Color display tube having tensioned color selection electrode and mounting arrangement

A flat color selection electrode is tensioned on a supporting frame. In order to compensate for differences in thermal expansion between the supporting frame and the color selection electrode and mislanding, spring constructions connect the supporting frame and the color selection electrode and permit only a maximum tension at the color selection electrode which is smaller than the elastic proof stress of the color selection electrode. Such a color selection electrode may be used in a color display tube having an at least substantially flat display screen.

BACKGROUND OF THE INVENTION 
The invention relates to a color display tube comprising in an evacuated 
envelope means to generate a number of electron beams and comprising an at 
least substantially flat display screen having areas luminescing in 
different colors and a color selection electrode having apertures for 
passing the electron beams and associating each electron beam with 
luminescent areas of one color, said color selection electrode being 
tensioned on a supporting frame of at least one set of oppositely located 
sides. 
The invention also relates to a color selection electrode tensioned on a 
supporting frame for use in a colour display tube according to the 
invention. 
Nowadays, nearly all commercial color display tubes have domed display 
screens. However, it is desired to provide a tube having a generally flat 
display screen. There are problems which have to be solved before a tube 
having a flat display screen is commercially available. A major problem 
concerns the color selection electrode. In known constructions of a tube 
having a domed display screen the color selection electrode is similarly 
curved and in such a manner that the color selection electrode varies 
slightly parallel to the contor of the display screen. Starting from such 
a known construction the color selection electrode of a tube having a 
substantially flat display screen should have a flat contor. However, such 
a color selection electrode has an insufficient non-deformability or 
rigidity. One manner of giving a color selection electrode rigidity is 
disclosed in U.S. Pat. No. 4,069,567. In this specification a method is 
disclosed of installing a color selection electrode in a color display 
tube in which the color selection electrode is kept tensioned in a 
supporting frame. The supporting frame may be of metal and may be placed 
in the evacuated envelope of The tube at a certain distance from the 
display screen. Alternatively the supporting frame may be substantially of 
glass, so that, for example, the frame may be constituted by the display 
screen or the evacuated envelope of the tube. In a preferred embodiment of 
this method of installing a color selection electrode, the color selection 
electrode is manufactured from a material which has a coefficient of 
thermal expansion exceeding that of the supporting frame. The color 
selection electrode and the supporting frame are heated together, for 
example in an oven, while the color selection electrode is tensioned. 
Simultaneously the color selection electrode is heated complementarily. 
The supporting frame and the color selection electrode consequently 
expand. However, as a result of the complementary heating the color 
selection electrode expands more than the supporting frame. The color 
selection electrode is then fixed to the supporting frame. The color 
selection electrode and the supporting frame are finally cooled to room 
temperature as a result of which the color selection electrode is extra 
tensioned. 
During the processing and the operation of the display tube, however, 
temperature differences occur between the color selection electrode and 
the supporting frame which can increase or decrease the tension of the 
color selection electrode. During the cooling portion of a number of 
process steps, for example bonding components of the envelope together and 
evacuating the display tube, the supporting frame will be warmer than the 
color selection electrode as a result of the difference in thermal 
capacity. This causes such a great difference in expansion between the 
supporting frame and the tensioned color selection electrode that the 
tension in the color selection electrode in this stage can become larger 
than the elastic proof stress (i.e. the elastic limit) of the color 
selection electrode, as a result of which it may be deformed permanently. 
After cooling the adhered or evacuated display tube the deformed color 
selection electrode is slack in the supporting frame. As a consequence 
mislanding occurs, i.e. each electron beam is not properly associated with 
luminescent areas of one color. 
It is an object of the invention to provide a color display tube having a 
color selection electrode which is secured to a supporting frame with a 
tension which is as large as possible, in which differences in thermal 
expansion between the color selection electrode and the supporting frame 
can be permitted without the tension in the color selection electrode 
becoming larger than the elastic proof stress of the color selection 
electrode. 
SUMMARY OF THE INVENTION 
According to the invention, a color display tube of the kind mentioned in 
the opening paragraph is characterized in that at least one side of each 
set of oppositely located sides of the color selection electrode connected 
to the supporting frame is connected to said supporting frame by means of 
a spring construction, the spring construction only permitting a mximum 
tension on the color selection electrode which is smaller than the elastic 
proof stress of the color selection electrode. The spring construction 
serves as a buffer to reduce changes in tension. Changes in tension which 
occur are not transmitted directly the between color selection electrode 
and the supporting frame but via the spring construction. By making the 
spring construction so that the maximum tension of the color selection 
electrode remains smaller than the elastic proof stress of the color 
selection electrode, the tension with which the color selection electrode 
is connected to the supporting frame may approach said elastic proof 
stress as close as possible. The spring construction then prevents a 
permanent deformation of the color selection electrode. Moreover as a 
result of this a maximum-tensioned color selection electrode is obtained. 
An embodiment of a color display tube in accordance with the invention is 
characterized in that each spring construction has a spring constant which 
is smaller than the spring constant of the color selection electrode. The 
color selection electrode may be considered as a spring which has a 
certain spring constant which depends inter alia on the intrinsic spring 
constant of the color selection electrode material and on the pretreatment 
to which the color selection electrode has been subjected, for example the 
etching of apertures therein. By exerting tensile stresses on the color 
selection electrode it is elongated elastically. When said tensile 
stresses reach the elastic proof stress of the color selection electrode, 
the color selection electrode is deformed permanently. When the tensile 
stresses between the supporting frame and the color selection electrode 
are transmitted via a spring construction whose spring constant is smaller 
than the spring constant of the color selection electrode, said spring 
construction is elongated more than the color selection electrode. The 
effective tensile stress which the color selection electrode experiences 
hence becomes smaller so that the point at which the color selection 
electrode would otherwise reach its elastic proof stess is changed. 
A further embodiment of a color display tube in accordance with the 
invention is characterized in that both sides of each set of oppositely 
located sides of the color selection electrode which are connected to the 
supporting frame are connected to said supporting frame by means of a 
spring construction. If only one side of each set of oppositely located 
sides which are connected to the supporting frame is connected to the 
frame by means of a spring construction, a possible mislanding may occur 
in the case of a change in size of the color selection electrode, which 
mislanding is non-symmetrical with respect to the centre of the color 
selection electrode since the color selection electrode is held by the 
spring construction only on one side. By providing a spring construction 
on both sides of each set of oppositely located sides of the color 
selection electrode, which construction is connected to the supporting 
frame, the possibly occuring mislanding becomes symmetrical. 
It is a further object of the invention to provide a color display tube 
having a tensioned color selection electrode in which no mislanding 
occurs. 
Thus, a further embodiment of the color display tube in accordance with the 
invention is characterized in that the spring constructions only permit 
movements of the apertures in the color selection electrode along the 
electron paths. When the possibly occurring mislanding becomes 
symmetrical, this can be removed successfully by converting all the 
movements of the apertures in the color selection electrode, for example, 
resulting from thermal loads, vibrations, impact or stroke, via the spring 
constructions into movements exclusively along the electron paths. 
The spring construction may be built up from a number of leaf springs 
extending over the length of a side of the color selection electrode or 
from one long leaf spring which is provided over the length of a side. 
A still further embodiment of a color display tube having a color selection 
electrode in accordance with the invention is characterized in that each 
leaf spring referred to above comprises a substantially flat surface to 
which a major surface of the color selection electrode is connected. Since 
a major surface, the upper surface or the lower surface, of the color 
selection electrode is connected to a flat surface of each leaf spring of 
which the spring construction is composed, the color selection electrode 
remains flat also during movements. Hence bending stresses which may 
change the shape of the color selection electrode in an undesirable manner 
do not occur. 
A still further embodiment of a color display tube having a color selection 
electrode in accordance with the invention is characterized in that each 
leaf spring comprises a Z-shape. Said Z-shape of a leaf spring is easy to 
produce. By means of a Z-shaped leaf spring it is easy both to convert the 
movements of the color selection electrode into movements along the 
electron paths and to connect the color selection electrode to each leaf 
spring. 
A yet further embodiment of a color display tube having a color selection 
electrode in accordance with the invention is characterized in that each 
spring construction has a coefficient of thermal expansion which is 
substantially equal to the coefficient of thermal expansion of the color 
selection electrode. The spring constructions are connected to the color 
selection electrode by means of welding or otherwise. During the 
processing and operation of the display tube temperature variations occur. 
When the thermal expansion of the spring construction differs from the 
thermal expansion of the color selection electrode, deformations along the 
welding seam may occur both in the color selection electrode and in the 
spring construions. One way to reduce said deformations to an acceptable 
level is to match the coefficient of thermal expansion of each spring 
construction to that of the color selection electrode.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The color display tube shown diagrammatically in FIG. 1 comprises in a 
glass envelope 1 three electron guns 5, 6 and 7 for generating three 
electron beams 8, 9 and 10 which envelope is composed of a substantially 
flat display window 2, a cone 3 and a neck 4. The display window 2 
comprises on its inside a great number of triplets of phosphor lines. Each 
triplet comprises a line 11 consisting of a blue-luminescing phosphor, a 
line 12 consisting of a green-luminescing phosphor, and a line 13 
consisting of a red-luminescing phosphor. All triplets together constitute 
the display screen 14. Positioned in front of the display screen 14 is a 
color selection electrode 15 in which a very great number of apertures 16 
is provided through which the electron beams 8, 9 and 10 pass to impinge 
only on phosphor lines of one color. The color selection electrode 15, 
which is flat, is tensioned on a supporting frame 17. Said tension is 
necessary to give the flat color selection electrode 15 sufficient 
non-deformability and rigidity. During operation of the color display tube 
a great part of the electron current on its way to the display screen 14 
impinges on the color selection electrode 15 so that heating of the color 
selection electrode 15 occurs. As a result of this the color selection 
electrode 15 expands so that a reduction of the tension of the color 
selection electrode 15 and consequently mislanding may occur. On the other 
hand, the tension on the color selection electrode 15 may be increased 
during the cooling phases of a number of manufacturing process steps of 
the color display tube, and by vibrations, impacts and the like. When the 
stess reaches a value above the elastic proof stess of the color selection 
electrode 15 the latter is deformed permanently. In order to prevent said 
permanent deformation, according to the invention a spring construction 18 
is placed between the tensioned color selection electrode 15 and the 
supporting frame 17 as is shown diagrammatically in FIG. 2. This spring 
construction 18 allows differences in expansion and movements between the 
supporting frame 17 and the color selection electrode 15. The spring 
construction 18 has a spring constant which is smaller than the spring 
constant of the color selection electrode 15. The spring constant of the 
color selection electrode 15 depends on the material from which the color 
selection electrode 15 is manufactured but also, for example, on the 
pattern of the apertures 16 in the color selection electrode 15. So in the 
case of a given color selection electrode 15 the spring construction 18 
should be constructed that, by a choice of the material and the shape, the 
spring constant of the spring construction 18 smaller than the spring 
constant of the color selection electrode 15. As a result of this the 
spring construction 18 has a longer travel than the color selection 
electrode 15 so that the increase of the tension caused by the difference 
in expansion between the supporting frame 17 and the color selection 
electrode 15 remains acceptable. By providing spring constructions 18 on 
both sides of oppositely located sides of the color selection electrode 15 
and by forming the spring constructions 18 in such a manner that the 
movements performed by the color selection electrode 15 and consequently 
also by the apertures 16 in the color selection electrode 15 take place 
only along the electron paths, for example the electron path 8 as is shown 
in FIG. 4, mislanding is also prevented. 
FIGS. 4a and 4b show two forms of the spring construction 18 in which 
movements of the apertures 16 in the color selection electrode 15 take 
place only along the electron paths. Two positions of the spring 
construction 18 and the color selection electrode 15 are shown in the 
FIGS. 4. One position is shown as a broken-line spring construction 18 and 
the other position is shown as a solid-line spring construction 18. 
Although the spring construction 18 shown in FIGS. 4 comprises a Z-shape, 
the invention is not restricted to such shapes. However, the spring 
construction 18 should comprise a substantially flat surface to which a 
major surface, the upper or lower surface, of the color selection 
electrode 15 can be connected. Movements of the color slection electrode 
15 now take place in such a manner that substantially no undesired bending 
stresses occur on the color selection electrode 15. In order to fully 
avoid such bending stresses, the material of which the spring construction 
18 consists may be locally weakened. For the FIG. 4b spring construction 
18, for example, said weakening should be present near the bending line 23 
of the spring construction 18. The spring construction 18 may be built up 
from a number of leaf springs 19 extending over the length of a side of 
the colour selection electrode 15, as is shown in FIG. 5a, or from one 
long leaf spring 20 which is provided over the length of a side, as is 
shown in FIG. 5b. The individual leaf spring 19 of FIG. 5a need not have 
the same shape. For making corrections towards the corner points of the 
color selection electrode 15 it is even useful to provide a variation in 
the shape of the leaf springs 19. The color selection electrode 15 is 
connected to the spring construction 18 by means of welding or by means of 
other known methods. During operation of the color display tube 
temperature variations occur. When the thermal expansion of the spring 
construction 18 differs from the thermal expansion of the color selection 
electrode 15, deformations in both the spring construction 18 and the 
color selection electrode 15 may occur as a result of said differences in 
expansion. By choosing for the spring construction 18 a material having 
substantially the same coefficient of thermal expansion as the color 
selection electrode 15, the possible occurrence of said deformations is 
prevented. 
When the spring construction 18 consists of a number of individual springs, 
for example as shown in fig. 5a, these deformations can also be prevented 
from occurring by making the width of the individual springs 19 to be 
sufficiently small irrespective of whether the spring construction has 
substantially the same coefficient of thermal expansion as the color 
selection electrode 15. In practice a width of approximately 1 cm proves 
to suffice. These deformations can also be prevented by providing notches 
in the edge of the color selection electrode which is connected to the 
spring contruction. Said notches may also be etched simultaneoulsy, for 
example, during etching the apertures in the color selection electrode. 
When a certain color selection electrode 15 is used each spring 
construction 18, in addition to a smaller spring constant, should also 
have, for example, a coefficient of thermal expansion substantially equal 
to that of the color selection electrode 15. When, by way of example, 
steel having a low carbon content, (C.ltoreq.0.004% and Al 0.02-0.06%) is 
used for the color selection electrode, a spring construction 18 
consisting of corrosion-resistant chromium-nickel steel with 16.0-18.0 % 
chromium, 6.50-7.75% nickel and 0.75--1.50% aluminium satisfies the 
desired requirements. 
The spring construction 18 is also connected to the supporting frame 17. 
When said connection is carried out by means of welding or in any other 
manner in which the spring construction 18 is rigidly connected to the 
supporting frame 17, deformations may occur in this case also due to the 
difference in expansion between the supporting frame 17 and the spring 18. 
By connecting the spring construction 18 to the supporting frame 17 in 
such a manner that the spring construction 18 and the supporting frame 17 
can move relative to each other, said deformations are prevented. FIG. 6 
shows an example in which a spring construction 18 is connected in the 
supporting frame 17 so as to be movable. The edges of the supporting frame 
17 are bent over so that slots are formed in which the spring construction 
18 can be inserted. The tension of the color selection electrode 15 pulls 
the edges of the spring construction 18 well into the slots of the 
supporting frame 17. It is feasible to construct the supporting frame from 
individual beams 21 as is shown in the plan view of FIG. 7. The spring 
construction 18 is connected on two oppositely located sides of the color 
selection electrode 15. A beam 21 is positioned over each spring 
construction 18. The advantage of a supporting frame constructed from 
individual beams 21 is that the tension can be produced at the color 
selection electrode 15 by pulling apart the two oppositely located beams 
21 in which the spring constructions 18 are provided. The resulting 
tensile force should be smaller than the proof stress of the color 
selection electrode 15 in the direction of drawing so as to prevent 
permanent deformation of the color selection electrode 15. The individual 
beams 21 are then connected together at their corners by means of plates 
22, as is shown in FIG. 7 for two corners. The individual beams 21 now 
together constitute the supporting frame 17. The supporting frame may be 
of metal and be placed at a given distance from the display screen in the 
evacuated envelope of the color display tube. Alternatively the supporting 
frame may be substantially of glass, so that, for example, the frame may 
be constituted by the display screen or the evacuated envelope of the 
tube. It is also possible, for example, to use as a supporting frame a 
substantially flat display window which comprises upright edges. The 
spring constructions which are present at the color selection electrode 
may then be connected on oppositely located edges of the display window. 
Dependent on the form of the color selection electrode, for example a wire 
grid, a shadow mask having a linear pattern of apertures or a shadow mask 
having a hexagonal pattern of apertures, a spring construction may be 
provided on two or on all sides of the color selection electrode. The 
color selection electrode may then be tensioned not only in one direction 
as is indicated in FIG. 7 by arrows, but may also be tensioned in a second 
orthogonal direction, it being ensured that said tensile stress also 
remains below the elastic proof stress of the color selection electrode in 
said direction.