US2926283A - Colour-television cathode-ray tube - Google Patents

Description

1960 H. BRUINING ETAL 2,926,283

COLOUR-TELEVISION CATHODE-RAY TUBE Filed July 22. 1957 a: a/ as)! #9- INVENTOR HAJO BRUINING EDWARD FOKKO DE HAAN BY am AGENT United States Patent COLOUR-TELEVISION CATHODE-RAY TUBE Hajo Bruining and Edward Fokko De Haan, Eindhoven,

Netherlands, assignors to North American Philips Company Inc., New York, N.Y., a corporation of Delaware Application July 22, 1957, Serial No. 673,363

Claims priority, application Netherlands August 13, 1956 12 Claims. (Cl. 315-21) The invention relates to a cathode-ray tube for the reproduction of colour images, more particularly colour television pictures, and to apparatus comprising such a tube.

For the reproduction of colour-television pictures quite different constructions of cathode-ray tubes have been suggested. Practically all tubes comprise a luminescent screen with three luminescent substances, which emit, upon electron impact, light in different colours. The addition of these colours provides the possibility of reproducing satisfactorily a very large part of the colours occurring in nature. To this end it is required to arrange the substances luminescing in different colours on the picture screen in separate surfaces arranged in a regularly repeating pattern. One of the simplest patterns consists of parallel strips of the different luminescent substances, applied for example to the glass faceplate of the cathode-ray tube. In this case, two adjacent strips emit light of different colours upon electron impact. In practice use is commonly made, as stated above, of three luminescent substances emitting red, green and blue light and for the sake of simplicity the invention will be described with reference to such a case, although it is not restricted thereto.

For the composition of the picture use is made, as is known, of a line deflection and a frame deflection, the directions of which are, as a rule, at right angles to one another (the line deflection direction will be assumed to be horizontal). Although, in principle, it is possible to choose any direction for the luminescent strips, it is common practice to have this direction coincide either with the direction of the line deflection or with that of the frame deflection.

There are various possibilities of composing the colour picture, but they may be grouped mainly into two categories, i.e. that in which three electron beams are produced in the cathode-ray tube (if it comprises, as is assumed, three different luminescent substances) and that in which only one electron beam is produced, which is directioned in succession to the three different luminescent substances in a given time schedule. In the first case a given electron beam always co-operates with a luminescent substance of a given colour and in the sec ond case the electron beam co-operates in succession with the three luminescent substances.

In a tube having three electron beams, produced by three electron guns having each a control-grid, the control-signals for the three colours are supplied simultaneously to the control-grids. With tubes having one beam, produced by one gun, the control-signals for the three colours are supplied to the control-grid in a given order of succession. In the two kinds of tubes the three luminescent substances may be arranged in substantially parallel strips on the picture screen; the direction of these strips may be horizontal or vertical.

Both the tubes having three guns and the tubes having one gun give rise to difliculties with the correlation of the spot where the electron beams strike the screen Patented Feb. 23, 1960 and the colour information signals supplied to the guns. Of course, as long as a given colour information is supplied to the gun, the beam must strike a luminescent strip which emits the corresponding light colour. This correlation may be disturbed by an incorrect direction of the beam. In order to avoid this difficulty and to restore the correct correlation, tubes have been made in which reference signals can be derived from the picture screen. In this case, in principle, two possibilities are available:

(1) Correction signals are produced on the screen, i.e. signals are derived from the screen only in the event of a deviation from the correct correlation.

(2) At regular instants electric pulses are produced via the screen, the frequency of these pulses being compared with the colour-information signal switching frequencies. The differences between these frequencies are employed to compensate deviations, if any.

The said reference signals are produced in a given kind of known tubes by applying to given areas of the picture screen materials of which the secondary emission deviates from the secondary emission at further areas, particularly from the secondary emission of the luminescent substances. Since, as a matter of course, the relationship between the areas of the screen portions with a deviating secondary emission and the areas of the various luminesent substances is known, the reference signals can be derived therefrom.

Three possibilities of arranging the screen portions with a deviating secondary emission are described.

In the first place between the luminescent strips may be provided strips of a material with a secondary emission which exceeds that of the strips of luminescent material.

Secondly, certain strips of luminescent material may be mixed with substances of which the secondary emission exceeds that of the luminescent substances.

Thirdly, certain luminescent strips may be covered with a material having a higher secondary emission than the luminescent substances of the uncovered strips.

During scanning of the picture screen, when an electron beam strikes the material having a higher secondary emission, a higher current of secondary electrons is abruptly produced. These secondary electrons are collected at an electrode, for example a ring, which is in the proximity of the picture screen. This electrode is connected to a voltage source through a resistor. Across the resistor are produced voltage variations owing to the higher current, which starts to flow when one of the secondary-emission surfaces on the picture screen is struck and these voltage variations are employed as reference signals.

The great disadvantage inherent in the use of secondary emission in this kind of tube is that the signals obtained exhibit a small difference from the background signal level, since all sorts of screen portions, particularly the luminescent substances also, exhibit secondaryemission phenomena. The difference between this secondary emission and that of the special surfaces for producing reference signals by means of secondary emission on the picture screen is small. Consequently, a tube in which reference signals are produced by means of secondary emission is very sensitive to interference. A further disadvantage is constituted by the transit time of the secondary electrons from the area where they are produced to the collecting electrode. A further disadvantage resides in the very presence of this collecting electrode. A cathode-ray tube for colour images is already very complicated and an additional electrode renders the manufacture, of course, still more difficult. The coating of particular strips of luminescent material with a layer of secondary-emissive material, has, furthermore, the disadvantage that the excitation energy available after the secondary-emissive layer has been pierced is thus reduced. Moreover, during the lifetime of the tube the secondary emission is variable.

The invention provides means for obtaining reference signals from the picture screen, which means do not exhibit the aforesaid disadvantages of secondary emission and which provide, moreover, advantages which will be described more fully hereinafter.

A cathode-ray tube according to the invention for the reproduction of colour images, more particularly colour television images, comprises an electrode system to produce at least one concentrated electron beam, deflection means and a beam-receiving picture screen wlth substantially parallel strips of luminescent substances, luminescing in different colours and united in groups, and is characterized in that in each group or in a multiple thereof provision is made of the same number of strips of a material of which the specific conductivity var es upon electron impact, these strips being provided with two electrodes.

In a tube according to the invention use is made of the phenomenon, known per se as electron-bombardment-induced-conductivity, that the specific conductivity of particular substances varies upon electron impact. Such substances are for example cadmium sulfide, zinccadmium sulfide, lead monoxide, arsenic trisulphide and antimony trisulphide, but any material having the effect of electron-bombardment-induced-conductivity may be used. Since in a tube according to the invention, the strips of this material lie between two electrodes, current variations may be produced by electron bombardment on these strips in a current circuit which includes these strips and the intermediate material.

If a load resistor is included in this circuit, the voltage variations produced by the resistance variation in the strip may be obtained from this resistor; subsequent to amplification, if necessary, this voltage variation may be used to act upon the impact area of the electron beam, i.e. the light spot on the screen.

It is not necessary to arrange the substance of which this specific conductivity varies in the form of a separate strip at the side of the luminescent strips, since the strips of phosphors, which emit the same colour upon electron impact may be mixed with a material of which the specific conductivity varies or a thin layer of such a material may be provided below or on these phosphor strips. As an alternative, for one of the luminescent strips use may be made of a substance of which the specific conductivity varies upon electron impact.

In order to avoid a large number of current supply wires, the corresponding electrodes of those between which the strips of the material with variable specific conductivity are arranged may be electrically connected to one another. In this case only two current supply wires are required for these electrodes. A particularly simple embodiment is obtained, as a matter of course, if the whole of the picture screen is arranged between two parallel electrodes. This is possible, for example, if on the cathode side is applied a thin metal layer, for example of aluminum, which is pervious to electrons and on the other side of the picture screen an electrode which is pervious to light. The latter may, for example, consist of a surface layer, rendered conductive by means of tin oxide, applied to a glass support of the picture screen, which support may, for example, be the wall of the tube.

Many constructions of a picture screen for use in a tube according to the invention are possible; particularly the arrangement of the electrodes may be varied. A few of these constructions will be described hereinafter with reference to a drawing.

It should be noted, however, that the picture screen to be described may be used both in tubes having a single gun, in which, consequently, a single electron beam is produced, and in tubes having a plurality of guns. For the sake of simplicity, if hereinafter reference is made to the production of a plurality of electron beams, a system of three electron beams will be described. The idea underlying the invention may, however, be applied without the need for further means to systems having two or more than three electron beams. In the drawing:

Fig. 1 is an elevation and a partial sectional view of a cathode-ray tube according to the invention, in which the principal component parts are shown.

Figs. 2, 3 and 4 serve to explain the principles on which tubes and devices according to the invention operate.

Figs. 5 to 8 are detail views of the constructions of various forms of the picture screen.

Referring to Fig. 1, reference numeral 1 designates the envelope of the cathode-ray tube, which comprises an electron gun 3, which produces an electron beam which is directed to and received by a picture screen 5 applied to one side of the bulb. The neck of the cathode-ray tube is surrounded by a focusing coil 7 and a deflection-coil system 9, which produces a deflection in two directions, i.e. one direction at right angles to the plane of the drawing and one direction lying in the plane of the drawing. The picture screen 5 is composed in a particular manner, which will be described with reference to the further figures.

Fig. 2 is a diagramamtical view of part of the picture screen 5, in which the phosphor strips are designated by 7, 9 and 11; the correspondingly numbered strips emit the same light colour upon electron impact. The strips 7 and 11 have between them strips 13 of a material, of which the specific conductivity varies upon electron impact. The strips 13 lie between two electrodes included in a current circuit which has a load resistor. It is assumed that the line deflection direction is parallel to the strips 7, 9, 11 and 13 and that three electron beams are produced, of which the cross-sections are indicated by the circles 15, 17 and 19. During the composition of the picture these three beams move simultaneously in the direction of the line deflection. The relative distance between these beams is determined by the electron gun and the focusing coil and can be kept satisfactorily constant. The spots of the three electron beams in common with respect to the phosphor strips can be kept constant less readily. The three electron beams in common may perform a movement which is no longer accurately parallel to the strips. At a given instant either the beam 15 or the beam 19 will then strike one of the strips 13. Thus the specific conductivity of the strip concerned varies and a voltage variation is produced across the load resistor. This voltage variation may be used to deflect additionally the three beams in common in the direction of the frame deflection, so that they are again accurately directioned on the correct phosphor strips. It will be set out hereinafter how it can be determined whether the three beams are too high or too low.

Fig. 3 shows an embodiment having only one electron beam, the cross-section of which is designated by 21. It will be assumed that this electron beam moves sinusoidally across the three colour lines 23, 25 and 27 with a frequency which is high with respect to the frame-deflectron frequency. (Instead of performing a sinusoidal movement, the beam may, for example, perform a square-wave or sawtooth-like movement.) It is known that with this system the colour information supplied to the electron gun must correspond always with the colour of the phosphor strip which is struck. A disadvantage of such a device is that sometimes the zero line of the sinusoidal movement does not lie exactly in the centre of the central colour strip. Consequently, the time during which a particular colour is produced does no longer correspond with the time during which the corresponding colour signal is supplied to the grid of the electron gun. It may then occur that the electron gun receives green information signals, for example, while the electron beam strikes a red or blue luminescing strip. In accordance with the invention the centre of the strip 25 is provided with a strip 29, of which the specific conductivity varies upon electron impact. Consequently, at any instant when the beam strikes this strip 29, a signal is produced. It is evident that the ditference in time between two such signals with successive passings of the electron beam across the strip 29 depends upon the position of the zero line of the sinusoidal movement.

Since the sinusoidal movement per se is obtained, as usual, by means of a constant frequency generator, the pulse frequency of the signal obtained from the strip 29 may be compared with the frequency of the generator and any difference between these frequencies may be utilized to readjust the zero line of the sine curve. In principle, the strip 29 may be provided at any position in the three phosphor strips; the position of the strip is always exactly known. It is then known beforehand what frequency is to be expected from the strip 29 with a correct position of the zero line of the sine curve.

Fig. 4 is a detail view of one embodiment of a screen for use in a tube according to the invention; the direction of the line deflection of the electron beam 31 is at right angles to the direction of the phosphor strips 33, 35 and 37. At the side of the strips 33 and 37 provision is made of strips 39 and 41 of a material, of which the specific conductivity varies upon electron impact. When the beam 31 passes, during its movement, the strips 39 and 41, a pulse is produced. The pulse frequency of the signal thus obtained varies with the velocity with which the beam moves across the phosphor strips. From the arrangement of the strips 33, 35 and 37 and of the strips 39 and 41 it is evident that it can be precisely determined at which instant the colour signals corresponding with the strips 33, 35 and 37 must be supplied to the grid of the electron gun, since this instant is At, 2At and 3A! respectively after the reference signal obtained from the strips 39 and 41. With this method, consequently, the pulse frequency obtained is also compared with the colour switching frequency, deviations, if any, being then corrected. As an alternative, at the side of each phosphor strip provision could be made of a reference strip. As a rule, it will be suflicient, however, to provide one reference-signal strip for each group or a multiple thereof.

Fig. is a cross-sectional view of a picture screen as shown in Fig. 2. Corresponding parts are designated by the same reference numerals. 7, 9 and 11 designate the various phosphor strips, 13 designates the strips of the material of which the specific conductivity varies upon electron impact. 15, 17 and 19 designate the three electron beams. The direction of line deflection is at right angles to the plane of the drawing. The strips 13 lie between electrically conductive coatings 43 and 45. The corresponding coating strips 43, which are pervious to electrons, are connected to one another, as well as the corresponding strips 45. The first-mentioned strips are provided with a current supply wire 47 and the latter with a current supply wire 49, which wires are passed externally of the tube. Between the wires 47 and 49 are connected a voltage source 51 and a load resistor 53. When the strips 13 are not struck by an electron beam, their conductivity remains low and a very low current will pass through the resistor 53. However, when one of the beams 15 or 19 strikes the strips 13, their conductivity increases and the current across the resistor 53 will vary. The pulse thus produced may be obtained from the capacitor 55 and supplied to the deflection device for the three electron beams 15, 17 and 19 to restore the correct position of the beams. If no particular measures are taken, it will not be possible to determine in this arrangement whether the three beams have a deviation to the left or to the right, since with both these deviations the capacitor 55 yields a pulse of the same polarity and, in the event of equal deviations of the beams, of the same value. This disadvantage may be obviated by modulating the intensity of the beams 15 and 19 with two very high frequencies, for example of more than 30 mc./s. The beam 15 may, for example, be modulated by 30 mc./s. and the beam 19 by 33 mc./s. When the capacitor 55 supplies a signal having a modulation of 33 mc./s., it is certain that the beams in common have a deviation to the right, so that a correction to the left is required. When the capacitor 55 produces a signal of a frequency of 30 mc./s., a correction to the right is required.

Since all electrodes 43 and 45 are interconnected, the strips 43 and 45 may be replaced by a conductive layer extending also over the phosphor strips. The strips 43 then constitute together a so-called metal backing, which is pervious to electrons. The strips 45 arranged to form a unit must then, of course, be pervious to light. To this end use may, for example, be made of conductive tin oxide, which is applied to the glass support of the phosphor screen. As an alternative, only the electrodes 43 or 45 may be united to form an uninterrupted layer.

An embodiment which substantially corresponds with the embodiment shown in Fig. 5, is shown in Fig. 6. The only diflerence is that the strips 57 of material of which the specific conductivity is variable are arranged between conductive strips 59 and 61, which extend in the direction of length of the electron beam.

One embodiment of a screen for use in a tube according to the invention, having three electron beams, the direction of the deviation of the three beams in common being determinable without the use of an additional intensity modulation of high frequency of at least one beam, is shown in Fig. 7. In this figure the reference signal strips are designated by 63. They are provided between an uninterrupted, electron-pervious aluminum layer 65 and strip-shaped counter-electrodes 67 and 69. The electrodes 67 are interconnected, as well as the electrodes 69. If the three beams 71, 73 and 75 deviate to 1 the left, a reference signal is obtained at the electrodes that a voltage pulse is obtained from the electrodes 67,

this pulse being equal to a voltage pulse from the electrodes 69, its polarity being, however, opposite. To this end, to the electrode 65 is applied a voltage V and to the electrodes 67 and 69 a voltage V+AV and VAV respectively.

Fig. 8 shows one embodiment of a picture screen according to the invention, in which no special strips of a material are provided, of which the specific conductivity varies. The phosphor strips are designated by 83, and 87. The phosphor material of the strips 83 is mixed with a material of which the specific conductivity varies upon electron impact. 89 and 91 designate two conductive layers, from which the reference signals are obtained.

Although a large number of embodiments of a tube according to the invention are described above, numerous other variations are possible within the scope of the invention, for example by combining the various embodiments. Particularly many embodiments having three electron beams described with reference to the figures may apply also to tubes having only one electron beam.

What is claimed is:

1. Apparatus including a color cathode-ray tube comprising electron-beam-producing means, beam-deflection means, a beam-receiving picture screen comprising substantially parallel luminescent strips luminescing in plural colors in response to electron bombardment and arranged in similar repetitive groups producing predetermined colors, plural strips of a material whose conductivity varies as a function of the extent of electron bombard.- ment associated with said groups in a similar repetitive pattern, means providing external circuit connections to two spaced points of each of said conductivity-varying strips, and means for establishing a potential difference between the two spaced points and thus across each of the conductivity-varying strips.

2. Apparatus as set forth in claim 1 wherein those luminescent strips producing a predetermined color each have admixed therewith the conductivity-varying material to form the conductivity-varying strips.

3. Apparatus as set forth in claim 1 wherein those luminescent strips producing a predetermined color are each constituted of a phosphor material that also possesses the property of its conductivity varying as a function of the extent of electron bombardment.

4. A color cathode-ray tube comprising electron-beamproducing means, beam-deflection means, a beam-receiving picture screen comprising substantially parallel luminescent strips luminescing in plural colors in response to electron bombardment and arranged in similar repetitive groups producing predetermined colors, plural strips of a material whose conductivity varies as a function of the extent of electron bombardment each associated with one of said groups and thus also arranged in a similar repetitive pattern, two electrodes coupled to opposite sides of the conductivity-varying strips, and means providing external circuit connections to the two electrodes whereby a potential may be applied thereto.

5. A tube as set forth in claim 4 wherein each electrode coupled to each of said conductivity-varying strips is interconnected with the corresponding electrodes coupled to the other strips forming two groups of interconnected electrodes.

6. A tube as set forth in claim 5 wherein all of the electrodes of each group are united to form a conductive layer contacting the whole of the screen.

7. A tube as set forth in claim 6 wherein the conductive layer adjacent the beam-producing means is electronpervious, whereas the other conductive layer is lighttransparent.

8. A tube as set forth in claim 5 wherein the group of electrodes adjacent the beam-producing means are united to form a conductive, electron-permeable layer contacting the luminescent strips.

9. A tube as set forth in claim 4 wherein corresponding electrodes coupled to corresponding sides of the strips are united to form a common conductive layer, and the other electrodes are alternately interconnected to form two groups of interconnected electrodes.

10. Television apparatus comprising a color cathoderay tube comprising plural electron-beam-producing means, beam-deflection means, a beam-receiving picture screen comprising substantially parallel luminescent strips luminescing in plural colors in response to electron bombardment and arranged in similar repetitive groups producing predetermined colors, plural strips of a material whose conductivity varies as a function of the extent of electron bombardment associated with said groups in a similar repetitive pattern, means providing external circuit connections to spaced points of each of said conductivity-varying strips, means for applying a potential difference to said external circuit connections, means for modulating one of the electron beams with a high-frequency signal, and means coupled to the external circuit connections for deriving an output signal correlated to the high-frequency modulating signal.

11. Apparatus as set forth in claim 10 wherein three electron-beam-producing means are provided arranged in a flat plane, and means are provided for modulating the two outer electron beams with different high-frequency signals.

12. Television apparatus comprising the tube of claim 9 wherein means are provided for applying a given voltage to the common conductive layer, for applying a voltage higher than the given voltage by a predetermined amount to one group of interconnected electrodes, and for applying a voltage lower than the given voltage by the same predetermined amount to the other group of interconnected electrodes.

References Cited in the file of this patent UNITED STATES PATENTS 2,446,440 Swedlund Aug. 3, 1948 2,723,361 Becker's Nov. 8, 1955 2,725,420 Zworykin Nov. 29, 1955 2,767,346 Hoyt Oct. 16, 1956 2,768,318 Bradley Oct. 23, 1956 2,790,930 Kalfaian Apr. 30, 1957 2,792,522 Welch May 14, 1957 FOREIGN PATENTS 417,679 Great Britain Oct. 10, 1934

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