GB865667A - Improvements in or relating to cathode ray tubes - Google Patents

Abstract

865,667. Cathode-ray tubes. MULLARD Ltd. June 4, 1957 [Aug. 1, 1956; Aug. 10, 1956], Nos. 23470/60 and 23471/60. Divided out of 865,663. Class 39(1). A cathode-ray display device comprises an evacuated envelope containing repeller and target electrodes having parallel plane or approximately plane inner operative surfaces spaced from one another by a distance smaller than the smallest dimension ofeither and defining between them a beam trajectory control space, a luminescent target substantially coincident with the target electrode, and means for directing an electron beam into the control space from an edge region thereof in a trajectory plane which intersects the target surface orthogonally. Means are provided for varying the instantaneous position of the trajectory plane so as to control the position of the point of impact of the beam on the target in a direction transverse to the trajectory plane while the point of impact in the trajectory plane is varied by applying an appropriate sawtooth potential between the repeller and target electrodes while maintaining constant the angle at which the beam enters the control space or, by varying this angle while applying a constant potential between the electrodes. In Fig. 2, the line scan is effected by varying the instantaneous position of the trajectory plane by means of field A between elongated electrodes 1, O. By varying the voltage on electrode 1 the beam b is constrained to pass through a slit S o in electrode O at a variable point but at a constant orientation. The beam then passes through a uniform accelerating field B and a field-free space C. The frame scan is effected by varying field D in a control space according to the invention between repeller and target electrodes 4. Alternatively the line scan may be effected by conventional deflection means L, Fig. 5, or D, Fig. 6, while the frame scan is effected by varying the field between electrodes O, 1. In Fig. 5 the beam is directed into the control space by a curved electron mirror M which renders all the reflected beam paths parallel. In Fig. 6 the beam from electron gun G enters the control space through a slit Sp in electrode O. Since a given voltage V 1 on electrode 1 would cause means D to scan the beam along a curved line a correction must be made, e.g. by ancillary deflection means or by superimposing a small correction component at line frequency on to the frame scan waveform of electrode 1. In each embodiment the target and repeller and control electrodes may be located on an actual wall of the device and would thus be slightly curved to enable the device to withstand atmospheric pressure. In a further embodiment of the invention the target and repeller electrodes R<SP>1</SP>o R1, Fig. 10, are uniformly resistive and have corresponding ends joined by elongated conductive electrodes O, 1 between which a variable field is applied to effect frame scanning. Preferably, the orientation of electrode R<SP>1</SP>o is chosen to give approximately normal incidence of the beam on the target. The side edges of electrodes R<SP>1</SP>o, R1 may be joined by further uniformly resistive surfaces to eliminate edge effects. To assist focusing of the beam a non-uniform control field may be used as in Fig. 14. The repeller electrode R is uniformly resistive and is provided with conductive edges R o , R n between which a constant D.C. potential is applied, the conductive target electrode being at the same potential as the edge R o . Line scanning is effected by means of a variable field set up between electrodes O, 1 and frame scanning by means of elongated deflection plates D.