Patent GB-723186-A (GB)
Title: Improvements in or relating to colour systems for television
IPC: H04N9/22
Filed: 1952-11-07
Abstract: 723,186. Colour television tubes. RAULAND CORPORATION. Nov. 7, 1952 [Nov. 10, 1951], No. 28089/52. Class 39 (1). [Also in Group XL (b)]. A discharge tube for converting monochrome images in a conventional receiver to colour images comprises a photocathode 17, Fig. 1, exposed to the light from the receiver screen and a multicolour fluorescent screen 22, means, such as an apertured mask 27, for restricting the possible area of simultaneous inpingement of electrons on the screen to one of the phosphor groups of one colour, and colour switching means 19, 20. The scanning means 13, 14 can be of conventional form if dot sequential transmission is used, but otherwise must be modified to take account of the form of the colour input signals. Screen 22 is preferably a tricolour screen and comprises manganese activated zinc silicate; titanium activated calcium magnesium silicate, and manganese activated cadmium borate elements for green, blue- and red emission respectively. The elements may be strips over the whole width or height of the screen, or dots. A backing layer 22 of aluminium is also provided, and the photocathode 17 which may be of antimony-caesium (SbCs,) or silvercaesium oxide-caesium (Ag-Cs 2 O-Cs) is earthed via conducting ring 23. 18, 19, 20 and 21 are accelerating electrodes with graduated potentials, and 19 and 20 have slanting adjacent edges to set up a transverse field component. Meshes 24, 25 prevent undesirable lens effects. Focusing solenoid 26 is optional. A " parallax mask " 27 comprising a metal grid or a glass plate with opaque and transparent areas, the opaque areas being formed by etching, is arranged preferably in the position shown or possibly inside tube 15 between 17 and the deflection system 19, 20. The mask apertures correspond to the tri-colour phosphor groups. The colour synchronizing components of the input colour signals are impressed on colour switching device 28 coupled to deflection system 19, -20. Line, dot or field transmission may be used. For field transmission a monochrome image in tube 15 representing one colour component is dissected by mask 27 so that the electrons from photocathode 17 correspond in cross section to one group of phosphors of one colour, and colour switching device 28 ensures the correct deflecting potential between 19, 20 so that the correct colour is produced on screen 16. One colour field may be excited when 19, 20 are at the same potential and the other two at positive and negative potentials of equal magnitude, and 28 may be a "stair step generator or a commutator system. The deflection system 19, 20 may be replaced by a pair of plates. A second deflection system in quadrature with 19, 20 may permit accurate beam registration with the phosphor groups, especially if of dot form; it is energized by 28 or by a separate adjustable bias. Additional accelerating and/or decelerating electrodes may improve the focussing. In a modification, Fig. 2 (not shown) the " parallax mask"  comprises a transparent base of glass or plastic with a pair of identical aligned mask elements on opposite sides thereof. The base member may be part of the tube wall, in which case one mask. element carries the photocathode 17, or it may be a separate member the composite mask'being outside the tube wall. In another modification, Fig. 3 (not shown) the electrodes 18-21 and an additional conical electrode inside a flared portion of the envelope are formed as wall coatings and meshes 24, 25 are dispensed with. The fluorescent screen may be carried on a separate support or on the tube end wall, and especially in the former case the flared envelope portion may be of metal. In another modification Fig. 4 (not shown) the photocathode is discontinuous being shaped to correspond to all but one of the groups of phosphors. It consists of an opaque member coated on the side facing the monochrome screen with photoemissive materials, so that photoelectrons drawn through the apertures of the opaque member correspond to one group of phosphors only. Alternatively the photocathode may be of a shape corresponding to one group of phosphors in which case it is formed on a conductive wall coating. The tube 15 may be mounted pivotally so that it can be withdrawn during intervals when monochrome reception is desired.