 and partially overlie vertical shading plates 61. The rack supports carry a pair of horizontal shading plates 51 which similarly are mounted for rectilinear motion toward and away from each other, but along a path parallel to the minor axis of faceplate 20. Each rack support includes a rack 57 with a mating pinion 55, mounted on a shaft 54, which is journalled in a pair of bearing supports 53 affixed to the horizontal shading plates.

A stationary guide plate 40 includes a plurality of guide slots, two for each vertical shading plate and one for each horizontal shading plate. The vertical guide slots 42 and the horizontal guide slots 41 are adapted to receive vertical guide pins 63 and the horizontal guide pins 58, respectively, on the bottom of each shading plate. Horizontal guide pin 58 has an extension terminating in a horizontal drive pin 59 engaging a horizontal cam 32 in cam plate 30. While each vertical shading plate 61 has two pairs of guide pins 63, only one of the guide pins has an extension terminating in a vertical drive pin 64 engaging a vertical cam 33 in cam plate 30.

Movable cam plate 30 includes a partial gear ring 31 attached to, or formed as part of its circumference. A reversible motor 25, mounted from the lighthouse by a bracket 19, powers a drive gear 26 in engagement with gear ring 31. A cycling pin 34 is mounted to the circumference of cam plate 30 for alternately engaging and operating a pair of reversing

switches

35 and 36 which are shown affixed to stationary guide plate 40. The reversing switches change the direction of rotation of motor 25 in a cyclical manner, and drive gear 25 rotates cam plate 30 back and forth between the positions determined by the switches.

Horizontal earns 32 comprise arcuate slots in cam plate 30, engaged by horizontal drive pins 59 attached to the underside of shading plate 51. The vertical cams 33 approximate L-shaped slots in cam plate 30 engaged by vertical drive pins 34 attached to the underside of shading plates 61. As cam plate 30 rotates clockwise, horizontal drive pins 59 are forced inward by the action of horizontal cams 32. The line of travel of horizontal shading plates 51 is controlled by the action of guide pin 58 in guide slot 41 and the provision of the rack and pinion arrangement which precludes all but rectilinear movement. Vertical drive pins 64 remain substantially stationary during the first portion of clockwise movement of cam plate 30 while the pins are in the arcuate shaped dwell portions of the vertical cams indicated by 33d. Responsive to continued rotation of cam plate 30, drive pins 63 are driven rapidly inward by the effective portions of the vertical cams indicated by 33c. Thus. as cam plate 30 rotates, horizontal shading plates 51 are brought towards each other at a constant rate, whereas the vertical plates are initially stationary and subsequently moved toward each other at a faster rate. When moved, the shading plates shadow portions of faceplate 20 from the light source.

When cam plate cycling pin 34 operates switch 36, motor reversal occurs and the cam plate is driven clockwise. Actions now reverse with the vertical shading plates being driven away from each other rapidly whereas the horizontal plates are driven at a slower rate.

The horizontal and vertical shading plates cooperate to define a window or opening whose shape and area vary with time. This window may be regarded and described as representing a two-dimensional, timevarying shading function. It is apparent that the vertically moving horizontal shading plates and also the horizontally moving vertical shading plates each accomplish an area-wise shading of the photosensitive coating on the faceplate, i.e., a shading in two dimensions. Each of the shading plate pairs alters the energy distribution pattern impressed on the photosensitive coating, the horizontal shading plates acting along a vertical axis and the vertical shading plates acting along a horizontal axis.

By shaping the frontal edge of each plate, for example parabolically, as shown, additional and compensating light energy is impressed along the faceplate diago' nals. This is seen more clearly in FIG. 2 where the four plates cooperate to form a time-varying window of pincushion shape. The window corners cause the portions of the faceplate along the diagonals to be exposed for a longer period of time than the portions along horizontal and vertical axes.

In a preferred embodiment, the plates are shaped to resemble parabolas and their movements timed to produce a light distribution pattern over the faceplate for yielding a locus of rectangles of similar area images, with the image areas diminishing from center to edge, centered about the faceplate. The continual movement of the plates produces gradual light distribution changes from point-to-point across the faceplate and thus results in a uniform transition in image area size.

The provision of four vertical guide pins 63 cooperating with a pair of parallel guide slots 32 precludes the vertical shading plates from traveling in other than a rectilinear path. It will be appreciated that the guidance arrangement for the vertical plates and for the horizontal plates may be used interchangeably, the criterion being that freedom from side movement be maintained.

It will be recognized that the size and shape of the plates may be varied along with timing of plate operations to produce a variety of shading patterns. This is of particular importance in a production area where adjustments may be made to compensate for the innumerable variables encountered in screening operations. This flexibility is not obtainable with fixed shader plates or lens coatings and is an extremely desirable attribute of the invention.

While particular methods of the invention have been shown and described, it will be obvious to those skilled in the art that changes and modifications may be made without departing from the invention in its broader aspects, and, therefore, the aim in the appended claims is to cover all such changes and modifications as fall within the true spirit and scope of the invention.

, What is claimed is:

1. In the formation of a screen for a color cathode ray tube, a method for exposing a photosensitive coating on a cathode ray tube faceplate comprising illuminating the coating with radiation actinic to said coating emanating from an effective point source through a shadow mask disposed adjacent to said coating having a predetermined pattern of apertures formed therein so as to impress on the coating a corresponding pattern of latent aperture images, said method including altering the energy distribution pattern impressed on the coating through the mask by shading said coating as a function of (i) time, (ii) the radial distance of latent images tst tx a sa at se tt n a (ii) tha l flt a easlwt latent images on the faceplate, such that there is defined on the faceplate coating a plurality of noncircular loci wherein each said non-circular locus consists of latent images which receive an equal time of exposure, wherein said shading comprises casting two shadow patterns on said mask which move with time across said mask in substantially orthogonal directions, each of said shadow patterns comprising first and second portions moving with time along the same axis in opposite directions.

2. The method defined by claim 1 wherein said loci are substantially rectangular.

3. The method defined by claim 2 wherein said loci are substantially concentric and centered about said faceplate, and the exposure times of successive loci di minish from faceplate center to faceplate edge.

4. The method defined by claim 1 wherein the confronting edges of the first and second portions of at least one shadow pattern are parabolic.

5. The method defined by claim I wherein the two shadow patterns create a border around a non-shaded area of said mask and said non-shaded area varies in size and shape with time.

6. The method defined by claim 5 wherein the nonshaded area exhibits a pincushion shape.

7. The method defined by claim 1 wherein said shading varies cyclically with time.

8. The method of manufacturing a rectangular screen assembly for a black surround color tube comprising the steps of:

a. applying a coating of photosensitive material to the faceplate of the tube;

b. mounting a shadow mask with a pattern of aper tures in spaced relationship to the faceplate;

c. providing a lighthouse having an exposure surface. a source of energy actinic to said photosensitive coating and an automatic shading mechanism between the energy source and the exposure surface, said shading mechanism casting two shadow patterns on said mask which move with time across said mask in substantially orthogonal directions, each of said shadow patterns comprising first and second portions moving with time along the same axis in opposite directions;

(1. mounting the faceplate and shadow mask on the exposure surface;

e. exposing the photosensitive coating from the source of energy through the shadow mask, while varying the time of exposure with said automatic shading mechanism such that time of exposure of coating elements associated with said mask apertures depends upon the radial distance of coating elements from faceplate center and upon azimuthal angle of coating elements on the faceplate, such that there is defined on the faceplate coating a plurality of substantially rectangular, faceplate centered, and concentric loci wherein each said locus consists of coating elements which receive an equal time of exposure;

f. developing the latent image of said pattern of apertures formed in said coating; and

g. applying a black pigment to the non-exposed portions of said faceplate.

9. The method set forth in claim 8 wherein said exposure step comprises repetitive exposures of said coating through said shadow mask with said energy source located in different positions corresponding to the three electron guns in a color tube.

10. The method set forth in claim 9 wherein said automatic shader mechanism comprises a set of plates, movable during exposure, for providing said variable time of exposure.

11. The method set forth in claim 10 wherein said set of plates include a pair of horizontal plates movable substantially along a path parallel to the minor axis of said faceplate and a pair of vertical plates movable substantially along a path parallel to the major axis of said faceplate.-

12. The method set forth in claim ll wherein control means are provided for moving said horizontal and vertical plates at different rates.

Claims

1. IN THE FORMATION OF A SCREEN FOR A COLOR CATHODE RAY TUBE, A METHOD FOR EXPOSING A PHOTOSENSITIVE COATING ON A CATHODE RAY TUBE FACEPLATE COMPRISING ILLUMINATING THE COATING WITH RADIATION ACTINIC TO SAID COATING EMANATING FROM AN EFFECTIVE POINT SOURCE THROUGH A SHADOW MASK DISPOSED ADJACENT TO SAID COATING HAVING A PREDETERMINED PATTERN OF APERTURES FORMED THEREIN SO AS TO IMPRESS ON THE COATING A CORRESPONDING PATTERN OF LATENT APERTURE IMAGES, SAID METHOD INCLUDING ALTERING THE ENERGY DISTRIBUTION PATTERN IMPRESED ON THE COATING THROUGH THE MASK BY SHADING SAID COATING AS A FUNCTION OF (I) TIME, (II) THE RADIAL DISTANCE OF LATENT IMAGES FROM FACEPLATE CENTER, AND (II) THE AZIMUTHAL ANGLE OF LATENT IMAGES ON THE FACEPLATE, SUCH THAT THREE IS DEFINED ON THE FACEPLATE COATING A PLURALITY OF NON-CIRCULAR LOCI WHEREIN EACH SAID NON-CIRCULAR LOCUS CONSISTS OF LATENT IMAGES WHICH RECEIVE AN EQUAL TIME OF EXPOSURE, WHEREIN SAID SHADING COMPRISES CASTING TWO SHADOW PATTERNS ON SAID MASK WHICH MOVE WITH TIME ACROSS SAID MASK IN SUBSTANTIALLY ORTHOGONAL DIRECTIONS, EACH OF SAID SHADOW PATTERNS COMPRISING FIRST AND SECOND PORTIONS MOVING WITH TIME ALONG THE SAME AXIS IN OPPOSITE DIRECTIONS.

3. The method defined by claim 2 wherein said loci are substantially concentric and centered about said faceplate, and the exposure times of successive loci diminish from faceplate center to faceplate edge.

5. The method defined by claim 1 wherein the two shadow patterns create a border around a non-shaded area of said mask and said non-shaded area varies in size and shape with time.

6. The method defined by claim 5 wherein the non-shaded area exhibits a pincushion shape.

8. The method of manufacturing a rectangular screen assembly for a black surround color tube comprising the steps of: a. applying a coating of photosensitive material to the faceplate of the tube; b. mounting a shadow mask with a pattern of apertures in spaced relationship to the faceplate; c. providing a lighthouse having an exposure surface, a source of energy actinic to said photosensitive coating and an automatic shading mechanism between the energy source and the exposure surface, said shading mechanism casting two shadow patterns on said mask which move with time across said mask in substantially orthogonal directions, each of said shadow patterns comprising first and second portions moving with time along the same axis in opposite directions; d. mounting the faceplate and shadow mask on the exposure surface; e. exposing the photosensitive coating from the source of energy through the shadow mask, while varying the time of exposure with said automatic shading mechanism such that time of exposure of coating elements associated with said mask apertures depends upon the radial distance of coating elements from faceplate center and upon azimuthal angle of coating elements on the faceplate, such that there is defined on the faceplate coating a plurality of substantially rectangular, faceplate centered, and concentric loci wherein each said locus consists of coating elements which receive an equal time of exposure; f. developing the latent image of said pattern of apertures formed in said coating; and g. applying a black pigment to the non-exposed portions of said faceplate.

11. The method set forth in claim 10 wherein said set of plates include a pair of horizontal plates movable substantially along a path parallel to the minor axis of said faceplate and a pair of vertical plates movable substantially along a path parallel to the major axis of said faceplate.

12. The method set forth in claim 11 wherein control means are provided for moving said horizontal and vertical plates at different rates.