JPH0139622B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an image pickup tube, and particularly to an image pickup tube using an electromagnetic focusing/electrostatic deflection type electron optical system.

Image pickup tubes using an electromagnetic focusing-electrostatic deflection type electron optical system (hereinafter referred to as MF type image pickup tubes) have, in principle, no focusing aberration, good deflection linearity, and vertical landing of the electron beam. This type of MF type image pickup tube has come to be widely used for this reason. However, this species
Even in MF type image pickup tubes, a false signal called a return beam image usually occurs. To explain this returned beam image, first, a normal MF type image pickup tube will be explained. As shown in FIG. 1, in this image pickup tube, an electron gun 2 is disposed within a tube body 1, and an electrostatic deflection electrode 5 for horizontal and vertical deflection of the electron beam from the electron gun 2 is provided on the inner circumference of the tube body 1, for example. It consists of a conductive film deposited on the surface. 6 indicates a focusing coil, and 7 indicates an alignment coil. For example, as shown in FIG. 2, the electron gun 2 has a cathode K and a first grid electrode _G1.
and a second grid electrode _G2 , and the second grid electrode _G2 is provided with an electrode plate 8 having a limiting aperture a in a plane substantially perpendicular to the tube axis.

In the image pickup tube with such a configuration, the electron gun 2
The emitted electron beam 9 is horizontally and vertically deflected by the electric field formed by the electrostatic deflection electrode 5 so as to scan the target 4 disposed on the inner surface of the face plate 3 of the tube body 1. It will be done. When the scanning area of the electron beam on the target 4 is shown as area A in FIG. 2, false signals occur in area B outside area A. That is, the so-called forward electron beam 9 emitted from the electron gun 2 hits the area A of the target 4.
Some of the electrons are pushed back toward the electron gun 2 as shown by the broken line 10, and this becomes a so-called return beam. Then, this return beam 10 mainly hits the electrode plate 8 of the second grid electrode G ₂ , and a part of it again heads toward the target 4 as shown by the broken line 11 . The energy of this beam 11 is small, and on the other hand, in the beam scanning area A of the target 4, the cathode potential is maintained close to 0V due to the discharge caused by the scanning of the beam 9. However, since the beam 9 is not scanned in the region B outside the scanning region A, the potential of the beam 9 is at a high potential of several tens of volts, which is close to the target applied voltage. enters the beam, and this is read out as a false signal, resulting in a so-called return beam image. Reference numeral 22 indicates a mesh electrode placed opposite the target 4.

The present invention makes it possible to effectively avoid the generation of false signals due to the above-mentioned return beam, especially in MF type image pickup tubes.

That is, in the present invention, the deflection sensitivity of each returning beam in the MF type image pickup tube, that is, the second
Focusing on the fact that the deflection sensitivities of the forward beam 9 shown in the figure, the return beam 10, and the beam 11 directed toward the target 4 by the return beam 10 are basically equal, it is assumed that the return beam 10 is a second electrode plate portion disposed to face the second electrode plate portion;
In the example shown in the figure, the target 4 hits the limiting aperture electrode plate 8 provided at the rear end of the cylindrical part of the second grid electrode _G2 so as to close it.
In particular, what is returned to area B other than area A is when the width of electrode plate 8 is 2/3 when the width of area A is Wt.
It is confirmed that this is caused by the return beam 10 hitting a point above Wt, and based on this, the return beam 10 is directed toward a position above 2/3 Wt of the position where the electrode plate 8 is arranged, at least in areas other than area B of the target 4. to encourage people to

An example of the present invention will be described with reference to FIG.
In FIG. 3, parts corresponding to those in FIG. 2 are designated by the same reference numerals, and redundant explanation will be omitted. In this example, an electrode plate part that constitutes the electron gun 2 and is arranged to face the target 4, in the illustrated example, the rear part of the electrode plate 8 of the second grid electrode _G2 has a length l, for example. A cylindrical auxiliary electrode 13 having a diameter of 0.05L to 0.4L (where L is the distance between the aperture a and the target 4) and an outer diameter of 2/3 Wt or less is provided. The auxiliary electrode 13 is, for example, 400 V higher than the average voltage applied to the grid electrode G ₂ and therefore to the electrostatic deflection electrode 5 which is electrically connected to and arranged outside the aperture electrode plate 8. A low voltage of, for example, 300 V is applied to form an electrostatic deflection electric field between the auxiliary electrode 13 and the electrostatic deflection electrode 5, and the return beam is returned in a direction that produces a beam 10 which is particularly directed towards the region B of the target 4. The return beam 9 is deflected outward so that it travels further away from the electrode plate 8 of electrode _G2 . Here, the length l of the auxiliary electrode 13 is:
If this is too short, sufficient deflection cannot be given to the returning beam, and if it is too long, the original deflection by the electrostatic deflection electrode 5 will be inhibited. This length l shall provide the electrode 13 with the voltage normally applied to the second grid electrode G ₂ and its limiting aperture electrode plate 8, while the electrostatic deflection electrode 5
It has been found that it is desirable to select the average voltage of 0.05Ll0.4L, assuming that the return beam is deflected by the electric field generated between the two, assuming that the average voltage is also a normally given voltage.

In the example shown in FIG. 4, an auxiliary electrode 13 is provided to make the deflection sensitivity different for the forward beam and the return beam, and the focusing coil 6 is placed biased toward the target 4 side. In this case, the length lc of the focusing coil 6 is such that if it is too long, it will not be able to give sufficient deflection to the returning beam, and if it is too short, the original focusing effect will be insufficient. It was found that it is desirable to select 0.05L≦lc<1.0L.

Note that when lc=0.8L, the deflection sensitivity for the forward beam and the deflection sensitivity for the return beam are 1:1.4.

In FIGS. 3 and 4, reference numeral 12 denotes a material layer provided around the electrode plate 8 for absorbing or scattering the return beam 10, so that the return beam impacts the periphery of the electrode plate 8 and returns to the target. This will prevent you from moving towards 4.

In addition, in each illustrated example, the outer diameter of the electrode plate 8 having the limiting aperture of _the electrode G ₂ is
Although the case is shown in which the diameter is larger than the diameter of the cylindrical portion of the electrode _G2 , it is also possible to have a structure having the same diameter as the diameter of the cylindrical portion of the electrode G2. Further, the configuration of the electron gun 2 can take various configurations, and it is needless to say that the electrode plate facing the target 4 is not limited to an electrode plate having a limiting aperture. Dew.

[Brief explanation of drawings]

FIG. 1 is a side view of an electromagnetic focusing/electrostatic deflection type image pickup tube used for explaining the present invention, FIG. 2 is a configuration diagram of its main parts, and FIGS. 3 and 4 are each of the image pickup tube according to the present invention. It is a block diagram of the main part of an example. 1 is a tube body, 2 is an electron gun, 3 is a face plate, 4 is a target, 5 is an electrostatic deflection electrode, 6 is a focusing coil, G ₁ and G ₂ are first and second grids,
8 is a limiting aperture electrode plate, 12 is an electron absorption or scattering layer, and 13 is an auxiliary electrode.

Claims (1)

[Claims] 1. Electromagnetic focusing - The electron gun of an electrostatic deflection type image pickup tube has a cylindrical auxiliary electrode at the rear stage of the electrode plate portion facing the target, and forms an electrostatic deflection electric field between it and the electrostatic deflection electrode. The image pickup tube is configured such that a return beam from the target directed to a position of 2/3 or more of the width of the electron beam scanning area of the target on the electrode plate portion does not return to the target again.