US3187650A - Compensation for vertical component of earth's magnetic field by color triad displacement - Google Patents

Description

J. L. HUDSON 3,187,650 TIC 'S MAGNETIC June 8, 1965 COMPENSATION FOR VER AL COMPONENT OF EARTH FIELD BY COLOR TRIAD DISPLACEMENT Filed Dec. 50, 1955 7'0 CWT I005 user/m1. compommr or m: 3/ EARTHJS 'MAg/vEr/c HEL o.

x. .lln d 5% w z INVENTOR. Hwso/v Joy/V l employed for the purpose of United States Patent Ofitice 3,187,650 COMPENSATION FOR VERTICAL COMPONENT OF EARTHS MAGNETIC FHELD BY CULGR TRIAD DISPLACEMENT John L. Hudsen, Lancaster, Pa., assignor to Radio Corporation of America, a corporation of Delaware Filed Dec. 30, 1955, Ser. No. 556,649 1 Claim. (Cl. 95-1) This invention relates to scopes and other cathode-ray tubes of the kind containing an apertured mask through which beam-electrons pass in their transit to preselected elemental areas on the mosaic target surface of a nearby screen, and to improvements in the art of making such tubes.

In the operation of cathode-ray tubes of the subject (masked-target) variety the accuracy with which the beam-electrons strike the individual elemental areas of the mosaic-screen may be adversely affected by the presence of the earths magnetic field in the paths of the electrons. This fact has long been recognized and it is now more or less standard practice to compensate for the beam-deflecting effect of the earths field by providing such tubes with a magnet-assembly or a purity coil capable of directing the electron-beams to paths which terminate, ideally, at the centers of the individual electron-sensitive dots (or lines) which make up the mosaic target-surface of the screen. Such auxiliary devices operate quite satisfactorily to perform their intended functions but their use is subject to certain disadvantages, In instances where the auxiliary magnetic field must be made of a relatively large magnitude, to effect resolution of the resulting image, picture or raster. Picture quality may be further adversely affected if the purity field interacts with the field of some other internal or external component of the set.

The present invention provides a cathode-ray tube of the masked-target variety wherein picture defects casioned by the presence of the earths magnetic field in the path of the beam-electrons are minimized by the provision of a target-assembly of a construction which permits the use of a correcting field of a magnitude considerably less than that of the auxiliary fields heretofore maintaining said electrons in their intended paths.

The principle upon which the invention is based will the more readily be understood when it is appreciated that the vertical component of the earths magnetic field, at any particular geographic location, has a constant eliect upon an electron-beam regardless of the orientation of the tube in which the beam is contained, provided only that the axis of the tube is substantially horizontal. (The effect of the horizontal component of the earths field upon an electron beam tube varies, at any given geographic location, with the orientation of the tube.)

The fact that the vertical component of the earths field has a constant effect upon an electron-beam permits a constant correction factor (heretofore supplied entirely by an auxiliary magnetic device) to be built in the tube. Stated generally, this is achieved bythe provision of a tube during the screen-plotting operation and are subsequently mounted Within the tube in a position whereat In making a screen-assernbly, in accordance with the 3,187,650 Patented June 8, 1965 invention, the relative location of the screen pattern, and of the pattern of apertures in the mask, with re spect to the location of the electron-gun or guns, is fixed during manufacture of the tube preferably by olfsetting the screen-assembly (i.e. the screen and its mask) with respect to the light source employed in plotting the location of the elemental areas of which the mosaic target surface of the screen is comprised. When a particular tube is to be employed at a particular geographic location the exact degree of off-set employed in the screen-plotting operation can be dictated by the magnitude of the vertical component of the earths magnetic field at that particular geographic location. In this case, no auxiliary magnetic field whatsoever is required to commagnetic device 18 ordinarily so small that it has pracor raster and, in any event, is less than is required for tubes made in accordance with the prior art.

The invention is described in greater detail in connection with the accompanying single sheet of drawings, wherein:

FIG. 1 is a partly diagrammatic longitudinal sectional view, as viewed from above, of a 3-gun tri-c0lor kinescope of the shadow-mask dot-screen variety; the drawing being marked with a line (yy) showing that the center of the mosiac pattern of the screen is off-set lateral- 1y, from the central axis (xx) of the tube, as dictated by the present invention;

FIG. 2 is an enlarged fragmentary rear-elevational view of the screen unit of the color-kinescope of FIG. 1;

FIG. 3 is a greatly enlarged front elevational view of the central portion of a color dot-screen similar to the one shown in FIG. 2 but with the drawing marked to illustrate a lack of register between the screen-dots and electron-spots occasioned, in a conventional tube, by the effect of the vertical component field upon the path of the electrons;

FIG. 4 is a partly diagrammatic sectional view of a photographic lighthouse showing the apertured mask and screen-plate of a color-kinescope set-up thereon in an off-axis position with respect to the light-source position,

invention. One type of cathode-ray tube to which the invention is applicable is a 3-gun tri-color kinescope of the shadow mask variety. As shown in mask variety.

The mosaic screen 17 (see FIG. 2) comprises a multiplicity (usually 300,000 or more) of triads (i.e. groupsof-three) 'of red (R), blue (B) and green (G) colorphosphor dots. The phosphor-dots are here arranged in a hexagonal pattern; that is to say each dot is surrounded by six other dots, alternate ones of said other dots being of a second color-response characteristic and the intermediate ones of said other dots exhibiting a third colorresponse characteristic. An electron-transparent, light reflecting, metallic (e.g. aluminum) film 17 (FIG. 1)

(eg 18 kv.) to provide a V g (green) are here shown in field designed to influence the target variety the relationship or phor areas.

3, covers the entire target surface of this dot-like mosaicscreen, and forms an electrical connection to the cap 13 of the envelope ll.

The other element or shadow mask of the bi-part screen- unit 17, 19 comprises a suitably curved thin-metal plate 19 containing a multiplicity of apertures 1% arranged in the same systemmatic (hexagonal) pattern as the phosphor screen-dots; there being one mask aperture for each triad (REG) of dots. As indicated by the single electrical connection 21 (FIG. 1) the metallized screen 1'7 and the mask 19 are here operated at the same potential field-free space therebetween, as is usual in color-kinescopes operating on the shadowmask principle. The mask has an integral rim 1% and is supported about said rim on three or more pegs 23 which project radially inward from the inner surface of the cap portion 13 of the envelope. The connection between the pegs 23 and the rim 19b is such as to permit the mask to be removed from the cap during the three (later described) emulsion-coating and developing operations incident to laying down the three color-phosphors (RBG) on the glass screen-plate 15.

The tubular glass neck 3 of the envelope 1 contains a battery of three electron-guns ZSr, 25b and 25g each of which is allotted to a particular screen-color. The guns are here shown arranged delta fashion about the hori- 'zontal axis x-x of the envelope (as in Schroeder USP 2,595,548) so that their beams converge near a surface of the mask 19, where their paths may cross and proceed to the different color-phosphor dots. Alternatively, a single beam may be employed, in which latter case auxiliary means may be provided as, for example, in Jenny 2,611,099, for sequentially shifting the single beam to positions corresponding to that of the several beams in a multi-gun tube.

Referring still to FIG. 1. Here, as in the Schroeder patent, the required horizontal and vertical scanning movements are applied to all three of the electron beams from the guns 25 by a common deflecting yoke 27 which will be understood to comprise two sets of electromagnetic coils (indicated by the double current leads) disposed at right angles to each other on the glass neck 3. The colorcenters r, b, g of the three beams r (red) b (blue) and the normal plane-of-deflection AA, i.e. approximately at the center of the yoke 27.

As in Friend USP 2,719,249 an auxiliary magnetic paths of the three beams is provided by a purity coi 29 mounted on the neck 3 surrounding the horizontal axis xx of the tube. and adjustably energized by a source of current, exemplified by the battery 31 and rheostat 33.

The color-tube of FIG. 1, more or less standard construction. However, it will be observed that the drawing is marked with a horizontal line or axis y-y which, as will hereinafter more fully appear, passes through the center of the mosaic pattern of phosphor dots on the screen, axis x-x, about which the electron guns 25 are grouped, a distance calculated to compensate for the lateral deflection forces to which the electron-beams are subjected by the vertical component of the earths magnetic field. the amount of, and the direction in which, the mosaic pattern of the screen is off-set from the horizontal axis xx of the tube is set forth in the following description of the manner in which the screen-unit is manufactured.

In the manufacture of cathode ray tubes of the masked phosphor dots (or lines) and the mask-apertures is determined, usually, by light rays emanating from a small source located, successively, at points corresponding to the color-centers of the completed tube. When the tube is I operated, electron-beams take the place of the light-rays that were used in plotting the location of the color phos- Hence one might assume that each beam would land squarely upon the particular dot toward which register" of the coloras thus far described, is of and is off-set from the V geographic location at which the it is directed. Such, however, is not the case because the electron beams, unlike the light-rays, are subject to lateral deflection by the presence of the earths magnetic field. Thus, referring now to FIG. 3, the effect of the vertical component of the earths magnetic field, in the absence of compensation, is to deflect the beams r, b and g an appreciable distance to the left, as viewed by an observer looking at the screen from the front of the tube. The amount or degree of misregister produced by the vertical component of the earths magnetic field is the vector summation of the misregister produced by said field in the gun region plus the misregister produced by said field in the space between the plane-of-defiection (AA) and the screen. These two misregister components are oppositely directed and the actual misregister observed in an uncorrected tube is a dilference error, in the direction of the larger component. At any given location the actual distance that the beams are moved oif the centers of the elemental screen-areas (RBG) by the vetrical component of the earths magnetic field is about twice as great in an unshielded glass tube than it is in a metal tube, or in one having a metal shield about its cone. was observed that at Lancaster, Pennsylvania (where the vertical component of the earths magnetic field is approximately 0.53 gauss), the amount of misregister at the center position of the screen of an unshielded 22 rectangular glass tri-color kinescope is approximately 8 mils whereas in a 21" tri-color tube having a metal envelope it is approximately 4 mils.

' The general formula for determining the distance (AS) that the top cap and light-source must be displaced with respect to each other in order to place the phosphor dots at the terminals of the three electron-beams when the latter are'subjected to the lateral deflecting force of the vertical component of the earths magnetic-field, is:

' ured along the axis of the tube.

Assuming now that a mosaic screen-pattern has been plotted or laid down in accordance with'the above formula, then the distance that said pattern should be off-set from the horizontal axis (x-x) of the finished tube is expressed by the formula:

AE= AS As a practical matter, the objects of the invention are achieved in an unshielded tube when AB is from 0.003 to 0.020 of an inch. In a shielded tube (eg one having a metal cone) AE may be of the order of from 0.0015 to 0.010 of an inch. As previously pointed out the direction of the off-set, north of the earths magnetic equator, is to the left of an observer looking directly at the obverse surface of the screen, and south of said equator the oif-setis in the opposite direction, i.e. to the right.

Where mass-production is involved it is not convenient to change the positionof the screen-pattern for each tubes'are to be used. Nor is it necessary to do so since the objects of the invention are fulfilled by the provision of a degree of off-set which permits a reduction in the magnitude of the magnetic field supplied by the, purity coil (or similar magnetic device) to a value which will not noticeably affect the resolution of the electron-images which the tubes are designed to handle. Accordingly, in applying Thus, it

(since AS=.00425 28.2)

the shift of the screen-assembly with respect to the axis (x x) of the tube, required to effect the desired comat a particular color-center, in the tubes plane-ofdeflection, or (b) by leaving it in its said usual position and placing the front-end or top-cap (13) of the tube the required distance off the central axis x-x of the to be mounted in the finished tube. The light-source 37, instead of being mounted for rotation about the cenmosaic pattern impressed thereon by reason of the presence of the apertured mask 19 in the path of the light rays from the point source 37. Assuming that the point 37, in the plane-of-deflection A-A, is the one to be traversed by the red beam of the finished kinescope then the emulsion coating 39 on the inner surface of the glass plate 15 should contain a red-phosphor such, for example as manganese activated zinc phosphate. Since, as previously mentioned, the apertured shadow mask 19 is removably supported on the inner surface of the cap 13 by three or more pins 23, it may be removed from the assembly during the three emulsion-coating and de veloping operations incident to laying down the three (red, blue and green) color-phosphors. When the tube cap 13 is restored pedestal 35 for the blue and the green exposures, the light-source must of course be moved about the new axis zz from exposure to bring green guns,

When the screen p 13 is sealed resp to its indicated axial position on the its position during the previous it to the position of the blue and ectively.

source in the previously described off-axis position the mosaic-pattern of the screen will be vertical component of the olf-set from the for the deflecting eifect of the earths magnetic field upon the beam-electrons employed in scanning the screen.

What is claimed is:

The method of compensating for misregister due to the beam-deflect ing effect of the vertical component of the earths magnetic field upon the beam-electrons in set from a colordefined by Where Pis the said aperture q is the distance between mosaic variety, ing photographically elemental screensaid method comprislaying-down a mosaic pattern of areas upon said target surface with the in said plane-of-deflection at a point oif center in said plane a distance (AS) the formula:

mponent of the earths magnetic field,

distance between said plane-of-deflection and d mask and said mask and screen as measured along said horizontal axis, and thereafter mou tube with dictated by the foregoing formula.

References Cited by the Examiner UNITED STATES PATENTS ROBERT SEGAL, Acting Primary Examiner. RALPH G. NILSON, GEORGE N. WESTBY,

Examiners.

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