US2457175A - Projection cathode-ray tube - Google Patents

Description

Dec. 28, 1948. L. w. PARKER' PROJEGTION CATHOD-RAY TUBE Filed Deo. 19, 1946V lll IN VEN TOR.

Patented Dec. 28, 1948 UNITED 4sfr-Ares PATENT orifice PROJECTION `CATHODE-RAY TUBE,

l Louis W. Parker, Linie Neck, N. Y., assigner tc Federal Telecommunication` Laboratories, Inc.,

, New York, N. Y., a corporation of Delaware Application December 19, 1946, scri-a1 Nc. 717,224.

I 1 This invention relates more particularly to such tubes which are to be used` as high intensity fluorescent light sources.

A principal object of the invention is to provide a novel construction of cathode-ray tube for image projection purposes.

` Another object is to provide a cathode-ray tube of high fluorescent light intensity by employing a special multiple electron gun construction for developing a high current `density in the final cathode-ray beam without lcorrespondingly increasing the second anode voltage,` or without correspondingly increasing the electron velocity at the screen.

A further object is to provide a fluorescent image projection tube wherein a plurality of electron guns are `mounted and correlated With a common electron baieeleotrode to produce 'what may be termed a single composite virtual cathode of elemental beam cross-section.

A feature of the invention relates to a :cathoderay tube having a plurality of similar electron guns which develop independent beams that converge at a common plane Where an apertured electron baille electrode is provided to'produce a composite virtual cathode of elemental cross-section. The electron image at this baille is then acted upon by a magnetic lens system to produce a focussed image thereof on the fluorescent screen of the cathode-ray tube. y

. Another feature relates to the novel organization and location of parts which cooperate to provide an improvedcathode-ray tube for the direct projection ofuorescentimages, such for example as in television projection tubes and the like.

Other features and advantages will be apparent after a consideration of the following detailed descriptions arid the appended claims.`

Referring to the drawing which shows in structural schematic and circuit form one preferred embodiment, there is illustrated an evacuated envelope I of glass or other material such as customarily employed in cathode-raytube construc-v tion. This envelope comprises `a plurality of diverging neck portions II, I 2,A which join a com-- mon substantially cylindrical or tubular body portion I3. Portion I3 joins anf enlarged or bulbous portion I4 which is closed by the usual at` tened window portion I5. Window 'I5 is coated interiorly with a layer I6 of any well-known iiuorescent material which emits fluorescent light when'boinbarded by cathode rays.

Preferably, the neckportions Il and`I2 are each` arranged at an acute angle with `respect to 4 creams. (o1. 25oism to cathode-ray tubes and the Acentral longitudinal axis of the envelope, sol

that the central longitudinal axis of these necks intersectr at the plane I'l-Il. suitably supported Within each neck is an electron-emitting cath-A ode I8, i9, which is of any Well-known type used in cathode-ray tubes and comprising for example a tubular metal sleeve Within which is insulatingly supported a heater filament or coil. The end of each sleeve facing the screen I6 is coated with electron-emissive material to develop a `beam of electrons when the associated sleeve is raised to a suitable temperature. `Adjacent each cathode is an apertured control electrode 20, 2l, which. electrodes are arranged to be excited in parallel with variable signals, such for example as those received .from the video amplifier of any wellknown television receiver. Y

The internal surface oi the envelope between the point 22 and point 23, is coated With a conductive paint such for example as Aquadag or colloidal graphite 24, which is contacted by a lead-in wire 2.5, sealedvthrough the Wall of the envelope and provided externally with a metal connector cap 26. Cap 20 is connected by conductor 21 to a predetermined point, for example `+1000 volts D. C. in a suitable source of D. C. supply. The cathodes I8 and I9 can be suitably biased positively with respect to the grounded end of the DQC. source by a potentiometer slider arm 28, and the voltage divider resistor 29. It will be observed that a suitable source of D. C. potential,

e. g. 8000 volts, is connected across resistors 29,' 30, 3l, 32 in series, so that the point 33 can be given any desirable bias, for example from 0 to 50 volts. The points 34, 35 and 36 are then respectively at approximately +1000 volts, +1500 volts and +8000 volts. In the Well-known manner, the coating 24 acts as the rst or accelerating anode which is common to the two cathoderay beams emitted from the cathode sleeves I8 and I9.

Suitably mounted Within the portion I3 of the envelope and at the plane Il-I'I, is a metal baffle electrode 31, which has a central opening 38,

located on the central longitudinal axis of the tube. This baille 3l is connected to the point 3'5 which is approximately 500 volts higher in positive potential than the coating 24. The bulb por-` is any well-known form of magnetic beam focussing unit 42. Likewise, surrounding the neck portion l2 is a similar beam focusing unit 43. These units 42 and 43 are adjustably mounted so as to change the orientation of the focussing magnetic field within the respective neck of the tube and thereby correspondingly control the trajectory of the focussed beam from the respective cathode in such a way that the two beams -converge and are focussed at the baille opening 38. One particular manner of providing this adjustability is illustrated in the drawing. For example, the device 42 may consist of the usual electromagnetic focussing coil which is pivotally supported for example at point 44 on a suitable rigid support 45, and the opposite side of the coil is adjustably supported from a corresponding xed support 46, through the intermediary of an adjusting screw 41, and a compression spring l48. Thus by turning screw k4I the orientation of the focusing coil 42 with respect to the central longitudinal axis of the neck Il, can be readily arranged. Likewise, the focussing unit 43 is pivotally supported at a point 49 and is provided on the opposite side with an adjusting screw 50 and a corresponding compression spring 5|. In order to facilitate the focussing of the two beams 52, 53 at the opening 38, the side of baille 3'I facing the cathodes I8 and I9, is coated with fluorescent material. The screws 47 and 50 are adjusted so that the portion of each of the beams which strikes the baffle 31 is at a minimum. In the well-known manner, the current through the windings of the focussing coils 42 and 43 can be adjusted to obtain an accurate focus of the beams 52 and 53 at the opening 38. The invention is not limited to an adjustment wherein the cross-section of the converging beams at the plane I1 is less than the size of the opening 38. If the two beams are improperly focussed, this will result merely in a lowering of the brilliancy of the picture on the screen I6, but will not affect its sharpness. This result is obtainable by reason of the fact that the intersecting beams form in effect a virtual cathode at the opening 38, and the size or crosssection of this Virtual cathode is xed by the size of the opening 38 regardless of the degree of focussing by units 42 and 43. Located between the member 31 and the screen I6 is any wellknown form of magnetic field producing device `54, which acts as an electron lens to form on the screen I6 an image of the virtual cathode at the opening 38. Also located between the magnetic lens 54 and the screen I-B is any wellknown form of beam-deecting arrangement 5'5 for deilecting the beam in coordinate directions so as to scan the screen I6 in the conventional manner. Instead of using electromagnetic focussing coil units 42 and 43, each of these units may consist of permanent magnets.

While the drawing shows a tube having two separate electron guns for producing the intersecting and converging cathode-ray beams, it will be understood that a greater number of such guns may be employed. For example, if six guns are employed, the tubular body portion lo will terminate in six separate neck portions similar to portions Il and I2, all these portions being symmetrically arranged around the longitudinal central axis of the tube so that each gun produces an individual cathode-ray beam which is focussed at the opening 38 to produce the desired composite virtual cathode thereat.

The above described arrangement has the ad- 4 vantage that it is possible to obtain an almost unlimited quantity of electrons from the virtual cathode at the opening 38. According to known theory, the maximum emission from the usual oxide cathode is two milliamperes per square millimeter. Since in the usual cathode-ray tube, the maximum size of the cathode area is limited to 1/4 square millimeters, only about 1/2 milliampere is available for cathode current, and the actual beam current may be even less. On the other hand, the brilliancy of the scanning spot on the screen I6 is determined by the watts of power dissipated on it by the beam. To obtain .as much as 10 watts dissipation on the screen of the usual projection type cathode-ray tube. it is necessary to use as much as kilovolts and about 330 microamperes beam current. With the arrangement above described, the voltage applied to the coating 39 may be reduced to 25% of its usual value, for example 8000 volts D. C., and the beam current can be increased, depending upony the number of separate guns that'l are employed for producing the single virtual cathode above descirbed. Probably the maximum beam current at which well-known saturation effects can be neglected, is about 2 milliamperes with 8000. volts electron velocity at the screen. A practical value would be 8000 volts electron velocity and 1.2 milliamperes beam current. This value is not difficult to obtain and produces the same brilliancy as other projection type tubes which require 30 kilovolts for the second anode potential.

While one particular embodiment has been described herein, it will be understood that various changes and modifications may be made Without departing from the spirit and scope of the invention.

What is claimed is:

1. A cathode-ray tubecomprising an envelope having a fluorescent screen, a plurality of electron guns for developing respective electron beams, an electron baille electrode having a central opening symmetrically disposed with respect toA said beams, and having a focus-indicator surface facing said guns which is coat-ed with fluorescent material to fluoresce when said beams impinge thereon, means to focus said beams at said opening to form a virtual composite cathode thereat, and an electron lens for forming on said screen an image of said composite Virtual cathode.

2. A cathode-ray tube comprising an envelope having a fluorescent screen, a plurality of electron guns for developing respective electron beams, a common electron baille electrode for said beams, said baffle having a focus-indicator surface facing said guns which is coated with fluorescent material to fluoresce when said beams impinge thereon, separate forcussingr means for said beams, means to adjust said focussing means to focus said beams at the electron opening in said baille, and a magnetic lens for imaging on saidgscreen the composite electron beam as it passes said baille opening.

3. A cathode-ray tube comprising an envelope having a fluorescent screen, said envelope having a plurality of diverging neck portions, an electron gun located in each neck portion to produce respective electron beams whose trajectories crossover at a point between the guns and the screen, an apertured electron baille electrode having its aperture located at said cross-over point and energized to produce a composite virtual cathode at said aperture, said baille having a focus-ndicator surface facing said guns which is coated with iluorescent material to fluoresce when said beams impinge thereon, and an electron lens for imaging upon said screen the said composite virtual cathode. r

4. A cathode-ray tube comprising an envelope having a fluorescent screen, said envelope having a plurality of diverging neck portions, an electron gun located in each neck portion to produce respective electron beams whose trajectories cross-over at a pointl between `the guns and the screen, rst and second accelerating anodes for said guns', an apertured electron baille electrode located between said anodes and having its aperture located at said cross-over point, said bafe having a focus-indicator surface facing said guns which is coated with fluorescent material to uoresce when said beams impinge thereon, and an electron lens for imaging upon said screen the composite electron beam as it passes the baille aperture.

LOUIS W. PARKER.

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

UNITED STATES PATENTS

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