JPH0131261B2 - - 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. In this image pickup tube, as shown in FIG. It is composed of a conductive film attached to the circumferential surface. 6 indicates a focusing coil, and 7 indicates an alignment coil. The electron gun 2 includes a cathode K and a first grid electrode, as shown in FIG.
_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 configured as described above, the electron beam 9 emitted from the electron gun 2 is horizontally and vertically deflected by the electric field formed by the electrostatic deflection electrode 5, and is deflected onto the face plate 3 of the tube body 1. The target 4 placed on the inner surface is scanned.

When the scanning area of the electron beam on the target 4 is shown as area A in FIG. 2, a false signal occurs in area B outside area A. That is, the so-called forward electron beam 9 emitted from the electron gun 2 scans the region A of the target 4, but some of the electrons are pushed back toward the electron gun 2 as shown by the broken line 10. This becomes the so-called return beam. and,
This return beam 10 mainly hits the electrode plate 9 of the second grid electrode G ₂ , and a part of it hits the broken line 1 again.
Head towards target 4 as shown at 1. The energy of this beam 11 is small, and on the other hand, in the scanning area A of the beam on the target 4,
The cathode potential is increased by the discharge caused by the scanning of the beam 9.
Since the voltage is maintained close to 0V, the beam 11 is not incident on this region A, but in the region B outside the scanning region A, the voltage is close to the target applied voltage because the beam 9 is not scanned. Since it is in a high potential state of about several tens of volts, the beam 10 is incident here, 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 beams, 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 going beam 9 and the returning beam 10 shown in FIG.
Furthermore, paying attention to the fact that the deflection sensitivities of the beams 11 directed toward the target 4 due to the return beam 10 are basically equal, an electrode plate portion is arranged such that the return beam 10 faces the target 4 of the electron gun 2;
In the example shown in FIG. 2, a limiting aperture electrode plate 8 is provided at the rear end of the cylindrical portion of the second grid electrode _G2 so as to close the second grid electrode G2. It was confirmed that the return to region B is caused by the return beam 10 hitting the electrode plate 8 at a point where the width of the electrode plate 8 is 2/3Wt or more, where the width of region A is Wt, and based on this,
At least the return beam 10 directed toward this position is deflected outward so that it avoids impinging on the electrode plate 8 or is directed at least to a region other than region B of the target 4.

In other words, in order to prevent the return beam 10 from hitting the electrode plate 8 at 2/3 Wt or more, the width of the electrode plate 8 should be kept to less than 2/3 Wt, but depending on the mechanical strength of the electrode, etc. The present invention is suitable for application when this width cannot be made sufficiently small due to problems, and in the present invention, means is provided for making the deflection sensitivities for the forward beam and the return beam different.

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, between the electron gun 2 and the target 4, that is, after the electrode plate 8 of the electrode _G2 , the length l is, for example, 0.05L to 0.4L (L is the aperture a).
A cylindrical auxiliary electrode 13 having a diameter φ of 2/3 Wt or less is provided. The auxiliary electrode 13 is, for example, a grid electrode.
G ₂ , therefore, the average voltage applied to the electrostatic deflection electrode 5 which is electrically connected to the aperture electrode plate 8 and arranged outside thereof is, for example, 400 V.
Auxiliary electrode 13 as a lower voltage e.g. 300V
An electrostatic deflection electric field is formed between the electrostatic deflection electrode 5 and the electrostatic deflection electrode 5. In this way, the forward beam 9 passes through the cylindrical auxiliary electrode 13 and is not deflected by it, but the return beam 10 passes through the radial static region between the auxiliary electrode 13 and the deflection electrode 5. Since it passes through an electric deflection electric field, it is deflected outward. Note that if the length l of the auxiliary electrode 13 is too short, sufficient deflection cannot be given to the returning beam.
If it is too long, the original deflection by the electrostatic deflection electrode 5 will be inhibited. This length l shall give the electrode 13 the voltage normally applied to the second grid electrode G ₂ and its limiting aperture electrode plate 8, while the average voltage of the electrostatic deflection electrode 5 is also the voltage normally applied. , it has been found that if the return beam is deflected by the electric field generated between them, it is desirable to select 0.05Ll0.4L.

Now, when making the outer diameter W of the electrode _G2 equal to the width Wt of the electron beam scanning area A of the target 4,
= 0.15L, with an auxiliary electrode 13 having a diameter φ of 1/3Wt,
When this voltage was set to the same potential as electrode _G2 and the potential difference with electrode 5 was set to 50V, the returned beam image could be completely eliminated.

In this example, an auxiliary electrode 13 is provided,
This is a case where the deflection sensitivities are made different for the forward beam and the return beam, as shown in FIG.
It is also possible to arrange the focusing coil 6 to be biased towards the target 4 side. In this case, the length of the focusing coil 6
If lc 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 0.5L≦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 addition, as a means for differentiating the deflection sensitivity for the forward beam and the return beam, as shown in FIG. 5, both the aforementioned auxiliary electrode 13 and the means using the focusing coil 6 explained in FIG. Depending on the situation, a material layer or a surface treatment layer 12 may be disposed on at least the outer peripheral surface of the electrode plate 8 facing the target 4 to prevent the return beam 10 from being absorbed or scattered and directed toward the region B of the target 4. can.

This material layer or surface treatment layer 12 can be formed by, for example, depositing an insulating material such as glass or ceramic, depositing a semi-insulating material such as carbon powder or aquadac, or depositing an electrode plate 8.
It can also be formed by roughening the surface itself.

In each illustrated example, the outer diameter of the electrode plate ₈ having the limiting aperture of the electrode _G2 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 to 5 are respective views 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)

[Scope of Claims] 1. In an electromagnetic focusing/electrostatic deflection type image pickup tube, a cylindrical auxiliary electrode is provided between the electron gun and the target at a later stage of the electron gun, and a cylindrical auxiliary electrode is provided between the auxiliary electrode and the electrostatic deflection electrode. An image pickup tube in which the deflection sensitivities of the forward and return beams toward the target are made different by the electrostatic deflection electric field that is formed.