US3790839A

US3790839A - Rectangular grade black surround screen

Info

Publication number: US3790839A
Application number: US00240931A
Authority: US
Inventors: L Dietch
Original assignee: Zenith Radio Corp
Current assignee: Zenith Electronics LLC
Priority date: 1972-04-04
Filing date: 1972-04-04
Publication date: 1974-02-05
Anticipated expiration: 1991-02-05
Also published as: JPS4985953A

Abstract

The screen structure of a black surround type rectangular color cathode ray tube has a plurality of viewable image elements which decrease in area from the center of the faceplate to the periphery. However, instead of having a uniform grade in all radial directions from the center, the viewable image elements of similar area are located along a substantially rectangular locus centered with respect to the face plate.

Description

United States Patent [1 1 Dietch RECTANGULAR GRADE BLACK SURROUND SCREEN [75] Inventor: Leonard lDietch, Skokie, Ill. [73] Assignee: Zenith Radio Corporation, Chicago,

Ill.

[22] Filed: Apr. 4, 1972 [21] App]. No.: 240,931

[52] US. Cl. 313/92 R, 313/92 CS [51] Int. Cl. H0lj 29/32 [58] Field of Search... 313/92 B, 92 R, 85 S, 92 CS [56] References Cited UNITED STATES PATENTS 2,755,402 7/1956 Morrell 313/85 S Feh.5,1974

8/1960 Kaplan 313/92 B 8/1964 Fiore et al 313/92 B Primary ExaminerRoy Lake Assistant Examiner-Siegfried l-l. Grimm Attorney, Agent, or Firm -Nicholas A. Camasto [5 7] ABSTRACT The screen structure of a black surround type rectangular color cathode ray tube has a plurality of viewable image elements which decrease in area from the center of the faceplate to the periphery. However, instead of having a uniform grade in all radial directions from the center, the viewable image elements of similar area are located along a substantially rectangular locus centered with respect to the face plate.

7 Claims, 4 Drawing Figures PAIENIEDFEB fi m 3.790.839

70 of Center Area 70 70 of Distance From Center Along Mninr AV 1 RECTANGULAR GRADE BLACK SURROUN SCREEN BACKGROUND OF THE INVENTION The present invention concerns the grade pattern of viewable image elements on phosphur 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 light emitting surface, which is normally limited by 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. These exposures are made from different positions, each corresponding to an electron gun position inthe finished tube, with the faceplate coated with a photosensitive emulsion bearing a phosphor of the 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.

In prior practice, the phosphor coverage on the tube faceplate has been maximized in an attempt to maximize theoretical light output. It can readily be shown that a screen with tangent phosphor dots achieves this criterion for a given aperture size and mask spacing.

The concept of grading the apertures of a shadow mask is quite old. The purpose of grading the apertures is to preserve purity in spite of the fact that the electron beam triangle suffersdistortion with scanning so that its registration with the phosphor dot triangles tends to become imperfect with increase in deflection angle. In other words, a condition of misregistration is encountered and a change of white field color, e.g. a tinting of the white field, is suffered whenever any one of the three beams rides off its assigned phosphor dot whether or not'the misregistration is so extreme that the beam impinges upon a non-assigned phosphor dot. It is clear that there is more tolerance available and therefore improved purity in spite of distortions of the beam triangle if the aperture diameter is decreased with radial displacement from the center of the field. While the reduced aperture diameter is accompanied by a brightness loss, that loss is accepted as a compromise in attaining better tolerance with respect to purity. Actually, the aperture grade is generally sufficiently gradual that the resulting brightness grade is hardly observable. Graded aperture masks of this type are currently in commercial use. When the effective area of the electron beam is smaller than the area of the phosphor deposit, the tube is said to have a positive purity tolerance or guard band.

More 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 US. Pat. No. 3,146,368 issued Aug. 25, 1964 in the name of Joseph 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 determine the viewable area of the latter applied phosphor deposits since the black surround matrix is opaque. Thus the maximum light emitting area is restricted by the matrix hole area even though the cross-sectional area of the electron beam impinging on the phosphor deposit maybe greater than that of the deposit. 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 above-mentioned Fiore et al. patent. Thus a negative tolerance or guardband 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.

In any case a compromise must be struck between purity tolerance and guard band, whether negative or positive, and effective light emission from the tube face. The present invention is an improvement in that a specifically different grading pattern of viewableimage elements defined by apertures in a black matrix is taught to further increase the total area of viewable image elements over the tube faceplate, thus increasing the brightness capability of the tube without loss of guard band. In fact, the guard band can actually be increased.

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

It is a specific object of the invention to provide a novel screen structure which enhances purity in a black surround color cathode ray tube.

It is yet another object of the invention to provide a novel screen structure for increasing the maximum brightness capability of a black surround color cathode ray tube without loss of guard band.

SUMMARY OF THE INVENTION The screen assembly of a color cathode ray tube, constructed in accordance with the invention, comprises a plurality of viewable image elements distributed throughout a rectangular field with an area grade characterized in that the viewable image element areas decrease in a predetermined manner with distance from the center of the field and further characterized by the fact that viewable image elements having the same area are disposed along a substantially rectangular locus centered in the field. Ceonsequently, the screen assembly has a light emission characteristic comprising a concentric series of substantially rectangularly shaped patterns of equal light emission in contradistinction-to devices of the prior art wherein the light emission characteristic is a concentric series of circular patterns of equal light emission.

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

FIG. 1 is a fragmentary showing of a color tube screen structure including a shadow mask and a black surround grille having a hole size grade pattern in accordance with the invention;

FIG. 2 is a plot used in explaining typical conditions of registration between the phosphor dot and beam triads;

FIG. 3 represents essentially the iso-transmission patterns of a black surround grille embodying the invention; and

FIG. 4 is a plot showing an illustration hole size grade for the black surround grille.

DESCRIPTION OF THE PREFERRED EMBODIMENT Shadow mask types of color cathode ray tubes are well known in the art and comprise a screen having in-.

terlaced and similar patterns of color phosphor deposits, usually green, blue and red, for excitation by assigned ones of three electron beams. The electron beams are developed in a triangular gun cluster in the neck of the picture tube opposite the screen which is on the inner surface of the tube faceplate.

A shadow mask having a multiplicity of small apertures is interposed in the electron beam paths close to the screen and the apertures thereof are arranged in a pattern essentially the same as the pattern of phosphor deposits on the screen. The shadow mask may be termed a color-selection electrode because the arrangement is such that each of the three beams, in passing through a given aperture of the mask, is angularly disposed to excite only that one of the three phosphors that is assigned to the particular beam. Between the mask and the gun cluster there is the usual deflection yoke and beam convergence arrangement provided so that the beams may scan a rectangular raster and in all positions thereof impinge on the screen with the angular relation required to maintain color selectivity. All of this is thoroughly understood in the art and applies whether the screen is circular or has the now popular rectangular shape. Since the invention may be fully understood by considering essentially only the screen assembly, which includes the phosphor deposits, black surround grille and shadow mask, the remainder of the description will be largely confined to such matters.

As mentioned previously, there is a choice with respect to the relative sizes of the phosphor deposits on the screen on the one hand and the exciting electron beams on the other hand, particularly where, as is usually the case, the screen is a mosaic structure defined by a multiplicity of phosphor triads each of which has a dot of green, a dot of blue and a dot of red phosphor. The phosphor dots in the early introduction of commercial television were sized to be in essentially tangential contact with one another over the screen area with the electron beams of smaller diameter. The difference in the dot and beam diameters provided a positive guard band or tolerance which permitted some migration of the beam over its assigned dot before a loss of purity was suffered.

As described in the Fiore et al. patent, the area of the phosphor dots may be smaller than the area of the electron beams and provide an equally large negative guard band or tolerance range for purity. A major advantage of small sized dots, separated from one another, is that a light-absorbing material may be placed on the faceplate areas separating the phosphor dots, which leads to enhanced brightness and contrast. FIG. 1 discloses this screen arrangement wherein a screen 10 has phosphor dots 11 separated from each other on the faceplate and varying in area in accordance with the invention. The phosphor deposits are arranged to define triads, some of which are delineated by the characters r,,, b,,, g,,, r,, b etc. The shadow mask 12, which is spaced from but substantially parallel to the screen, is a foraminous or perforated member having a multiplicity of circular apertures with the apertures of the mask in alignment with corresponding phosphor dot triads in the conventional manner. The mask apertures are larger than the phosphor deposits. For convenience, it is assumed that the apertures are of the same area A, although this is not essential to the invention.

Certain series of phosphor triads are designated by reference characters Y Y Y Y and Y.,. A common subscript indicates triads of dots of approximately the same diameter and the series are graded from the largest value Y at the center of the screen. The particulars of the grading will be discussed hereafter.

First it is informative to consider the plot of FIG. 2 which depicts the manner in which misregistration is usually encountered in a conventional color tube of the three-gun shadow mask type. FIG. 2 indicates a portion of the upper right quadrant of a tube screen and the clear circles designate assumed tangent components g, b and r of phosphor triads while the fitted-in smaller circles represent electron beams with each beam shown impinging upon or exciting its assigned phosphor dot. Since the screen is rectangular, it has both a minor and a major axis with the latter usually disposed in the direction of horizontal scan while the other is disposed in the direction of the vertical scan. These axes intersect at the center of the screen where, as illustrated in FIG. 2, perfect registration is obtained. A condition of perfect registration is one wherein the beam triangle is in precise registration with the phosphor triangle, that is to say, each beam is concentric with its assigned phosphor dot. In this condition both the phosphor dots and the beams define equilateral triangles which, being in proper registration, establish the desired condition of color purity. Unfortunately this ideal geometrical relation holds only in the central portion of the screen and, because of such factors as astigmatism of the yoke, suffers deterioration and a loss of registration as well as purity tolerance with deflection angle. At the ends of the major axis of the screen, usually termed the 3 and 9 oclock positions, the red and green beams tends to become grouped, that is to say, they tend to be closer to one another while the blue beam experiences a displacement in the opposite or in the degrouping sense; that is, the blue beam tends to be further displaced vertically from the remaining two beams than is the case at the center of the screen, as indicated in FIG. 2. This is the most serious condition of beam triad distortion and results in the least purity tolerance because, comparing the conditions at the center and at the end of the major axis, less movement of the beam is permissable at the 3 and 9 oclock positions before suffering loss of purity than is true at the center of the screen.

Another screen position that is difficult from the standpoint of purity tolerance, although perhaps not quite as serious as the 3 and 9 o'clock positions, occurs at each end of the minor axis, normally referred to as the 12 and 6 oclock positions. In these locations, the

red and green beams tend to be separated or degrouped while the blue beam shifts in a grouping sense. This is apparent from comparison ofthe 12 oclock and center positions in FIG. 2. It again represents a condition of imperfect beam-phosphor registration where purity loss is apt to be encountered. Other screen locations of particular interest are those which occur at the ends of the diagonal axes of the screen; one such location is also indicated in FIG. 2.

If the mask apertures are of uniform diameter, there is obviopsly a much greater likelihood of color distortiori ends of the riiajoi and minor axes than is the case at the center of the screen simply because of the influence of yoke astigmatism and other tube parameters responsible for th'eloss' of registration with deflection angle. It is to minimize thatpossibility and to provide a guard band or purity tolerance that the art has adopted the technique of a graded mask. In prior art tubes, however, the grading is such that the aperture diameter decreases uniformly along all radial directions from the center of the screen toward the periphery. That, of course, means that there is a severe brightness loss at the ends of the diagonal axes which gives rise to .what was known as a hot center or dark corner screen effect.

, This objectionable effect is avoided and a noticeable change in screen brightness is attained with the present invention by adopting a distinctly different grading pattern for the holes or apertures in the black surround grille 20. Here the areas of the viewable image elements are determined by the areas of the phosphor dots capable of being usefully excited by the electron beams.

In FIG. 3, by way of illustration, the central hole in the black surround grille is designated Y and one series of holes Y, having a substantially common but smaller area is disposed along the rectangular path bearing the same designation. The rectangles Y and Y; of FIG. 3 are similarly loci of holes having a common area although the area decreases from one rectangle to the next in the direction of the edge of the screen. The grille may be said to' have an area characteristic comprisinga concentric series of rectangularly shaped isotransmission patterns or equal brightness patterns on the face of the tube, assuming the electron beams are always larger than the holes and the gun currents match to provide white light of a given color temperature. For example, the transmission or brightness at the center of the screen may be I and the iso-transmission patterns Y, to Y have the following percentages of the transmission or brightness at Y namely, 95, 84, 68 and 50, respectively. Expressed somewhat differently, the hole area grade of black' surround grille 20 in accordance with the invention is such that equal brightness patterns are disposed at the same percentages of distance from thecenter of the screen. The broken-construction rectangles of FIG. 1 are also an effort. to represent these patterns and, while shown over only a part of the screen, it will be understood that the grade continues in this fashion throughout'the entirety of the screen.

An important practical result of fabricating a screen assembly with a black surround grille with this hole grade is that a greater screen brightness range, that is center-to-edge brightness, is possible. Recognizing that misregistry problems are minimal at the center of the screii, allows greater utilization of this area for light emitting materials without loss of purity tolerance or guard band. This change in grade pattern, moreover, has resulted (with a typical 25-inch diagonal black surround color tube) in an increase of 0.4 rnils in the guard band at the sides of the tube, a 0.7 mil increase in guard band at the top and bottom of the tube and a 0.2 mil increase in guard band at the tube corners.

The manner in which the black surround grille hole size varies along the major axis of the screen is not critical. One illustrative variation in area with distance from the center is represented by the curves of FIG. 4. Others may, of course, be employed.

A screen assembly having a black surround grille may be fabricated in at least two distinct ways. The preferred form of fabrication is to initially deposit on the clear faceplate of the picture tube a black surround grille with its pattern of hole areas varying in accor dance with the teachings of the invention. Thereafter, appropriate phosphors are deposited in corresponding holes in the black surround grille. In this technique the physical dimensions of the phosphor materials are not critical in that the black surround grille holes determine the viewable phosphor areas.

Such a screen assembly may be prepared by initially forming clear dots of polyvinyl alcohol (pva), sensitized with ammonium dichromate, over the elemental areas of the faceplate which are to receive phosphor deposits. This is accomplished by photoexposure of the sensitized pva coating from all three lighthouse positions, which results in insolubilizing the pva in areas where the phosphor deposits are to be. The faceplate is then developed by washing away the soluble areas of pva in the spaces intervening the insoluble dots. Thereafter a light-absorbing material such as an aqueous solutionof graphite mixed with an inorganic binder is coated over the faceplate and dried. The inorganic binder causes the graphite to adhere to both the glass and insoluble dots of pva. The pva dots are then caused to swell and break the graphite coating thereover by application of hydrogen peroxide to the faceplate. Then a water wash is used to remove the pva and adherent graphite to leave a black grille with a plurality of holes in appropriate locations for reception of proper phosphors. Screening with appropriate phosphor materials is then carried out in accordance with conventional techniques.

A different method of applying a black surround grille is to initially screen the faceplate with appropriate color phosphor materials. Thereafter the spaces intervening the phosphor deposits are filled with a black, light absorbing material. One technique is to photoexpose, through the front of the faceplate, with a photosensitized solution of such material, for example, causing insolubilization thereof in the intervening spaces. Subsequent washing removes the material in .the areas protected from the solubilizing radiation, namely over the deposited phosphor materials. In this technique, the phosphor deposits themselves would initially be graded in area in accordance with the invention.

It will be seen that in either case the viewable image elements are the areas of phosphor available for light emitting purposes. In the former example the available phosphor is determined by the area of the holes in the black surround grille, whereas in the latter case the phosphor deposit itself determines the available area since the grille merely fills in the spaces between such deposits.

it is well known in the color tube manufacturing art that the size of the image formed by exposure of photosensitive materials to actinic radiation through apertures, such as those found in a conventional shadow mask, may be substantially determined by controlling the intensity and duration of such exposure. Thus the areas of exposed pva, whether including phosphor materials or not, may be controlled by controlling the extent of exposure to actinic energy. The art has long recognized this principle and the use of shader plates, in lighthouses for controlling the distribution of exposure energy over the faceplate is well known. Shader plates comprise glass plates which have deposited thereon a light attenuating medium, such as metal, which varies in intensity in accordance with the light distribution pattern desired. The deposited metal is of micro thickness and introduces very little refraction to the exposure rays passing therethrough. In many instances the coating is put on a surface of the lens usually associated with lighthouses. The use of such a shader plate in lighthouses used to expose faceplates for forming a black surround grille in accordance with the instant invention can facilitate obtaining the desired rectangular grade pattern for the hole areas. Specifically, a shader plate may be fabricated to produce an exposure intensity across the faceplate which will yield appropriate size deposits of insoluble pva (with or without phosphor material added) in accordance with the desired graded area pattern.

While a particular embodiment of the invention has 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. A black surround color television picture tube having a substantially rectangular faceplate and a screen assembly including an aperture mask spaced from a plurality of viewable image elements distributed in a rectangular field over the faceplate and characterized in that said viewable image elements diminish gradually in area from the center to the edge of the faceplate such that elements having the same area are disposed on concentric loci which are centered in said field and which have a substantially rectangular geometry.

2. A black surround color television picture tube as set forth in claim 1 wherein said viewable image elements comprise, light emitting color phosphors arranged in groups or red, blue and green.

3. A black surround color television picture tube as set forth in claim 2 wherein said viewable image elements comprise dots of said light emitting phosphors arranged in triads, said dots being spaced apart from each other on said faceplate and having areas diminishing in accordance with said pattern.

4. A rectangular screen assembly for a color television picture tube including a matrix of light absorbing material on said faceplate, said matrix having holes arranged in a rectangular field over said faceplate coextensive with the maximum viewable areas of light emitting phosphors, said holes having areas diminishing in accordance with distance from the center of said faceplate and arranged in a pattern characterized in that lines of holes of substantially the same areas occur at the same percentages of distance from said center.

5. A screen assembly for a color cathode ray tube having a faceplate with a black surround grille including a multiplicity of holes, each defining the viewable area of an associated deposit of light emitting phosphor, distributed throughout said faceplate with an area grade characterized in that the hole areas decrease in a predetermined manner with distance from the center of the faceplate and further characterized in that holes having substantially the same areas are disposed along a substantially rectangular locus centered in said faceplate so that the screen assembly, when incorporated into a tube, will have a viewable phosphor area characteristic comprising a concentric series of substantially rectangularly shaped patterns of equal brightness.

6. A screen assembly as set forth in claim 5 wherein said holes are circular and said phosphors comprise deposits of red, green and blue light emitting phosphor materials.

7. A black surround color television picture tube having a substantially rectangular faceplate and a screen assembly including an aperture mask spaced from a plurality of viewable image elements distributed in a rectangular field over the faceplate and characterized in that in at least one common coordinate direction said viewable image elements diminish gradually in dimension from the center to the edge of the faceplate such that elements having the same value of said dimension in said common coordinate direction are disposed on concentric loci which are centered in said field and which have a non-circular geometry.

Notice of Adverse Decision in Interference In Interference No. 98,668, involving Patent No. 3,790,839, L. Dietch, RECTANGULAR GRADE BLACK SURROUND SCREEN, final judgment adverse to the patentee was rendered Oct. 29, 1976, as to claims 1-7.

[Ofiicial Gazette July 5, 1977.]