WO2003052787A2

WO2003052787A2 - Cathode ray tube and picture display device

Info

Publication number: WO2003052787A2
Application number: PCT/IB2002/005407
Authority: WO
Inventors: Marcellinus P. C. M. Krijn; Willem L. Ijzerman
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2001-12-17
Filing date: 2002-12-12
Publication date: 2003-06-26
Also published as: TW200411708A; WO2003052787A3; AU2002366421A8; AU2002366421A1

Abstract

The invention relates to a cathode ray tube (CRT) having an electron gun (1), which is provided with a beam-forming part (20) for forming an electron beam (EBR, EBG, EBB). The electron beam is imaged onto a display screen (3) by means of the electron gun (1) and deflection means (2). In operation, the deflection means (2) deflect the electron beam (EBR, EBG, EBB) towards any position on the display screen (3), thereby constituting a deflection lens (2'), the lens strength of which varies with the deflection angle. The electron gun (1) is further provided with an electron lens (30) near the beam-forming part (20), for forming the electron beam (EBR, EBG, EBB) into a crossover (10R, 10G, 10B) in the vertical direction. Said crossover is shiftable along the gun axis (MA) so as to compensate for the varying power of the deflection lens (2'). The crossover (10R, 10G, 10B) may be located anywhere between the electron lens (30) and the deflection means (2). This is particularly advantageous for large deflection angles, such as angles of 120 degrees or more.

Description

Cathode ray tube and picture display device

The invention relates to a cathode ray tube comprising: a display screen for receiving an electron beam at the location of a landing position; an electron gun for generating said electron beam, having: a beam- forming section; an electron lens whose lens strength is changeable in dependence on the landing position of the electron beam so as to give the electron beam a crossover in a first direction at right angles to a central axis of the electron gun, which crossover can be shifted along the central axis, and a main lens between the electron lens and an end of the electron gun which faces the display screen; and comprising deflection means arranged between the electron gun and the display screen, which deflection means are self-convergent in a second direction that is transverse to the first direction and the central axis, and are used for adjusting the landing position of the electron beam on the display screen by deflecting the electron beam through a predetermined deflection angle in order to obtain an electron-optical image on the display screen, said deflection means forming a deflection lens acting on the electron beam. The invention also relates to a picture display device equipped with such a cathode ray tube.

An embodiment of such a cathode ray tube is disclosed in US-A-5,262,702.

In this cathode ray tube, a triode comprising a cathode for emitting electrons is used as the beam-forming section, h general, the electron beam converges within the triode to a punctiform crossover, also referred to as object point.

In this application, a crossover is to be taken to mean a location where the diameter of the electron beam in at least one direction is such that said location is suitable for being electron-optically displayed on the display screen, at least in said direction, with a sharpness desirable for picture display.

The electron lens gives the electron beam a line-shaped crossover by focusing the electron beam in the vertical direction. Said crossover can be shifted along the central axis and is situated within the electron gun, i.e. between the electron lens and the main lens. In the vertical direction, the shiftable, line-shaped crossover is then displayed on the display screen, and, in the horizontal direction, the object point in the triode is displayed on the display screen.

During deflecting the electron beam, the deflection means act as an electron lens on the electron beam, which electron lens is hereinafter also referred to as deflection lens. Predominantly the main lens and the deflection lens are responsible for displaying the electron beam on the display screen.

The lens strength of the deflection lens varies with the deflection angle. The deflection angle is defined as an angle that corresponds to a certain landing position, which angle is defined as twice the angle enclosed by the electron beam, near the landing position on the display screen, and an imaginary electron beam landing in the center of the display screen.

The deflection means are self-convergent in the horizontal direction, as a result of which the defocusing effect of the deflection lens in the horizontal direction is such that the display of the object point in the horizontal direction remains substantially in focus throughout the display screen.

For the deflection lens, use is customarily made of a quadrupole lens, so that the deflection lens has a focusing effect in the vertical direction. As the line-shaped crossover can be shifted, the variation of the lens strength of the deflection lens in the vertical direction can be partly compensated for.

It is desirable for a cathode ray tube to be not very bulky. In particular, it is desirable for a picture display device, provided with said cathode ray tube, to comprise a display screen having a comparatively large screen surface area, while the depth dimension of the picture display device is comparatively small. The larger the surface area of the cathode ray tube display screen is at a given depth dimension, or the smaller the depth dimension of the cathode ray tube is at a given surface area of the display screen, the higher the values of the deflection angle must be in operation to make sure that the electron beam covers the entire display screen. The maximum deflection angle, i.e., the deflection angle at a landing point of the electron beam in the corners of the display screen, is comparatively large.

Cathode ray tubes having a maximum deflection angle of 120 degrees will be available shortly, and, in novel designs, it is the endeavor to obtain a cathode ray tube whose depth in centimeters is equal to the display screen diagonal in inches. In this case, the maximum deflection angle is approximately 135 degrees.

A drawback of the known cathode ray tube resides in that at such deflection angles, the display of the crossover, which display is also referred to as "spot" in this patent application, changes in the vertical direction across the display screen. Particularly in the corners of the display screen the spot is comparatively unsharp.

It is an object of the invention to provide a cathode ray tube of the type mentioned in the opening paragraph, which is capable of producing a comparatively sharp display of the crossover throughout the display screen.

In the cathode ray tube in accordance with the invention, this object is achieved in that the electron gun is arranged to shift the crossover to a position between the main lens and the deflection means, near an object plane of the deflection lens.

At a limited value of the deflection angle, for example 60 degrees or less, the deflection lens is weak and the crossover is imaged onto the display screen mainly by the main lens. In this case, the crossover lies between the electron lens and the main lens, near an object plane of the main lens.

As the deflection angle increases, the crossover is imaged onto the display screen to an increasing extent by the deflection lens. The distance between the crossover and the main lens becomes smaller, as a result of which the contribution of the main lens to imaging the crossover decreases.

The invention is based on the recognition that, at a comparatively large deflection angle, in general approximately 120 degrees and more, the lens strength of the deflection lens is such that, in the first direction, the object plane of the deflection lens is situated between the main lens and the deflection means if the image plane of the deflection lens coincides with the display screen.

To improve the sharpness of the spot, particularly in the corners of the display screen, the crossover must be situated, in accordance with the invention, near the object plane of the deflection lens. In this manner, a comparatively sharp spot can be obtained for each deflection angle.

At a deflection angle of approximately 120 degrees, the crossover in the cathode ray tube in accordance with the invention then lies in or beyond the main lens, so that the contribution of the main lens to the imaging of the crossover is zero, and the crossover is imaged on the display screen only by the deflection lens.

In the known cathode ray tube, the line-shaped crossover cannot be shifted to a position in or beyond the main lens, so that the increase in strength of the deflection lens can be insufficiently compensated for. In the known cathode ray tube, the main lens contributes substantially to the display of the crossover, for each value of the deflection angle, and the spot is comparatively hazy particularly in the corners of the display screen.

The cathode ray tube in accordance with the invention also has the advantage that the size of the spot on the display screen is comparatively small in the first direction, and particularly uniform throughout the display screen. In the crossover, the dimension of the electron beam is comparatively small in the first direction, and the crossover is displayed comparatively sharply on the display screen, as a result of which also the spot is comparatively small.

At increasing deflection angles, the angle of opening of the electron beam becomes smaller near the display screen in the first direction. This generally causes the spot to grow due to space-charge effects near the display screen.

However, since an increase of the deflection angles is accompanied by a simultaneous reduction of the diameter of the electron beam in the first direction at the location of the main lens, spherical aberrations in the main lens are reduced. By virtue thereof, the growth of the spot due to space-charge effects is compensated for. In the case of the cathode ray tube in accordance with the invention, a substantially complete compensation is achieved, so that the diameter of the spot is very uniform throughout the display screen in the first direction.

In a further advantageous embodiment, the beam-forming section of the electron gun is arranged so as to generate three electron beams, which are juxtaposed in the same plane in the second direction, for example the horizontal direction, and the lens strength of the electron lens is different for each of the electron beams. The plane in which the electron beams are situated is generally referred to as the "in-line" plane.

This electron gun can be used in a color cathode ray tube, with the three electron beams generally corresponding to the colors red, green and blue. As the three electron beams in the "in-line" plane are spaced some distance apart, for example 6 mm at the location of the deflection means, they pass through the self- convergent magnetic field of the deflection means along different paths. On the display screen, the electron beams converge however substantially in the same landing point, so that the deflection angle is different for each of the electron beams.

As a result, the lens strength of the deflection lens is different for each one of the three electron beams, in particular for comparatively large deflection angles of the electron beam, for example 120 degrees. The spot sharpness on the display screen is not the same for the three electron beams. This effect will hereinafter be referred to as "color- dependent defocusing".

In this case, it is advantageous for the lens strength of the electron lens to be different for each one of the three electron beams. This makes it possible to shift the crossover of each individual electron beam to a position of its own, near the object plane of the deflection lens corresponding to the electron beam. In this manner, color-dependent defocusing can at least partly be compensated for, and the color cathode ray tube operates particularly well.

In an advantageous embodiment of the cathode ray tube, the electron gun of the beam-forming section successively comprises, viewed in the direction of the electron gun's end facing the display screen, a first focusing electrode, a dynamic focusing electrode for forming the electron lens, a second focusing electrode and an anode. In operation, the main lens is formed in general between the second focusing electrode and the anode.

The electron gun comprises, for example, a triode as the beam-forming section. In this case, the gun has six electrodes. It has been found that, in this case, the electron gun operates well. The electron gun can be manufactured more economically and more readily than the electron gun of the known cathode ray tube, which comprises ten electrodes.

Alternatively, the electron gun has, for example, a diode as the beam-forming section.

The dynamic focusing electrode may have end faces which extend transversely to the central axis, which end faces are provided with an elongated opening in the second direction for allowing passage of the electron beam. The first focusing electrode and the second focusing electrode have end faces facing the dynamic focusing electrode, which end faces are provided with a similar opening. In this manner, the dynamic focusing electrode forms the electron lens, which has a cylindrical lens portion on either side of the dynamic focusing electrode. The strength of the electron lens is variable in operation owing to the fact that the dynamic focusing electrode receives a dynamic voltage that depends on the deflection angle. The electron beam can thus be focused in the first direction without the electron lens acting on the electron beam in the second direction. This is advantageous because the deflection means are self-convergent, as a result of which the electron beam is substantially in focus in the second direction throughout the display screen.

By increasing the dynamic voltage for increasing deflection angles during operation, the electron lens becomes weaker in dependence on the deflection angle and the crossover shifts along the central axis to the display screen. In this manner, the crossover is held substantially in the object plane of the deflection lens, or, in the case of comparatively small deflection angles, in the object plane of the assembly of the deflection lens and the main lens, and the display of the crossover on the display screen is substantially in focus throughout the display screen.

When such an electron lens is used, comparatively few aberrations occur.

An embodiment of the cathode ray tube comprising an electron lens having a different lens strength for each one of the electron beams includes a dynamic focusing electrode with a sub-electrode for each of the electron beams, said sub-electrodes being electrically insulated from each other. The lens strength of the electron lens can be varied independently for each beam during operation, because each one of the sub-electrodes receives an individual dynamic voltage that depends on the deflection angle. This cathode ray tube enables color-dependent defocusing to be at least partly compensated for.

The dynamic voltage can be varied between 350 and 2500 volts. In an embodiment of the cathode ray tube, the dynamic voltage is for example 800 volts if the electron beam lands in the center of the display screen, and the dynamic voltage is 1650 volts for a deflection angle of 120 degrees.

Such voltages are low in comparison with the dynamic voltage in the known cathode ray tube, which ranges between 3000 and 4000 volts, and particularly in comparison with conventional DAF guns wherein the dynamic voltage is superposed on a fixed focusing voltage of, for example, 6000 volts.

In a further embodiment, an astigmatic correction element is present near the side of the main lens which faces the beam-forming section. In general, the main lens is designed such that the electron beam properly fills the main lens in the second direction. As a result, the quality with which the object point in the beam-forming section can be displayed on the display screen is comparatively good.

If the crossover is close to the electron lens, i.e., if the electron beam lands near the center of the display screen, the main lens may not be strong enough to focus the spot on the display screen.

This effect can be reduced by providing the main lens with the astigmatic correction element, which is, for example, a cylindrical lens. This enables the electron beam to be prefocused already in front of the main lens in the vertical direction, so that the spot can also be focused on the display screen for small deflection angles. It has been demonstrated that in a cathode ray tube in accordance with the invention, the diameter of the display of the crossover on the display screen in the first direction is smaller than 300 micrometers for each deflection angle of the electron beam, i.e. throughout the display screen, for each one of the electron beams. In this case, the beam current is maximally about 0.5 mA. In the cathode ray tube in accordance with the invention, the electron beams are in focus throughout the display screen. By virtue thereof, a display device provided with the cathode ray tube in accordance with the invention has a comparatively good picture quality.

These and other aspects of the cathode ray tube and the display device in accordance with the invention will be elucidated with reference to the embodiments.

In the drawings:

Fig. 1 diagrammatically shows the cathode ray tube in accordance with the invention, provided with a first embodiment of the electron gun;

Fig. 2 shows a detail of the first embodiment of the electron gun;

Fig. 3 shows an equivalent lens system of the electron lenses in the cathode ray tube, in the horizontal direction;

Fig. 4 shows an equivalent lens system of the electron lenses in the cathode ray tube, in the vertical direction;

Fig. 5 A shows an equivalent lens system of the first embodiment of the electron gun, which is additionally provided with the astigmatic correction element;

Fig. 5B is a front view of the electrodes of the correction element;

Fig. 6 diagrammatically shows a second embodiment of the electron gun; Fig. 7 shows an equivalent lens system of the cathode ray tube, provided with the second embodiment of the electron gun;

Fig. 8 shows in greater detail the dynamic focusing electrode in the second embodiment of the electron gun, and Fig. 9 shows a picture display device in accordance with the invention.

The color cathode ray tube in accordance with the invention, as shown in Fig. 1, is provided with the first embodiment of an electron gun 1. The electron gun generates a first outermost electron beam EBR that corresponds to the color red, a central electron beam EBG that corresponds to the color green, and a second outermost electron beam EBB that corresponds to the color blue. The electron beams EBR, EBG, EBB are co-linear in the plane of the drawing, which is also referred to as the "in-line plane" in this patent application.

In this first embodiment of the electron gun 1, a beam-forming section 20 is formed by a triode comprising the electron sources 10R, 10G, 10B, a first electrode Gl and a second electrode G2. The electron gun 1 additionally comprises a first focusing electrode G3A, a dynamic focusing electrode G3B, a second focusing electrode G4 and an anode G5.

The electron sources 10R, 10G, 10B emit the electrons, which, in the beam- forming section 20, are formed into the corresponding electron beams EBR, EBG, EBB. Within the beam-forming section 20, the electron beam EBR, EBG, EBB has a smallest diameter at the location of the object point 21 R, 21 G, 21B.

The electron beams EBR, EBG, EBB are guided towards the display screen 3 of the cathode ray tube via the electron lens 30, the main lens 50 and the deflection lens 2'.

The first focusing electrode G3 A and the second focusing electrode G4 receive a fixed focusing voltage Nf. This focusing voltage is, for example, approximately 5.7 kN. The gun anode G5 receives the anode voltage Va of, for example, approximately 30 kV. The main lens 50 is formed, during operation, between the second focusing electrode G4 and the gun anode G5.

The dynamic focusing electrode G3B receives a dynamic focusing voltage Ndyn. This dynamic focusing voltage increases, during operation, in dependence upon the deflection angle of the electron beams EBR, EBG, EBB.

The first focusing electrode G3A, an embodiment of the dynamic focusing electrode G3B and the second focusing electrode G4 are shown in more detail in Fig. 2. The electron lens 30 comprises two cylindrical lenses 31, 32. The first cylindrical lens 31 is formed between the opening 33 A in the end face 36 of the first focusing electrode G3A and the opening 33B in the end face 35 of the dynamic focusing electrode G3B which faces the first focusing electrode G3A. The second cylindrical lens 32 is formed between the opening 33D in the end face 36 of the second focusing electrode G4 and the opening 33C in the end face 35 of the dynamic focusing electrode G3B which faces the second focusing electrode G4.

The spacing between juxtaposed electrodes G3A, G3B; G3B, G4 is 0.8 mm, the length of the dynamic focusing electrode G3B in the direction along the central axis MA being approximately 3.6 mm.

The openings 33 are of rectangular shape, which is elongated in the horizontal direction. The electron beams EBR, EBG, EBB, which are juxtaposed in the horizontal direction, pass through the openings 33. For illustration, this is shown in the drawing by indicating, in opening 33D, the points 37R, 37G, 37B where the respective electron beam EBR, EBG, EBB passes through the opening 33D. The electron beams EBR, EBG, EBB pass through the other openings 33A-33C in a similar manner.

The length of the openings 33 in the horizontal direction is such that the cylindrical lenses 31, 32 do not act on the electron beams EBR, EBG, EBB in the horizontal direction. For example, the length is 18.6 mm. The diameter of the openings 33 in the vertical direction is, for example, 3.6 mm, so that, in operation, the cylindrical lenses 31, 32 focus the electron beams EBR, EBG, EBB in the vertical direction in such a manner that the shiftable crossovers 10R, 10G, 10B are formed.

In this embodiment, all openings 33 are equally shaped. As a result, the strength of the first and the second cylindrical lens is the same. The lens strength depends on the voltage difference between the fixed focusing voltage Nf and the dynamic focusing voltage Ndyn.

As the dynamic focusing voltage Ndyn increases, the voltage difference Vf- Ndyn, and hence the strength of the electron lens 30, decreases. The distance between the crossover 10R, 10G, 10B and the electron lens 30 increases. The operation of the electron lens 30 will now be illustrated by means of equivalent lens systems, as shown in Fig. 3 for the horizontal direction and in Fig. 4 for the vertical direction. In these Figures, only one of the electron beams, namely the central electron beam EBG, is shown. For the other electron beams EBR, EBB, the equivalent lens system is identical in this embodiment of the dynamic focusing electrode G3B. If the electron beams EBR, EBG, EBB converge at a landing point near the center C of the display screen 3, the crossover 10R, 10G, 10B lies within the second focusing electrode G4, near the end face 36 thereof which faces the dynamic focusing electrode G3B. The dynamic voltage Ndyn is approximately 800 N. The deflection lens 2' has no effect. The main lens 50 displays the object point

2 IR, 21 G, 2 IB in the horizontal direction on the display screen 3, as shown in Fig. 3 A, and it displays the crossover 10R, 10G, 10B in the vertical direction on the display screen 3, as shown in Fig. 4A. The distance between the crossover 10R, 10G, 10B and the associated object point 21R, 21G, 21B in the direction of movement of the electrons is preferably as small as possible.

The deflection angle increases and the deflection lens 2' becomes stronger as the electron beam EBR, EBG, EBB lands at a greater distance from the center C of the display screen 3. In the vertical direction, the deflection lens 2' has a positive lens effect, and, in the horizontal direction, the deflection lens 2' has a negative lens effect, hi addition, the distance covered by the electron beam EBR, EBG, EBB between the main lens 50 and its landing point on the display screen 3 increases. hi Figs. 3B and 4B, the deflection angle is approximately 90 degrees. The crossover 10R, 10G, 10B is shifted along the central axis MA over a distance LI. The crossover 10R, 10G, 10B now lies near the object plane OP' of the assembly of main lens 50 and deflection lens 2' in the vertical direction.

In the horizontal direction, the deflection means 2 are self-convergent. The increase of the distance between the main lens 50 and the place on the display screen 3 where the electron beams EBR, EBG, EBB converge, as well as the negative lens effect of the deflection lens 2', can be compensated for each value of the deflection angle by shifting the crossover 10R, 10G, 10B.

In the horizontal direction, the electron beam EBR, EBG, EBB is substantially in focus throughout the display screen 3 as a result of the fact that the deflection means 2 are self-convergent. In the vertical direction, the electron beam EBR, EBG, EBB can be held substantially in focus by shifting the crossover 10R, 10G, 10B, in dependence upon the deflection angle, in the direction of movement of the electrons. The distance between the crossover 10R, 10G, 10B and the object plane OP' is comparatively small, so that a comparatively sharp display of the crossover 10R, 10G, 10B on the display screen 3 is obtained. In the case of a further increase of the deflection angle, displaying the electron beam EBR, EBG, EBB on the display screen 3 is done to an increasing extent by the deflection lens 2', and the contribution of the main lens 50 gradually decreases to zero as the crossover is situated closer and closer to the main lens 50. In Fig. 4C, the deflection angle is approximately 120 degrees. The crossover

10R, 10G, 10B has now been shifted over a distance L2 as a result of which the crossover lies between the main lens 50 and the deflection lens 2 in the object plane OP of the deflection lens 2'. The crossover 10R, 10G, 10B is displayed on the display screen 3 exclusively by the deflection lens 2'. The dynamic voltage Ndyn applied to the dynamic focusing electrode G3B now is approximately 1650 volts.

For small deflection angles, the main lens 50 may possibly be too weak to focus the electron beam EBR, EBG, EBB on the display screen 3. The outermost rays of the electron beam EBR, EBG, EBB then enclose an angle with the central axis MA which is too large. As a result, the quality of the display of the electron beam EBR, EBG, EBB on the display screen 3 is adversely affected, i.e. the spot is not in focus on the display screen 3.

This is shown in Fig. 5 A for the central electron beam by means of the electron beam EBG' indicated by means of broken lines.

To limit this effect, a prefocusing cylindrical lens 40 is arranged near the side of the main lens 50 which faces a beam-forming section 20. This cylindrical lens 40 is much weaker than the electron lens 30 and is effective only for small deflection angles.

The electron beam EBG, represented by a continuous line, is prefocused by the cylindrical lens 40 before entering the main lens 50, as a result of which the angle that the outermost rays of the electron beam EBG enclose with the central axis MA at the location of the main lens 50 is reduced. The electron beam EBG can now be focused in a spot SG of comparatively high quality on the display screen 3 by the main lens 50.

The cylindrical lens 40 can be formed, for example, as a result of the fact that the second focusing electrode G4 is divided into two sub-electrodes G4A, G4B, the facing ends of the sub-electrodes G4A, G4B being indicated in Fig. 5B as electrode 41. In this case, in particular the cylindrical lens is formed between the two electrodes 41.

The electrode 41 has an elongated opening 42 in the horizontal direction to allow passage of the electron beams EBR, EBG, EBB. In the horizontal direction, this opening 42 has approximately the same diameter as the openings 33, and the correction element 40 has substantially no effect on the electron beams EBR, EBG, EBB. In the vertical direction, the diameter is much larger than the diameter of the openings 33, so that the lens effect of the cylindrical lens 40 is weaker than that of the electron lens 30. For example, in the vertical direction the diameter of the opening 42 is 8.4 mm.

A second embodiment of the electron gun 101 is shown in Fig. 6, which is a sectional view of the electron gun in the "in-line" plane. The second embodiment is substantially identical to the first embodiment, the dynamic focusing electrode being divided into three sub-electrodes Gr, Gc, Gl in the horizontal direction. Each of the sub-electrodes Gr, Gc, Gl corresponds to one of the electron beams EBR, EBG, EBB.

The second embodiment is particularly advantageous for use in a color cathode ray tube.

In operation, each of the sub-electrodes Gr, Gc, Gl forms a distinguishable electron lens 130R, 130G, 130B. By applying an associated, independent voltage Vdynl, Ndyn2, Ndyn3 to each of the electrodes Gr, Gc, Gl, respectively, it is achieved that the lens strength of the electron lenses 130R, 130G, 130B is different. As shown in Fig. 7, this enables the crossovers 110R, HOG, HOB for each of the three electron beams EBR, EBG, EBB to be formed in an individual position.

In particular, the crossover HOB for the blue electron beam EBB lies between the electron lens 130B and the main lens 150, the crossover HOG for the green electron beam EBG lies in the main lens 150, and the crossover 110R for the red electron beam EBR lies between the main lens 150 and the deflection lens 102R. In the Figure, the effect of color- dependent defocusing is made to appear stronger than it actually is. In reality, the crossovers 110R, 110G, HOB are situated at a smaller distance from each other along the central axis MA.

At the location of the deflection means 2, the spacing between the electron beams EBR, EBG, EBB is, for example, 6 mm. If the electron beams EBR, EBG, EBB are deflected, for example towards the north-east corner of the display screen 3, the deflection angle for the blue electron beam EBB is smaller than the deflection angle for the green electron beam EBG, and the deflection angle for the green electron beam EBG is smaller than the deflection angle for the red electron beam EBR. As a result, the lens strength of the deflection lens 102B for the blue electron beam EBR is smaller than the lens strength of the deflection lens 102G for the green electron beam EBG, and the lens strength of the deflection lens 102G for the green electron beam EBG is smaller than the lens strength of the deflection lens 102R for the red electron beam EBR. As each crossover 110R, HOG, HOB can be shifted over a different distance, each crossover 110R, 110G, 110B can be held substantially in the object plane OPR, OBG, OPB of the associated deflection lens 102R, 102G, 102B by suitably choosing the corresponding dynamic focusing voltage Ndynl, Ndyn2, Ndyn3. As a result, the occurrence of color-dependent defocusing is substantially precluded in this embodiment of the electron gun 101, even at comparatively large deflection angles of for example 120 degrees.

By way of example, for each of the three electron beams EBR, EBG, EBB a suitable value of the associated dynamic voltage Ndynl, Ndyn2, Ndyn3 is listed in a Table for a landing point in the center of the display screen 3 with an associated deflection angle of 0 degrees as well as for a landing point in the northeast corner of the display screen 3 with an associated deflection angle of 120 degrees.

Said Table also shows the diameter x, y of the spot SR, SG, SB of the electron beams EBR, EBG, EBB, in the horizontal (x) direction as well as in the vertical direction (y), which spot is formed on the display screen 3 in this case. For all values listed, the fixed focusing voltage applied to the first focusing electrode G3A and the second focusing electrode G4 is 5.66 kN, and the beam current of the electron beam EBR, EBG, EBB is 0.5 mA.

Table 1 : Exemplary values of the dynamic voltages Ndynl, Ndyn2, Ndyn3 with associated diameters x,y of the spot SR, SG, SB for the three electron beams EBR, EBG, EBB.

Centre EBR Vdynl = 829 N x(SR)=783 um y(SR)=269 um

EBG Ndyn2 = 829 N x(SG)=1.25 mm y(SG)=277 um EBB Ndyn3 = 829 N x(SB)=783 um y(SB)=269 um

Northeast

EBR Vdynl=1736 V x(SR)=4.05 mm y(SR)=275 um

EBG Vdyn2=1639 V x(SG)=5.10 mm y(SG)=262 um

EBB Vdyn3=1557 V x(SB)=3.69 mm y(SB)=286 um

For a deflection angle of 120 degrees, the dynamic voltages Vdynl, Ndyn2, Ndyn3 are much smaller than 2 kV and, unlike the dynamic focusing voltage in conventional so-termed DAF electron guns, they do not have to be superposed on the fixed focusing voltage.

The diameter y of the spot SR, SG, SB in the vertical direction is smaller than 300 μm throughout the display screen 3. As a result, the second embodiment of the electron gun 101 can very suitably be used in a cathode ray tube of the so-termed FIT-type, as is known from EP-A-1 058 942.

In such a cathode ray tube (FIT-CRT) a shadow mask for color selection is absent. The landing points of the three electron beams EBR, EBG, EBB on the display screen 3 can be corrected with respect to a color of the phosphor corresponding to the electron beam, i.e. red, green and blue, respectively. The phosphors are provided on the display screen 3, for example, in horizontal paths. On either side of the phosphor paths there are index elements that generate a first and a second correction signal. The landing point of an electron beam on a corresponding phosphor path can be determined from the difference between the first and the second correction signal. During writing an image, the electron beams generally do not travel over the phosphor paths in a straight line; instead, disturbances occur which are predominantly caused by fluctuations in the magnetic field of, for example, the deflection means. As a result, an electron beam travelling in a direction perpendicular to the phosphor paths can be undesirably deflected and land on a phosphor path of a color that does not correspond to the electron beam, leading to color errors in the light image.

If, in operation, the electron beam does not land completely on the phosphor path but partly on one of the index elements, the difference between the correction signals is comparatively large. In this case, a magnetic correction coil is activated which causes an additional deflection of the electron beam in the direction perpendicular to the phosphor paths, so that the effect of the magnetic fluctuations on the landing point of the electron beam can be corrected.

Since a shadow mask can be dispensed with, a FIT cathode ray tube is comparatively inexpensive, and an electron beam with a comparatively low beam current can be used. For example, the beam current is maximally about 0.5 mA. A FIT cathode ray tube imposes high demands on the size of the spot on the display screen. If the electron beam lands correctly on the phosphor path, the electron beam does not overlap the index elements, so that both correction signals are zero. The spot size in the direction perpendicular to the phosphor paths on the display screen, i.e. in general the vertical direction, should, for this purpose, not be larger than a diameter of the phosphor paths in this direction. Said diameter is approximately 325 micrometers in a 32" Wide Screen Real Flat (WSRF) FIT-cathode ray tube with 535 RGB lines (PAL format).

Table 1 shows that, in the second embodiment of the electron gun 101, the spot size in the vertical direction (y) meets this requirement throughout the display screen 3 at a customary maximum beam current in a FIT cathode ray tube of 0.5 mA.

An example of a dynamic focusing electrode with three sub-electrodes Gr, Gc, Gl, which is suitable for use in the second embodiment of the electron gun 101, is shown in greater detail in the front view of Fig. 8.

The sub-electrodes Gr, Gc, Gl are each composed of two segments 142r, 143r; 142c, 143c; 1421, 1431, which are situated at a certain distance, for example 0.2 mm, from each other, viewed in a direction perpendicular to the "in-line" plane. As a result, between the two segments 142r, 143r; 142c, 143; 1421, 1431 of an associated sub-electrode Gr, Gc, Gl, openings are formed having a diameter of for example 3.6 mm, through which the electron beams EBR, EBG, EBB pass, during operation, at the indicated locations 141R, 141G, 141B. The segments 142r,c,l; 143r,c,l, which are situated next to each other on the same side of the "in-line" plane, are, in this example, attached to a common glass plate 144A, 144B, respectively, so as to be mutually electrically insulated. Via multiforms 145 A, 145B, the dynamic focusing electrode G3B is secured in the electron gun 101.

A picture display device comprising a cathode ray tube with the second embodiment of the electron gun 101 is shown in Fig. 9.

A control unit A in the picture display device is arranged to receive a picture signal VID and to generate modulation signals MR, MG, MB and position signals Px and Py from this picture signal VID, during operation.

The modulation signals MR, MG, MB can be supplied to the respective electron sources CR, CG, CB for modulating the current density of the electron beams EBR, EBG, EBB and hence change an intensity with which, respectively, the red, green and blue phosphors luminesce on the display screen 103 at the location of a landing point of the electron beams EBR, EBG, EBB.

The position signals Px and Py can be supplied to a deflection circuit D which forms a line-frequency deflection current I] and a frame-frequency deflection current I_f from said position signals. The deflection means 102 can be coupled with the deflection circuit D to receive the deflection currents I_\, I_f. In particular, the deflection means 102 comprise a line- deflection coil L], which receives the line-frequency deflection current Ii for deflecting the electron beams EBR, EBG, EBB in the horizontal direction during operation. Furthermore, the deflection means 102 comprise a frame-deflection coil L_f, which receives the frame- frequency deflection current I_f for deflecting the electron beams EBR, EBG, EBB in the vertical direction during operation.

The position signals Px and Py can also be supplied to a focusing circuit F for the synchronized generation of dynamic focusing voltages Vdynl, Vdyn2, Vdyn3. In operation, the left sub-electrode Gl of the dynamic focusing electrode G3B receives the first dynamic focusing voltage Vdynl, the central sub-electrode Gc receives the second dynamic focusing voltage Vdyn2 and the right sub-electrode Gr receives the third dynamic focusing voltage Vdyn3. The dynamic focusing voltages generally are fourth-order signals, which are dependent on the landing point of the electron beams EBR, EBG, EBB on the display screen 3.

The dynamic focusing electrode G3B having the three sub-electrodes Gl, Gc, Gr is shown in the drawing. The other electrodes in the electron gun 101 are equal to those shown in the first embodiment of the electron gun 1 and, for the sake of clarity, are not shown in this Figure.

The drawings are diagrammatic and not to scale. It will be obvious that the invention is illustrated by means of the Figures and the descriptions of the associated embodiments, which are not to be interpreted as limiting the scope of the invention. The invention particularly comprises modifications of the embodiments described herein, which modifications could be carried out by a person skilled in the art within the scope of protection of the appended claims.

Claims

CLAIMS:

1. A cathode ray tube (CRT) comprising: a display screen (3) for receiving an electron beam (EBR, EBG, EBB) at the location of a landing position (BL); an electron gun (1) for generating said electron beam (EBR, EBG, EBB), having: a beam-forming section (20); an electron lens (30) whose lens strength is changeable in dependence on the landing position of the electron beam (EBR, EBG, EBB) so as to give the electron beam (EBR, EBG, EBB) a crossover (10R, 10G, 10B) in a first direction at right angles to a central axis (MA) of the electron gun (1), which crossover can be shifted along the central axis (MA), and a main lens (50) between the electron lens (30) and an end of the electron gun (1) which faces the display screen (3); and comprising deflection means (2) arranged between the electron gun (1) and the display screen (3), which deflection means are self-convergent in a second direction that is transverse to the first direction and the central axis (MA), and are used for adjusting the landing position of the electron beam (EBR, EBG, EBB) on the display screen (3) by deflecting the electron beam (EBR, EBG, EBB) through a predetermined deflection angle in order to obtain an electron-optical image on the display screen (3), said deflection means (2) forming a deflection lens (2') acting on the electron beam (EBR, EBG, EBB), characterized in that the electron gun (1) is arranged to shift the crossover (10R, 10G, 10B) to a position between the main lens (50) and the deflection means (2), which position is near an object plane (OP) of the deflection lens (2').

2. A cathode ray tube as claimed in claim 1, characterized in that the beam- forming section (20) of the electron gun (1) is arranged so as to generate three electron beams (EBR, EBG, EBB), which are juxtaposed in the same plane in the second direction, and in that the lens strength of the electron lens (30) is different for each of the electron beams (EBR, EBG, EBB).

3. A cathode ray tube as claimed in claim 1, characterized in that the electron gun (1) of the beam-forming section (20) is successively provided, viewed in the direction of the display screen (3), with a first focusing electrode (G3A), a dynamic focusing electrode (G3B) for forming the electron lens (30), a second focusing electrode (G4) and an anode (G5).

4. A cathode ray tube as claimed in claim 3, characterized in that the dynamic focusing electrode (G3B) has end faces (35) which are situated transversely to the central axis (MA), which end faces are provided with an elongated opening (33B, 33C) in the second direction for allowing passage of the electron beam (EBR, EBG, EBB), and the first focusing electrode (G3A) and the second focusing electrode (G4) have an end face facing the dynamic focusing electrode (G3B), which end faces are provided with an elongated opening (33A, 33D) in the second direction.

5. A cathode ray tube as claimed in claims 2 and 3, characterized in that the dynamic focusing electrode (G3B) comprises a sub-electrode (Gr, Gc, Gl) for each of the electron beams (EBR, EBG, EBB), said sub-electrodes (Gr, Gc, Gl) being electrically insulated from each other.

6. A cathode ray tube as claimed in claim 3, 4 or 5, characterized in that the dynamic focusing electrode (G3B) receives a dynamic voltage (Vdyn) during operation, which depends on the deflection angle.

7. A cathode ray tube as claimed in claim 6, characterized in that the dynamic voltage (Vdyn) can be varied between 350 V and 2500 V.

8. A cathode ray tube as claimed in claim 2, characterized in that an astigmatic correction element (40) is present near a side of the main lens (50) which faces the beam- forming section (20).

9. A cathode ray tube as claimed in any one of the preceding claims, characterized in that a diameter of the display of the crossover (10R, 10G, 10B) on the display screen (3) in the first direction is smaller than 300 micrometers at least substantially throughout the display screen (3) at a beam current of the electron beam (EBR, EBG, EBB) of at the most approximately 0.5 mA.

10. A display device comprising a cathode ray tube as claimed in at least one of the claims 1 through 9.