US2739260A - Cathode-ray tube for color television - Google Patents

Description

March 20, 1956 o, LAWRENCE 2,739,260

CATHODE-RAY TUBE FOR COLOR TELEVISION Filed March 20, 1950 3 Sheets-Sheet l SCANNING OSCILLATORS I c Q 5 I E a O. 3 Y o z 3 N g 5 [LI .J LIJ INVENTOR. ER/VfST 0. LAWRENCE A TTORNEYS March 20, 1956 E. o. LAWRENCE 2,739,260

CATHODE-RAY TUBE FOR COLOR TELEVISION Filed March 20, 1950 5 Sheets-Sheet 2 &

O GREEN PHOSPHOR A RED PHOSPHOR El BLUE PHOSPHOR IN VEN TOR. ERNEST 0. LAW/PE NOE 25 A TTORNE'YS March 20, 1956 E. o. LAWRENCE 2,739,260

CATHODE-RAY TUBE FOR COLOR TELEVISION Filed March 20, 1950 3 Sheets-Sheet 3 INV EN TOR. ERNEST O. LAWRENCE 4'0. swstl A T TORNE Y5 United States Patent 6 CATHODE-RAY TUBE FOR COLOR TELEVISION Ernest 0. Lawrence, Berkeley, Calif., assignor, by mesne assignments, to Chromatic Television Laboratories, Inc., New York, N. Y., a corporation of California Application March 20, 1950, Serial No. 150,732

4 (Ilaims. (Cl. 313-73) This invention relates to cathode-ray tubes for displaying television images in polychrome, and particularly to tubes for displaying such images directly upon the luminescent screen or target of the tube by either a two-color or three-color additive system without the necessity for interposing between the target and the observer any optical system for superimposing the images or portions of images which are representative of the primary colors employed in the system.

Among the objects of the invention are to provide a cathode-ray tube wherein the color displayed upon any elementary area of the displaying surface can be controlled by purely electrical means; to provide a tube of the character described wherein the potentials employed for the control of the color of the display are but a few percent of the total potential used for accelerating the electron beam; to provide a polychrome cathode-ray display tube wherein the color instantaneously visible upon the display surface is independent of the path followed by the beam in scanning the image, so that no means need to be provided for insuring absolute linearity of scan as in the case where (as has been proposed in the past) phosphors emissive of ditferent colors are laid down upon a single display area in strips or spots of sub-elemental size; to provide a tube of the character described which is applicable to either simultaneous or sequential methods of transmitting the signals representative of the different primary colors and, among the sequential systems, is equally applicable to field sequential, line sequential, dot sequential or dot-sequential multiplex systems; and to provide a tube the major portion of which is of standard and well known construction and wherein the luminescent target and the color control appurtenances thereof are capable of construction within the limits of accuracy necessary to achieve the required results by relatively simple means so that the overall cost of the completed device is within economical limits. Monochrome or black-and-white television transmissions in the United States are, at the present time, standardized on a basis which is referred to as a 525-line picture with 2:1 interlace transmitted at thirty frames per second. To accomplish a transmission of this character the cathode-ray beam or other scanning element which traces out the television images is deflected in the vertical dimension of the picture field at 60 cycles per second and in the horizontal direction at 262% times that rate or 15,750 cycles per second, thus producing two fields or rasters of scanning lines each containing nominally 262 /2 lines. The lines of the second raster fall between those of the first so as to produce a total of 525 horizontal deflections for each two vertical deflections. Actually the scanning beam is blanked out for approximately 8% of the total vertical scanning time to take care of the fly-back or return of the beam from bottom to top of the image field, and as a result the actual picture shows approximately 480 lines in the visible field instead of the nominal 525 lines.

Other countries have adopted other standards both for vertical and horizontal rates of deflection. The tube 2,7392% Patented Mar. 20, 1956 of my invention can be constructed and operate under any of the standards that have so far been proposed in any country, but purely for illustrative purposes and in order to provide a norm against which the results obtainable in the color tube of this invention can be compared as to resolution or for other purposes, the United States black-and-white standard will be used and the modifications required satisfactorily to transmit color will be referred to in terms of the necessary modification of that black-and-white standard.

Theoretically if polychrome television pictures are to be transmitted, with the same amount of detail and resolution in all colors as is presently utilized in black-and-white, by any three-color additive process three times as much information must be transmitted within a given interval as is required to produce the monochrome picture. Twocolor processes have been proposed but the color fidelity obtainable therewith is inferior to the processes using three additive primary colors. Still greater fidelity could be obtained by the transmissions of additional primary colors, but the gain in fidelity is small in comparison to the added complexity and the channel widths required for their transmission.

Owing to the large demands for channel space even a channel of three times the width required for black and white is presently considered economically unfeasible and therefore various expedients and compromises have been adopted to permit the transmission of pictures on a narrower channel than that indicated by theory as set forth above. For the purposes of the present application it is unnecessary and, perhaps, undesirable to go into the expedients that have been adopted for the purpose mentioned and it is sufficient to state that the systems which have been seriously considered have been classified gen erally in accordance with the manner in which the channels have been divided between the signals representing the various primary colors.

Of these systems there is a general division, first, between simultaneous and sequential systems, and, second, a subdivision of sequential systems as between field-sequential, line-sequential, and dot-sequential systems.

In the various simultaneous systems that have been suggested three separate video signals, representative respectively of the red, green and blue primaries are transmitted. This system is the most profiigate of channel widths and although some saving can be eifected by transmitting less detail in the red and the blue signals than is transmitted in the green, as of the date of this application simultaneous systems appear to be in abeyance. It is to be noted, however, that the tube of this invention can be used with simultaneous systems as will hereinafter be described in brief.

The sequential systems diifer primarily in the rate at which the color represented by the signal is changed, all such systems attempting to transmit signals representative of the intensity of the illumination of each portion of the picture within the spectral band comprising each of the three component primaries in such rapid succession that the colors are blended in the eye to give the effect of a picture in substantially its true colors. As is suggested by the names by which these systems have become known, in a field-sequential system the entire picture is traced to generate a wave representative of a single primary color, and then retraced rapidly in each of the other two primary colors in succession. In general only one of the two fields corresponding to the interlaced lines of a frame is transmitted before the color is changed, so that, for example, as the odd lines of a complete frame (i. e., the first field) are traced in red, the even lines of the first frame or the second field are .traced in blue, the odd lines, again, in green, and the even lines in red, so that, the color cycle progresses,

all lines of the picture are scanned in each of the three colors.

In line-sequential systems successive lines in each field are traced in different colors; the fields are interlaced as before and again there is a shift between successive frames so that eventually each line is scanned in different colors. In dot-sequential systems the change from primary color to primary color is even more'rapid, the colors changing at a rate which is of the general order of magnitude of that required to transmit a single picture element. Again there is a shift or progression in the position of the dot representing any individual color, so that elements of the picture surface which are depicted in the initial scanning in one primary color appear in successive scanning's in different color.

In field-sequential systems the method-employedin the past has been to use a rotating color filter in front of the display tube, the latter being provided with a phosphor or mixture of phosphors-emissive of all of the primaries. The filter disk or drum removes from the emitted light the components of the colors not momentarily being transmitted. A rotating mechanical filter offers a complication which is undesired and, as will be shown, which may be avoided by the use of the tube of this invention.

In both line and dot sequential systems the method employed in the past has been to form the images representing the three different primaries either on different tubes or upon different areas of the screen of a single tube, and to combine the three images optically so that they appear to be superposed, either by means ofmirrors, semi-silvered or dichroic, or by optical projection methods. Each of these systems requires that exact registration of the images be obtained, both with respect to the electronic scanning of the luminescent target upon which they are produced and optically by the instrumentalities used for superposing them as they appear to the eye. Again, as will be shown hereinafter, the tube of this invention avoids all of the registration problems mentioned; the target or luminescent area is scanned exactly as in the case of a black-and-white image and the color produced by such scanning is altered by an electrical potential which may be varied at any rate desired, either to produce field, line, or dotsequence.

Broadly considered, the tube of my invention comprises the usual evacuated envelope of glass or glass and'metal which contains a cathode ray gun of any suitable type for directing a beam of cathode rays against a target area formed at the end of the tube opposite the gun. The usual deflecting devices, either electrostatic or electromagnetic, for sweeping the beam across the target area in two directions are either built into the tube or are externally supplied.

A transparent target area may either be formed upon a window in the end of the envelope itself or it may be.

formed upon a sheet of glass or other suitable material closely adjacent to the window. In either case the target area is supplied with a coating of phosphor which, under bombardment by cathode rays, fluoresces (or phosphoresces with a short period) in one of the primary colors chosen for the transmission of the polychrome pictures. Closely adjacent to this coating butinsulated therefrom, is positioned a grating formed of a plurality of mutually insulated conductive strips mounted substantially edge-on with respect to the target surface. Each of these strips is coated, preferably on both sides, with a phosphor emissive of another of the primary colors used in the picture transmission; it a three-colorsystemis employed alternate strips carry phosphors emissive of the two other primaries. Strips carrying phosphors of the same color are electrically connected, and external connections are provided so that deflecting potentials can be applied between the strips carrying different color phosphors. The strips forming the grating are in concentric circles, ellipses or the like.

preferably separated by the distances no greater than.

and preferably less than the width of the lines which form the television image rasters, and the width of the individual strips is of the order of ten times their separation, although this value is not critical and narrower strips can be used at the expense of the use of higher potentials to control the colors to be displayed. The alinement of the strips is not important, but they should, in general, be approximately parallel if this term he considered as broad enough to include their disposition The surfaces in which the strips lie should, in general, be parallel to or substantially coincident with a surface includingv the path of the cathode ray as it enters the grating; i'. e., the strip should lie edgewise to the orifice of the cathoderay gun or else means should be provided to deflect the cathode ray beam just prior to its entering into the grating to such a degree that in the absence of any deflecting potential on the grating it will strike only the proximal edge of the strips and will have no appreciable component of velocity normal to the flat surface of the strips.

With a device of this character, if it be assumed that the primary chosen for emission by the coating of the target surface itself be green, and that alternate strips of the grating are coated with phosphors which luminesce in red and blue respectively, if the beam be deflected across the surface of grating and target in the ordinary manner for scanning a television field, the surface of the target area itself will receive the entire electron flow from the beam and will luminesce in green as long as no differential potential is applied between the strips forming the grating.

If, however, a differential potential is applied between the adjacent strips of the grating the beam will be deflected after it enters the space between the strips toward the one which carries the relatively positive potential and away from that carrying the negative potential. If the width of the grating strips be ten times their separation a potential difference of approximately 4% of the total voltage used to decelerate the beam will cause all of the electrons of the beam to fall upon the strip which is positive. The strip thus receiving the beam will fluoresce in the primary color proper to the phosphor with which it is coated, and because the light thus emitted is confined between surfaces which are largely reflective a major portion of the emitted light will be transferred to the target area and will be transmitted through the translucent screen. The adjacent strips confine the light thus emitted and reflected to a narrow strip which is approximately equal in width to the separation between the strips.

It will be seen that since each red strip is between two blue ones it makes no difference whether the beam happens to fall wholly on one or the other side of the red strip if the latter is positive the beam will always be defiected in passing through the grating so that it hits the strip "which will fluoresce in red. Similarly, each blue strip is adjacent two red ones, and if the blue strips be positive with respect to the red only blue luminescence willbe produced. By making the separation of the strips less than the theoretical size of the elementary areas of the television picture to be produced, and preferably of the order of one-half the size, a beam may fall upon the target and its grating at random and the right color will always be produced and in the right spot. Furthermore, it makes no difference whether the strips forming the grating run parallel to the scanning lines used to form the television picture, normal to those lines, or at some random angle, the result will always be the same and the right color "produced as long as the deflecting voltage as between the alternate strips is in the proper direction. it is for this reason that parallelism of the strips is not important, and that the grating can be formed of interlaced spiral coils or otherwise and still obtain the desired result. There is, of course, some scattering of the light produced by the phosphors on the grating. This is of minor importance, however; if the green. phosphor be used to coat the target area itself as it is well known the eye is most sensitive in the green and most receptive to detail carried by the green image. The blue and red may therefore spread to a limited degree without visible degradation of the image.

The invention may be better understood by the following detailed description considered in conjunction with the accompanying drawings, wherein:

Fig. 1 is a schematic diagram of a tube in accordance with this invention, together with elementary circuits illustrative of what is required to cause the display thereon to appear in any one of three primary colors;

Fig. 2 is a diagram showing the display end of a tube similar to that shown in Fig. 1 more in detail, but still in schematic form;

Fig. 3 is a view showing the disposition and, to some extent, the construction of the grating of the tube of Fig. 2, relative planes of section in views 2 and 3 being indicated by the appropriately designated lines in the two views;

Fig. 4 is an enlarged cross-sectional view indicating one method of constructing a grating of the type described;

Fig. 5 is another enlarged detailed view, in perspective, illustrating another possible method of construction;

Fig. 6 shows stilt another modification of construction of the grating;

Fig. 7 shows a fourth method of constructing the grating;

Fig. 8 is illustrative of a grating formed of concentric spiral strips rather than the plane parallel strips indicated in other figures;

Fig. 9 shows a grating formed of wedge-shaped strips as distinguished from the plane strips indicated in the preceding figures;

Fig. 10 shows a form of grating wherein the phosphors are deposited on beads carried on the edges of the grating strips rather than strips themselves; and

Fig. 11 is a diagram indicating the deflection of the cathode-ray beam under different deflecting voltages.

Considering first the tube in general, as shown schematically in Fig. l and in slightly greater detail in Figs. 2 and 3, it comprises the usual evacuated envelope 1, which may be of either glass or metal. The tube is, as is customary, of generally conical or even rectangular form, having at the base of the core a transparent window 3.

Mounted in the smaller end of the cone is an electron gun of conventional form. This is shown as comprising a filament or heater 5, which raises a thermo-emissive cathode 7 to its emitting temperature. Electrons emitted by the cathode are amplitude modulated by a grid 9 and are successively accelerated by first and second anodes 11 and 13. The electron beam thus formed may either be focused by electron lenses established by the fields between the various elements or external focusing coils (not shown) may be employed. The proper voltages for exciting the various electrodes mentioned are supplied by the television receiver indicated schematically by the block 15.

The electron beam produced by the gun is deflected in two dimensions, in this instance, by the coils 17 and 19 which carry, respectively, sawtooth currents of the two frequencies utilized to produce the vertical and horizontal scannings as produced by scanning oscillators of proper form indicated by the block 21. Thus, assuming that the same scanning frequencies are utilized as in the current black-and white standards, the coils 17 will carry sawtooth waves having a fundamental frequency of 60 cycles per second, while the coils 19 will carry similarly shaped waves at a frequency of 15,750 cycles per second. The currents in these coils are adjusted to cause the oathode-ray beam to trace upon the Window of the tube rectangular rasters of the proper shape and interlace.

Formed upon the Window of the tube is a translucent layer of a phosphor which, when excited by the electron beam, will emit light corresponding to one of the primary colors chosen for use in the system wherein the tube is to be employed. Actually, of course, the particles of the phosphors which are deposited upon the window to form the screen are of microscopic size. They are illustrated in the drawing, however, by discrete circles, and as, for reasons which will be gone into more fully hereinafter, I prefer to deposit the green phosphor directly upon the target area of the tube, circles of this type will be used throughout the drawings to indicate the green phosphor. Other phosphors used in the construction of the device will normally be emissive of red and blue light respectively, and like the green phosphor will be formed of particles of microscopic size. They will be illustrated, however, by small triangles and rectangles respectively, such representation, of course, being purely symbolic.

The luminescent coating 23, deposited directly upon the Window 3 is rendered conducting by any of a number of well known methods, perhaps the best of which is the deposition upon it, on the side facing the cathode-ray gun, of an extremely thin metallic layer such as may be formed, for example, by the evaporation of aluminum. A lead 25 connects this coating with the receiver which is arranged to apply suitable potential thereto to give the cathode-ray beam its final acceleration.

Closely adjacent luminescent target area 23 there is positioned a grid or grating formed of alternate mutually insulated strips or" conducting material coated on both sides with phosphors emissive of one of the other two primaries employed in the system. Strips 27 carrying the red phosphor are connected together, as are the strips 29 which carry the blue phosphor. A lead 31 connects all of the red phosphors to one pole of a switching mechanism 32 which is here shown, for illustrative purposes only, as being of the mechanical type, while a second lead 33 connects all of the blue bearing strips to another pole of the same or an interlocked switch 32'. Means, here illustrated as batteries 35, 35, connect from the lead 25 to the contacts of the switches. The batteries 35, 35' are so poled that when the switches are thrown in one direction all of the strips 27 will be positive with respect to the target 23 while all of the strips 29 will be negative to the target, whereas, when the switches are thrown in the opposite direction, the reverse will be the case. When the switches are in their intermediate position all of the strips and the target area will be at the same potential.

It is to be emphasized that this mechanical switching arrangement is shown merely for illustrative purposes and to indicate the nature and direction of the potentials employed. In any practical case electronic switching will undoubtedly be used, but such electronic switching arrangements are no part of this invention.

The diagram of Fig. l, as well as those which will be described later, are not to be considered as representative to scale of the relative sizes or separations of the strips of the color grating, but are merely a showing of the relative positions of the grating and of the strips comprising it. Means are provided for causing the electron beam to enter the grating at any point in a direction parallel to the wider dimension of the strips, so that when the latter are at the same potential as the target area itself they will have no tendency to deflect the beam, which, accordingly, will strike the strips at barely grazing incidence if at all. This can be accomplished in two different manners; either auxiliary focusing means may be used adjacent the display end of the cathode ray tube for bending the beam into a path parallel with the strips, or the strips can be arranged in planes slightly tilted with respect to each other and which intersect in a common line substantially at the orifice or effective exit aperture of the electron gun, as is indicated in Fig. 1. The two procedures are considered to be equivalent; means for setting up corrective fields of the character first mentioned are well known and therefore arenot here'shown. Arrange ments for tilting the strips as mentioned are illustrated in Figs. and 7.

The relative widths and spacings of the strips forming the grating are subject to rather wide variations. It can be shown that if the width of the strips is ten times their separation, a potential difference of 4% of the voltage employed to give the electrons their velocity upon entering the grating, applied between the two sets of strips, will be sufficient to cause all of the electrons of the beam which enter the space between two adjacent strips to impinge on whichever of the two is the more positive; i. e., if the voltage accelerating the beam is 10,000 volts a 400-volt differential between the two sets of strips. will cause all of the electrons to strike the strips carrying either the blue or the red phosphors as the case may be.

Substantially all of the phosphors now used commercially are white or nearly so. An electron beam entering the space between the adjacent strips and striking entirely upon the red or blue phosphors will produce light of the corresponding color and a large portion of this light will fall, either directly or after reflection from the opposing surface, upon a portion of the translucent target area defined between the adjacent strips. There will, it is true, he a considerable loss of light in this process, but by suitable choice and distribution of phosphors and accelerating potentials the light which finally passes through the window 3 can be made to have the proper radiation intensity and relative luminosity so that when the beam is modulated in succession by signals representative of all three primaries the result is effectively a white light emitted from the screen and the proper color balance may thus be secured. Owing to the fact that the strips of the grating are substantially perpendicular to the main window through which the light emitted from them is viewed there may be some tendency for the color balance to change as the angle of observation changes. This is largely neutralized by the scattering of the light by the coating of phosphor on the window, but if this is not sufiicient frosting of the window surface prior to the deposition of the phosphor will accomplish the desired result.

The ratio of separation of the strips to their widths is not critical, but it is directly connected with the voltage required to cause complete switching as between the primary colors emitted, this voltage being proportional to the square of the ratio between separation and width; thus while a separation-width ratio of requires about 4% of the beam accelerating voltage to cause complete deflection, a ratio of 5% will cause complete color transfer upon application between the strips of 1% of the accelerating voltage. The choice of separation-width ratio is therefore a compromise between electrical and mechanical features and is subject to variation at the option of the designer of the equipment.

The absolute dimensions of the strips constituting the grating are a function of the size of the screen and of the resolution which is to be required of it. Preferably the separation between the adjacent strips should be materially smaller than the dimension of the picture elements which it is desired to resolve. A separation equal to onehalf of diameter of a single picture element is satisfactory, but a separation of one-third of the diameter of the picture element to be resolved gives somewhat improved resolution. Further reduction of the separation gives no improvement in resolution if the beam diameter is of the proper size.

What this means in terms of atubedesigned to produce color pictures conforming to the present 525" line blackand-white standard can readily be shown; a cathode ray tube having a window 16 inches in diameter will show a picture roughly 9 /2 by 13 inches in dimension, although actually the picture shown is usually a little larger since it is customary to cut ed the extreme corners of'a picture. As has been pointed out above the lines actuallyshown in a so-called 525-line picture number approximately-480,

so that each lineon the maximum size picture which may be displayed on a 1.6-inch diameter screen is 8/ of an inch wide. A separation between adjacent strips of ,5 of aninch will therefore give a one-half element separation, which has been indicated above as the maximumdesirable. The strips themselves have, of course, finite thickness, and experiment has shown that copper strip 3 mils in thickness is appropriate for forming the grating. Subtracting the thickness of the strips from the 10 mil theoretical separation gives an actual separation of 7 mils, and strips of an inch wide. With such stripsa satisfactory grating may be formed by any of the methods next to be described.

One out of several methods of constructing the grating is illustrated in Figs. 2, 3 and 4, still in semi-diagrammatic form, the dimensions again being exaggerated in order to show more clearly the actual construction. Where this structure is used the strips 27 and 29 which form the grating are laced or woven together by means of fiber glass cords 37 which are laced around the strips at intervals along their length. In the showing of Fig. 3 only two of these cords are shown but as the gratings are actually constructed they are placed at intervals along the strips which are of the same order of magnitude as the spacing between the strips. The ends of the strips are set in slots formed at insulated end supports 39. The strips are slightly staggered lengthwise so that strips 27 project beyond strips 29 at one end where they are electrically connected to the lead 31, while at the other end the strips 29 project and are connected to the lead 33. The same convention as before is used to indicate the different colored phosphors.

Figs. 5, (Sand 7 show alternative methods of fastening the strips together'intoa continuous grating. In the method shown in Fig. 5 notches 41 are formed at in tervals along the strips. Similarly notched insulating blocksor separators 43 are positioned between the strips with notches alined with the notches in the strips themselves, and binding cords of fiber-glass 45, positioned by the notches, fasten the whole structure together. The separators are slightly wedge shaped to give the proper tilt to permit parallel entry of the beam, but the angle between any two successive strips is too small to show in the figures.

In the modification shown in Fig. 6 transverse corrugations 47 are formed in the strips, preferably before the deposition of the phosphors upon them. Insulating beads 49 are fused to these corrugations and serve to determine the relative positions of the strips. As before, the ends of the strips aresecured to support rods 39.

In the method of construction shown in Fig. 7 the strips areperforated at intervals and glass threads 51 are passed through the perforations. Glass beads 53 are strung on these threads between the strips to act as separators. The tilt to permit parallel entry of the beam is secured by using beads of slightly different diameter at front and rear of the strips.

In the construction shown in Fig. 8 the strips themselves are similar in form to those of Fig. 6, but instead of being linear they are wound into a spiral form. This construction has a considerable advantage from the point of view of rigidity and ease of manufacture but because of a considerable inductance and distributed capacity in .the structure as a whole the rate at which the color can be switchedis .limited. .The structure of Fig. 8 is practicalfor either field sequential or line sequential. methods of transmission but mayor may not be suitable for dot sequential systems, depending upon the rate at which the colors are changed.

Fig. 9 shows a modificationof the device wherein the strips are wedge-shaped instead of flat as in the other modifications which have been described. The wedgeshaped strips 27' and 29 are mounted with their apices facing the translucent target 23 and their bases toward the electron gun. The grating can. be formed by sub stantially any of the methods that have been described for fiat strips. This structure has the disadvantages that it cuts off a greater portion of the electron beam, the bases of the strip acting as a diaphragm which intercepts a considerable number of the electrons, and, further, that a higher voltage is required to deflect that portion of the beam which does pass through the grating and cause it to strike entirely against the proper phosphor. The construction has the countervailing advantage that the transfer of light from the phosphors on the strips is more effective than in the other cases mentioned.

Still a further modification is shown in Fig. 10. Here again the strips 27 and 29 are slightly wedge-shaped in form, but in this case transparent glass beads 56 are fused to the apices of the wedges and the phosphors are deposited upon these beads. With this construction the beam does not have to be deflected so that it actually hits against the sides of the strips, but only far enough so that it hits the phosphors. As in the case of the modification last described the optical efiiciency is somewhat higher than in the case of the flat strips. Control voltages are also required to be somewhat higher than before, but not quite as high as in the case of the modification of Fig. 9.

Whatever construction is used for the grating the electrical effect is much the same. What happens is, perhaps, best shown in Fig. 11. In this figure the dotted line 55 indicates the path of the beam when the strips of grating are at the same potential as the target 23 itself, the beam passing undeflected through the grating and exciting the luminescent target only. When a control voltage is applied to the gratings through the leads 31 and 33 electric fields are set up between each adjacent pair of strips. Fig. 11 shows the strips 27 as positive and strips 29 as negative. The fields between the strips are oppositely directed as between successive pairs of strips; i. e., a field between the upper strip 291 and strip 271 is directed (conventionally) downward, while the field between strips 271 and 292 is directed upward. As a result whichever side of strip 271 the beam may fall, it is deflected toward that strip as shown by the dotted lines 55' and if the control voltage be properly chosen with respect to the strip separation and beam velocity it will strike on that strip only and excite only the correspond ing phosphor. If the strip sizes and separation are of the magnitude here recommended the beam will strike at least two strips of the same color phosphor, and the light falling on and transmitted by the translucent target will be all of one color. Reversal of the control voltage will change the color from red to blue or vice versa as the case may be.

Since the spacing between the strips is of sub-pictureelement dimensions the strips do not injure the definition procurable from the tube. that the colors when reproduced are in superimposed position with respect to the observer, rather than adjacent one another as with many of the suggested tube forms.

One of the major advantages of the tube of this invention is that no necessity arises for having the beam follow the strips. Wherever it may fall it will produce a spot of light of the proper color and of approximately the dimension of the cross-section of the beam. The strips may therefore be mounted either parallel to, transverse to or diagonal to the scanning line without any material effect upon the picture. It is this fact also which makes the spiral construction of the screen posible. In order that the deflecting potentials may always be equally effective and to prevent distortion of the spot shape and size it is desirable that the distances between successive strips be substantially a constant, i. e., that the strips be parallel, either in the rigorous sense of that term or substantially concentric.

Throughout this specification it has been assumed that the green phosphor would be the one to be deposited directly upon the transparent target and that red and It also must be emphasized,

blue phosphors would be deposited on the grating strips. This arrangement is chosen purely from practical consideration; theoretically phosphors emissive of any three colors, none of which can be formed by additive combinations of the other two, may be used and it is immaterial which of these phosphors be deposited upon the target itself and which upon the grating strips. Actually it may be shown that the best color fidelity can be secured by the use of red, green and blue phosphors, and because of the much greater sensitivity of the eye to green light than to light of the other two primary colors detail is much more visible in green than it is when depicted in the other primary colored lights.

Light emitted from the grating strips is somewhat scattered, although not seriously so. It has been shown by other experimenters that if the detail be sharply depicted in green a considerable lack of resolution in the red and the blue may be easily tolerated without perceptible degradation of the image as viewed by the eye. This is the reason for the choice of the phosphor deposition here recommended but satisfactory results can be obtained by other depositions of other primaries as has been indicated above.

It was stated in the first part of this specification that the tube of this invention is applicable not only to field, line or dot sequential systems but also to simultaneous systems of television transmission. It is believed that the method of application to any sequential system is self-evident. Its use for simultaneous systems involves the reception of the three simultaneously transmitted color signals and their switching as between the target and the strip areas at a high rate which is independent of any transmitted signal but which is determined within the receiver itself. This method of simultaneous reception is covered by a copending patent application filed simultaneously with this; such simultaneous method of reception is not considered to be a part of this invention but is mentioned here merely to emphasize the versatility of the tube of this invention.

A few of the points which have merely been alluded to in the foregoing description may perhaps need some further explanation. It has been stated that the distance between the separators between the grating strips should be of the same order of magniude as the separation between the strips themselves. This serves to confine scattered light and increase definition, and is desirable irrespective of the type of separator used.

Moreover,although it has been mentioned that the deflecting voltages applied between the grid strips is not critical, it should be noted that any voltage above the minimum defined by the strip separation will secure the desired result, excess voltage merely shifting slightly the point on the strips from which maximum light emission occurs. By appropriately grading the density of the phosphors the loss of light caused by the greater distance of this point from the screen may be compensated.

Finally, it should be pointed out that although the drawings show an actual electrical connection to the target 23, this connection can be omitted as it frequently is in tubes of ordinary construction as secondary emission from the screen will result in its finding a proper mean potential. Its omission may, in fact, ofler a definite advantage in that even the thinnest conductive coating will result in a loss of light through the screen.

I claim:

1. A cathode-ray tube for the production of television images in polychrome comprising an envelop, an electron gun within said envelop for generating a beam of cathode rays, a target area within said envelope formed of transparent material and positioned to receive said beam of cathode rays, a translucent coating of a phosphor emissive of light of one primary color on said target area, a grating comprising a plurality of mutually insulated conductive strips mounted edge-on adjacent to said translucent coating, said strips being wedge-shaped in cross section Withthebases, of .the wedges directed toward the cathode-ray gun and the apices of the wedges directed toward the target area, a coating of aphosphor .ernissive of .light of a difierentprimary color from that of said first mentioned coating on each of saidtstrips, and electrical connections to all of said strips for applying deflecting potentials therebetween.

2. A cathode-ray tubevfor the production of television images in polychrome comprising an envelop, anelectron gun Within said envelop for generating a beam of cathode rays, a target area within said envelop formed of transparent material and positioned to receive said beam of cathode rays, a translucent coating of a phosphor emissive of light of one primary color ontsaid target area, a grating comprising a plurality of mutually insulated conductive strips mounted edge-on closely adjacent to said translucent coating, glass beadssecured to the edges of said strips comprising the grating which are apposed to the target area.

3. The cathode-ray tube claimed in claim 2 wherein the glass heads at theregion of beam entrance andbeam 1 2 exittof the gratingstripare of different diameter thereby permit para1lel:entry of the. beam.

4. The cathode-(ray tube claimed in claim 2' wherein phosphor coatings aredeposited uponthe beads.

References Citedin the tile of this patent UNITED STATES PATENTS 2,307,188 2,446,249 Schroeder Aug. 3, 1948 2,446,440 Swedlund Aug. 3, 1948 2,461,515 Bronwell Feb. 15, 1949 2,498,705 Parker Feb. 28, 1950 2,518,200 Sziklai et'al Aug. 8, 1950 2,529,485 Chew Nov. 14, 1950 2,571,991 Snyder, Jr Oct. 16, 1951 2,579,705 Schroeder Dec. 25, 1951 2,635,203 Pakswer Apr. 14, 1953 FOREIGN PATENTS 443,896 Great Britain Mar. 10, 1936 Bedford Jan. 5, 1943

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