US2404046A - Television transmitting apparatus - Google Patents

Description

July 16, 1946. E. FLORY ET AL 2,404,046

TELEVISION TRANSMITTING APPARATUS Filed June 21, 1941 2 Sheets-Sheet 1 INVENTO RS LESLIE E. FLO/Z), ERNEST A. MASSA AND GEORGE A. M0 zfoN WI ATTORNEY 2 Sheets-Sheet 2 L- E- FLORY ET AL Filed June 21, 1941 INVENTOR-S LESLIE E. FLORY,

TELEVIS ION TRANSMITTING APPARATUS July 16, 1946.

55A AND GEORGEALMORTON BY I W I ATT O ERA/ESTA. MA

W RN'EY Patented July 16, 1946 TELEVISION TRANSMITTING-APPARATUS Leslie E. Flory, Oaklyn, N. .J., Ernest A. 'Massa, Osborn, Ohio, and George A. Morton, Haddon Heights, N. J assignors to Radio Corporation of America, a corporation of Delaware Application June 21, 1941, SerialiNo..399,110

8 Claims. (01. 178-123 Our invention relates to television apparatus, tubes and systems and is concerned primarily with apparatus incorporating a tube in which an electron beam having substantially zero velocity is scanned over a target to generate signals representative of an optical image for television transmission.

It has been customary to provide electronic image transmitting tubes of either the storage or non-storage type and to use either the resultant electrostatic charges produced from storage or to use the current image signals from the nonstorage type to develop signalling impulses for transmission of an optical image replica. The storage type of tube offers considerable increase in the magnitude of the output signal energy obtained when a mosaic of photoemissive elements is scanned with a high velocity electron beam, however, the non-uniformity of secondary electron collection and emission saturation results in a signal-to-noise ratio which is not as high as might be desired.

Local variations in the potential distribution on the mosaic generate spurious signals which are usually referred to as dark spot signals in that they produce a non-uniform gradation of shading corresponding, not to the desired distribution of light and shade of the optical image formed on the mosaic, but to random areas of light and shade which do not correspond to those of the desired image. Since the field utilized for collecting secondary electrons emitted by a mosaic electrode under scansion is modified by the electron scanning beam and consequently is of unequal intensity or efiect over the scanned area of the electrode, it has been found difiicult to collect uniformly the secondary electron emission from the surface of the mosaic electrode whereby it might be amplified such as by secondary emission amplification.

The above difliculties may be obviated by providing a tube and system wherein the tube generates an electron beam which is directed upon the target or mosaic electrode with a velocity approaching zero so that no secondary electrons are liberated which would inherently be distributed non-uniformly over the surface of the mosaic electrode and cause what has been referred to above as dark spot signals. A low velocity beam scanning tube and system is described by Albert Rose in his U. S. Patent 2,213,174 which utilizes a magnetic field to direct the beam along paths normal to the target. The use of magnetic field generating means is often disadvantageous for use in compact television camera units and especially in application where it is desired to combine the Rose tube with reflection type optical systems, inasmuch as the magnetic means which surrounds the Rose tube occupies considerable space and intercepts considerable light.

Tubes which we :have designed .for'use without a longitudinal magnetic field and wherein the electron; beam approaches the target or mosaic electrode with substantially"zero'velocity have exhibited considerable distortional elTects which we have termed crawling or fcree'ping inasmuch as the recreated image replica continuously shifts or moves especially over areas remote from the center of the image; A fullycomprehensive theory of why this crawling o creeping occurs has not been formulated but. whenever, tubes utilizing low velocity beam scanning have been operated without a longitudinal magnetic-field these crawling effects have been most objectione ab-le. .1

.It. is an object of our invention to" provide highly sensitive television apparatus and syse tems capable of developing picture replicas without "distortion wherein a low velocity electron bea'mris directed upon a target by purely -elec'- trostati-c *means. It is :another object to provide a lighttranslating and scanning system which will simplify the translation of optical effects into televisionsignalling impulses with a; minimum of dlstortionalefi'ects. It is a further object to provide a television transmitting tube wherein .elec- .trostatic charges representative of the light and shade areas of an'optical image may be neutralized over a relatively'large extended area,and it is a still further object to minimize orjsub stantially eliminate distortional effects occasioned by grazing incidence of an electron beam on a target. 7 r t In accordance with our invention we provide television apparatus incorporating a tube wherein the electron beam is directed along paths oblique with respect to the surface of a target and then directthe electrons of the beam along paths substantially normal to the elemental surface areas of the targetby correlating theobliquity of the original paths, the position and contour of the target, and the effects of an electrostatic field immediately adjacent the target. These and other objects, features and advantages of our invention will become apparent upon consideration of the following description and the accompany: ing drawings in which Figure 1 is a longitudinal view of television transmitting apparatus incorporating our invention; g g j Figure 2 is a graph showing electron beam trajectories in a conventional television transmitting tube; and q Figure 3 is a graph similar to that; of Figure} showing trajectories ofthe electron beam in a tube of the type of Figure 1. In the'illustrative embodiment of our invention as shown in Figure 1, the apparatus includes a discharge device or tube comprising a highly evacuated glass envelope or bulb l with a tubular in accordance with our invention, is of a prede:

termined contour. The target is symmetrically positioned in the enlarged portion of the envelope so that its projected surface is substantially perpendicular to the longitudinal axis of the bulb I and so that it may be scanned by a beam of electrons from the electron gun and may also have projected thereon an optical image of which a signal replica is to be transmitted. As shown, the mosaic electrode 2 is preferably deposited or formed on the inner surface of the end wall of the envelope'which may in this case determine the predetermined contour of the mosaic electrode.

The electron gun is of a conventional type and comprises a cathode 5 from which an electron stream may be drawn, a control electrode 6 connected to the usual biasing battery, and a first anode 1 maintained positive with respect to the cathode 5. The electron stream leaving the first anode I is accelerated and concentrated into an electron scanning beam and directed toward the surface of the target or mosaic electrode 2 facing the electron gun by a second anode 8 which is preferably a conductive coating on a portion of the inner surface of the envelope I. The first anode I and the second anode 8 are maintained at the desired positive potentialsby a potential source represented as the battery 9. Although we have shown one form of a suitable electron gun other forms may serve to equal advantage. Intermediate the first anode I and mosaic electrode 2 and at a predetermined distance from the mosaic electrode as measured along the'electron gun axis We provide conventional electron beam deflection means such as the coils l0 and II to deflect the beam from its axial path and sweep the beam in horizontal and vertical planes from a center point of deflection such as the point l2. It will be noted that as the electron beam leaves the electron gun 2, its path is coaxial with the first anode, but upon reaching the center of deflection I2 the beam is deviated or deflected from this path and approaches the target from this point as a deflection center. It will likewise be appreciated that in the absence of further deflection means or electrostatic lenses between the center of deflection l2 and the mosaic electrode, the electron beam trajectories would be straight lines. As the electrons comprising the electron beam leave the junction of the first and second anodes they are directed along converg- 'ing paths, only the central or average path of which is shown in Figure 1. Furthermore, the average potential of the mosaic electrode is maintained substantially equal to the potential of the cathode 5, as explained in greater detail hereafter, so that the electrons are decelerated in the vicinit of the mosaic electrode to a velocity of substantially zero.

In accordance with our invention we so modify the electron beam trajectories intermediate the center of deflection and the target and likewise provide the target of predetermined configuration preferably of spherical form with its center of curvature located on the electron gun axis and preferably between the center of deflection and the target surface. More particularly, the means to modify the electron beam trajectories may comprise one or more electrodes surrounding the electron beam between the second anode 8 and the target or mosaic electrode 2. Re er i g to adjacent the target, such as the electrodes l5- and I5 maintained at the desired positive potentials with respect to the cathode by the battery 9. These electrodes may comprise electrically conductive coatings on the inner wall of the bulb l adjacent the mosaic electrode, although it is within the spirit of our invention to provide these electrodes in the form of wire mesh screens longitudinally displaced along and transverse to the longitudinal axis of the tube. The function of the electrodes !5 and I6 will be considered in considerable detail in connection with Figures 2 and 3.

Referring more particularly to the configuration and construction of the mosaic electrode 2. this electrode is preferably of spherical form with its center of curvature intermediate the center of deflection and the target surface, and while this electrode may be supported within the bulb l, we have shown the structure as being formed directly on the inner wall of the bulb I which in this case has a curvature corresponding to the desired curvature of the electrode. While any form of light responsive electrode capable of resolving individual elemental'areas of light and shade into elemental 'chargeareas may be used, we prefer to pro'videan electrode of the semitransparent type including alight permeable electrically conducting film, such as the metal film 20 deposited directly on the inner end wall of the bulb l. The electrically conducting film 20 serves as a signal plate in capacitive relation with the photosensitized particles of the mosaic 22 and is connected to a translating device such as the amplifier 25 and to a potential at or near that of the cathode 5 through an output impedance 26. This connection, as indicated above, causes the mosaic 22 to assume an average potential which is equal to or near the cathode potential, thereby decelerating the electron beam adjacent the target. The metal film 20 may be provided with a non-conducting coating 2|, such as of aluminum oxide, china clay, glass or'othe'r dielectric material. The coating 2| is likewise substantially transparent and bears on its exposed surface a mosaic 22 of phot osensitized metallic particles so that an optical image such as represented by the arrow 23 may be focused thereon through a lens system 24. Inasmuch as the mosaic electrode is of spherical configuration, the lens system may be designed to provide a spherical image field, or reflection optical systems inherently producing a spherical image fleldmay be substituted for the particular lens system 24, as shown in the drawings.

For a better understanding of the principles underlying our invention, reference may be had to Figure 2 which shows a graph of electron beam trajectories in a tube incorporating a, plane target and an electron lens immediately adjacent the target. As shown in Figure 2, the target represented at 30 is of planar form, only a portion of the target being shown to one side of the axis 31 on which the center of deflection I2 is located at a predetermined distance from the target. The positions of the second anode 8 and the electron lens forming electrodes l5 and I6 are shown, it being assumed that potentials of 1000, 600 and 450 volts with respect to cathode are applied to these electrodes respectively} Assuming a, unit charge on the surface of the target 30 such as might be occasioned by light representative of a uniform illumination over the surface of the taradjacent the surface.

get, an electron beam approaching the target along the axis 3| of unit intensity is collected 'by the target to neutralize the unit charge. If, however, the beam is deflected at the center of deflection l2 through an angle of approximately 3 with the axis, the electron beam trajectory will be as shown at 32. The electrostatic field developed by the electrodes l5 and I6 for the potentials referred to above is a field of such intensity as to form equi-potential field gradients substantially corresponding to the normal deceleration field gradients through which the electron beam approaches the target. Thus the potentials of the electrodes I5 and is are here chosen such that they have little if any effect on the electron beam trajectories. The axial velocity of the electron beam decreases as the electrons approach the target, but the radial velocity, that is, a velocity transverse to the axis, remains substantially constant so that the electron beam, instead of approaching at the original 3 angle of deflection, increases its effective deflection adjacent the target so that the beam is shifted from the intended point of contact with the target and fails to reach the target because when the electrons reach their nearest point 33 of approach to the target, the electrons are reflected and return in the general direction of the electron gun. This effect is even more pronounced with larger angles of deflection. For a deflection of approximately 5% the trajectory as shown at 34 approaches the target at its nearest point 35 which is even farther removed from the target than point 33. Similarly, for larger angles of deflection, such as approximately 7 and 8, the electron beam follows the trajectories 3t and 38, the nearest points of approach to the target being the points 31 and 39. It will be noted that as the angle of deflection increases, the distanc between the target and the nearest point of approach for the various trajectories increases correspondingly, and it will be evident that while a unit picture charge may be neutralized by the beam having an axial trajectory, the beam will have progressively less effect upon a unit charge as the angle of deflection is increased. Actual measurements have shown that a unit charge capable of being neutralized by an axial trajectory beam must be increased to 12, 20, 46 and 70 units for the four trajectories respectively, as shown in Figure 2.

While it is possible to partially compensate the effects produced by non-uniform distances of approach of the beam to the target by increasing the strength of the electron lens formed between the electrodes 8, l5 and I6, we have found that lens strengths sufficient to overcome even a major portion of the difficulties introduce severe spherical aberration elfects so that the resulting picture replica, while of more uniform response, becomes distorted with respect to the original picture content. However, in accordance with our invention, these difiiculties may be overcome by providing only a small lens action immediately adjacent the target by forming the target in such a manner that its surface is substantially coincident with the electrostatic field gradient In this manner the radius of curvature of the target is made substantially equivalent to the radius of curvature of adjacent equi-potential field gradients and preferably less than the radius of deflection which may be defined as the axial distance between the center of deflection and the target. This results in the electron beam approaching the elemental surface area :along paths substantially normal thereto. Figure'3 is .a'graph similar to thatshown in Figure 2 wherein the radius of curvature of the target has been correlated with and chosen with respect to the voltages applied to the electrodes l5 and I6 and with respect to the initial center of deflection. In accordance with our invention and as shown by Figure 3 the electron beam approaches the mosaic along paths which are sub-' stantially normal to elemental surface areas. It will be noted that the trajectories 32, 34, 36 and 38' have points of nearest approach 33, 35, 31' and 39' which are substantially coincident with thesurface of the target. It is likewise noted that the radius of curvature of the equi-potential line marked 25 along the axis of the tube is substantially equal to the radius of curvature of the target over the principal portion of its length. Thus, while the graph of Figure 3 applies to' a tube having a potential of 200 volts applied to the electrode It, a slight increase in this potential will produce an even closer symmetry between the'target curvature and that of the adjacent equi-potential field.

As a particular example of a tube made in accordance with our invention and of which the graph of Figure 3 is representative, the axial distance between the center of deflection and the target was made equal to 9. inches with aradius of curvature of the target of '7 inches, the potential of the electrodes 8, l5 and I6 beingequal,

to approximately 1000, 500 and 200 volts respectively, the target being rectangular and measuring 1 x 2 inches. This tube, made in accordance with our invention, exhibited a minimum of distortion due to non-normal approach of the electron beam and resultant crawling effects, as well as a minimum of spherical aberration effects occasioned by the electron lens between the second anode and target.

While we have described our invention with respect to the provision of a tube and system wherein more uniform approach of the electron beam to the target is obtained, it should be understood that the principal criterion for satisfactory operation is the path along which the electron beam approaches the target surface, this path being normal to the elemental surface areas of the target. We have observed that the rate of the.

crawling or creeping effect in both horizontal and vertical directions is the same which seems to indicate that this effect is associated with the instability of accumulations of charges over the mosaicsurface this instability resulting in the moving charge excesses and deficiencies causing the effect. Therefore, while we have not sought to set forth a positive explanation of the efl'ect it will be appreciated that we have overcome the difliculty by so correlating the target contour and the desired beam trajectories that the beam approaches along a path normal to the elemental target surface and thereby eliminating the crawling or creeping distortion. Furthermore while we have indicated the preferred embodiments of our invention of which we are now aware, and have trically conducting electrode on one surface of said end portion, a mosaic of light sensitive photoemissive particles adjacent said electrode and in capacitive relation therewith the exposed surface of said mosaic conforming to the spherical concavity of said end portion, an electron gun oppositely disposed from said mosaic to develop an electron beam, scanning means to direct said beam along paths oblique with respect to the radii of said curved end portion and centered at a point more remote from said mosaic than the center of spherical concavity thereof, and means to divert said beam from the oblique paths immediately adjacent said mosaic to paths radially incident thereon to develop signals in response to light projected upon said mosaic.

2. Television signal generating apparatus comprising an evacuated envelope, an electron gun within said envelope to develop a high velocity electron beam, a target having a great multiplicity of electrostatic charge retaining mosaic elements arranged on a spherical surface which is concave with respect to said electron gun, the said target being symmetrically disposed and transverse to the axis of said beam, means to deflect said beam at a predetermined rate through a vertical angle and means to deflect said beam at a predetermined higher rate through a horizontal angle said last two mentioned means being positioned about the beam axis at a region more remote from said target than the center of concavity thereof, means to develop an electrostatic image of varying electrostatic charge intensity over the surface of said target, a plurality of electrodes between said deflection means and target, and means to decelerate and direct said beam following said deflection along paths radial to the surface of said target to develop signals substantially proportional to said charges and in a time sequence determined by the uniform rates of deflection.

3. Apparatus for television transmission comprising a tube having an electron gun to develop an electron beam, an oppositely disposed light sensitive target positioned transversely to the axial path of said beam to receive electrons of said beam, said target being concave with respect to said gun, means to deflect said beam from a center of deflection further removed from the said concave target than the center of curvature thereof whereby said beam follows trajectories oblique to said target, and a plurality of electrodes adjacent said concave target to deflect said beam to follow trajectories substantially radially incident on said concave target.

4. Apparatus for television transmission comprising a tube having an evacuated envelope one portion of which is spherically concave with respect to its interior, an electron gun oppositely disposed from said concav portion to develop an electron beam, a light sensitive target adapted to develop elemental charges representative of the light and shade areas of an optical image deposited on and conforming to the concavity of said portion to receive electrons flowing along a path from said electron gun, beam deflection means surrounding a portion of said beam path at a greater distance from said target than the center of curvature of said concave portion to deflect said beam from a center of deflection removed from the center of curvature of said concave portion whereby said beam follows non-.radial trajectories toward said target, and means adjacent said target to modify the trajectories of said beam to 8 trajectories substantially radially incident on said target.

5. Apparatus for television transmission comprising a tube having an electron gun to develop an electron beam, an oppositely disposed light sensitive target positioned transversely to the axial path of said beam to receive electrons of said beam, said target being spherically concave with respect to said gun and having its center of curvature lying on said axial path, means more remote from said target than said center of curvature to deflect said beam over said spherically concave target whereby said beam follows trajectories oblique to the radii of said target, and means adjacent said target to deflect said beam to follow trajectories substantially radially incident on said target said last-mentioned means including said target and a plurality of electrodes adjacent said target.

6. Apparatus for developing signals for television transmission comprising a tube having an electron source, an anode to accelerate electrons from said source, a target having a finite radius of curvature, the ,center of curvature lying on the axis between said source, anode and target, means surrounding the said axis at a region more remote from said target than the center of concavity thereof to scan said electrons over paths obliquely incident on said target, means including said target to decelerate said electrons to a substantially zero velocity in the vicinity of said target, said means including an electrode adjacent said target to direct electrons from said paths to paths radially incident on said target.

7. Apparatus for developing signals for television transmission comprising a tube having an electron source, an anode to accelerate electrons from said source, a target having a finite radius of spherical curvature, the center of curvature lying on the axis between said source, anode and target, means surrounding the said axis at a region more remote from said target than the center of concavity thereof to scan said electrons over paths normally oblique to the surface of said target, means including said target to develop an electrostatic equipotential field directly adjacent said target which is of spherical curvature conforming substantially to the curvature of said target to direct electrons from said paths to paths radially incident on said target.

8. Apparatus for developing signals for television transmission comprising a tube having an evacuated envelope a portion of which is adapted to bear a light sensitive target, an electron source, an anode to accelerate electrons from said source, a target having a finite radius of spherical curvature between said envelope portion and said electron source, the center of curvature of said target lying on the axis between said source, anode and target, means surrounding a portion of said axis and between said source and said center of curvature to scan said electrons over portionsof paths normally oblique with respect to said target, means including said target to decelerate said electrons to a substantially zero velocityin the vicinity of said target, said means including an electrode adjacent said target to develop a spherical equipotential field which substantially con-- forms to the curvature of said target immediately adjacent the surface thereof whereby electrons are deflected from said paths to paths radially incident on said target.

LESLIE E. FLORY. ERNEST A. MASSA. GEORGE A. MORTON.

Images