US3600628A

US3600628A - Cathode ray tube projection display system

Info

Publication number: US3600628A
Application number: US859978A
Authority: US
Inventors: Charles R Corpew; Paul H Gleichauf
Original assignee: Stromberg Datagraphix Inc
Current assignee: ANACOMP Inc 11550 NORTH MERIDAN STREET CARMEL INDIANA 46032 A CORP OF INDIANA
Priority date: 1969-09-22
Filing date: 1969-09-22
Publication date: 1971-08-17
Anticipated expiration: 1988-08-17

Abstract

A display system using a cathode ray tube of the shaped beam type is disclosed. A Schmidt-type optical system is used to project an image from the tube face onto a large surface. In order to provide the desired projected image brightness and definition, a shaped beam tube having three electromagnetic deflection yokes positioned along the tube in a substantially unipotential region to accomplish aperture selection, beam convergence and beam realignment is provided.

Description

United States Patent Inventors Charles R. Corpew La Mesa; 1 1 Paul H. Gieichauf, La Jolla, both of, Calif. 859,978

Sept. 22, 1969 Aug.l7, 1971 Stromberg Datagraphix, Inc. San Diego, Calif. Continuation-impart of application Ser. No. 694,686, 119 22119 Rat -1 N 3,473,077.

Appl. No. Filed Patented Assignee CATHODE RAY TUBE PROJECTION DISPLAY SYSTEM r 9 Claims, 3 Drawing Figs.

US. Cl 315/27, l78/7.92, 315/18, 315/17 Int. Cl H01] 29/70 Field of Search..; 315/17, 18,

27; ITS/7.92

[56] References Cited 1 UNITED STATES PATENTS 2,803,769 8 1957 McNaney 315/17x 2,811,668 10/1957 McNaney 315/17 3,473,077 10/1969 Corpew 315/18 Primary Ekaminer- Rodney D. Bennett, Jr. Assistant Examiner -J. M. Potenza Attorney-John R. Duncan ABSTRACT: A display system using a cathode ray tube of the shaped beam type is disclosed. A Schmidt-type optical system is used to project an image from the tube face onto a large surface. in order to provide the desired projected image brightness and definition, a shaped beam tube having three electromagnetic deflection yokes positioned along the tube in a substantially ,unipotential region to accomplish aperture selection, beam convergence and beam realignment is provided. I

PATENTED AUBI new 3.600.628

SHEEI 2 OF 2 INVENTORS.

CHARLES R. CORPEW BY PAUL H. GLEICHAUF #MLNW ATTORNEY CATHODE RAY TUBE PROJECTION DISPLAYSYSTEM CROSSREFERENCE TO RELATED APPLICATIONS IIOII.

BACKGROUND OF THE INVENTION Cathode ray tubes of the shaped beani type are used for generating alphanumeric characters, line segments of graphs and charts, and similar configurations. In cathode ray tubes of the shaped beam type, one or more electron beams are shaped as'they pass from an'electron gun to a target such that the resulting cross section of each shaped beam is of predetermined configuration. At the target which, for example, may be a phosphor coated screen, an area is energized or illuminated corresponding in shape to the shape of the beam. The target inay be constructed, for example, to provide a visible display or for photographing onto film, such as microfilm. The display may also be utilized in connection with printing apparatus, such as an electrostatic printer. I

The desired beam cross sectional configuration is attained by passing the beam through one of a plurality of shaping apertures in an electron opaque plate or stencil disposed perpendicularly to the initial axial path of the beam. The apertures usually are distributed on the stencil in the form of a matrix, the electron beam being deflected from its initial axial path to pass through the selected aperture. After passing through the aperture, the beam is then redirected to the axis. In addition, the beam is generally focused, in some convenient region after passing through the aperture, to provide sharp imaging on the target. The beam is then deflected to a predetermined position on the target by an electromagnetic deflection yoke or similar means. The various elements which accomplish the directing and focusing of the electron beam may be disposed inside or outside of the cathode ray tube envelope. As used in this application, the term cathode ray tube is intended to include associated elements disposed either inside or outside of the tube envelope.

Should the beam be off axis or of large cross sectional area at the point of final deflection to a position on the target, the deflection usually causes undue distortion of the character on the target, thereby rendering such character partially or totally unintelligible.

It is therefore desirable that the beam be redirected from its initial deviating path from the tube axis back toward the axis to cross the axis near the point of final deflection. To do this, previously known systems have generally utilized a convergence lens. The convergence lens, which may be either electrostatic or electromagnetic, not only redirects the electron beam to the convergence or crossover point on the axis but, in addition, effects lens action upon the beam to focus the beam to a minimal cross sectional area at the crossover point. The beam is then redirected along the axis by a suitable reference deflection system. A cathode ray tube of the foregoing described type is shown and described in U.S. Pat. No. 2,824,250, assigned to the assignee of the present invention.

In order to minimize distortion, convergence lenses in shaped beam tubes of the type described have heretofore been made relatively large. This is because, under most conditions, only a small portion near the center or electro-optical axis of an electrostatic or electromagnetic focusing field is of sufficient uniformity to avoid distortion. Although relatively large diameter convergence lenses are satisfactory in many cases, and have been successfully applied in many shaped beam tubes where wide beam deviations are desired, relatively large diameter convergence lenses may be undesirable under some conditions. Reduction of convergence lens size, however, may

introduce undesired focusing aberrations unless the beam deviation is reduced. This limitation on the amount of deviation of the beam from the axis may result in a corresponding limitation in matrix size, thus limiting the number of apertures which may be used. Large aperture size would, however, be desirable since it increases the display brightness. The lowest possible magnification from the matrix aperture character to the character displayed on the screen will produce the greatest possible brightness when the limiting current density has been reached. The convergence lenses used in previous tubes have provided electric or magnetic fields having their principal components parallel to the axis of the tube and having cylindrical symmetry about the tube axis.

As mentioned above, shaped beam cathode ray tubes have been used for a wide number of purposes, including visible display of data for direct observation. Because of the abovedescribed problems, however, shaped beam tubes have not been satisfactory for use in display systems of the Schmidt projection type, such as is described in U.S. Pat. No. 2,273,801 to Landis.

Such display systems are highly desirable for displaying information and data on large screens for entertainment, educational and business purposes. While shaped beam tubes produce images having excellent resolution, they have not in the past given sufiiciently bright images, nor been able to provide the large number of different characters often required.

Conventional video tubes have also generally been unsatisfactory for use in such large screen display systems because of their relatively low image resolution and short tube life when operated at the necessaryvery high electron acceleration levels.

Thus, there is a continuing need for improved large-screen cathode ray tube display systems.

SUMMARY OF THE INVENTION It is, therefore, an object of this invention to provide a display system overcoming the above-noted problems.

Another object of this invention is to provide a cathode ray tube large screen display system of improved resolution and brightness.

A further object of this invention is to provide a cathode ray tube large screen display system having an increased useful tube life.

The above objects, and others are accomplished in accordance with this invention by providing a Schmidt-optics projection system including a cathode ray tube image source, a concave mirror receiving the image from said tube and projecting it onto a large surface, and optical aberration correcting means in the path of said image; said tube being of the shaped beam type in which aperture selection, beam convergence and beam realignment are accomplished by three electromagnetic deflection yokes positioned along the tube in a substantially unipotential region.

This tube provides the desired high image resolution characteristic of shaped beam tubes. Ordinarily, though such tubes have insufficient brightness for use in projection systems. Surprisingly, however, the present tube has much greater brightness, when constructed and operated as described herein, and is eminently satisfactory for use in such systems. In addition, this tube has a surprisingly long useful life, apparently due to the fact that, since the electron beam is spread out over the entire alpha-numeric shape rather than being concentrated in a conventional spot which results in a lower duty cycle of phosphor excitation. Therefore, even at very high electron acceleration phosphor burn is substantially reduced.

DETAILED DESCRIPTION OF THE INVENTION Other details of the invention will become apparent to those skilled in the art from the following description, taken in connection with the accompanying drawings wherein:

FIG. 1 is a schematic sectional view of a cathode ray tube constructed for use in accordance with the invention;

FIG. 2 is a schematic sectional view of an alternative embodiment of the cathode ray tube; and

FIG. 3 shows a schematic representation of the cathode ray tube image projection system of this invention.

Basically, the cathode ray tube of the invention comprises an elongated envelope having at one end therein an electron gun 11 for producing an electron beam aligned on an electro-optical axis 12. A target or screen 13 is provided at the other end of the envelope 10 and a stencil 14 is disposed between the electron beam gun and the screen. The stencil has a plurality of apertures 16 therein for shaping the electron beam cross section, each aperture producing a predetermined shape. A first deflection yoke 17 is positioned between the electron gun and the stencil for deflecting the electron beam from the electro-optical axis to select one of the apertures. A second deflection yoke 18 is positioned at the stencil for deflecting the electron beam in a path convergent with the electro-optical axis. A third deflection yoke 19 is positioned between the stencil and the screen fordeflecting the electron beam from the convergent path to a path coincident with the electro-optical axis. A fourth deflection yoke 21 is positioned between the third deflection yoke and the screen for deflecting the electron beam to impinge on a predetermined area of the screen.

The cathode ray tube includes the envelope 10, which may be made of glass, having an .enlarged bell-shaped section 22 and a long slender neck section 23 appended thereto. For purposes of description, the end of the tube toward the right in the drawings is referred to as the rear end. The generally frustoconical section 22 of the glass envelope is provided with a face plate 24, and the target or screen 13, of a suitable phosphor or electro-luminescent material, is deposited on the inside of the face plate 24.

The rear end of the neck section 23 is provided with a glass end cap 28 through which suitable electrical connectors 29 extend in a sealed relationship thereto. Electrical leads, not illustrated, extend from the connectors to the various internal elements of the tube for maintaining potentials thereon, as will be described. Some of the connectors 29 are also utilized to supply a heating current for the filament, not illustrated, inside the cathode described below.

The electron beam gun 11 is disposed at the rear end of the neck section 23 and includes a cup-shaped cathode 31 coated with electron emissive material, such as barium oxide, on the exterior surface of its closed end. The cathode is supported by suitable means, not shown, on that its electro-optical axis 12 of the tube. A heater wire or filament, not illustrated, positioned inside the cathode cup 31, raises the temperature of the cathode and causes emission of electrons from the coated closed end surface. A cup shaped control electrode 32 is positioned coaxially about the cathode and is supported by means, not shown, so that an aperture 33 therein is positioned along the axis 12. By applying suitable potentials to the control electrode 32, the electron beam may be turned on or shut off as desired. With the control electrode slightly negative with respect to the cathode 31, for example -l0 volts, an electron beam passes through the aperture 33. This beam is accelerated by a suitable positive potential on a generally cylindrical accelerating electrode or anode 36 disposed in the neck section in front of the control electrode.

A generally cylindrical focusing electrode 34 is provided inside the neck section 23 after the first anode 36, and two additional

cylindrical electrodes

37 and 38 are provided after the focusing electrode 34. The

electrodes

34,37 and 38, together with the anode 36, are suitably supported in axial alignment and are maintained at appropriate potentials to form a series of convergent electrostatic lenses. These lenses focus the electron beam so that its cross sectional area is slightly larger than the cross sectional area of a trim aperture 41 provided in an electron opaque trim aperture plate 42. The plate 42 is supported within the electrode 38 by a generally annular plate support member 45. The electron beam, as it leaves the aperture 41, has a cross section conforming to the shape of the aperture cross section, and is desirably square or rectangular.

The rectangular or square electron beam is then focused by a pair of electrostatic lenses established between the forwardmost focusing electrode 38 and another cylindrical focusing electrode 39 and between the latter mentioned focusing electrode 39 and still another cylindrical electrode or second anode 43. The

electrodes

39 and 43 are supported in axial alignment by suitable means, not shown. The focusing action is such that the beam cross section at the electron opaque plate or stencil 14 is slightly greater than the area occupied by one of a plurality of apertures 16 in the stencil. in this manner, the beam will flood the entire aperture at which it is directed and emerge on the other side of the stencil 14, having a cross section conforming to the shape of the aperture in the stencil through which it passed. The stencil 14, which may be'of conventional construction but generally with larger or a greater number of apertures, is supported perpendicularly to the beam axis 12 by an annular stencil support structure 46 positioned within the second anode 43.

In order to cause deviation of the electron beam from the axis 12 to select a predetermined aperture 16, and to return the beam to its former axial path, the three

deflection yokes

17, 18 and 19 are provided. The deflection yokes are supported externally of the neck section 23 by means, not shown, so that the

yokes

17, 18 and 19 are respectively disposed rearward of, at, and forward of the stencil 14, the rearward and forward yokes 17 and 19 being equally spaced from the center yoke 18. Each

deflection yoke

17, 18 and 19 is provided with an X deflection winding and a Y deflection winding, schematically illustrated, as is well known in the art. Each winding establishes a magnetic field having a principal component generally perpendicular to the axis 12 of the tube. Suitable currents supplied to the X and Y windings of each deflection yoke determine the amount of deflection of the electron beam as it passes through the magnetic fields established by the yokes. Such currents are provided, in the illustrated embodiment, by suitable character selection circuits, indicated generally at 47. Although the character selection circuits may include separate amplifiers for driving each of the X and Y windings in each yoke, a substantial saving in cost may be achieved by connecting the X windings of the

yokes

17, 18 and 19 in series such that they may be driven by a single amplifier. Similarly, the Y windings of the

yokes

17, 18 and 19 are also connected in series to be driven by a single amplifier. The X windings in the yokes 17 and 19 are identically wound, whereas the X winding in the yoke 18 is wound oppositely. Similarly, the Y windings in the yokes 17 and 19 are identically wound whereas the Y winding in the yoke 18 is wound oppositely.

The magnetic field established by the currents in the rearward deflection yoke 17 causes the electron beam within the magnetic field of the yoke to deflect from its axial path. Application of the proper currents in yokes 17 and 18 causes the beam to select any one of the character apertures 16 in the stencil 14.

After passing through the selected aperture in the stencil 14, the beam is turned from its divergent path back toward the electro-optical axis 12 of the tube by the magnetic field established by the center yoke 18. In the illustrated configuration, such turning or deflection occurs both before and after the beam passes through an aperture in the stencil, since the stencil is at about the midpoint of the second yokes field. If desired, however, the second yoke may be shifted to a position on either side of the stencil to provide all of the deflection of the beam either before or after it passes through an aperture.

The angle through which the beam is deflected by the center yoke 18 is nominally twice the deflection angle imparted by the rearward yoke 17, and is in the opposite direction. Since in series connected yokes the current is the same in each yoke, the yoke 18 is provided either with twice the number of turns on its coils, or with twice the axial length of yoke 17, to give it twice the deflection sensitivity. The field of the yoke 18 is made opposite in direction either by winding its coils in the opposite sense or by crossing the connecting wires between the yoke 17 and the yoke 18. The correction provided to the beam by' the center deflection yoke 18 is sufficient to cause the beam to intersect the axis 12 at about the center of the forward yoke 19.

The forward deflection yoke 19 operates to deflect the electron beam an amount sufficient to cause itto be coaxial, once again, with the electro-optical axis 12 of the tube. Since the windings in the forward deflection yoke 19 are identical with the windings in the rearward deflection yoke 17, the direction of deflection is the same. The amount of current flowing in the forward yoke 19 is also identical to that flowing in the rearward yoke 17 and, consequently, the amount of correction is just sufficient to compensate the beam for its initial deviation. In producing the previously described deflections for aperture selection, focusing action on the beam is held to minimum levels. This is accomplished by maintaining the third anode 48, which define the region wherein the beam is deflected, at substantially the same potential, thereby creating a drift region or unipotential region wherein virtually no acceleration, deceleration, or focusing occurs. Distortions introduced by lens aberrations are thereby minimized. If high high speed operation is required, the second and

third anodes

43 and 48 may be provided with a number of longitudinal gaps (not shown) to prevent the anodes from acting as shorted turns.

The second and

third anodes

43 and 48 may be replaced by a single cylindrical electrode if individual amplifiers and gain controls are provided for driving each of the

yokes

17, 18 and 19. Such gain controls permit adjustment to compensate for deviation of the fields established by the yokes within manufacturing tolerances. In the illustrated embodiment, however, the corresponding X and Y windings of

the'yokes

17, 18 and 19 are connected in series to reduce the required number of yoke driving circuits. To allow for variation in the gain of the

yokes

17, 18 and 19, an adjusting circuit49 is connected to the third anode 48. The adjusting circuit 49 is constructed to permit adjustment of the potential of the third anode over a very narrow range near the potential of the second anode 43, to thereby adjust the velocity of electrons in the electron beam. By changing the velocity of the electrons in the electron beam, they may be made more or less subject to the influence of the magnetic field through which they are passing. Accordingly, the deflection of the beam may be adjusted to compensate for variation in field strength due to variation of the fields of the yokes within manufacturing tolerances. The effect of varying the deflection sensitivity of the center and forward yokes 18 and 19 by variation of the potential on the third anode 48 produces little or no significant focusing action where only a slight difference in potential between the third anode 48 and the second anode 43 exists as shown in FIG. 1.

After passing the forward deflection yoke 19, the electron beam, which is once again on the axis 12 of the tube, is focused by a pair of suitably supported cylindrical focusing electrodes 51 and 52, acting with the third anode 48 and an aquadag coating 53 provided on the inner surface of the frustoconical section. The

electrodes

48, 51, and 52 and the aquadag coating 53 form convergent electrostatic lenses which focus the character-shaped beam on the screen 13. The size of the displayed characters may be selected by varying the voltages on the electrodes 51 and 52 to vary the focusing of the lenses. The final deflection yoke 21 is coupled to character positioning circuits 54 to deflect the shaped electron beam to any desired position on the screen 13.

The cathode ray tube of the invention provides satisfactory operation with lowdistortion. It may be noted that each time the electron beam passes through a focusing region or electrostatic lens, the beam is on the axis 12 of the tube and hence, in the region of minimal distortion. Accordingly, distortion of displayed characters is minimized. In the region where the electron beam is displaced from the axis of the tube, the potential is uniform, with virtually no acceleration, deceleration or focusing. As a result, little distortion occurs in the regron.

Because the region of off-axis deviation of the beam is unipotential, further deviation of the beam is unipotential, further deviation of the beam from the axis is possible than where convergence lenses are utilized. Thus,- the foregoing described design makes possible the use of a stencil with a much larger matrix of apertures therein for a given tube-neck diameter. A greater number of character shapes or much larger characters for increased brightness are therefore possible. Moreover, with the series connected arrangement of deflection yokes, as above described, the number of driving circuits required is reduced over that required in many prior art arrangements. In addition, the diameter of the neck section 23 is smaller than in many prior art arrangements, increasing deflection sensitivity, since the diameter of yoke 21 may be correspondingly smaller.

In a display system, this increased number of available characters is especially important since it permits the display of a large number of specialized symbols in addition to the usual alphanumeric characters. Also, larger character openings may be used in the matrix, increasing the brightness of the display image. For example, where prior shaped beam tubes often used matrix character heights of about 9-12 mils, the present tube is capable of utilizing a complete matrix having character heights of, typically, 3540 mils. An increase of character height from 12 to 40 mils increases the area of the character aperture by a factor of more than 11. Thus, when beam current density at the matrix 14 is at its maximum level, the beam current of the present tube is at least ll times greater than for prior tubes. This 1 l-fold increase in beam current and, hence, intensity makes the use of the present tube in a Schmidt projection system practical.

Another advantage accruing from the invention is that power supply requirements are reduced over the requirements of many prior art devices. In prior art devices utilizing electrostatic deflection of the electron beam for aperture selection, it has frequently been desirable to operate the deflection plates near ground potential in order that their driving power supplies need not be floated" at high potential. The fixed potentials of the cathode and the aquadag coating therefore are considerably minus or plus, respectively, from the operating potential of the plates. As a result, two high-voltage power supplies may be required, one for maintaining the cathode potential at a high negative value and one for maintaining the aquadag coating potential at a high positive value. In the cathode ray tube of the invention, because electromagnetic deflection is utilized, either the aquadag coating or the cathode (usually the cathode) may be maintained at ground potential. Accordingly, only a single power supply is needed that required for driving the high negative elements (cathode) or the high positive elements (aquadag coating).

Referring now to FIG. 2, an alternative embodiment of the cathode ray tube is illustrated. Components of the device in FIG. 2 having functions substantially the same as corresponding elements in the embodiment of FIG. 1 have been given the same reference numerals preceded by a 1. Operation of the deflection yokes 117, 118 and 119 is as described in connection with FIG. 1. The same is true for the cathode and grid, 131 and 132, and the accelerating anode 134 and plate 142. The deflection yoke 121 for character positioning is also operable as described in FIG. 1.

The difference in the embodiment of FIG. 2 over that of FIG. 1 is that, in place of electrostatic focusing elements, all focusing is accomplished electromagnetically. An electromagnetic focusing coil 156 is provided between the cathode 131 and the deflection yoke 117. Similarly, an electromagnetic focusing coil 157 is provided between the deflection yoke 1 19 and the deflection yoke 121. The focusing action on the electron beam produced by the coil 156 is similar to the focusing action produced on the beam by the arrangement of the

electrodes

37, 38 and 39 in FIG. 1. Similarly, the focusing action provided on the electron beam by the focusing coil 157 is similar to that provided by the focusing electrodes 51 and 51 in FIG. 1.

By utilizing electromagnetic focusing as well as electromagnetic deflection, as described in connection with FIG. 2, the initial cost of the total system may be higher than some prior art devices. The system, however, is less complex since the power supply need not contain the high-voltage elements required for typical electrostatic focusing arrangements. The latter are often unreliable and expensive. Moreover, the tube itself is easier and less expensive to build, since there are fewer internal elements. Such a tube is consequently less expensive to replace and, over the life of the equipment, this may provide an advantage that more than offsets a higher initial cost.

Referring now to FIG. 3, there is seen a schematic representation of a Schmidt-type projection system according to this invention. In this embodiment, the cathode ray tube is arranged with face plate 224 facing downwardly towards a concave reflecting surface 250. Although surface 250 is preferably spherical, suitable aspheric surfaces may be used, if desired. The neck 223 of the tube extends upwardly through a housing 251 which contains the various deflection yokes shown in FIGS. 1 and 2. A corrector lens 252 is located around housing 251 to correct spherical aberration introduced by concave reflector 250.

When an image is produced in the phosphor layer on face plate 224, light rays typified by

rays

255 and 256 leaving point 253 on face plate 224 are reflected and focused by concave reflector 250. The reflected rays reach a first surface reflector 254 which directs them to focus at point 260 on screen 261. As seen in FIG. 3, screen 261 is a translucent rear projection screen, viewed from the side opposite to that upon which light rays 256 impinge. The optical axis of the system is indicated by centerline 262.

Face plate 224 is preferably curved to match the curvature or reflecting surface 250 to eliminate distortion caused by curvature of field.

While the arrangement of components shown in FIG. 3 is preferred, other arrangements of cathode ray tube, concave reflecting surface, correcting lens and screen may be used, if desired. For example, construction such as those shown in US. Pat. No. 2,295,779 to Epstein may be used if desired. Similarly, if desired a small portion of reflecting surface 250 may be made nonreflecting immediately opposite face plate 224 to prevent any objectionable direct reflection of light back to the face plate from the reflecting surface.

A typical shaped beam cathode ray tube of the sort described above may have a face plate diameter of about 5 inches and an image area of about 3 by 4 inches. If it is desired to project this image to fill a 30 by 40 inch screen, the area enlargement is about 100 to 1, resulting in a 100 to 1 reduction in brightness. Projection systems using lens systems rather than reflective systems of have transmission efiiciencies of less than 10 percent. I-Iowever, reflective systems of the sort shown in FIG. 3 have light transmission efficiencies of 25 percent or greater. Even with such a system, the reduction in brightness will be nearly 400 to l with the area enlargement described above. Prior cathode ray tubes were not capable of producing projected images of acceptable resolution and brightness under these conditions. However, it has now been found that satisfactory brightness and excellent image resolution may be obtained where the high brightness shaped beam tube described above is used in a Schmidt optical system as is shown in FIG. 3.

Various modifications of the invention, in addition to those shown and described herein, will become apparent to those skilled in the art from the foregoing description and accompanying drawings. Such modifications are intended to fall within the scope of this invention as defined by the appended claims.

We claim:

1. A projection system comprising a shaped beam cathode ray tube; a concave reflecting surface spaced adjacent the face of said tube; a screen positioned to receive images reflected by said concave reflecting surface, and optical aberration correcting means located in the optical path between said tube face and said screen; said tube comprising an electron gun for producing an electron beam directed along an electro'optical axis, a target adjacent the tube face, a stencil disposed between said electron beam gun and said target and having a plurality of apertures for shaping the cross section of the electron beam, a first deflection yoke positioned between said electron beam gun and said stencil for deflecting the electron beam from the electro-optical axis to a selected one of said apertures, a second deflection yoke positioned near said stencil for deflecting the electron beam to a path convergent with the electro-optical axis, and a third deflection yoke positioned between said stencil and said target for deflecting the electron beam from the convergent path to a path substantially coincident with the electro-optical axis.

2. The projection system according to claim 1 wherein said concave reflecting surface is spherical.

3. The projection system according to claim 2 wherein said target is supported on said tube face and has a spherically curved surface concentric with said spherical reflecting surface.

4. The projection system according to claim 1 wherein said aberration correcting means consists of an annular corrector lens surrounding the neck of said tube.

5. The projection system according to claim 1 wherein each of said deflection yokes includes an X deflection winding and a Y deflection winding, wherein said X deflection windings are connected in series and wherein said Y deflection windings are connected in series, and wherein the windings of said second deflection yoke are wound and connected to produce magnetic fields opposite in directions to the directions of the corresponding fields of said first and third deflection yokes.

6. The projection system according to claim 1 including a fourth deflection yoke positioned between said third deflection yoke and said target for deflecting the electron beam to impinge on a predetermined position on said target, and focusing means positioned between said third deflection yoke and said fourth deflection yoke.

7. A projection system comprising a shaped beam cathode ray tube; a concave spherical reflecting surface spaced adjacent the face of said tube; a screen positioned to receive images reflected by said spherical reflecting surface, and spherical aberration correcting means located in the optical path between said tube face and said screen; said tube comprising an electron beam gun for producing an electron beam directed along an electro-optical axis, a target adjacent to the tube face, a stencil disposed between said electron beam gun and said target and having a plurality of apertures for shaping the cross section of the electron beam, a first deflection yoke positioned between said electron beam gun and said stencil for deflecting the electron beam from the electro-optical axis to a selected one of said apertures, a second deflection yoke positioned near said stencil for deflecting the electron beam to a path convergent with the electro-optical axis, a third deflection yoke positioned between said stencil and said target for deflecting the electron beam from the convergent path to a path substantially coincident with the electro-optical axis, and focusing means positioned between said third deflection yoke and said target.

8. The projection system according to claim 7 wherein said target is supported on said tube face and has a spherically curved surface concentric with said spherical reflecting surface.

9. The projection system according to claim 1 wherein said yokes are positioned along the neck of said tube within a housing, and said spherical aberration correcting means consists of an annular corrector lens surrounding said housing.

Claims

1. A projection system comprising a shaped beam cathode ray tube; a concave reflecting surface spaced adjacent the face of said tube; a screen positioned to receive images reflected by said concave reflecting surface, and optical aberration correcting means located in the optical path between said tube face and said screen; said tube comprising an electron gun for producing an electron beam directed along an electro-optical axis, a target adjacent the tube face, a stencil disposed between said electron beam gun and said target and having a plurality of apertures for shaping the cross section of the electron beam, a first deflection yoke positioned between said electron beam gun and said stencil for deflecting the electron beam from the electro-optical axis to a selected one of said apertures, a second deflection yoke positioned near said stencil for deflecting the electron beam to a path convergent with the electro-optical axis, and a third deflection yoke positioned between said stencil and said target for deflecting the electron beam from the convergent path to a path substantially coincident with the electro-optical axis.