Abstract:
A display arrangement consists of a tubular envelope with fluorescent stripes running along its length, localized portions of which are illuminated by different electron guns which produce flood beams of electrons. Field electrodes which surround each electron gun determine the angular direction at which the flood beam leaves the gun and hence which one of the fluorescent stripes is caused to emit light. The stripes can be of three different primary colors, to produce a colored display.

Description:
BACKGROUND AND SUMMARY OF THE INVENTION 
     This invention relates to display arrangements which are capable of producing bright, readily alterable displays. 
     According to this invention, a display arrangement includes an evacuated envelope having a fluorescent screen and an electron gun which is capable of producing a flood beam of electrons which falls upon said screen, the screen having three distinct adjacent localized areas which emit light of three different primary colors respectively in response to incident electrons, the electron gun comprising a cathode and two field electrodes positioned one on each side of the cathode and arranged to shape the flood beam which emerges from said gun, the three localized areas of the screen being such that the undeflected flood beam falls upon one of them, and such that the flood beam is deflected to fall upon the other two localized areas respectively in response to potentials of said field electrodes of the same value but of opposite effect. 
     Three different localized areas of the screen can be associated with a particular flood beam, and each of these areas carries a different color phosphor, e.g. red, green, blue, so that by altering the angle at which the beam emerges from the gun, the color of the display can be changed. This angle is selected by applying predetermined potentials of low magnitude to the two field electrodes. 
     Preferably a mesh electrode is positioned between the screen and the cathode, and carries a relatively low potential, so that the customary very high potential which is applied to the screen does not influence the operation of the gun. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1 and 2 illustrate cross-sectional views of a display arrangement in accordance with the present invention and 
     FIG. 3 is a longitudinal section view. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to the drawing, the display arrangement consists of a long tubular glass envelope 1 of approximately rectangular cross-section, a portion of which constitutes a fluorescent screen 2 and carries three longitudinal stripes 3, 4 and 5 of red, green and blue phosphor respectively. The envelope is sealed at both ends (not shown) and is evacuated to a high level of vacuum. A single elongate cathode 6 is positioned towards the end of the rectangular section which is away from the screen 2, and on either side of the cathode 6 is a respective field electrode 7 and 8. The two electrodes together with the cathode 6 constitute an electron gun 9 which is arranged to produce a flood beam of electrons, the width of which is determined primarily by the opening 10 between the mouths of the two electrodes 7 and 8. A mesh electrode 11 is positioned between the electron gun and the screen 2. 
     In operation, emission of electrons from the electron gun 9 can be controlled by the potentials applied to the field grids 7 and 8. By controlling the angle at which the electron beam emerges, it can be caused to strike just one of the three stripes 3, 4 and 5 so that a red, green or blue patch of light can be selected at will. 
     FIG. 1 shows the trajectory of the electron flood beam when no lateral deflection is applied to it so that it strikes the green stripe 4 and produces a correspondingly colored patch of intense illumination. Under these conditions, typical voltages are as follows. A very high potential is applied to the inner surface of the screen 2 and is typically about +7 KV. The mesh electrode is held at +10 V and both field electrodes 7 and 8 are held at the same potential of +10 V. All potentials are with respect to the nominal earth potential of the cathode 6. 
     As both field electrodes 7 and 8 are at the same potential, the electrons which are emitted from the cathode 6 experience a net positive electrostatic field, and are accelerated towards the mesh electrode 11. It will be noted that the width or spread of the flood beam in a direction transverse to the axis of the cathode, is dictated by the width of the slotted opening 10, and that the flood beam electron continues to diverge in an almost linear manner until it reaches the mesh electrode 11 which is held at +10 V. The extensions 12, 13 of the field electrodes 7 and 8 assist in controlling the profile of the flood beam as it leaves the electron gun. When the electrons reach the mesh electrode 11, they are greatly influenced by the very high potential on the screen and are accelerated in a very rapid manner so that they strike the screen with high energy. 
     In practice, the brightness of the display is determined by pulse width modulating the potential on the field electrodes, i.e. controlling the duration of the pulses applied to it. In this case, the cathode is a directly heated filament, that is to say its temperature is raised to that at which copious emission of electrons takes place by passing an electric current through it. The resistance of the filament is chosen so as to provide the required temperature rise. By pulsing the current along the filament instead of passing it continuously, the variation of potential along the filament can be prevented from causing brightness variations across the screen. Thus, the current pulses are applied to the filament only while the electron beam is not permitted to leave the electron gun. The device is turned off, i.e. the electron beam is contained within the electron gun by applying a potential of -2 V to the field electrodes 7 and 8 with respect to the cathode 6. The pulse repetition rate of the pulses applied to the field electrodes should be well above the flicker threshold of the eye, so that an observer sees a continuously present display. 
     When it is desired to illuminate a different one of the phosphor stripes, for example stripe 5 which is blue, all of the previously stated potentials, which were applied while a green patch of light was produced, remain the same except for the values of the two potentials which are applied to the field electrodes 7 and 8. In order to produce the angular deflection illustrated in FIG. 2, a potential of about +12 V is applied to field electrode 7 and a potential of +8 V is applied to field electrode 8. 
     By interchanging the application of the potentials to the two field electrodes, the red phosphor stripe 3 can be illuminated. 
     Alternatively, the device as a whole can be turned on and off by applying a suitably negative potential to the mesh electrode 11, but this is not preferred if the mesh electrode is common to a plurality of electron guns which are to be operated independently of each other. 
     Referring to FIG. 3, a longitudinal view of the display device is shown and it will be seen that a plurality of electron guns 20, 21 and 22 are mounted within the common elongate envelope along which the single continuous cathode 6 passes. Each electron gun consists simply of a respective pair of field electrodes in combination with the cathode, but in FIG. 3 only the field electrodes 8 are visible. By applying common potentials to the field mesh 11 and to the fluorescent screen 2, it is necessary only to apply selectively switchable potentials to the field electrodes 7 and 8 of each electron gun in order to either turn that gun on or off or select a particular color. 
     It will be noted that the field electrodes constitute the entire electron gun in combination with a filamentary cathode but they can nevertheless produce a flood beam of electrons of controlled intensity and beam width. In addition, the angle at which the electron beam emerges from the gun can be finely controlled entirely by adjusting the relative potentials on the two field electrodes. Once the potentials have been adjusted, it is merely necessary to apply one of the three predetermined sets of potential values to the field electrodes 7 and 10 to produce a visible display of the required color. A large number of separate electron guns can be mounted in a single tubular envelope 1, and a large number of tubular envelopes can be mounted side by side to produce a large two-dimensional display area with extremely good optical resolution, and excellent control over the color of the separate pixels in the display.