ELECTRON GUN AND CATHODE RAYTUBE.

FIELD OF THE INVENTION

The present invention relates to an electron gun for a cathode ray comprising a main lens section and a Dynamic Astigmatism and Focusing (DAF) lens section. The invention further relates to a cathode ray tube comprising such an electron gun. Electron guns have a wide use in cathode ray tubes in television sets and computer monitors. A prior art electron gun usually consists of at least a triode for forming electron beams, and a main lens section comprising at least two electrodes. The triode comprises a cathode and two electrodes. A voltage difference is applied between the electrodes of the main lens section, whereby an electric field is defined in the gap between these electrodes. Said electric field accomplishes a focusing lens action on the electron beams, which lens action is such that the electron beams are in focus on the display screen of a cathode ray tube (CRT).

BACKGROUND OF THE INVENTION Major trends in CRT development are striving towards larger display screens that are flat or nearly flat, a reduction of the depth of the CRT, a higher resolution and lower cost. For a flat or almost flat display screen, i.e. a non-spherical display screen, the focal length should vary with the landing position of the electron beams on the display screen. In the corners of the display screen, the focal length should be larger than in the center of the display screen. In addition, the deflection unit is generally a magnetic deflection unit, which has the side effect of also acting as an astigmatic electron-optical lens on the electron beams. The strength of this deflection lens increases with increasing deflection angle, and thus varies with the landing position of the electron beams on the display screen. As a depth reduction generally requires the electron beams to be deflected over a larger angle, the lens action of the deflection lens gets stronger in a CRT with reduced depth. In addition, a viewer, generally, perceives an increased sharpness if the spot size, i.e. the size of the electron beam at the location of the display screen, is reduced. This also allows for a higher image resolution.

A solution adopted over the past few years is the introduction of a so-called Dynamic Beam Forming (DBF) lens section. The DBF lens section has an inverse polarity

as compared to the DAF lens section. Therefore it can be used to compensate the side effect of the DAF modulation of the beam diameter in the main lens section.

A disadvantage of introducing a DBF lens section in the electron gun is that it comprises an expensive option: at least two base plates and a decoupling pot must be added. Attempts to integrate the DBF to the preceding lens in the gun (the so-called pre- focus lens) have not been successful. In addition, the DBF and DAF, from a focusing point of view, have opposite actions. This may lead to stronger DAF and/or higher amplitude of the dynamic voltage.

SUMMARY OF THE INVENTION

The invention has for its object to eliminate the above disadvantage wholly or partly. According to the invention, this object is achieved by an electron gun as mentioned in the opening paragraph wherein the dynamic astigmatism and focusing lens section is arranged on the main lens section for forming an integrated dynamic astigmatism and focusing main lens section.

Instead of adding more grids, in the electron gun according to the invention the DAF lens section is arranged on the main lens section. The electron gun according to the invention comprises a so-called Integrated DAF main Lens section (IDL).

The integrated DAF main lens section according to the invention has a number of advantages. The total length of an electron gun comprising the integrated DAF main lens section may be considerably smaller than that of the known electron guns. A shorter electron gun enables the manufacturing of cathode ray tube with a considerably reduced depth; the depth is measured as the distance between the front display screen and the base of the electron gun. In addition, the manufacturing of the electron gun according to the invention is considerably simplified giving rise to a reduction of the costs of the electron gun. In the integrated DAF main lens section according to the invention a (metal) pot, and also the corresponding brackets, that usually decouple the DAF lens section from the main lens section, are omitted. In addition, at least one base plate can be dispensed with. In case of a DBF-DAF electron gun, the introduction of an integrated DAF main lens section also triggers the removal of the DBF lens section.

An additional advantage of the integrated DAF main lens section according to the invention is that the spot performance is largely improved: the DAF lens section being arranged on the main lens section does no impact the beam diameter anymore. Yet another advantage of an electron gun comprising an integrated DAF main lens section is that the DAF lens section can be produced with less strength, as it is

positioned right on the main lens section (the closest possible with respect to the deflection unit).

A preferred embodiment of the electron gun according to the invention is characterized in that, the dynamic astigmatism and focusing lens section and the main lens section are mounted on a single base plate. Preferably, the base plate of the main lens section forms a part of the DAF lens section.

Mechanically (short gun + low cost) as well as optically (no modulation of the beam opening due to DAF), a goal is to achieve the a relatively very compact electron gun system (DAF + main lens). To this end, a preferred embodiment of the electron gun according to the invention is characterized in that the dynamic astigmatism and focusing lens section and the main lens section overlap each other. By this measure the total length of the electron gun is further reduced. This is realized, in particular, when the DAF section and the main lens share one plate. In this situation, the DAF and main lens have maximum mechanical and optical overlap. Preferably, the shape of the DAF holes is (semi-)elliptical to ensure a good behavior with respect to spherical aberrations.

A favorable embodiment of the electron gun according to the invention is characterized in that at least one grid is added to the dynamic astigmatism and focusing lens section for increasing the strength of the dynamic astigmatism and focusing lens section. Preferably, more than one grid is added to the DAF lens section to increase its strength. A favorable example of this embodiment of such an electron gun is an IDL having a DAF lens section comprising three grids.

Yet another favorable embodiment of the electron gun according to the invention is characterized in that the integrated dynamic astigmatism and focusing main lens section comprises at least two dynamic astigmatism and focusing plates added to the main lens section. Preferably, the integrated dynamic astigmatism and focusing main lens section further comprises a beam forming region.

A very cost effective DAF electron gun can be constructed by combining an integrated DAF main lens section having two DAF plates added to the main lens section and a beam forming region (triode + pre-focus).

The invention further relates to a cathode ray tube comprising such an electron gun.

BRIEF DESCRIPTION OF THE DRAWINGS These and other aspects of the invention are apparent from and will be elucidated with reference to the embodiments described hereinafter.

In the drawings:

Figure 1 is a main lens section focusing a beam at the center of the display screen; X and Y denote the horizontal and vertical directions;

Figure 2 shows an electron beam focusing at a corner of the display screen for the configuration as shown in Figure 1 ; the deflection coil ensures that the horizontal beam remains in focus whereas the vertical beam is over-focused;

Figure 3 is an electron gun comprising a DAF lens section and a main lens section;

Figure 4 shows an electron beam focusing at a corner of the display screen for the configuration as shown in Figure 3; the DAF lens sections brings the vertical beam in focus;

Figure 5 is a DBF-DAF electron gun;

Figure 6 shows an electron beam focusing at a corner of the display screen for the configuration as shown in Figure 5; the beam is focused at the corner position of the display screen;

Figure 7 is electron gun with a DAF lens section arranged on the main lens section according to the invention;

Figure 8A shows electron beam formation at the center of the display screen; Figure 8B shows electron beam formation at the corner of the display screen;

Figure 9 shows an example of an IDL gun having a DAF lens section comprising three grids;

Figure 10 shows an Integrated DAF main lens section with a DAF lens section comprising three grids, and

Figure 11 shows a cathode ray tube (CRT).

The Figures are purely diagrammatic and not drawn to scale. Notably, some dimensions are shown in a strongly exaggerated form for the sake of clarity. Similar components in the Figures are denoted as much as possible by the same reference numerals.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Figure 1 represents a main lens section focusing an electron at a center of the display screen. Both the horizontal and vertical directions are specified and are denoted by X and Y, respectively.

In the corner of the display screen, the (self-convergent) deflection coil ensures that the beam remains in focus in the horizontal direction (X). However, the beam undergoes the inverse effect in the vertical direction (Y). As a result the electron beam at a corner of the display screen tends to be over-focused (see Figure 2).

In a known electron gun a Dynamic Astigmatism and Focusing (DAF) lens section is introduced to counteract the above-described side effect of the deflection coil. The DAF lens section is separated from the main lens section by a (metal) pot; an example of such a DAF electron gun is shown in Figure 3. The main reason for this separation is, probably, avoiding an overlap of the electric fields generated by the DAF lens section and the main lens section. In addition, the DAF lens section and the main lens section are normally designed separately (modular design). The (metal) pot between the DAF lens section and the main lens section does not prevent optical interactions between the DAF lens section and the main lens section. Generally, such interactions will deteriorate the spot size as well as the spot uniformity on the display screen.

Figure 4 shows the shape of the electron beam in the corner of the display screen for an electron gun comprising a Dynamic Astigmatism and Focusing (DAF) lens section, a main lens section and a deflection coil. The common voltage of the DAF lens section and the main lens section is modulating such that the vertical beam is brought in focus all over the display screen while all dynamical effects cancel out in the horizontal direction.

When the beam is well focused on the display screen, the spot size is minimized by increasing the beam diameter in the main lens section until the spherical aberration prevents further reduction.

In the horizontal direction, the beam diameter in the main lens section is normally optimized in this way. However, the compromise is different in the vertical direction. In reality, the DAF lens section in the known electron gun as shown in Figure 3 and Figure 4 does not bring the beam totally in focus, especially when the deflection angle is relatively large. This would imply high amplitude of the dynamical voltage, which is not desired by the TV-set makers. To minimize the spot in the corner of the display screen, one must then reduce somewhat the beam diameter to create depth of focus. A compromise is found between a (large) optimal diameter of the beam focused in the center of the display screen and a flat beam, which is best for the corner of the display screen. Probably, the best compromise is found between the (large) optimal diameter of

the beam focused in the center of the screen and a flat beam, which is best for the corner of the screen.

From Figure 4 it can be learned that the DAF lens section actually modulates the beam diameter in the main lens section. Unfortunately, the modulation of the beam diameter by the DAF lens section achieves the opposite of what would be desirable. The beam has a minimal vertical diameter in the main lens section when it is perfectly in focus (center of the display screen) and has a maximal diameter when more depth of focus is needed (corner of the display screen). All over the display screen, the vertical spot is not minimized. In addition, the beam has a minimal horizontal diameter in the main lens section when it is focused in the corner position of the display screen. That is to say, where the display screen is the farthest away from the electron gun and where the negative lens of the deflection coil minimizes the beam diameter as seen by the display screen. As a result, the horizontal spot is not minimized in the corner of the display screen. Moreover the uniformity between the center and the corner of the display screen is decreased.

The solution adopted so far is to introduce a Dynamic Beam Forming (DBF) section. A typical example of such an Dynamic Beam Forming and Dynamic Astigmatism and Focusing (DBF-DAF) electron gun is shown in Figure 5. The DBF lens has an inverse polarity as compared to the DAF lens. Therefore the DBF lens can be used to compensate the side effect of the DAF modulation of the beam diameter in the main lens. The net effect of the DBF-DAF electron gun can also be that the modulation is opposite of what is delivered by the DAF section alone.

Figure 6 schematically shows an electron beam focusing at a corner of the display screen for the configuration as shown in Figure 5; the beam is focused at the corner position of the display screen. In the embodiment of Figure 6, spot performance, as well as spot uniformity, is improved. However, there are disadvantage for introducing a DBF section in the electron gun. One disadvantage is that it is a relatively expensive option: at least two base plates and a decoupling pot must be added to the electron gun. Attempts to integrate the DBF to the preceding lens in the gun (the pre-focus lens) were not very successful. From a focusing point of view, the DBF and DAF have opposite actions resulting in stronger DAF and/or higher amplitude of the dynamic voltage. A solution would be to add more grids into the electron gun. However, this would have a undesired effect on the length of the electron gun.

Figure 7 schematically shows electron gun with a DAF lens section arranged on the main lens section according to the invention. In the electron gun according to the invention the DAF section is placed directly on the main lens: in this manner an Integrated DAF main Lens (IDL) is obtained. The advantages of an electron gun with an integrated DAF main lens are substantial. Usually the extra pot (+ brackets) for decoupling the DAF and the main lens can be avoided as well as one base plate. Preferably, the dynamic astigmatism and focusing lens section and the main lens section are mounted on a single base plate. In addition, the dynamic astigmatism and focusing lens section and the main lens section, preferably, overlap each other. In addition, the dynamic astigmatism and focusing lens section and the main lens section, preferably, overlap each other.

In case of a DBF-DAF gun, the electron gun according to the invention also triggers the removal of the DBF section. Another advantage of the electron gun according to the invention is the improved spot performance.

The DAF section, being directly on the main lens does no longer have an impact on the beam diameter anymore. This is exemplified in Figure 8A and 8B. Figure 8A schematically shows electron beam formation at the center of the display screen; whereas Figure 8B schematically shows electron beam formation at the corner of the display screen. The net effect of the dynamical voltage is the modulation of the lens strength in vertical direction.

Figure 9 schematically shows an example of an IDL gun having a DAF lens section comprising three grids.

Figure 10 schematically shows an Integrated DAF main lens section with a DAF lens section comprising three grids. In this embodiment, the pre-focus lens is connected to the dynamic voltage and produces some DBF effect.

Figure 11 schematically shows a cathode ray tube (CRT). The CRT comprises an electron gun 30, and deflection means 33 and 34, which may be electromagnets. The deflection means are usually integrated in one unit. The CRT also comprises a shadow mask 35 in front of a luminescent display screen 36. All these parts are contained within a vacuum tube 37. The three electron beams corresponding to the three colors on the display screen are represented by the dotted lines 31, 32, 38. In

operation, electrons are emitted from the cathodes with help from the electrodes in the triode and voltages thereon. The electrons are then accelerated and focused by the focusing system to a focused electron beam. The focused electron beam is then deflected by the deflection means 33, 34. The electrons are finally spatially filtered by the shadow mask 35, so that after filtration they hit the luminescent display screen 36.

It should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be able to design many alternative embodiments without departing from the scope of the appended claims. In the claims, any reference signs placed between parentheses shall not be construed as limiting the claim. Use of the verb "comprise" and its conjugations does not exclude the presence of elements or steps other than those stated in a claim. The article "a" or "an" preceding an element does not exclude the presence of a plurality of such elements. The invention may be implemented by means of hardware comprising several distinct elements, and by means of a suitably programmed computer. In the device claim enumerating several means, several of these means may be embodied by one and the same item of hardware. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.