Cathode ray tube having improved curvature characteristics and method of fabrication thereof

A face panel (12) constituting a part of a vacuum envelope (20) has a substantially rectangular effective area (10), the inner surface of which is formed with a phosphor screen (14). The effective area has a long axis (X) extending in the horizontal direction and a short axis (Y) extending in the vertical direction. The outer surface of the effective area is cylindrically curved with an infinitely large radius of curvature along the long axis and a predetermined radius of curvature along the short axis. The vacuum envelope has arranged therein a shadow mask (15) in opposed relation to the phosphor screen. A mask body of the shadow mask has a substantially rectangular effective surface cylindrically curved with an infinitely large radius of curvature along the long axis and a predetermined radius of curvature along the short axis thereof. In manufacturing the shadow mask, a flat mask is subjected to a plastic deformation into a cylindrical shape curved along the short axis and then subjected to an elastic deformation in such a manner that the radius of curvature thereof is larger than at the time of plastic deformation. The mask body thus formed by elastic deformation is fixed to a rectangular frame.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a cathode ray tube having a substantially 
flat face panel and a method of manufacturing the same. 
2. Description of the Related Art 
Generally, a color cathode ray tube is provided with a vacuum envelope 
having a glass face panel and a glass funnel. A phosphor screen having 
three-color phosphor layers is formed on the inner surface of the 
effective area of the face panel, and an electron gun is arranged in the 
neck of the funnel. Three electron beams emitted from the electron gun are 
deflected by the magnetic field generated by a deflector mounted on the 
outside of the funnel, and scan horizontally and vertically on the 
phosphor screen through a shadow mask, thereby displaying a color image. 
The face panel of a color cathode ray tube having this configuration 
generally includes a substantially rectangular effective area and a side 
wall erected along the peripheral portion of the effective area. The face 
panel is formed with inner and outer surfaces curved so differently that 
the central portion of the effective area is thinner than the peripheral 
portion thereof in order to secure a sufficient strength to resist the 
atmospheric load applied to the vacuum envelope. 
Generally, the outer surface of the effective area is formed with such a 
curvature that the height with respect to the sealed surface between the 
face panel and the funnel is greatest at the central portion of the 
effective area and lower toward the peripheral portion. Specifically known 
face panels include the one with the outer surface of the effective area 
having a spherical curvature, the one having a cylindrical outer surface 
with a substantially infinitely large radius of curvature along the 
vertical axis and a curvature along the horizontal long axis, and the one 
having a curved outer surface expressed by a high-order polynomial. 
With respect to the shape of the outer surface of the effective area of the 
face panel, the recent trend is toward the flattening in order to improve 
the visibility. Depending on the curved geometry of the outer surface of 
the effective area of the face panel, a generally known method of 
indicating the flatness of the effective area includes the index R. The 
index R is given as the ratio of the average radius of curvature of the 
corners determined by the difference (the fall of the corners) between the 
height of the central portion of the face panel and the height of the 
corners of the face panel to the diagonal length of the effective area 
multiplied by a factor of 1.7. In the case where the flatness expressed by 
this index R remains the same, the fall at the corners is the same for any 
shape of the curved outer surface of the effective area, and though 
somewhat depending on the geometry of the curved surface, the feeling of 
flatness of the effective area of the face panel is substantially equal. 
With the increase of the flatness of the face panel, however, the 
atmospheric strength of the glass vacuum envelope decreases. The flatness 
of the outer surface of the effective area, therefore, is at most about 
2.0R even for a large cathode ray tube. 
On the other hand, various shapes are available for the inner surface of 
the effective area of the face panel. The inner surface of the effective 
area, however, is often formed in the same type of curvature as the outer 
surface of the effective area so that the effective area is thinnest at 
the central portion thereof and thicker toward the peripheral area in 
order to maintain the atmospheric strength required of the glass vacuum 
envelope. 
In recent years, the atmospheric strength of the glass vacuum envelope has 
improved due to an improved design accuracy of the face panel and an 
improved performance of the reinforcing band to such an extent that a 
predetermined strength is secured even with a flattened face panel. In the 
case where the inner and outer surfaces of the face panel are configured 
of the same type of curvature as described above, however, a still higher 
strength of the vacuum envelope against the atmospheric pressure is 
required if the effective area of the face panel is to be flattened more. 
This in turn requires reinforcing by increasing the glass thickness 
greatly or attaching a reinforcing film on the outer surface of the 
effective area of the face panel at the sacrifice of a remarkably higher 
cost. 
On the other hand, there exists a cathode ray tube comprising a face panel 
having a substantially flat outer surface of the effective area. In this 
cathode ray tube, however, the inner surface of the face panel is formed 
by a combination of curved surfaces like the well-known face panel. For 
this reason, the vacuum envelope is reinforced by thickening the effective 
area of the panel or attaching a reinforcing film on the outer surface of 
the effective area of the face panel in order to secure the atmospheric 
strength of the vacuum envelope. This leads to a considerably higher cost 
as in the above-mentioned case. 
With the color cathode ray tube, the shadow mask is configured of a 
substantially rectangular flat mask body about 0.1 to 0.3 mm thick and a 
substantially rectangular frame fixed on the peripheral portion of the 
mask body. The effective surface of the mask body is opposite to the 
phosphor screen and the effective surface is formed with a number of 
apertures allowing the electron beams to pass therethrough. 
Generally, the effective surface of the mask body is shaped in conformance 
with the inner surface of the effective area of the face panel and has at 
least a curved central portion protruding toward the phosphor screen. The 
shape of the curved surface conventionally used includes a cylindrically 
curved surface having a predetermined curvature along the horizontal axis 
and a substantially infinitely large radius of curvature along the 
vertical axis or a curved surface expressed by a high-order polynomial. 
Regardless of the shape of the curved surface of the shadow mask, the 
electron beam apertures of the shadow mask and the phosphor layer are 
required to be in specified relative positions in order to assure the 
accurate landing of the electron beams on the phosphor layers. The same 
relative positions are always required to be maintained through the whole 
operation of the cathode ray tube. In other words, the distance between 
the shadow mask and the phosphor screen must always be within a 
predetermined tolerance. 
The amount of the electron beams that reaches the phosphor screen through 
the electron beam apertures of the shadow mask, however, is not more than 
one third of all the electron beams emitted from the electron gun, and the 
remaining electron beams bombard the shadow mask. The electron beams that 
have thus bombarded the shadow mask are converted into thermal energy to 
heat and expand the shadow mask. 
The thermal expansion of the shadow mask increases the displacement of the 
beam landing and the deterioration of the color purity. The magnitude of 
the mislanding caused by the thermal expansion of the shadow mask is 
greatly varied with the image pattern displayed and the time during which 
an image pattern is sustained. Especially in the case where a locally 
high-luminance image pattern is displayed, the local doming of the shadow 
mask occurs so that the mislanding of the electron beam is caused within a 
short time resulting in a great displacement of the electron beam. The 
mislanding is most conspicuous in the case where the doming of the shadow 
mask occurs at a portion located toward the center from the horizontal end 
of the effective surface of the shadow mask by about one third of the 
horizontal length. 
The two methods of forming the curved surface of the shadow mask include 
using the press work and (2) applying a tension. In the method using the 
press work, a planar mask plate (flat mask) made of a thin metal with a 
number of electron beam passage apertures is subjected to plastic 
deformation in the press. This method is used mainly for forming a 
spherical surface or a curved surface expressed by a high-order 
polynomial, as described above. 
The second method of forming the curved surface of the shadow mask under 
tension is used for producing a cylindrical surface with a predetermined 
radius of curvature along the horizontal axis and a substantially 
infinitely large radius of curvature along the vertical axis. In this 
method, a planar plate of a thin metal with a number of electron beam 
passage apertures is arranged along the frame. The frame has a 
mask-mounting surface curved along the horizontal axis with a 
substantially infinitely large radius of curvature along the vertical 
axis. This mask plate is fixed to the frame under a tension applied along 
the vertical axis of the mask plate. 
As described above, the curved surface of the shadow mask has been 
flattened and has an increasingly larger radius of curvature with the 
flattening of the face panel. With the increase in the curvature of the 
shadow mask, the strength of holding the curved surface of the shadow mask 
is reduced. As a result, the effective surface of the shadow mask is 
easily deformed under a shock or other external forces applied to the 
color cathode ray tube. Also, in the case where the color cathode ray tube 
is exposed to a vibration, the shadow mask is liable to develop a 
resonance (howling). In either case, the color purity of the displayed 
image is deteriorated. 
The strength of holding the curvature of the flattened shadow mask can be 
improved by increasing the thickness of the shadow mask. An increased 
thickness of the shadow mask, however, makes it difficult to form the 
electron beam passage apertures by photoetching and difficult to obtain 
beam passage apertures with desired shape and size. Further, the cost of 
the material of the shadow mask increases. 
As a measure for improving the curved surface-holding strength, a method is 
conceivable in which the shadow mask is mounted under a tension applied 
along the direction of the vertical axis having an infinitely large radius 
of curvature. In this case, however, the requirement of applying a very 
large tensile force to the shadow mask necessitates a very high strength 
of holding the shadow mask. As a result, the production cost of the color 
cathode ray tube increases. At the same time, the increased frame weight 
greatly increases the whole weight of the cathode ray tube. 
SUMMARY OF THE INVENTION 
The present invention is designed in consideration of the above 
circumstances, and its objective is to provide a cathode ray tube and a 
method of manufacturing the same in which the flatness of the effective 
area of the face panel and the visibility can be easily improved without 
increasing the production cost substantially. 
In order to achieve this objective, according to one aspect of the present 
invention, a cathode ray tube comprises a vacuum envelope having a face 
panel with a substantially rectangular effective area and a funnel; a 
phosphor screen formed on an inner surface of the face panel; and an 
electron gun arranged in a neck of the funnel for emitting electron beams 
toward the phosphor screen. The effective area of the face panel has a 
long axis extending in the horizontal direction and a short axis extending 
in the vertical direction. The outer surface of the effective area is 
formed in a cylindrically curved shape having a substantially infinitely 
large radius of curvature along the long axis and a fixed radius of 
curvature along the short axis over the entire outer surface. 
In this aspect of the invention, the outer surface of the effective area 
has a radius of curvature along the short axis expressed by a high-order 
polynomial. 
Also, the ratio of the size along the long axis to the size along the short 
axis of the effective area is set to 16:9. 
According to another aspect of the present invention, a cathode ray tube 
comprises a vacuum envelope having a face panel with a substantially 
rectangular effective area and a funnel; a phosphor screen formed on an 
inner surface of the face panel; and an electron gun arranged in a neck of 
the funnel for emitting electron beams toward the phosphor screen. The 
effective area of the face panel has a long axis extending in the 
horizontal direction and a short axis extending in the vertical direction. 
The outer surface of the effective area is formed in the shape of a curved 
surface having a substantially infinitely large radius of curvature along 
the long axis and a radius of curvature along the short axis which is 
different between the portion on the short axis and a portion near the 
short side of the effective area. 
The cathode ray tube having the above-mentioned configuration, can improve 
the strength of the vacuum envelope over the conventional cathode ray tube 
by taking advantage of the difference between the lateral and longitudinal 
sizes of the face panel even in the case where the panel has substantially 
the same flatness as conventional cathode ray tubes. Further, if the 
strength is the same as the conventional cathode ray tube, it is possible 
to provide a cathode ray tube improved in the flatness of the face panel. 
According to another aspect of the invention, a cathode ray tube comprises 
a vacuum envelope having a face panel with a substantially rectangular 
effective area and a funnel; a phosphor screen formed on an inner surface 
of the face panel; and an electron gun arranged in a neck of the funnel 
for emitting electron beams toward the phosphor screen. The effective area 
of the face panel has a long axis extending in the horizontal direction 
and a short axis extending in the vertical direction. The outer surface of 
the effective area is formed substantially flat, and the inner surface of 
the effective area has a cylindrically curved shape with a substantially 
infinitely large radius of curvature along the long axis and a fixed 
radius of curvature along the short axis over the whole inner surface. 
The ratio between the sizes along the long axis and the short axis of the 
effective area is set to 16:9. 
In the cathode ray tube according to this aspect of the invention, the 
effective area of the face panel has a long axis extending in the 
horizontal direction and a short axis extending along the vertical 
direction, and the outer surface of the effective area is curved with a 
substantially infinitely large radius of curvature along the long axis and 
a predetermined radius of curvature along the short axis. 
The inner surface of the effective area is cylindrically curved and has a 
substantially infinitely large radius of curvature along the long axis and 
a predetermined radius of curvature along the short axis over the whole 
inner surface. 
With a cathode ray tube having this configuration, the flatness of the 
outer surface of the effective area of the face panel can be improved to 
configure a color cathode ray tube with a superior visibility without 
reinforcing the face panel considerably. 
According to still another aspect of the invention, a cathode ray tube 
comprises a vacuum envelope having a face panel with a substantially 
rectangular effective area and a funnel; a phosphor screen formed on an 
inner surface of the face panel; a shadow mask arranged in the vacuum 
envelope to oppose the phosphor screen and include a mask body having a 
substantially rectangular effective surface opposite to the phosphor 
screen and a number of electron beams passage apertures formed in the 
effective surface, and a substantially rectangular frame supporting the 
peripheral edge of the mask body; and an electron gun arranged in a neck 
of the funnel for emitting electron beams toward the phosphor screen. 
The effective surface of the mask body has a long axis extending in the 
horizontal direction and a short axis extending in the vertical direction. 
The mask body is formed in the shape of a cylindrically curved surface 
having a substantially infinitely large radius of curvature along the long 
axis and a substantially fixed radius of curvature along the short axis 
over the whole effective area. 
Also, with a cathode ray tube according to another aspect of the invention, 
the effective surface of the mask body has a long axis extending along the 
horizontal direction and a short axis extending along the vertical 
direction. The effective surface of the mask body is formed in a curved 
surface with a radius of curvature along the long axis is substantially 
infinitely large and a radius of curvature along the short axis expressed 
by a high-order polynomial. 
With the cathode ray tube having the above-mentioned configuration, the 
effective surface of the mask body is formed as a curved surface having a 
substantially infinitely large radius of curvature along the long axis and 
a fixed radius of curvature along the short axis or as a curve surface 
expressed by a high-order polynomial. The strength of holding the curved 
surface of the shadow mask is thereby considerably improved. Also, the 
flatness is improved and the face panel can be readily flattened while 
holding a curved surface-holding strength of the shadow mask equivalent to 
the conventional shadow mask. Further, the thickness of the shadow mask 
can be reduced for the same flatness as that of the conventional shadow 
mask. 
According to a still another aspect of the invention, a method of 
manufacturing a cathode ray tube comprises: preparing a substantially 
rectangular flat mask formed with a number of electron beam passage 
apertures; forming a mask body by subjecting the flat mask to plastic 
deformation into a cylindrical shape which has an infinitely large radius 
of curvature along the long axis and curved along the short axis; 
subjecting the plastically-deformed mask body to an elastic deformation in 
such a manner as to make the radius of curvature along the short axis 
larger than at the time of plastic deformation; and fixing the peripheral 
edge of the elastically-deformed mask body to the frame. 
The above-mentioned manufacturing method can produce a shadow mask having a 
high strength of holding a curved surface in view of the fact that the 
mask body is fixed on the frame under a stress applied thereto in such a 
direction that the radius of curvature of the mask body along the short 
axis is reduced.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
A color cathode ray tube according to a first embodiment of the invention 
will be explained with reference to the accompanying drawings. 
As shown in FIG. 1, a color cathode ray tube is configured with a vacuum 
envelope 20 including a substantially rectangular face panel 12 made of 
glass and a funnel 13 of glass coupled to the face panel. The face panel 
12 has a substantially rectangular effective area 10 with a curved surface 
described below and a skirt 11 erected along the peripheral edge of the 
effective area. The funnel 13 is coupled to the skirt. 
On the inner surface of the effective area 10 of the face panel 12 is 
formed a phosphor screen 14 having three color phosphor layers of blue, 
green and red. Also, a shadow mask 15 is arranged in the vacuum envelope 
20 to face the phosphor screen 14. The shadow mask 15 is supported on the 
skirt 11 of the face panel 12 by means of a plurality of holders 20. 
An electron gun 18 for emitting three electron beams 17 is arranged in the 
neck 16 of the funnel 13. The three electron beams 17 emitted from the 
electron gun 18 are deflected by a magnetic field generated by a deflector 
19 mounted on the outside of the funnel 13, and scan the phosphor screen 
14 horizontally and vertically through the shadow mask 15, thereby 
displaying a color image. 
As shown in FIGS. 1 and 2, the effective area 10 of the face panel 12 is 
formed in a laterally long rectangle which has a long axis (X axis) 
perpendicular to the tube axis Z and extending in the horizontal direction 
and a short axis (Y axis) perpendicular to the long axis and tube axis and 
extending in the vertical direction. In FIG. 2, the outer surface 10a of 
the effective area 10 is shown with a multiplicity of matrix lines to 
define the shape thereof clearly. 
As shown in FIGS. 2 to 4B, the outer surface 10a of the effective area 10 
is formed as a cylindrically curved surface having a center axis parallel 
to the long axis X. Specifically, the outer surface 10a of the effective 
area 10, as shown by line 21 in FIG. 3, is formed as a curved surface 
having an infinitely large radius of curvature along the long axis (X 
axis) as shown by straight line A in FIG. 3 and a radius of curvature 
along the short axis (Y axis) in such a manner that the radius of 
curvature on the short axis Y and on an arbitrary line IV--IV parallel to 
the short axis assume a predetermined value, as shown by curves 22a, 22b 
in FIGS. 4A and 4B. 
In a color cathode ray tube having a substantially rectangular effective 
area 10 with the lateral length larger than the longitudinal length, the 
shape of the curved surface shown in FIG. 2 has the largest average radius 
of curvature as far as the diagonal average radius of curvature determined 
by the fall of the diagonal corners remains the same. The average radius 
of curvature K represents the sum of the minimum radius of curvature 
(1/Rmin) and the maximum radius of curvature (1/Rmax) and is given as 
EQU K=1/Rmax+1/Rmin (1) 
where Rmax is the maximum radius of curvature and Rmin the minimum radius 
of curvature among the various radii of curvature in all the directions at 
an arbitrary point on the outer surface 10a of the effective area 10. 
The atmospheric strength of the vacuum envelope 20 is determined by the 
geometry of the outer surface and the inner surface of the face panel 12. 
Another important factor of determining the atmospheric strength of the 
vacuum envelope is the average radius of curvature K. Also, the sum of the 
square of the minimum radius of curvature (1/Rmin) and the square of the 
maximum radius of curvature (1/Rmax) shown in equation (2) below is still 
another indicator for determining the atmospheric strength of the vacuum 
envelope. 
EQU (1/Rmax).sup.2 +(1/Rmim).sup.2 (2) 
The effective area 10 having the shape of the curved surface shown in FIG. 
2 permits both equations (1) and (2) to assume a maximum value for all 
shapes of curved surface and thus to improve the strength of the face 
panel 12. As a result, a strength equivalent to other face panels in 
common use can be secured even after improving the flatness of the face 
panel 12. It is thus possible to flatten the face panel 12 without 
increasing the thickness of the face panel 12, without attaching a 
reinforcing film to the outer surface 10a of the effective area 10 of the 
face panel or without taking any other measure for reinforcing the face 
panel 12. The successful flattening of the face panel contributes to a 
configuration of a color cathode ray tube having a higher atmospheric 
strength and an improved visibility. 
A few actual examples of the invention will be explained. 
ACTUAL EXAMPLE 1 
Explanation will be made about an actual example 1 of a face panel 
including the above-described curved outer surface applied to a color 
cathode ray tube having an effective area with an aspect ratio of 16:9 and 
a diagonal length of 66 cm constituting the main stream of the cathode ray 
tubes commercially available in recent years. 
Generally, an index R is used to indicate the flatness of the face panel as 
an expression based on the ratio between the diagonal average radius of 
curvature and the diagonal length of the effective area multiplied by a 
factor of 1.7. The color cathode ray tubes commercially available now have 
flattened the index R to about 2.0R. In the case of the index R of 2.0R, 
the diagonal average radius of curvature is R2244. The fall at the 
diagonal corners thus is 24.4 mm. 
In contrast, the face panel 12 as shown in FIG. 2, has a surface 10a and 
effective area 10 that is configured of a cylindrical curved surface 
having an infinitely large radius of curvature along the long axis X and a 
predetermined radius of curvature along the short axis Y. The flatness of 
the outer surface 10a of the effective area 10 is 2.0R. 
Table 1 shows the characteristics including the radius of curvature, the 
average radius of curvature, etc. of the outer surface 10a of the 
effective area 10 of the face panel 12 of the color cathode ray tube 
according to the actual example 1 in comparison with a face panel having a 
spherical outer surface (reference 1) and a face panel having a 
cylindrical outer surface with an infinitely large radius of curvature 
along the short axis Y and a predetermined radius of curvature along the 
long axis X (reference 2). 
TABLE 1 
______________________________________ 
Actual 
Example 1 Reference 1 Reference 2 
______________________________________ 
R index 2.0R 2.0R 2.0R 
Average radius of curvature R2244 R2244 R2244 
at diagonal corners 
Fall of diagonal corners 24.4 24.4 24.4 
Max. radius of curvature .infin. R2244 .infin. 
Min. radius of curvature R549 R2244 R1707 
Average curvature 1.82 .times. 10.sup.-3 8.91 .times. 10.sup.-4 5.86 
.times. 10.sup.-4 
______________________________________ 
As clear from Table 1, the outer surface 10a of the effective area 10, 
exhibits different appearances depending on the shape of the curved 
surface, and has a substantially equal flatness for the same diagonal 
average radius of curvature. The actual example 1 has the same index R of 
2.0R as the references 1 and 2 as a flatness determined by the diagonal 
average radius of curvature of the effective area 10 as described above. 
As compared with the references, however, this example has a considerably 
large average radius of curvature and a large sum of squares of the radius 
of curvature. This is because the face panel 12 according to the actual 
example 1 is laterally elongate with an aspect ratio of 16:9. 
Because the average radius of curvature and the sum of squares of the 
radius of curvature constitute indexes for determining the atmospheric 
strength of the vacuum envelope, a face panel that has such a large 
average radius of curvature and a large sum of the squares of the radius 
of curvature maintains a considerably large strength of the vacuum 
envelope as compared with the face panels of references 1 and 2. 
Although the actual example 1 shows the case wherein the strength of the 
face panel is increased relative to the conventional face panels, the 
thickness of the panel according to the actual example 1 can be decreased 
arbitrarily to about the same strength as the conventional face panels. 
ACTUAL EXAMPLE 2 
An actual example 2 will be explained with reference to the case in which 
the outer surface having the above-described shape is applied to a face 
panel having an effective area with an aspect ratio of 16:9 and a diagonal 
length of 66 cm as in the actual example 1. The actual example 2, however, 
has the same average radius of curvature as the reference 2. 
Table 2 shows the characteristics, such as the radius of curvature, and the 
average radius of curvature of the face panel according to the actual 
example 2 as compared with those of the references 1 and 2. 
TABLE 2 
______________________________________ 
Actual 
Example 2 Reference 1 Reference 2 
______________________________________ 
R index 6.3R 2.0R 2.0R 
Average radius of curvature R7084 R2244 R2244 
at diagonal corners 
Fall of diagonal corners 7.69 24.4 24.4 
Max. radius of curvature .infin. R2244 .infin. 
Min. radius of curvature R1707 R2244 R1707 
Average curvature 5.86 .times. 10.sup.-4 8.91 .times. 10.sup.-4 5.86 
.times. 10.sup.-4 
Sum of squares of radius of 3.43 .times. 10.sup.-7 3.97 .times. 
10.sup.-7 3.43 .times. 10.sup.-7 
curvature 
______________________________________ 
As shown in Table 2, the face panel according to the actual example 2 has 
the same average radius of curvature and the same sum of squares of radius 
of curvature as the second reference. The reference 2 has a radius of 
curvature of R1707 along the long axis and an infinitely large radius of 
curvature along the short axis. In contrast the face panel according to 
the actual example 2 has an infinitely large radius of curvature along the 
long axis and a radius of curvature of R1707 along the short axis. 
As described above, the face panel according to the actual example 2 has 
the same average radius of curvature as the reference 2. With its aspect 
ratio of 16:9, however, the actual example 2 has a fall at the diagonal 
corners considerably different from the reference 2 and has a remarkably 
improved flatness of 6.3R as compared with the figure 2.0R of the 
reference 2. 
FIG. 5 shows the geometry of the outer surface 10a of the effective area of 
the face panel 12 corresponding to the actual example 2. The outer surface 
10a of the effective area 10 has the same average radius of curvature as 
the reference 2 but an apparently different flatness with a remarkably 
improved over the reference 2. Also, as seen from Table 2, the flatness of 
the outer surface of the effective area is determined not by the average 
radius of curvature but substantially by the diagonal average radius of 
curvature depending on the fall at the diagonal corners. 
The vacuum envelope of the color cathode ray tube is fabricated by first 
coupling the face panel of glass to the funnel of glass and then 
exhausting the air from the interior of the envelope into a vacuum state. 
The pressure difference between inside and outside caused by the vacuum 
develops a deformation and an internal stress of the vacuum envelope. In 
order to alleviate this internal stress, the vacuum envelope is reinforced 
by a metal reinforcing band. Even the reinforcing band, however, cannot 
completely alleviate the internal stress of the vacuum envelope. 
The atmospheric strength of the vacuum envelope, which depends to a large 
measure on the shape and the thickness thereof, is also affected by the 
shape of the outer surface of the face panel. Generally, the larger the 
average curvature of the outer surface of the effective area, the larger 
the atmospheric strength. The face panel according to the actual example 2 
has a remarkably improved flatness as compared with the reference 2 but 
the same average radius of curvature as the reference 2, resulting in 
substantially the same atmospheric strength as that of the reference 2. 
ACTUAL EXAMPLE 3 
A face panel according to an actual example 3 is configured with an outer 
surface of the effective area having an infinitely large radius of 
curvature along the long axis X and a constant sectional shape for every 
point of the effective area in a plane parallel to the plane containing 
the short axis Y and the tube axis. Further, the same sectional shape is 
not arcuate having a single radius of curvature like the actual examples, 
but provides a curved surface expressed by a high-order polynomial. 
Specifically, the outer surface of the effective area is curved as to be 
expressed by Z=.SIGMA.a.sub.i Y.sup.2i, where a is a coefficient and i=0, 
1, 2, . . . ,n in a coordinate system having an origin at the center of 
the outer surface of the face panel, a long axis along the X axis, a short 
axis along the Y axis and a tube axis along the Z axis with the skirt end 
(junction with the funnel) of the face panel down. 
More specifically, according to the actual example 3, the outer surface of 
the effective area is formed in the shape given by the formulae below, 
assuming that n=2. 
EQU Z=a1Y.sup.2 +a2Y.sup.4 
EQU a1=-2.350.times.10.sup.-4 
EQU a2=-2.245.times.10.sup.-9 
This face panel is equivalent to the face panel of the actual example 2 
with a slightly larger radius of curvature of the peripheral portion of 
the effective area and including the second-order components of 80% and 
the fourth-order components of 20%. The face panel has an outer shape 
given as 
EQU Z=-2.350.times.10.sup.-4 Y.sup.2 -2.245.times.10.sup.-9 Y.sup.4 
Thus, the sectional shape with the effective area cut away in a plane 
containing the short axis Y and the tube axis Z, i.e., the sectional shape 
along the short axis, is as shown by a curve 22a in FIG. 6. The same 
curved surface is shared by the sectional shape obtained with the 
effective area cut away in a plane parallel to the plane containing the 
short axis Y and the tube axis Z. The diagonal average radius of curvature 
dependent on the fall at the diagonal corners of this face panel is 6.3R. 
Assume that the outer surface of the effective area is curved as described 
above. As clearly seen from FIG. 6, the radius of curvature in the 
direction of the short axis Y in the neighborhood of the long axis X of 
the effective area can be slightly increased, and the radius of curvature 
in the neighborhood of the long side of the effective area can be slightly 
decreased. As a result, it is possible to improve the strength of the 
vacuum envelope which generally has a lower strength at the peripheral 
portion than at the central portion of the effective area. 
ACTUAL EXAMPLE 4 
In the face panels according to the actual examples 1 and 2, the effective 
surface has a cylindrically curved outer surface with an infinitely large 
radius of curvature along the long axis X and a predetermined radius of 
curvature along the short axis Y. In the actual example 4, on the other 
hand, the face panel of the second actual example is slightly modified to 
have an outer surface with a slight radius of curvature along the long 
axis X as shown by a curve 24 in FIG. 7 due to the manufacturing problems, 
etc. Also, the radius of curvature of the outer surface of the effective 
area along the short axis is slightly different between the portion on the 
short axis and in the neighborhood of the short side, as shown by a curve 
25a for a portion on the short axis Y and by a curve 25b for a portion in 
the neighborhood of the short side of the effective area. 
More specifically, as shown in Table 3, the outer surface of the effective 
area has a radius of curvature of R41363 along the long axis X in such a 
manner as to secure the fall .DELTA.26 of 1 mm at the end 26 of the long 
axis X. Also the radius of curvature along the short axis Y is slightly 
smaller on the short side of the effective area than on the short axis Y. 
In FIG. 7, the central portion of the outer surface 10a of the effective 
area is designated by reference numeral 27, the diagonal corner by numeral 
28, the fall at the diagonal corner by .DELTA.28, and the fall at the end 
of the short axis Y by numeral .DELTA.29. 
TABLE 3 
______________________________________ 
Actual Actual 
Example 4 Example 2 
______________________________________ 
Index R 6.3R 6.3R 
Average radius of curvature at diagonal R7084 R7084 
corners 
Fall .DELTA. 28 at diagonal corners 7.69 7.69 
Radius of curvature along long axis R41363 .infin. 
Fall .DELTA. 26 at long axis end 1 0 
Radius of curvature along short axis R2303 R1707 
Fall .DELTA. 29 at short axis end 5.69 7.69 
Radius of curvatures of long side R20682 .infin. 
Radius of curvature of short side R1960 R1707 
Radius of curvature of central portion 4.58 .times. 10.sup.-4 5.86 
.times. 10.sup.-4 
Radius of curvature of diagonal corners 5.96 .times. 10.sup.-4 5.86 
.times. 10.sup.-4 
______________________________________ 
The outer surface of the effective area of the face panel curved as 
described above does not constitute a cylindrically curved surface and the 
average radius of curvature is reduced at both the central portion 27 and 
the diagonal corners 28. The basic curved form, however, suits the 
geometric requirement of a curved surface according to the present 
invention, so that the atmospheric strength of the vacuum envelope of this 
example is substantially equivalent to that of other examples described 
above. 
A face panel with an equal effect can be obtained also by modifying the 
face panel of the actual example 3 in the same manner as the actual 
example 4 is modified in the curve of the outer surface of the effective 
area. 
A few actual examples were explained above. The outer surface of the 
effective area of the face panel of the cathode ray tube according to the 
invention, however, is not limited to a cylindrically curved surface or 
the one expressed by a polynomial. For example, the atmospheric strength 
of the face panel can be improved relative to a face panel having the 
conventional outer surface, even when the outer surface has a radius of 
curvature more than that of the actual example 2 or a radius of curvature 
intermediate between the actual examples 1 and 2, as far as the flatness 
is the same. 
Although the actual example 3 refers to a curved surface expressed by a 
high-order polynomial, say, a fourth-order function, the invention is not 
limited to such a curved surface but is applicable to a curve adjusted in 
accordance with the desired characteristics by a formula including fourth- 
or higher-order polynomial. 
The inner surface of the effective area of the face panel can be set to an 
arbitrary curve regardless of the shape of the outer surface. 
Then, a color cathode ray tube according to a second embodiment of the 
invention will be described in detail. 
A general configuration of the color cathode ray tube is identical to that 
of the color cathode ray tube according to the first embodiment described 
above. The same component parts in this embodiment as the corresponding 
ones in the first embodiment are designated by the same reference 
numerals, respectively, and will not be described in detail. The second 
embodiment is different from the first embodiment in the shape of the 
effective area 10 of the face panel 12. The configuration of the face 
panel 12 will be described in detail. 
As shown in FIGS. 8 to 11B, the face panel 12 includes a substantially 
rectangular effective area 10, the outer surface 10a of which is formed 
substantially flat with a substantially infinitely large radius of 
curvature along both the long axis X (horizontal direction) and the short 
axis (vertical direction). As compared with the substantially flat outer 
surface 10a, the inner surface 10b of the effective area 10, as shown in a 
sectional view taken along the long axis X in FIG. 10, has a substantially 
infinitely large radius of curvature along the long axis X and, as shown 
in a sectional view taken along the short axis Y in FIG. 11A, is formed 
into a cylindrically curved surface having a predetermined radius of 
curvature along the short axis Y. 
On the basis of the shape of the effective area 10 described above, the 
outer surface 10a of the effective area may be curved with a slight 
curvature along the short axis Y, and the inner surface 10b of the 
effective area 10 can be arbitrarily curved with a slight radius of 
curvature along the long axis X. 
In the face panel 12 including the substantially flat outer surface 10a and 
the curved inner surface 10b of the effective area 10 with the 
above-mentioned configuration, the thickness of the peripheral portion of 
the effective area is determined in accordance with the curvature of the 
inner surface 10b. In the color cathode ray tube in which the length of 
the effective area 10 along the long axis X is larger than the length 
along the short axis Y, i.e., the lateral length is larger than the 
longitudinal length, the inner surface 10b of the effective area 10 can be 
curved with the largest average curvature if the inner surface of the face 
panel 12 has the same fall at the diagonal corners and the thickness of 
the face panel is the same at the diagonal corners. 
The average curvature K is defined as the sum of the minimum radius of 
curvature (1/Rmax) and the maximum radius of curvature (1/Rmim) and given 
as 
EQU K=1/Rmax+1/Rmim (3) 
where Rmax is the maximum radius of curvature and Rmim is the minimum 
radius of curvature in all the directions at an arbitrary point on the 
inner surface 10b of the effective area. 
The atmospheric strength of the vacuum envelope 20 is determined by the 
shape of the outer surface and the inner surface of the face panel 12. 
With the face panel 12 having a substantially flat outer surface 10a of 
the effective area 10, the average radius of curvature K of the inner 
surface 10 constitutes one of the crucial factors for determining the 
atmospheric strength of the vacuum envelope. Also, the sum of the squares 
of the minimum radius of curvature (1/Rmax) and the maximum radius of 
curvature (1/Rmim) shown in equation (4) below is another indicator for 
determining the atmospheric strength of the vacuum envelope. 
EQU (1/Rmax).sup.2 +(1/Rmim).sup.2 (4) 
The effective area 10 having the curved surface as shown in FIG. 9, with 
which the values of equations (3) and (4) can be maximized for all curves, 
can improve the strength of the face panel 12. Even in the case where the 
flatness of the face panel 12 is improved, therefore, a strength 
equivalent to that of other face panels now in common use can be secured. 
For this reason, the face panel can be flattened without taking any 
reinforcing measures such as thickening the face panel 12 or attaching a 
reinforcing film on the outer surface 10a of the effective area of the 
face panel. The successful flattening can configure the color cathode ray 
tube with a higher atmospheric strength and an improved visibility. 
A few actual examples will be explained below. 
ACTUAL EXAMPLE 1 
An actual example 1 will be explained with reference to the case in which 
the present embodiment is applied to a color cathode ray tube having an 
aspect ratio of 16:9 and a diagonal length of 66 cm constituting the 
recent main stream of the color cathode ray tubes. 
The face panel used in the actual example 1 has a substantially flat outer 
surface of the effective area as shown in FIGS. 9 to 11b, and the inner 
surface 10b of the effective area is configured of a cylindrical curve 
with an infinitely large radius of curvature along the long axis X and a 
single radius of curvature along the short axis Y. 
Specifically, as shown in FIG. 11A, the outer surface 10a of the effective 
area 10 has an infinitely large radius of curvature along the short axis Y 
and only the inner surface 10b has a predetermined radius of curvature 
along the short axis Y. The thickness of the face panel 12 is maximum at 
the end of the short axis Y. The sectional view of the inner surface 10b 
of the face panel 12 along the direction parallel to the short axis Y, as 
shown in FIG. 11A, is arcuate, while the sectional view of the inner 
surface of the face panel along the line XI--XI parallel to the short axis 
Y has the same arcuate form as that on the short axis Y as shown in FIG. 
11B. 
As shown in FIG. 10, the radius of the curvature of the inner surface and 
the outer surfaces 10b, 10a of the effective area 10 along the long axis X 
are both infinitely large, so that the thickness of the face panel 12 is 
substantially constant along the long axis X. 
Table 4 shows the characteristics including the radius of curvature and the 
average radius of curvature of the inner surface 10b of the effective area 
of the face panel 12 of a color cathode ray tube according to the actual 
example 1, as compared with those of a face panel having a spherical inner 
surface (reference 1) and those of a face panel having a cylindrical inner 
surface (reference 2) with an infinitely large radius of curvature along 
the short axis and a predetermined radius of curvature along the long 
axis. In the actual example 1, the outer surface 10a of the effective area 
10 is assumed to be formed substantially flat. 
The actual example 1 and the references 1 and 2 have a thickness difference 
of 7 mm between the central portion and the diagonal corners of the face 
panel (this difference constitutes the fall at the diagonal corners of the 
inner surface of the panel). Only the shape of the inner surface is 
different between the actual example 1 and the references 1 and 2. 
TABLE 4 
______________________________________ 
Actual 
Example 1 Reference 1 Reference 2 
______________________________________ 
Index R for inner surface 
6.9R 6.9R 6.9R 
Average radius of curvature at R7782 R7782 R7782 
diagonal corners of inner 
surface 
Diagonal corner fall of inner 7 7 7 
surface 
Max. radius of curvature .infin. R7782 .infin. 
Min. radius of curvature R1873 R7782 R5912 
Average radius of curvature 5.34 .times. 10.sup.-4 2.57 .times. 
10.sup.-4 1.69 .times. 10.sup.-4 
Sum of squares of radius of 2.85 
.times. 10.sup.-7 3.30 .times. 
10.sup.-8 2.86 .times. 10.sup.-8 
curvature 
______________________________________ 
As clearly seen from Table 4 above, in the face panel according to the 
actual example 1, the fall at the diagonal corners of the inner surface of 
the panel is the same as that of the references 1 and 2. Nevertheless, the 
average radius of curvature of the inner surface of the effective area is 
remarkably large with a larger sum of the squares of the radius of 
curvature. This difference conspicuously presents itself with the face 
panel of the actual example 1 which has a great difference between the 
lateral and longitudinal lengths as indicated by the aspect ratio of 16:9. 
As described above, the atmospheric strength of the vacuum envelope is 
affected by the average radius of curvature and the sum of squares of the 
radius of curvature of the face panel. Generally, the larger these values, 
the higher the atmospheric strength of the vacuum envelope. Consequently, 
the face panel according to the actual example 1, as compared with the 
references 1 and 2, can have a considerably larger atmospheric strength of 
the vacuum envelope. 
The conventional face panels having a flat outer surface of the effective 
area never has an inner surface of the effective area curved like that of 
the actual example 1. As a result, the color cathode ray tube of the 
actual example 1, as compared with the conventional face panel, can 
advantageously have an improved atmospheric strength of the vacuum 
envelope. In this color cathode ray tube, therefore, the atmospheric 
strength can be increased to the desired level with a lesser reinforcement 
such as thickening of the face panel. 
ACTUAL EXAMPLE 2 
An actual example 2 will be explained with reference to the case in which 
the shape of the inner surface described above is applied to the face 
panel having an aspect ratio of 16:9 and a diagonal length of 66 cm, as in 
the actual example 1. The actual example 2, however, has the same average 
radius of curvature as the second reference. 
Table 2 shows the characteristics such as the radius of curvature and the 
average radius of curvature of the inner surface of the effective area of 
the face panel according to the actual example 2, as compared with those 
of the actual example 1 and the reference 2. 
TABLE 5 
______________________________________ 
Actual Actual 
Example 2 Example 1 Reference 2 
______________________________________ 
Index R for inner surface 
22.0R 6.9R 6.9R 
Average radius of curvature at R24639 R7782 R7782 
diagonal corners of inner 
surface 
Diagonal corner fall of inner 2.2 7 7 
surface 
Max. radius of curvature .infin. .infin. .infin. 
Min. radius of curvature R5912 R1873 R5912 
Average radius of curvature 1.69 .times. 10.sup.-4 5.34 .times. 
10.sup.-4 1.69 .times. 10.sup.-4 
Sum of squares of radius of 2.86 
.times. 10.sup.-8 2.85 .times. 
10.sup.-7 2.86 .times. 10.sup.-8 
curvature 
______________________________________ 
As seen from Table 5, the face panel 12 of the actual example 2 has the 
same average radius of curvature and the same sum of squares of the radius 
of curvature of the inner surface of the effective area as those of the 
reference 2. In the reference 2, the radius of curvature along the long 
axis of the inner surface is R5912 and the radius of curvature along the 
short axis is infinitely large. Conversely, with the actual example 2, the 
radius of curvature along the long axis of the inner surface is infinitely 
large, while the radius of curvature along the short axis thereof is 
R5912. With the face panel of the actual example 2, the average radius of 
curvature is the same as that of the reference 2, but due to the aspect 
ratio of 16:9 of the effective area, has a considerably different fall at 
the diagonal corners of the inner surface from that of the reference 2, so 
that the actual example 2 can remarkably reduce the difference in 
thickness between the central portion and the peripheral portion of the 
face panel 12. As a consequence, the difference of light transmittance 
between outside and inside the effective area 10 can be reduced for an 
improved uniformity of the displayed images. Further, in the face panel 12 
of the actual example 2, the panel reinforcement required for flattening 
the outer surface of the effective area, such as thickening of the face 
panel, can be minimized as in the case of the reference 2. 
ACTUAL EXAMPLE 3 
A face panel according to a actual example 3 has a substantially flat outer 
surface of the effective area and an inner surface of the effective area. 
The inner surface which has a substantially infinitely large radius of 
curvature along the long axis X and a fixed sectional view on a plane 
parallel to the plane containing the short axis Y and the tube axis Z for 
any point in the effective area. In addition, the sectional view of the 
face panel of the actual example 3 is not arcuate having a single radius 
of curvature like those of the actual examples 1 and 2, but is curved as 
expressed by a high-order polynomial. 
Specifically, the inner surface of the effective area constitutes a curved 
surface expressed by Z=.SIGMA.a.sub.i Y.sup.2i where a is a coefficient 
and i=0, 1, 2 . . . ,n in a coordinate system with the end of the skirt 11 
(junction with the funnel) of the face panel down, having a long axis as X 
axis, a short axis as Y axis and a tube axis as Z axis with an origin at 
the center of the inner surface of the face panel. 
In particular, the inner surface of the effective area of the third actual 
example is formed in a manner satisfying the relation specified below, 
assuming n=2. 
EQU Z=a1Y.sup.2 +a2Y.sup.4 
EQU a1=-2.139.times.10.sup.-4 
EQU a2=-2.919.times.10.sup.-10 
This face panel, as compared with the face panel of the actual example 2, 
is equivalent to the case in which the radius of curvature of the 
peripheral portion of the effective area is slightly increased and 
configured of the second-order components of 80% and the fourth-order 
components of 20%. The shape of the inner surface of this face panel is 
given as 
EQU Z=-2.139.times.10.sup.-4 Y.sup.2 -2.919.times.10.sup.-1 Y.sup.4 
FIG. 12 is a sectional view of the effective area 10 cut away in a plane 
containing the short axis Y and the tube axis Z, i.e., a sectional view on 
the short axis, in the case where the fall at the diagonal corners of the 
inner surface 10b of the effective area 10 is 7 mm. The same curved 
surface is observed in a sectional view of the effective area 10 cut away 
in a plane parallel to the plane containing the short axis Y and the tube 
axis Z. 
Assuming that the inner surface of the effective area 10 is as described 
above, as seen clearly from FIG. 12, the radius of curvature along the 
short axis Y in the neighborhood of the long axis X of the effective area 
can be increased while the radius of curvature in the neighborhood of the 
long side of the effective area can be slightly reduced. As a result, it 
is possible to improve the strength of the vacuum envelope generally 
having a lower strength at the peripheral portion than at the central 
portion of the effective area thereof. 
ACTUAL EXAMPLE 4 
In the face panel of the actual example 1, the outer surface 10a of the 
effective area 10 is substantially flat. A face panel of an actual example 
4, on the other hand, has the same inner surface of the effective area as 
the face panel of the actual example 1, and a curved outer surface of the 
effective area with a small curvature along the short axis. Specifically, 
the outer surface of the effective area is cylindrically curved with an 
infinitely large radius of curvature along the long axis and a 
predetermined radius of curvature R6545 along the short axis. The fall at 
the diagonal corners of the outer surface of the effective area is 2 mm. 
The face panel according to the actual example 4 having a configuration as 
described above, which is a slight modification from the face panel of the 
actual example 1 taking into consideration the problems encountered in 
manufacturing the face panel, can produce the substantially same function 
and effects as the face panel according to the actual example 1. 
ACTUAL EXAMPLE 5 
A face panel according to an actual example 5 is configured of a combined 
shape of the actual examples 3 and 4. Specifically, the outer surface of 
the effective area of the face panel is cylindrically curved with an 
infinitely large radius of curvature along the long axis and a small 
curvature along the short axis, while the inner surface of the effective 
area is curved in a form expressed by a high-order polynomial. 
The face panel having this configuration can also produce substantially the 
same function and effects as the face panel of the actual example 4. 
ACTUAL EXAMPLE 6 
A face panel according to an actual example 6 has a substantially flat 
outer surface of the effective area and an inner surface not exactly 
cylindrically curved but with a slight curvature along the long axis. 
Specifically, as shown in FIG. 13, the radius of curvature is set to 
R41363 so as to secure the fall .DELTA.26 of 1 mm at the end 26 of the 
long axis X of the inner surface. 
Also, the radius of curvature along the short axis Y of the inner surface 
of the effective area 10 is slightly varied between a portion on the short 
axis and a portion in the neighborhood of the short side as indicated by a 
curve 25a and a curve 25b, respectively. In FIG. 13, the central portion 
of the inner surface 10b of the effective area is designated by reference 
numeral 27, the diagonal corners thereof by numeral 28, the fall at the 
diagonal corners by numeral .DELTA.28, and the fall at the end of the 
short axis Y by numeral .DELTA.29. 
Table 6 shows the characteristics of a face panel according to the actual 
example 6 as compared with those of the face panel according to the actual 
example 1. 
TABLE 6 
______________________________________ 
Actual Actual 
Example 6 Example 1 
______________________________________ 
Index R for inner surface 
6.9R 6.9R 
Average radius of curvature at diagonal R7782 R7782 
corners of inner surface 
Fall .DELTA. 28 at inner surface diagonal 7 7 
corners 
Radius of curvature along long axis R41363 .infin. 
Fall .DELTA. 26 at end of long axis 1 0 
Radius of curvature along short axis R2620 R1873 
Fall .DELTA. 29 at end of short axis 5 7 
Radius of curvatures of long side R20682 .infin. 
Radius of curvature of short side R2184 R1873 
Average radius of curvature of central 4.06 .times. 10.sup.-4 5.86 
.times. 10.sup.-4 
portion 
Average radius of curvature of diagonal 5.06 .times. 10.sup.-4 5.34 
.times. 10.sup.-4 
corners 
______________________________________ 
The inner surface of the effective area of the face panel described above 
is not cylindrically curved, and therefore both the central portion 27 and 
the diagonal corners 28 are smaller in average radius of curvature. The 
basic geometry, however, is suited to the curved form intended by the 
invention, and the atmospheric strength of the vacuum envelope is also 
substantially equal to those of the above-mentioned actual examples. 
A face panel having a similar effect can also be obtained by adding a 
modification similar to the actual example 6 to the basic curved structure 
of the face panel of the actual example 3. 
A few actual examples of the second embodiment were explained above. The 
shape of the inner surface of the effective area of the face panel of the 
cathode ray tube according to the invention, however, is not limited to a 
cylindrically curved surface or a curved surface expressed by a 
polynomial. A face panel having a curved inner surface with a radius of 
curvature intermediate the actual examples 1 and 2 or a face panel having 
a curved inner surface with a radius of curvature not less than that of 
the actual example 2, for instance, can improve the atmospheric strength 
of the face panel having a conventional shape of the inner surface, as far 
as the peripheral portion of the effective area has the same thickness. 
Also, apart from the curved surface expressed by a high-order polynomial, 
say, a fourth-order function according to the actual example 3, the 
invention is not limited to such a case, and it is possible to adjust the 
shape of the curved surface in accordance with the desired characteristics 
by a formula including a fourth- or higher-order polynomial. 
The actual example 2 was explained above with reference to the case where 
the outer surface of the effective area of the face panel is substantially 
flat, and the case in which the outer surface is curved with a small 
radius of curvature along the short axis. Any shape of the outer surface, 
however, can be used as far as the visibility can be improved to achieve 
the object of the invention. Also, the outer surface of the effective area 
can be curved with a slight radius of curvature, and the inner surface of 
the effective area can be a combination of finely-adjusted curved surfaces 
but not an exactly cylindrical shaped, taking the manufacturing problems 
into consideration. 
According to the second embodiment, as shown in FIGS. 8, 14 and 15, the 
shadow mask 15 includes a substantially laterally long rectangular mask 
body 21 arranged in opposed relation to the phosphor screen 14 formed on 
the inner surface of the effective area 10 of the face panel 12, and a 
substantially laterally long rectangular frame 22 supporting the 
peripheral edge portion of the mask body 21. The mask body 21 is formed 
with a number of electron beam passage apertures 32 arranged in a 
predetermined fashion. The frame 22 is supported on the skirt 11 of the 
face panel 12 through a plurality of holders 24. 
The mask body 21 has a rectangular effective surface 23, which is formed as 
a curved surface having an infinitely large radius of curvature along the 
long axis X (horizontal direction) and having a predetermined curvature 
only along the short axis Y (vertical direction). 
Specifically, as shown in FIGS. 14, 16 to 17B, the effective surface 23 is 
formed as a cylindrically curved surface with an infinitely large radius 
of curvature along the long axis X and a single radius of curvature fixed 
over the entire surface along the short axis Y, or as a curved surface 
with an infinitely large radius of curvature along the long axis X and a 
radius of curvature along the short axis Y expressed by a high-order 
polynomial. 
In the shadow mask 15 having the effective surface 23 of a laterally long 
rectangle as described above, a curved surface with a maximum radius of 
curvature can be obtained by a curved surface having an infinitely large 
radius of curvature along the long axis X and a predetermined radius of 
curvature only along the short axis Y, as far as the fall at the diagonal 
corners and the flatness of the effective surface 23 are the same. 
The strength of holding the curved surface of the shadow mask 15 is 
determined by the curve of the effective surface 23 of the mask body 21, 
the thickness of the mask body 21, the shape and size of the electron beam 
passage apertures 32 and the arrangement of the electron beam passage 
apertures. Assuming that the thickness of the mask body 21 and the shape, 
size and arrangement of the electron beam passage apertures 32 are 
constant, on the other hand, the strength of holding the curved surface is 
determined by the curve of the mask body. 
One index for determining the strength of holding the curved surface of the 
shadow mask 15 is provided by the sum of the squares of the maximum radius 
of curvature 1/Rmax and the minimum radius of curvature 1/Rmim shown 
below. 
EQU 1/Rmax.sup.2 +1/Rmim.sup.2 
With the shadow mask 15 having a surface curved only along the short axis Y 
with an infinitely large radius of curvature along the long axis X as 
described above, the average radius of curvature expressed as the sum of 
1/Rmax and 1/Rmim and the sum of squares thereof can both be maximized of 
all the shapes of curved surfaces, resulting in an improved strength of 
holding the curved surface. 
The shadow mask 15 described above further can facilitate the flattening of 
the face panel by improving the flatness thereof while maintaining the 
strength of holding the curved surface equivalent to the conventional 
shadow mask. Furthermore, the thickness of the shadow mask can be reduced 
while maintaining the same flatness as the conventional shadow mask. 
In manufacturing the mask body 21 of this shadow mask 15, as shown in FIG. 
18A, a flat mask 25 formed with a number of electron beam passage 
apertures 32 in a predetermined arrangement by photoetching is prepared as 
in the ordinary shadow mask. Then, as shown in FIG. 18B, the flat mask 25 
is rounded using a roller or the like so that the cylindrical mask 26 
curved only along the short axis Y is subjected to plastic deformation. 
After that, as shown in FIG. 18C, the cylindrical mask 26 is subjected to 
an elastic deformation, and the radius of curvature thereof along the 
short axis Y is increased to the desired value described above. The 
peripheral edge of the mask 26 subjected to the elastic deformation into a 
predetermined shape in this way is fixedly welded to the frame 22 having 
the same shape as the peripheral potion of the mask body 21 of the shadow 
mask 15 to be formed. 
This method of manufacturing the shadow mask 15 can retain the internal 
stress in the mask body 21 of the shadow mask 15 in such a direction as to 
reduce the radius of curvature of the mask body 21 along the short axis Y, 
whereby a shadow mask is provided with a high strength of holding the 
curved surface. 
This method of manufacturing the shadow mask is not applicable to the 
conventional shadow mask having a spherically curved surface, but is 
effective for the fabrication of a shadow mask curved along the long axis 
or the short axis. Especially, the manufacturing method described above 
can exhibit a superior function and effect in an application for forming 
the shadow mask 15 according to the present embodiment having a curved 
surface with an infinitely large radius of curvature along the long axis X 
and with a predetermined curvature only along the short axis Y. 
Now, actual examples of the shadow mask 15 will be explained. 
ACTUAL EXAMPLE 1 
An actual example 1 will be explained with reference to a shadow mask 
applied to a color cathode ray tube having an aspect ratio of 16:9 and a 
diagonal length of 66 cm constituting the main stream of color cathode ray 
tubes in recent years. 
In this shadow mask 15, as shown in FIGS. 14, 16 to 17B, the effective 
surface 23 of the mask body 21 is formed as a cylindrically curved surface 
having an infinitely large radius of curvature along the long axis X and a 
fixed radius of curvature along the short axis Y over the entire range of 
the effective surface 23. 
Table 7 shows the characteristics including the radius of curvature and the 
average radius of curvature of the effective surface of the shadow mask 
according to the actual example 1, as compared with those of a shadow mask 
having a spherical effective surface (reference 1) and a shadow mask 
having an infinitely large radius of curvature along the short axis and 
curved only along the long axis (reference 2). 
TABLE 7 
______________________________________ 
Actual 
Example 1 Reference 1 Reference 2 
______________________________________ 
Mask index R 6.9R 6.9R 6.9R 
Diagonal average radius of R7782 R7782 R7782 
curvature of mask 
Diagonal corner fall of 7 7 7 
Mask 
Max. radius of curvature .infin. R7782 .infin. 
Min. radius of curvature R1873 R7782 R5912 
Average radius of 5.34 .times. 10.sup.-4 2.57 .times. 10.sup.-4 1.69 
.times. 10.sup.-4 
curvature 
Sum of squares of radius of 2.85 .times. 10.sup.-7 3.30 .times. 
10.sup.-8 2.86 .times. 10.sup.-8 
curvature 
______________________________________ 
In Table 7, the shadow masks of the actual example 1 and the references 1 
and 2 are tabular and 0.2 m thick formed in press as conventionally 
practiced. These shadow masks are formed with a curved surface having a 
fall of 7 mm at diagonal corners. 
Comparison between a plurality of shadow masks having the same fall at the 
diagonal corners and different curved surfaces shows that the shadow mask 
according to the actual example 1 has a considerably larger average radius 
of curvature than those of the references 1 and 2. The sum of the squares 
of the radius of curvature is also large for the shadow mask of the actual 
example 1 as compared with those of the references. As described above, 
the strength of holding the curved surface of the shadow mask is affected 
by the average radius of curvature and the sum of squares of the radius of 
curvature. Generally, the larger these values, the higher the strength of 
holding the curved surface of the shadow mask. Further, the shadow mask 
shown in Table 7 has a large aspect ratio of 16:9 with a large difference 
between the horizontal size and the vertical size thereof. The shadow mask 
according to the actual example 1, therefore, has an especially large 
average radius of curvature and an especially large sum of the squares of 
radius of curvature as compared with the shadow masks of the references 1 
and 2. The strength of holding the curved surface thus is seen to be 
considerably larger for the shadow mask of the actual example 1 than that 
for the references 1 and 2. 
It is generally known that the local doming of the shadow mask can be 
controlled by increasing the radius of curvature of the portion of the 
shadow mask having a large deterioration of color purity. The shadow mask 
according to the actual example 1, as compared with those of the 
references 1 and 2, has a large radius of curvature in the particular area 
and therefore is exposed to a smaller local doming. 
In the shadow mask 15 according to this embodiment, each electron beam 
passage aperture 32 formed in the mask body 21 has an elongate form 
extending along the short axis Y as shown in FIG. 15. A plurality of the 
electron beam passage apertures 32 are arranged in juxtaposition through 
bridges 33 along the short axis Y. Further, the electron beam passage 
apertures extending along the short axis Y are arranged in a plurality of 
lines at predetermined spatial intervals along the long axis X. 
The mask body 21 having the electron beam passage apertures 32 arranged as 
mentioned above has continuously linear portions extending along the short 
axis Y but lacks such continuous linear portions along the long axis X. As 
a result, the mask body 21 has such an anisotropic property that the 
strength along the short axis thereof is higher than that along the long 
axis X. Taking into consideration the fact that the distance between the 
long sides of the mask body 21 is small in comparison with the distance 
between the short sides of the mask body 21 and that the mask body has an 
anisotropic property, therefore, curving the mask body along the short 
axis higher in strength than the long axis can produce a higher effect of 
controlling the doming for the same radius of curvature. Consequently, the 
shadow mask according to the actual example 1 has a higher effect of 
suppressing the local doming than the first and second references. 
Further, the fabrication of the shadow mask by the above-mentioned 
fabrication method can retain the internal stress in the mask body in such 
a direction as to reduce the radius of curvature along the short axis 
higher in strength, and thus can maintain a sufficient strength of holding 
the curved surface. 
In view of these facts, a color cathode ray tube can be obtained which can 
resist the shocks and vibrations which may be exerted on the color cathode 
ray tube and which hardly deteriorate in color purity. 
ACTUAL EXAMPLE 2 
The actual example 1 relates to a shadow mask of a color cathode ray tube 
having an aspect ratio of 16:9 and a diagonal length of 66 cm and 
including a mask body 0.2 mm thick. In an actual example 2, on the other 
hand, a flat mask as thin as 0.18 mm is pressed to make a shadow mask 
having a cylindrically curved surface. 
With a thin mask body as mentioned above, the uniformity of the shape and 
size of the electron beam passage apertures formed in the mask body by 
photoetching can be improved while at the same time reducing the 
production cost. In addition, in spite of a smaller thickness, it is 
possible to secure a strength of holding the curved surface at least equal 
to that of the shadow masks of the references 1 and 2 having the strength 
of holding the curved surface as shown in Table 7. 
ACTUAL EXAMPLE 3 
Like in the actual examples 1 and 2, a shadow mask applicable to a color 
picture ray tube having an aspect of 16:9 and a diagonal dimension of 66 
cm is fabricated with the same average radius of curvature and the same 
sum of squares of the radius curvature as in the reference 2 shown in 
Table 7. 
Following Table 8 shows the characteristics including the radius of 
curvature and the average radius of curvature of the effective surface of 
the shadow mask according to the actual example 3, as compared wit those 
of the actual example 1 and the reference 2. 
TABLE 8 
______________________________________ 
Actual Actual 
Example 3 Example 1 Reference 2 
______________________________________ 
Mask index R 22.0R 6.9R 6.9R 
Diagonal average radius of R24639 R7782 R7782 
curvature of mask 
Diagonal corner fall of 2.2 7 7 
Mask 
Max. radius of curvature .infin. .infin. .infin. 
Min. radius of curvature R5912 R1873 R5912 
Average radius of 1.69 .times. 10.sup.-4 5.34 .times. 10.sup.-4 1.69 
.times. 10.sup.-4 
curvature 
Sum of squares of radius of 2.86 .times. 10.sup.-8 2.85 .times. 
10.sup.-7 2.86 .times. 10.sup.-8 
curvature 
______________________________________ 
The shadow mask of the actual example 3 shown above is configured to have 
the same average radius of curvature and the same sum of squares of the 
radius of curvature as the reference 2 taking into consideration of the 
geometry of the inner surface of the effective area of the face panel. The 
shadow mask according to the reference 2 has a radius of curvature of 
R5912 along the long axis and an infinitely large radius of curvature 
along the short axis, whereas the shadow mask according to the actual 
example 3 conversely has an infinitely large radius of curvature along the 
long axis and a predetermined value R5912 of radius of curvature along the 
short axis. 
In other words, the shadow mask of the actual example 3 has a radius of 
curvature along the short axis similar to the radius of curvature along 
the long axis of the reference 2, and therefore has a greater effect of 
suppressing the local doming as described above, thereby reducing the 
deterioration of the color purity of the color cathode ray tube. 
Also, as shown in Table 8, the average radius of curvature of the shadow 
mask according to the actual example 3 is identical to that of the 
reference 2. Because of the aspect ratio of 16:9, however, the fall of the 
diagonal corners of the shadow mask according to the actual example 3 
assumes a value of 2.2 considerably different from the FIG. 7.0 for the 
shadow mask of the reference 2. As a result, the effective area 23 is 
considerably flattened. The effective area of the face panel can thus be 
flattened in accordance with the curve of the shadow mask surface. 
ACTUAL EXAMPLE 4 
An actual example 4, like the actual examples 1 to 3, concerns a shadow 
mask applicable to a color picture tube having an aspect ratio of 16:9 and 
a diagonal length of 66 cm, in which the curved surface of the effective 
area is configured to have an infinitely large radius of curvature along 
the long axis X and a predetermined radius of curvature along the short 
axis Y expressed by a high-order polynomial unlike the arcuate surface 
with a single radius of curvature along the short axis Y in the actual 
examples 1 to 3. Also, the shadow mask according to the actual example 4 
has a fall of 7 mm at the diagonal corners. 
Specifically, the effective surface of the shadow mask forms a curved 
surface expressed as Z=.SIGMA.a.sub.i Y.sup.2i, where a is a coefficient 
and i=0, 1, 2, . . . ,n, in a coordinate system with an origin at the 
center of the effective surface, a long axis as the X axis, a short axis 
as the Y axis and a tube axis as the Z axis, and having the side thereof 
opposed to the effective area of the face panel turned up. 
In particular, the actual example 4 is formed in a shape expressed by the 
following equations, assuming that n=2. 
EQU Z=a1Y.sup.2 +a2Y.sup.4 
EQU a1=-2.139.times.10.sup.-4 
EQU a2=-2.919.times.10.sup.-10 
This shadow mask has the fourth-order components of 20% and the 
second-order components of 80% given by the fourth-order polynomial 
described above, leading to a slightly larger radius of curvature of the 
peripheral edge of the effective surface than that of the shadow mask of 
the actual example 3. The geometry of the mask along the short axis on the 
short axis is expressed by the fourth-order function as 
EQU Z=-2.139.times.10.sup.-4 Y.sup.2 -2.919.times.10.sup.-10 Y.sup.4 
The curved surface in the direction parallel to the short axis at an 
arbitrary point of the mask body also assumes the same shape as mentioned 
above. 
In the case where the effective area of the shadow mask is formed as a 
curved surface as mentioned above, the radius of curvature along the short 
axis in the neighborhood of the long axis can be reduced slightly and the 
radius of curvature along the short axis in the neighborhood of the long 
side of the mask body can be slightly increased. Especially, the strength 
of holding the curved surface of the mask body can be increased to such an 
extent as to be balanced appropriately over the effective surface of the 
shadow mask. 
The present invention is not limited to the above mentioned embodiments, 
but also various changes and modifications may be applied within the scope 
of the invention. For example, in a cathode ray tube according to the 
present invention, the effective area of the face panel may be formed with 
the outer and inner surfaces having the shapes described in the first and 
second embodiments, and this effective area may be combined with the 
shadow mask described in the second embodiment.