TELEVISION CAMERA-TUBE

Abstract: A television camera-tube comprising in an evacuated envelope (1) an electron gun (4) for generating at least one electron beam which is focused onto a target (3) and which is deflected over the said target, a grid electrode (7) having hexagonal apertures being provided in front of said target. The apertures in the grid electrode (7) are arranged in a honeycomb structure, which apertures have the form of an equilateral hexagon having angles of 120°, and as a result the susceptibility to Moiré effects is reduced, the microphonic properties are improved and the mesh obtained as a high transmissivity while it can more readily be tensioned and is easier to handle.

Description: The invention relates to a television camera-tube comprising in an evacuated envelope an electron gun for generating at least one electron beam which is focused onto a target having a photosensitive layer and which is deflected over said target, a grid electrode having hexagonal apertures being provided directly in front of said target. Such a television camera-tube is known from Japanese Kokai 58-7752. In the tube described therein, a grid electrode is provided a few millimeters away from the photosensitive layer. The electric field between the grid electrode and the photosensitive layer ensures that a substantially perpendicular landing of the electron beam is obtained over the entire photosensitive layer. Moiré effects, which arise when the grid electrode is scanned by an electron beam along a line pattern, are less liable to occur if the apertures in the grid electrode are hexagonal. However, the process of imparting a hexagonal shape to the apertures, as described in Japanese Kokai 58-7752 which is considered to be incorporated herein by reference, results in a reduction of the electron transmission as compared with the transmission of the commonly used grid electrode having square apertures. The grid electrode is tensioned in order to reduce microphonics in the tube. The maximum tension is then limited by the tensile strength of the grid electrode. It is an object of the invention to provide a television camera-tube in which the electron transmission of the grid electrode is at least equal to the electron transmission of a grid electrode comprising a mesh having square apertures, the pitch remaining the same. Another object of the invention is to provide a television camera-tube having improved microphonic properties. According to the invention, a television camera-tube of the type described in the opening paragraph is characterized in that the apertures in the grid electrode are arranged in a honeycomb structure (a dense structure of hexagonal apertures), which apertures have the form of an equilateral hexagon with angles of 120°. If the pitch and the wire width remain the same, the transmission of a grid electrode having a honeycomb structure is equal to that of an electrode having square apertures. The grid electrode as shown in Japanese Kokai 58-7752, just like grid electrodes having square apertures, has two mutually perpendicular directions in which the strength is greatest. Moreover, the wires of two adjacent apertures adjoin each other. The grid electrode in accordance with the invention has three directions in which the strength of the electrode is greatest. Moreover, the wires which extend in one direction are co-axial, but they are not interconnected. This results in a better distribution of the mechanical stress in the grid electrode, consequently, the microphonic properties improve and the stress may be higher. This hexagonal pattern therefore results in a grid electrode of great strength. Consequently, a mesh having hexagonal apertures in accordance with the invention can more easily be tensioned in an electrode support without the mesh being damaged. The increased strength makes it possible to reduce the width of the wires, which results in a greater transmission. A type of mesh which is very suitable for use in television camera-tubes, is characterized in that the apertures in the grid electrode have a pitch between 10 and 50 µm, preferably approximately 17 µm, and that the width of the wires is between 2 and 6 µm, preferably approximately 4 µm. A mesh aperture of 17 µm and a wire width of 4 µm corresponds to a transmission of 60% in the case of 1500 lines per inch (60 lines/mm). The grid electrode is preferably made of a material or an alloy from the group formed by nickel, copper, platinum, gold. Such a grid electrode can be obtained by means of a method known from British Patent Specification 2,063,299 which is considered to be incorporated herein by reference. This method employs a mould in which grooves having a depth corresponding to the desired electrode thickness are formed by a photolithographic process in a pattern which is a negative of the desired mesh pattern, the mesh being formed in said grooves by means of electro-deposition, after which it is removed from the mould. A variant of this known method uses a mould of non-conductive marerial, particularly quartz glass, in which the requisite groove pattern is RF-sputter etched. Subsequently, the bottom of the grooves is rendered electrically conductive by sputter depositing with metallic palladium, silver or silver-palladium alloys, the excess of which is then rubbed off the surface, leaving only metal in the grooves. Next, the electrode is formed by electro-deposition and removed from the mould. Electrodes having a honeycomb structure are stronger than square-aperture electrodes having the same wire width. Consequently, the former can more readily be stripped off the mould without cracking the electrode, even when the wires are less thick. Another variant of the known method employs a mould of a semiconductor material, namely a single-crystal silicon wafer. A silicon oxide layer is vapour deposited thereon by means of a chemical process (CVD). Subsequently, by applying a photoresist and etching with a hydrofluoric acid solvent in a photolithographic process, the silicon oxide layer is formed into a mask whose pattern is the negative of the desired groove pattern. Using a boiling NaOH solution, the silicon is etched to a depth of 5 µm via this mask, after which, in the same way as the first-mentioned variant, the electrode is formed in the mould pattern obtained by means of electrodeposition and subsequently removed from the mould. This method, however, is rather complicated and time-consuming: for each individual electrode the vacuum sputtering process and the removal of the excess metal have to be repeated. United States Patent Specification 3,878,061 which is considered to be incorporated herein by reference, discloses a method which is much simpler and which employs a mould for developing the grid electrode, which mould can be used several times without requiring any intermediate treatment. For this purpose, the grooves in the mould have side walls which are not electrically conductive and a bottom which is electrically conductive. By way of example, the invention will now be described with reference to the accompanying drawing, in which Fig. 1 is a longitudinal sectional view of a television camera-tube in accordance with the invention and Fig. 2 shows a part of a grid electrode having a honeycomb structure, for use in a television camera-tube in accordance with Fig. 1. The television camera-tube as shown in Fig. 1 comprises a glass envelope 1 which is closed at one end by a glass disc 2 having a target 3. In the tube there is an electron gun 4 to which the desired voltages can be applied via a number of lead-through pins 5. The inner wall of the envelope 1 is coated with a thin nickel layer 6 by means of a known process, such as electroless nickel plating. The tube further comprises a grid electrode 7 of nickel and a diaphragm 8 having an opening 9 through which passes an electron beam generated by the electron gun 4 prior to landing on the photosensitive layer 3. The nickel layer 6 is interrupted near the grid electrode 7 and near the diaphragm completely therearound so that this layer is divided into three parts. Each of these parts forms a wall electrode which contributes to the formation of a target of the electron beam on the photosensitive layer 3 of desired form and dimensions. By means of indium balls 10 and 11 the grid electrode 7 which is tensioned in an annular supporting member and the diaphragm 8 are mechanically and electrically connected to the nickel layer 6 at the sides facing away from the bearing surface. Fig. 2 shows a part of the grid electrode 7 having apertures 12 which are arranged in a honeycomb structure. The apertures 12 have the form of equilateral hexagons having sides of 16 µm and angles of 120°. The width b of the wires 13 is, in this case, 6 µm and the apertures have a pitch of 34 µm, and consequently the transmission is 68 %. The thickness of the wires is 4 µm.

Claims: 1. A television camera-tube comprising in an evacuated envelope an electron gun for generating at least one electron beam which is focused onto a target having a photo-sensitive layer and which is deflected over the said target, a grid electrode having hexagonal apertures being provided directly in front of which target, characterized in that the apertures in the grid electrode are arranged in a honeycomb structure, which apertures have the form of an equilateral hexagon having angles of 120°. 2. A television camera-tube as claimed in Claim 1, characterized in that the apertures in the grid electrode have a pitch between 10 and 50 µm and that the width of the wires between the apertures is between from 2 and 6 µm.

Inventors: VAN GORKUM, AART ADRIANUS, VRIJSSEN, GERARDUS ARNOLDUS HERMAN MARIA
Applicants: N.V. PHILIPS' GLOEILAMPENFABRIEKEN
Application year: 1986
Main classification: H01J29
IPC codes: H01J29, H01J31