US2879444A - Display screen for television tubes - Google Patents

Description

l 24, 1.959 I c. NUNAN Filed D eo 29, 1.955.V

Al- AAA DISPLAY SCREEN FOR TELEVISION TUBES 2v Sheets-Sheet 1 Mar'ch 24, 1959 c. s. NUNAN v2,379,444

- DISPLAY SCREEN FOR' TELEVISION TUBES- Filed Dec. 2"9, 1955 I 2 sheets-sheet 2 2,879,444 Patented Mar. 24, 1.959

l. DISPLAY SCREEN FOR TELEVISION TUBES Craig S. Nunan, lalo Alto, Calif., assigner to Chromatic Television Laboratories, Inc., New York, N.Y., a corporation of California Application December 29, 1955, Serial No. 556,338

13 claims. (c1. 315-14) This invention relates to display screens for use in television cathode-ray tubes and particularly in display screens for use in tubes intended for the display of television images in color.

Among the objects of the invention are to provide display screens capable of giving color images having a high degree of saturation in the color displayed, to provide means for reducing the apparent size of the spot upon lthe screen produced by the impact of the cathode-ray beam, to provide means for reducing the intensity of halos surrounding the spot, and, generally, to provide tubes wherein the television image produced has a maximum degree of crispness, contrast, and color definition. In cathode-ray tubes generally there are a number of causes which either increase the size of the spot of the light produced by the impact of the cathode-ray beam upon the display screen or produce halos of light around the vspot so produced. One cause of such effects is the multiple reflection, from the surfaces of the glass base upon which the display screen is formed, of light emitted from phosphors in o-ptical contact with the screen and at relatively large angles from the perpendicular to those surfaces. The intensity of this halo is reduced, in accordance with known practice, by tinting the screen; reflected light must then traverse the screen at least three times and at relatively high angles before it can reach the eyes of the viewer and its relative intensity, incomparison with unrefiected light, is therefore greatly reduced.

A second source of halo, increase in spot size, and undesired increase in the general level of the illumination of the screen is secondary electrons emitted by the screen. Such electrons are emitted with only a few electron-volts energy, but they fall back against the screen and produce an over-all low level of illumination. It is also common practice to avoid this effect, and, at the same time, to increase the light reaching the eye of the viewer by depositing a reflecting layer of aluminum on the surface of the screen facing the electron source. It is customary to minimize the thickness of this conducting layer so that the loss in energy of the beam in penetrating it is rninimized While the low energy secondary electrons thus emitted have not enough energy to penetrate the aluminum and reach the phosphors with suicient energy^to excite luminescence.

Several other sources of increase in spot size and halo are effective in colo-r television tubes, which frequently have either a mask or an electron lens structure interposed between the source of the electron beam and the screen. The electrons striking the mask or electron lens structure may be scattered without material loss of energy, being effectively retracted at the edges of the apertures in the structure, or they may cause the emission of low energy secondaries which, particularly in the case of electron lens structures, may be accelerated between the structure and the screen and strike it with sufficient energy to penetrate the aluminum, as in the case of the primary beam, and excite luminescence. Finally, there are scattere'd `electrons which emerge from the screen at considerable angles from the perpendicular and with a large proportion of their initial energy. In monochrome television tubes, without a mask or lens structure, these electrons would normally reach the walls of the tube and do no harm. Where a mask or lens structure is present, however, they may be refracted back to the screen itself, and strike it with sufiicient energy to penetrate the usual thin metallic film, causing a general increase in the level of background illumination displayed by the screen. Such electrons may strike the phosphors emissive of any of the primary colors used in the system, and therefore the light emitted by the scattered electrons will be whitish in overall effect. The result is a general desaturation of the colors displayed, so that even though the background level or D.C. component of the picture image may be proper, all of the colors may be paler than the original v1ew.

It is the effect of these high energy scattered electrons which has proved in practice to be most difficult to overcome or minimize, and it is to the suppression of the effects of such electrons that this invention is particularly directed.

ln using a film of aluminum over the phosphors of the display screen to provide a conducting surface, to reflect light emitted from the phosphors and to absorb the energy of secondary electrons,`it has been the practice to make the film as thin as possible and still have it continuous, to .provide a conducting mirror surface. Electrons penetrating a film of this kind suffer a loss of energy, almost all of the energy thus-lost being expended in ionizing the penetrated material. This loss of energy causes a diminution in .thelight energy emitted by the phosphors; at least to a first approximation, the diminution in light emitted is proportional to the ionization loss of energy by the beam. The loss of energy is related to the density of the material which the beam penetrates, and the range of the beami.e., the distance which the beam will penetrate-into any material of relatively small atomic number, before all of its energy is lost, can be expressed by the approximate equation:

R=2.1V2 (Equation l) where R is in terms of micrograms per square centimeter of the material from which the film is formed and V is the kinetic energy of the electrons, expressed in kilovolts.

The films normally employed in metallizing television display screens are formed of a quantity of aluminum that would result in a thickness'of film of about 1500 Angstroms, or 40 micrograms per square centimeter'. This is about l@ of the lelectron range in monochrome tubes operated at 13 to 14 kv. anode potential; it is from I 1A? to 1/1 of the range ofl8 kv. to 20 kv. electrons. Such higher potentials are being increasingly used in color tubes, where the substantially same film thicknesses as for monochrome tubes has vcustomarily been used.

The equation for electron range is based on the fact that electrons of thevbeam lose energy almost entirely through ionizing the material through which they pass.- The density of materialsof fairly low atomic number, Z, is approximately proportional to their atomic number, which is equal to the number of orbital electrons4 available to be ionized. The range given is along the actual path of the beam electrons, and these are repeatedly deflected or scattered, the scattering being attributable to the nuclear charge. The beam electrons therefore follow tortuous paths, and it comes out that except in the very thinnest films, in which few scattering events occur, the loss of energy by beam electrons passing through .a .film of fractional-range thickness is a nearly linear function of the film thickness. It is for this reason that it is the practice vto make the films as thin as possible to obtainy 'the maximum illumination from the tube.

which the electrons are scattered follow substantially a Gaussian distribution curve In thicker films, and after entering the phosphor layer," the beam electrons are subjected to many scattering deflections through 'random angles, resulting in an angular distributionfthat is still a 'probability function but no longer follows the Gaussian law. In entering any material there will be some electrons which are subjected to single scattering, and are reflected without material loss of energy. 4Most of the 'electrons will be subject to multiple scattering, accompanied by a loss of energy due to multiple collisions. Eventually the scattering will be through a solid angle of 41r radians.

There is a possibility of electrons being back-scattered, '(meaning deflected through an angle that will result in their re-emergence from the face'of the screen) at any depth of penetration in any material. The probability of such back-scattering is roughly proportional to Z.

The angular distribution of such back-scattered electrons is substantially independent of the scattering material. The number so back-scattered, however, is nearly directly proportional to Z, for the more deeply the electrons have penetrated before undergoing enough scattering defiections to reverse the general direction from their initial course the greater the probability of their losing their energy before they can re-emerge from the surface of the screen.

The effective Z of a compound in causing scattering is substantially the arithmetic mean of the atomic numbers of the atoms entering into it. Most phosphors are salts of heavy metals, many' containing zinc, Z=30, or cadmium, Z=48 as a base. A frequently used blue phosphor is zinc sulfide, ZnS, with an effective Z of 23. In one color tube the average effective value of Z is 16.8, using the best phosphors, as regard color emitted, presently, available; in another the effective value of Z is 17.5.

In accordance with the present invention a phosphor screen is employed having the desired characteristics as to color emitted and pattern. Upon the phosphor screen there is deposited a coating of material of effective atomic number lower than that of the phophors and of a thickness in terms of electron range R, several times that normally employed in the aluminum coatings, provided to give conduction and light-reflection. The thickness of this coating is preferably from Ms R to 1/s R, with an optimum of about l/s of the range of electrons having the energy in electron volts for which the tube is designed, in which the screen is used leading, in a tube designed for 18 to 2O kv. beam energies, to films from 3 to 6 times as thick as those normally employed; preferably about 3.5 times normal thickness for an 18 kv. tube or 4.5 times normal thickness for a 20 kv. tube. Aluminum (Z=13) having an atomic number only slightly less than the average of the phosphor coatings, will give a materially increased ratio of total light to halo, with a consequent increase in picture contrast and color saturation; preferably, however, the material chosen for the coating is one from the first group of the periodic table; beryllium (Z=4) or boron (2:5) being among the preferred materials.

The above will be more evident with a consideration of the accompanying drawings and the ensuing description thereof. In these drawings:

Fig. l is a diagrammatic illustration of a tube of the type to which this invention primarily relates;

Fig. 2 is a fragmentary cross-sectional view through the display screen and the focusing electrode structure of the tube of Fig. 1, illustrating the path of the primary electron beam as well as the electron and light paths which are responsible for the various halos, spreading of the light spot, and over-all raising of the background level which has been discussed above; and

Fig. 3 is a diagramof the face of the .screen,.i1lustrating In passing through very thin films the angles through 4 v the areas impacted by the various electrons whose trajectories are shown in Fig. l.

The tube of Fig. 1 is shown symbolically to illustrato the conditions giving rise tothe particular problem which the present invention is designed to alleviate. Characteristically, a tube of this character `will comprise an evacuated envelope 1, which is generally of funnel-shape and can be either metal or glass. For present purposes it will be assumed that this envelope is of metal', if of glass it will be provided with a conductive lining. The large end of the funnel is provided with a transparent window 3 through which the television images are displayed. In the neck of the funnel is an electron gun 5. Only one such gun is shown, although in certain forms of the tube for which the present invention is adapted a plurality of guns may be used.

Within the window 3 is the display screen 6 upon which the television images are projected. The screen comprises a transparent base 7, which may, in certain instances, be the window itself. Deposited upon the surface of the screen which faces the electron gun is a layer 9 of phosphors which are deposited (for a color tube) in a repeating pattern of color cells which, in one dimension at least, are of the order of magnitude of a single picture point or elemental area of the image to be reproduced. These cells may be of like size in the other dimension, or they may be of strips extending entirely across the screen area. In the present instance it will be assumed that they are of this latter form. Each color cell includes sub-areas of the individual phosphors which upon electron impact emit light of the various primary colors used in the system for which the tube is designed, usually red, green, and blue.

Deposited upon the layer of phosphors 9 is a film 11 which will be assumed for the present purposes to be of aluminum, although, as will be shown hereinafter, beryllium is even better. It is with this film that the present invention is particularly concerned and its characteristics will be described in detail later.

Mounted adjacent to this screen is a lens-grid or focusgrid structure 13, which, since a strip-type phosphor screen has been assumed, comprises a multiplicity of elongated linear electrodes, the interspaccs between which form the apertures of the multiplicity of electron lenses each of which is electron-optically alined with a corresponding color cell on the screen. With a single gun tube, of the type shown, the electrodes forming the grid are divided into two sets which are mutually insulated and interleaved.

In the operation of the tube shown, the various elements within the tube are operated at definite relative potentials. This is symbolized in the drawing by conventionally shown potential source 15, connected across a voltage divider 17 which has various taps connected to these elements. The most negative point on the divider connects to a control grid 19 of the electron gun. The gun cathode 21 is connected to a slightly more positive point on the divider, the voltage with respect to the grid usually being somewhere in the neighborhood of volts or less. Taking the other elements of the tube in order of increasing positive potential, the lens grid 13 may be operated somewhere in the neighborhood of 5 kv. positive to the cathode, the gun anode 22 and the envelope of the tube somewhere in the neighborhood of 200-400 volts positive to the lens grid, and the film 11 about 18 kv. positive to the cathode or 13 kv. positive to the grid 13. It is to be understood that these figures are given for the purpose of'` illustration only, and that tubes may be designated to operate at relative voltages quite different from those here suggested. Furthermore, it will be realized that the lens-grid structure here shown is one of the simplest, and

that, in addition to the single grid here described, a lensf,

gridcan include one or more additional grids, which maybe either nearer to or farther from the screen thanthatV the apertures of 'the electron' lenses. In the`pres` ent 'case the electron lenses are formed by the electric iields between the film 11 `and the 'grid wires 13. In multiple-.plane grid 'structures the focusing fields may be set up between the elements ofthe 'gurir-self, in which case the nlm 11 need not necessarily be of conducting material, as far as focusing action is concerned.

With the particular 'form of lens here described, and with the relative voltages mentioned applied to the electrodes, a beam of 'cathode rays 'from the 'gun 5, directed through any of the apertures in the lens-grid'st`ructure, will be focused on 'the center of the corresponding color cell. ISo focused', its cross-sectional width, normal to the 'electrodes of the grid, will be small in comparison to the 'width of the aperture and the beam will fau on a parriclnar phosphor; -'i.e., that which occupies "the mid-position in thfear'ry of strips vof diierent phosphors iforming the cell. For various reasons which are not pertinent to the present invention the central phosphor strip will usually be that 4emitting green.

In order to change the color displayed by the tube, a potential difference is applied between the two sets of electrodes, 13b and 13r of the grid. This potential diiferen'ce 'may be applied vfrom `a color oscillator 23, connected across a` resistor 25, the `center tap of which connects to the voltage divider 17 so that the llatter supplies the mean potential of the grid. The oscillating voltage thus applied deilects thev electrons passing through the interspac'e 'between the electrodes 'to one side or the other, S'o ythat the beam 'is focused Yon either the red-or blueemitting 'phosphor 'as the `case may be, depending upon the lthe foc'al point 'effected -by the oscillator 23.

yIf -a television image is to be displayed on a tube of 'the 'character here "described in natural-appearing colors, itis obvious that 'the various phosphors must be excited at Asuch intensity and for 'such relative periods of time that "the Veye will integrate `the -light emitted by them to show both'the 4vproper "hue and the proper degree of saturation. Tf, yfor example, it is desired to display a picture element in green, if 'any electrons from the beam fall on either the red-emitting 'or 'the 'blue-emitting 'phosphors there will be some 'color degradation; vif the light excited from both phosphore is of equal energy they'ove'r-all 'elect will be that the primary-'color green is diluted or desatur'ated, tending y'to produce a pastel shade instead of a saturated one. yIf the red ,phosphor is more eiiicient than the blue,

.the `green 4will have a yellowish 'cast while if the blue is the 'more efficient vthe dilution will be bluish, tending to turn a 'pure green into a pale cyan.v

'What happens in a tube of conventional construction (having the usual `minimum thickness aluminum layer) when fa high energy :beam vsuch as is produced by the tube described strikes a phosphor coating is illustrated in Figs. 2 and 3. The primary beam yfrom the electron gun, the 'center line 'of which is designated by the reference character 29, approaches 'the screen at an angle p lfrom the normal tothe screen surface. The focused beam strikes the "screen at the point designated as A in both Figs; '2 and '3. `yShown asa short black yline in Fig. 3, this illustrates the relative size, on a greatly exaggerated scale, of the particular color element the vbeam is intended to illuminate. g

Light is radiated .from those illuminated phosphors which are in `optical contact with thescreen in accordance with -Lamberts ylaw. .Light emanating vfrom the point A more widely as it passes through the front surface 31 'of the screen base. Light emanating from beyond the critical angle is reflected from the front surface of the base back to the rear surface 33, where it is re-reflected, and probably scattered in part, to be visible probably as a halo, this effect being illustrated at B in Figs. 2 and 3.

In Fig. 3 the circle B marks the inner edge of this halo. This will be the brightest portion, the halo fading from this point outward due to the Lamberts law emission'from the phosphors. The color of the halo will be the same as that emitted by the phosphor instantaneously impacted, but it will be constantly changing, as the spot A is deected from phosphor to phosphor and as the beam is scanned over the surface of the screen. Therefore, except in the case of a large area of a single primary color, the eiect of the halo will be to dilute the colorsrdisplayed and tend to raise the over-all level of the white background. L

As the beam passes between the grid wires 13b'andf13y, a small percentage will strike the conductors and cause the emission of low-velocity secondary electrons. These electro-ns, under the influence of the perhaps 13 kv. field between the grid and the film 11, will fall directly to the screen, exciting illumination from the points designated as C in Figs. 2 and 3. Again it will be seen that the effect Wil-l be a general dilution of the color displayed, fo'r`it is evident, as shown in Fig. 3 particularly, that the phosph'orsoccupying these points have no constant relationship, effective throughout the screen, to the phosphor t point A. The color of this dilution will vary from `point to point, over the -surface of the screen, due to the relative displacement of the various phosphors with respect 'to the location of their corresponding apertures and to the angle of incidence at the beam. y

In addition to the low velocity secondaries causing the effects described at C, a certain percentage will be scattered and emerge from the grid conductors at fairly high velocity and at random angles to the beam path. These electrons, too, will be accelerated by 'the field between the grid and the screen, and will follow parabolic courses, exciting illumination in semi-lunar areas designated as Din Fig. 3. Since, in a practical tube, the total physical area of the grid conductors will be only somewhere in the neighborhood of I20% of 'the screen area and their effective 'area is even less, and since only a relatively small proportion of the electrons which do strike 'the grid conductors will be scattered at high energy, the illumination from the electrons forming the areas D, `is not as important as some o'f 'the other sources which have been mentioned.

The final source of color dilution, 'and the one which has proved most troublesome in practice, is the result of relatively high-Velocity scattered electrons from the screen itself. The electrons reaching the screen have an energy which is proportional to the voltage through which they have fallen between the cathode of the lgun and the screen. The back-scattered 'electronsmay emerge from the screen at any angle and with any energypup to substantially that of the incident beam. 'Experimentally it has been shown that the distribution in angle is substantially independent of the material doing 'the scattering. With a perpendicularly incident beam it is clear that the electrons must be deflected through a total cumulative angle of over in order to emerge. The distribution in angle or the emergent electrons rises from substantially zero at 90 to a maximum at 104, falls to less than 20% of maximum at 120., and then falls olf nearly linearly, approching zero again at The energy distribution in the back-scattered electrons also varies. The maximum energy increases from near zero at 90 to amaximum in theregion lfrom 103 to 113, and finally decreases approximately linearly.

Fig. 2 illustrates a number of possible trajectories o 'f the scattered, high-'energy electrons. Certain of these atlelativelywideanglesfrom-the'normal.isrefractedfstill 76 electrons, o'fmaximurn energy and-scattered through Wide king to the screen.

- "ian'glesywill penetrate the' vgrid andy enter the 'space be- L tween it and the electron gun, as is illustrated kby the ytrajectory 35e Oncethey Ahave ydone so they will be attracted to the wall of the envelope instead of' return-'f An electron of equal energy, but

, scattered through any angle slightly lower than that which would cause it tofollowv trajectory 35,50 that'its com-y ponent of energy normal to the screen is less thanthel f,

l screen-to-grid voltage, ywilll be refracted along a path subrstautially as is indicated by the trajectory 37l and fall .backr .to the screen with an energy and at an angley tothe'y screen surface substantially equal to that at which it lemerged, although .its component of energyynormal f to` f the screen will be equal, inthe limit, to the grid-screeny Noitage. f rStill morefelectrons, scattered through smaller and ysmaller angles, ywill have; trajectories `such as yarey illustrated bythe curves designated by the referencey char v l acters 38 and'39. rSuch electrons willfall in the area lgenerallydesignatedat E. of Fig.v 2- and within theA bound'- ary represented by the circle rE of' Fig'. 3..' f ,Electronsv emitted yfrom deeper lwithin the screen structure pene v-trated byr ther beam; at any of vthese various angles,-r ywill 8 asscattering by the phosphors is concerned, such, ,hlm

thicknesses are substantially'aseffective: for suppressing f halo as thicker films would be. f f f v Although, ,as has .beenr mentioned, some scattering oc.- curs from the. first penetration :of-the electrons andy with any material, the probability'ofscattering and ythe angles f through whichfit occursvaries inversely as the electron energy; The deeper the electronspenetrate intor layer f 11 and the phosphors underlying ity the greater will be the probability of back-scattering and the greater the probable angle through `rwhich they are rcleiiected in each scattering event.

f occurs while the rays yare .penetrating through the first f layerv itself. -It is for this reasonthat an aluminum layer i of l/s f R -or more will cut down theettective halo mayterially;r to :a muchgreater extent than' the difference' 2O With -high kvoltage tubes such ask are. usedffor ycolor reproduction, relatively little scattering quarter orso of their range, ymost of yit being effected when the electrons have penetrated intoV the phosphor between its -fatomicr number .fZ and the effective f Z of the phosphors wouldindicate. lf, however, thelot thev overlying layer were equal to or greater than the effec- `emerge with lower energy, and may impact the screen*r f at any point within thecircle E. The outer limit of this' vcircle isd'eiinedv by maximum energy electrons emitted 'at a 45"` angle to the normal to ther screen.r rThe regiony f rwithin, which they impact the screen. is therefore inde'- f rpendent of the angle tb, but their'concentration will `vary f f l.with this` angle, being kgreatest in yazirnuths opposite to 'that of 'the entering beam.

f As vhas beenindicated above,fanypractical display tuber f for 'television purposes must bedesigned tofopera'tey with- 'in 'a very limited rangey of voltages.. If this range isex f ceeded iny use, insulation'y breaks down,- iiash-overoccurs,r and the life of `the tube is shortened below practical;

limits. Ify yoperated at materially. lower voltages, the

` rbrilliancyj ofv theirnage 'ffallsofl so vrapidly as .to become f The energy of ytheele'ctronv v beam and,y therefore, its range in. any of the materials f which are employed-in the constructionof display screens,

unacceptable to the public.

is therefore a fundamental design parameter of any tube which can be commercially marketed. It `-has also been pointed out that in order to provide screens of maximum brilliancy it is accepted practice to make the aluminum layer 11, when used, as thin as `possible and still secure continuity and thus provide a mirror surface; i.e., its thickness has been minimized." In accordance with the present invention the layer 11, instead of being minimized is maximized; its thickness is so proportioned that substantially all of the electrons which are back-scattered from the phosphor surface have their remaining energy absorbed in the layer 11 in passing back through it twice before they can again reach the screen to cause halo. On its face, this would appear to mean that the thickness of the tlm should be equal to one-third of the range of the beam electrons, but actually this is neither necessary nor desirable. Both theory and practice show that by far the greater number of the back-scattered electrons have been subjected to many scattering deflections before they emerge and have therefore traversed paths longer than the actual thickness of the lm. Excess lm thickness causes unnecessary loss of illumination, and hence it has been found that in practice the optimum thickness of the film Vis from l/s to fs of the theoretical range. As already `stated of electrons back-scattered from the phosphors, the maximum of both energy and number occurs in the neighborhood of 104, which corresponds to an angle of 14 to the surface. Such electrons, assuming no further scattering in the layer 11, would have to traverse a path equal to substantially the entire range of the electrons lin their three passages through the layer even if the thickness is only 1A R. Actually, of course, they are subject to further scattering in the layer, more and morelas their'velocity decreases and therefore, so 4far ztiveZ of the phosphors the totalscattering ywould be the same as though the layer were omitted andvalthough l it would be only those scattered in the upper portions of f v the layer that would `reach `the screen the loss of primary l illumination wouldv4 be suchy ythat 'little relativemprove ment in color saturation wouldbe obtained. f *Because some scattering does occur inthe overlyingl layer, however, itis pfeferable to make, this layer ofy as f low Z material as possible, consistentwith the'k processing whichv the tube must undergo inv construction. l)Foi-.this reason materials-having a lowerjatomic numberr give even f better'resultsthanan aluminum coating.` `Thus if the layer-.11v is formed'of beryllium the number of electrons back-scattered ,by the layer itself willfbereduced substan tiallyr in the ratio lof the atomic number of .beryllium to that ofv aluminumv or' to. alittle-more than a quarte-ref 'the number back-,scattered by aluminum.r This results inl a further decrease in the ratio of halo intensity to intensity` `to illumination. It will be apparent that the few electrous which are back-scattered by the protective layer will have traversed a smaller proportion of their total range than those penetratingdeeper and will therefore have, more energy upon emergence from thelayer and excite more halo. Therefore, while a layer of a given mass per square centimeter will be almost equally effective in suppressing halo due to back-scattering by the phosphors irrespective of its atomic number, lower Z materials will reduce halo resulting from back-scattering in the protective layer itself. l

The reduction through the use of low Z materials, in average scattering angle of electrons which reachthe phosphor layer is just as important, as the reduction in back-scattering in the layer itself. This reduction implies that even though a certain percentage of the beam electrons will be back-scattered lintthe phosphors they will have to be deflected to Wider angles before this occurs and therefore, on the average, have to be subjected to more scattering deflections and traverse greater lengths of path within the phosphor itself, accomplishing more ionizing events within the phosphor, with greater consequent light emission. With 'the same proportional thickness of layer, in terms of electron range, their average energy on enteringthe phosphors will be` the same, but because of their narrower dispersion .in angle they will, on the average, have to suffer more scattering events before they are back-deflected widely enough to cause them to re-enter the overlying layer, penetrateit, and escape in'to the space above.' Once having entered this space they are subjected to the eld which accelerates them back toward the display screen, to re-enter it with the same velocity they had when they escaped.

As has been pointed out; the. beam' loses energy 'sub-'- stantially in proportion to .thedep'th toiwhich ithaspenetrate'd and th'el probability ofseatteiflngv to 'a given angle is approximately proportional to Z. With a coating of a lower effective Z than the average of the phosphors and of a thickness of 1k R to 1/5 R it is therefore-quite ap.` parent that most of the back-scattering must take place in the phosphor layer and that in this layer the electrons must be back-scattered far enough to leave it if they are to re-enter the layer 11 and escape through it.

It has also been mentioned that the greatest density of back-scattered electrons occurs at an angle of about 104 and that the peak in energy of such electrons occurs between 103 and 113?, roughly at the angle of maximum density. A thickness of J/5 R is just about sufficient to stop all electrons leaving the phosphors ati peak'energy and density, assuming no further scattering occurs in the layer 11 and that the sole effect of this layer is to absorb their energy. This assumption, of course, is nottrue; further scattering, with consequent increase in path length, occurs in layer 11 and therefore thinner layers, down to about 1/6 R, are quite effective.

Because, however, some of the scattering that occurs in the third traversal of the layer 11 is in a direction toward the phosphors, neither a layer of Ve R nor Vs' R will completely prevent electrons from re-entering the phosphors. Not even a layer of the theoretical thickness of l R, which would require that all electrons reachingv the phosphors would have to traverse the complete electron range to reach them again, will do this, since some very small proportion of 4the beam will be bask-scattered ,in they Outer portion 0f the layer 1.1- and. thus retain enough energy to penetrate it upon their return to the screen. The maximum theoretical thickness of l/a R thereforeresults in; a .less in illumination which iS not usually justified by the reduction in halo. Layers of 1/6 R to 1/5 R in thickness have proved to be the best compromise. Such layers reduce the relative brilliance of the halos which are formed by a large factor, much greater than the proportional loss in screen illumination.

As has been pointed out, both the energy and the density of the back-scattered electrons fall off rapidly beyond the peak angle of about 104 and it therefore cornes out that the layer thickness which is sufficient to stop electrons of maximum energy and greatest number is also effective in stopping those scattered by the phosphors through greater angles.

There are several criteria which may be used in choosing the material for the layer 11. Preferably, of course, it will be one which will form a mirror surface to reecr. light back out of the screen to increase its apparent luminance. Preferably, also, it will be a material of as low an effective atomic number as possible and this combination of properties indicates beryllium as first choice. Other factors which enter from a practical point of view are cost and complexity of apparatus required for depositing the layer upon the screen and since beryllium fumes are highly toxic, manufacturing precautions are necessary where it is used that are not required if non-toxic substances are employed.

Since the total number of back-scattered electrons is roughly proportional to the effective Z of the material penetrated by the beam and therefore some halo reduction can be attained by the use of any material having a lower Z than the phosphor layer. A large part of the effective Z of the phosphors commonly employed is due to the heavy-metal atoms entering into their composition, even though such atoms may form a minority of the total number entering into the compounds used. The phosphors mentioned above are characteristic, and it will be noted that taken over the entire screen their effective Z is something over half of the atomic number of the heavy-metal atoms entering into them. The total number of electrons scattered from near the outer surface of thelayer 11 and which, therefore, retain enough energy to penetrate it when returned by the accelerating field, will therefore be decreased by any layer having an effective Z'less thanone-half of thexZ'of' the heavy-metal constituent of the phosphor. This can'be taken Aas 'a rule-of-thumb criterion of the materialswhich will give a definite improvement. Amore accurate criterion is that the material chosen should have an effective Z lower than the average of the phosphors, considered over the entire screen. Any material meeting these criteria will give a reduction in halo below those in evidence where layers of conventional thickness and materials are used.

For best results, however, materials having the lowest possible Z should be employed.

It should be evident that the phenomena which the present invention prevents do not generally occur in screens used for theA presentation of pictures in monochrome. The aluminum films employed in such tubes serve to reflect the light emitted by the phosphor and to block negative ions from reaching the phosphor. Any film thick enough to form a conductive mirror over the phosphors will suffice to stop true secondary electrons, emitted with an energy of only a few electron volts and falling back to the screen with the same maximum energy, and due to the very short range of negative ions will also block them and prevent the formation of an ion spot on the screen. In tubes which do not have a powerful eld in the neighborhood of the screen, tending to return higher velocity electrons to it, the-great majority of scattered electrons escape and are collected by the conducting walls of the tube.' in tubes employing electrode structures adjacent to the screen that halos of the type suppressed by the present invention occur to any material extent. v

Numerous types of electron lens systems have been suggested for use in tubes intendedfor theprojection of television images in color, and as has been set-forth above various materials may be used which will give a material suppression of halos resulting from back-scattered elev trons. The particular structures and materials, shownA in the drawings and discussed yin the specification are therefore to be considered asrnerely illustrative, not as limiting the scope of the invention, all intended limitations being specifically expressed in the following claims.

What is claimed is:

1. In a cathode-ray tube including means for developing a beam of electrons having a maximum kinetic energy of V kilo electron volts against a target area, said means including an electrode structure positioned generally in the path of said beam toward said target area and adapted for operation at a potential less than V with respect to the source of said electron beam, a display screen disposed in said target area comprising a transparent base, a phosphor coating comprising an element of relatively high atomic number covering said base, and a layer deposited over said coating of a material all components whereof have an atomic number less than one-half that of said element and having a thickness corresponding to at least a mass per square centimeter of 035V2 micrograms.

n 2. A display screen in accordance with claim 1 where- 1n said layer is formed of a material all components whereof have atomic numbers less than 14.

3. In a cathode-ray tube including means for developing a beam of electrons having a maximum kinetic energy of V kilo electron volts against a target area, said means including an electrode structure positioned generally in the path of said beam toward said target area adapted for operation at a potential less than V with respect to the source of said electron beam, a display screen disposed in said target area comprising a transparent base, a phosphor coating comprising an element of relatively high atomic number covering said base and a metallic layer deposited over said coating of a material all components whereof have an atomic number less than 14 and of a mass per square centimeter of at least 0.35V2 micrograms.

lt is therefore onlyl "11 j ,4. A display screen as dened in claim 3 wherein said metallic layer is formed of beryllium.

5. A display screen as dened in claim 3 wherein said metallic layer is formed of aluminum.

6. In a cathode-ray tube including means for developing a beam of electrons having a maximum kinetic energy of V kilo electron volts against a target area, said means including an electrode structure positioned generally in the path of said beam toward said target area adapted for operation at a potential less than V with respect to the source of said electron beam, a display screen disposed in said target area comprising a transparent base, a phosphorcoating comprising an element of relatively high atomic number covering said base and a layer deposited ,over said coating of a material all components whereof have an atomic number less than one-half that of said element and of an average thickness of between 1/6 and 1/3 the range of electrons of V kilo electron volts energy, said range being defined as a thickness corresponding to a mass of 2.1V2 micrograms per square centimeter.

7. A display screen in accordance with claim 6 wherein said layer is formed of beryllium.

. 8. A display screen in accordance with claim 6 wherein said layer is formed of aluminum.

9. In a cathode-ray tube which comprises means for developing a beam of cathode rays of known maximum kinetic energy and structure within said tube which is subjected to electron bombardment by said beam and the composition of which includes elements of relatively high atomic number, the said structure including a luminescent screen', means for suppressing luminescence from said screen resulting from electrons of said beam scattered by said elements of the structure which comprises a dcposit of material thereover each component whereof has 'an atomic number less than one-half that of the scattering element covered thereby and of an aggregate mass per square centimeter of at least 0.352 micrograms, where V'is said known maximum kinetic energy expressed in kilo electron volts.

' 10. Ina cathode-ray display tube which includes means for directing a beam of electrons of known maximum kinetic energy to impact a target area and structure adjacent to said area normally operative at a potential relative Ato the source of the electrons of said beam which is materially less than that necessary to impact such maximum kinetic energy to said electrons a display screen mounted within said target area comprising a light-transmitting base, a coating of phosphors on said base, the average value of the atomic numbers of the atoms constituting said phosphors being Z, and a layer of material covering said coating having an effective value of atomic number which is less than of Z and having a mass per square centimeter of at least 0.35V2 micrograms, where V is said maximum kinetic energy in kilo electron volts.

11. A display screen as defined in claim 10 wherein said layer of material is metallic.

, 12. A display screen as defined in claim l0 wherein the thickness of said layer of material is less than 0.7V2.

13. A display screen adapted for use in cathode-ray tubes wherein there is vdeveloped an electronl beam of known maximum kinetic, energy comprising a light-permeable base, a phosphor coating on said base, and a metallic layer overlying said coating and of a thickness corresponding to4 from `/sto Vs of the range of electrons of said known kinetic energy in the material of said metallic layer, said range being taken as equal to 'a thickness having a mass per square centimeter of 2.1V2, where V is said lmaximum kinetic energy in kilo electron volts.

References Cited in the file of this patent UNITED STATES PATENTS 2,226,567 Le Van Dec. 31, 1940 2,303,563v Law Dec. 1, 1942 2,374,311 Schaefer Apr. 24, 1945 2,692,532 Lawrence Oct. 26, 1954 2,762,943 Mayer Sept. 11, 1956

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