Color cathode ray tube having an in-line electron gun

A color cathode ray tube having an integrated electron gun structure including mirrored main focusing and accelerating electrodes (46B, 52). The electron gun comprises a triode section formed by three in-line arranged cathodes (27, 28, 29) and first and second grid electrodes (30, 36) whose apertures are symmetrically disposed about respective central (21) and outer (32, 34) axes which pass through their respective cathodes. A prefocusing electrode (38) having eccentric outer apertures is positioned next to the second electrode (36). The field produced by the prefocusing electrode (38) serves to converge the electron beams produced in the triode section. At least one, but more conveniently two, further electrode(s) (40, 46A) are provided between the prefocusing electrode (38) and the main focusing lens electrode (46B). The outer apertures (say the apertures 44) of at least one of the further electrodes are elongated to provide an asymmetrical lens field which is used to neutralize spot errors, beam displacement and beam asymmetry. The shapes of the elongated apertures (44) are such that a portion of their peripheries are concentric about the axes (32, 34).

BACKGROUND OF THE INVENTION 
The present invention relates to a colour cathode ray tube having an 
in-line electron gun. 
Electron guns for colour cathode ray tubes are arranged to generate three 
electron beams whose paths of propagation lie in a plane which is 
generally horizontal. The electron guns may be constructed so that there 
is one discrete electron gun for each beam or so that they have a number 
of electrodes in common, a so-called integrated electron gun structure. 
Integrated electron gun structures are inherently more compact and in 
consequence are popular for use in those colour cathode ray tubes, such as 
narrow-necked and mini-necked colour cathode ray tubes in which space is a 
premium. When designing and constructing an electron gun for a colour 
cathode ray tube various types of errors have to be taken into account and 
an optimum comprimise has to be decided upon in order to minimize the 
errors. The types of errors which are of interest are core haze 
eccentricity (CHE), beam displacement (BD) and free fall error (FFE). Core 
haze eccentricity occurs when the haze which surrounds the spot proper at 
the screen is located eccentrically with reference to the centre of the 
spot. Beam displacement occurs in respect of relative positions of the 
outer electron beams to the center electron beam. Free fall error (FFE) is 
effectively the convergence error at the screen. FFE can be corrected by 
altering the pitches of the outer apertures with respect to the central 
aperture in the electrodes of the triode part of the electron gun to 
obtain a desired angle of trajectory. However this also has an effect on 
CHE and BD. CHE can be reduced by ensuring that the converging electron 
beams pass through the centers of their respective focusing lenses. In 
simple terms these errors can be grouped in two classes namely focusing 
errors and convergence errors. Furthermore unless special precautions are 
taken, measures to reduce the effects of one type of error make the other 
type of error worse. 
British Patent Specification No. 2031221 A (PHN 9215) discloses an in-line 
electron gun assembly in which focusing and convergence are independently 
adjustable. In the embodiments of the electron guns disclosed the 
convergence of the outer electron beams takes place in the prefocusing 
part of the electron gun and the electron beam focusing is carried-out 
using a bipotential electron lens. An embodiment of an integrated electron 
gun assembly shown in FIG. 4 of Specification 2031221A has three in-line 
arranged cathodes, a first grid, a second grid, a prefocusing grid, a 
focusing electrode an an accelerating electrode, all the grids/electrodes 
being orthogonal to the central longitudinal axis of the electron gun. 
Each grid/electrode has three in-line apertures of which the central ones 
are co-axial about said central longitudinal axis. However in order to 
obtain the required degrees of freedom the outer apertures in the 
prefocusing grid, the focusing electrode and the accelerating electrode 
are not only of differing sizes but their pitches, that is the distance 
from their centers to the central longitudinal axis, are different. 
Consequently no two grids/electrodes are the same. 
Specification U.S. Pat. No. 4612474 discloses an in-line integrated 
electron gun having mirrored main focusing and accelerating electrodes. A 
pre-focusing electrode is provided between the triode (or beam forming) 
section of the electron gun and the main focusing lens. The outer 
apertures of the electrodes of the triode section are concentric about 
respective axes. The axes of the outer apertures in the pre-focusing 
electrode are displaced outwards relative to the first mentioned axes. 
Lastly the axes of the apertures in the main lens electrodes are displaced 
inwards relative to the first mentioned axes. By offsetting the axes in 
this way the outer electron beams are converged by the prefocusing lens. 
Such an arrangement provides two degrees of freedom, namely the 
eccentricity of the outer apertures in the pre-focusing electrode and the 
offsetting of the respective axes for optimising the spot error, beam 
displacement and beam asymmetry. Hence a compromise has to be made. 
Another aspect to be considered is the assembly of the electrodes 
comprising the electron gun. Normally a jig is used having three 
substantially parallel insertion pins. Each pin has a plurality of steps 
of different cross-sectional area thereon which steps act as abutments for 
the mutual spacing of some of the electrodes in the axial direction, the 
mutual spacing of others of the electrodes being obtained by the use of 
spacers. Offsetting the axes of outer apertures in one or more electrodes 
requires the pins to be specially formed. This is both troublesome because 
the pins have to be specially formed and this constitutes an additional 
cost item because each type of electron gun requires its own jig. 
SUMMARY OF THE INVENTION 
An object of the present invention is to avoid having to compromise between 
FFE, BD and CHE. 
According to the present invention there is provided a colour cathode ray 
tube having an electron gun structure for producing three electron beams 
whose paths of propagation constitute a single plane, the gun structure 
comprising a triode section consisting of in-line arranged central and two 
outer cathodes and first and second grid electrodes each having central 
and two outer apertures which are symmetrically disposed about respective 
axes passing through the cathodes; a third electrode having in-line 
arranged central and outer apertures, the outer apertures being 
eccentrically disposed about the respective axes passing through the outer 
apertures of the first and second grid electrodes; mirrored main focusing 
and final accelerating electrodes and means disposed between the third 
electrode and the main focusing electrode for producing asymmetrical 
electrical fields in the beam paths of the outer electron beams. 
The invention is based on the recognition of the fact that at least three 
degrees of freedom for optimising FFE, BD and CHE are obtainable in an 
electron gun having mirrored lens and accelerating grid components by 
constructing the electron gun so that convergence is determined in the 
prefocusing section of the electron gun and so that other asymmetries are 
corrected by said means thereby enabling the outer electron beams to pass 
through the centers of their respective focusing lenses. By being able to 
provide at least three degrees of freedom compromises which have been 
necessary in some prior electron guns having only two degrees of freedom 
are unnecessary. 
In embodiments of the present invention the asymmetrical electrical field 
producing means may comprise one or two further electrodes. The outer 
apertures in the one or at least one of the two further electrodes are 
elongate in the plane of the electron beams. 
To facilitate assembly of the electrodes of the electron gun structure on 
insertion pins, at least a portion of the periphery of each of the 
elongate apertures which intersects and crosses the in-line plane, is 
concentric about its respective one of the axes passing through the outer 
apertures in the first and second grid electrodes. The direction of 
elongation is either towards or away from the central aperture of the 
relevant further electrode. By elongating the holes in this manner, a 
standard set of mounting pins can be used to assemble several different 
types of electron guns which not only introduces an element of flexibility 
but also a cost saving.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
FIG. 1 is a cross-sectional view of a colour cathode ray tube including a 
glass envelope 10 having a neck 14, a display window 12 and a conical part 
13. An integrated in-line electron gun 16 is provided in the neck 14 to 
generate three electron beams 18, 19, and 20. The axes of these electron 
guns are situated in one plane, the plane of the drawing. The longitudinal 
axis of the electron gun 16 coincides with the main axis 21 of the 
envelope. A display screen 22 comprising a large number of triplets of 
phosphor lines is provided on the inside of the display window Each 
triplet comprises a line consisting of a green luminescing phosphor, a 
line consisting of a blue luminescing phosphor and a line consisting of a 
red luminescing phosphor. The phosphor lines extend perpendicularly to the 
plane of the drawing. A shadow mask 23 having a large number of elongate 
apertures 24 parallel to the phosphor lines, through which apertures the 
electron beams 18, 19 and 20 pass, is placed before the display screen 22. 
Since the electron beams enclose a small angle with each other and 
converge on the display screen, each beam is incident only on phosphor 
lines of one colour via the elongate apertures. 
Referring now to FIG. 2, the integrated in-line electron gun 16 shown may 
for convenience of reference be regarded as a quadri-potential focusing 
electron gun because of the manner in which the electrodes are connected. 
The electron gun 16 comprises a triode section formed by three cathodes 
27, 28 and 29 and first and second grid electrodes 30, 36. The grids 30, 
36 have central and outer apertures of substantially the same size. The 
central apertures in the first and second grids are symmetrically disposed 
about the main axis 21 and the side or outer apertures in the first and 
second grids are symmetrical about their respective axes 32, 34. A third, 
prefocusing grid electrode 38 is provided and has central and two outer 
apertures. The central aperture is coaxial about the axis 21 whereas the 
outer apertures are eccentric with respect to the axes 32, 34 thereby 
introducing the major part of the convergence to the electron beams 
passing therethrough. A fourth grid electrode 40 follows the third grid 
38. In the presently described example this grid 40 has a circular central 
aperture 42 which is co-axial of the main axis 21 and asymmetrical outer 
apertures 44 whose axes of symmetry are not coincident with the axes 32, 
34. These apertures 44 are made asymmetrical by elongating an otherwise 
circular aperture outwardly (FIG. 3) or inwardly (FIG. 4) in the direction 
of the in-line plane. In either case the elongate apertures 44 are of 
greater area than the central aperture 42. The non-elongated peripheral 
portions of the apertures 44 which intersect and cross the in-line plane 
are co-axial with respect to their axes 32, 34. 
A fifth electrode 46 comprises two cup-shaped members 46A, 46B which are 
joined together at their rims. The central aperture 48 and the outer 
apertures 50 of the member 46A are coaxial about their respective axes 21, 
32 and 34. In the present example the apertures 48 and 50 are of the same 
size as the aperture 42 in the electrode 40. 
The electrodes 40, 46A co-operate to produce asymmetrical electric fields 
for the outer electron beams, which fields provide two extra degrees of 
freedom to those already provided by the pitches of the apertures in the 
first and second electrodes 30, 36 and the eccentricity of the outer 
apertures in the pre-focusing electrode 38. These extra degrees of freedom 
can be used to neutralise spot error, beam displacement and beam 
asymmetry. These extra degrees of freedom are obtained by varying the 
pitch which is achieved by the elongate shape of the holes, when present 
in the electrodes 40, 46A and suitably adjusting the mutual distances 
between the electrodes 40, 46A. For convenience of illustration the 
apertures 44 have been made elongate. However in alternative 
non-illustrated embodiments of the present invention the apertures 50 in 
the electrode 46A are asymmetric and of greater area than that of the 
central aperture whilst the apertures 44 are circularly symmetrical and 
coaxial about the axes 32, 34 the outer apertures 44 and 50 in both the 
electrodes 40, 46A are asymmetric and are of greater area than that of the 
respective central apertures or the asymmetric electric field is produced 
by a single electrode, say the electrode 46A, the electrode 40 having been 
omitted. 
The cup-shaped member 46B constitutes the main focusing electrode and 
together with an accelerating electrode 52 forms lens fields for the final 
focusing of the electron beams. The member 46B and the electrode 52 are 
mirrored electrodes so that any distortion introduced into the electron 
beam(s) due to an imperfection in one of these electrodes is compensated 
at least in part by the corresponding imperfection in the other of these 
electrodes. Each electrode 46B and 52 is formed as a "bath tub" electrode 
comprising a peripheral rim and a base portion in which three in-line 
arranged apertures are provided. The apertures may be of polygonal shape, 
for example as disclosed in European Patent Specification No. 0134059 
corresponding to U.S. Pat. No. 4,626,738 (PHN 10.752), details of which 
are incorporated by way of reference. 
By electrically interconnecting the electrodes 36 and 40 and the electrodes 
38 and 46, the electron gun can be operated as a quadri-potential electron 
gun by applying 0 V to the electrode 30, 500 V to the electrodes 36 and 
40, 7750 V (31% of the final anode voltage) to the electrodes 38 and 46 
and 25 kV to the accelerating electrode 52. 
In the embodiment illustrated in FIGS. 2 and 4 the spacings (S) between the 
respective electrodes are 
EQU S.sub.27,30 =0.08 mm 
EQU S.sub.30,36 =0.405 mm 
EQU S.sub.36,38 =1.0 mm 
EQU S.sub.38,40 =1.0 mm 
EQU S.sub.40,46 =1.0 mm 
EQU S.sub.46,52 =0.9 mm 
The axial thicknesses (or axial lengths) (d) of the electrodes are 
EQU d.sub.30 =0.085 mm 
EQU d.sub.36 =0.30 mm 
EQU d.sub.38 =0.40 mm 
EQU d.sub.40 =0.80 mm 
EQU d.sub.46 =20.00 mm 
The nominal pitch, that is, the distance between the central axis 21 and 
the outer axis 32 or 34 is 4.86 mm. However the pitch of the eccentric 
apertures in the third grid electrode 38 with respect to the axis 21 is 
4.91 mm. In the case of the elongate apertures 44 in the grid electrode 
40, the pitch is measured to the axis of symmetry of the elongate hole and 
in this example the pitch has a value of 4.77 mm. The outermost surfaces 
of the apertures are circular having their centers of curvature coinciding 
with the axis 32, 34, respectively. The diameter of the apertures in the 
electrodes 30, 36 is 0.6 mm, that of the apertures in the electrodes 38 
and 46A are 1.15 mm and 3.0 mm, respectively. In the case of the aperture 
42 in the electrode 40, its diameter is 3.0 mm whereas the elongate 
apertures 44 are effectively formed by two overlapping circles of 3.0 mm 
diameter, with a distance of 0.18 mm between their centers. 
FIG. 5 illustrates a jig 60 on which the electrodes constituting an 
integrated electron are assembled prior to their being fixed together by 
means of glass rods (not shown). The jig 60 includes a base member 62 on 
which three upstanding insertion pins 64, 66, 68 are provided. The steps 
formed on each of the pins 64, 66 and 68 are such that some of the grids 
and electrodes can rest against an abutment thereby ensuring their 
relative axial positions whilst others are separated from each other by 
spacers 72, 74 and 76. Additionally in order to obtain the correct 
alignment it is necessary to ensure that there is no lateral misalignment 
and/or rotational misalignment. These possible misalignments can be 
avoided by machining accurately the correct profiles on the pins 61, 66 
and 68. However, this would mean that each jig is only suitable for a 
particular electron gun and not for a range of electron guns. This need 
not be the case in respect of the electron gun used in the colour cathode 
ray tube made in accordance with the present invention because by 
elongating the apertures 44 in the electrode 40 so that at least a portion 
of their peripheries are concentric with the respective axes 32, 34 it is 
possible to effect the necessary changes required to obtain the desired 
extra degrees of freedom but at the same time obtain the required 
alignment of the electrodes. In order to obtain this flexibility, the 
relevant step 70 on the outer insertion pins 66, 68 is circular having a 
diameter corresponding to the nominal diameter of the concentric portion 
of the apertures 44 that is 3.0 mm in the numerical example given above. 
Thus if the apertures 44 are elongated outwards as shown in FIG. 3, the 
inner peripheral portions bear against the steps 70 on the pins 66, 68 and 
if the apertures 44 are elongated inwards as shown in FIG. 4 then their 
outer peripheral portions bear against the step 70 on the pins 66, 68. In 
either case lateral displacement and rotational displacement of the 
electrode 40 is prevented.