JPH0318295B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an image pickup tube, and particularly to a vidicon type image pickup tube.

In general, vidicon-type image pickup tubes equipped with photoconductive targets suffer from capacitive lag (or
beam discharge lag). This capacitive afterimage causes a trailing phenomenon in the image when a bright subject moves, impairing the dynamic resolution, and therefore needs to be reduced as much as possible. To this end, it is desirable to reduce the capacitance of the target and to reduce the axial velocity dispersion of the electrons in the electron beam reaching the target, that is, to lower the electron temperature of the electron beam as much as possible. Here, the afterimage refers to a residual signal expressed three fields after the incident light is stopped, expressed as a percentage of the initial value.

On the other hand, in order to improve the resolution characteristics of the vidicon type image pickup tube shown in FIG. 1, it is desirable to form an object point with as high a current density as possible and project it onto a target with a low magnification electron lens. For this reason, the electron beam emitted from the cathode 1 is strongly focused at the triode, forming a crossover with high current density, and a beam disk (reflection disk) with a hole diameter of, for example, about 20 to 30 μm is placed near the crossover. In many cases, a focusing aperture (2) is placed and the aperture is used as an object point to project onto a target (3).

In FIG. 1, 4, 5 and 6 are first, second and third grids, respectively, and 7 is a mesh. Further, 8, 9, and 10 are a deflection coil, a focus coil, and an alignment coil, respectively, and 11 is a heater.

The beam disk 2 is used to set the incident angle of the electron beam when it enters the main lens system within an appropriate range depending on the positional relationship with the crossover. Only a small portion of the beam can pass through this beam disk 2. If we take the crossover made up only of the electron beam that has passed through this as an object point, the crossover is seen through the narrow hole in beam disk 2, so the apparent crossover is smaller than the crossover actually formed by the full current. It is used as a method to achieve high-resolution characteristics because it is possible to determine the overflow.

In conventional vidicon-type image pickup tubes that form object points in this way, the current density of the electron beam at the object point or crossover is extremely high, reaching 10 A/cm ²
It often happens that more than that is reached. Therefore,
In this high current density part of the electron beam, energy is exchanged through collisions between electrons.
The velocity dispersion increases and the electron temperature increases. For example, in a vidicon using an ordinary oxide cathode with a cathode operating temperature of 1080°C, the electron temperature in the electron beam reaching the target increases to 4000-6500°C, and it is known that the afterimage characteristics deteriorate. .

From the above, in the vidicon type image pickup tube,
It is difficult to achieve both high resolution characteristics and low afterimage characteristics because mutually contradictory factors are involved.

In view of this, an object of the present invention is to provide an image pickup tube with low afterimage and high resolution.

In order to achieve this objective, the present invention is characterized in that a beam disk is placed at the position of the cathode image.

That is, since the diameter of the electron beam on the target is dominated by the so-called Langmuir condition, all the electrons passing through the beam disk can be limited to only the electrons emitted from a specific point on the cathode. Moreover, if all the electrons emitted from this specific point pass through the beam disk, the loading of the electron beam on the cathode ρc will not be impaired at all due to the existence of the beam disk, and the beam passing through the beam disk will be The effective cathode load can be maximized.

Incidentally, the electrons emitted from the cathode are focused by the lens action of the electric field created by the first grid and the second grid, creating a beam stream as shown in FIG. The group of electrons that shoot out in all directions with thermal velocity dispersion from a "single point" on the cathode becomes a Gaussian beam group (Gaussian beamlet) with a certain spread, which is brought back to a single point by the above-mentioned lens action. Focus. This position is the position of the cathode image as shown in FIG. The electron beam emitted from the cathode is a collection of beamlets emitted from each point on the entire electron emitting surface of the cathode, and the position where the diameter of the entire electron beam is minimum is called the crossover, where the current density is maximum. . In the cathode image, an image of the cathode is formed according to the lens magnification of the triode section. Therefore, in the present invention, a beam disk is placed at the position of this cathode image so that all of the Gaussian beamlets due to thermal velocity emitted from the center of the cathode can pass through, thereby increasing the effective cathode load of the passing electron beam. .

Most conventional vidicon type image pickup tubes have a structure that allows only 20 to 30% of the electrons to pass through the beamlet. Therefore, according to the configuration of the present invention, the cathode current during operation is reduced to 1/1 of the value according to the conventional design.
When the ratio is 3 to 1/5, the effective cathode loading by the electron beam after passing through the beam disk can be made equal, and the ratio of the electron beam passing through the beam disk to the cathode current can be increased. In fact, by doing this, we were able to reduce the current from 200 μA to about 50 μA, reducing the operating current to 1/5.

Thus, reducing the cathode current during operation can thereby also reduce the crossover current density, thereby reducing the electron temperature of the electron beam reaching the target. Therefore, a vidicon-type image pickup tube with low afterimage characteristics can be realized without impairing resolution characteristics. The present invention will be explained in detail below.

Generally, in the triode section of an image pickup tube electron gun, the initial velocity distribution of thermionic electrons emitted from the hot cathode (thermal velocity dispersion) and the space charge effect due to the repulsion of charges acting between electrons coexist, so that normal electron optics is It is not possible to determine the crossover position, current density, or cathode image position or magnification using standard methods. This is because in a normal oxide cathode, which draws electron flow from the cathode under space charge limitation,
The axial potential gradient E _z = dV/dz in front of the cathode becomes zero near the cathode, and d ² V/dz ² diverges, so the effect as an electro-optical lens cannot be obtained, and the space charge This is because the cathode image itself becomes blurred due to the effect.

However, directly heated cathodes can be used with limited temperature, making it possible to limit the potential gradient near the cathode. In this case, the lens effect can be calculated, making it possible to calculate the position, magnification, etc. of the cathode image. It becomes possible.

When converging an electron beam emitted from a cathode into a small diameter, the diameter is usually limited by the following three factors.

In other words, it is limited by the initial thermal velocity of electrons. This effect is called Langmuir's condition and is expressed by the formula ρ _s =ρ _c (1+eV/kT) sin ² θ.

Here, ρ _s is the current density of the focused beam, ρ _c is the current density at the cathode, e and k are physical constants, T is the cathode temperature, and V is the target voltage.

Therefore, the larger the current density at the cathode, the smaller the diameter of the electron beam for the same current/voltage and the same convergence angle θ.

Limited by space charge effects. This is a phenomenon in which the electrons in an electron beam repel each other due to electric charge, causing the beam to expand in diameter.

Limitations due to aberrations of the converging lens system.

Among these factors, in order to achieve high resolution in an image pickup tube, the above factors are dominant, and as a result of studies conducted by the present inventors, it has been found that it is desirable to satisfy the following two conditions. That is, in order to ensure low afterimage characteristics, the crossover current density is made as low as possible. Specifically, it is preferably 10 A/cm ² or less, for example 2 to 5 A/cm ² . On the other hand, in order to maintain high resolution characteristics, it is desirable that the loading of the electron beam passing through the beam disk on the cathode be as large as possible, for example, 0.2 A/cm ² to 0.5 A/cm ² .

The above two conditions were obtained from the following study results. First, the low afterimage characteristic will be explained with reference to FIG. In the figure, the horizontal axis shows the current density at crossover in the electron beam, and the vertical axis shows the afterimage value in the third field (50 ms after the light incident on the target is blocked). The dotted line and solid line on the diagram correspond to signal currents of 50 nA and 200 nA, respectively. As shown in the figure, in any case, as the crossover current density decreases, the afterimage decreases almost proportionally. Current density for signal current 50nA
It can be seen that the afterimage decreases rapidly from around 2A/ ^cm2 . Next, resolution characteristics will be explained.
From Langmuir's conditions, a beam diameter of 15 μm to 20 μm is required to obtain sufficient resolution on a mesh electrode just in front of a 500 volt target, for example. At this time, when calculated for the actual size and shape of a vidicon of 18 mm, the effective cathode loading is 0.2 to 0.3 A/cm ² .

The position of the cathode image is determined by the following method.
First, a Laplace solution is obtained using an electronic computer, and the on-axis potential is approximated as φ=pz ³ +qz. Here, p and q are coefficients for polynomial approximation of the potential in the z direction. Next, a group of electrons (Gaussian beamlets) are emitted from a single point in the center of the cathode in the lateral direction with various initial thermal velocity components.
Find the position on the axis where the diameter of the beamlet) is the minimum. FIG. 3 shows the position of the cathode image obtained in this manner and the change in the diameter of the Gaussian beamlet emitted from the center of the cathode depending on the axial position. The positional relationship of the electrodes constituting the triode part is as shown in the upper part of the figure.
0.47mm, 2.1mm, first and second
The diameter of the grid is 0.65 mm, and the diameter of the beam disk is 50 μm. The voltage applied to each electrode is -100 to 0V for the first grid 4, with the potential of the cathode 1 being 0V;
The second grid 5 and beam disc 2 are at 300V.

As shown in FIG. 3, in the present invention, by placing the beam disk at the position of the cathode image during operation, it was possible to realize an image pickup tube with low afterimage and high resolution.

In Fig. 3, the solid line indicates the cathode current I _K
The graph shows the change in the Gaussian beamlet diameter when 58 μA and double beam setting (applied voltage of the first grid is -70 V), and the curve shown by the dashed line shows the cathode current.
The curve shown by the dashed line shows the change in the Gaussian beamlet diameter when I _K is 140 μA and the quadrupled beam setting (the applied voltage of the first grid is -59 V) _.
shows the Gaussian beamlet diameter when is set to 64μA and just beam. The positions indicated by arrows in the figure are the positions of the crossover and cathode images, respectively. As shown in the figure, the position of the cathode image when I _K =64 μA is 2.1 mm from the cathode. In the present invention, the beam disk 2 is placed at the position of this cathode image.

Furthermore, as shown in Fig. 3, when a beam disk with a radius of 25 μm or more is used, the effective cathode loading of the passing electron current to the center of the cathode is reduced when the transmittance of the central Gaussian beamlet is 60% or more, for example, when the beam is doubled. 98%, and it is possible to make the effective cathode loading equal to the cathode loading.

Note that in order to operate the image pickup tube, the beam current must be adjusted from the just beam value, which is equal to the output current, so that the peak of the output signal current, that is, the signal current for the brightest part of the screen, will not cause a beam shortage.
Since a 1.5 beam or even a 4x beam setting is often used outdoors, it goes without saying that the configuration should be such that a cathode image is formed on the beam disk within this range.

Figure 4 shows the TV that can be identified on the monitor on the horizontal axis.
The number of beams and the (resolution) amplitude characteristics are plotted on the vertical axis, which shows the resolution characteristics when the beam current is 600 nA. In the figure, the black circles are for the image pickup tube according to the present invention, and the white circles are for the conventional image pickup tube, and the resolution characteristics of both are almost the same.

FIG. 5 shows the bias write current on the horizontal axis and the afterimage of the third field on the vertical axis, and shows the afterimage characteristics of the image pickup tube of the present invention and the conventional image pickup tube when the beam current is 600 nA. As shown, signal current 50nA
When , the afterimage decreases as shown by curve A to B, and when the signal current is 200 nA, it decreases as shown by curve A' to B'. Specifically, the bias write current is
When the signal current is 10nA and the signal current is 50nA, the conventional afterimage is about 5
%, but according to the present invention, it has been reduced to 2%, and with a signal current of 200 nA, the afterimage was conventionally about 1.5%.
However, according to the present invention, the amount is reduced to about 1%. Practically speaking, it is desirable for the afterimage to be 2% or less, but according to the present invention, this can be achieved even with a signal current of 50 nA.

Therefore, according to the present invention, it is possible to provide an image pickup tube in which the afterimage is reduced to approximately 1/2 and the resolution characteristics do not deteriorate.

[Brief explanation of the drawing]

Figure 1 is a diagram showing the outline of the structure of a conventional vidicon type image pickup tube, Figure 2 is a diagram showing the relationship between crossover current density and afterimage, and Figure 3 is a diagram showing the change in the diameter of the electron group depending on the axial position. 4 is a diagram showing resolution characteristics according to the present invention, FIG. 5 is a diagram showing afterimage characteristics according to the present invention, and FIG. 6 is a diagram for explaining the positional relationship between the crossover and the cathode image.

Claims (1)

[Claims] 1. In a vidicon type image pickup tube consisting of a triode section consisting of a cathode, a first grid, and a second grid, and an electron gun having a beam disk, the beam disk controls the position of the cathode image created by the electron beam of the electron gun. A vidicon-type image pickup tube characterized in that it is arranged in a.