GB739496A - Improvements in or relating to cathode ray tubes - Google Patents

Abstract

739,496. Cathode-ray tubes. NATIONAL RESEARCH: DEVELOPMENT CORPORATION. July 1, 1953 [Sept. 15, 1952], No. 18221/53. Class 39 (1). In a cathode-ray tube with a plane fluorescent screen the electron beam is projected in a plane parallel to the screen and is deflected in one dimension of the plane, and the end portion of the beam is deflected towards and into impact with the screen at a position which is controllable in another dimension of the plane of the screen by an electrode array in a plane parallel to but spaced from the screen. The electrode array sets up an electrostatic field having a component normal to the screen. Deflection plates 12, Fig. 2, control the line scan and the electrode array 3 in front of the screen 2 controls the frame scan. The elec- 'trode array consists of parallel linear conductors insulated from one another and associated with a common capacitive backing plate 4, Fig. 3. An electric field wave travelling with constant speed across the array successively discharges the conductors so that the beam is thrown on the screen after a diminishing length of travel. The shape of the wave is such that the beam is simultaneously focused on the screen. A structurally similar electrode array may be disposed at or in close proximity to the screen. The conductors of the two arrays are connected together, one by one or in small groups. Corresponding conductors are not directly facing one another but are offset. The two arrays may be a folded array. The discharging field wave is set up by an electron valve extending over the whole length of the electrode array. Its outer electrode 6, Fig. 3, consists of the conductors of the array which are bent back and into the form of a U-shaped surface. Within the outer electrode 6 of the control valve, Figs. 3 and 18, are a cathode 23, a control electrode 7, a collector electrode 8 and secondary electron collecting rod electrodes 26. The control electrode 7 has a partition 25 with a slot aperture or a series of round apertures. At the far end of the array, as seen by the main beam, the array is terminated by a plate permanently at or near the potential of the main electron gun, and at the near end by a second plate permanently at maximum positive potential. In the scanning phase electrons emitted by the cathode 23 at the negative end of the array are deflected on to conductors of the electrode array to discharge them and so initiate a potential wave which runs down the array. In the fly-back phase the array is raised to the maximum positive potential by releasing from its conductors secondary electrons which are collected by the electrodes 26. The conductors on opposite sides of the U-shaped electrode 6 are axially offset, an axial potential gradient down the array giving rise to a transverse component of electrostatic field. The electrode arrays are formed from a fabric comprising the conductors interwoven with insulating threads. The fabric is cemented to an insulating support and all or some of the insulating threads are destroyed. Suitable destructible threads are of cotton, nitrocellulose or rayon. Glass threads may be retained for additional support. The cathode-ray tube may be used for colour television. The fluorescent screen then comprises colour strips which are selectively excited by the electron beam falling on the screen at varying angles of incidence. To vary the angle of incidence the electron beam may be displaced in a direction normal to the plane of the screen magnetically or electrostatically. Three electron guns may be used. The colour strips 28, 29, 30, Fig. 21, are formed on the inside walls of asymmetrical grooves 27 in a supporting plate 2. The grooves are formed by etching. The surface layer of the plate 2 may be of opal glass. The colour strips may be pigments acting as colour filters coated with white fluorescent powder. In a modification, Fig. 26, the colour strips 28, 29, 30 all face the screen surface. The surface layer 33 of the screen plate is of a more easily soluble glass than the base 2. The base 2 has ridges 34 to prevent under-cutting. In another screen., Fig. 28, the ridges 34 of the base 2 are first coated with metal to form projections 35. These projections are then sprayed from one side to form the projections 36. In one form of tube, Fig. 5, the electron gun 9 is disposed behind the electrode array-fluorescent screen structure 2, 3. The beam is turned back upon itself by an electrostatic mirror formed by cylindrical electrodes 13 at positive potential, adjacent the limbs of a U-shaped electrode 15 at zero potential and a central electrode 14 at the potential of the electrodes 13. The tube envelope 1 is made in two dish-shaped halves, and the complete electrode structure is supported by studs which project between the two halves in pre-moulded grooves. The electrode leads are also sealed in the joint between the two halves. A magnetic field may be provided in the region of the electrodes 13 to compensate for tilting of the plane in which the beam moves due to imperfect positioning of the deflecting electrodes 16. Provision is made for the correction of trapezium distortion. Line scanning is preferably effected magnetically.