US3179836A

US3179836A - Compensator for earth's magnetic field by color dot displacement

Info

Publication number: US3179836A
Application number: US600545A
Authority: US
Inventors: Giuffrida Joseph
Original assignee: Columbia Broadcasting System Inc
Current assignee: CBS Broadcasting Inc
Priority date: 1955-04-27
Filing date: 1956-07-27
Publication date: 1965-04-20
Anticipated expiration: 1982-04-20
Also published as: US3222172A; GB836008A; DE1017647B; FR1148169A; NL206671A

Description

April 20, 1965 J. GIUFFRIDA 3,179,336

COMPENSATOR FOR EARTH'S MAGNETIC FIELD BY COLOR DOT DISPLACEMENT 2 Sheets-Sheet 1 Original Filed April 27, 1955 INVENTOR. Joseph Giuffrido ATTORNEY J. GIUFFRIDA April 20, 1965 GOMPENSATOR FOR EARTH'S MAGNETIC FIELD BY COLOR DOT DISPLACEMENT 2 SheetsSheet 2 Original Filed April 27, 1955 DEFLECTION YOKE PURITY COIL PLANE INVENTOR. Joseph Giuffridcl ATTORNEY United States Patent 8 Claims. (Cl. 313--92) The invention relates to color television in general and to color television picture tubes in particular.

This application is a division of US. patent application entitled, Color Television Picture Tube, Serial No. 504,287, filed April 27, 1955.

The most practical color television picture tube presently manufactured is the so-called shadow mask tube. The shadow mask tube is a modified cathode ray tube having a multi-colored luminescent screen, a perforated shadow mask and three electron guns. Auxiliary components, not ordinarily considered to be part of the tube itself, are usually mounted on the outside of the tube to control and deflect the electron beams emanating from the electron guns.

The screen is composed of three diflferent luminescent materials or phosphors, each producing one of the additive primary colors. In the tubes presently used, the first phosphor produces the color red when excited by electrons; the second, blue; and the third green. There are many phosphors known to the art which have the desired color response when bombarded with electrons. Each of the phosphors is separately deposited on discrete portions of the viewing end or face of the tube envelope to form a symmetrical mosaic. Although the shape of each phosphor element in the mosaic may be varied, it is preferred that each element be circular. The individual elements or dots in the mosaic are arranged in closely spaced triads, each dot in each triad producing one of the primary colors upon excitation.

The shadow mask is usually a thin metallic sheet disposed between the luminescent screen and the electron guns. Apertures are formed in the shadow mask in such a manner that the elements or dots on the screen which produce red may be energized only by electrons which arrive at the screen from a first direction, those which produce green may be energized only by electrons arriving from a second direction and those which produce blue by electrons arriving from a third direction. In a practical tube, there is one aperture in the shadow mask for each triad of dots on the viewing screen.

The electron guns may be of well-known design, but it is preferred that they be mechanically modified for mounting adjacent each other. The electron beams from the guns will then be substantially parallel to each other and equally spaced around the longitudinal axis of the tube. External means are usually provided to cause the electron beams to converge simultaneously on the same aperture in the shadow mask. Thus, each electron beam approaches the shadow mask from a different direction. The proper proportioning of the electron guns and the relative size and position of each aperture in the shadow mask with respect to each of the phosphor dots so restricts each electron beam that it energizes only one typ of phosphor dot.

Although the theory of operation of a shadow mask tube is relatively simple, experience has proven that a practical tube is very diliicult to manufacture. While each component contributes to the difficulty of manufacture, perhaps the most difficult component to make is the multi-color viewing screen. Some idea of the difficulties involved in making the viewing screen may ddlhdlifi Patented Apr. 26, 1965 ice be obtained if one considers that there are approximately 4,000 phosphor dots per square inch of viewing screen and that the tolerance in position of each dot is approximately one one-thousandth of an inch.

Several processes, such as silk screening or printing, have been developed to produce luminescent screens for color television picture tubes. However, the presently preferred method of producing a luminescent screen is shown in detail in the co-pending application of Perry and Rowe, Serial No. 387,192, entitled, Process For Forming Color Screens, and assigned to the same assignee as the present application. The cited application discloses a photographic process whereby images of each aperture in the shadow mask are formed on the luminescent screen by light emanating from a point source. Each dot on the screen, an aperture in the shadow mask and the beam deflection center of the corresponding electron beam are in an optically straight line. Such positioning, in turn, allows registration of any given electron beam on only one color-producing array of dots, provided the trajectory of each electron beam is a straight line.

Unfortunately, the trajectory of each electron beam in a finished tube is not a straight line. The magnetic field of the earth exerts a force on each electron beam of such magnitude and direction that each beam follows a curved path. In all practical tubes used in the northern hemisphere, the path is essentially circular, bending to the right as observed from the electron guns looking toward the screen. The approximate circularity of the path is caused by the vertical component of the earths magnetic field.

Various ways have been tried to minimize the effect of the earths magnetic field. The most obvious way, of course, is to shield the electron beams from the earths field. However, experience has shown that the best magnetic shields, even those made from very permeable metals such as mu metal, are expensive to make and difiicult to use. In fact, adequate shielding has never been attained in practice.

Two known expedients may be cited to illustrate compensation of the curvature of the electron beams in a color picture tube by planned distortion of the electron beams, rather than bymagnetic shielding.

One of these expedients involves the placing of magnets on the outside of the tube adjacent the periphery of the viewing screen to form a so-called color equalizer. The electron beams are allowed. to bend under the influence of the earths field until they come into the field of the peripheral magnets. The peripheral magnets are so arranged and polarized that the path of each electron beam may be sharply bent near the viewing screen to direct each beam to its corresponding phosphor dot. An absolutely uniform field having a predetermined magnitude and direction must be maintained across an area approximately the size of the viewing screen by adjusting the strength and position of a number of independent magnets. Adjustment of the peripheral magnets to obtain an absolutely uniform field is diflicult even under laboratory conditions; after installation in a practical color television receiver, proper adjustment is almost impossible. Moreover, the size of the compensating magnets and their position around the viewing screen forces enlargement of the cabinet in which the tube is installed.

The other expedient referred to involves the use of a so-called purity coil to obtain color registry. The purity coil is an electromagnet mounted on the neck of the picture tube, so that its field is concentrated along a short distance near the electron guns. The coil is energized with direct current to produce a magnetic field.

The intensity and direction of the magnetic field may be adjusted by changing the current and the polarity of the coil current or by rotating the coil around the neck of the tube. The intensity and direction. of the magnetic field, in turn, affect the direction of the electron beams. Although each electron beam is allowed to bend under the influence of the earths field along the greater part of its length, color registration may still be obtained by the proper adjustment of the purity coil field.

The use of a purity coil has some advantages over peripheral magnets, but there are also several different objectionable effects. Perhaps the most objectionable effect of a purity coil is that known as neck shadow. Neck shadow causes a serious problem, namely, the loss of a portion of the visible raster. This loss results from interception of the electron beams by the inner wall of the tube at the junction of the neck and the conical part of the tube whenever a certain critical deflection angle is exceeded. Unfortunately, use of a purity coil makes it more likely that the, critical deflection angle be exceeded. In fact, in an unskilled, all-glass, wide angle tube, neck shadow is almost invariably observed when the purity coil is adjusted to obtain optimum color registration and this phenomenon may even be found in metal cone tubes.

The purity coil is in proximity to and interacts with the deflection, convergence and centering electromagnets usually used with color television picture tubes. The resultant magnetic field is very complex; the design and adjustment of each electromagnet being greatly affected by that of each other electromagnet. Such complexity makes adjustment diflicult for even trained personnel; it is evident that widespread commercial acceptance of color television requires that adjustments be simplified so that untrained persons. may perform them.

The shifting of at least one electron beam away from the tube axis caused by the purity coil imposes greater restrictions on the design of the deflection yoke used with the tube. It is almost impossible in practice to obtain an absolutely uniform magnetic field across an area the size of the neck of a color picture tube. Experience has shown that the field intensity of any practical deflection yoke decreases an unpredictable amount as the distance from the geometric center of the deflection yoke increases. Since the purity coil usually shifts one beam farther from the geometric center of the deflection yoke, such a beam will experience less deflection at any given instant than the beams which are closer to the geometric center of the yoke. As a result, the beams do not converge on the shadow mask at all times and the quality of the picture deteriorates. It should be noted that the same diificulty is encountered regardless of the type of deflection utilized in. the tube.

Even if the deflection yoke is designed so that an absolutely uniform field is obtained, proper convergence of the electron beams cannot be attained at all times if one beam is moved farther from the tube axis than the others. A moments thought will show that convergence can be attained only when the length of each electron beam is kept equal to the length of each of the other beams at all times. If one of the beams is moved farther from the axis of the tube than the others, such a condition is impossible to meet.

A general object of the invention is to obtain color registration in a color television picture tube without apparatus to compensate for the earths magnetic field.

A further object of the invention is to eliminate neck shadow in a color television picture tube.

A still further object of this invention is to improve convergence of the electron beams in a color television picture tube.

For a better understanding of the invention together with other and further objects, features and advantages, reference is made to the following description with reference to the drawing, in each of which corresponding parts are similarly numbered, and in which:

FIG. 1 is a perspective view, partly cut-away, of a color television picture tube embodying the principles of the invention;

FIG. 2 is a diagram of an electron beam path in a color television picture tube showing in particular how the viewing screen may be offset to compensate for curvature of the electron beams in a color television picture tube; and

FIG. 3 is a diagram of a color television picture tube showing how the invention minimizes neck shadow and improves scannlng.

In general, the invention consists in a color television picture tube wherein no compensation for the eflects of the earths magnetic field is required after the tube is fabricated. In the finished tube, the centers of the phosphor dots do not bear a straight line relationship to the electron gun with which they are associated. Rather, a line traced from a given gun to any of the phosphor dots which it is designed to energize follows a curved path. The curvature of each line is such that the electron beams from each gun pass through each aperture in the mask at the same angle as the light beam which originally formed the dot, if the photographic process was used.

Referring now to FIG. 1, an embodiment of the invention in a practical tube may be seen. It should be noted that several simplifications, not material to the invention have been made. The electron beanrforming, focusing and accelerating elements have not-been shown. Standard means to shape and direct the electron beams are well-known in the art and may be applied Without change in a tube embodying the concepts of the present invention. Well-known current waveshapes may be applied to the deflection yoke shown in FIG. 1 to obtain horizontal and vertical deflection of the beams. Only one gun and one beam are depicted to prevent undue complication of the figure. The tube shown in FIG. 1 operates in the following manner. An electron gun 11 is positioned in the neck 12 of the tube 13. The electron beam 14 is directed along the length of the neck 12. A magnetic deflection yoke 15 may be mounted on the outside of the neck 12 as shown, although the type of deflection means used is not material to the invention. The electron beam 14 is shown for convenience, deflected toward the observer in a horizontal plane. After deflection by the deflection yoke 15, which may be considered, without significant error, to take place at point A, the electron beam 14 passes along the length of the tube 13 through an aperture 16 formed in the shadow mask 17 and impinges on a phosphor dot 13. Both apertures and phosphor dots have been greatly exaggerated in size to show more clearly the working of the invention. The electron circuit is completed through a conducting surface (not shown) on the phosphor dot 18 back to the electron gun H. The electron beam 14- follows a substantially circular path from the point A through the aperture 16 to the phosphor dot 18. The electron beam 14 also follows an approximately circular trajectory between its point of origin at the electron gun It and the point A, the path having the same radius of curvature as the path from the point A to the phosphor dot 13.

The earths magnetic field which imparts the curvature to beam 14 is represented by the arrows 19. The inclination of the arrows 19 to the horizontal representsthe dip angle of the earths magnetic field at a latitude of approximately 40 and the direction of the arrows represents the direction of the earths magnetic field 19 in the northern hemisphere. The cross-sectional area of the tube 13 is so small that the field 19 may be assumed to be uniform throughout the tube. A well-known principle of electron motion in a uniform magnetic field may now be applied to determine the trajectory of the electron beam 14. The principle is that if the original velocity of an electron is not normal to the direct-ion of a magnetic field, the velocity component of an electron parallel to the field is not affected either in direction or magnitude while the velocity component normal to the field continually changes direction, remaining constant in magnitude. Strict application of this principle leads one to the conclusion that the trajectory of the electron beam 14 in the tube 13 would be helical. However, consideration of the size of tube 13 and the relative size of the normal and parallel components of the initial electron velocity in the beam 14 shows that a helical path will not result in this practical case. The normal component of the velocity, remaining constant in magnitude, but continually changing direction, causes the beam to describe an approximately circular path. The same considerations indicate that the beam 14, when deflected upwardly or downwardly in tube 13, will follow an essentially circular path which, for any given angle of deflection, occurs entirely in a plane.

The curvature of the electron beam 14 which is aimed at the center of screen 211 may be determined empirically by measuring the distance of the point of impingement of beam 14 from the center of the viewing screen 20 when no deflection waveform is applied to the deflection yoke 15. It is not necessary to actually determine the geometric center of the viewing screen 20 to measure the desired distance. If the point of impingement of the electron beam '14 be noted and the tube be rotated 180, a second point of impingement of the beam 14 on the viewing screen 21) will be observed. One half the distance between the two points of impingement corresponds to the offset of the electron beam 14 due to its curvature. In a practical tube having a beam length of approximately 21", actual measurements have been made to show that the offset of the beam is approximately /2". Of course, to obtain a working value, a number of difierent tubes such as tube 13 should be tested to reduce experimental errors due to misalignment of the individual tubes.

The deflection center of the electron beam, marked A, the center of given aperture 16 on the shadow mask 17 and the center on the corresponding phosphor dot 18 lie on a curved line. This line represents the trajectory of the electron beam 14. Curvature of the electron beam 14 is emphasized by the straight dotted line passing from the center of the phosphor dot 18 through the aperture 16 in the shadow mask and intersecting the deflection plane of the deflection yoke 15 at the point B. Point B represents the position of the dot forming light source which would be used to form the dot 18 and the other similar color generating dots on the viewing screen 20 in the photographic process described above. The de termination of the distance between point A and point B will be explained in detail in connection with FIG. 2.

Referringto FIG. 2, a practical method of calculation of the offset of the point B may be seen. In FIG. 2, the line GAX represents an ideal electron beam emanating from an electron gun at the point G, passing through the point A in the deflection plane of the tube and impinging on a viewing screen at the point X; the arc GY represents the path of an actual electron beam emanating from an electron gun at the point G, passing through a uniform transverse field and impinging on the viewing screen at the point Y; the line GY represents a chord drawn from the point G to Y; the line BY represents a tangent to the trajectory of an actual electron beam at the point Y intersecting the deflection plane of the tube a the point B; the lines (30 and Y0 represent radii of curvature of an actual electron beam; the angle represents the observed displacement of the point of impact of an actual electron beam from the ideal; the line PO represents a construction line drawn from the intersection of the line GAX and the line BY to the intersection of the lines GO and Y0; and the line AB represents the required offset.

To derive a general formula:

(1) Since the triangle OGY is isosceles, angle OGY equals the complement of one-half the angle (2) By construction, the angle XGY is equal to the complement of the angle OGY.

(3) In the tetragon OGPY, the angle GPY is equal to the supplement of the angle (4) Therefore, the angle XPY is equal to the angle (6) Therefore, PX=GY (tan /2/ tan (7) Simplifying, PY=GX (cos /1+cos 5) (9) Since triangle EPA is similar to triangle Y1,

AB X Y=AP/ PX 10) or AB=XY(AP/PX) (11) Substituting, AB:XY(AXGY(cos /1+cos GY(cos /1+cos 41) (12) Simplifying, AB:XY(AX(1+cos qt) -GY cos GY cos In a practical tube,

XY=.5 inch AX 16 inches GY=21 inches Substituting in Equation 12, AB=.26 inch It should be noted that Equation #12 above, may be simplified still further in a practical case. Since the angle is very small, the cosine of the angle is very nearly equal to unity and Equation #12 reduces to the form:'

. FIG. 3 shows how the neck shadow is overcome in a color television picture tube embodying the invention. It should be noted that the electron beam 14a represents the trajectory of an electron beam which has been properly changed by a purity coil to obtain color registration on the viewing screen. The electron beam 14 represents the trajectory of a properly registered electron beam in a tube constructed according to the invention. It is evident that the electron beam 14:! is more likely to be intercepted by the inner wall of the tube in the area D than the electron beam 14. In fact, the point C is ordinarily so close to the area D that the beam 14a is deflected more than the critical angle so that it is partially blocked from reaching the viewing screen 21).

The electron beam 14 cannot be partially blocked since I of electron beams passing through the point C in the manner of beam 14a. It is obvious that a greater correction is required to center such a raster.

The practical difliculties attendant on producing an absolutely uniform magnetic field by a deflection yoke are well-known. Uniform fields are most easily attained over a small area near the geometric center of any coil. In FIG. 3 it may be seen that the electron beam 14 passes through the deflection coil closer to its geometric center than the electron beam 14a. Since the electron beam 14 passes through an inherently more uniform portion of the magnetic field produced by the deflection yoke, the deflection angle of the electron beam 14 may be more closely controlled.

If it be assumed that the electron beam 14 rotates at a constant angular speed about the point A to trace out each horizontal line in the raster in the preferred embodiment of the invention and that the electron beam 14a traces out each horizontal line in the raster of a presently known tube by rotating at a constant angular speed about the point C, it may be seen that the point of impingement of each electron beam on the viewing screen moves at a varying speed as the horizontal line is generated. However, the variations in speed of the point of impingement of the electron beam 14 are less than the variations of the speed of the point of impingement of the beam 14a.

Although the embodiment of the invention shown and described counteracts the effect of the vertical component of the earths magnetic field, only simple modifications would be required to counteract other magnetic fields, such as the horizontal component of the earths magnetic field. Such fields are of minor importance insofar as color registry is concerned. It should also be noted that the effects of electric fields may also be minimized by adaptations of the techniques disclosed herein. Furthermore, anyone having skill in the art will determine means obtaining results similar to those obtained by the invention by dis lacing the neck axis of the tube from the shadow mask and viewing screen. Variations of the invention to compensate for such external fields are within the scope and spirit of the invention.

The invention claimed is:

1. A color television picture tube comprising, an elongated evacuated bulb, at least one source of electrons, means for forming an electron beam adjacent each of said sources of electrons, a shadow mask having a plurality of apertures formed therein and a luminescent screen having a plurality of elements, said source of electrons, said beam-forming means, said shadow mask and said luminescent screen being sequentially disposed inside said bulb, eachof said elements of said luminescent screen being disposed on a line of predetermined curvature corresponding to the curvature imparted to said electron beam by the vertical component of the earths magnetic field passing from one of said sources of electrons through one of said apertures in said shadow mask.

2. In combination with a television picture tube having a plurality of electron beam sources each capable of producing an electron beam and a shadow mask having apertures formed therein, a viewing screen comprising, a pre- 7 determined pattern of luminescent materials deployed in elemental fashion, the elements of said pattern being disposed on predetermined curved lines passing from said electron beam sources through different areas of said apertures in said mask, the curvature of said curved lines corresponding to the curvature of said electron beams when influenced by the vertical component of the earths magnetic field. V

3. In a television picture tube having a plurality of electron beam sources, electron beams emanating from said sources and impinging on a luminescent viewing screen, said electron beams being curved by the earths magnetic field in transit between said sources and said luminescent screen, and a shadow mask having predetermined apertures formed therein, a viewing screen comprising a predetermined array of luminescent materials, each element of said array being disposed on said viewing screen on a straight line passing through each of said apertures in said shadow mask and the apparent origin of each of said electron beams.

4. A color television picture tube having at least one source of electrons, a shadow mask having a plurality of apertures formed therein, and a luminescent viewing screen adjacent said shadow mask and having active portions similar in shape to said apertures, said active portions of said luminescent screen being ofifset a predetermined distance from the point of intersection with said screen of straight lines drawn from each said source through said apertures in said mask to compensate for the curvature of the path traced by unshielded electrons from each of said sources when subjected to the vertical component of the earths magnetic field.

5. A cathode ray tube comprising an evacuated envelope having a horizontal axis, a source of beam electrons mounted adjacent to said axis near one end thereof, a screen unit extending across said axis adjacent to the opposite end thereof in a position to be scanned by beamelectrons from said source, said screen-unit comprising a mask containing a multiplicity of systematically arranged apertures and a screen having a mosaic target surface made-up of a multiplicity of groups of elemental areas arranged in a pattern which is systematically related to the pattern of apertures in said mask, the pattern of apertures in said mask being centered on said horizontal axis and the mosaic pattern on the target surface of said screen being off-set from said horizontal axis in a direction and to the extent required substantially to compensate for the deflecting effect of the vertical component of the earths magnetic field upon said beamelectrons. V

6. The invention as set forth in claim 5 and wherein said cathode ray tube is located to the north of the earths magnetic equator and the direction in which the mosaic pattern of said screen is offset from said horizontal axis is to the left as viewed by an observer looking directly at the obverse surface of said screen from a point on said horizontal axis.

7. The invention as set forth in claim 5 and wherein said cathode ray tube is located to the south of the earths magnetic equator and the direction in which the mosaic pattern of said screen is offset from said horizontal axis is to the right of an observer looking directly at the obverse surface of said screen from a point on said horizontal axis. 7

8. A color television picture tube as set forth in claim 1 wherein said tube is located north of the equator and said elements of said luminescent screen are offset to the left, as viewed from the front of the tube, with respect to the said shadow mask such that straight lines from one of said sources of electrons through the apertures in said shadow mask intersect said luminescent screen to the right, as viewed from the front of the tube, of said elements of said luminescent screen.

References Cited by the Examiner UNITED STATES PATENTS 2,733,366 1/56 Grimm et a1 313-925 2,755,402 7/56 Morrell 3l392.5 2,917,276 12/57 Epstein 3l392.5 X

GEORGE N. WESTBY, Primary Examiner. RALPH G. NELSON, Examiner.

Claims

3. IN A TELEVISION PICTURE TUBE HAVING A PLURALITY OF ELECTRON BEAMS SOURCES, ELECTRON BEAMS EMANATING SAID SOURCES AND IMPINGING ON A LUMINESCENT VIEWING SCREEN, SAID ELECTRON BEAMS BEING CURVED BY THE EARTH''S MAGNETIC FIELD IN TRANSIT BETWEEN SAID SOURCES AND SAID LUMINESCENT SCREEN, AND A SHADOW MASK HAVING PREDETERMINED APERTURES FORMED THEREIN, A VIEWING SCREEN COMPRISING A PREDETERMINED ARRAY OF LUMINESCENT MATERIALS, EACH ELEMENT OF SAID ARRAY BEING DISPOSED ON SAID VIEWING SCREEN ON A STRAIGHT LINE PASSING THROUGH EACH OF SAID APERTURES IN SAID SHADOW MASK AND THE APPARENT ORIGIN OF EACH OF SAID ELECTRON BEAMS.