US2266621A - Cathode ray tube system - Google Patents

Description

H. B.DE`VORE` GATHDE RAY TUBE SYSTEM Dec. 16, 1941.

Filed Aug. 23, 1940 /ENR Y 5. DE /RE BY vI Patented Dec. i6, lgl

stars ere maan CATHGDE RAY TUBE SYSTEM Henry B. De Vcre, Newark, N. J., assigner to Radio Corporation of America, a corporation of Delaware 6 Claims.

My invention relates to television transmitting tube systems of the cathode ray type and is more particularly directed to tubes and systems utilizing low velocity electron scanning beams.

In television pick-up tubes using low-velocity electron scanning, such as the type described by Rose in United States Patent 2,213,174J it is customary to employ as a source of electrons the combination of a thermionic cathode, a grid, and a small limiting aperture which is maintained at a positive potential. This limiting aperture serves to denne the electron beam by selecting from the total electron emission from the cathode a small fraction which traverses this aperture and forms the scanning beam. These electrons may have a distribution of radial velocities relative to the direction of the beam, owing to curvature of the accelerating field and to the radial components of their emission velocities from the cathode surface. tribution of longitudinal velocities possessed by the electrons of the beam.

1n order to focus the beam to a small spot at the surface of the target, a uniform longitudinal magnetic iield is provided. This has the characteristic that the paths of the individual electrons are helices having a diameter proportional to the radial velocity. The beam as a Whole expands to some maximum diameter which is determined by these spirals and again contracts to approximately its original size at a number of points of focus between the cathode and target. I have found that these spirals are executed in a time which is uniform for al1 electrons, and the effect of the variation in longitudinal velocity is that the electrons do not all return to their original radial distance from the beam axis at the same distance from the limiting aperture, and consequently each successive beam focus is less sharp than the preceding. Thus, the electron beam as focused at the target forms a spot which is considerably larger than the limiting aperture at which the beam originates. The size of this spot depends on the radial velocities of the electrons in the beam and in the corresponding departure from uniformity of their longitudinal velocities.

A large scanning spot is undesirable since, as the electron density decreases toward the edge of the spot, the rate of discharge of a charged element of the mosaic is comparatively slow, and a signal corresponding to the element is trans* mitted during a comparatively long time. This results in a reduction of resolution in the transmitted image.

There is also a corresponding dis- It is an object of my invention to provide a tube having a target on which an electron beam is magnetically focused wherein the electron beam may be focused toja smaller effective diameter on the target, whereby the resolution capability oi the tube is increased; and it is a further object to provide a tube of the type described wherein the eifective diameter of the electron beam and consequently the tube resolution is substantially independent of electron beam current.

In accordance with my invention I provide an apertured electrode structure for use in magnetie cally focused cathode ray tubes wherein the apertures of the electrodes are so proportioned and the electrodes so spaced as to limit the diameterof the beam and select the electrons having the desired radial and longitudinal velocities in order to focus the beam to a small spot atthe surface of a beam receiving target.

A better understanding of my invention will beA obtained and other objects, features and advantages will appear from the following description taken in connection with the accompanying drawing in which:

Fig l is a longitudinal sectional view of a television transmitting tube embodying my invention, Fig. 2 is a cross-section of the tube shown in Fig. 1 taken along the lines 2 2, and

Figs. 3 and 4 are greatly enlarged views of a portion of the structure shown in Fig. 1 showing certain electron trajectories. Y v In general,l the tube incorporating my inven-` tion comprises an evacuated envelope having a target preferably of the photosensitive mosaic type at one end and an electrode assembly including an electron emitting cathode and an electron collecting electrode at the opposite end of the tube. The target, if of the mosaic type, is provided on its front surface with an extremely large number of mutually insulated photosensitive particles and is so positioned that it may be scanned by an electron beam from the gunuand may also have focused upon it an optical image of the object of which a picture is to be transmitted;-

coaxialwith the center line of the gun and the target to scan the electrons from the gun-over the mosaic electrode. The potentials between the cathode and target are so adjusted that the electron beam is projected at relatively low velocity and impinges upon the target at extremely vlow or substantially zero velocity, that is, with a velocity approaching zero at the point of impact. In operation, the electrostatic image, consisting of charges proportional to the intensity of the incident light, is neutralized by the low velocity beam to generate electrical signals representative of the optical image.

Referring specifically to my tube structure shown in the drawing, the tube comprises an evacuated envelope I enclosing at one'end a target or mosaic electrode 2 and at the opposite end an electrode assembly 3 adapted to project electrons from a cathode toward the mosaic electrode.

The mosaic electrode 2 which faces the electron gun 3 preferably comprises a substantially transparent sheet of insulation suchy as the mica sheet 4 having on its rear surface a translucent or semitransparent electrically conducting lm 5, the opposite or front surface ofthe mica sheet facing the electron gun being provided with an exceedingly great number of mutually separated photosensitive particles E. In making the mosaic electrode I coat one side of the mica sheet 4 with ay film 5 of metal of sufficient thinness as to be substantially transparent so that an optical image such as represented by the arrow 1 may be t focused on the photosensitive particles 6 by a lens system 8. ticles may be made by depositing on the front surface of the mica sheet 4 finely -divided silver oxide which is reduced to provide ay surface of individually separated silver particles or globules 6 which are subsequently oxidized and sensitized with caesium or other alkali metaly during the evacuation process. Such an electrode structure and method of sensitization is well known in the art.

Intermediate the electrode assembly 3 and the mosaic electrode I provide two deflection plates 9 which are curved to provide afuniformly increasing and decreasing electrostatic deflection field, a pair of shield plates Iil--I I, one on either side of the deflection plates 9, having slots I 2-I3 for the passage of the electron beam from the cathode and a conductive wall coating I4. The plates 9 are connected to ground and to the shield plates I Il-II and coating I4 through a centertapped resistance of one to ten megohms and to a source of deflection potential. To'produce the desired deection and focus of the electron beam I provide means to wholly immerse the deflection plates in a uniform magnetic field which is preferably generated by a magnetic coil I5 of slightly larger diameter than the envelope I extending over and beyond the space between the electron gun 3 and the mosaic electrode 2. The deflection plates 9 in combination with the eld produced by the coil I5 deflect the beam in a plane midway between the plates and through the center line of the electrode assembly 3 and target. Delection of the electron beam in a direction normal to that produced by the plates 9 is accomplished by a pair of deflection coils I6. This deiiection is preferably the frame or vertical deiiection, and since in standard television systems the frame deection is of lower frequency than the horizontal line deiiection, the coils I6 should preferably be operated at the lower of the two frequencies. The deflection coils I6 may, of course, be replaced by a second pair of deflection plates. In operation, the electron beam from the cathode is focused on and scanned over the mosaic electrode 2 to discharge the electrostatic charges developed by light focused thereon. The semi-transparent film 5 is maintained at substantially cathode potential by a connection from the cathode than the grid 22 there isy provided an electron accelerating electrode or anode 23 similarly apertured and aligned with the grid 22 and cathode 20.

In accordance with my invention and between f the anode 23 and shield electrode II I provide The mosaic of mutuallyseparated pary two apertured electrodes having selected aperture diameters and predetermined spacing between adjacent surfaces of the electrodes. More particularly, the electrodes 24 and'25 are ypositioned between the anode 23 and shield electrode I I with their apertures 26 and 21 axially aligned with the cathode and the apertures in the yother electrodes to allow electrons to flow from said cathode through said apertures and toward the target 2. it will be understood that the function of this anode may be served by the apertured electrode 24, since said electrode is maintained positivey with respect to the cathode by the battery or other potential source I8. The aperture 26 in electrode 24 will be' referred to hereinafter as the beam limiting aperture 26, and the aperture 21 in the electrode 25 as the electron selecting aperture 21. In operation, athefdiameter of the electron beam is limited by the aperture 2 and the desired electrons passing through this aperture are selected and passed by the aperture y21 and directed toward and scanned over the target 2. Electrons of the beam not reaching the target return in the direction of and are collected by the electrode 25. For this reason the electrode 25 is preferably elongated to collect the electrons returning through the slot I2 in the shield plate I0. The returning electrons are modulated in intensity in accordance with the light and shade areas of an optical image on the target 2, and consequently the impedance I1 may be located in the electrode 25 to potential source circuit but the previously described connection for this impedance is preferred.

Referring to Fig. 3, I have shown the apertured electrodes 24 and 25 separated by an axial distance designated Lr. The aperture 26 which is for the purpose of limiting the electron beam diameter may be circular, rectangular, or of other shape, but when I refer to the diameter or radius of this Vaperture 25, it will be understood that this is the diameter of a circular aperture or of a circle in which the rectangular or other-shaped aperture may be inscribed, this being likewise true of the electron selecting aperture 21 in the electrode 25.

I have found that electrons liberated, such as from a cathode located in an axial magnetic neld which are not emitted in a direction parallel with the field, acquire a motion which is not parallel with the eld. Thus, electrons emitted with an initial radial velocity describe helical paths about an axis through the point of emission and parallel with the magnetic eld. rIhe electrons of the beam which possess radial velocities return to a point of focus following each helical revolution While I have 'shown a separate anode r23,

about the axis. This point of focus may be referredto as a focal point of the beam or as a nodal point of the envelope of the paths traversed by the electrons of the beam emitted from the point of origin. Two such nodal points are indicated in Fig. 3, one lying within the aperture 26 and the other lying along the axis such as at a point 2S, at which point the target 2 on which the beam is focused may be assumed as being located. The target 2 is shown in Fig. 3 at the nodal point 28 for purposes of explanation, although in practice the target 2 will actually be located at a subsequent nodal point. The distance between these focal or nodal points is determined by the strength of the magnetic field and the velocity of the beam electrons and may be termed the length of one loop of focus. In Fig. 3 I have shown an envelope of electrons bounded by the full lines Z9, these being the selected electrons capable of passing through the electron selecting aperture 2l. The electrons having greater radial velocities than the electrons bounded by the lines 29 such as represented by the envelope enclosed between the lines 3D are collected by the selecting apertured electrode and prevented from owing to the target. In the absence of the selecting apertured electrode 25, these electrons would continue to the target along paths within the envelope bounded by the lines 3l. It will be noted that while the electrons of the beam bounded by the lines 29 return to a focused condition at the nodal point 28, the electrons bounded by the lines 3b would, if allowed, continue towards the target to be focused at a different nodal point 32 displaced from the targ While Fig. 3 shows the eifect of the electron selecting aperture on an electron beam assumed as having a nodal point on the axis within the aperture, in practice, the electrons nowing through the aperture 26 may be considered as originating at various points within the aperture 26. Fig. 4 shows a limiting condition where one of these points of origin is at the edge of the limiting aperture. The full line 33 represents the projected trajectory on a plane through the focal pointsof an electron having the max imum radial velocity which it is-desired to retain in the electron beam. It will be observed that the electron trajectory is terminated at the target 2, displaced by a distance equivalent to the radius of the aperture 2%. An electron having a greater radial velocity and correspondingly lower axial velocity than the electron whose projected trajectory is represented by the line 33 will follow such a path as 35 which is intercepted by the selecting apertured electrode 2t? and prevented from continuing along its normal path 36 to a point on the target which may be displaced from the axis by a greater distance than the radius of aperture 2e. Therefore, in accordance with my invention, I reject from the electron beam all electrons which have a radial velocity in excess of a small predetermined value. From a number of tubes made in accordance with my invention I have found considerable improvement in resolution and fidelity of reproduction of the optical image replica when electrons having a radial volt velocity in excess of 0.6 volt are rejected, and even greater improvement when electrons having a radial volt velocity in excess of 0.2 volt are rejected from the electron beam. ln the latter case I have also found little or no decrease in resolution with increasing beam current.

The following calculations will serve to illustrate my method of determining the proper radii of the apertures and the spacing between the beam limiting and electron selecting electrodes, in accordance with my invention.

If an electron enters a space in which there exists a uniform magnetic field and no electrostatic leld, its path is a helix having its axis in the direction of the magnetic field. The proe jection of this helix upon a plane perpendicular to the field is a circle which is tangent to the radial component of the initial electron velocity at the point at which the electron enters the field. The speed of the electron in the projected circle is constant and equal to the initial radial velocity component. The radius of this projected circle is, (see Malcff and Epstein, Electron Optics in Television, page 151).

where vrzinitial radial velocity component Hzmagnetic field intensity Combining (l) and (3) Hence combining (l), (2) and (4) Hi m S111 2 e Ha if the electron has an initial velocity component f v0 in the direction of the field, then as this velocity component is unaiected by the magnetic eld, the distance LT which the electron moves in the direction of the eld in the same time t is Substituting for t it is seen that an electron in moving a distance Lr along the field suffers a radial displacement 'r of eLr 1) 1': Z-T sin and target. From the above cited reference the length of one loop of focus is known to be It is well known that the velocity I an electron is proportional to the one-half power of the accelerating potential. Therefore, with a potential of Vo volts on the limiting and selecting apertured electrodes 23-34 with respect to cathode potential and a radial volt Velocity or" Vr volts The above expression for r under these condi-- tions may be simplified to r Vo In the above analysis the longitudinal velocity was represented by 11o (corresponding to Vo volts) which is the longitudinal velocity of the electron beam between the apertured electrodes 24 and 25, assuming the electrodes 24 and 25 at a common potential such as obtained when the switch S in Figure l is thrown to the right-hand position. If, however, the two apertured electrodes are maintained at diierent potentials with respect to the cathode, such as by throwing the switch S to the left-hand position, the above equations must be modified because ii the electrons acquire or lose velocity in the space between these electrodes, the time these electrons are between the electrodes will vary. Therefore, the expression sin Hi in must be modified if the two electrodes are maintained at different potentials. lf the limiting apertured electrode is at a potential of Vo' and the selecting apertured electrode is at a potential of Vo, which is the final accelerating potential. Equation l1 becomes L (is) If, however, the potential Vo is not the final accelerating potential, but a potential different from Vo or Vo with respect to the cathode is applied to electrodes eiTective on the electron beam between the selecting apertured electrode 24 and the target and close to the electrode 24 such as to the shield plate Il, the above Equation 14 is not modified.

An electron in order to pass through both the limiting and selecting apertures may not have a displacement greater than the sum of the radii of these apertures. If the selecting and limiting apertures are separated by a distance Lr to remove electrons with a displacement greater than r, then the sum of the radii of the two apertures is properly determined equal to i'. For this condition, electrons having a radial volt velocity equal to or less than Vr will be selected and passed by the selecting aperture, and all electrons having a velocity in excess of Vf will be rejected from the beam.

Assume that it is desirable to reject electrons having radial velocities greater than 0.6 volt from a beam having an average mean longitudinal velocity of 200 volts. Then, from Equation mi 1 LT- 20D-18.2

As shown in connection with Fig. 4, the maximum displacement of an electron is represented by the case of an electron just grazing and leaving the edge of the limiting aperture and grazing the opposite edge of the selecting aperture. Then, if in the case considered, the limiting aperture has a diameter of 0.002 inch (radius=0.001 inch), and a diameter of 0.006 inch (radius=0.003 inch) is chosen for the selecting aperture, the maximum displacement of an electron permitted by this system is and the proper separation of the two apertures is Thus, with a spacing of 0.073 inch between the limiting aperture and selecting aperture, electrons having a radial velocity in excess of 0.6 volt will be withdrawn from the beam, thereby providing an electron beam which, when focused on the target by the magnetic field, has a smaller eective diameter than a beam merely limited by a limiting aperture, and consequently the resolution of which the tube is capable will be increased. Similarly, if it be desired to reject electrons having a radial volt velocity greater than 0.2 volt from an electron beam having a mean longitudinal velocity of 200 volts, the spacing between the electrodes 23 and 24 will be 0.126 inch.

In the above examples it was assumed that electrons having Vr 0.2 volt, Equation l comes fw/n2 2 Lf 25o' /555 1 +R, 20o

L Lf`l2/i Therefore for limiting and selecting apertures ci 0.002" and 0.006 diameter respectively, the separation between the apertured electrodes '2d-E5 should be equivalent to Lf or 0.050 inch.

In the above description and calculations it is evident that the apertures in the limiting and selecting electrodes were chosen of unequal diameter with the limiting apertured electrode closer to the cathode, and that the limiting aperture was located at one of the points oi focus oi the beam. The tube will operate equally as well if the selecting apertured electrode is closer to the cathode than the limiting aperture. In addition, the diameters of these apertures may be equal or unequal, but for equal diameters the desired spacing between the electrodes is small and consequently such a structure is more difcult to construct.

I have constructed tubes in accordance with my invention designed to select and utilise only the electrons of the beam having radial velocities less than 0.6 volt and also less than 0.2 volt and have found that considerable improvement in resolution compared with tubes not utilizing my invention is obtained. I have also found that the resolution of which the tubes are capable is, in the latter case, substantially independent of the electron beam current for beam intensities usually used in such tubes.

While I have indicated the preferred embodiments of my invention of which I am now aware and have indicated the specific application as directed to cathode ray transmittingr tubes having target electrodes of the photosensitive mosaic type, it will be apparent that my invention is by no means limited to the purpose of television transmission, to the exact forms illustrated, or to its use in cathode ray tubes incorporating target electrodes of the mosaic type, but that many variations may be made in the particular structure used, such as by replacing the mosaic electrode with target electrodes of the fluorescent, photoconductive, or photovoltaic type, without departing from the scope of the invention as set forth in the appended claims.

I claim:

l. A cathode ray tube system comprising a tube having an electron emitting source, a target to receive electrons from said source, a pair of separated apertured electrodes with their apertures lying along an axis between said source and said target to limit the flow of electrons therebetween, means to generate a longitudinal magnetic eld extending between said source and said target, and means to maintain said two apertured electrcdes at the same positive potential with'respect to said source, the sum of the radii of the two apertures of said apertured electrodes being represented by the expression l T V0 where Lr is the separation no greater than onehalf inch between adjacent surfaces of said electrodes in inches, Vo is the potential diierence in volts between said electrodes and said cathode, and V1' is the maximum radial volt velocity 0I" electrons in said beam and less than 0.6.

2. fr cathode ray tube system as claimed in claim l wherein the radii of the apertures in said electrodes are unequal.

3. A cathode ray tube apparatus comprising a tube having an electron emitting cathode, an oppositely disposed target to receive electrons from said cathode, two apertured electrodes having their apertures aligned with an axis extending from said cathode to said target, means to generate a magnetic lield extending along lines substantially parallel with said axis and between said cathode and sai target, and potential means to maintain each of said apertured electrodes at positive potentials with respect to said cathode, the sum of the radii in inches of the two apertures in said electrodes being represented by where L1- is the separation no greater than onehalf inch along said axis between said electrodes measured in inches, Vo and Vo are the potentials in volts at which the said electrodes in their order of distance from the cathode are maintained by said potential means, and Vr is the maximum radial volt velocity, less than 0.6, of electrons passing through the apertured electrode nearer said target.

fl. Television transmitting apparatus comprising a cathode ray tube having an evacuated envelope, a magnetic coil surrounding said envelope, a cathode within said eld to emit electrons of which a portion have radial volt velocities greater than 0.6, causing said portion of electrons to follow helical paths in said field and to describe loops of focus between said cathode and target, two apertured electrodes having selected aperture diameters between said cathode and target and means to maintain said two electrodes at a single positive potential with respect to said cathode, the sum of the radii of said apertures in inches being substantially equal to E w Sli). L

where I.. is the distance in inches and n the number of loops of focus between said electrode nearer said cathode and said target, Lr is the separation in inches between said apertured electrodes measured in the direction of said ield, Vo is the single potential in volts at which said two electrodes are maintained, and Vr is the maximum radial volt velocity, less than 0.6, of electrons passing through the apertured electrode nearer said target.

5. Television transmitting apparatus comprising a tube including an evacuated envelope, a magnetic coil surrounding said envelope to gencrate a substantially uniform magnetic field, a target in said envelope positioned in a plane substantially normal to said field, a cathode opposite said target and within said eld to emit electrons in the direction of said target, a portion of said emitted electrons having a radial volt velocity in excess of 0.6 causing said portion of electrons to describe helical paths in said eld and to follow loops of focus between said cathode and target, a pair of apertured electrodes having selected aperture diameters between said cathode and target, and means to maintain said apertured electrodes at positive potentials with respect to said cathode, the sum of the radii of the apertures of said electrodes in inches being represented by the expreswhere Lr is the separation in inches along said iield between said apertured electrodes, Vo' is the potential in volts with respect to said cathode applied to the said electrode nearer the cathode, Vo the potential applied to the electrode nearer the target, L is the distance in inches, and n the number of loops of focus between the said electrode nearer said cathode and said target, and Vr is the maximum radial Volt Velocity, less than 0.6, of electrons passing through the apertured electrode nearer said target.

6. Apparatus as claimed in claim 5 wherein the apertures in said pair of electrodes are of unequal diameter, the electrode having the aperture of smaller diameter being nearer said cathode.

HENRY B. DE VORE.

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