US2627043A - Image-reproducing device - Google Patents

Description

Jan. 27, 1953 J. J. ocALLAGHAN IMAGE-REPRODUCING DEVICE Filed July 3, 1951 w UIOH .obnnamwa om IN V EN TOR.

HISYATTORNEY.

Patented Jan. 27, 1953 IMAGE-REPRODUCIN G DEVICE Jerome J. OCallaghan, Skokie, Ill., assigner to The Rauland Corporation, a corporation of Illinois ApplicationJuly 3, 1951,. Serial No. 235,045`

3 Claims.

This invention relates to image-reproducing devices and more particularly to electrostatically focused cathode-ray tubes for use as picture-reproducing devices in television receivers and the like.

In the copending application of Constantin S. Szegho, Serial No. 229,013, led May 31, 1951, for Image-Reproducing Device, and assigned to the present assignee, there is disclosed and claimed a novel cathode-ray tube employing a unipotential electrostatic focusing system which requires no operating potential between the B-supply Voltage of the associated receiver apparatus and the iinal anode voltage. The unipotential electrostatic focusing system comprises three electrodes, the outermost two of which are operated at final anode voltage, while the intermediate one is operated at a' voltage between ground potential and theB-supply voltage of the associated apparatus to provide the desired electrostatic field distribution comprising convergent and divergent lens components and having a net convergent eiect on the electron beam. Gne of the outermost electrodes is connected to the conductive coating on the inner wall of theenvelope by means of conventional metal spacer springs or the like, and conductive connecting strips are provided between the two outermost electrodes -so that both of these electrodes are maintained at a common operating potential with the conductive coating. The getter rings are supported on the screen side of the last electrode.

While. the cathode-ray tube described inthe Szegho application accomplishes the objective of satisfactory focusing without employing any operating potential between that of theV receiver B-supply and the final anode voltage, certain difliculties of a practical nature are sometimes encountered. Because the intermediate focusing electrode is operated at or near cathode potential, there is a high potential gradient between that electrode and the conductive wall coating, and undesirable spark discharge may result. Moreover, flashing of the getter material sometimes results in conductive deposits on the insulating spacer-support pillars for the focusing electrodes, thereby decreasing the length of the` effective leakagepath across these pillars and occasionally resulting in breakdown of the insulating material.

It is an important object of the present invention to provide an improved cathode-ray` tube of the electrostatically focused type in which these .difficulties are substantially avoided.A

Toaccomplsh this. and other objects, anelectrostatically focused. image-reproducing device of the cathode-ray type comprises an` evacuated envelope having a iiuorescent-screen-bearing,target portion and a neck portion. An electron gun isV housed in the neck portion of the envelope for projecting an electron beam towards the target portion, the electron. gun including a cathode. a control electrode, and first and second accelerating electrodes. Following the electron gun is an electrostatic focusing system including a lens electrode and an additional electrode. Acentrally apertured conductive disc is disposed in the neck portion of the envelope between the focusing system and the target portion. A conductive coating on the inner wall of the envelope extends from the direction of the target portion into the neck portion substantially only to the plane of the conductive disc. Means are provided for establishingan electrical contact between the conductive disc and the conductive coating, and an electrical connection is provided withinl the envelope between the additional electrode and the conductive disc.

The features of the present invention which are believed to be novel are set forth with particularity in the appended claims. The invention, together with further objects and advantages thereof, may best be understood, however, by reference to the following description taken in connection with the accompanying drawing, in the several figures of which like reference. numerals indicate like elements, and in which:

Figure l is a fragmentary sideelevation, partly in cross-section and partly cut away, of an imagereproducing device constructed in accordance with the present invention; and

Figure 2 is a schematicl diagram of a television receiver embodying the image-reproducing kdevice of Figure l.

The image-reproducing device of Figure l-comprises a fluorescent screen I0. aixed to the glass target portion II of a cathode-ray tube envelope which also comprises a glass neck portion Ilenelosing an electron gun and an electrostatic focusing system. The electronA gun comprises. a cathode I3, a controlelectrode I4, and rstand secondaccelerating electrodes I5 and I6 respectively. A diaphragm I'I having a central aperture I8 is disposed across the outlet end of'second accelerating electrode I6; 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 I S is laterally offset fromrst accelerating electrodeIE to. provideiatransverse electrostatic'- deflection field 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 I6 including diaphragm I1, a lens electrode I9, and an additional electrode 20 which are all coaxially mounted with respect to the tube axis A-A. A centrally apertured conductive disc 2l is disposed in the neck portion of the envelope between the focusing system and target portion I I. A conductive coating 22, of colloidal graphite such as aquadag or the like, extends from the direction of target portion II into the neck portion of the envelope substantially only to the plane of conductive disc 2 I, and conductive disc 2| is maintained at a common potential with conductive coating 22 by means of metal spacer springs 23. For convenience, electrodes I4, I5, I5, E@ 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 2l) which is the fth grid. All iive grids are supported 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 supplyrleads 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 2l 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 fth 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. Moreover, a fluorescent coating 39 on the outer surface of the second accelerating electrode I (grid 3) is provided for facilitating alignment of the ion-trap beam-bender magnet 23.

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 2l facing uorescent screen I@ to absorb residual gases after evacuation.

In operation, a mixed beam of electrons and Vnegative ions originating at cathode I3 is projected through the aperture in rst accelerating electrode 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 upwardly in the view of Figure 1. The magnetic field imposed by beam-bender magnet 28 serves to deflect the electrons in a downward direction as viewed in Figure 1 without substantially affecting the path taken by the negative ions. Thus, when beam-bender magnet 28 is accurately adjusted, the beam of electrons is projected centrally through aperture I8 of diaphragm I1 in aV direction along the tube axis A-A, while the negative ions are intercepted by the metallic portions of grid 3 and diaphragm I?. 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 U. S. 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 elds produced by diaphragm I'I, lens electrode I9 and the fifth grid 2Q 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 ldimensions of and the spacing between the electrodes constituting the unipotential lens. Although the relationships are not necessarily linear7 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 deiiecting influence exerted by the asymmetrical electrostatic field established between lead wire 25 and grid 3 becomes objectionable.

Unipotential electrostatic lens systems have previously been employed in cathode-ray tubes. Such lens systems have been found quite satisfactory and readily adaptable to mass production techniques when the electron gun and the focusing system are coaxial and the path of the beam is restricted to the tube axis during its entire progress from the cathode through the focusing system. However, according to present commercial practice, nearly all television picture tubes are provided with an ion-trap mechanism of one sort or another for removing negative ions from the electron beam in order to avoid deterioration of the fluorescent screen. Such types of ion-trap arrangements as are commonly employed provide ion separation by subjecting the mixed beam. to opposed electrostatic and magnetic fields; both electrons and ions are transversely deflected by the electrostatic field, while the electrons only are substantially deflected in the opposite direction by the impressed magnetic field. It is apparent that the practical requirement for ion trapping results in a displacement of the electron beam from the tube axis, and it is necessary that the beam be directed to the axis before it enters the focusing system. The accuracy with which this is accomplished is of importance when magnetic focusing is employed or when a unipotential electrostatic lens system is employed with a focusing voltage source which may be varied over a relatively wide range, but slight misalignment may` be` compensated by varying: the;` focusing voltage: However, theion. trapv alignment becomes much more critical whenv satisfactory operation off theA electrostatic. focusing. system. is requiredv Within a narrow focusingrvoltagefrange between ground; potential andthe: B-supply. voltage'- of the' associated; apparatus', owing to the increased strength. of: the individual lens: components constituting the unipotential focusing system..`

In general, ifthe electron beam is'inaccurately centered. or approaches'thefocusing system in an' angular manner, multiplicity of focus.: is en'- countered.. The most troublesomaform is which this` manifests itself is. that.' of astigmatism and/orl coma. In order to. obtain. focusing. com;-V

parable with thatproyidedby magneticfocusing systems, while. operating the' lens electrode: at or near cathode potential, it has been. found essential that` the electron` beam. be centrally directed` along the axis in its passage through the focusing: system.. In other words. the". ion trap must be precisely. adjusted for satisfactory focusings' with a low-potential electrostatic lens system.. Such precise alignment' of the. ion trap is facilitated by providing fluorescent coating 36 on the outer wall of second accelerating electrode I6. This fluorescent coating serves as an ion-trap adjustment indicator; in practice, permanent magnet 28 is moved both axially and rotationally until the glow from uorescent coating' 30 is reduced to a minimum, thus indicating precise ion trap alignment; The. fiuorescent coating must of course be so situated that it is excited into uorescence whenever the electron beam is entirely or partially intercepted by diaphragm I'I, and the fluorescentglow must be visible, either directlyor by reflection, through the transparent'neck portion I2 of the. tube envelope. Other possible locations for fluorescent coating 30 include the inner wall of neck portion I2 near the space between grids 2 and 3, the inner surface of diaphragm I'I, and the. surface of diaphragm 20 facing the cathode. The ion-trap indicator is described andclaimed in the copending applications of Constantin S. Szegho, Serial No. 134,725, filed December 23, 1949, now U. S. Patent No. 2,564,737, issued August 21, 1951, and of Constantin S. Szegho et al., Serial No. 162,906, filed May 19, 1950, both entitled Cathode-Ray Tube, now U. S. Patent; No. 2,565,533, issued August 2.8, 1951i, and both assigned to theA presentf. assignee.

In, order. to. obtain satisfactory focusing. with thesystem: shown in `Figure 1, it is necessary to i maintain. rather stringentv manufacturing 'toler- Aanceswith respect to the. dimensions and spacings, of the several electrodes constituting the focusing system. In addition, sinceL grid 4 is to be-operated ata potential substantially equal to that of the cathode, extremely high voltage gradients are-produced between grid 4 and grids 3 and 52 In orderto avoid undesirable corona effects and eld emission, grids 3 and 5 are each provided with corona rings 32 and 33 in the form of` rolled anges ofy 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 inA a metal disc 35. The construction of grid dis specifically disclosed and claimed in the copending application of Robert W. Shawfrank, Serial No. 234,920 filed concurrentlyl herewith. for Cathode-Ray Tube Electrode', and assignedV to the present assignee.

CFI

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

As compared. with. the somewhat similar constructions. now employed in magnetically focused television picture tubes, the electron gun of the tubeof Figure. 1 has been modied in two further structural respects.. In. the rst place, the angle 0 byv which the entire gun is tilted with respect to the tube axis A-A is reduced from about 6 degrees to about 4% degrees, and the amount of lateral offset of grid 3 with respect to grid 2 is reduced` from 1.9. mm. to 1.6 mm. In the second place, the length of the tubular portion of grid 3 is'reduced. from aboutv 38 millimeters to about 30 millimeters;

These two parameters influence the performance of the electrostatic focusing system in the following' manner. In order to obtain satisfac tory focusing with the reduced focusing-voltage range, it is desirable to make the electron beam more nearly parallel as it enters the focusing system.. This may be accomplished by locating the unipctential lens system nearer to the cathode and thereby'decreasing its focal length; for this reason grid 3 is shortened. However, it is not possible to shorten. grid 3 indefinitely because a. certain. minimum length is required to insure ion. trapping at normal operating voltages. To compensate` for the impaired performance of theiontrap occasioned by shortening grid 3, the amount of lateral offset between grids 2 and Sand the angle 0 by which the entireelectrongun is tilted with respect to the tube axis. A-A may each be increased; however, increasing the angle ofv gun tilt results in an increase in the angle at which the beam enters the focusingsystem, resulting in greater astigmatism and/or coma, so that from this point of View the angle. of gun tilt should be made as small as possible.- consistentl with the requirement for: iontrapping with a single beam-bender magnet. The condition represented'by the relationships set. forth in the preceding paragraph has been found toV provide good focusing while retaining ion trapping at reasonable operating voltages. In this connection; it is observed that the particular type of offset ion trap employed in the tube of Figure 1 has theimportant advantage over other types of ion-trap guns, such as thatemploying coaxial electrodes with a slanted aperture between grids 2 and 3, that a greater angle of gun tilt may be employed for a given amount of lens distortion since the cathode is closer to the: tube axis. Moreover, the offset type of ion trap is free of the characteristic elliptical distortion associated with the coaxial slanted-aperture type.

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 par- .allel to the tube axis A-A. In other words, all

of these apertures should intercept an imaginary straight line parallel to reference axis A--A, and the apertures in grids I and 2 should intercept that line asymmetrically. Fulllment of this condition is dependent upon the angle by which the entire electron gun is tilted with respect to the tube axis, and yalso 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 2, 3 and 5 are no longer in marginally overlapping alignment in ya 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 trapping 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|, 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 and conductive dise 2| in which the boundary potentials are not denitely 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 f grids 3 and 5.

Moreover, conductive disc 2| serves as an effective getter shield to avoid conductive deposits on glass support pillars 24 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.

In the television receiver schematically illustrated in Figure 2, incoming composite television signals are received and separated into video-signal components and synchronizing-signal components by means of conventional receiver circuits 40 which may include a radio-frequency amplifier, an oscillator-converter, an intermediate-frequency amplier, a video detector, a video amplifier and a synchronizing-signal separator,

as well as suitable circuits for reproducing the sound portion of the received signal.

tected -composite video signal from receiver cir-v The decuits 40 is applied between the control electrode I4 and the grounded cathode I3 of an imagereproducing device 4I of the type shown in Figure 1. Synchronizing-signal components of the detected composite video signal are employed to drive a synchronizing system 42 of conventional construction which supplies the line-frequency and field-frequency deilection coils 43 and 44 with suitable Scanning currents to control the scansions of the cathode-ray beam of device 4 I. A high-voltage power supply 45, which may also be of conventional construction, is employed to provide a suitable high operating voltage for the conductive coating 22 to which grids 3 land 5 are internally connected. Lens electrode I9 (grid 4) is connected to a variable tap 46 associated with a potentiometer resistor 41 connected between the receiver D, C. voltage supply source, conventionally designated B+, and ground. With the tube construction shown and described in connection with Figure 1, optimum focus of the cathode-ray beam is achieved when the minimum glow of fluorescent coating 30 indicates precise ion-trap alignment and when the lens electrode I9 is operated at a potential substantially equal to that of the cathode I3.

While the invention has been described as hav.- ing particular advantages in connection with a cathode-ray tube employing a, unipotential electrostatic focusing lens system of the type in which the lens electrode is operated at a low potential equal to 5% or less of the nal anode voltage, it is apparent that the invention may be applied to advantage in cathode-ray tubes employ- Ving other types of unipotential electrostaticfocusing arrangements. Moreover, the invention may be employed in a cathode-ray tube having any type of electron gun, with or without provision for ion trapping.

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 an-d modications as fall within the true spirit and scope of the invention.

I claim:

l. An electrostatically focused image-reproducing device of the cathode-ray type comprising: an evacuated envelope having a neck portion and enclosing a target portion; an electron gun housed in said neck portion of said envelope for projecting an electron beam `toward said tar,- get portion and including a cathode, av control electrode, and first and second accelerating electrodes; an electrostatic focusing system including a lens electrode and an additional electrode; a centrally apertured conductive disc disposed in said neck portion between said focusing system and said target portion; a conductive coating on the inner wall of said envelope extending from the direction of said target portion into said neck portion substantially only to the plane of said conductive disc; means establishing an electrical contact between said conductive disc and said conductive coating; and an electrical connection within said envelope between said additional electrode and said conductive disc.

2. An electrostatically focused image-reproducing device of the cathode-ray type comprising: an evacuated envelope having a neck portion and enclosing a target portion; an electron gun housed in said neck portion of said envelope for projectingan electron beam toward said target portion and including a cathode, a control electrode, and first and second accelerating electrodes; a unipotential electrostatic focusing system including said second accelerating electrode, a lens electrode, and an additional electrode; a centrally apertured conductive disc disposed in said neck portion between said focusing system and said target portion; a conductive coating on the inner wall of said envelope extending from the direction of said target portion into said neck portion substantially only to the plane of said conductive disc; means establishing an electrical contact between said conductive disc and said conductive coating; and an electrical connection within said envelope between said second accelerating electrode, said additional electrode, and said conductive disc.

3.l An electrostatically focused image-reproducmg device of the cathode-ray type comprismg: an evacuated envelope having a neck portion and enclosing a target portion; an electron gun housed in said neck portion of said envelope for projecting an electron beam toward said target portion and including a cathode, a control electrode, and first and second accelerating electrodes; and electrostatic focusing system including a lens electrode and an additional electrode; a centrally apertured conductive disc disposed in said neck portion between said focusing system and said target portion; a conductive coating on the inner Wall of said envelope extending from the direction of said target portion into said neck portion substantially only to the plane of said conductive disc; means establishing an electrical contact between said conductive disc and said conductive coating; an electrical connection within said envelope between said additional electrode and said conductive disc; and a getter ring supported on the surface of said conductive disc facing said target portion.

JEROME J. OCALLAGHAN.

REFERENCES CITED The following references are of record in the le of this patent: Y Y

UNITED STATES PATENTS Number Name Date 2,058,194 Rudenberg Oct. 27, 1936 2,070,319 Rudenberg Feb. 9, 1937 2,210,127 Rogowski Aug. 6, 1940 2,363,359 Ramo Nov. 2l, 1944 2,452,919 Gabor Nov. 2, 1948 2,454,345 Rudenberg Nov. 23, 1948 2,555,850 Glyptis June 5, 1951 n OTHER REFERENCES Industrial Electronics and Control, by R. G. Kloetfler (copyright 1949), published by John W. Wiley & Co., p. 453, Fig. 2.

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