US2507958A - Mosaic screen for cathode-ray tubes - Google Patents

Description

May 16, 1950 H. cAssMAN MOSAIC SCREEN FOR CATHODE-RAY TUBES Filed July 5, 1947 f f l, ff,

l .lille l wllllln, l Willi! u v Patented May 16, 1950 MOSAIC SCREEN FOR CATHODE-RAY TUBES Harry Cassman,-'Harlngton,England, assigner to Electric and Musical Industries Limited, Middlesex, England Application July 5, 1947, Serial No.3759,237 In Great Britain February 21, 1946 "Section l, Public Law 690, August 8,1946

Patent expires February 21, 1966 io claims. l

This 4invention relates to mosaic screens such as areemployed in televisiontransmitting tubes andv for vother purposes.

In manufacturing mosaic screens it is necessary to provide a multiplicity ofinsulated mosaic elements and for .this ,purpose it has been proposed to evaporate metal, such as silver, through r the intersticesof a mesh .onto an insulating surface forming thereon a multiplicity of mosaic elements. .In manufacturing mosaic screens in this fmanner it is desirabletoimaintain themesh in close contact with the insulating surface but it is found Vthat when the v-mesh is so arranged the .mosaic elements tend to adhere to metal deposited on thesides of theinterstioes of the mesh.

Incertain casesthis.isrdisadvantageous and the object of the-.present -inventionis .to'provide an improved .manner vof making a mosaic screen with. a view to avoiding thisdisadvantage.

According `tothe invention there is provided a method of manufacturing .a mosaic screen wherein a mesh is arranged in contact vwith an insulating surface, and metal sevaporated in vacuo through the interstices of said mesh onto saidinsulating surface forming thereon a multiplicity of Ymosaic elements, the interstices yin .said mesh beingsuch thatthe area ofthe intersticesis greater` at the Aside of the mesh in contact with the insulating surface than at a section of the mesh towards the side exposed to the evaporated metal so that themetal deposited on the insulating surface does not extend into contact with the sides ofthe interstices of said mesh.

The mesh can subsequently be removed from contact with said surface without danger of dislodging the deposited mosaic elements, or alter natively the ,mesh ,may be retained ,in contact with said surface without destroying theinsulation of the mosaic elements from one another. A preferred feature ofthe invention is that the sadmethod may be applied tothemanufacture of mosaic screens in electron discharge devices such as television transmission tubes, and the saidmesh then retained in said device so that it can be used as an electrode in said device. The mesh may for example, be retained in contact with said surface or it may be moved toa small distance from said surface after saidelements are formed.

This feature of the invention is especially applicahle to television transmission tubes operating with cathode potential stabilisation. The said .mesh may then be employedas the signal plate in said tube, whereby the transparent signal plate usually employed insuch tubescan bett dispensed with and the consequent loss of light Y avoided.

In order that the said invention may be clearly understood and readily carried into eifect, the same will now be more fully described with reference `to the accompanying drawings, in which:

Fig. l illustrates vdiagrammatically one example of the-method of manufacturing a mosaic screen according to the invention;

VFig..2 illustrates a preferredfeature of the said example;

Fig. Sillustrates diagrammatically a television transmission tube embodying a mosaic screen manufactured in accordance with the said ex ample of the invention; and

Fien shows a cross section of a mosaic screen accordingto a modification of the invention.

In the example illustrated, a suitable low-shadow ratio'mesh I of ne pitch and of which the bars, as indicated at fi, have a triangular or wedge-.shaped section is held so that the ridges or edges '2 of the mesh are in close contact with an insulating surface 3, for example of glass or mica. Due to the cross-sectional shape of the i bars of the mesh the area of the interstices is smaller at the side ofthe mesh-5 than at the side in contact with the surface 3. The insulated surface 3 with the mesh i held in contact therewith is then exposed to metal evaporators in a suitably evacuated chamber and metal is evaporated through the mesh Ion the insulating surface 3. It is known that when metal is evaporated in vacuo the trajectories of the evaporated particles are straight lines, and if the evaporators are 1ocated at a considerable distance from the surface 3 itis possible to make these trajectories approximately normal to the surface 3, as indicated by the arrows ii, and hence due to the fact that'the area of the interstices at the side 5 of the mesh issmaller than at the other side, the

evaporated metal-is deposited on to the insulating surface inthe form of discrete elements 5 without extending into contact with the side walls i of the vbars of the mesh, and also leaving an area 5 of the surface 3 surrounding each element B clear of metal by reason of the said area being in the shadow of the side walls l. The mosaic elements .d are therefore insulated from one another although, as shown, the metal mesh l is not insulated. Metal is of course also deposited on the exposed surface 5 of the mesh, as indicated at il, there forming a conducting grid but the deposited metal does not destroy the insulation of the mosaic elements 8 from one another. The evaporated metal is preferablyantimony, but other metals for example a stibide or silver, may be employed. The mosaic elements 8 may be subsequently rendered photosensitive in any known manner. Meshes having other crosssections than the mesh I may be employed, provided the interstices have a smaller area at some section towards the side of the mesh 5 than at the side in contact with the surface on which the mosaic elements are to be formed.

As a result of the above method, the mesh I can be retained in position in contact with the insulating surface 3 provided with the mosaic elements 8, and can be used subsequently as an electrode in capacitative relationship with the mosaic elements, in a television transmission tube or other electron discharge where a mosaic screen is employed. In some cases however where the mesh I is employed as an electrode it may be desirable to remove it to a very small distance (for example from 0.04 to 0.001 inch) from the surface 3, and this can be done after the evaporation of the metal without disturbing the mosaic elements 8. When it is intended that the mesh I should be employed subsequently as an electrode at such a distance from the mosaic screen, it may in the first instance be mounted at the appropriate distance from the surface 3 and brought into contact with the surface by electrostatic attraction during the evaporation of the metal the mesh being normally suiciently flexible to permit this. Alternatively the mesh I may be mounted in contact with the surface 3 in the first instance and moved to the appropriate distances after the evaporation.

Instead of an un-insulated metal mesh I as illustrated in Fig. 1, which may be of nickel, the mesh may if desired be insulated. For example, as shown in Fig. 4, the mesh 24 may be made of aluminium which is anodised to provide an aluminium oxide coating 23 for the required insulation, or the mesh may be made of woven glass, or other materials provided with any suitable form of insulating coating. Where an insulated mesh 24 is employed, the conducting grid II deposited on the exposed side 5 enables the mesh t to be employed subsequently as an electrode as in the case of where the mesh is not insulated.

Fig. 2 of the drawing illustrates a preferred method of mounting the mesh I in contact with the surface 3 which is shown as a flat glass disc. v

A cement consisting of lead borate powder is applied to the glass disc by painting a suspension of the powder in acetone or water on to the disc, so as to form a frame conforming to the periphery of the mesh I, and the cement is then glazed by heat treatment at a temperature of about 400 to 500 C. A similar frame I0 of the cement is applied in a similar way to the periphery of the mesh I and while the disc is maintained at glazing temperature the mesh is placed on the glass disc. Heating is continued until the two cement frames have fused. On cooling of the cement, the mesh contracts relatively to the glass disc and becomes taut. A similar method may be employed for mounting the mesh where the surface 3 is of mica or other material having a smaller coefficient of expansion than that of the material. In addition other cements than lead borate having the property of softening when heated, for example lithium borate, may be employed, and where it is intended that the mesh I be subsequently employed as an electrode at a small distance from the surface 3 the mesh may be cemented as above described to an annulus or frame which in turn is secured or otherwise held in position against the surface 3, electrostatic attraction being employed to hold the mesh in contact with the surface 3 during the evaporation.

Alternatively the mesh I mounted on a frame in any suitable way may be supported in position against the surface 3 or the whole of the ridge side of the mesh I may be sprayed with lead borate cement and the mesh I caused to adhere to the surface 3 over the entire area of the mesh by the same heat treatment as described with reference to Figure 2. An advantage of fixing the mesh I on the surface 3 is that leakage under the mesh is prevented so that the insulation of the mosaic elements 0 from one another is improved. Where the mesh I is mounted on a frame and supported in position in contact with the glass surface it may be advantageous for the surface 3 to be slightly convex towards the mesh in order to ensure close contact.

As stated, the invention is especially applicable to the manufacture of mosaic screens for use in television transmission tubes operating with cathode potential stabilisation, and in Figure 3 there is illustrated a television transmission tube of this kind, parts which are not relevant to the present description being omitted. The tube comprises an electron gun I 3, a solenoid I5 which produces the enveloping axial magnetic eld, two pairs of deiiecting coils, 0f which one pair II is shown for imparting horizontal and vertical deflections respectively to the beam of electrons I4 from the electron gun, a wall anode I6, a decelerating electrode IS and electron multiplier I9. These parts are arranged and function in the same way as in a television transmission tube such as known by the registered trade-mark Orthieon and need not be further described. The tube is, however, provided with a mosaic screen by the method described with reference to Figure 1 or Figure 4 and the mesh I is retained in position and employed as the signal plate of the tube. It is therefore possible to dispense with the transparent signal plate usually provided in such tubes and during its manufacture the mosaic elements 8 of the mosaic screen are evaporated through the mesh I directly on to the inner surface of the transparent end wall I2 of the tube, said inner surface constituting therefore the surface 3.

During the operation of the tube, an optical image of the object for transmission is projected on to the mosaic screen on the surface 3 and charges the capacities provided between the mosaic elements 8 of the mosaic screen, and the mesh l, so as to produce a positive charge image of the object for transmission in known manner. and the screen is then scanned by the beam I4 at such low velocity that said capacities are discharged and the mosaic elements 8 of the screen are restored to an equilibrium potential corresponding substantially to the cathode of the electron gun I3, unrequired electrons forming a return beam 2! which is directed into the electron multiplier i9 from which a signal output is obtained. The mesh i is arranged to have a suiiciently fine pitch as not to be resolved in the picture.

The mesh I is preferably biassed to a potential of a few volts positive (for example 4 volts) with respect to the cathode of the gun I3, and hence with respect to the potential at which the mosaic screen is stabilised in the absence of light thereon. The mesh I then serves to conduct away photo-electrons liberated from the mosaic screen, but at the same time also functions as a control electrode and preventsfmosaic elements 8; on. the mosaic screen from charging toapo-` tential; appreciably higher thany the potential ol the meshV I.l Thecharge onthe mosaic elements which intensely bright .parts ofthe object for transmission may cause is therefore limited and the limit can bevaried by varying the-.bias applied to the mesh. I. It is thereforepossible to improve the signal gamma ofthe tube and to eliminate-or reduce picture distortion duetodeflection of the incident beam ofelectrons I4 by highly charged mosaic elementsthereby reducingV th geometrical distortion which arises in`V tubes operating with cathode potential stabilisation.

I-f the mesh. I is maintained-as stated,.at a

positive potential it will absorb electrons fromthe beam I4 Whereas in the absence-of the mesh- I- the insensitiveinsulating areas of thesurface 3-of the mosaic,- scrcen between the mosaic elements 3, would be at cathode potential and would re turn all the incident beam I4 to the multi-plier I9. Therefore the-employment of the mesh i maintained at a positive potentiallwill'vreduce the current from the insensitive areas of the mosaic screen and hence the modulation of the return beam Will'be improved With a corresponding improvement in the signal/noise ratio.

During tube operation the electron bea-m scans the-surface of the target Iv--3l on they endoftube It; rEhe-screen I-is sufliciently fine that the beam spot or cross-sectional area of the beam I4 at the target covers an area'of the screen I embracing a plurality of interstices or apertures. Thus, the portion of the incident beam I 1I intercepted by the positive mesh screen I will not be returned and only those portions of the beam Irl which pass through the'mesh and are turned back by the discharged mosaic elements 8 will constitute the return beam 2|. Without the presence ofV a positive mesh I, thereturn beam, ZI would include not only the portions turned back. by the discharged mosaic elements 8. but also alarge component turned back by theinsulating areas of the insulator surface 3'which would be at Acathode potential or more negative than cathode poi tential during tube operation.

In some cases it may be necessary or preferable to remove the mesh I after the evaporation of the mosaic elements B, to a small distance from the surface 3 of the mosaic screen, (for example about .04.- inch) While maintaining the mesh and the surface parallel. In this case the capacity of the mosaic elements 8 to the mesh I, may be too small to use the mesh I as a signal plate and it may Abe necessary to employ the usual transparent signal plate.

A further metal mesh indicated at in Figure 3 may also be included in the tube, parallel to the surface 3 and about l. cm. distant from it, the mesh 2Q being maintained at a potential of about 10 volts positive with respect to the Wall anode IB and the anode of the gun I3, which will be maintained for example, at about 100 to 200 volts positive with respect to the thermionic cathode of the gun I3. The mesh 29, in such a case serves to prevent positive ions reaching the mosaic screen on the wall IZ thus reducing or removing the ion-spot which is troublesome in television transmission tubes employing cathode potential stabilisation. It has also been found that such a mesh, by causing uniform deceleration of the electron beam I4 near the mosaic screen, reduces distortion and causes the area scanned by the return beam 2| to be very small. In this case by the use of lift plates, the

return Vbeam can be diverted into anelectron multiplier 'separate `from' the electronzgun. lf3., rather than a multiplier such asv IzS whichemploys the'anodeof thev gunv I3 as.. the irst multiplying;

electrode.

While the invention has been described, as applicable especially to a television transmission tube operating with cathode potential stabilisation it 'will be understood that mosaic screens manufactured-in raccordance with the invention can be applied to other electron` discharge devices employing mosaic screens.

Having now particularly described andascern tained the nature of' my said invention and in what manner the-:same is to be performed, I declare that what I claim is:

l. For use in a cathode ray discharge device, a mosaic screen comprising a transparentinsulator backing plate, a metal mesh including bars and intersticesv therebetween having one side xed against onesurface .of said, backing plate, an insulating material coating the surface of'said metal mesh, aplurality of photosensitized metallic elements on Said one surface of` said backing plate, each one of said metallic elementsfposi tioned in alignment with an interstice Vof said mesh and spaced from the others of` said elements and from said mesh, and-a conductive metallm covering the bars on thecther side ot said mesh.`

2. For use in aV cathoderay 'discharge device, a mosaic screen comprising a glassfbacning plate, an aluminum metal mesh including bars and interstices having one side-xedf against one surface of -said backing plate, aninsuiating oxide layer'coatingsaid aluminum mesh, a pluralityy of photosensitized antimony metalv elements on-.said one surface of said backing plate, each one of said-v'antimonyy elements positioned in alignment with an interstice or said mesh and spaced from the others ofsaid` elements and from said'. mesh, and a metal 'film covering the bars on the other side offsaid mesh.

3. For usein a cathode ray discharge-device, a-mosaicscreen oomprisingfa glass backing plate, a metal mesh including a plurality of bars and interstices therebetween fixed against one surface of said backing plate, said bars being Wedgeshaped and arranged with the edges thereof in contact with said one surface of said backing plate, a photosensitized metallic element xed on said one surface of said backing plate in alignment with each one of said interstices, said bars insulatingly spaced from said elements.

4. For use in a cathode ray discharge device, a mosaic screen comprising a glass backing plate, an aluminum metal mesh including a plurality of bars and interstices therebetween having one side thereof xed against one surface of said backing plate, said bars being of triangular cross-sectional area to form an edge contacting said one surface of said backing plate, a plurality of photosensiu tized transparent elements Xed on said one surface of said backing plate, each one of said photosensitized elements positioned in alignment With an interstice of said mesh, an oxide coating on the bars of said mesh to insulate said mesh from said photosensitive elements, and a metal film covering the other side of said mesh.

5. A discharge device comprising an evacuated envelope, means for producing a beam of electrons along a path within said envelope, a mosaic screen positioned Within said envelope transverse to the path of said electron beam, said screen including an insulator backing plate having one surface positioned to intercept the electron 7 beam, a conductive mesh in contact with said one surface of said backing plate and a plurality of photosensitive metallic elements xed on said surface of said backing plate, each one of said metallic elements insulatingly spaced from the others of said elements and said mesh.

6. A discharge device comprising an evacuated glass envelope, means for producing a beam of electrons along a path within said envelope, a mosaic screen within said envelope, said screen including a portion of said glass envelope intercepting the path of said electron beam, a metal mesh xed against the inner surface of said envelope portion, a plurality of transparent photosensitive elements fixed on the inner surface of said envelope portion, each one of said photosensitive elements positioned in alignment With an interstice of said mesh and insulatingly spaced from others of said elements and from said mesh.

7 A discharge device including a tubular evacuated glass envelope, means for producing a beam of electrons parallel With the axis of said tube, a mosaic screen within said envelope, said screen comprising the end Wall of said tubular envelope intercepting the path of said electron beam, a metal mesh fixed against the inner surface of said end wall and a plurality of transparent photosensitive elements fixed on the inner surface of said end wall, each of said photosensitive elements positioned in alignment with an interstice of said mesh and insulatingly spaced from others of said elements and from said mesh.

8. A discharge device including a tubular evacuated glass envelope, means for producing a beam of electrons parallel with the axis of said tube, said means including a cathode electrode mounted within said envelope, a mosaic screen within said envelope intercepting the path of said electron beam, said mosaic screen comprising a portion of an end Wall of Said envelope, a metal mesh xed against the inner surface of said end wall and adapted to be maintained positive relative to said cathode electrode during tube operation and a plurality of transparent photosensitive elements on the inner surface of said end Wall, each one of said elements in alignment with an interstice of said mesh to intercept said electron beam, each of said elements insulatingly spaced from others of said elements and from said mesh.

9. A mosaic screen for an electrical device, said screen comprising an insulator backing plate, a metal mesh including a plurality of bars and interstices therebetween xed against one surface of said backing plate, said bars being wedgeshaped and arranged with the edges thereof in contact with said one surface of said backing plate, a photosensitized metallic element fixed on said one surface of said backingr plate in alignment with each one of said interstices, said bars insulatingly spaced from said elements.

10. A discharge device comprising an envelope, means including a cathode electrode for producing a beam of electrons along a path within said envelope, a target plate electrode within said envelope transverse to the path of said electron beam, a metal mesh xed against the surface of said target plate electrode intercepting said beam path and adapted to be maintained positive relative to said cathode electrode during tube operation, a plurality of photo-sensitive elements fixed to said intercepting surface of said target electrode, said photo-sensitive elements insulatingly spaced from each other and from said metal mesh.

HARRY CASSMAN.

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

UNITED STATES PATENTS Number Name Date 2,077,442 Tedham Apr. 20, 1937 2,237,681 McGee et al Apr. 8, 1941 2,244,365 Iams June 3, 1941 2,250,721 Moller et al July 29, 1941 2,401,220 Bonner May 28, 1946

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