Color display tube including a shadow mask sheet with a bulged portion

A shadow mask (20) for a color display tube is formed from one piece. Each side of the shadow mask (20) comprises a bulged portion (23) with a collar (24). Having a center of mass which when viewed in a cross-section perpendicular to the side, is situated substantially in the plane of the shadow mask sheet (21). As a result of this no moments are exerted on the shadow mask (20) when vibrations and shocks occur, so that the mask is not deformed. The angle which the collars (24) make with the longitudinal axis of the display tube is such that electrons reflected at a collar (24) do not land within the pattern of apertures (22) on the shadow mask (20). A magnetic screening cap adapted to the shadow mask construction is also constructed so that electrons reflected at the cap do not land within the pattern of apertures (22) of the shadow mask (20).

BACKGROUND OF THE INVENTION 
The invention relates to a colour dispay tube comprising an envelope having 
an electron gun system to generate a number of electron beams and a 
substantially rectangular display window comprising an upright edge and a 
substantially rectangular shadow mask sheet provided with a pattern of 
apertures and connected in the corners of the upright edge of the display 
window. 
Such a colour display tube is disclosed in U.S. Pat. No. 3,548,235. In this 
patent the shadow mask does not comprise the usual rigid carrier frame. 
The shadow mask is formed by a substantially rectangular mask sheet 
manufactured from thin metal plate and having a mask ring of substantially 
the same thickness as the mask sheet connected to the edge. The free end 
of the mask ring is bent over and gives the shadow mask a certain rigidity 
at the sides of the shadow mask. The shadow mask is connected to the four 
corners of the upright edge of the display window. However, the shape of 
the mask ring causes moments to be exerted on the mask sheet in the case 
of vibrations and shocks of the tube. These moments produce deformation of 
the shadaw mask which causes fading of the displayed picture. Moreover, 
welding of the mask ring to the mask sheet is expensive and welding 
sputters may land on the shadow mask sheet. 
SUMMARY OF THE INVENTION 
It is therefore an object of the invention to provide a colour display tube 
in which the shadow mask is of a simple construction, is manufactured from 
the smallest possible number of components, has a large resistance against 
shocks and vibrations and does not substantially deform as a result of 
thermal effects. 
It is another object of the invention to provide a screening cap which is 
adapted to the new shadow mask construction and which is used to screen 
the electron beams from the earth's magnetic field. 
For that purpose, a colour display tube of a kind mentioned in the opening 
paragraph is characterized in that the each side of the shadow mask sheet 
comprises a bulged portion thereof extending toward the display window and 
including a collar extending away from the display window. The centre of 
mass of the bulged portion at any cross-section taken perpendicularly to 
the longitudinal direction of the portion is situated substantially in the 
plane of the pattern of apertures. 
In a display tube in accordance with the invention the shadow mask is 
manufactured from one sheet. The shadow mask is rigid in its own plane. 
The bulged portions with collars give the sides of the shadow mask, and 
hence the entire mask a certain rigidity perpendicular to the plane of the 
shadow mask. Since the centre of mass of a cross-section of the bulged 
portion in a direction perpendicular to the longitudinal direction of the 
portion is situated substantially in the plane of the mask sheet, no 
moments are exerted on the mask sheet in the case of the occurrence of 
shocks and vibrations, so that the mask sheet remains undeformed. As a 
matter of fact the occurring forces lie in the plane of the mask sheet 
which is rigid in its own plane and are transmitted to the bulged portions 
where the forces act in the centres of mass of the portions. 
It is to be noted that a shadow mask having a peripheral embossment 
extending toward the display window is known per se from U.S. Pat. No. 
3,005,921. In this case, however, it concerns a post-acceleration tube 
having a circular shadow mask in which the peripheral embossment serves to 
counteract interferences of the post-accelerating field at the edge of the 
shadow mask. 
An embodiment of a display tube in accordance with the invention is 
characterized in that the collar makes such an angle with the longitudinal 
axis of the display tube that electrons reflected by the collar fall on 
the mask sheet outside the pattern of apertures. As a result of this there 
is no need for a diaphragm, which in conventional shadow mask display 
tubes is connected to the upright edge of the mask sheet to prevent 
electrons reflected at the upright edge from landing on the mask within 
the pattern of apertures. Moreover, the omission of the diaphragm has the 
advantage that electrons land on the whole mask so that the mask is more 
uniformly warmed-up. 
Another embodiment of the display tube is characterized in that the collar 
makes such an angle with the longitudinal axis of the display tube that 
electrons reflected by the collar are reflected in a direction which is at 
least perpendicular to the longitudinal axis of the tube, so that the 
electrons reflected at the collar do not land on the mask sheet. In this 
embodiment also no diaphragm is necessary since the electrons reflected by 
the collar do not land on the mask sheet. 
Another embodiment of a display tube is characterized in that the tube 
comprises an internal conical magnetic screening cap which on its side 
facing the display window overlaps the collar of the shadow mask and 
extends substantially parallel to the collar. The screening cap serves to 
screen the electron beams from the earth's magnetic field. As a result of 
the overlap of the screenig cap and the collar of the shadow mask the 
screening cap and the shadow mask are short-circuited magnetically without 
the two components necessarily making mechanical contact with each other. 
Another embodiment is characterized in that the overlap of the screening 
cap and the collar of the shadown mask is at least ten times as large as 
the distance between the screening cap and the collar. It has been found 
experimentally that good magnetic screening is obtained in this case. 
Another embodiment of a display tube in accordance with the invention in 
which the collar makes such an angle with the longitudinal axis of the 
display tube that electrons reflected by the collar fall on the mask sheet 
outside the pattern of apertures is characterized in that the portion of 
the screening cap near the collar of the shadow mask comprises a shoulder 
which covers the aperture between the screening cap and the shadow mask. 
The shoulder prevents electrons between the screening cap and the collar 
of the shadow mask from landing on the display window via reflections. 
Another embodiment of a display tube in accordance with the invention in 
which the collar makes such an angle with the longitudinal axis of the 
display tube that electrons reflected by the collar fall on the mask sheet 
outside the pattern of apertures is characterized in that the magnetic 
screening cap, in a cross-section along a plane through the longitudinal 
axis of the display tube, has the form of a part of an ellipse of which 
one focus lies in the mask sheet just outside the pattern of apertures and 
the other focus lies in the deflection point of the electron beam which in 
the direction of the edge of the pattern of apertures makes the largest 
angle with the longitudinal axis of the tube. The magnetic screening cap 
should have a shape which ensures that electrons reflected by the 
screening cap land on the shadow mask outside the pattern of apertures. 
This situation can be most effectively accomplished if the screening cap 
has the form of an ellipse which is rotated about the focus lying just 
outside the pattern of apertures in a direction towards the longitudinal 
axis of the display tube or if the screening cap has the form of an 
ellipse which is moved in a direction away from the longitudinal axis of 
the display tube. The impinging electrons having the largest angle of 
incidence will then land on the shadow mask at a larger distance outside 
the pattern of apertures.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The colour display tube according to the invention shown in FIG. 1 is 
formed by a glass envelope 1 which comprises a substantially rectangular 
display window 2 which has an upright edge, a cone 3 and a neck 4. A 
pattern 12 of phosphors luminescing in the colours red, green and blue is 
provided on the display window 2. At a short distance in front of the 
display screen a shadow mask 5 is connected with the aid of suspension 
means 6. An electron gun 7 to generate three electron beams 8, 9 and 10 is 
mounted in the neck 4 of the tube. These beams are deflected by means of a 
system of deflection coils 11 placed around the tube. The beams intersect 
each other substantially at the area of the shadow mask 5, after which 
each of the electron beams impinges on one of the three phosphors provided 
on the display screen. The electron beams 8, 9 and 10 are screened in the 
tube from the earth's magnetic field by means of a magnetic screening cap 
13. 
FIG. 2a is a perspective view of an embodiment of a shadow mask of the tube 
shown in FIG. 1. The shadow mask 20 is formed from a thin metal plate the 
central portion 21 of which has a large number of apertures 22. The shadow 
mask 20 is dished and corresponds with the shape of the display window. A 
bulged portion 23 terminating in a collar 24 is provided in each of the 
four rectangular sides of the mask 20. The collars 24 are bent inwardly 
and make an angle of 5.degree. with the longitudinal axis 28 of the 
display tube. 
FIG. 2b is a sectional view along a part of the line A'A' of FIG. 2a. The 
shadow mask 20 having apertures 22 comprises a bulged portion 23 of height 
h and width b and a collar 24 of length l. The length l of the collar 24 
should be selected relative to the height h and the width b of the bulged 
portion 23 such that the centre of mass of a cross-section of the bulged 
portion 23 and collar 24 taken perpendicular to the longitudinal direction 
of the bulged portion is situated substantially in the plane of the mask 
sheet 21 as is shown by the broken line 29. As a result of this, no 
moments are exerted on the mask 20 when vibrations and shocks occur in the 
plane of direction of the mask 20. The occurring forces lie in the plane 
of the mask sheet which is rigid in its own plane and are transmitted to 
the bulged portion 23 with collar 24. As a result of these forces the 
bulged portion 23 with collar 24 may not experience a moment causing 
rotation since in that case the mask sheet is deformed and fading of the 
displayed picture occurs. These moments are prevented because the centre 
of mass of any cross-section of the bulged portion 23 with collar 24 
perpendicular to the longitudinal direction of the bulged portion 23 with 
collar 24 is situated substantially in the plane of the mask sheet 21 and 
the center of mass is where applied forces are opertive. 
The shadow mask 20 is manufactured, for example, from iron and has a 
thickness of approximately 0.15 mm. With a bulged portion 23 of height h=5 
mm and width b=2.5 mm, the length l of the collar 24 should be 
substantially equal to l=8.6 mm in order that the centre of mass of a 
cross-section of the bulged portion 23 with collar 24 be situated 
substantially in the plane of the mask sheet 21. 
For collars 24 which are bent inwardly it has been found that the maximum 
deflection angle in a direction along the line A'A' is decisive of the 
angle which the collar 24 should make with the longitudinal axis 28 of the 
display tube in order to prevent electrons which reflect at the collar 24 
from landing within the pattern of apertures 22 of the mask 20. In the 
embodiment shown of a 110.degree. display tube the maximum deflection 
angle of the electron beams which impinge on the end of the collar along 
the line A'A' is 51.5.degree.. The outermost mask aperture 22 is situated 
at a distance of substantially 11 mm from the outside of the bulged 
portion 23. The collar 24 makes an angle of 5.degree. with the 
longitudinal axis 28 of the display tube. Angles exceeding 5.degree. are 
also possible, which will be explained in detail with reference to FIG. 
2c. The electrons reflected at the collar 24 do land on the mask 20 so 
that the whole surface of the mask 20 is impinged upon by electrons which 
results in a more uniform warming-up of the mask 20. The radii of the 
curvatures 25, 26 and 27 are equal to 3.5 mm and 0.5 mm, respectively. By 
choosing the radii to be equal to these measurements and using a mask 
material having a tensile strength of .sigma..sub.B =170.sup.N /mm.sup.2 
it has been found that the bulged portion 23 after drawing the shadow mask 
20 deforms elastically in such manner that the collar 24 makes an angle of 
5.degree. with the longitudinal axis 28 of the display tube. By choosing 
the radii of the curvatures and/or the tensile strength of the mask 
material to be different, larger angles can be obtained in the same 
manner. 
It will be described with reference to FIG. 2c which angle the collar 
should make with the longitudinal axis of the display tube in accordance 
with the various parameters in the display tube so as to prevent electrons 
reflected at the collar from landing within the pattern of apertures on 
the mask. The Figure shows diagrammatically a part of the shadow mask 20 
with bulged portion 23 and collar 24. The collar 24 has a length l and 
makes an angle .beta. with the longitudinal axis of the tube. The mask 
sheet makes an angle .gamma. with the axis perpendicular to the 
longitudinal axis 28 of the tube. An electron beam deflected over a 
maximum angle .phi. impinges on the collar 24 at an angle .alpha..sub.i 
and is reflected at the same angle .alpha..sub.i. The electron beam 
reflected by the collar 24 impinges on the shadow mask at an angle .theta. 
with the plane of the mask sheet at a distance .omega. from the outside of 
the collar 24. If the distance from the outermost mask aperture 22 to the 
outside of the collar 24 is known, the maximum value which thedistance 
.omega. may have is also fixed. At a given collar length l and a given 
maximum deflection angle .phi., the angle .beta. of the collar 24 with the 
longitudinal axis 28 is determined by the relationship: 
EQU l sin (.phi.-.beta.)=.omega. sin (90.degree.+2.beta.-.gamma.-.phi.). 
For example, for a 110.degree. display tube with .phi.=51.5.degree., l=8.6 
mm, .omega.=10 mm and .gamma.=16.degree. this leads to a minimum angle 
.beta. of 5.degree.. 
FIG. 3a is a diagrammatic drawing to explain the construction of the 
screening cap. The display window 30 and a part of the cone 31 are shown. 
Within the envelope the shadow mask 33 and the screening cap 34 are 
secured. The screening cap 34 serves to screen the electron paths from the 
earth's magnetic field, so as to maintain a good colour purity. The 
collars 35 of the shadow mask 33 make and angle of +5.degree. with the 
longitudinal axis 36 of the tube. The edge 37 of the screening cap 34 
follows the contour of the shadow mask 33 and thus makes an angle of 
-5.degree. with the axis 36. In order to obtain good magnetic screening, 
the shadow mask 33 and the screening cap 34 should be short-circuited 
magnetically. As a result of the overlap of the shadow mask 33 and the 
screening cap 34, these are short-circuited magnetically without 
mechanical contact. 
A detail hereof is shown in FIG. 3b. It has been found that a good 
screening is obtained if the overlap of the edge 37 of the screening cap 
34 and the collar 35 of the shadow mask 33 is at least 10 times as large 
as the distance between the edge 37 and the collar 35. In order to prevent 
electrons between the edge 37 and the collar 35 from landing on the 
display screen by reflections, the screening cap comprises a shoulder 38 
which covers the opening between the collar 35 and the edge 37. 
The collars make an angle of 5.degree. with the longitudinal axis 36 of the 
display tube so as to ensure that the electrons reflected by the collars 
35 land on the mask 33 beyond the pattern of apertures. It should also be 
ensured that electrons reflected by the screening cap 34 land on the mask 
33 outside the pattern of apertures. For this purpose the screening cap 
should have a particular shape which will be explained with reference to 
FIG. 3c. 
A part of the shadow mask 33 with outermost mask aperture 32 and the 
screening cap 34 are shown diagrammatically. Reference numeral 40 denotes 
the deflection plane in which the deflection points 41, 42 and 43 of the 
electron beams R, G and B, respectively, are situated after passing 
through the shadow mask 33, the beams impinge upon phosphor regions on the 
display screen luminescing in red, green and blue, respectively. Of the 
electron beams falling through the outermost mask aperture 32 the electron 
beam R makes the largest angle with the longitudinal axis 36 of the tube. 
Of the overdeflected electrons, the electrons of the electron beam R will 
consequently impinge on the screening cap 34 at the largest angle and 
after reflection impinge on the mask 33 at the largest distance from the 
collar 35. In order to ensure that these reflected electrons land on the 
mask 33 outside the last mask aperture 32, the screening cap 34 should 
have the shape of an ellipse, the foci of which are situated in the point 
39 situated just beyond the pattern of holes and the deflection point 41 
of the electron beam R. When the screening cap 34 is turned about the 
focus 39 in a direction denoted by the arrow 44, the electrons deflected 
at the screening cap 34 land on the mask 33 farther from the outermost 
mask aperture 32. The screening cap 34 may also be parallel to the 
position shown in FIG. 3c at a larger distance from the longitudinal axis 
36 of the display tube. 
FIG. 4a is a perspective view of another embodiment of a shadow mask in 
accordance with the invention. Like the mask shown in FIG. 2, the shadow 
mask 50 is formed by a thin metal sheet the central portion 51 of which 
comprises a large numbr of apertures 52. A bulged portion 53 with collar 
54 is provided in each of the four rectangular sides of the mask. The 
collars 54 are bent outwardly and make an angle of 25.5.degree. with the 
longitudinal axis of the display tube. 
FIG. 4b is a sectional view along a part of the line B'B' of FIG. 4a. The 
height of the bulged portion 53 is again 5 mm, the width 3.5 mm and the 
length of the collar 54 is 8.6 mm. For collars 54 bent outwardly it has 
been found that the maximum deflection angle in a direction along the line 
B'B' is decisive of the angle which the collars 54 should make with the 
longitudinal axis 55 of the display tube so as to prevent electrons 
reflected at the collars 54 from landing on the mask sheet. In the 
embodiment of a 110.degree. display tube shown the maximum deflection 
angle of the electron beams along the line B'B' which impinge on the end 
of the collar is 39.degree.. The collar 54 makes an angle of 25.5.degree. 
with the longitudinal axis 55 of the display tube. Angles exceeding 
25.5.degree. are also possible, which will be explained in detail with 
reference to FIG. 4c. The electrons reflected at the collar 54 are 
reflected in a direction perpendicular to the longitudinal axis of the 
display tube and do not land on the mask sheet. 
FIG. 4c shows diagrammatically a part of the shadow mask 50 with bulged 
portion 53 and collar 54. The collar makes an angle .beta. with the 
longitudinal axis 55 of the display tube. An electron beam deflected over 
a maximum angle .phi. impinges on the end of the collar 54 at an angle 
.alpha..sub.i and is reflected at the same angle .alpha..sub.i. The 
minimum angle .beta. which the collar 54 should make with the longitudinal 
axis 55 so as to prevent reflected electrons from landing on the shadow 
mask 50 is determined by the relationship: 
EQU .beta.=1/2(90.degree.-.PHI.) 
For example, for a 110.degree. display tube with .phi.=39.degree. this 
leads to a minimum angle .beta. of 25.5.degree.. 
FIG. 4d shows diagrammatically a part of the magnetic screening cap 58. The 
edge 59 of the screening cap 58 extends parallel to the collar 54. The 
magnetic screening is optimum if the overlap of the edge 59 is at least 10 
times as large as the distance between the edge 59 and the collar 54. 
A shadow mask in accordance with the invention is formed from a 
comparatively flexible mask sheet and includes a mask frame in the form of 
four bulged portions with collars extending along the sides of the mask. 
The bulged portions are rigid in the direction perpendicular to the plane 
of the mask sheet. The mask sheet is rigid in its own plane. The shadow 
mask as a whole can easily be twisted about the diagonals and consequently 
has four hinge points at the corners of the shadow mask. The position of 
the shadow mask is fixed unambiguously with respect to the display window 
having an upright edge if eight and only eight degrees of freedom of the 
shadow mask are fixed. A first embodiment of a suspension of the shadow 
mask with which this is realized will be explained with reference to FIG. 
5a which is an exploded perspective view of a suspension of the shadow 
mask in a corner of the display window. It is to be noted that this 
suspension forms the subject matter of a simultaneously filed patent 
application Ser. No. 270,444. The Figure shows the shadow mask 60 with a 
pattern of holes 61 and the bulged portions 62 with collars 63 provided 
along the sides. A brace 64 is connected to the collars 63 in the corner 
of the shadow mask 60. the brace 64 is folded about the lines 65 and 66 
and has a hole 67 of triangular shape. The display window 70 comprises 
face plate 71 and an upright edge 72. The display screen 73 luminescing in 
three colours and covered by an aluminium coating is provided on the face 
plate 71. A chamber-like recess 74 is provided in the corner of the 
upright edge 72. A metal strip 75 having a flat metal spring 76 is 
connected in said recess 74. The strip 75 has an aperture 79 used for the 
connection of the magnetic screening cap in the tube, which will be 
explained in detail with reference to FIG. 6. The strip 75 is fixed in the 
corners of the chamber-like recess 74 by means of a glass enamel or a 
cement 77. The spring 76 is connected to the strip 75 at such an angle 
that the spring 76 is substantially perpendicular to the path of the 
electron beams towards the corner of the display window 70, so that during 
warm-up of the display tube the shadow mask will move towards the face 
plate 71, which is necessary to maintain good colour purity. A positioning 
member 78 is provided in the spring 76. In manufacturing the display tube 
the shadow mask 60 is placed at the correct distance from the face plate 
71 by means of four spacing members placed in the corners. The braces 64 
are clamped onto the springs 76 by means of temporary clamping members, 
the positioning member 78 falling in the hole 67. In this position the 
strips 75 are fixed by means of a glass enamel or a cement 77 in the 
chamber-like recesses 74. After providing the luminescent phosphor pattern 
on the display window 71, in which the shadow mask 60 is repeatedly 
removed and remounted, the positioning member 78 is permanently connected 
to the brace 64 by means of a number of laser welds or other contactless 
welds. As a result of this suspension construction the position of the 
shadow mask 60 relative to the display window 71 is unambiguously fixed. 
The distance from the four corner points to the display window and hence 
the distance from the shadow mask 60 to display window 71 is fixed 
unambiguously, with which four degrees of freedom are fixed. As a result 
of the connection of the positioning member 78 of the spring 76 to the 
brace 64, a movement of the corner points of the shadow mask 60 in a 
direction perpendicular to the diagonals in the plane of the shadow mask 
60 is impossible, while all other directions of movement are possible. As 
a result of this four degrees of freedom are fixed and thus for the shadow 
mask 60 a total of eight degrees of freedom are fixed. 
FIG. 5b shows diagrammatically the connection of the magnetic screening cap 
in the display tube in which for clarity non-essential components are not 
shown. The screening cap 80 comprises in each of the corners a bent-over 
strip 81 which has a part-spherical embossment 82. The strip 81 is placed 
against the strip 75 connected in the corner of the display window, the 
embossment 82 falling through the aperture 79 of the strip 75. In this 
position the embossment can be connected to the aperture 79, for example, 
by means of a cement. 
FIG. 6 is an exploded perspective view of a second embodiment of a 
suspension of the shadow mask in a corner of the display window. It is to 
be noted that this suspension also in itself is the subject matter of a 
simultaneously filed patent application Ser. No. 270,285. In the corner of 
the shadow mask 90 again a brace 91 is connected. A flat spring 93 is 
connected to the brace 91 by means of a thin metal plate 92. A carrier 
plate 94 having a rectangular aperture 95 and a triangular aperture 96 is 
connected to the spring 93. A strip 103 is fixed in the chamber-like 
recess 101 in the edge 102 of the display window 100 by means of a glass 
enamel or a cement 108. A supporting plate 104 having three embossed 
portions 105, 106 and 107 is connected to the strip 103. The embossments 
105 and 106 are larger than the embossment 107. The shadow mask 90 is 
connected in the display tube by placing the carrier plate 94 on the 
supporting plate 104. The carrier plate engages the embossed portion 107. 
The embossments 105 and 106 fall partly through the apertures 95 and 96, 
the embossment 105 engaging the aperture 95 at two points and the 
embossment 106 engaging the aperture 96 at three points. The supporting 
plate 104 and the carrier plate 94 are held together by a clamping member 
110 the bent-over end 111 of which engages the carrier plate 94 in a point 
which corresponds to the centre of mass of the three embossments 105, 106 
and 107. The shadow mask 90 can be detached from the display window 100 by 
removing the clamping member 110. As a result of the shape of the carrier 
plate 94 and the supporting plate 104 the shadow mask after repeated 
assembly and disassembly, as is necessary to provide the display screen, 
always assumes the same position. The distance from the four corner points 
of the shadow mask to the display window is thus fixed unambiguously so 
that four degrees of freedom are fixed. The substantially punctiform 
connection between the spring 93 and the brace 91 by means of the metal 
plate 92 ensures that the corner points of the shadow mask cannot move in 
a direction perpendicular to the diagonals of the shadow mask, while the 
remaining directions of movement are permitted. Thus a total of eight 
degrees of freedom of the shadow mask are fixed.