Picture display device provided with an electron gun, and electron gun for use in such a device

A picture display device comprises an evacuated envelope and is provided at a first side with an electroluminescent display screen and at an opposite side with an electron gun (6). Between the electron gun (6) and the display screen, the device comprises deflection means with which an electron beam generated by the electron gun (6) can be deflected during operation. The electron gun (6) has at least an electron beam-generating portion (20) and is provided with a main lens system (40) having a first electrode (41), a final electrode (45) and at least one intermediate electrode (42-44) across which a main lens voltage is gradually applied step-wise during operation so as to form an electron-optical main focusing lens. The gun is further provided with means for presenting a dynamic potential (V.sub.d) to at least an electrode (41) of the main lens system (40), which is the first electrode viewed in the direction of propagation of the electron beam (7-9). For an increased dynamic focusing, a coupling capacitor (50) is arranged at least between said first electrode (41) and the subsequent electrode (42) of the main lens system (40).

BACKGROUND OF THE INVENTION 
The invention relates to a picture display device comprising an evacuated 
envelope, a first side of which is provided with an electroluminescent 
display screen, an opposite side is provided with an electron gun and in 
which deflection means are arranged between the electron gun and the 
display screen with which means, at least during operation, an electron 
beam generated by the electron gun can be deflected, the electron gun 
having a portion generating at least an electron beam and being provided 
with a main lens system having a first electrode, a final electrode and at 
least one intermediate electrode across which a main lens voltage is 
gradually applied step-wise during operation so as to form an 
electron-optical main focusing lens. The invention also relates to an 
electron gun for use in such a device. 
Such a device is known from U.S. Pat. No. 3,932,786. The electron gun 
described in this Patent comprises six intermediate electrodes between the 
first electrode and the final electrode of the main lens system and, in 
comparison with other, more conventional electron guns, it comprises a 
relatively large number of electrodes. For this reason such a main lens is 
commonly referred to as DML (Distributed Main Lens), MSFL (Multi-Stage 
Focus Lens) or MEL (Multi-Element Lens). The separate electrodes of the 
main lens system in the known device are interconnected by means of a 
resistive voltage divider so that the main lens voltage is gradually 
distributed step-wise across the electrodes during operation in order to 
reduce the magnitude of potential jumps in the main lens system. This 
leads to considerably improved lens properties as compared with more 
conventional guns in which the main lens voltage is entirely applied 
across only two electrodes. Notably spherical aberrations can be 
adequately suppressed to relatively large electron beam currents without 
an increase of the lens diameter being required. 
Although spherical aberrations can be reduced to an acceptable level in a 
picture display device of the type described above, spot errors may 
nevertheless occur due to dynamic focusing errors. Such spot errors arise, 
inter alia because the path length of the electron beam varies, dependent 
on the position of the spot on the display screen. This becomes notably 
manifest with pixels which are further remote from the centre of the 
display screen. 
SUMMARY OF THE INVENTION 
It is, inter alia an object of the present invention to provide a picture 
display device of the type described in the opening paragraph in which 
also dynamic focusing errors are adequately inhibited. 
According to the invention, a device of the type described in the opening 
paragraph is therefore characterized in that the first electrode of the 
main lens system is provided with means for applying a dynamic potential 
thereto, at least during operation, and in that a coupling capacitor is 
arranged at least between the first electrode and the electrode of the 
main lens system which is the subsequent electrode viewed in the direction 
of propagation of the electron beam. 
The dynamic voltage which is applied to the first electrode of the main 
lens system in the device according to the invention has a time-dependent 
variation which is adapted to the sweep of the electron beam across the 
display screen. With this dynamic signal, the static main lens voltage is 
constantly corrected for the changing path length of the beam. Thus it is 
achieved that a voltage which is always optimum for focusing the beam is 
present across the main lens system. The invention is based on the 
recognition that such a dynamic focusing is considerably increased by 
coupling the dynamic potential via a coupling capacitor to at least the 
electrode subsequent to the first electrode of the system. According to 
the invention, an extremely accurate spot formation is achieved in a 
device of the type described in the opening paragraph. 
A special embodiment of the device according to the invention is 
characterized in that a coupling capacitor is arranged both between the 
first electrode of the main lens system and the subsequent electrode of 
the system, and between said subsequent electrode and the next electrode 
of the system. In this embodiment the dynamic focusing is further 
increased because a coupling capacitor is now arranged not only between 
the first electrode and the subsequent electrode of the main lens system 
but also between said subsequent electrode and the next electrode of the 
system. Coupling capacitors may also be arranged between further pairs of 
adjacent electrodes of the system so as to further spread the dynamic 
potential across the main lens system. A better dynamic focusing and 
consequently a better lens action always appears to be achieved as 
compared with the full absence of coupling capacitors in the main lens 
system. The plurality of coupling capacitors may each have the same 
capacitance or be separately adjusted to a lens action which is optimum 
for specific cases. 
As a larger number of coupling capacitors is used in the main lens system, 
the dynamic potential has a larger spread across the electrodes of the 
grid so that the electron beam will be subject to fewer large potential 
jumps. The dynamic effect of the main lens system thereby decreases 
eventually. An optimum dynamic lens action is achieved in a preferred 
embodiment of the device according to the invention in which a coupling 
capacitor is arranged exclusively between the first electrode of the main 
lens system and the subsequent electrode of the system, and between the 
subsequent electrode and the next electrode of the system. In this 
embodiment a coupling capacitor is arranged exclusively between the first 
two pairs of adjacent electrodes of the main lens system. 
Exclusively capacitors resistant to the potential differences which are 
customary in electron guns should be used as coupling capacitors. In a 
special embodiment of the device according to the invention only 
capacitors having a dielectric of barium titanium oxide and being 
resistant to potential differences of up to about 15 kV are used.

The Figures are purely diagrammatic and not to scale. For the sake of 
clarity, some dimensions are exaggerated. Corresponding components in the 
Figures have been given identical reference numerals as much as possible. 
DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The embodiment of the picture display device according to the invention 
shown in FIG. 1 is provided with a cathode ray tube 1 having an evacuated 
envelope 2 with a display window 3, a cone 4 and a neck 5. The neck 5 
accommodates an electron gun 6 for generating, in this embodiment, three 
electron beams 7-9. It is to be noted that within the scope of the 
invention the term electron gun should be considered to have a wide 
meaning so that it does not only include a single gun suitable for 
generating only one electron beam, but also integrated or non-integrated 
systems of often three electron guns which are described, for example in 
the present embodiment. 
An electroluminescent display screen 10 which, in this embodiment comprises 
red, green and blue phosphor elements is present at the inner side of the 
display window 3. The outer side of the envelope 2 is provided with 
deflection means 11 which are only shown diagrammatically and generally 
comprise a deflection unit in the form of a system of magnetic coils. On 
their path to the display screen 10, the electron beams 7-9 can be 
deflected by means of the deflection unit so that the entire display 
screen 10 can be scanned, and the beams pass a colour selection means 12 
which in this embodiment comprises a shadow mask in the form of a plate 
having apertures 13. The beams 7-9 pass the apertures 13 at a small mutual 
angle and thus only impinge on phosphor elements of the colour associated 
with the relevant beam 7, 8, 9. The picture display device further 
comprises means 14, shown diagrammatically, for applying electric voltages 
to the electrodes of the electron gun, which means are connected to the 
electron gun 6 in the final product by means of lead-through electrodes 
15. The assembly further has a housing (not shown). Particularly, means 14 
may be used for applying a dynamic potential to the main lens system. 
The electron gun 6 of the device of FIG. 1 is shown in perspective and 
greater detail in FIG. 2. The gun 6 comprises an electron beam-generating 
portion 20 referred to as the triode, in which three juxtaposed electron 
sources are incorporated which are provided with a common electrode 21, 
often referred to as G1 which is connected to ground during operation. 
Similarly as all other electrodes of the electron gun 6, the common 
electrode 21 is provided with three apertures 16 aligned in a row and 
having a diameter of approximately 5.5 mm for passing the electron beams. 
The gun 6 also comprises a prefocusing section 30 which is constituted by 
two successive electrodes 31, 32 having operating potentials of typically 
400-500 volts and 5-6 kV, respectively which are usually denoted as G2 and 
G3, respectively. The electron-optical prefocusing lens which is 
constituted by this system 31, 32 of electrodes provides a virtual image 
of the electron sources which serves as an object for a main focusing lens 
constituted in a subsequent main focusing section of the gun. 
The main focusing section comprises a main lens system 40 having a first 
electrode (41), a final electrode (45) and three intermediate electrodes 
(42-44) across which a main lens voltage of typically 25-30 kV is applied 
during operation. In this embodiment, a potential V.sub.g of 5-6 kV is 
present during operation at a first electrode 41 of the system, while the 
potential of the last electrode 45 which is generally referred to as anode 
is 30-35 kV during operation. 
In the main lens system of the device described, the main lens voltage is 
distributed gradually and step-wise across the five electrodes (41-45) of 
the main lens system (40). To this end the intermediate electrodes 42-44 
are interconnected by means of a resistive voltage divider 46 and 
connected to the outer electrodes 41, 45 of the system. By this uniform 
and step-wise spread of the main lens voltage across the five electrodes, 
the potential jump between adjacent electrodes in the main lens system may 
remain limited to 5-15 kV, which has an extremely favourable effect on the 
lens action of the main lens. Thus, for example spherical aberrations can 
be adequately inhibited, even at larger beam currents without an increase 
of the lens diameter being required. 
The different components of the gun are held together at both sides by 
means of an insulating support 47, often referred to as multiform rod or 
beading rod and fixed with respect to each other. The assembly further 
comprises a plurality of radially positioned centring springs 49 with 
which the gun is centred in the neck 5 of the envelope 2 and with which 
also the high voltage of the anode 45 can be taken up at the tube wall. At 
the opposite side, the gun is provided with lead-through electrodes 15 
which for the sake of clarity have been omitted in the relevant Figure, 
but with which the other potentials required in the gun can be supplied. 
The first electrode 41 of the main lens system 40 is provided with means in 
the form of an electric connection shown diagrammatically in the Figure 
for applying, during operation a dynamic potential V.sub.d of the order of 
0.5-2.0 kV in addition to a static potential V.sub.g of 5-6 kV to these 
means. The time-dependent variation of the dynamic voltage V.sub.d follows 
the sweep of the electron beam 7, 8, 9 in the deflection field of the 
deflection means 11 in such a way that the total focusing voltage is 
always adapted to the changing path length of the beam. Thus, in the 
device according to the invention, dynamic focus errors and related 
spherical aberrations of the ultimate spot of the beam 7, 8, 9 on the 
display screen 10 can be inhibited. 
According to the invention, the dynamic voltage V.sub.d is also applied to 
at least the subsequent electrode 42 of the main lens system 40. This is 
achieved by arranging a coupling capacitor 50 at least between the first 
electrode 41 and the subsequent electrode 42. The coupling capacitor is 
not visible in FIG. 2 but is represented diagrammatically. 
FIG. 3 shows diagrammatically a plurality of alternative configurations of 
the main lens system of the gun of FIG. 2 with one or more coupling 
capacitors 50 between adjacent grids. Moreover, the known situation is 
shown in which there is no coupling capacitor between the grids but only a 
small parasitic coupling. In this embodiment a capacitor having a 
dielectric of barium titanium oxide which is breakdown-resistant to high 
voltages of up to about 15 kV is used for the coupling capacitor(s) (50). 
Moreover, in this embodiment substantially identical capacitors 50 having 
a relatively large capacitance of approximately 2 nF are used. Within the 
scope of the invention it is readily possible to vary the capacitances of 
the different coupling capacitors 50 in the system 40 so as to further 
improve the dynamic focusing and use smaller capacitances which, however, 
should exceed the value of the parasitic capacitance of typically 3-50 pF. 
Of the different configurations of FIG. 3, FIG. 4 shows a computer 
simulation of the dynamic focusing across the display screen in terms of 
the reciprocal value of the focal length 1/b as a function of the dynamic 
focusing voltage V.sub.d. The curve indications correspond to the numbers 
of the configurations in FIG. 3. All configurations 3B, 3D-F in which, in 
accordance with the invention, a coupling capacitor is arranged between 
the first electrode 41 and the subsequent electrode 42 of the main lens 
system appear to have a stronger dynamic focusing than the reference 
system 3A in which no coupling capacitor are used. A further improvement 
of the dynamic focusing is obtained in all configurations 3D-F in which a 
coupling capacitor 50 is arranged both between the first electrode 41 of 
the main lens system 40 and the subsequent electrode 42 of the system, and 
between said subsequent electrode 42 and the next electrode 43 of the 
system, and possibly between further pairs 43-44, 44-45 of electrodes. In 
contrast, the coupling between the first electrode 41 of the system and 
only the third electrode 43, cf. configuration 3C, provides a poorer 
dynamic focusing than the reference system 3A. 
Moreover, it appears that configuration 3D is preferred in which the 
dynamic focusing voltage is coupled through in the main lens system 40 but 
still not "spread" so much that its effect becomes smaller. This 
configuration shows the strongest dynamic focusing and is therefore used 
in a preferred embodiment of the invention. 
Although the invention has been described with reference to a single 
embodiment, it will be evident that it is by no means limited thereto and 
that those skilled in the art will be able to conceive many variations and 
designs within the scope of the invention. For example, the invention is 
not only suitable for a gun of the above-described bipotential type but 
also for unipotential and three-potential guns in which a potential is 
associated with at least one of the electrodes of the main lens system. 
Moreover, within the scope of the invention, a larger or smaller number 
than three electrodes can be used between the first and last electrodes of 
the main lens system. Moreover, the invention is not only suitable for a 
colour display device having an integrated colour gun but also for a 
colour display device having (three) separate electron guns, and for 
monochrome picture display devices. 
Generally, the invention provides a picture display device of the type 
described in the opening paragraph with an electron gun comprising a 
multiple main lens in which in comparison with conventional devices of the 
same type a considerably better focusing is achieved by presenting a 
dynamic focusing voltage and by capacitive intercoupling of adjacent 
electrodes.