US2836753A - Dark trace cathode-ray tubes and screens therefor - Google Patents

Description

May 27, 1958 G. HoDowANEc 2,836,753 DARK TRACE cATHoDE-RAY TUBES AND SCREENS THEREFOR Filed March 25, 1954 25 AMoRPHous) 1'? 'f' 8 1:- a ZOGCOTOPHQR) 7 f l@ i? Zweers-mums) 25 (CRYSTALLINE) gi ZOCOTOFHOR) Tici 20 Kc 21e y INVENToR. ad 2f GRE-Gom HODOWQNEC 216 21g Byjq T0 PUMP* AT1-o @Nav bombardment.

United States arent DARK TRACE CATHDE-RAY TUBES AND SCREENS THEREFR Application March 2S, 1954, Serial No. 418,624 8 Claims. (Cl. SLS-92) This invention relates to electron tubes of the electronoptical transducer kind, and more especially it relates to tubes of the so-called dark trace variety, especially those employing an alkali-halide crystal screen as the transducer element. Such transducer elements are referred to herein as scotophors.

A principal object of the invention is to provide an improved cathode-ray tube of the scotophor screen kind, having a high degree of optical contrast and capable of rapidrecord erasure.

It is known that such normally are a ditfuse dark trace tube screens, which white, darken under electron This is due to the creation of an absorption band for light in the visible spectrum. In the case of the ionic alkali-halide crystals, this absorption band is attributed to the trapping of electrons by the electrostatic fields of anion vacancy sites within the crystal. These trapping sites are commonly called F-centers, and the induced y.absorption band is called F-band. In the particular case of dark trace tubes using potassium chloride as the scotophor, the induced F-band has a peak at a wavelength of approximately 5600 angstroms or the green region of the visible spectrum. When a crystal of potassium chloride containing F-centers is exposed to white light, it appears magenta in color, since the reiiected light is now enhanced'in red and blue.

The electrons which become trapped at anion vacancy sites to form F-centers are mainly those ejected from the anions of the crystal by electron bombardment. These ejected or internal secondary electrons are free to move within the crystal as conduction electrons. As such they have energies over a range of values generally termed the conduction band of the crystal. At room temperature or lower, there is a high degree of probability that they will be trapped by the electrostatic iields of anion vacancy sites with the formation of F-centers. The anions (chlorine ions in the case of potassium chloride) which have lost an electron will be essentially neutral halogens and are commonly called holes within the crystal. ln a scotophor of stoichiometric composition, there will be a hole for every F-center.

Since the F-center is an electron trapped in the electrostatic ield of a net positive charge created by the anion vacancy, the energy levels of the various trapping states are determined mainly by the eongurational coordinates of the surrounding ions. ln the particular case of the potassium chloride scotophor, the F-center has, at normal room temperature, but two prominent rnetastable trapping states. One is the lowest energy or ground state, and the other is a first excited state which eventually stabilizes just below the conduction band level for the crystal. The electron is raised from the ground state to this first excited state by the absorption of quanta in the F-band. In the excited state, the electron needs to absorb but a small amount of energy (which can be supplied by the surrounding ions) in order to enter the conduction band of the crystal. ln the conduction band the electron can either return to a hole or again be trapped as an F-center.

Patented May 27, 1958 .2 At normal room temperature, where there is but random thermal agitation of the crystal lattice, re-trapping as an F-center is very much more probable than return to a hole. However, if the ions surrounding the F-center are agitated in some way, the probability of return to holes increases tremendously since the F-centers will now tend to be unstable. The decay of F-centers with the return of electrons to holes in the crystal is termed erasure, for then the crystal reverts back to the original uncolored state.

Since thermal agitation or the crystal lattice at room temperature is insufficient for fast erasure, decay of F- centers can be accelerated simply by increasing the lattice agitation by some external means. Erasure devices of the prior art operated mainly by the diffusion of thermal energy (heat) through the crystal in a process of heat conduction. One method of doing this in the prior art was to subject the scotophor to very strong F-band light. The formation of F-centers is accompanied by the generation of a certain amount of lattice vibrations (heat). Under continued exposure to strong F-band light, the crystal temperature eventually rises and therefore decay of F-centers proceeds. However, this is a very slow process involving much power and thus is somewhat impracticable. Another method of the prior art is to excite the crystal lattice by heat conduction from a heated substrate supporting the scotophor, or from a suitable film placed in contact with the scotophor. This has the disadvantage that in many cases the heating element is heated by electronic conduction, i. e., resistance or joule heating. This requires that the conducting element (and thus the scotophor) be rectangular in shape and of uniform resistance for uniform current distribution. Yet another method of the prior art is to irradiate the scotophor with ultra-violet quanta which are strongly absorbed by the surface layers of the crystals of the scotophor. Such absorptions also generate lattice vibration (heat) which then diffuses to within the body of the crystals. Yet another method of the prior art is to bombard the crystal lattice with suicient electron density so that the crystal temperature rises rapidly through inelastic collisions. Both the ultra-violet method and electron bombardment method of erasures result in irreversible damage to the alkali-halide scotophor. ln these methods, some anions are lost as halogen gas, therefore the crystals are no longer of stoichiometric composition. The excess alkali creates additional unwanted and permanent absorption bands.

The primary object of the present invention is to provide an erasure device for use in dark trace cathode-ray tubes using alkali-halides as the scotophor which is relatively fast and uniform in operation, and which is not destructive to the scotophor.

Another object is to provide an erasure device which is suitable for dark trace screens of any practical size or shape.

Still another object is to provide an erasure device which acts uniformly throughout the volume of the scotophor of the screen, and which does not therefore primarily depend upon a general diftusion eifect.

A still further object is to provide an erasure device which is simple and which does not require additional equipment or power other than regular commercial electric supply lines.

A feature of the invention relates to a scotophor screen which has, as an integral part thereof, a radiation wavelength converter for converting received radiations in the short infra-red range, for example approximately 7000 A.- 14,000 A., into the long infra-red range, for example, 20a to 1001i, whereby erasure heat energy can be efciently and rapidly applied directly to the scotophor material without destroying its transducer qualities and Without deleteriously alecting its life.

. which cooperate to provide Y me s sfhereof.

` amount of energy in this process.

a transparent, low heat capacity backing for a scotophor, and a coating oILthe scotophor comprisingat Yleast twov superposed layers,` one of which comprises a light-Weight element of the group .onsstingof aluminum, magnesium, beryllium, and carbon in amorphousl or randomly oriented particle form, and-another layer of which consists of one of said elements in micro-crystalline form, the layers acting as a wavelength converter for converting .impinging short. Wavelength infra-red into long wavelength'infra-red. Y

A further feature relatesV to aV scotophor screen comprising alayer of yscotophor crystals upon which are deposited in successive overlay a layerV of a metal from the group consisting of aluminum, beryllium and magnesium, and atlayer of carbon.` j y YA still further feature relates to the novel organization, arrangement, Vlocation an improved scotophor screen.

Otherfeatures and advantages not particularly enumerated will beY apparent after a consideration of the following detailed descriptions and the appended claims.

The above and other objects may beaccomplished by practicing this invention which makes use of the long wave infra-red radiant energy absorption characteristic of alkali-halide crystals in a manner hereafter` more fully disclosed.

and composition Vof. parts, j

t long wave infra-red regions of Fig. l is a longitudinal sectional view of a dark trace cathode-ray tube according to the invention.

Fig, 2 is a magnied sectional'view ofthescreen of Fig. l.

the-screen of'Figs.V l and 2.

Fig.r 7 is a schematic view of one form Vof apparatus for preparing'the screen according to the invention. Fig. S-Vis a sectional view Vof Figrl, taken along the Figs. 3, 4, 5 and 6 represent respective modications of fore ,the infra-red absorbing lrn The anni-handeerystaishave Strengebsorpsenbands in the long wave Vinfra-red region of the electro-'magnetic spectrum which are due to Vvolume polarizationof the crystal.A 'The anions and cations of the crystaljlattice are subject to the electric fields of the incoming electroappreciable amount of: energy from-the; high velocity el'eci magnetic waves of long wave infra-red radiation:` When 21 the infra-red radiation frequency approaches-,the natural vibrational frequency of the cation-anion lattice structure, then a resonance effect takesplace and the lattice vibrates with'maximum amplitude. Much of the incoming radiant energy is therefore converted to lattice vibrations, which, after all, is thermal energy. In the particular, case of the screen using potassium chloride as the scotophor, there is appreciable absorption in the region Vof wavelength ZOn to 100g with a peak at 63g for the4 average screen thickness of 8p to 10p. Since the incoming waves have Vlong wavelength compared tothe scotophor thickess, the processor polarization is relatively uniform throughout the volume of thea screen. This means that al1 portions of Vthe'sc'otophor gain approximately thesame v This thermalenergy is of itself su'icient to internally ionize the `excited,1J-centers. In addition to this general lattice vibration, the sites` of cation and anion vacancies are regionslofshifting effective dielectric'constant. If the anion vacancy happens -to have an electron, i. e., it is an F-ce'nter, the effect of an increasing effective dielectric constant isto raise the energy level of the F-center nearer the continuum of the conduction band, thus increasing the probability of ionization of vthe center, Thus, it is seen that F-centers be'corne very unstable under these conditions.

In the region ofthe scotophor originally containingv Fi centers, the electrons from 'the now unstable F-'cen'ter's are in the conductionbarid of Vthe crystal and returning to holes in the'crystal. Duririgthistim'e, vthe crystal is essentially a-con'ductor and thefre'eelectrons of thec'onduction band are subject to excitation by both long Wave and short wave infra-red'radia'tion 'and also by Vvisible t 4 V Y spectrum radiation. These excited conduction `band electrons add to the crystal heat byffrequent collisions with the vibrating ions in .the lattice structure. VIn ad-A dition, these same free electrons tremendously increase the amount of heat conducted from the long wave infrared radiator Yif it is in intimate contact with the scotophor.

Fastest erasure time is achieved only when anradequate amount of F-band quanta isalso present, for this radiation places the F-center inthe excited state, where the crystal heat developed by polarization is sucient tor-destroy F-centers. The additional heat developed by shorter wave radiation Vabsorption and accelerated heat conduction is more than sutlicient to destroy. or disperse any other more stable centerswhich may be for-med in theprocess y of erasure or coloration. In the practice of this invention, a krelatively strong source of long wave'infra-'red radiation is secured without large expenditure of power by placing the longrwaveinfra-VK red radiatorjin intimate contact with the scotophor of the screen. The long Wave infra-red radiator can be a thin film of material which hasa high absorption characteristic for radiant energy in the nlm can absorbV the short wave infra-red energy of a readily available source, for instance a hot tungsten fila ment.-V `As the' temperature ofV this iilm risesi inxthis process, it reradiates the absorbed energyA at muchlnger wavelengths. f

Itghas beenffound that there is a transfer` of energy'as long wave radiationwith maximum eiciency when the radiator. reacheszatemperature of approximately 809/ C. This is: in: agreement with Plancks radiation law. Thereis in eilecta converter device, injthatit'absorbsthe short wave infra-.red radiation which is. readily available'but isnot absorbed vby the scotophorand converts'- it into radiation of longer wavelength Vwhich'. is .absorbed directly bythe alkali-halide scotophor. Y. f v

Suitable Vmaterials for this converter-radiator `filmV are ouasi-amorphousV deposits of the' conductingV elements. of low. density. `rThese' deposits would abs'tra'c't'butan-introus of thefcathode-ray beam, and yet they :would-have agood absorption characteristic for the 'radiant energy concerned VAmong'suchmaterials `suitable 'for use with alkali-halide 'scotophorswould bei beryllium, carbon, magile-"sittin,-Y and aluminum.

Deposition of the above quasi-amorphous hline, which are gryto black ih` appearance, vdirectly. on the scotophor of the screen gives a gray appearance to the screen. In addition, these; deposits ductor'sl offelectricity and also have poor-secondary'emission,V characteristics. 'Therefore dark Vtrace-tubescreens coated-'With such deposits as the erasurefdevicearelimited in operation to -thatj anode voltage at which the screen sticks, i: Ye., 'the screen potential no Vlonger increases as the anode `Ivoltage is raised. i Y

Some improvement in the above 'basic radiation-conveiterrlayercan be achieved in vthecase vof quasi-amorphous deposits of Vmagnesium and aluminum by placing a very thin `conducting but highly translucentilaye'rof vthe metal vover the quasi-amorphous deposit. Y operation-of vthe screen at any anodefpotential'and also whitens th'e appearance ofthe screen somewhat.l How- "ever, erasure time `suierssin'ce the thin metallielm reilects 'a 'substantial portion of theradiationimping'ing upon it; A l f Amore suitable arrangement for'the erasure device is to make the radiation-converter lm in composite form. in this arrangement a thin quasi-amorphousgdeposit of magnesium or aluminum fis placed 'phor of the .dark trace tube screen. a thin 'but highly reflecting (metallic) depositfoffthe same material. f 'the quasi-amorphous deposits mentioned ab'ov'elis short wave infra-red and the spectrum. Thus-this are essentially'very poor 'conl This 'enables` directly onV the 'scotoj Y This isfollowerlibyy A third 'thin layer. (whichcan be any.

then deposited. The metallic reecting layer of magnesium or aluminum serves three purposes. First, it serves to retiect whatever light is transmitted through the scotophor back towards the observer, thus increasing the amount of scattered light in the screen. This whitens the appearance of the screen considerably in spite of the very thin quasi-amorphous deposit of metal at the scotophor-metal interface. Second, since it is a conducting layer, it serves to keep the screen at anode potential so that the tube can be operated at any potential desired. Third, it serves as a soft X-rays generated by cathode rays in the scotophor. These soft X-rays can contribute to the coloration of the scotophor, i. e., increase screen contrast.

The operation of such a composite radiation-converter device can be described as follows: Short wave infrared radiation is absorbed by the third quasi-amorphous deposit and the heat generated is transferred to the conducting metallic iilm by heat conduction. As the temperature of the metallic film rises, long wave infra-red radiation is emitted by the quasi-amorphous deposit of this same metal which faces the scotophor of the screen. This long wave radiation is directly absorbed by the scotophor. If F-centers are present in the scotophor, erasure takes place in a manner described earlier in this disclosure.

In general, the method of dark trace tube erasure stated here is to use easily generated short wave infrared radiant energy which is not directly absorbed by the alkali-halide scotophors to be converted into long wave infra-red radiant energy which is directly absorbed by the scotophor by means of the composite radiation-converter device described above. The long wave infra-red radiation plus the short wave infra-red and visible spectrum radiation available both from a radiation lament and ambient lighting, rapidly and effectively erase the dark trace of the alkali-halide dark trace cathode-ray tube.

Referring to the drawing, merely by way of example, the invention, wherein the numeral indicates any suitable evacuated enclosing bulb or envelope. Mounted within the neck of the bulb is any well-known form of electron gun 11 for developing a sharply focussed electron beam 12. This gun may comprise the electron-emitting cathode 13 with its internal heater 14; a centrally perforated metal disc 15 which may constitute the control grid and upon which are impressed the electric signals to be recorded. In the well-known manner, the grid 15 controls the intensity of the electron beam 12 which is arranged to act as a writing or recording beam and scans in a point-by-point fashion, the electro-optical light transducer screen 16. The beam 12 should preferably be a high intensity beam, for example of 8 kv. to 14 kv. For this purpose the usual beam focusing and accelerating anodes 16 and 1S are provided and the usual coordinate beam deflector elements, such as the deflector plates 19, are mounted Within the bulb. It will be observed that the screen assembly 16 serves as the final anode. Preferably also the neck portion of the bulb is coated with the usual conductive coating which may be connected through the same high direct current potential as that applied to the lsecond anode 18.

The screen 16, in accordance with one embodiment of the invention, may comprise a thin glass or mica backing sheet 17, for example of the order of .0012 inch, which is mounted in any suitable circular or rectangular frame 19a, suitably anchored within the bulb 10 adjacent the front end wall. As shown more clearly in the sectional view of Fig. 2, the light transparent backing 17 has on the side facing the gun, a coating 20 of scotophor material which may consist of any of the alkali metal halides, preferably potassium chloride. The thickness of the scotophor material 20 should preferably lbe correlated there is shown in Fig. l, one typical embodiment of reecting layer for theV preferably with Lthevelocifyof' the'beam-'12- For example, if the beam 12 isa 14 kv.-beam, the scotophor material20 may be deposited with an overall thickness of approximately 8 to l0 microns. Preferably also the scotophor material 20 is deposited in successive depositions. For example, it may be deposited in successive steps with each step depositing a thickness. of approximately 21/2 microns, and each deposition is allowed to cool before proceeding t0 mica sheet 17 with the scotophor material should preferably be done before the screen is assembled within the bulb 10 and preferably in a vacuum. For example, as indicated in Fig. 7, the glass orA mica sheet 17 can be supported on a suitable frame 21 within a bell jar -22 which is arranged to be evacuated, and when the desired degree of vacuum -is reached the potassium chloride ma# terial may be deposited in any well-known manner on the glass sheet 17. For example, a quantity of potassium chloride can be placed in a cup 21a which can be heated by applying electric current to the lead-ins 21b, 21C. A After the depositionof the appropriate thickness of the scotophor material it is coated with a layer 23 of a light weight element in amorphous or quasi-amorphous form, as distinguished from'the crystalline or electrically conductive form. 'Inother words, the particles of the metal are deposited in randomly oriented form, as distinguished from a regular. crystal formation. This element should be chosen from the group consisting of aluminum, beryl.` lium, magnesium and carbon. This amorphous or quasiamorphous material should also be deposited in a vacuum, for example as described above in connection with the bell jar method of Fig. 7, and the thickness of the deposited amorphous metal should be limited so that it is transparent to the electron beam 12. For example with a 14 kv. beam, its thickness may be .25 micron.

I have found that Vsuch an amorphous layer of the above elements is capable of acting as a wavelength con# verterfor infra-red radiation, and this characteristic can be taken advantage of for rapidly erasing the record which has been previously produced on the scotophor screen by the' Writing beam 12.' For this purpose there is employed within the bulb 10 in front of the screen a line wire tungsten filament 24 which may be supported in zig-zag formation so as to extend across the screen as schematically represented in Fig. S. This iilament, for example, may consist of tungsten wire of approximately 0.008 inch diameter and is sufficiently fine so that it casts negligible electron shadow on the screen 16. ln order to reduce any such shadow effect, the ilament 24 can be biased positively with respect to the screen 16, as indicated schematically in Fig. 1. lin any event the filament 24 is designed so that when connected to a commercial 115 volt supply line its temperature can be raised to approximately 2000 K. at which it emits maximum infra-red radiation inthe relatively short wavelength region, for'example 7000 A.l4,000- A'. The coating 23 can be deposited by applying heating current to the lead- ins 21d, 21e, which can be adjusted to control the vaporization of small aluminum pellets 21j on the filament 21g.

As pointed out hereinabove, the efficient and rapid erasure ofi'the record inthe scotophor screen is obtained by the conversion of this relatively short wavelength infra-red incident radiation' to much longer wavelength infra-redv-radiation, vfor example, in the region 20u to e, and this conversion takes place in the layer 23, which is in direct and overall uniform surface contact with the scotophor. In other words, the layer 23 of amorphous light-weight material, which is transparent to the cathode-ray beam 12, acts directly as a wavelength converter for the incident short wavelength infra-red radi- ,t'ion and it becomes itself a radiator of long Wavelength infred-red radiation which is in direct and complete surface contact with the scotophor 20. f

Fig @shows a modification of Fig. 2, `wherreinth'ej the next step. The coating of the glass or V' nation, an enclosing the setonher, 20, and.v morpheus. light-'weight lenient nal.. coating. 259i.. e of 23,; iS. proyided with. an. add l the-abovefmeutioned lightawel nements., Qf;y Suiiient thinness to be transparentt. electitqnbeam 1.2. but whiciiis depositediu a. Cry'StaH.' @forni SQ, astcfbehighly reiiectine. 0.1.1. the. side facing the. sCQtOuhQrzZQ- layer 25,.ferfexamp1e, may.V have. a thickness. of; approximately 7.00.: l-9.00. A. Eurthenmcre, being. in the Crystalline States as. distinguished; from the amorphous Statali! iS; a gOQd, electric co'iarluctor.A

Fia- 4 Shows a still further mqdication Wherilh@ elements 17, 20.-.23. andfZS; may be the Same as there of. Eig. 3, vand in additionanother. layerlof the abovenotedfligh't-weight materialfinamorphous torni, Yis applied tothe coating 23; VHere again, the layerlisof jsniicient thinness to. be transparentto the electron beam 12, and it acts' as a wavelength converter. inthe saine way that the layer 23 actsas a converter. One of'theadyantages of the embodimentsof Fig. 3 .and Fig. 4, i'sthatrthe intervening micro-crystalline layer 25. is a good. electrical conductor, whereas the amorphous layers 23.' and; 2,6.. are relativelypoor'electrical conductors and the. tendency of the screen to stick at a given potential .is avoided. .This sticking as is well known arises by. the secondary electron emission when the screen is struck bythe beam 172. If the material is a poor conductor, it tends to assume an equilibrium or sticking'potential,

Fig. 5: shows a further modification of. Figs. l and 2, wherein the screen consists backingV 17 with the scotophor coating 20 and with a single amorphous coating Fig. 2. V,nlthis embodiment'howevenan additional, coat,- ing 27 of the scotophor material'is applied over the amorphous light-weight coating 23.

In the embodiments "of Figs. 2 and. 5, where thescreen has only the amorphouslight-weight element coating in contact with the s'cotophor, the writingvoltage for the tube is restricted to a value below the"sticking potential of the screen. Furthermore,` when the ,scotophor iscoated only with the amorphous light-weightV conductor material 23, the screen may assume'. a. grayish or'blackish appearance, and the amorphousV metal coating may become oxidized during the production and processing of the tube after thescreen has'benenV assembled therein; TheV embodiment of Fig. 4, wherein an additional highly rellecting microcrystalline coating 25, of the light-weight material. is used over the. amorphous light-weight material,` the screen becomes whitishA in appearance.

Fig. shows'a modication of Fig. 4,'Wherein. an additional. layeror coating 28. ofthe scotophor isv depositedjover the. second layer `26.of"the amorphous lightweight material. lm protectsthe underlying metal against oxidation, and

also increasesv thecontrastwhen the tube is viewed with back lighting.l yreference 'ismade'to aj lightweight amorphousl coating In all thepreceding embodiments, Where or film, it is understood that. this film mayA consist of aluminum, beryllium, magnesium, or carbon.

Various changes and modicat'ionsmay be made in the disclosed embodiments without departingfrom the spirit and scope. of the invention. In all of therforegoing embodiments, in orderto pre1/entA damage to the screen, a Suitable .Switching arrangement VWell-kilow-nY in. the. art be prende@ for blankina Qi, the. beamJZ during. the period that the erasure ilamentZl-y is being lighted.

nWhat is claimed is: l' Y' f l. Cathodelray tube apparatus comprising in combiev'acu'ated ,eliyelopgY means to devel@ a beam. 0f electrons, ia scrlectifiuwn which said beamv impi'ng'es to maire. a record, s aid Vscreen comprising a scotophor materialwhich develops opacity centers when said beam i1 'Ypinges'thereon and means to erase said centers, the lastlmentioned means including a1 Aheata'ble filament i Yof the thin light-transparentY 23 similar to coating 23 ofv This iinal layer '28 ofV the scotophorV within said 'envelope'v vfor emitting ak a beam 0i elections, screen non ,includingI a' layer,A of

substantial amount oiyshortawavelength illf-red radiation n i0. Saidsien alidin'eahs ioimins Pit J0i Said screen to Vc onire'rt;'said.;A incidentfshort wavelength infraredradiation vinto long' wauelen'gth' infra-red radiation and Y g (amophosfparticles fof an element chosen, from` the 'groin/a.4 consisting digaluminurn, beryllium, magnesium and,.carbon Vsurface V'ccmtact'with said scotophonf'saidl'layer bein v substantiallyf'transparent. tousaid electron beam.A Cathode-ray tube. apparatus; .comprising inj'combination, an 'enclosing evacuated envelope, means VK'to develop 'Whichflsi beam im: pinges to make a record, said'screen includingn a scotophor material whichdevelops opacityfcfenteiis'when said .beam impinges thereon, andurneans toferas'esa'id. centers, the last-mentioned. -n'leansL includin v particles of an. Velernexnt selected from thefgroupfconsis'ting., 0f' aluniiiiufn, bryiim; and in direct heat exchange relation with-the scotophor, and overlayerH Aofvcrystalline; particles of; 'said selected element, Said. layer au@ Qrely'r, beine' Sbsfa'ntially transparent to said beam. Y 'Y 3f. @ethnie-farai@ ibaiatie comprisinein Combinaaan, an. enclosing. evacu a beam of` e'lectronvs,` a` screenv uponl said beam impinges to make a` recordfsaid'screen including a scotoplainY material whichdevelops-opacity centers when'said beam, impiega. theorti. and' mesf t0y is Said. Centers, ille. last-muddled Hns uliClildine a layr-Qi'swtphor material, arcoatingv offamorphous particles bilan' ele-V merit .frm the ardu?. ntinecihimiulm beryllium, magnesium and, carbon, indirect contact withI said layer oftvscotophor, a .seconc'l'coatingbff crystalline particles of said Vselected element on theirst coati and a; third coatselectedelement rvon ing cf.- morphine habidas. fsaid,

' 4. YCathode-ray ,tubeV apparatus, comprising in comb ination, anenclosingY evacuatedA envelope, means. to develop a beam of. electrons, a5 screen" uponl Awhich .said beamimpinges -to'VV A alie a record, screen including a scotophor material which develops. PEQY `centerswhen said beam impingdes thereon, and means toferas'ev said centers includingV ay coating` of; ar'ruarphousy particles `of an element selectedfromthefgroup. consisting of aluminum, beryllium, magnesium, carbon direct contactlwith said. scotophorQand second layer -of scotophor material on said.coating,'said coating beingfsubstatiallytransparent to said'electronbeam.k 'l Y `5 Cathodefray tube. apparatus,z.compris ing.in combination, an'enclosing .evacuated..eilvelope, means to develop ia beamV of electl'ra vscreen .iipnqwhichfsaid i beam imping'es toV make record,j said screen. including a scotophor 4material, which .develops opacity centers `.when said beani impinges 'trl1`ere'on`, and Vrr'ieans toweraseY said centers, including .a lfirst coatingfc'af amorphous,` particles of an element selected in amorphousformffrom the group Consisting ci.' aluminum." beryllium, magnesium. and carboni'ai second.Y coating,V Qf` ysialline/Plartides; of. Vsaid Selected element iufctysialli affirm, a' third; coating 0f amorphous. particles 0f* l Selected.' element," and a fourli'cfiatins which is af'SsQtOPhQrs ailysidfwaiiiigs beingtranspa'rent'to the cathode-'ray beam; 5 f

6K. A. cathode-raymbe comprising anevacuatedenvelope Acontaininl'gnan ailrali` Yalide crystal 'isre-eil- `which develops opacity centers "in f response Vto vanfincident cathode-ray beam, anelectron gun-foi' developing said beam and facing Isaid, screen, i a: line wirel fllarnentfsup# ported, between said gun and screenl and: havingy leadins for connection. to a current:supply for said lilarnent to a Ftenn eratu`e-j at vw `ch vitfjemit's vaf. substantial means including a multi-layerfand beantransparent coat 'ofathinness'. whichV is g afla'yer of.v amorphous magnesium Y and f carbon,Y

ated'e'nvelope, riie'aiisA to develop on said screen on the side facing said gun for converting said infra-red to infra-red in a longer Wavelength range, said multi-layer coat including at least two layers of an element transparent to said beam and chosen from the 2535817 group consisting of aluminum, beryllium, magnesium and 5 2,615,821 carbon, one layer being of amorphous particles and an- 6161057 other of crystalline particles. 2,661,437

7. A cathode-ray tube according to claim 6, in which 25651220 the said amorphous particle metal layer is in direct con- 6731816 tact with the alkali-halide crystals of said screen. 10 21676413 8. A cathode-ray tube according to claim 6, in which the screen is constituted of potassium chloride crystals and said multi-layers are constituted of aluminum.

UNITED STATES PATENTS Skellett Dec. 26, Levy Oct. 28, Coltman Oct. 28, Beckers Dec. 1, De Gier Ian. 5, Neuhaus Mar. 30, Jervis Apr. 20,

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