US2661437A - Dark trace cathode-ray tube - Google Patents

Description

De- 1 1953 A. E. BEcKERs 2,661,437

` DARK TRACE CTHODE-RY TUBE Filed June 19, 1951 nanna uurpn oooocooo 90000000.0000..

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' ALBERT E. Blawgs ATTORNEY Patented Dec. 1, 1953 DARK TRACE CATHODE-RAY TUBE Albert E. Beckers, East Orange, N. J., assigner to National Union Radio Corporation, Orange, N. J., a corporation of Delaware Application June 19, 1951, Serial No. 232,319

(Cl. S13- 71) 14 claims. l

This invention relates to electron trace tubes, and more especially it relates to tubes of the socalled dark trace or tenebrescent kind.

As is Well-known, dark trace tubes employ a screen consisting of a layer of crystals which exhibit tenebrescence or formation of dark color centers when bombarded by relatively high velocity electrons from an electron gun. Examples of such crystal screens are those oi' the alkali halide type formed of crystals of an alkali halide or mixture of such halides, for example potassium iodide, potassium chloride and the like.

One of the disadvantages of the dark trace tube as compared with tubes using a iluorescing screen, is the fact that once an opacity center is formed it tends to remain dark or opaque. and special procedures are required to restore the individual darkened areas to their normal light conduction or light reflection condition. Heretofore two methods have been used to effect the erasure of the dark trace record. One such method employs a source of heat adjacent the dark trace screen, but that method is relatively slow in operation and requires a period of several minutes to restore the screen to its normal condition. The second method uses an electron beam to effect the erasure. 'I'hus the customary way of effecting erasure by an electron beam is to use the same electron gun that is used for the dark trace recording, and to greatly increase the beam current for erasure as compared with the beam current for recording. Because of the extraordinarily high beam current, the life and uniformity of the gun are severely curtailed. Thus the usual cathode ray gun employs a cathode emission surface which is very small in area, for example, less than one millimeter and only the actual central portion of that area is useful in developing the cathode ray or electron beam. Therefore in order to secure uniformity of beam current for a given control voltage and in order to provide a reasonably long life, it is necessary to protect the cathode during recording and at other times, against excessive current drain. An average emission current of not more than 100 microamperes has been found admissible. Since the cathode is subjected to its greatest strain or load during erasure it has been proposed heretofore to employ a second gun siznilar to the recording or writing gun but which comes into operation only during erasure. However the use of a separate erasure gun introduces a number of disadvantages. First the additional space needed for location of the separate gun is as a practical matter not available in the customary cathode ray tube construction and a separate tube neck must be used to house the separate gun. This requires separate deflection and beam focussing coils on the two necks with the likelihood of undesirable mutual interaction. Secondly the provision of two separate necks complicates the manufacture and assembly and greatly increases the cost of the finished tube. The tube becomes unwieldy to handle and requires extensive change in the mechanical and electrical design of the associated equipment with which it is to work.

Therefore it is one of the principal objects of the present invention to provide an improved arrangement for enabling the record on the screen of a dark trace tube to be erased without reducing the useful life or uniformity of operation of the tube and Without greatly increasing the cost of manufacture.

Another object is to provide an improved construction of dark trace tube wherein the erasure can be effected with rapidity and wherein the erasing action does not introduce any undesired coloring effect on the screen.

Another object is to provide an erasing arrangement for dark trace tubes. which does not require the action of beam focussing and beam deilecting during the erasure.

A feature of the invention relates to a dark trace cathode ray tube having a novel construction and location of electron nood gun for effecting erasure of a dark trace record.

Another feature relates to an improved dark trace cathode ray tube having mounted relatively close to the dark trace screen. a special erasure iilamentary cathode for subjecting the screen to a flood of erasing electrons uniformly over its entire area and Without requiring an abnormally high voltage gradient between the filament and screen.

Another feature relates to a novel construction and mounting arrangement for an erasing flood gun for dark trace cathode ray tubes.

A further feature relates to the novel organization, arrangement and relative location of parts which cooperate to provide an improved cathode ray tube of the dark trace recording type.

Other features and advantages not particularly enumerated, will become apparent after a consideration of the following detailed descriptions and the appended claims.

In the drawing, which shows by way of example one preferred embodiment,

Fig. 1 is a plan View. partly sectional, of a dark trace cathode ray tube according to the invention.

Fig. 2 is a magnified sectional view of part of Fig. 1.

Fig. 3 is a perspective view of one half of the screen with its mounting arrangement and associated erasure flooding filament.

Fig. 4 is a front View of the erasure flooding filament itself.

Fig. 5 is a schematic diagram explanatory of certain features of the invention.

Referring to Fig. l, there is indicated by the numeral .LILany well-known form of cathode ray tube envelope, comprising the elongated neck portion II joined to the flared bulb portion I2, which is closed off by the transparent end wall I3. Attached to the neck II is any well-known plug-in base I4 carrying the usueilcontactprongs I5. The various lead-in wires .andelectrode support wires (not shown) are sealed through the stem or header (not shown) which closes off `the neck I I and through which the bulb can be evacuated in known manner. Mounted ywithin the neck is any well-known form of electron gun comprising for example the .electron-emitting cathode I6, control grid I1, rst accelerating anode I8, second accelerating anode I9. These electrodes, for simplicity of showing, are indicated schematically in Fig. l, since their actual structure is well-known. Sufice it to say that by suitable heating of the cathode, and by suitable energization of the anodes by direct current voltage, the electrons from the cathode are formed into a beam which can be focussed upon the recording screen by the conventional focussing yoke 20. If desired, the inner surface of the bulb I2 can be coated with a conductive material which can be connected to the second anode potential for purposes well-known in the art. Surrounding the neck are the usual magnetic yokes 2 I, 22, for subjecting the focussed beam to the desired pointby-point scanning motion over the screen.

Mounted at the end of the tube adjacent the wall I3 is the dark trace recording screen and erasure assembly according to the invention. The screen proper, comprises a transparent sheet or support 23 (Figs. 2 and 3) of mica or thin glass which is coated on the side facing the electron gun with a layer 24 of tenebrescent material such as potassium iodide crystals, potassium chloride crystals, or crystals of other tenebrescent halide or halides. This tenebrescent layer is coated with, for example by having evaporated thereon, a Vlayer 25 of aluminum of sufficient thinness as to be transparent to the electron beam 2B which is focussed on layer 24.

The screen comprising elements `23, 24, 25, is mounted in an annular metal supporting frame comprising the metal ring 21 having an integral nange 28, and the said screen is pressed against the ring 2T through the intermediary of a compressible metal ring 29 and another metal ring 30 which ls fastened to the flange 28. The ring 30 has a pair of oppositely extending flanges 3l, 32. Flange 3l acts as a retainer for the compressible metal ring 29, and the flange 32 is for attaching to the flange 28. This latter fastening can be effected in any suitable way, for example by a plurality of straps 33, which can be clinched around the adjacent edges of flanges 28, 32, after the ring 3U has been pressed against the compressible ring 29 to insure good, but somewhat yieldable, electrical contact between the supporting frame and the coating 25.

In accordance with one feature of the invention, the compressible ring 29 may be formed from a sheet or ribbon of aluminum or similar metal of about 0.00025" thickness, and which after being rolled up longitudinally is shaped to form an annular washer which extends around the entire margin of the coating 25 and is capable of being compressed to provide the requisite resilient; pressure to hold the screen in place while making good electrical contact to the said coating, when the ring 30 is adjusted in place. Electrical contact can be made to the coating 25 by a 4 fine wire 34 sealed through the wall of bulb I2 and provided with any well-,knownexternal contact button-'3 5.

Welded or otherwise fastened to the ring 30 1s aseries of metal tabs 3E, each of which carries an individual glass bead 31 and with each bead carrying an individual lament supporting hook 38 which hooks are therefore insulated from ring 30. Strung A.backend forth in zig-zag fashion between the successive hooks and in substantial coplanar form, is a fine wire electron emissive filament 39 :for example of 1% thoriated tungsten wire oi' approximately 0.0016" diameter. This filament constitutes the cathode of the erasure flood gun and is mounted by means of the hooks 38 re1- atively close to the dark trace screen, for example at a distance of one inch. The two ends of the filament are welded to respective lead-"ln wires 40, sealed through the bulb and provided with conventional contact buttons 4I on the external surface of the bulb, Thus the flood ,gun filament can be connected to a suitable source of current to raise it to the requisite electron-emitting temperature for example 2000" K, and at the same time a suitable direct current potential. for example about 500 volts can be applied to the dark trace screen through contact button 35.

In one tube that was found to have the desired recording and erasure characteristics, the screen had a diameter of 6.5 inches and the flood gun erasure filament was stretched in a zig-zag form as shown in Fig. 4, so as to be substantially uniformly effective over the entire screen area. This filament during the erasure interval only, was connected to a source of approximately 450 volts A. C. to produce a filament current of approximately 395 milliamperes, thus raising it to a temperature of approximately 2000 K. At the same time a direct current voltage of about 500 volts with respect to the filament center, was applied to the screen through contact 35. As a result, the screen was bombarded by an electron flow of 150 milliamperes and all the previously recorded dark trace areas were uniformly and completely restored to their normal state Within 7 seconds. It was found that when the filament was heated but without applying a Voltage to the screen, while erasure took place it required a period of from to 120 seconds.

It will be understood that the filament 39 can be of sufficient thinness so as not to cast any appreciable electron shadow on the screen vduring the Writing or recording operation. If however such a shadow should tend to appear, I have found that it can be avoided by applying a relatively small positive potential to the filament during the recording. Thus in the particular tube mentioned above barely perceptible shadows existed when there was applied to the filament a positive potential of 50 volts with respect to the screen and second anode I9.

As shown in Fig. 5, the width W of the electron flow from each one of the parallel strands of the filament to the screen is determined `by the distance d" between the filament and screen. by the well-known work function of the cathode material expressed in E volts, and by the voltage U between the cathode and screen. Therefore, the width when it is heated for erasure it uniformly sprays the entire area of the screen.

Various changes and modifications may be made therein without departing from the spirit and scope of the invention.

What is claimed is:

1. A cathode ray tube having an enclosing envelope containing a screen which becomes tenebrescent in response to impinging electrons in a beam of one range of intensity and which becomes non-tenebrescent in response to impinging electrons in a different range of intensity, a rst electron emitter for developing tenebrescence in said screen, a second electron emitter closely adjacent the screen for erasing said tenebrescence, and lead-in means for said second emitter for supplyingr heating current therethrough.

2. A cathode ray tube having an enclosing envelope containing a screen which becomes tenebrescent in response to impinging electrons in a beam below a certain intensity, a first electron emitter for developing s( id beam, a second electron emitter located between the first emitter and the screen so as to be traversed by the electrons from the first emitter, and means to apply a heating current through to said second emitter to simultaneously flood substantially the entire area of said screen with electrons to erase the tenebrescent record produced thereon by said beam.

3. A cathode ray tube having an enclosing envelope containing a screen which becomes tenebrescent in response to an impinging recording electron beam, and means to erase said tenebrescence comprising an electron flood gun consisting solely of a heatable fine wire electron emitter mounted closely adjacent to said screen, and means to supply heating current through said filament to simultaneously flood said screen with electrons of markedly higher beam intensity than the first beam.

4. A cathode ray tube having an enclosing envelope containing a screen which becomes tenebrescent in response to an impinging recording electron beam, an electron gun for developing said recording electron beam, means to move said beam in a desired scanning pattern over said screen, and another electron emitter mounted closely adjacent to said screen, lead-in means for supplying said other emitter with heating current for producing simultaneously a multiplicity of adjacent electron beams to flood said screen with electrons of much higher beam intensity than the first beam to erase the tenebrescent record on said screen.

5. A cathode ray tube having an enclosing envelope containing .a halide crystal screen of the type which becomes tenebrescent in response to an impinging electron beam, an open-work electron emitter mounted closely adjacent to said screen, and lead-in means to supply heating current to said emitter to flood substantially the entire screen at much higher intensity to erase a previously recorded tenebrescent condition on said screen.

6. A cathode ray tube having an enclosing envelope containing a halide crystal screen of the type which becomes tenebrescent in response to an impinging electron beam, an erasing flood gun including a foraminous electron emitting cathode mounted closely adjacent to said screen, and lead-in means for supplying heating current to said foraminous cathode.

7. A cathode ray tube according to claim 6 in which said cathode is filamentary and with a multiplicity of spaced strands, the ends of the filament being connected to contact terminals for connection to a source of heating current.

8. A cathode ray tube according to claim 6 in which said cathode is fllamentary and consists of a flne Wire strung in zig-zag form adjacent said screen.

9. In a cathode ray tube of the kind described, a halide crystal screen having an electron transparent metal coating, a metal frame for supporting said screen Within said tube, and an open- Work filamentary cathode insulatingly carried by said frame closely adjacent to the screen, and lead-in means to supply heating current to said open work cathode for flooding the screen with erasing electrons to erase a previously recorded tenebrescent condition on said screen.

10. A cathode ray tube according to claim 9 in which said frame is provided with an external connection for applying a voltage difference between the screen and cathode, and said lamentary cathode is provided with external terminals for applying heating current thereto.

1l. A cathode ray tube comprising a recording screen to be scanned by an electron beam, said screen having an electron transparent metal coating and a frame for supporting said screen in electrical contact with said frame, said frame comprising a pair of metal rings and an elastically compressible metal member arranged to be clamped between said rings and in conductive Contact with said coating.

12. A cathode ray tube according to claim 11 in which said yieldable metal member comprises a sheet of thin metal which has been rolled on itself longitudinally and is formed to washer shape.

13. A cathode ray tube according to claim 12 in which said yieldable metal member is formed of aluminum sheeting.

14. A recording screen and erasure assembly for dark trace cathode ray tubes, comprising a light transparent plate having an electron transparent coating and a layer of tenebrescent halide crystals between said plate and coating, a metal frame for supporting said plate within a cathode ray tube envelope, and a multi-strand llamentary cathode carried by said frame in predetermined closely spaced relation to said plate, the said strands being spaced apart to produce a series of electron beams impinging on said screen, the width W of each beam with respect to the spacing of said cathode from said screen being deflned by the formula where E is the work function of the cathode material, d is the spacing between the screen and said cathode, and U is the voltage between the cathode and screen.

ALBERT E. BECKERS.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 2,228,388 Farnsworth Jan. 14, 1941 2,239,887 Ferrant Apr. 29, 1941 2,259,507 Iams Oct. 21, 1941 2,276,359 Von Ardenne Mar. 17, 1942 2,402,762 Leverenz June 25, 1946 2,438,668 Koch et al Mar. 30, 1948 2,533,381 Levy et al Dec. 12, 1950 2,582,843 Moore Jan. l5, 1952

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