US2658160A - Image-reproducing device - Google Patents

Description

Nov. 3, 1953 R. s. PETERMAN IMAGE-REPRODUCING DEVICE Filed Nov. 23, 1951 INVENTOR. RUSSEL S. 'PETERMAN HIS ATTORNEY.

Patented Nov. 3, 1953 IMAGE-REPRODUCING DEVICE Russel S. Peterman,

Des Plaines, Ill., assignor to The Rauland Corporation, a corporation of Illinois Application November 23, 1951, Serial No. 257,692 Claims. (01. 313-82) invention relates to image-reproducing devices and more particularly to cathode-ray tubes for use as picture-reproducing devices in television receivers and the like.

One of the problems often encountered in the production of television receivers stems from the susceptibility of commonly employed cathode-ray tube constructions to exhibit extraneous cold emission and spark discharge in the electrode system. Whenever two closely spaced electrodes are maintained at large potential differences, these undesirable phenomena are apt to be encountered. For example, most electron guns comprise a cathode, a control grid, a so-called second grid which is maintained at a low constant positive potential with respect to the oathode, and a tubular anode closely spaced from the second grid and maintained at a much higher positive potential. Due to the large potential difference and the close spacing between the tubular anode and the second grid, cold emission and spark discharge may lead to a condition commonly referred to as popover which is manifested by a momentary loss of brightness or a tearing out of minor portions of the reproduced image. The problem is further aggravated in the case of electrostatically focused cathode-ray tubes employing unipotential focusing lens systems, particularly in the case of such systems wherein the focusing electrode is maintained at or near cathode potential, due to the extremely high voltage gradients between the focusing electrode and the other two elements of the focusing lens.

It is an important object of the present invention to provide a new and improved image-reproducing device in which cold emission and spark discharge in the electrode system are effectively inhibited.

It is a further object of the invention to provide a new and improved image-reproducing device in which the objective of reduced cold emission and spark discharge is accomplished by means of a simple and inexpensive modification of the tube structure.

In accordance with the present invention, a new and improved cathode-ray tube comprises an electrode system for projecting an electron beam. The electrode system includes a cathode and a plurality of tubular electrodes supported within an evacuated envelope, and a conductive coating on the inner wall of the envelope. An additional conductive coating on the inner wall of the envelope, substantially shielded from the path of the elect on b am. at least partially lencompassing the tubular electrodes and electriconnection with the accompanying drawing, in

which the single figure is a fragmentary side elevation, partly in cross-section and partly cut away, of an image-reproducing device constructed in accordance with the present invention.

The image-reproducing device of the figure comprises a fluorescent screen l0 afiixed to the glass target portion ll of a cathode-ray tube envelope which also comprises a glass neck portion I 2 enclosing an electron gun and an electrostatic focusing system. The electron gun comprises a cathode I 3, a control electrode l4, and first and second tubular accelerating electrodes l5 and I6 respectively. A diaphragm I! having a central aperture I8 is disposed across the outlet end of second accelerating electrode l6, and aperture I8 is symmetrically centered with respect to the tube axis A-A perpendicular to the center of the fluorescent screen I0. Second accelerating electrode 5 is laterally offset from first accelerating electrode F5 to provide a steady transverse electrostatic-deflection field component in the region between these two electrodes,

and the entire electron gun structure is tilted with respect to the tube axis A-A by an angle 0.

An electrostatic focusing system of the unipotential lens type is disposed between the electron gun and the fluorescent screen. The focusing system comprises the outlet end of second accelerating electrode l6 including diaphragm IT, a lens electrode I9, and an additional electrode 20 which are all coaxially mounted with respect to the tube axis AA. A centrally apertured conductive disc 2| is disposed in the neck portion of the envelope between the focusing system and target portion H. A conductive coating 22, of

:colloidal graphite such as aquadag or the like.

extends from the direction of target portion ll into the neck portion of the envelope, and conductive disc 2| is maintained at a common potential with conductive coating 22 by means of metal contact springs 23.

For convenience, electrodes 14, I 5, I6, I 9 and 20 may be termed grids and may be designated by number starting with control electrode I4 as the first grid and progressing in the direction of beam travel to additional electrode 20 which is the fifth grid. All five grids are suported in predetermined mutually spaced relation by means of a pair of glass pillars 24, of which only one is shown, in a manner which will be apparent to those skilled in the art. Separate leads for grids I, 2 and 4 extend through the base 25 of the tube, as do the supply leads for the cathode I3 and its associated heater element (not shown). Lead 26 from grid 4 through the base of the tube may be provided with an insulating glass bead (not shown) to inhibit spark discharge to electrode I6. Conductive disc H is mechanically supported from and electrically connected to grids 3 and 5 by means of metal connecting strips 21. Operating potential for the conductive coating 22, and therefore for the third and fifth grids, may be supplied by means of a conventional contact button if the envelope is of the all glass type, or directly to the metal cone member if the tube is of the glass-metal variety.

An external permanent magnet 28, supported in a spring clamp 29 which fits snugly around the neck of the tube and is movable both axially and rotationally, is provided to develop a magnetic field within the tube to provide separation of the negative ions from the electron beam.

The tube is evacuated, sealed and based in accordance with well known procedures which require no further explanation, and suitable getters 3| are supported from the surface of conductive disc 2I facing fluorescent screen III to absorb residual gases after evacuation.

In operation, a mixed beam of electrons and negative ions originating at cathode I3 is projected through the aperture in second grid I5. When the mixed beam emerges from grid 2, it encounters an electrostatic field having a transverse component due to the lateral offset of grid 3 with respect to grid 2. Consequently, electrons and ions are both deflected to the left in the view of the figure. The magnetic field imposed by beam-bender magnet 28 serves to defiect the electrons to the right as viewed in the figure without substantially affecting the path taken by the negativ ions. Thus, when beam-bender magnet 28 is accurately adjusted, the beam of electrons is projected centrally through aperture I8 of diaphragm IT in a direction along the tube axis AA, while the negative ions are intercepted by the metallic portions of grid 3 and diaphragm II. The ion-trap mechanism is disclosed and claimed in the copending application of Willis E. Phillips et al., Serial No. 156,746, filed April 19, 1950, for Electron Gun for Cathode- Ray Tubes, now Patent No. 2,596,508, issued May 13, 1952, and assigned to the present assignee.

The axially directed electron beam is subjected to the focusing action of the electrostatic fields produced by diaphragm II, lens electrode I9 and the fifth grid 20 which together constitute a unipotential electrostatic focusing lens system. The general construction and operation of lenses of this type are well understood by those skilled in the art as indicated by an article entitled Measured properties of strong unipotential electron lenses by G. Liebmann, Proceedings of the Physical Society, Section B, volume 62, part 4, pages 213-228 (April 1, 1949) The required operating potential difference between the lens electrode (grid 4) and the other electrodes of the focusing system (grids 3 and 5) is determined by the dimensions Di and the spacing between the electrodes constituting the unipotential lens. Although the relationships are not necessarily linear, the required focusing potential difference varies directly with the length and inversely with the diameter of grid 4, and inversely with the separation between grid 4 and grids 3 and 5. Certain limitations on these parameters are imposed by practical considerations; if the diameter of grid 4 is made too small, excessive spherical aberration is encountered, and if the separation between grids 3 and 4 is made too great, the deflecting influence exerted by the asymmetrical electrostatic field established between lead wire 26 and grid 3 becomes objectionable. The focusing system is preferably constructed and arranged to obtain focusing with grid 4 operated at or near cathode potential, in order to avoid the necessity of providing a source of operating potential intermediate the B-supply voltage and the final anode voltage.

In order to obtain satisfactory focusing with grid 4 operated at a potential between cathode potential and the B-supply voltage, it is necessary to maintain rather stringent manufacturing tolerances with respect to the dimensions and spacings of the several electrodes constituting the focusing system. In addition, when grid 4 is operated at a potential substantially equal to that of the cathode, extremely high voltage gradients are produced between grid 4 and grids 3 and 5. In order to suppress undesirable corona effects and field emission, grids 3 and 5 are each provided with corona rings 32 and 33 in the form of rolled flanges of stainless steel or the like which are welded or otherwise secured to the respective electrodes, and grid 4 is constructed by rolling the two ends of a metal cylinder 34 over the edge of a large aperture in a metal disc 35.

Corona rings 32 and 33 also perform an additional function in facilitating the maintenance of the required close manufacturing tolerances by mechanically reinforcing the circular flanges to which they are attached against warping or bending during the assembly of the electrode system. The electrodes are assembled by means of accurately constructed jigs and are all rigidly supported by means of opposed common glass pillars 24, the gun assembly being properly oriented in the tube neck by means of other jigs in the usual manner. It has been found that these precautions suflice to insure satisfactory operation of the completed structure, any small deviations in dimensions and spacings being readily compensated by adjustment of the ion-trap magnet 28.

For best results, it has been found that the apertures in grids I, 2, 3 and 5 should be in marginally overlapping alignment in a direction parallel to the tube axis AA. In other words, all of these apertures should intercept an imaginary straight line parallel to reference axis AA, and the apertures in grids I and 2 should intercept that line asymmetrically. Fulfillment of this condition is dependent upon the angle 0 by which the entire electron gun is tilted with respect to the tube axis, and also upon the length of the electron gun from the cathode to aperture I8 in diaphragm I1. If the angle 0 and/or the length of the gun is increased to such an extent that the apertures in grids I, 2, 3 and 5 are no longer in marginally overlapping alignment in a direction parallel to the tube axis, increased multiplicity of focus is encountered, and the performance of the focusing system is inferior. On the other hand, if the angle 0 is decreased so that the apertures are in complete coaxial alignment, ion trappin may no longer be conveniently accomplished.

By employing a separate conductive disc 2! for establishing electrical contact to conductive coating 22, and by terminating conductive coating 22 at substantially the plane of the conductive disc 2 1, high potential gradients and undesirable spark discharge between the low-potential lens electrode i9 and the high-potential conductive coating 22 are substantially avoided. Moreover, even though this construction results in a space between grid 5 and conductive disc 2i in which the boundary potentials are not definitely established, no observable distortion or defocusing of the beam is encountered. The size of the aperture in conductive disc 2| is not critical but should be large with respect to the apertures in grids 3 and 5.

Moreover, conductive disc 2! serves as an effective getter shield to avoid conductive deposits on glass support pillars 2 when the getter 3! is flashed during the processing of the tube. In this manner, excessively high potential gradients along the insulating pillars and possible insulator breakdown are substantially avoided.

Due to the close spacings and the high potential differences between grids 2 and 3 and between grid :3 and grids 3 and 5, undesirable cold emission and spark discharge between these electrodes may be encountered. This undesirable situation may be avoided by observing every precaution to insure that the glass envelope and all of the electrodes are kept clean and free from grease or other organic matter, and by avoiding sharp edges on the closely spaced opposing portions of the several electrodes. However, it has been found in practice, particularly in the production of electrostatically focused picture tubes embodying unipotential focusing lens systems, that the requisite degree of cleanliness cannot be efiectively maintained by the use of commercially feasible production methods.

In accordance with the present invention, undesirable cold emission and spark discharge in the electrode system are effectively inhibited by providing a second conductive coating 49, separate from the conductive coating 22 which is connected to grids 3 and 5 and maintained at high potential, within the glass neck portion of the envelope in a position at least partially encompassing the tubular electrodes 1 5 and it of the electron gun. Conductive coating 39 is maintained at a constant low positive potential with respect to cathode i3, preferably by means of contact springs 4! connected to the second grid l 5. It has been found that a cathode-ray tube constructed in this manner may be operated with final anode voltages considerably in excess of those required in actual use of the tube in a television receiver or the like without encountering spark discharge or popover in the electrode system.

The technical reasons underlying the success of the construction of the present invention are not fully understood. One theory which seems in agreement with numerous observations is predicated on the hypothesis that cold emission and spark discharge are only encountered at normal operating voltages in the presence of extraneous gas molecules. In a conventional image-reproducing device not provided with the conductive coating 19 of the present invention, primary or secondary electron bombardment of the glass neck portion of the envelope is thought to result in the liberation of occluded gases from grease,

' vice, and Robert W.

dirt, or other extraneous organic matter which may be present. These gas molecules then become ionized and are caused to bombard the gun electrodes, leading to cold emission which ultimately results in spark discharge. This theory is supported by the observation that troublesome spark discharge generally takes place along the glass wall of the envelope in the first instance and only later across the shorter intervening space between the gun electrodes. By providing internal conductive coating 48, and by maintaining that coating at a low constant positive potential by means of contact springs 41 connected to second grid l5, gas liberation is avoided for two reasons. In the first place, coating 39 acts as a physical barrier between the offending primary and/0r secondary electrons and the glass wall of the envelope, thus rendering extraneous grease or dirt on the glass wall inaccessible. In the second place, such stray electrons as reach the wall of the tube are collected by coating 40 and conducted oil through the external circuit associated with the second grid.

While it is preferred to employ internal contact springs 4| to connect coating 49 to second grid [5, in order to avoid the necessity for an additional external lead from the envelope, the potential at which conductive coating 40 is maintained is not critical, and it is within the scope or" the invention to provide other means independent of the gun electrodes for connecting coating 40 to an associated constant potential source.

Coating 49 may be formed of silver paint or any other conductive material which is amenable to application in the form of an internal coating. The length of coating 40 is not critical, although it is essential that the tubular gun electrodes be at least partially encompassed by the coating, and that coating 46 be maintained physically and electrically separate from the final anode coating 22.

Thus the present invention provides a new and improved image-reproducing device in which cold emission and spark discharge are effectively inhibited by the simple expedient of providing a conductive coating on the inner wall of the glass neck, physically and electrically separate from the final anode coating and at least partially encompassing the tubular gun electrodes. It has been found that image-reproducing devices constructed in accordance with the present invention, whether of the magnetically focused or the electrostatically focused type, exhibit a greatly re-v duced tendency towards undesirable popover, even at operating voltages greatly in excess of those normally employed.

Certain features of the construction illustrated and described in the present application are disclosed and claimed in the copending applications of Constantin S. Szegho, Serial No. 229,013, filed May 31, 1951, for Image-Reproducing Device, Jerome J. OCallaghan, Serial No. 235,045, filed July 3, 1951, now Patent No. 2,627,043, issued January 27, 1953, for Image-Reproducing De- Shawfrank, Serial No. 234,920, filed July 3, 1951, now Patent No. 2,627,049, issued January 27, 1953, for Cathode- Ray Tube Electrode, all of which are assigned to the present assignee.

While a particular embodiment of the present invention has been shown and described, it is apparent that various changes and modifications may be made, and it is therefore contemplated in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of the invention.

I claim:

1. A cathode-ray tube comprising: an evacuated envelope; an electrode system within said envelope for projecting .an electron beam, said electrode system including a cathode, a plurality of tubular electrodes, and a conductive coating on the inner wall of said envelope; and a second conductive coating on the inner wall of said envelope, separate from said first-mentioned coating, substantially shielded from the path of said electron beam, and electrically connected to one of said tubular electrodes, for inhibiting cold emission and spark discharge in said electrode system.

2. A cathode-ray tube comprising: an evacuated envelope; a fluorescent screen supported within said envelope; an electrode system within said envelope for projecting an electron beam, said electrode system including a cathode, a plurality of tubular electrodes, and a conductive coating on the inner wall of said envelope between said tubular electrodes and said fluorescent screen; means for electrically connecting said conductive coating to one of said tubular electrodes; and another conductive coating on the inner wall of said envelope, separate from said first-mentioned coating, substantially shielded from the path of said electron beam, and electrically connected to another of said tubular electrodes, for inhibiting cold emission and spark discharge in said electrode system.

3. A cathode-ray tube comprising: an evacuated envelope; a fluorescent screen supported within said envelope; an electrode system within said envelope for projecting an electron beam, said electrode system including a cathode, a plurality of tubular electrodes, and a conductive coating on the inner wall of said envelope between said tubular electrodes and said fluorescent screen; means including a plurality of contact springs for electrically connecting said conductive coating to one of said tubular electrodes; another conductive coating on the inner wall of said envelope, separate from said first mentioned coating, substantially shielded from the path of said electron beam, and at least partially encompassing said tubular electrodes, for inhibiting cold emission and spark discharge in said electrode system; and additional contact springs for electrically connecting said other conductive coating to another of said tubular electrodes.

4. A cathode-ray tube comprising: an evacuated envelope; a fluorescent screen supported within said envelope; an electrode system within said envelope for projecting an electron beam, said electrode system including a cathode, first and second grids, a tubular electrode, and a conductive coating on the inner wall of said envelope between said tubular electrode and said fluorescent screen; means including a plurality of contact springs for electrically connecting said conductive coating to said tubular electrode; another conductive coating on the inner wall of said envelope, separate from said first-mentioned coating, substantially shielded from the path oi! said electron beam, and at least partially encompassing said second grid and said tubular electrode, for inhibiting cold emission and spark discharge in said electrode system; and additional contact springs for electrically connecting said other conductive coating to said second grid.

5. A cathode-ray tube comprising: an evacuated envelope having a glass neck portion; a fluorescent screen supported within said envelope; an electrode system for projecting an electron beam, said electrode system, including a cathode, first and second grids, and a tubular electrode all supported within said glass neck portion, and a conductive coating on the inner wall of said envelope electrically connected to said tubular electrode, for projecting an electron beam toward said fluorescent screen; and another conductive coating, on the inner wall of said neck portion, separate from said first-mentioned coating, substantially shielded from the path of the electron beam, and electrically connected to said second grid, for inhibiting cold emission and spark discharge in said electrode system.

RUSSEL S. PE'IERMAN.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 2,058,914 Rudenberg Oct. 27, 1936 2,070,319 Rudenberg Feb. 9, 1937 2,170,251 Schlesinger Aug. 22, 1939 2,210,127 Rogowski Aug. 6, 1940 2,213,175 Iams et al Aug. 27, 1940 2,250,927 Davisson July 29, 1941 2,264,274 Broadway Dec. 2, 1941 2,275,864 Record Mar. 10, 1942 2,363,359 Ramo Nov. 21, 1944 2,409,514 Pratt Oct. 15, 1946 2,414,881 Law Jan. 28, 1947 2,452,919 Gabor Nov. 2, 1948 2,454,345 Rudenberg Nov. 23, 1948 2,496,127 Kelar Jan. 31, 1950 2,555,850 Glyptis June 5, 1951 2,562,242 Pohle July 31, 1951 2,567,893 Pohle Sept. 11, 1951 2,572,858 Harrison Oct. 30, 1951

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