Cathode ray tubes

Abstract

 In a cathode ray tube provided with a limiting aperture (LA) in an electron gun, the axial potential between the limiting aperture (LA) and an electrode (SG) arranged at the rear stage of the limiting aperture (LA) is lower than a potential of the limiting aperture (LA) by several tens of volts, so that secondary electrons generated from the limiting aperture (LA) are hindered from reaching a fluorescent screen (1) or a target screen of the cathode ray tube.

Description

[0001] This invention relates to cathode ray tubes including an electron gun with a limiting aperture.

[0002] In, for example, a high definition picture tube or a beam index type colour picture tube, it is necessary for the electron beam spot diameter on the fluorescent screen to be small, so as to obtain a good picture image.

[0003] Generally, the primary ways of obtaining a small beam spot diameter are to reduce the diameter of a cross-over point which forms the object point to be projected, or to reduce the aberration of a projection lens.

[0004] As an alternative, we have proposed that a limiting aperture be provided in an electron gun. Figure 6 of the accompanying drawings shows a typical arrangement of the limiting aperture. The cathode ray tube includes a cathode K, first and second grid electrodes Gl and G2, third and fourth grid electrodes G3 and G4 forming a focusing electrode, and a fluorescent screen 1. A limiting aperture LA is arranged at the rear stage of the electrode G2.

[0005] By the provision of the limiting aperture LA, the apparent diameter of the cross-over point may be reduced, and the divergence angle of the electron beam may be reduced (which is equivalent to enlargement of a lens diameter). As a result, the beam spot diameter may be made small.

[0006] A limiting aperture LA is usually adopted in a pick-up tube. However, if used in a picture tube, it has been found that a serious problem arises. Namely, when the electron beam strikes the limiting aperture LA, a large number of secondary electrons e' are generated. In a pick-up tube of cathode potential stabilizing type, the voltage on the target screen is low, and accordingly the secondary electrons e' do not land on the target screen. However, in the case of a picture tube, the voltage on the fluorescent screen 1 is high, and accordingly the second electrons e' land on the fluorescent screen 1 as shown in Figure 6. As a result, there is generated a so-called halo phenomenon because of a defocused beam Bm2 of secondary electrons e', as also indicated in Figure 7, and this substantially degrades the picture image. Figure 7 also indicates a main electron beam Bml.

[0007] Moreover, in a high speed beam scanning pick-up tube, the halo phenomenon is similarly generated.

[0008] According to the present invention there is provided a cathode ray tube including an electron gun with a limiting aperture; characterized in that:

an axial potential between said limiting aperture and an electrode arranged at the rear stage of said limiting aperture is lower than the potential of said limiting aperture by several tens of volts.

[0009] Thus in an embodiment of the invention, as the axial potential between the limiting aperture and the electrode arranged at the rear stage of the limiting aperture is lower than the potential of the limiting aperture, the secondary electrons generated from the limiting aperture are hindered from reaching the fluorescent screen or the target screen. Accordingly, the halo phenomenon due to the secondary electrons may be prevented.

[0010] The invention will now be described by way of example with reference to the accompanying drawings, throughout which like parts are referred to by like references, and in which:

Figure 1 is a schematic illustration of an embodiment of cathode ray tube according to the present invention;

Figure 2 is a graph showing the distribution of an axial potential in the embodiment of Figure 1;

Figure 3 is a cross-sectional illustration of an example of part of the structure of an embodiment;

Figures 4 and 5 are schematic illustrations of modified embodiments of the present invention;

Figure 6 is a schematic illustration of a previously proposed cathode ray tube; and

Figure 7 is a diagram for explaining the halo phenomenon.

[0011] Referring to the embodiment of Figure 1, as corresponding parts of Figure 6 are designated by the same reference numerals, their detailed description will not be repeated.

[0012] In the embodiment, there is provided a disc-like secondary electron control electrode SG between the limiting aperture LA and the electrode G3. The potential on the axis in the vicinity of the control electrode SG, for example, at a centre point P of an aperture of the electrode SG, is lower than that of the limiting aperture LA by several tens of volts, for example, 50 V or more, and the voltage is applied to the control electrode SG.

[0013] For instance, assuming that the voltage EK of the cathode K is 0 V; the voltage EG1 of the first grid electrode Gl is approximately -20 V; the voltage EG2 of the second grid electrode G2 is approximately 400 V; the voltage ELA of the limiting aperture LA is approximately 400 V; the voltage EG3 of the third grid electrode G3 is approximately 1.5 kV; the voltage EG4 of the fourth grid electrode G4 is approximately 6 kV; and the voltage Ep of the fluorescent screen 1 is approximately 6 kV, the voltage of the control electrode SG becomes 0 to 300 V. The range of voltages results from differences in the axial potential in the vicinity of the control electrode SG, dependent upon the position of the control electrode SG and the diameter of the aperture of the control electrode SG.

[0014] Figure 2 is a schematic graph showing distribution of the axial potential, and Figure 3 shows part of the structure of the embodiment of Figure 1. The aperture diameter Ø₁ of the electrode Gl is 0.2 mm; the aperture diameter Ø₂ of the electrode G2 is 0.25 mm; the diameter of the limiting aperture LA is 0.1 mm; the aperture diameter Ø₅G of the control electrode SG is 1.5 mm; the distance d_{(K -} Gl) between the cathode K and the electrode Gl is 0.1 mm; the distance d_(G1 - _G2) between the electrodes Gl and G2 is 0.25 mm; the distance d_{(G2 - LA)} between the electrode G2 and the limiting aperture LA is 0.55 mm; the distance d_{(LA - SG)} between the limiting aperture LA and the control electrode SG is 0.2 mm; and the distance d_{(SG - G3)} between the control electrode SG and the electrode G3 is 0.2 mm. The voltage EK of the cathode K is 0 V; the voltage EG1 of the electrode Gl is -25 V; the voltage EG2 of the electrode G2 is 550 V; the voltage ELA of the limiting aperture LA is 550 V; the voltage ESG of the control electrode SG is 0 V; and the voltage EG3 of the electrode G3 is 1.6 kV.

[0015] As the axial potential in the vicinity of the control electrode SG arranged at the rear stage of the limiting aperture LA is lower than the potential of the limiting aperture LA by several tens of volts, secondary electrons e' generated by the electron beam Bml from the cathode K striking the limiting aperture LA are forced back towards the limiting aperture LA as shown, and are thereby prevented from reach the fluorescent screen 1.

[0016] Accordingly, the halo phenomenon due to the secondary electrons e' may be prevented. Therefore, the beam spot diameter on the fluorescent screen 1 may be made small.

[0017] Figures 4 and 5 show modified embodiments of the present invention.

[0018] Referring to Figure 4, the second electron control electrode SG is cylindrical, and the other members are constituted in the same manner as in Figure 1 to show the same operation and effect.

[0019] Referring to Figure 5, in this embodiment the secondary electron control electrode SG is not specifically provided, but the voltage of the electrode G3 is appropriately selected to obtain the same operation and effect. For instance, the voltages of the components may be as follows:

(1) EK is 0 V; EG1 is -30 V; EG2 is 400 V; ELA is 1000 V; EG3 is 800 V; and EG4 is 6 kV

(2) EK is 0 V; EG1 is -30 V; EG2 is 400 V; ELA is 400 V; EG3 is 200 V; and EG4 is 6 kV

[0020] In these cases, since the axial potential between the limiting aperture LA and the electrode G3 arranged at the rear stage of the limiting aperture LA is lower than the potential of the limiting aperture LA, the secondary electrons e' generated from the limiting aperture LA are forced back to the limiting aperture side, and are thereby prevented from reaching the fluorescent screen 1.

Claims

1. A cathode ray tube including an electron gun with a limiting aperture (LA);
characterized in that:
an axial potential between said limiting aperture (LA) and an electrode (SG; G3) arranged at the rear stage of said limiting aperture (LA) is lower than the potential of said limiting aperture (LA) by several tens of volts.

2. A cathode ray tube according to claim 1 wherein said electrode (SG) arranged at the rear stage of said limiting aperture (LA) is a disc-like secondary electron control electrode (SG).

3. A cathode ray tube according to claim 1 and claim 2 wherein said several tens of volts are at least 50 V.

4. A cathode ray tube according to claim 1 wherein said electrode (SG) arranged at the rear stage of said limiting aperture (LA) is a cylindrical secondary electron control electrode (SG).

5. A cathode ray tube according to claim 1 wherein said electrode (G3) arranged at the rear stage of said limiting aperture (LA) is a grid electrode (G3).

Drawing