US2943230A - Storage-type color display tube - Google Patents

Description

E O. LAWRENCE HAI. 2,943,230

STORAGE-TYPE COLOR DISPLAY TUBE June 28, 1960 2 Sheets-Sheet 1 Filed March 11, 1958 INV ENT ORS 7A/57' 0. ANPE/VCE @4V H. LEE

BY I

June 28, 1960 E 0- LAWRENCE ETAI- 2,943,230

' soRAGE-TYPE COLORA DISPLAY TUBE Filed March 1l, 1958 2 Sheets-Sheet 2 irren/ir.;

2,943,230 STORAGE-TYPE coLoR DISPLAY TUBE Ernest O. Lawrence, Berkeley,` Calif., and Ray H. Lee,

New York, N.Y., assignors to Chromatic Television Laboratories, Inc., New York, N.Y., a -corporation of California Filed Mar. 11, 195s, ser. No. 720,714

@Claims-Lg (cl. 315-12) This invention relates to cathode-ray tubesof the storage type, adapted for the display of images in color for television, radar, and like purposes.

A storage-type cathode-ray display tube, as the term is used herein, meansacathode-ray `tube wherein the image tracedY by a scanning cathode-ray beam, instead of`being displayedinstantaneously, a point at a time, is Veffectively stored or retained for a greater or less period so that thepath traced bythe beam can be reproduced and displayed after the beam has passed. 'I'he period for which the image is retained can vary between wide limits; say, from go second, corresponding to the period of field repetitionof television signals transmitted in accordance with present United States standards, or'itA can be l for muchlongerperiods, such as minutes, hours, or even days. Display of the` image can be substantially co'ncurrent with its production or jwriting, the display of any individual areaA1 of' the image iield beginning aty the instant `that it is stored and continuing until that elementary area is again scanned by the` electron beam, or the image may be held, invisibly, or remembered until it is reproduced at a later time.

Numerous storage-type monochrome display tubesy have been suggested in the past; many of theseY arev described in Storage Tubes by Knoll andgKazan (lohn Wiley and Sons, Inc., 1932), together withtheir general principles of operation and including the theoretical discussion of various ways of writing, reading, and erasing the images produced and reference is made to this workfor matters not discussed in detail herein. Obviously tubes of the storage type are considerably more complex than the simple type of cathode-ray display tubes as used in television receivers generally. Numerous types of cathoderay tubes for the display of television or like images in color have also been proposed. These, too, have involved greater complexities than monochrome display tubes. Furthermore, in both storage tubes and color tubes the positioning of their various elements and the relative potentials at which these elements are operated are usually quite critical. Certain of these requirements of the two types of tubes have proved to be diiiicult to combine in a single devicel that would unite the purposes and advantages of both.

The broad purpose of the present invention is to pro` vide a cathode-ray tube eiectively combining the characteristics of storage and color type display tubes. More specifically, among the objects of the inventionv (not necessarily listed in the order of their importance) are the following:

(1) To provide a storage-type tube which will display a polychrome image with a definition substantially equal to that attainable by direct display;l

(2) To provide a cathode-ray tube that will` display images in color at greatly increased brilliancy as compared with that attainable withrdirect display;

('3) To provide a storage-type tube for displaying images in color which is capable of either displaying Patented .lune 28, -1960 Y such images concurrently with their tracing or of retaining such images for a more or less indefinite period and later displaying them in color with substantially n'o degradation in resolution;

(4) To provide a tube adapted, without structural change, to usesubstantially any of the known techniques of writing, reading and erasure of the images; depending upon the particular useto which the tube is to be put; and 5 To'provide a tube which can readily be constructed in either two color or three color versions. w In order to describe-the present invention in general terms it is simplest to consider rstthe known type of color television tube as proposed by one of the present inventors and described in his Patent No. 2,692,532, Such a tube comprises the usual evacuatedV envelope,

, preferably of generally funnel-like shape, having a view-,-

`beam bidirectionally toV scan the viewing area may be ing area or Window inthe largeend' of the funnel and an electron gun in the neck of the funnel and adapted to direct aj narrow, collimated beam of electrons toward the center of the viewing area. Means for deiiecting the incorporated within the tubev in the form of deflecting plates, but are more usumly provided by auxiliary coils encircling the neck tothe tubewhen in use; the same isv true of-` means for focusing the beam into its narrow I pencil-like form. The display screen in such a` tube is comprised of narrow strips of pliosphors emissive of light of different colors upon electron impact, these strips extending: across the full 'widthof the viewing area in one dimension and each strip beingl less than one elemental area offth'e picture t`o be reproduced in width.` Deposited over the `layer`-`of'phos`phor strips is a thin film' ofconduct-y ing material; preferably of aluminum or some other metal having a lowl atomic number, this film being sothin 'as to be` electron-permeable.`

Mounted between the electron gun and the screen, closely adjacent Ito and parallel tothe latter, isa structure adapted to set up electric fields'that will form a n1ulti`v plicity of cylindrical electron lenses; i.e., electron lenses which tend to converge inf` one dimension, electrons entering' 'the pupils thereof, leaving the 4component of the direction of their paths in the other Vdimension either unchanged or, sometimes, slightly divergent. This electron lens structure may take several forms,` but it necessarily includes a color-control grid comprising two mutually insulated, interleaved sets of tightly stretched linear conductors, the conductors in each set being electrically connected. These conductors are so arranged that those of one set are electron-optically centered over phosphor strips emissive of one color upon the display screen while those of the second set are similarly centered over or alined with the centers of the strips emissive of a dilerent color. By electron optically centered is meant that when normal operating potentials are appliedvto the'variplied to the two sets of conductors of the `color grid, thus deflecting the focal point toward oneor the other of the strips o f phosphor above'which the grid vconductors are.A

centered.

InY accordance with the present inventionthe electrn lens system for accomplishing color control and focusing comprise rst, a colorcontrol grid as above described;

second, ascreen grid, which may be either a'rriesh of' Y ne wires or, preferably, a series of parallel wires extending transversely to those of the color control grid. Beyond these two grids, positioned in the plane that would normally be occupied by the display screen in a tube of the non-storage type, is a storage mesh comprising a foraminated sheet or netting of conductive material, coated, at least on the side facing the electron gun, with an insulating lm having good secondary electronemissive properties. The display screen itself is of the same type and conformation as would be used were it positioned in the plane of the storage mesh; it is spaced slightly beyond the storage mesh in a position as though moved back without rotation from the plane of the latter in a direction normal to its own plane.

Mounted between the screen ygrid and the storage mesh are iirst, a source of flood electrons, so directed as to form a space charge between the ,screen grid and the storage mesh, and second, closely adjacent to the storage mesh, a collector grid of ne wires. The preferred forms of the various elements described and the mode of the operation of the device will be explained in connection with a detailed description that follows, this description being illustrated by the accompanying drawings wherein:

Fig. 1 is a diagrammatic illustration of one embodiment of the invention, not to scale, the dimensions of the various elements being distorted in order to show more clearly the construction and disposition of the parts;

Fig. 2 is a diagram illustrative of the voltage gradients within the tube of Fig. 1; Y

Fig. 3 is a schematic diagram showing the connections of one form of flood electron source;

Fig.` 4 is an'elevation, partly in section, showinganother type of ilood electron source; and

Fig. 5 is a cross-sectional view of the source illustrated in Fig. 4, the plane of section being indicated by the lines 5-5 of the preceding ligure. l

The form of the present invention illustrated in Fig. l comprises the usual, generally funnel-shaped envelope 1, which may -be either of metal or glass; if of glass, it is provided with an inner conductive layer. The larger end of the envelope is closed by a glass window 3; the small end terminates in a tubular neck 5, also usually of glass, within which is mounted an electron gun. The latter comprises a cathode 7, usually an intensity controlling grid 9, and one or more anodes 11, the last of which connects to the envelope or its conductive lining. Many forms of the electron gun are known, any of which may be used provided it is adapted to form a collimated narrow beam of electrons -directed normally toward the viewing area. Connections from the various elements in the electron gun are made to pins 12, carried by a base on the neck of the tube. In use, the usual focusing and deflecting coils surround the neck between the end of the anode 11, and the start of the are of the funnel.

The remaining elements within the envelope are all substantially planar in form and nearly equal in area and are mounted very closely adjacent to the viewing window 3, in planes substantially normal to the undeilected path of the beam from the electron gun to the center of the window. The irstof these elements is a color-control grid, comprising two mutually insulated interleaved sets of tautly-stretched, fine linear conductors 131 and 132, the conductors of each set being electrically connected together as shown schematically in the diagram. wires of about 4 mils diameter, spaced 10 to 2O mils on centers, depending on the total size of the tube and the area of its display screen.

The second element in the'electron lens system isa screen grid 15, formed of linear conductors similar to those forming the color-control grid but extending transversely to the latter across the tube.

they need not be quite as closely spaced as the control In a typical tube these conductors will be l All of the screenV grid conductors are connected together, however, and

grid conductors. If desired, a wire mesh may be used for the screen grid instead of the transverse conductors only, but the latter arrangement is preferred. The spacing between the color-control grid and the screen grid Will typically be approximately about 1/2 inch.

Behind the screen grid again is a flood cathode 17. In Fig. 1 this cathode is indicated symbolically as a resistor; actually it is formed as a grid or network of thermionically emssive wire, e.g., ne thoriated tungsten, connected as shown' in Fig. 3. In Fig. 3 the emissive wires are those running laterally of the ligure and indicated at 171. These are cross-connected to form a network through equalizers K172, the whole being supplied by leads 19. As will be seen, the structure and method of connection is such as to put many short laments in parallel connection so as to minimize the voltage diierence between the various parts of the network and make its average potential as nearly uniform as possible, the total variation in voltage over the entire area of the cathode being only the order of 2 or 3 volts at most. Further, to minimize such voltage variations, the equalizers are made of low resistance conductor. The heating current for the elements is provided by a pair of batteries 21 and the bias source that establishes a mean operating potential of the cathode, in relation to the other elements of the tube, connects at a point'23 between the two batteries. In a typical tube, from which the dimensions of that herein describedrare taken, the spacing of the cathode from the screen grid is approximately 0.2 inch.

The next element of the tube, spaced, in the present case, 0.2 inch from the flood cathode 17, is a collector grid 25. This should be a mesh of very ne wire, 3 mils in diameter or less, the openings betweenv the wires being relatively large in comparison With'the wire diameter; e.g., 0.10 inch. Alternatively, it may be stretched parallel wires running unidirectionally, without the transverse members of the mesh.

A storage mesh 27, upon which the operation of the tube primarily depends, is spaced 0.1 inch from the collector grid in a plane parallel thereto. This mesh is very tine. It may be woven wire, 250 or more meshes to the inch, or it may be a thin foraminated foil, formed, for example, by the etching process, with its foraminations spaced on centers by distances ot the same order of magnitude as the Woven mesh just mentioned. The storage element itself is deposited on the conductive base formed by the mesh or foil. It comprises an insulating layer which may, for example, be deposited on the conductive oase by evaporation. Any of the numerous materials that have been suggested for the purpose may be used; these materials have in common that they have a relatively high secondary emission ratio and high insulation resistance. Among those that have been suggested and are, in fact, suitable, are the phosphors, such as the silicates, evaporated silica and aluminum oxide. This coating need only be deposited on the side of the mesh facing the electron gun.

The nal element of the tube is a display screen, generally indicated by :reference character 29, mounted just within the viewing window 3 and approximately 0.25 inch from the storage mesh. It comprises a glass or other transparent baseplate 31, on which, facing the electron gun, is deposited a layer of phosphors 33 covered by a conducting film 35. The phosphor layer comprises strips of phosphors emissive of different colors upon electron impact. These strips run substantially parallel to the wires of the color grids 131, 1&3. The number of different phosphors employed may be either two or three, preferably three if the device is to be used to reproduce colorrtelevision signals, although in certain cases two color reproduction may be used for certain military, scientic, or other purposes. For a bi-color tube s trips of one color, say red, are electron-optically centered behind conductors 131 of the color grid, while those: of a. secondrcolor, say; green, are similarly centered behind: conductors 132. For a tri-color tube strips of` the third. color, say blue, are positioned midway between each pair of red and green strips. The order `of the strips is, underthe above assumptions, red, blue, green, blue, red, etc., there being twice the number of blue strips as of red or green. Which strip is chosen. as the center strip is `to a large degree arbitrary and d'epends on the particular system with which the tube is used. The order described is purely illustrative.

The strips are electron-optically centered beneathV the color control grid conductors, not physically centered; except` at the 'center of the screen a perpendicular dropped from the conductor 131 would not strike the center of. the corresponding strip. What electron optically centered means, in thepresent instance, must Ybe derivedindirectly from the position. such .strips would occupy were the display screen moved to the position of `the storage-meshfzlg; As will `be described in more detail. later, the electrons streamingv from the `gun emerge from it at a velocity corresponding to approximately 3 kilovolts, and are acceleratedto 5 kv. at' the screen grid, beyond. which-their velocities are substantially constant until theyy strike'the storage mesh. The acceleration does not aiect the vpaths of the electrons traveling down the tube axis. iWhen` deflected toward the edges of the Screen in the scanning process, however, the accelerating forces. are predominantly normal tothe surfaces of the grids, the transverse components ofthe velocities being but littleaffected. .Their trajectories can be computed; as has. been des'cribedin the prior art, andthe pointi at which? an electron `of the beam which would pass through the positionv occupied by one ofthe grid con-- ductors.;131.. (were it; not there) would strike the storage mesh is therefore electron-optically centered or alined behind that conductor.

The electrons used to excite the screen, however, are not those of the beam but thoseA emitted trom the floodz cathodes 17 They are Ynot subject to dellection buttravel ini-paths perpendicular to the surfaces of the storage mesh` and screen. The disposition of the strips` on theA screen is therefore what it would be were the screen moved axially of the tube without rotation'` about its axisinto the-position lof the storage mesh.

Separate connections are brought out of the tube for each of the sets of color-grid conductors, the screen grid, the flood cathode (as has already been described), the collector grid, the base conductor of the storage meshand the conducting lm 35 of the displayscireen. Some of these connections can be brought out through the pins 12 on the base; in other instancesit is more convenient to bring them out through thewalls of the tube. These connections are shown schematically, without necessarily indicating how they would be handled in any individual tube.

There are various ways of writing the signal on the` storage mesh, all described in Storage Tubes by Knoll and Kazan, cited above. For purposes of the present'description the. operating parameters rst described will be those suitable to non-equilibrium writing and grid control readingVas described therein, it being funderstood that grid control reading will be utilized irrespective of the writing method actually employed. There are two setsvof conditions that must be met which dictate the biases imposedupon the various electrode structures; rst,the electron lens system established by the colorcontrol grid l13 and the screen grid V15 must have a focal length of the proper magnitudefto converge electrons passing between adjacent wires 131, 132 into a line focus,

or approximate focus, giving a beam width lnot greater than one-.half'lthe width "ofone of the phosphorl strips. and preferably less. The focal length of the lens'system' here described depends upon the ratio between the initial acceleratingvoltage applied-tothe beam and the voltage betweenthecolor'control,grid and the screen, grid. v It is'Y but slightly affected byV the presence of Hood cathode"` 1'7- and collector grid 25. However, the voltage` gradients within the space between the screen grid and storage mesh must be such as to permit proper writing or collection of stored charges without interfering with the reading of the signals as they appear on the display screen.

Thevoltage gradients used to accomplish these results are indicated by the plot of Fig. 2. It is convenient, for various reasons, to operate the conductive base of "the storage mesh at Vground potential. VThe plot o f` voltage gradients is based' on the assumption that it is so operated but it is to be understood that under other circumstances any point of the system c an be. grounded, as long as the voltage differences between thevarious elements are in the proper ratio. i i In the operation'here illustrated the cathode7 is biasedy at about 5 kv. negative toground andY the control grid -9 about 30 volts negative to the, cathode. The final anode 11 of the electron gun is biased approximately Skv. negative to ground; in practice betweenabout 2.7 and 3.3, it being this voltage that is varied to obtain the best focus of the beam by the color grid, both sets offconductorsv of the latter being operated at the same mean potential. TheV screen grid is grounded, so that there is approximately a 3 kv. dierence in potential between it and the average potential of the color control Vgrid conductors. In use, the color switching voltage may be appliedto the .color-control grid conductors through a transformer 37, the secondary-whereofis provided" with a center tapthrough which the bias potential is supplied.

With the voltage gradientsjand spacing of .the parts as here described, the switching can be accomplished with an applied .A.C. voltage across the color grid conductors of approximately l00"volts.V

The flood cathode 1'7 is operated at a bias potential at about. l0'volts positive and the collector grid Vabout 150 volts positive -to ground. The base potential of'the sto.- age meshe'is, as before stated, zero. Except for the charges stored on` the individual elements of the storage mesh, there is/thereforeV no` net potential difference between the screen grid 15 and the storage mesh 27. The maximum voltage difference within this space Vis there- Vfore only in the neighborhood of volts." This is so Small in comparison with the 5000 electron-volt energy ofthe incoming electrons that it may be disregarded and the beam considered as though it were traveling in a eld-free space; i.e., the eiectV of the ood cath'ode and the collector grid on the path of the high velocity beam is so small that it may bev neglected.

With the materials used fo'r coating the storage grid, the 5 kv. energy applied .to the electron beam is substantially that required to operate it at the so-called second cross-over point of secondary electronic emission of the coating material (Vm, to use the notation generally employed); i.e., at the impact energy whereat the secondary electron gun. `Under the circumstances here considered these charges are always positive.` In scanning, therefore, the beam deposits, on the storage mesh,A acharge pattern that corresponds inintensity to the' light patternV thatwould be exhibited vwerethe display screen moved' to the same position. The film 35VA on the surface of'the display screen. is

biased to a relatively high potential positive toV ground;` somewhere in the order ofV 10 kv., but this is not critical,

The flood cathode 17 liberates electrons which, since the ood cathode is positive to both the screen grid and the base of the storage mesh, but negative to the collector grid, are attracted toward the latter, most of them passing through the meshes to form a substantially uniformly distributed space charge immediately adjacent to the secondary-emitting surface of the storage mesh and thus forming a virtual cathode between the storage mesh and the collector grid. This virtual cathode, the individual charges on the storage mesh,'and the conductive film on the display screen therefore form, in effect, a triode, with the stored charges acting as control signals on the grid to control the electron flow.

The graph of Fig. 2 illustrates, only very roughly to scale, potential gradients within the tube, the dotted lines indicating the approximate vpositions along the Vaxis of the tube whereat the various bias voltages are applied. The difference between the cathode voltage E and the bias Ei at the intensity-control grid is too small to show on the scale selected.` The potential of the anode 11 is substantially equal to the bias voltage El applied to the color control grid, so that through the major portion of the tube the beam travels thro'ugh a substantially fieldfree space. A steep potential gradient accelerates the beam between El, the color-control grid potential and E2, the screen-grid potential at ground. The storage mesh is also biased to the same potential E'2, but between these elements a very small potential gradient exists between the screen grid and the collector grid biased at EC. This gradient is shown on a somewhat exaggerated scale, but even at this scale, the volt rise from E2 to' E, is too small to show. Beyond the storage mesh is the steep gradient rising to l0 kv. at E3.

Because of the extended parallel surfaces of the storage i mesh and the display screen, the lines of force between the two are normal to both and the paths of electrons penetrating the mesh of the storage screen follow the lines of force, so that the image displayed upon the display screen is substantially identical with that which would be shown upon direct scanning. It should be mentioned that the display screen may be formed directly on the window 3. The window is almost always slightly curved to enable it better to withstand the air pressure on its surface. If the curvature is slight it will have little efect on the electron trajectories between the storage mesh and the screen, as the lines of force are nearly normal to the -storage mesh until the electrons have reached nearly their terminal velocity. If the screen is more deeply curved the trajectories can be computed and allowance made in positioning the phosphor strips.

The pattern displayed may, however, be very much brighter than that of a directly scaned display screen, depending upon the way in which the tube is used. Any of the various known methods of erasing an image that has been written upon the storage mesh in one scanning, preparatory to' the re-storage of a new image, may be used. These have been described in the text above referred to.

One method, involving a different mode of writing, is of particular interest; i.e., equilibrium writing.

In this method of writing the cathode 7 is operated at a potential negative to the storage mesh somewhat greater than Vm, and an unmodulated beam of high intensity is employed, the grid 9 being coupled back to' the cathode 7 through a condenser in order to `insure that no material A C. potentials appear between them to cause variation of beam intensity, while still permitting the proper dilference in bias potential between the two. The collector grid is operated at about the same or at a slightly lower potentialdiference with respect to the grounded base of the storage mesh as in the case previously described.

Under these circumstances the elementary areas of` the storage mesh that are under bombardment from the scanning beam will tend to assume an equilibrium potential such that the secondary electrons to the collector grid are equal in number to the primary electrons from the beam. This equilibrium potential is a constant for a given structure and cathode potential. The quantity of the charge required to bring any element of the storage surface to equilibrium depends upon the potential difference between the collector grid and the conducting base of the storage mesh, still assuming constant cathode potential; it will vary with the latter.

The signal voltage can therefore be applied either to the cathode, the collector grid, or the storage mesh, therebeing some slight advantage in applying it to the cathode as this does not aiect the triode operation of the ood cathode-storage-mesh-display screen combination. Any one of the three will work, however, provided the signal is applied in the proper polarity.

lf cathode modulation is used a positive signal will decrease the impact velocity of the beam, and since operation is beyond the cross-over voltage Vm, will increase the secondary emission, and thus tend to drive the impacted area positive, increasing the brightness of the corresponding area of the screen.

Applying the signal to the collector grid, the charged surface 4tends to assume the same potential as the grid and in this case also a positive signal corresponds td a positive charge and increased brightness of the corresponding point on the screen.

Applying the signal to the storage mesh the reverse is the case; with a positive signal the charge collected on the storage surface must be negative to restore equilibrium, resulting in fewer electrons passing the mesh in the triode operation of the device. The signal polarity must therefore be reversed as compared with a gridmodulated direct display tube or the other two methods of operation.

In this last-described mode of operation, the oodelectron flow will vary constantly with the variation in storage-mesh voltage. Over the period of `a frame, however, the electron ow will correspond to the average voltage of the mesh, varying above and below the zero axis of the A.C. component of the signal. This method of applying the signal is therefore operative, although less elegant than the others.

With all three modes of equilibrium writing the storage-mesh elements are recharged (or discharged) to their new equilibrium potentials each time they are scanned by the beam, thus erasing their former charges. The image on the screen is therefore displayed without interruption, giving a picture without icker or color crawl.

In equilibrium writing one of the principle concerns is to obtain a suiciently intense beam to obtain complete equilibrium. High current beams tend to diverge, because of the Coulomb effect. ln the present device the electrons passing between any pair of color grid wires are re-focused to an area much smaller than the initial cross-section of the beam. Therefore higher densities and better approach to equilibrium can be attained than with most devices using equilibrium writing.

Another method of introducing the ood electrons into the space between the screen grid and the collector grid is illustrated in Figs. 4 and 5. In this arrangement the source of the flood electrons surrounds, at least partially, the area into which the electrons are introduced. The electron source is a gun comprising a glass frame 41 of U- or channel-shaped cross-section, with the open end of the U directed inward; this member 0r frame may either be mounted within the envelope or sealed in its walls substantially in the plane of the cathode 17 of Fig. 1. Within the U section of the frame, starting at the bottom of the channel, is a heater element 43, an

electron emissive cathode 45 and an anode 47, slotted to permit egress of the electrons from the cathode.

In operation the anode 47 Vis biased a few volts posanni; reg-heeft 9. itive to ground potential, the` cathode'Y slightly :less positive, the actual values of the biases being'ldetetminedkby expeimentfat the best valuesto'give annifonr distributiontotheinjected-'electrons'. Y.

" arangementhas considerable exibility Ainloperationf. Thev cathode can be sectionalized v.so that the various sections can bei-operated' at different `'biasesjto secure the `best and most` iunifnrrn` distribution of- =elec mms :within the region between the 'sc/reenggridfandfthe grid. in someiu'stances `it possible to omit entirely the cathode 'sections'othe' shorter sidesy of the vrectang'ular'frame, provided Va rectangular `tube is used. lWith the cathode operated at a potentialslightly positive to the `bias potential -of the storage mesh,rand laccelerated t'owardftheilatte'r -by the collectorl grid, electrons will approach-the storage-'meshfasfa result of their acceleration but not strike yit as long'as n'o' part` offitsxsurface reaches `a .potential thatfisasufar positive as that of-the cathode;4 As a result,-if thesupply `f electrons Vfrom the flood cathode is adequate-fthe desired virtual cathode will form immediately infront of 'thel storage fmeshbut will penetrate the latter only to the extent that the iield from the display screen 4penetrates it, :the effective penetration, ofcourse, being moditedby, the charges collected by the storage plate. 4

"It will be appreciated that the electron flow through thestorage mesh `need not be large in Aorder to produce fdisplay that is very brilliant in comparison with 'the one produced. by a scanning electron beam. In'they ordinary television display tube the current carried by the electron beam is only a fraction of a milliampere. At the rate of scanning dictated by present U.S. standards of transmission `the beam falls upon any individual area for only about Immo@ of the total time during which the picture is displayed. For intervals of the order of magnitude of the frame frequency the eye integrates the light it receives from a given area. 'Ihe light emitted from the display screen is, to at least a first approximation, proportional to the energy input to the screen. It will thus be seen that even if the display screen is operated at a voltage of only one-half that utilized between the cathode and screen of a tube using direct scanning, individual areas of the storage mesh need pass only about 174200,00() of the current in the scanning beam to produce equal brightness. The actual current density may be a great deal higher than this, giving a high degree of brilliancy. Some electrons of the scanning beam will, of course, penetrate the storage mesh and strike the screen but these are so small in number in comparison with the electrons from the iiood gun or cathode that 'their effect `on the display is negligible.

If the equilibrium method of writing and erasure are employed, television images can be produced without ilickers; because the mesh is' always negative to the flood cathode it collects no electrons from the latter and the image fades or deteriorates only as a result of the leakage through the insulating, secondary-emissive material. Since the effective resistance of the material is very high and the voltage differential between the charges collected upon it and its biasing potential are small, the rate of leakage can be very low and the stored image maintained for long periods. This is advantageous in many applications of the device where a record is to be produced for later viewing. It is also possible to use alternate writing and reading and erasure in the same manner as has been proposed for monochrome storage display tubes. Since the techniques for this type of operation are well-known it appears unnecessary to describe them in detail here. p

While specific voltages, electrode spacings, grid wire i dimensions and phosphor stripwidths have been given in this description, this has been done for illustrative purposes only. Diierent secondary-emissive materials have different cross-over points and since -in most methods Itif-operation the-potential diderence between the cathode of the electron gun and-.the conductive base.` of thestorage-niesh should besubstantiall'y equal to the cross-over `point voltage, .different voltages' maybe required for different secondary emissive materials. In this casethe' bias voltages applied to other elements within the `tube will have to be changed. The focal length. of an' electron lens depends upon the ratio of Ithe voltages applied .to its various elements and not upon their a-bsolute values. Thus, if the second cross-over voltage of-a particular material usedl for the insulating coating of Vthezstora-ge meshwere found to be, say, 3000 vol-ts instead-'of 5000; the only: change required to produce the results here, described `would be a scaling down of the vol-tages, Boland E1 to 3/ 5 ofthe valuesgiven; Le., to 3 vkvlvffor Bland 1.8 kv. instead of 3 as, the normal value of,E1`.cff."f.sV .fr

'Thefspacings between. the color grid, screen grid, and storage mesh may. be changedif 4sui-table changes are made` in the relative potentials appliedV to them. A steeper,potentialgradient between color grid and screen grid will shorten the effective focal length of the electron lenses and per-mit closer spacings. 'Lower velocities between the electron gun and the color grid will permit lowerV switching voltages. All of these relationships are now Well-known and electron lens systems can be devised in accordance with known principles to meet specific ments (a) to (g), mounted in succession between said V electron Igun and said viewing area and defined as follows:

Element-t (a): a color-control grid of substantially equal dimensions to said viewing area comprising two interleaved and mutually insulated sets of parallel linear conductors, the conductors of each set being interconnected and the spacing between adjacent conductors of .the respective sets .bein-g of the order of magnitude of an elementary area of the image to be displayed;

Element (b): a screen grid mounted parallel and adjacent to element (a) and comprising parallel conductors eX- tending in a direction .transverse to the conductors of element (a);

Element (c): a source of dood electrons a space charge element (a);

Element (d): a collector grid mounted in a plane parallel to elements (a) and (b) and of substantially like dimensions thereto;

Element (e): a charge-storage mesh mounted parallel to element (d) iand comprising a foraminated base .of conductive material having a coating of insulating secondary-emissive material adherent thereto on at least the side thereof vfacing element (d);

Element (f): a display screen mounted parallel .to element (e) and comprising a light transmissive base, a layer of phosphors thereon comprising parallel strips of phosphors emissive on electron impact of light of different colors, strips of one color being alined electron optically behind the conductors of one of the sets `of element (a) and strips emissive of a different color being so aligned behind the conductors of the other set thereof, and `an electron-permeable conductive layer overlying said phosphor layer; and i l for establishing behind `element (b) as viewed from Element (g): leads for applying different electrical poy element (a), to elements (b), (c), (d), (e), and the conductive layer of element (f).

2. The combination as defined in claim 1 including,- in addition, means for varying Ithe intensity of said electron beam.

3. In a storage-type cathode-ray tube for the display of images in color, t-he combination, within an evacuated envelope including a viewing area at one end thereof and an electron gun at the opposite end thereof adapted to develop a concentrated beam of cathode rays directed toward said viewing area, of elements designated as elements (a) to (g), mounted in succession between said electron gun and said viewing area and dened as follows:

Element (a): a color-control ygrid of substantially equal v dimensions tosad viewing area comprising two inter- Y leaved .and mutually insulated sets of parallel linear conductors, Ithe conductors of each set being interconnected and lthe spacing between adjacent conductors of Ithe respective sets being of the order of magnitude of anelementary area of the image to be displayed;

Element (b) a screen grid mounted parallel and adjacent to element (a) and comprising parallel conductors eX- tending in a direction transverse to the conductors of element (a) Element (c): a cathode comprising a grid of thermoemissive iilaments, said cathodehaving over-all dimen sions substantially similar to elements (a) and (b) and being mounted in a plane parallel thereto;

Element (d): a collector grid mounted in a plane parallel to elements (a) and (b) and of substantially like dimensions thereto; Element (e): a charge-storage mesh mounted parallel to element (d) and comprising a foraminated base of v conductive material having a coating of insulating secondary-emissive material adherent thereto on at least the side thereof facing element (d); Element (f): a display screen mounted parallel to element (e) and comprising a light transmissive base, a layer of phosphors thereon comprising parallel strips of phosphors emissive on electron impact of light of diierent colors, strips of one ycolor being alinednelec- VItron optically behind the conductors of one of the sets of element (a) and strips emissive of a diiferentcolor v being so alined behind the conductors of the other set thereof, and an electron-permeable conductive layer overlying said phosphor layer; and Element (g): leads for applying diiferent electrical potentals respectively to the ytwo sets of conductors of felement (a), to elements (b), (c), (d), (e), and the conductive layer of element (f).

References Cited in the le of this patent UNITED STATES PATENTS 2,721,293 Gow Oct. 18, 1955 2,748,312 Beintema May 29, 1956 2,795,727 Hae June l1, 1957 2,867,686 Hafner Jan. 6, 1959

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