US2203483A - Cathode ray tube - Google Patents

Description

June 4, 1940. G. B. BANKS 2,203,483

CATHODE RAY TUBE Filed Jan. 15, 1937 lZig-i 06ft iCTIA/f (OILS INVENTOR GEORGE BALDWIN BANKS ATTORNEY Patented June 4, 1940 PATENT OFFICE GATHODE RAY TUBE George Baldwin Banks, Chelmsford, England, assignor, to Radio Corporation of America, a corporation of Delaware Application January 15, 1937, SerialNo. 120,706 In Great Britain January 24, 1936 r 10 Claims. (01. 250-453) This invention relates to cathode ray tubes and is applicable both to tubes of the oscillograph type, such as are commonly employed for television reception and like purposes, and also totubes of the type sometimes termed transmitter tubes such as are commonly employed for television transmission purposes. In the former type of tube thescreen electrode towards which the cathode ray is. projected is usually prepared with or carries a layer of fluorescent material and in the latter type of tube (so-called iconoscope) the screen electrode consists of a mosaic of minute photo-sensitive cells.

In cathode ray tubes of the types referred to,

it is commonly desired to apply a relatively high potential to the screen electrode towards which the cathode ray is projected. For example, in the oscillograph type of tube with a fluorescent screen, it is common to constitute the fluorescent 1; screen by a layer of fluorescent material upon a metal plate in order that highintensity of illumination may be obtained without burning of the screen. For television receiver cathode ray tubes which are to be utilized for projection purposes, this use of a metal plate as part of the screen structure is, in practice, almost a necessity. Again, in the iconoscope type of tube a metal back plate forms part of the screen. structure, this back plate cooperating with the tiny parconstitute a plurality of minute condensers having a common electrode In this and in similar cases where the metal plate forms part of the screen it is common to tilt the screen at an. angle of about 45" to the axis of the electron gun in order to facilitate. the. projection of imageson to the mosaic screen, or to facilitate viewing of or projection of images built up on the fluorescent screen, as the case may be. t 40 There are a number of serious objections to the use of a screen structure tilted at 45 to the gun axis. Referring to the drawing, Fig. 1 shows diagrammatically the path travel. of a cathode ray beam for purposes of explaining the invention while Fig. 2 shows schematically one embodiment of the invention. One of these objections is as maybe seen from Figure 1 of the accompanying drawing that the length of the path of the cathode ray from gun to screen: is: not constant (compare lengths OA and OB for positions A and Bof the ray) as the ray explores the screen and this involves that sharp focusing of the cathode ray is not obtained over the whole screen area, This ,obj ection is particularly serious jinz the fluorescentl screen type of tube and comticles of photo-electric material in the mosaic to pensation for such focusing error is difficult, especially where magnetic focusing of the ray is employed. A second disadvantage is that if the deflecting forces applied to produce scanning action of the ray are such as would give a rectangular scanned area upon a screen perpendicular to the gun axis, the area actually scanned upon a tilted screen is not a rectangle but a trapezium, and although this type of distortion can be corrected for, the correction involves a certain amount of difiiculty and/or provisionof additional apparatus.

The present invention avoids the disadvantages and difficulties above referred to.

According to this invention a cathode ray tube comprises an electron gun and a screen structure which is mounted transversely withrespect to the gun axis and is also off-set from the gun axis and means are provided for setting up between the gun and the screena ray deflecting field having lines. of force running parallel to the surface of the screen, the section of this field (viewed in the direction of the said lines of force) being v such that it bends the cathode rays through the angle necessary to cause it to fall upon the screen and in such manner that the path traced upon the screen as the result of the application of predetermineddeflecting force to the ray (other than the forces due to the aforementioned field) is the same as would be traced upon a screen perpendicular to the gun axis were the same deflecting forces applied but the aforementioned field omitted. 9

Preferably the screen surface is parallel to the gun axis and preferably also the aforementioned field is a magnetic field of substantially uniform .drical portions l and 2 merged into one another,

the axis of the two cylinders being perpendicular. That. envelope portion 2 which corresponds to the hammer head is considerably shorter than the other and the said shorter cylindrical portion of theenvelope lies for its greater part to one side of the axis of the longer cylindrical portion. In other words the hammer head portion is not symmetrical to the hammer shaft portion the two portions joining near one end of the hammer head portion. At that end of the hammer shaft portion remote from the ,hammer head? portionis carried the usual elec tron gun system not shown but indicated, as to approximate position by the reference 0, the axis of the gun corresponding to the axis of the cylindrical envelope portion l in which it is mounted. The screen 4, which in, the case of a perpendicularly to the axis or the hammen head portion of the envelope near that end of said portion which is furthest from the axis of the other envelope portion, That end wall of the hammer head portion of the envelope which is furthest from and parallel to the screen is preferably constituted by a separate circular plate 5 of good optical qualities to facilitate the projection of light on to the screen (where the said screen is a mosaic) or the projectionof light from the screen (where the said screen is fluorescent) In use, ray deflecting forces, shown in the drawing as conventional magnetic deflecting coils, well known in the art, it being understood that static deflection could be used optionally, are appliedto thecathode ray, as in the usual way, insuch manner that a rectangular area would be traced upon a screen perpendicular to the gun axis and means are provided for bending the cathode ray round the corner between the two cylindrical envelope portions in such manner that a similarly shaped rectangular area is traced upon the actual screen, which is, as stated, parallel to and not at right angles to the gun axis. The said means are constituted by an electromagnetic field of triangular section this field being generated, by an electromagnet having triangular pole pieces'only one of which, namely 6, is shown by broken lines. mer head portion of the envelope is positioned between these triangular pole pieces so that the electromagnetic lines of force run through the hammer head portion of the envelope in a direction parallel to the surface of the screen (per-- pendicular to the plane of the paper in Fig. 2). The section of the triangular field is that of a right angled isosceles triangle and the relative position of the tube and of the pole pieces is such that the hypotenuse of the triangle runs at 45 to boththe mutually perpendicular axes of the mutually perpendicular portions of the tube envelope. Itwill thus be seenthat the triangular magnetic field acts in arnanner some:- what analogous to that of a prism of optics. The

cathode ray in positions (0A, QB, 00) entering] this field are bent round more or less in circular manner until they proceed towards the screen. 1

A ray projected from the gun in a direction DA on theside of the gun axis 00 remote from the screen'will be bent round by-the triangular field, and will, upon leaving said field, proceed in a direction towards the edge A of the screen which is nearest the gun. Similarly a ray projected from the gun'in a direction OB on the side oi'the gun axis towards the screen will be bent round by the field in such manner as to 'reach thescreen adjacent that edge Bffwhich is furthest from the gun. By correctly positioning the tube with relation to the pole pieces,rthe result may be achieved, that the shape of the scanned area on the screen is the same as that which would be obtained were there no triangular field and the screen were perpendicular to the gun axis. In practice, an external lens system 8 may be placed with its axis, corresponding to.

The hamdeposition 9 is provided in the hammer head portion of the tube envelope at least over the. cylindrical walls thereof, and means may be prov vided for applying a desired fixed, potential to this coating which may, for example, be constituted by a silver deposit. Such a coating will serve to ensure a'uniiorm electric field within the corresponding part of the tube.

Having described my invention, what I claim is:

l. A cathode ray tube comprising in combination, an envelope of two approximately cylindrical portions having their axes mutually perpendicular, an electron gun at the, end of one said portion remote from the other said portion the axis of the said gun corresponding to the axis of one said portion, a screen at'the end of the other said-portion remote'from the one said portion mounted perpendicularly to the axis of the other said portion, an; optical end w'all mounted at the end of the other said portion parallelto the screen and means for establishing an external magneticfield of substantially uniform intensity and of triangularlsection, the

tions remote from theother said portion, the

axis of'said gun corresponding to the axis of one said portion,a screen at the other end of the other said portion remote from the one said portion mounted perpendicular to the other said portion, an optical end wall mounted at the end of the other said portion parallel to the screen, and means for establishing an external magnetic field of substantially uniform intensity and of triangular section, the hypotenuse of said triangle section being placed at an angle of 45 degrees" to the axis of both portions. 7

3. An electronic device comprising 'afirst cylindrical envelope, anpptical end wall at one end of said envelope, 9. target electrode at the other end of saidenvelope, a second'cylindrical envelope joined to'fthe first cylindrical envelope intermediate the optical end wall and, target, an 3 electron gun structure positioned within and at the unjoined end of the second envelope and beyond theconfines of said first cylindrical envelope the axis of said electron gun coinciding with the axis of said second envelope, both of said axes being perpendicular to the axis of the first cylindrical envelope, and means for establishing an electromagnetic field of constant intensity intermediate the target and end wall, the direction of said field being perpendicular to the axis of both of said cylindrical envelopes.

4. An electronic device comprising a first cylindricalenvelope-an optical end wall at one end of magnetic field of constant intensity and triangular cross-section intermediate the target and end diate the optical end wall and target, an electron gun structure positioned within and at the unwall, the direction of said field being perpendicular to the axis of both of said cylindrical'en velopes.

5. An electronic device comprising a first cylindrical envelope, an optical end Wall at one end of said envelope, a target electrode at the other end of said envelope, a second cylindrical envelope joined to the first cylindrical envelope intermejoined end of the second envelope and beyond the confines of said first cylindrical envelope, the axis of said electron gun coinciding with the axis of said second envelope, both of said axes being ofsaid envelope, a target electrode at the other end of said envelope, a second cylindrical envelope joined to the first cylindrical envelope intermediate the optical end wall and target, an electron gun structure positioned within and at the unjoined end of the second envelope and beyond the confines of said first cylindrical envelope, the

axis of said electron gun coinciding with the axis of said second envelope, both of said axes being perpendicular to the axis of the first cylindrical envelope, means for establishing an electromagnetic field of constant intensity intermediate the target and end WalLthe direction of said field being perpendicular to the axis of both of said cylindrical envelopes, and acylindrical focusing electrode positioned onthe inner surface of each perpendicular to the axis of the first cylindrical envelope, means for establishing an electromagnetic field of constant intensity and triangular cross-section intermediate the target and end wall, the direction of said field being perpendicular to the axis of both of said cylindrical envelopes, and a cylindrical focusing electrode positioned on the inner surface of each of the cylindrical envelopes.

8. An electronic device comprising a first cylindrical envelope, an optical end Wall at one end of said envelope, a targetelectrode at the other end of said envelope, a second cylindrical envelope joined to the first cylindrical envelope intermediate the optical end wall and target, an electron gun structure positioned within and at the unjoined end of the second envelope and beyond the confines of said first cylindrical envelope, the axis of said electron gun coinciding with'the axis of said second envelope, both of said axes being perpendicular to the axis of the first cylindrical envelope and beyond the confines of said first cylindrical envelope, means for establishing an electromagnetic field of constant intensity and isosceles cross-section intermediate the target and end wall, the direction of said field being perpendicular to the axis of both of said cylindrical envelopes, and a cylindrical focusing electrode positioned on the inner surface each of the cylindrical envelopes.

9. An electronic device comprising a first cylindrical envelope, an optical envelope at one end of said envelope, a target electrode at the other end of said envelope, a second cylindrical envelope joined to the first cylindrical envelope intermediate the end wall and target, the axis of said second cylindrical envelope being perpendicular to the axis of the first cylindrical envelope, means to direct a beam of electrons along the axis of said second cylindrical electrode, said means being located at'the unjoined end of the second cylindrical envelope, means for setting up an electromagnetic field of constant intensity Whose direction is perpendicular to the axis of both of said cylindrical envelopes for projecting the directed beam of electrons onto the target electrode, and focusing cylindrical electrodes positioned on the inner wall of said cylindrical envelope.

10. An electronic device comprising a first cylindrical envelope, an optical envelope at one end of said envelope, a target electrode at the other end of said envelope, a second cylindrical envelope joined to the first cylindrical envelope intermediate the end wall and target, the axis of said second cylindrical envelope being perpendicular to the axis of the first cylindrical envelope, means to direct a beam of electrons along the axis of said second cylindrical electrode, said means being located at the unjoined end of the second cylindrical envelope, means for setting up an electromagnetic field of constant intensity and triangular cross-section whose direction is perpendicular to the axis of both of said cylindrical

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