The present invention relates to a color display tube comprising an electron gun having a beam-forming part for generating a plurality of electron beams and a main lens for focussing said electron beams; a display screen for generating an image, said screen being provided with phosphor tracks flanked by index elements for controlling a position of an electron beam relative to a corresponding phosphor track; first electron lens means acting on the electron beams in a direction perpendicular to the phosphor tracks, and second electron lens means acting on the electron beams in said direction perpendicular to the phosphor tracks, said second electron lens means having opposite action as said first electron lens means, and comprising a magnetic quadrupole between the main lens and the display screen.

Cathode ray tubes (CRTs) conventionally comprise an electron gun that creates an electron beam and a display screen that displays an image when the electron beam is scanned over the screen. The electrons are emitted from a cathode and transported through the tube towards the display screen. The electron beam forms a spot on the screen when hitting the screen.

The electron gun comprises a main lens for focussing the electron beam on the screen. The display screen is provided with phosphor elements that emit light when hit by the beam. The electron beam has to be controlled so that it hits the screen correctly in order to have the correct image displayed by the screen.

In particular, color display devices traditionally comprise an electron gun that generates a plurality of electron beams, usually three electron beams in line. Each beam then represents a specific primary color (red, green and blue) and impinges on a phosphor element matching that color. Color selection means are required to ensure that each beam lands on its corresponding phosphor color, and thus to obtain color purity of the displayed image.

In conventional CRTs a shadow mask is used for proper color selection for the three electron beams. When the beam is deflected towards the corners of the screen, the spot shape of the beam on the screen becomes distorted with an enlarged horizontal axis. The shadow mask prohibits the beam from hitting the wrong phosphor elements, but the beam enlargement in the horizontal direction results in a loss of resolution in this direction.

Although use of the shadow mask is a reliable method for color selection, there are several drawbacks. Since the shadow mask blocks typically 80 % of the electron beam, a high beam current is needed to obtain the prescribed brightness. This high beam current causes problems of local heating of the mask. The high beam current also results in large space-charge effects due to the electrons in the beam repelling each other, which limits the electron-optical performance. The most important drawback of the shadow mask, however, is that it is an expensive part of the CRT.

For these reasons a new type of CRT has been developed, called the beam- index tube. This tube does not have a shadow mask, thus solving the problems associated with the shadow mask. However, proper color selection now has to be ensured in another way. In the beam-index tube, the phosphor elements are arranged in tracks on the screen.

Each phosphor track on the screen is flanked by index elements that act as detectors for the position of an electron beam relative to its corresponding phosphor track.

The index elements produce a signal that is dependent on the position of the beam relative to the phosphor track in the direction perpendicular to the phosphor track. By inserting this position dependent signal into a feedback loop, which is able to actively steer the beam, the relative position of the electron beam can be corrected. Thus, the electron beam is made to keep on the phosphor track while scanning the track.

An embodiment of a beam-index tube is known from Japanese Kokai 08-212947. This cathode ray tube has an electron gun for generating a single electron beam.

The electron beam is deflectable so as to sequentially scan the phosphor tracks corresponding to the different primary colors red, green and blue.

The index elements ensure that, during scanning, the beam is aligned with the phosphor tracks as well as possible. Since the phosphor lines are positioned in the conventional order, i. e. red, green and blue, and there is no shadow mask to block the signal from the wrong gun, the requirements on the spot size of the electron beam on the screen are high.

In order to obtain color purity, the size of the spot in the direction perpendicular to the direction of the phosphor tracks should not exceed the width of the

phosphor track. Otherwise, the beam may partially land on a phosphor track corresponding to a different color, even if the positioning of the electron beam on the phosphor track is in itself correct. This is unacceptable, for in this case the colors of the image can't be controlled.

Therefore, the spot size of the beam needs to be so small that an adjacent phosphor track is not hit by any part of the electron beam. For this purpose, the opening angle of the electron beam near the display screen has to be relatively high. A large opening angle reduces space-charge effects within the beam, so that mutual repulsion of electrons in the beam is reduced.

In the known beam-index tube, a first quadrupole (Qpl) is arranged between the cathode and the main lens of the electron gun. This first quadrupole focuses the electron beam in the direction perpendicular to the phosphor tracks. A second quadrupole (Qp2) is arranged between the main lens and the display screen, which quadrupole focuses the electron beam in the direction perpendicular to the phosphor track. The action of the second quadrupole is such, that the action of the first quadrupole is substantially compensated for.

By applying these quadrupoles, the opening angle of the electron beam near the display screen can be increased. The first quadrupole focuses the beam, so that it has a relatively large diameter in the second quadrupole. The second quadrupole focuses the electron beam onto the display screen. Thereby, a smaller spot size in the direction perpendicular to the phosphor tracks is obtained. The color purity of the beam-index type cathode ray tube is improved.

However, it is a problem that, when a multi-beam electron gun generating a plurality of electron beams is used in a beam-index type CRT as previously described, satisfactory color purity is still not obtained over the entire display screen.

It is an object of the invention to provide a color display tube of the type as described in the opening paragraph, having improved color purity.

The object of the invention is achieved by means of a color display tube according to the independent claim 1. Preferred embodiments of the color display tube according to the invention can be derived from the dependent claims 2-8.

Thus, a color display tube according to the invention is characterized in that the first electron lens means is arranged for influencing the convergence of the electron beams and positioned between the main lens and the second electron lens means.

The second electron lens means influence a convergence of the electron beams, i. e. the mutual angle between the electron beams is altered. If this change is not corrected for, the electron beams do not land substantially at the same location of the display screen, so that color purity is negatively affected.

It is therefore required that the first electron lens means also influences the beam convergence, The first electron lens means have opposite action as the second electron lens means, so that, for instance, the first electron lens means converge the beams (i. e. decrease their mutual angle) and the second electron lens means diverge the beams (i. e. increase their mutual angle).

If the first electron lens means would be positioned between the beam-forming part and the main lens, as is the case in the prior art, at least some of the electron beams pass the main lens substantially off-axis and/or fill the main lens insufficiently. This leads to a deterioration in spot size and/or shape for these electron beams, which negatively influences image quality and/or color purity.

Therefore, the first electron lens means is positioned between the main lens and the second electron lens means. Thereby, the convergence of the electron beams on the display screen is ensured, while the filling of the main lens is good, so that a sufficiently small spot of the electron beam is obtained. The displayed image has a relatively high quality and relatively good color purity.

Generally, it is desirable to space the electron lens means apart as far as possible, so that a relatively large action on the electron beams is obtained using relatively low drive voltages/currents.

Preferably, the first and second electron lens means comprise magnetic quadrupoles, for instance 45-degree 4px quadrupoles. Magnetic quadrupoles have the desired focusing/defocusing lens action in the direction perpendicular to the phosphor tracks, whereby they influence the convergence of the beams.

The first and second electron lens means may be dynamically adjustable in dependence of a landing position of the electron beam on the display screen.

Usually, the electron beams are deflected over the entire display screen by a self-convergent magnetic deflection unit, in order to display an image. When the beam is deflected towards the edges and/or the corners of the screen, the deflection unit forms a deflection lens focusing the electron beams in the direction perpendicular to the phosphor tracks. The spots of the electron beams thereby become more elliptically shaped as the beams

land closer to the edges of the display screen, the long axis of the ellipse being in the direction parallel to the phosphor tracks.

By varying the strength of the first and second electron lens means dynamically, depending on the landing position of the electron beams on the screen, the effect of the deflection lens is counterbalanced, and spot uniformity over the screen is improved. A desired spot shape can be better maintained over the entire display screen.

Preferably, the first and second electron lens means are dynamically adjustable essentially at line frequency. The ellipticity of the spot is strongest in the direction of the phosphor tracks, i. e. the line-frequent scanning direction. Therefore, compensation of the ellipticity should for a large part be carried out in this direction.

Good color purity requires all electron beams to land substantially at the same location on the display screen, so that the different beams have a relatively small mutual distance near the display screen. Such a small distance causes, apart from the effect of space- charge repulsion of electrons within each individual beam, a significant mutual repulsion between the different beams. This influences the correct landing of the electron beams on their corresponding phosphor color and affects color purity.

This may be counterbalanced by increasing the mutual angle with which the electron beams land on the display screen, and thus increasing the virtual gun pitch between the electron beams. The virtual gun pitch is the mutual distance between the electron beams at the location of the deflection unit. By increasing the mutual angle near the display screen, electron beam repulsion is reduced and spot performance and color purity are improved.

Increasing the virtual gun pitch using the two electron lens means requires the first lens to converge the beams, and the second lens to diverge the beams. However, this implies that the opening angle of each separate beam is decreased, so that the space-charge repulsion within each beam is stronger. This causes spot growth in the direction perpendicular to the phosphor tracks and reduced color purity, which is undesired in a beam- index type CRT. The dimension of the spot in the direction perpendicular to the phosphor tracks should be as small as possible.

Thus, when applying the two electron lens means, these two effects have to be balanced as well as possible.

In a preferred embodiment, the display tube is characterized in that the second electron lens means focus the electron beams in the direction perpendicular to the phosphor tracks, at least when the electron beams land, in operation, in the central portion of the display screen.

It has been found that the best spot performance in the central portion of the display screen is obtained by having the lenses decrease the virtual gun pitch and increase the opening angle of the individual electron beams, thereby shrinking the spot size in the direction perpendicular to the phosphor tracks. This operation mode is similar in nature to the aforementioned Japanese Kokai 08-212947.

As explained before, during dynamical operation the action of the electron lens means may be weakened as the electron beams land closer to the edges of the display screen. Near the edges and/or in the corners, the lenses may even have zero or inversed action.

Preferably, the electron gun comprises an electric quadrupole acting on the electron beams in a direction perpendicular to the phosphor tracks.

The presence of an electric quadrupole allows for a further improvement in the uniformity of the desired spot shape over the entire display screen, since the filling of the two electron lens means can now be made as good as possible. It is easier to keep the electron beam in focus over the entire screen.

In a preferred embodiment, said electric quadrupole is arranged for dynamically compensating astigmatism and focusing of the electron beam occurring in dependence of the landing position of the electron beam on the display screen.

An electron gun with such an electric quadrupole is commonly known in the art as a DAF-type electron gun. The electric quadrupole before the main lens is arranged to cooperate with the main lens to compensate for electron beam defocusing, and with the deflection lens to compensate for astigmatism caused thereby. The electric quadrupole is generally addressable by means of a separate dynamic voltage, the so-called DAF voltage.

In conventional DAF electron guns, it is a problem that at a certain deflection angle of the electron beam, the so-called DAF explosion angle, focusing and astigmatism can no longer be corrected, no matter how large the DAF voltage applied. This is caused by the fact that the electron beam, when traced back from the display screen, crosses the electronoptical main axis of the electron gun at the location of the DAF quadrupole, so that said quadrupole hardly has any lens action on the beam.

In the present invention, the beam path can be changed by focusing and focusing each beam using two magnetic quadrupoles. It has been found that the electron beam has a larger diameter in the DAF quadrupole when traced back from the screen, if the second quadrupole lens has a focusing action in the direction perpendicular to the phosphor tracks. At least part of the electrons in the beam then pass the DAF quadrupole off-axis, so

that the desired lens action of the DAF quadrupole is restored. By applying two magnetic quadrupoles, DAF explosion occurs at a larger value of the deflection angle.

Therefore, preferably, an action of the first and second electron lens means on the electron beams when landing in an edge portion of the display screen is opposite to an action of the first and second electron lens means on the electron beams when landing in a center portion of the display screen.

Near the edges of the display screen, the operation of the quadrupoles is now inversed as compared to the center of the screen. Although the quadrupoles decrease the opening angle of the electron beams in the direction perpendicular to the phosphor tracks, which in itself causes a spot growth, in practice the spot size has been reduced. This is because the deflection lens at least partially compensates for the inversed action of the magnetic quadrupoles, and especially because of the improved functioning of the DAF section of the electron gun.

In general, this embodiment allows for a uniformity in spot size that is as good as possible and good color purity, whereby the required DAF voltage is relatively low and the beams can be deflected over relatively large angles, so that the CRT can have a relatively small depth.

Preferably, the electron gun is of the so-called in-line type, in which three electron beams are generated, which are substantially in line within the electron gun. Each electron beam represents one of the primary colors red, green and blue.

These and other aspects of the invention will be apparent from and elucidated with reference to the appended figures. Herein: Fig. 1 shows schematically a longitudinal section of a preferred embodiment of a color display tube according to the invention; Fig. 2 is an equivalent lens model of the preferred embodiment, when the electron beams land in the central portion of the display screen, and Fig. 3 is an equivalent lens model of the preferred embodiment, when the electron beams land in the corner of the display screen.

In Fig. 1, a color display tube 1 is schematically shown. An electron gun 2 has a beam-forming part that generates three electron beams (not drawn) in one end of the tube.

The beam is generated from electrons that are emitted from a cathode 3.

The electron gun 2 comprises electrodes, Gl, G2, G3a, G3b and G4, to which different voltages are supplied for accelerating and focussing the electron beam. The electrons emitted from the cathode 3 form a cross-over between the electrodes G1 and G2, i. e. the beam is focussed in the cross-over. The electric field between electrodes G3b and G4 forms a main lens 4 for focussing the electron beam onto a display screen 5 at the other end of the tube. The spot of the electron beam on the screen 5 is an image of the beam cross-over.

A magnetic deflection unit 6 is arranged in the tube for deflecting the beam across the screen 5. The deflection unit 6 is implemented as a combined structure of two deflection coils, surrounded by a core, for deflecting the beam in the horizontal and vertical directions. The deflection unit 6 forms, in operation, a deflection lens causing astigmatism in the electron beams. When the electron beams are deflected to the edges and corners of the screen 6, the distance between the main lens and the landing position of the beams increases which focuses the electron beams.

Therefore, a so-called dynamic-astigmatism-and-focus (DAF) section 7 is arranged in the electron gun 2. The DAF section 7 is controlled by means of a dynamic DAF voltage and compensates for astigmatism and focusing of the electron beams.

The display screen 5 comprises horizontal phosphor tracks that are arranged in groups of three different color tracks, i. e. red phosphor tracks 1 OR, green phosphor tracks lOG and blue phosphor tracks lOB. Two index elements 11 flank each phosphor track l OR, 1OG, 1OB. The index elements 11 comprise either tracking electrodes generating a voltage difference indicating the position of the electron beam relative to its corresponding phosphor track, or separate tracking phosphors that generate backscattering light, from which a signal indicating the beam position is generated by means of a photodetector.

The tracking signal is fed back to the deflection unit 6 to control and dynamically adjust the deflection of the beam so that the beam hits the screen 5 correctly.

In a beam-index tube, each electron beam should only hit the corresponding phosphor track, and thereby preferably be centered on said track. Thus, the spot size of the electron beam in the direction perpendicular to the phosphor track, indicated as the y-direction hereinafter, should be smaller than the dimension of the track itself.

The size of the spot on the screen 5 can be calculated using the Helmholtz- Lagrange law:

Here, ro is the size of the spot, ao the opening angle of the beam and Vo the electrical potential at the beam cross-over point and ri, ai and Vl the same parameters at the position of the screen 5. Since the electrical potentials Vo and Vl are fixed for one particular tube design, it could be derived from (1) that, taking a fixed value for ro and ao, the size of the spot at the screen 5 directly depends on the opening angle al at the screen 5 side of the tube. It is noted that the opening angle can have different values for the y-direction and the x-direction, the x-direction being the direction parallel to the phosphor tracks.

The opening angle ai can be increased by placing a positive electron lens between the main lens 4 and the display screen 5, such as the second magnetic quadrupole 8.

To keep the beam in focus on the display screen 5, it now should be diverged first with a negative lens. A first magnetic quadrupole 9 is therefore arranged in the tube between the cathode 3 and a second magnetic quadrupole 8. The first magnetic quadrupole 9 always has opposite lens action as the second magnetic quadrupole 8.

The magnetic quadrupoles 8,9 are preferably 45° magnetic 4Px quadrupoles.

By applying two magnetic quadrupoles, good convergence of the three beams on substantially the same landing position on the display screen is ensured.

Such a quadrupole may for instance be realized by a device with four magnetic poles, two positive poles and two negative poles. The poles are placed in a circle, in which the positive and negative poles are alternately arranged and displaced by 90° to each other.

Thus, an electron beam passing through the quadrupole will be affected by the magnetic fields of the poles surrounding the beam. The effect of the quadrupole lens is a positive lens in a first direction and a negative lens in a second direction perpendicular to the first direction.

The quadrupoles 8,9 are arranged in a plane perpendicular to the longitudinal axis of the CRT so as to act on the electron beam in a direction perpendicular to the movement direction of the electrons in the beam. The magnetic poles are arranged in the periphery of the tube in order to surround as large an area as possible. The formation of magnetic poles could be achieved by an annular core, around which wires leading electric currents are wound.

As has already been stated, the second quadrupole 8 should be placed as close as possible to the screen 5. This can, for example, be accomplished by integrating the second quadrupole 8 with the deflection unit 6. The second quadrupole 8 may be integrated in the

deflection unit 6 as a toroidal coil, wound around the core of the deflection unit 6 in such a way that a magnetic quadrupole is achieved.

The first quadrupole 9 is positioned between the main lens 4 and the second quadrupole 8. Since with only two quadrupoles it can be difficult to achieve focus in both the x-and the y-direction, the DAF section 7 can also be arranged to compensate for this. The DAF section 7 comprises an electric quadrupole. The use of three quadrupoles generally allows for better lens filling of the second magnetic quadrupole 8 that is close to the display screen 5.

In Fig. 2, the electron beam path in a conventional cathode-ray-tube (CRT), i. e. without the two quadrupoles, and the electron beam path in the CRT according to Fig. 1 are shown, whereby the electron beans land in the central portion of the screen. The dotted trace shows the path of the beam in a conventional CRT. The solid trace shows the path in the preferred embodiment of a CRT according to the present invention.

First, an electron beam is created in an electron gun 2. Next, the beam is passed through a main lens 4 for focussing the beam on the screen 5. After passing through the main lens 4, the beam is passed through the first magnetic quadrupole 9. At least for the central portion of the display screen 5, the first quadrupole 9 diverges the beam. The second quadrupole 8 converges the beam, so that it has a relatively large opening angle. Because of both quadrupoles being magnetic quadrupoles, a relatively good convergence of the three electron beams is maintained.

In the center, the opening angle of the beam is increased in the y-direction from al to a2, which is the critical direction. It is also shown that the opening angle in the x- direction is decreased and thus the spot size is increased in this direction. This increase is not so critical as the phosphor tracks are arranged horizontally and an increase of the spot size in this direction will thus not result in the electron beam hitting a track of the wrong color.

The DAF section 7, comprising an electric quadrupole, can also be used for this purpose. By tuning the DAF voltage, good filling of the first and second magnetic quadrupoles can be obtained.

When the electron beams are deflected towards the corners of the display screen 5 by the deflection unit 6, the spot will become more elliptically shaped, the long axis of the ellipse being in the x-direction. The deflection unit 6 is generally of the self-converging type, so that the deflection lens has no action on the electron beams in the x-direction.

Also, the electron beam is no longer in focus on the display screen 5, since the distance between the main lens 4 and the landing position of the electron beam on the display screen 5 increases as the beam lands closer to the edge or the corner of the display screen 5.

To compensate for these effects, the DAF section 7 is provided. The electric quadrupole allows for an effective weakening of the main lens, so as to compensate for defocusing. Also, in the y-direction, a negative DAF lens is formed compensating for the deflection lens.

The DAF section 7 is dynamically adjustable in dependence of the landing position of the electron beams. The quadrupoles 8, 9 are now also dynamically adjustable, whereby their strength is weakened and even inversed towards the corners of the screen 5.

Since the x-deflection causes the largest contribution to the ellipsoidity of the beam, the quadrupoles 8,9 are dynamically adjusted at line frequency.

By means of a dotted line, the path of one of the electron beams landing in the corner of the display screen is indicated as traced back from the screen, in the case that the quadrupoles 8,9 would not be present. It can be seen that, in the y-direction, the beam crosses the electronoptical axis of the electron gun at the position of the DAF quadrupole 7. The DAF quadrupole 7 is no longer able to fully compensate the action of the deflection lens, no matter the DAF voltage being applied. This situation is called DAF explosion.

The quadrupoles 8,9 are arranged such that the second quadrupole lens 9, being the quadrupole lens closest to the screen, now has negative lens action in the y-direction. It can be seen that the DAF quadrupole 7 acts on the electron beam in this case.

The spot of the electron beam is improved, even though the opening angle of the individual electron beams is decreased.

Thus, by dynamically adjusting the quadrupoles 8,9 at line frequency, such that the action of the quadrupoles is inversed when going from the center of the screen towards the edges, the spot uniformity is particularly good over the entire display screen.

The required spot size in the y-direction for a PAL beam-index type color picture tubes (CPTs) is set at 300 um. An experiment was performed to check whether the spot size could meet this requirement.

A 32"Wide Screen Real Flat FIT-CPT is equipped with a DAF electron gun.

The maximum deflection angle of this tube is 110 degrees. The spot size for the central electron beam is measured in the y-direction, for different landing positions on the display screen.

The tube is provided with two magnetic quadrupoles as described above, which are formed by means of coils, through which a suitable current is fed. The direction of the current is defined such that a positive current causes a positive lens to be formed by the second quadrupole 8 in the y-direction. This situation corresponds to Fig. 2. A negative current thus causes a negative lens to be formed by the second quadrupole 8 in the y-direction. This situation corresponds to Fig. 3.

The results of the experiment are shown in Table 1.

It can be seen that the spot size requirement can be met. Since all beams of the gun are affected in the same way by the quadrupoles 8,9, it is expected that both outer beams can also meet the spot requirements.

If the quadrupoles 8,9 are not used (I = 0 A), the spot size is just acceptable in the center of the screen. In the east, the spot size in y-direction would be very small, but a substantial increase in the voltage supplied to the focus bus (VG3A) and the DAF voltage are required. However, in the northeast corner, the electron beam cannot be brought into focus on the display screen. This is the situation of DAF explosion.

Table 1: Spot size of the central electron beam in the y-direction, for different landing positions on the display screen and magnetic quadrupole coil currents. landing position loi (A) IQ2 (A) VG3A (V) VDaF (V) spoty (urn) CENTER - 0. 5-0.5 9150 9010 404 - 0. 25-0.25 9150 8975 354 0 0 9150 8935 305 0.25 0.25 9150 8910 270 0.5 0.5 9150 8935 227 EAST - 0. 5-0.5 9150 9560 250 - 0. 25-0.25 9150 9820 212 0 0 10550 11650 191 NORTHEAST - 0. 5-0.5 9150 9620 303 0 0 Not in focus (DAF explosion occurs)

In the center of the screen it is desirable to have positive currents for best spot performance, for instance +0.5 A. The required DAF voltage is then 8935 V. However, if we deflect the beam towards the edges of the screen while maintaining this current, the required DAF voltage increases rapidly and focusing of the electron beam becomes difficult.

Therefore, the current should be dynamically adjustable, thereby weakening the quadrupoles.

It is most advantageous if the quadrupole strength, and thus the current, is reversed when going from the center to the edge of the screen. This can for example be done by means of a sinusoidal current having the line frequency, varying between +0.5 A in the center and-0.5 A near the edges of the screen. Now, the required DAF voltage at the east border of the screen is 9560 V, which is an increase of 625 V as compared to the center. In the northeast corner, the DAF voltage needs to increase further to 9620 V, which is an increase of 685 V as compared to the center.

In this case, the requirement on spot size in the y-direction can be met over the entire display screen of the beam-index cathode ray tube. Thereby, the amplitude of the DAF voltage applied is lower than 700 V, which is a particularly small value as compared to prior art DAF guns.

In principle, the magnetic quadrupoles could also be used to reduce DAF voltages in conventional shadow mask tubes. However, this would require a decreasing shadow-mask-to-screen distance when going from the center of the screen to the edges, because of the changing virtual gun pitch. It is questionable that this solution is technologically feasible.

The drawings are schematic and were not drawn to scale. In the drawings, the preferred embodiments of the display tube is, for simplicity reasons, shown with only a few phosphor tracks, whereas an actual PAL display tube would have, for instance, 540 x 3 phosphor tracks.

While the invention has been described in connection with preferred embodiments, it should be understood that the invention should not be construed as being limited to the preferred embodiments. It includes all combinations of elements described therein, and variations which could be made thereon by a skilled person, within the scope of the appended claims. The term"comprising"in the claim does not exclude the presence of additional elements in the display tube.

In summary, a beam-index type color display tube is disclosed, using index elements to control the position of an electron beam relative to a phosphor track on the display screen. In order to ensure color purity, the spot size of the electron beam in the

direction perpendicular to the phosphor screen needs to be so small, that an adjacent phosphor track is not hit by any part of the beam. The color display tube comprises a first and a second magnetic quadrupole lens for varying the opening angle of the beam close to the screen. The first quadrupole is positioned between the second quadrupole and the main lens of the electron gun, in order to ensure good color purity. A particularly good spot uniformity over the entire display screen is obtained when the two quadrupoles are dynamically driven.