Patent Publication Number: US-6703776-B1

Title: Cathode ray tube

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a cathode ray tube having (1) an electron source having a cathode for emission of electrons, (2) an electron beam guidance cavity having an input and an output for concentrating electrons emitted from the cathode, (3) a first electrode being connectable to a first power supply for applying, in operation, an electric field with a first field strength E I between the cathode and the output of the cavity so as to allow electron transport through the electron beam guidance cavity, and (4) an accelerating grid for accelerating the electrons leaving the cavity and a main electron lens for focusing the accelerated electrons on a display screen. 
     Such a cathode ray tube may be used in television displays, computer monitors and projection TVs. 
     2. Description of the Related Art 
     A cathode ray tube of the type described in the opening paragraph is known from U.S. Pat. No. 5,270,611. This patent describes a cathode ray tube that is provided with the cathode, the electron beam guidance cavity and the first electrode which is connectable to a first power supply for applying the electric field with a first field strength E 1  between the cathode and the output aperture. The electron beam guidance cavity has walls in which, for example, a part of the wall near the output has an insulating material having a secondary emission coefficient δ 1 . Furthermore, the secondary emission coefficient δ 1  and the first field strength E 1  have values that allow electron transport through the electron beam guidance cavity. The electron transport within the cavity is possible when a sufficiently strong electric field is applied in a longitudinal direction of the electron beam guidance cavity. The value of this field depends on the type of material and on the geometry and sizes of the walls of the cavity. The electron transport then takes place via a secondary emission process so that, for each electron impinging on a cavity wall, one electron is emitted on average. The circumstances can be chosen to be such that as many electrons enter the input aperture of the electron beam guidance cavity as will leave the output aperture. When the output aperture is much smaller than the input aperture, an electron compressor is formed which concentrates the luminosity of the electron source by a factor of, for example, 100 to 1000. An electron source with a high current density can thus be made. The accelerating grid accelerates electrons leaving the cavity towards the main electron lens. The main electron lens images the exit hole of the cavity on the display screen and, via a deflection unit, a raster image is formed on the display screen of the tube. 
     The spot size of the electron beam determines the resolution of the tube. Especially for computer monitor tubes and also television picture tubes, the resolution may be an important feature. 
     SUMMARY OF THE INVENTION 
     It is, inter alia, an object of the invention to provide a cathode ray tube in which the spot size of the electron beam on the display screen is reduced. This object is achieved by the cathode ray tube according to the invention, which is characterized in that the cathode ray tube comprises a further electron lens between the cavity and the main lens for adapting the diameter of the electron beam to the entrance of the main lens, said further electron lens comprising the first electrode and the accelerating grid. The electron beam entering the main lens is then less divergent and the spherical aberrations caused by the main lens are reduced. The invention is based on the recognition that the electrons leaving the electron beam cavity have a relatively high velocity compared to electrons leaving a conventional cathode, and therefore the diameter of the electron beam entering the main lens is too large. With the prefocussing effect of the further electron lens between the electron beam cavity and the main lens, and given a fixed relationship of the distances between the cathode, main lens and display screen, the diameter of the electron beam entering the main lens can be optimized for a small spot size and minimal spherical aberrations. 
     Further advantageous embodiments of the invention are defined in the dependent claims. 
     A particular version of the cathode ray tube according to the invention is characterized in that the first electrode comprises a first and a second part, placed behind each other along an axis of the main lens, the diameter of the first part being smaller than the diameter of the second part. A so-called cup lens is then formed for prefocussing the electron beam before entrance into the main lens. An advantage of the cup lens is its economic design. Moreover, the cup-lens is robust against flashes which occur during the manufacturing process of the cathode ray tube or during operation. The first and second parts may have different symmetric shapes. The shape of the parts can also be adapted in order to reduce astigmatism of the spot on the display screen, for example, the shape of the cup lens may be a rectangle or ellipsoid. 
     A further version of the cathode ray tube according to the invention is characterized in that the further electron lens further comprises a second electrode which is concentric with the first electrode, the second electrode being connectable to a second power supply for applying, in operation, an electric field with a second field strength E 2  between the first and the second electrodes, the voltage of the second power supply being lower than that of the first power supply. An electron lens is then formed having a special shape for prefocusing the electron beam in the entrance of the main lens. An advantage of this electron lens is that some of the electron lens characteristics can be adjusted when the cathode is mounted in the cathode ray tube. This is in contrast with the above-mentioned cup lens, which has characteristics that are completely determined when the cathode is mounted in the cathode ray tube. Furthermore, the first and second electrodes may have a symmetrical shape. 
     A further version of the cathode ray tube according to the invention is characterized in that the first and second electrodes are substantially in the same plane. A planar electron lens is thus obtained. These planar lenses can be easily made by removing parts of metal forming the first electrode. Furthermore, a planar lens design allows a large degree of freedom in the prefocussing characteristics of the electron lens. 
     A further version of the cathode ray tube according to the invention is characterized in that the cathode ray tube comprises a third electrode placed between the cathode and the cavity, said third electrode being connectable to a third power supply for applying, in operation, an electric field with a third field strength E 3  between the cathode and the third electrode for controlling the emission of electrons. In this way, relatively small modulation voltages can be applied for modulating the electron beam. For example, when the distance between the cathode and the third electrode amounts to 100 micrometers, an amplitude modulation of 5 Volts is sufficient for modulating a current between 0 and 3 mA when conventional oxide cathodes are used. This modulation gauze is described in the unpublished EP patent application 9920199.6. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other aspects of the invention are apparent from and will be elucidated with reference to the embodiments described hereinafter. 
     In the drawing: 
     FIG. 1 is a schematic diagram of a cathode ray tube, 
     FIG. 2 shows a first embodiment of a cathode structure with a cup lens according to the invention for use in a cathode ray tube, 
     FIG. 3 shows a first example of a cup lens, 
     FIG. 4 shows a second example of a cup lens, 
     FIG. 5 is a cross-section of a planar electron lens, and 
     FIG. 6 is a top view of a planar electron lens. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 is a schematic diagram of a known cathode ray tube. This cathode ray tube is known from the cited U.S. Pat. No. 5,270,611. The cathode ray tube  100  comprises an electrode structure  101  having cathodes  105 , 106 , 107  for emission of electrons, and electron beam guidance cavities  120 , 121 , 122 . Preferably, the cathode ray tube comprises heating filaments  102 , 103 , 104 . Furthermore, the cathode ray tube comprises an accelerating grid  140 , a conventional main lens  150  and a conventional magnetic deflection unit  160  and a display screen, for example a conventional color phosphor screen  170 . All these parts are known from conventional color cathode ray tubes. The cathode ray tube according to the invention may be applied in television, projection television and computer monitors. 
     FIG. 2 shows a first embodiment of the cathode structure in accordance with the invention, which cathode structure may be applied in the cathode ray tube shown in FIG. 1 The cathode structure  200  comprises a frame  201 , heating filaments  202 ,  203 ,  204  and cathodes  205 , 206 , 207  corresponding to each of the heating filaments. The cathodes are provided in triplicate so that the cathode ray tube may be used for the display of color images represented by red, green and blue signals. Furthermore, the cathode structure  200  comprises electron beam guidance cavities  220 , 221 , 222  each having input apertures  208 , 209 , 210 , output apertures  223 , 224 , 225  and first electrodes  226 , 227 , 228 . The input apertures  208 , 209 , 210  may have a square shape with dimensions of 2.5×2.5 mm. At least a part of the interior around the output apertures  223 , 224 , 225  of the electron beam guidance cavities  220 , 221 , 222  is covered with an insulating material having a secondary emission coefficient δ 1 &gt;1 for cooperation with the cathodes  205 , 206 , 207 . This material comprises, for example, MgO. The MgO layer has a thickness of, for example, 0.5 micrometer. Other materials that may be used are, for example, glass or Kapton polyamide material. The first electrodes  226 , 227 , 228  are positioned around the output apertures  223 , 224 , 225  on the outer side of the electron beam guidance cavities  220 , 221 , 222 . The first electrodes consist of a metal sheet. The metal sheet has a thickness of, for example, 2.5 micrometers and may be provided by metal evaporation of, for example, a combination of aluminum and chromium. The output apertures  223 , 224 , 225  may have a circular shape with a diameter of, for example, 20 micrometers. Furthermore, each filament  202 , 203 , 204  for heating the cathodes  205 , 206 , 207  can be coupled to a first power supply V 1  (not shown). In operation, each filament  202 , 203 , 204  heats a corresponding cathode  205 , 206 , 207 . The cathode comprises conventional oxide cathode material, for example, barium oxide. 
     In operation, the first electrodes  226 , 227 , 228  are coupled to a second power supply VA 1  for applying an electric field with a field strength E 1  between the cathodes  205 , 206 , 207  and the output apertures  223 , 224 , 225 . The voltage of the second power supply is, for example, in the range between 100 and 1500 V, typically 700 V. The secondary emission coefficient δ and the field strength have values which allow electron transport through the electron beam guidance cavity. This kind of electron transport is known from the cited U.S. Pat. No. 5,270,611. 
     According to the invention, an electron lens is formed by the first electrode  226  and the accelerating grid  140  between the output apertures  223 , 224 , 225  of each cavity  220 , 221 , 222  and the main lens  150  for reducing the diameter of the electron beam which enters the main lens. Preferably, the electron lens comprises a so-called cup lens, which is formed by the first electrodes  226 , 227 , 228 . The cup lens comprises a first and a second part, which parts are situated behind each other along an axis of the main lens  150 . The diameter of the first part of the cup lens is smaller than the diameter of the second part of the cup lens. Preferably, both parts are circularly symmetric. However, by applying non-circularly symmetric parts, for example, an ellipsoidal or rectangular shape, the cup lens can be made astigmatic to correct the spot shape on the phosphor screen even more. 
     For a further explanation of the operation of the cathode ray tube, reference is made to FIG.  1 . After the electrons have left the output apertures  223 , 224 , 225  of the electron beam guidance cavities  220 , 221 , 222 , the electron lens formed by the first electrodes  226 , 227 , 228  and the accelerating gauze  140  prefocuses the electron beam in the main lens  150 . In this way, the diameter of the electron beam entering the main lens is reduced and can be optimized for a minimal spot size on the display screen. Via the main lens  150  and the deflection unit  160 , the three electron beams corresponding to the red, green and blue signals are directed to the phosphor screen  170  in order to build the image represented by the red, green and blue signals. The cup lens may be formed by various shapes of the first electrode as for example shown in FIG.  3  and FIG.  4 . 
     FIG. 3 is a cross-section of a first example in accordance with the invention, of a cup lens electrode  302  situated at the output aperture  300  of the cavity of the cathode structure. The electrode  302 , situated at the wall  301  of the cavity, comprises a first part  304  and a second part  306 , the first and second parts being placed behind each other along an axis of the main lens  150  (not shown). The first and second parts have, for example, a cylindrical shape. The first part  304  has a length L 1  of, for example, 3 μm and a diameter D 1  of 200 μm. The second part  306  has a length L 2  of, for example, 250μ and a diameter D 2  of, for example, 600 μm. 
     Rectangular and ellipsoidal shapes of the first and second parts are also possible. By adapting the shape of the first and second parts, the spot shape on the phosphor screen can be adapted. FIG. 4 is a cross-section of a second example of a cup lens electrode  402  which may be situated at the output apertures of the cathode structure according to the invention. The cup lens electrode  402  comprises three parts  404 , 406 , 408  situated along a main axis of the main lens. Preferably, the first and second parts have a cylindrical shape and may have the same dimensions D 1 ,L 1  and D 2 ,L 2  as in the first cup lens electrode  302 . Preferably, the third part has a frusto-conical shape. The side of the frusto-conical part  408  that has the largest diameter faces the main electron lens. The largest diameter D 3  of the frusto-conical part of the cup lens electrode is, for example, 900 μm. The length L 3  of the third part  408  is, for example 600 μm. 
     A second embodiment of the cathode ray tube according to the invention comprises an electron lens formed by the first electrode, a second electrode and the accelerating grid. The second electrode is concentric with the first electrode. Preferably, the first and second electrodes are in the same plane. An example of a planar electron lens formed by the first and second electrodes is shown in FIG.  5  and FIG.  6 . 
     FIG. 5 is a cross-section of a planar lens electrode which may be situated at the output aperture  500  of the cavity of the cathode structure. The planar electron lens electrode  502  comprises the first and second electrodes  504 , 506 . Preferably, the first and second electrodes  504 ,  506  are concentric and circularly symmetric. Both electrodes  504 , 506  can be made by providing a metal layer on the insulator and by etching the desired electrode patterns. 
     FIG. 6 is a top view of the output aperture  500  and the first and second electrodes  504 , 506  of the planar electron lens electrode  502 . Instead of a circularly symmetric shape of the first and second electrodes, a rectangular or ellipsoidal shape may also be applied. A further planar electrode may be applied for creating a further degree of freedom to manipulate the electron lens characteristics even more. 
     In operation, the first electrode  504  is connected to the second power supply VAI for applying an electric field with a field strength E 1  allowing electron transport through the cavity. The second electrode  506  is connected to a third power supply VA 2  for applying an electric field with a field strength E 2  between the first and the second electrode  504 , 506 . The voltage VA 2  of the third power supply VA 2  is determined in such a way that a desired rate of prefocusing is obtained for adapting the diameter of the electron beam entering the main lens. Furthermore, the voltage VA 2  of the third power supply is lower than that of the second power supply VA 1 . For example, the voltage VA 1  of the first power supply is 1000 V and the voltage VA 2  of the second power supply is 600 V. Furthermore, the accelerating grid is connected to a fifth power supply (not shown) for applying an electric field having a sufficient field strength E 3  for accelerating the electrons. The voltage VA 3  is higher than that of the second power supply. For example, this voltage VA 3  is 6000 Volts. 
     Preferably, third electrodes  230 , 231 , 232  are placed before the input apertures  208 , 209 , 210  between the cathodes  205 , 206 , 207  and the cavities  220 , 221 , 222  for modulating the current of the electron beam. The third electrodes  230 , 231 , 232  are coupled to a sixth power supply VA 4  (not shown) for applying, in operation, an electric field with a fourth field strength E 4  between the cathodes  205 , 206 , 207  and the third electrodes  230 , 231 , 232  for controlling the emission of electrons. Preferably, the third electrodes  230 , 231 , 232  comprise a gauze with a 60% transmission of electrons. The gauze can be made of a metal, for example molybdenum, and may be electrically coupled to the frame  201 . In practice, the three gauzes  230 , 231 , 232  are electrically coupled to the frame  201 . A voltage difference between the cathodes  205 , 206 , 207  and the gauzes  230 , 231 , 232  is determined by applying a fixed voltage to the frame and varying voltages to the gauzes. In operation, a pulling field due to the voltage difference applied between the gauzes  230 , 231 , 232  and the cathodes  205 , 206 , 207  pulls the electrons away from the cathodes  205 , 206 , 207 . The voltage differences between the cathodes  205 , 206 , 207  and corresponding gauzes  230 , 231 , 232  corresponds to the respective red, green and blue signals which represent the image. 
     Now, referring to cathode structure of FIG. 2, where the distance between the gauzes  230 , 231 , 232  and the cathodes  205 , 206 , 207  is small enough, for example, in a range between 20 and 400 micrometers, a relatively low voltage difference between the cathodes  205 , 206 , 207  and the gauzes  230 , 231 , 232  can modulate the emission of the electrons towards the input aperture of the electron beam guidance cavities  220 , 221 , 222 . For example, when the distance between the cathodes  205 , 206 , 207  and the gauzes  230 , 231 , 232  is 100 micrometers a voltage swing of 5 volts can modulate an electron current of between 0 and 3 mA to the electron beam guidance cavities  220 , 221 , 222 . 
     It should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be able to design many alternative solutions without departing from the scope of the claims.