US2435435A - Cathode-ray screen - Google Patents

Description

Fgb. 3, 1948. 7 2,435,435

CATHODE RAY SCREEN Filed April 6, 1943 MODIFIED HALIDE SCREEN snom' 'PERSIVSTENCE on HIGH CONTRAST Invent of: Gov'bon RT-onda,

y 'His Attorney.

Patented Feb. 3, 1948 CATHODEFRAY SCREEN Gorton R: Fonda; SchenectadmN. -Y., assignor to GeneraL-iElecti-ic Company,

wii mk a corporation. of 1' Application April 6,1943, Serial No. 482,025 lorfllaims; (Cl. 250:.-16l)i The 1 present invention relates -to cathode --ray devices of the type. having screens which-respond to, excitation byicathoderays; ire; .abeamof electrons, .and; in. particular,- to screens in which a marked darkening is producedbythe-impact ofelectrons. These devices are sometimes referred to as darktrace tubes-because the path of the cathode ray beam is reproducedvin theform oi a.

dark trace, the intensity of which varies with alterations in the intensity of thec'athode ray beam. They are also-knownas-skiatrons. The screens of such devices commonly consist of an alkali metal halide, for example potassium chloride.

Itxisthe object of-my present invention to improve alteration of the composition and characteristics of. the electron-sensitive screen In skiatron tubes, suchasaused fo'rexample in rangeefinding :.and.- direction-finding equipment, a beam of. electrons or cathode rays moves over the screenin responseto an externallyap plied deflecting'forcegrsuchas the-variation of an" electromagnetic field; Reception of electromagnetic. radiations which-are I reflected fronran obj act. to.- be located results in. appropriate current: variations in the-control field of the equipment. As a result the cathode beam is caused to travel over thescreeni-of the skiatron,--thereb3' producing a darki-imageor trace. This image or dark trace 'may'beviewed by light which falls on. the. screen; fromtan. external source.- If desired, t the image may be projected ona distant viewing surfaces. The 4externaliillumination of the screennottonly permits -the-"dark -trace-- to be viewed,.but. also has the-important function of causingthe .dark traceto decayand disappear,

thus restoring the original condition of the screen. In other'words; impinging light wipes out the dark-trace after it has served its purpose.

and extend I the applications to- Whichskiatrons can beput; this result beingattained-by For some purposes it is desirable that thelife or persistence .of xsuch-trace-should be reduced.-

Commonly,; the: dark: trace :remainsnvisible for about a minute, or:even 1onger depending upon the intensity of the iilluminationj For some purposes-itisxdesirable that the dark trace shoulddisappear .in aboutfive seconds. For still other purposes,: a decay period I of no longer than one second is needed;

It'isv also desirable that thecontrast intensity between the .dark trace and the lighter background should be as marked as-possible to permit" ready recognition of the'trace or image? It is the primary object o--my invention to viewed-in a darkened room,

reduce the decay periodof traces "or images on skiatron fili'ns- Such a reduction not only widens the field' ofusefulness 'inrange-findingand direction-finding equipment, but also renders it adaptable formse in other: fields.

tant field foninstance, would be television A- darktrace screen havingasuificiently short--per-. sistence offers the noteworthy-advantage over thescreen" that it need-' notbe J as-is necessary for of screen, but may be observed present fluorescent the latter type I equally well in a well-lighted room';

This shorter persistence is obtained iZbY aSSO ciating a screen consisting of an. alkali halide with l a =minor amount J of a =-modifier -consisting of a example the halides of alkaline earth metaI including .magnesium, barium, cal

foreign halide, for

ciu'm and others which will be "mentioned here after.

I findthat the presence oithe foreign halide in an amount suflicient to reduce the decay-period I of "the dark trace alsoresults .in some sacrifice ofintensity of the dark trace. A decrease in the amount of foreign halide is accompanied by 4. a lessening of botheficcts; that .is,.the decrease in decay period and the decrease ofiintensity both:-

become less as the amount of modifier is reduced.- Surprisingly, these. variations .do not change continuously with lessening amounts of f-oreig n additions.--v The presence of a very small amount of the foreign halide varying from a few -hundredths of one per cent to a per cent or more, depending upon the 1 activity oi the particular halide used, may result ina markedincreaseinintensity when the concentrationof the foreign halide has been reduced to such a point-thatitno longer afi'ectsthe lengt'hof the decay period.-

These and other: features of my invention will be described with greater -particularity hereinafter.- The accompanying drawing shows conventionallya cathode 1 ray" tubecontaining *a screen embodying my invention.

The cathode ray tube here shown has as usual a relatively slender tubular portion 3 which communicates with'a bulbous portion 4; one wall 5 "of which approximates a flattened shape. The dark trace screenfifwhich responds to electron impact,

is-coated on the inner surface of the wall 5. At,

the opposite end of the tubular portion remote from the screen 6 is a so-calledelectron gun 1 for generating a desired cathode ray beam.

This electron gun, which of, course is sealed within the enclosing envelope, may comprise an oxide-coated cathode 8 which is sh'own as a. small cylinder receiving heat "from an electric heater embodying my invention as a control grid; Itiis connected to. an inter, mediate, more positive point of potentialf the source l2 by a conductor l5. An electrode which may be a film (not shown) )on theinterior of f the envelope wall is connected by a conductor. [6.

The device of the drawing is" i shown only as a schematic representation'il'lus-' to the source l2.

trating generally the type of device in'which films are used and, therefore, beam-,defiecting means and vices have not been shown.

The composition comprising halide and 'modi-i fier, the preparaion of which will be described, may be applied as a film 6 on the inside surface of the end wall of the skiatron shown in the draw-,

ing bysublimation, from a holder l9, indicated schematically as being held by a spring wire support 20. The modifier mixture, after fusion as hereafter described, may conveniently be ground and compressed into arpellet for insertion in the holder the exhaust of the device, heat may be applied by,high frequency induction to effect the'sublimation..

In the preparation of the material to be volatillzed and applied as a skiatron or dark trace film an alkali halide is mixed inwthe finely divided state. with a desired amount of chosen finely The mixture is subjected to; fusion which is sufiiciently prolonged to bring divided modifier.

about complete homogeneity. I A fusion, temperature should be maintained until the melted mass has become clear, transparent, and free from the agitation which occurs when a fusion heat is first applied. However, the fusion should not-be prolonged so long as to cause undesired .chemical changes, such as oxidation. Also it is undesirable to havethe temperature rise much above the melting point .ofthe mixture. Potassium chloride, potassium bromide, rubidiumchloride, lithium iodide and sodium chloride maybe employed satisfactorily as the main constituent of the screen. ,I prefer touse potassium chloride be cause it yields the highest intensiy of dark trace. 1 The modifying material, particularly when. employed with potassium chloride, advantageously may consist. of various, halides such, for example, as the chlorides of Samarium, thallium, manganese, sodium, magnesium, cadmium, lead, chromium, nickel, platinum, caesium, calcium, aluminum, iron thorium, lithium, rubidium, copper,

barium, zinc, zirconium or tin.. The extent of the 6O and alsowith the halogen chosen. Frequently activity varies with the metallic element chosen the iodide is more active than the bromide, and the latter more active than the chloride, but this is notinvariably so. in activity which increase with concentration over a certain but variable range, it is generally pos-' sible to obtain the same shortening ofpersistence by adding a small amount of anactive halide of a foreign metal as by adding a. relatively large amount of a less active one. Ordinarily, the addition of such modifier will be less than ten per cent by weight. A

Such mixtures of only because of the extensive reductions in perother-auxiliary doy forme d by a beam of l9. At an appropriate point during Because of these variations 7 there is no longer any'particular on the persistence-but the intensityis increased f V V H, that;;'obtainable with potassium chloridewith mag-V nesium chloride are especially-advantageous not sistence that are obtainable'but also because, these reductions are accompanied by smaller losses in the intensity of the dark trace. A further advantage of films prepared from these mixtures with 5 magnesium chloride lies in the speed with which the "after trace disappears; that is, a long persistent trace of low -intensity characteristic of films of potassium chloride {alonejajfsfwell' as of films prepared from many of its mixtureswith 10 foreign halides.

- Because of the variations in activity character- *'istic of the difierent modifiers, the proportion of modifier should be chosen to give the resultsdesired, theperiodofdecay (or persistence) becoming progressively shorter (within limits) with increase of modifier. "For example, the dark trace V electrons on a film of potassium chloride, unmixed with a modifienpersists,

F under'a certain intensityof illumination, at least about two minutes. The following tableshows the effect on persistence, under the same intensity of illumination, of films vaporized -from fused mixtures of potassium chloride withprogressively largeradditions'of hydrated magnesium chloride:

' Length in Calculated as Per Cent MgCla by Weight I As above stated, the additionof a small amount 5ml platinumhalide to potassium chloride results in a composition which, when volatilized, forms a-film of shorter persistence thanunmodified potassiumchloride.

Shorter persistence also. is exhibited by 1 ;,made by volatilizing as described a fusion ofa mixture of potassium chloride and "potassium hydroxide in a platinum containerJ The fusion is enveloped either in a hydrogen atmosphere or in air during preparation. The fusion-may result in some complex volatile compound ofpotassium chloride, potassium 1 hydroxide: and platinum. Such compoundappears to 'be the active agent in. volatilization'of suchfusion a film prepared bythe as above described. a j As already observed; the intensity of: the initial centrations of these foreign halides ,much below these required for iobtaining' any notable shortening of persistence. For mixtures of potassium chloride .with magnesium chloride, for instance,

to 0.02 per cent magnesium chlorideresults, in only*a:normal persista decrease in concentration once but the intensity of trace fincreases 25 :per cent.

, by about 4 0,per cent over potassium chloride alone,- .7

Seconds of 1 Decay Period dark trace may beincreasedby reducing the con- A still greater'intensity or contrast'is obtained 1 potassium chloride with barium effeot observable I a What I claim as new and desire-Rte secure by ,7 5- Letters 'Patent of the United States. is; :j

saunas 1. A cathode ray device containing a screen consisting of a blend of alkali halide and a halide or alkaline earth metal, the latter halide constituting less than ten per cent of the entire composition.

2. A cathode ray screen consisting mainly of halide of an alkali metal and an associated chloride of the alkali earth metal constituting no more than about five per cent by weight of said alkali halide.

3. A cathode ray screen consisting mainly of potassium chloride and no more than about five percent of magnesium chloride.

4. A cathode ray screen consisting mainly of potassium chloride and no more than about five per cent of barium chloride.

5. The method of preparing a cathode ray screen which consists in subjecting to fusion 9. mixture consisting mainly of halide of an alkali metal and up to about ten per cent or an alkaline earth halide, maintaining the temperature adjacent the melting point until agitation ceases and a clear, quiescent fusion results, and subliming the resulting product on a receiving surface in a cathode ray tube.

6. The method of preparing a cathode my screen which consists in subjecting to fusion a mixture consisting mainly of potassium chloride and up to about five per cent of magnesium chloride, maintaining the temperature adjacent the melting point until agitation ceases and a clear, quiescent fusion results, and subliming the resulting product on a receiving surface in a cathode ray tube.

7. A cathode my device comprising a glass envelope. 9. source of cathode rays which are oriented to impinge on a portion of the inner surface of said envelope and a coating on said surface consisting of a pre-fused mixture of alkali halide and alkaline earth metal halide, the latter constituting about 0.3 to 5 per cent by weight of said coating.

8. A cathode ray screen consisting mainly of a mixture of potassium chloride and magnesium chloride, the latter constituting about .3 to 1.0 per cent by weight of said mixture.

9. A cathode ray screen consisting mainly of potassium chloride and approximately five percent of magnesium chloride.

. 10. A cathode ray screen consisting mainly of potassium chloride and approximately 5% of a chloride of a member selected from the group consisting of magnesium, zinc, and barium.

\ GORTON R. FONDA.

REFERENCES CITED The following references are of record in the

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