Color cathode ray tube having a shadow mask of improved strength

A color cathode ray tube has a shadow mask. The shadow mask is composed of an almost rectangular mask body and an almost rectangular mask frame. The mask body has an effective surface facing a fluorescent screen, a non-aperture section surrounding the effective surface, and a bent skirt section formed on the periphery of the non-aperture section. The mask frame has a sidewall section provided on the skirt section of the mask body. In the shadow mask of such a configuration, a step section having a step in the direction perpendicular to the skirt section of the mask body is formed. With this configuration, the strength with which the curved surface of the effective surface of the shadow mask is enhanced, which prevents the color purity from deteriorating due to deformation and facilitates the formation of the shadow mask.

BACKGROUND OF THE INVENTION
 This invention relates to a color cathode ray tube, and more particularly
 to a color cathode ray tube which alleviates the degradation of color
 purity due to the deformation of the shadow mask and facilitates the
 formation of the shadow mask.
 Generally, a color cathode ray tube has a vacuum envelope composed of an
 almost rectangular panel 3 with a curved-surface effective region 1 on
 whose periphery a sidewall section 2 is provided, and a funnel 4 connected
 to the sidewall section 2 is provided, and a funnel 4 connected to the
 sidewall section 2. On the inside face of the effective region 1 of the
 panel 3, a fluorescent screen 5 is provided. The fluorescent screen 5 is
 composed of a black non-emitting layers and three-color fluorescent layers
 formed so as to fill up the spacing of the black non-emitting layer.
 Inside the fluorescent screen 5, an almost rectangular shadow mask 6 is
 provided on a mask frame, so as to face the fluorescent screen 5. The
 shadow mask 6 is supported in a detachable manner by elastic supports 8
 hooked on stud pins 7 provided on the sidewall section 2 of the panel 3.
 In the neck 10 of the funnel 4, an electron gun 12 that emits three
 electron beams 11B, 11G, 11R is provided. The three electron beams 11B,
 11G, 11R emitted from the electron gun 12 are deflected by a deflection
 unit 13 provided on the outside of the funnel 4 and directed toward the
 fluorescent screen 5 through the shadow mask 6. The three electron beams
 11B, 11G, 11R scan the fluorescent screen 5 horizontally and vertically,
 thereby producing color images on the fluorescent screen 5.
 The shadow mask 6 has the function of selecting the three electron beams
 11B, 11G, 11R for the three-color fluorescent layers constituting the
 fluorescent screen 5 and landing the three electron beams 118, 11G, 11R on
 the corresponding three-color fluorescent layers. The shadow mask 6 is
 composed of an almost rectangular mask body 18 and a mask frame 20. As
 shown in FIGS. 2A and 2B, the mask body 18 is composed of an effective
 surface 15, a non-aperture section 16 enclosing the effective surface 15,
 and a bent skirt section 17 formed on the periphery of the non-aperture
 section 16 which has no through holes. The effective surface 15 is a
 curved surface, has a large number of electron beam through holes or
 apertures formed therein, for allowing the electron beams therethrough,
 and faces the fluorescent screen 5. The mask frame 20 has a sidewall
 section 19 provided on the skirt section 17 of the mask body 18.
 In general, to display an image with a high color purity on the fluorescent
 screen 5 of the color cathode ray tube, the three electron beams 11B, 11G,
 11R have to be selected by the electron beam through holes in the mask
 body 18 so that the three electron beams 11B, 11G, 11R may land on the
 three-color fluorescent layers properly. To do this, the positional
 relationship between the panel and shadow mask 6 needs to be maintained
 properly. The clearance (q value) between the inside face of the effective
 region 1 of the panel 3 and the effective surface 15 of the mask body 18
 particularly has to be kept within specific permitted limits.
 In recent years, to improve the visibility of color cathode ray tubes,
 effort has been directed toward making the radius of curvature of the
 outside face of the effective region 1 of the panel larger to bring the
 outside face close to a plane. In such a panel, the radius of curvature of
 the inside face of the effective region 1 also needs to be made larger
 from the viewpoints of the formation of the panel 3 and the visibility.
 With an increase in the radius of curvature of the inside face of the
 effective region 1, the radius of curvature of the effective surface 15 of
 the shadow mask 6 also needs to be made larger to achieve a suitable beam
 landing.
 However, as the radius of curvature of the effective surface 15 of the
 shadow mask 6 is made larger, the strength with which the curved surface
 is retained decreases. As a result, a local deformation in manufacturing
 the shadow mask 6 or a thermal deformation in manufacturing the color
 cathode ray tube is liable to take place. This may cause a shift in the
 beam landing, making color purity liable to deteriorate. When the color
 cathode ray tube has been incorporated into a television set, the sound
 from the speaker is liable to cause the shadow mask 6 to resonate. The
 resonance may cause the deterioration (howling) of color purity.
 To improve the strength with which the curved surface of the shadow mask 6
 is retained, a method of providing a reinforcing bead on the effective
 surface has been disclosed in Jpn. Pat. Appln. KOKAI Publication No.
 7-161306.
 However, when a reinforcing bead has been provided on the effective surface
 with a larger radius of curvature to produce a sufficient curved-surface
 retaining strength, the spacing between the inside face of the effective
 region of the panel and the effective surface of the shadow mask falls
 locally away from the permitted limits. This permits an image of a step
 caused by the formation of the reinforcing bead to appear on the screen,
 degrading the picture quality seriously. For this reason, there is a limit
 to the height of the reinforcing bead. Usually, the limit ranges from
 about 0.1 to 0.2 mm, which causes the problem of being unable to make the
 curved-surface retaining strength sufficiently high.
 As described above, to improve the visibility of a color cathode ray tube,
 the radius of curvature of the outside face of the effective region of the
 panel is made larger to bring the outside face close to a plane. This also
 requires the radius of curvature of the inside face of the effective
 region to be made larger. As a result, the radius of curvature of the
 effective surface of the shadow mask has to be made larger. Making the
 radius of curvature of the effective surface larger decreases the
 curved-surface retaining strength.
 As a result, a local deformation in manufacturing a shadow mask or a
 thermal deformation in manufacturing a color cathode ray tube is liable to
 take place. This may cause a shift in the beam landing and the
 deterioration of color purity. When the color cathode ray tube has been
 incorporated into a television set, the sound from the speaker is liable
 to cause the shadow mask to resonate. The resonance may degrade the color
 purity.
 To improve the strength with which the effective surface of the mask body
 with a larger radius of curvature is retained, a method of providing a
 reinforcing bead on the effective surface has been proposed. When a
 reinforcing bead has been provided on the effective surface with a larger
 radius of curvature to produce a sufficient curved-surface retaining
 strength, the spacing between the inside face of the effective region of
 the panel and the effective surface of the mask body falls locally away
 from the permitted limits. This permits an image of a step caused by the
 formation of the reinforcing bead to appear on the screen, seriously
 degrading the picture quality. For this reason, the curved-surface
 retaining strength cannot be made sufficiently high by use of a
 reinforcing bead in a shadow mask whose effective surface has a larger
 radius of curvature.
 BRIEF SUMMARY OF THE INVENTION
 The object of the present invention is to provide a color cathode ray tube
 which heightens the curvedsurface retaining strength of the effective
 surface of a shadow mask to prevent color purity from deteriorating as a
 result of the deformation of the shadow mask and facilitate the formation
 of the shadow mask.
 (1) In a color cathode ray tube of the present invention, an almost
 rectangular shadow mask is provided so as to face a fluorescent screen
 formed on the inside face of a rectangular panel on whose periphery a
 sidewall section is provided, the shadow mask being composed of an almost
 rectangular mask body and an almost rectangular mask frame, the mask body
 having an effective surface in which a large number of electron beam
 through holes are made and which faces the fluorescent screen, a
 non-aperture section enclosing the effective surface, and a bent skirt
 section formed on the periphery of the non-aperture section, and the mask
 frame having a sidewall section to be provided on the skirt section of the
 mask body, and a step section is formed which has a step at the skirt
 section of the mask body in the direction perpendicular to the axis of the
 tube.
 (2) In a color cathode ray tube of the present invention, an almost
 rectangular shadow mask is provided so as to face a fluorescent screen
 formed on the inside face of an almost rectangular panel on whose
 periphery a sidewall section is provided, the shadow mask being composed
 of an almost rectangular mask body and an almost rectangular mask frame,
 the mask body having an effective surface in which a large number of
 electron beam through holes are made and which faces the fluorescent
 screen, a non-aperture section enclosing the effective surface, and a bent
 skirt section formed on the periphery of the non-aperture section, and the
 mask frame having a sidewall section to be provided on the skirt section
 of the mask body, the boundary between the non-aperture section of the
 mask body and the skirt section is made closer to the fluorescent screen
 than the boundary between the effective surface and the non-aperture
 section, and a step section having a step in the direction perpendicular
 to the axis of the tube is formed at the bent skirt section formed on the
 periphery of the non-aperture section.
 (3) In the color cathode ray tube in item (1) or (2), the step section is
 formed continuously in the direction perpendicular to the direction of
 width of the skirt section.
 (4) In the color cathode ray tube in item (1) or (2), the step section is
 formed discontinuously in the direction perpendicular to the direction of
 width of the skirt section.
 (5) In the color cathode ray tube in item (3) or (4), the height of the
 step in the step section is set to 1 to 10 mm.
 (6) In the color cathode ray tube in any one of items (1) to (5), the step
 section is formed near the boundary between the skirt section and the
 non-aperture section.
 (7) In the color cathode ray tube in any one of items (1) to (6), the step
 section is formed at the skirt section at least on either the long sides
 or the short sides of the mask body.
 (8) In the color cathode ray tube in item (7), the size of the step section
 is changed according to a position on the skirt section.
 (9) In the color cathode ray tube in item (7), the size of the step section
 on the long sides of the mask body differs from that on the short sides.
 (10) In a color cathode ray tube of the present invention, an almost
 rectangular shadow mask is provided so as to face a fluorescent screen
 formed on the inside face of an almost rectangular panel on whose
 periphery a sidewall section is provided, the shadow mask being composed
 of an almost rectangular mask body and an almost rectangular mask frame,
 the mask body having an effective surface in which a large number of
 electron beam through holes are made and which faces the fluorescent
 screen, a non-aperture section enclosing the effective surface, and a bent
 skirt section formed on the periphery of the non-aperture section, and the
 mask frame having a sidewall section to be provided on the skirt section
 of the mask body, and a step section having a step in the direction
 perpendicular to the axis of the tube and slits are formed at the skirt
 section of the mask body.
 (11) In a color cathode ray tube of the present invention, an almost
 rectangular shadow mask is provided so as to face a fluorescent screen
 formed on the inside face of an almost rectangular panel on whose
 periphery a sidewall section is provided, the shadow mask being composed
 of an almost rectangular mask body and an almost rectangular mask frame,
 the mask body having an effective surface in which a large number of
 electron beam through holes are made and which faces the fluorescent
 screen, a non-aperture section enclosing the effective surface, and a bent
 skirt section formed on the periphery of the non-aperture section, and the
 mask frame having a sidewall section to be provided on the skirt section
 of the mask body, the boundary between the non-aperture section of the
 mask body and the skirt section is made closer to the fluorescent screen
 than the boundary between the effective surface and the non-aperture
 section, and a step section having a step in the direction perpendicular
 to the axis of the tube and slits are formed at the bent skirt section
 formed on the periphery of the non-aperture section.
 (12) In the color cathode ray tube in item (10) or (11), the step section
 is formed discontinuously in the direction perpendicular to the direction
 of width of the skirt section near the boundary between the skirt section
 and the non-aperture section and the slits are made in the edge of the
 skirt section in the portions where the step section has and has not been
 formed.
 (13) In the color cathode ray tube in any one of items (10) to (12), the
 length of the slits is made about one-half the width of the skirt section.
 (14) In the color cathode ray tube, an almost rectangular shadow mask is
 provided so as to face a fluorescent screen formed on the inside face of
 an almost rectangular panel on whose periphery a sidewall section is
 provided, the shadow mask being composed of an almost rectangular mask
 body and an almost rectangular mask frame, the mask body having an
 effective surface in which a large number of electron beam through holes
 are made and which faces the fluorescent screen, a non-aperture section
 enclosing the effective surface, and a bent skirt section formed on the
 periphery of the non-aperture section, and the mask frame having a
 sidewall section to be provided on the skirt section of the mask body, and
 a step section having a step in the direction perpendicular to the axis of
 the tube, which is formed near a welding portion between the mask boy and
 the bent skirt.
 Additional objects and advantages of the invention will be set forth in the
 description which follows, and in part will be obvious from the
 description, or may be learned by practice of the invention. The objects
 and advantages of the invention may be realized and obtained by means of
 the instrumentalities and combinations particularly pointed out
 hereinafter.

DETAILED DESCRIPTION OF THE INVENTION
 Hereinafter, referring to the accompanying drawings, embodiments of the
 present invention will be explained.
 FIG. 3 shows a color cathode ray tube according to an embodiment of the
 present invention. The color cathode ray tube has an almost rectangular
 panel 32 on the periphery of whose effective region 30 a sidewall section
 31 is provided. A funnel 33 connected to the sidewall section 31 and the
 panel 32 constitute a vacuum envelope. The inside face of the effective
 region 30 of the panel 32 is flat or slightly curved. Its inside face
 takes the form of a curved surface with a larger radius of curvature than
 that of the inside face of the effective surface of a conventional panel.
 Namely, the panel 32 is a flattened panel.
 On the inside face of the effective region 30 of the panel 32, a
 fluorescent screen 35 is provided. The fluorescent screen 35 is composed
 of black non-emitting layers called black stripes, and three-color
 fluorescent layers provided so as to fill out the spacing of the black
 non-emitting layer. The three-color fluorescent layer emits blue, green,
 and red rays of light. Inside the fluorescent screen 35, an almost
 rectangular shadow mask 36 is provided so as to face the fluorescent
 screen 35. Stud pins 37 are provided on the sidewall section 31 of the
 panel 32. A wedge-shaped elastic support 38 is provided on the sidewall
 section of each corner of the frame of the shadow mask 36. The
 wedge-shaped elastic supports 38 are hooked on the stud pins 37, thereby
 enabling the panel 32 to support the shadow mask 36 in a detachable
 manner. Inside the neck 40 of the funnel 33, an electron gun 42 that emits
 three electron beams 41B, 41G, 41R is provided.
 The three electron beams 41B, 41G, 41R emitted from the electron gun 42 are
 deflected by a magnetic field caused by a deflection unit 44 provided on
 the outside of the funnel 33 and directed to the fluorescent screen 35
 through the shadow mask 36. The three electron beams 41B, 41G, 41R then
 scan the fluorescent screen 35, thereby producing color images on the
 fluorescent screen 35.
 As shown in FIGS. 4A and 4B, the shadow mask 36 is composed of an almost
 rectangular mask body 50 and an almost rectangular mask frame 52. The mask
 body 50 has an effective surface 47 composed of a curved surface facing
 the fluorescent screen and a non-aperture section 48 provided so as to
 surround the periphery of the effective surface 47. In the effective
 surface 47, a large number of electron beam through holes 46 are made. On
 the periphery of the non-aperture section 48, a bent skirt section 49 is
 formed. The mask frame 52 has a sidewall section 51 on the inside of which
 the skirt section 49 of the mask body 50 is provided.
 In the embodiment, as shown in FIG. 4B, at the boundary between the skirt
 section 49 of the mask body 50 and the non-aperture section 48, there is
 provided a step section 53 stepped in the direction of width, i.e., height
 (the direction of Z) of the skirt section 49 and extended in the direction
 (the directions of X and Y) perpendicular to the direction of width (the
 direction of Z). The open end of the skirt section 49 where the step
 section 53 has not been formed is welded to the mask frame 51. The step
 section 53 is formed continuously around all the circumference of the
 skirt section 49 so as to enclose the effective surface 47. The lateral
 length d of the step in the direction (the directions of X and Y)
 perpendicular to the tube axis (Z-axis) is set to 1 to 10 mm.
 The step section 53 of the mask body 50 can be easily formed at the same
 time that a flat-plate mask in which electron beam through holes have been
 made by photoetching is press-molded.
 When the step section 53 is provided at the boundary between the skirt
 section 49 of the mask body 50 and the non-aperture section 48, the
 strength of the skirt section 49 is increased by the plastic deformation
 caused by forming the skirt section 49 by press molding. This improves the
 strength with which the curved surface of the effective surface is
 retained. The step section 53 can absorb deforming stress in manufacturing
 a shadow mask or thermal deforming stress in manufacturing a color cathode
 ray tube, thereby preventing the effective surface 47 from being deformed.
 Furthermore, when a color cathode ray tube is incorporated into a
 television set, the improvement of the curved-surface retaining strength
 makes the sound from the speaker less liable to cause resonance.
 Therefore, use of the step section 53 makes the color cathode ray tube
 less liable to permit color purity to deteriorate.
 For instance, as a result of providing a step with d=4 mm for the skirt
 section in a shadow mask incorporated in a color cathode ray tube with a
 76-cm-long diagonal, it has been verified that the movement of the beam
 landing on the three-color fluorescent layer caused by the speaker sound
 resonance was reduced about 50%.
 The reason why the length d of the step in the step section 53 in the
 direction perpendicular to the axis of the tube was set to 1 to 10 mm is
 that use of the length d less than 1 mm cannot improve the curved surface
 retaining strength for the effective surface 47 effectively. Another
 reason is that when the length d exceeds 10 mm, not only is a balance
 between the outside diameter of the shadow mask and the dimensions of the
 effective surface lost for practical use, but also the effect of improving
 the curved-surface retaining strength of the effective surface 47
 decreases.
 Next, a shadow mask according to another embodiment of the present
 invention will be explained. The shadow mask of this embodiment differs in
 structure from the shadow mask shown in FIGS. 4A and 4B.
 In the embodiment shown in FIGS. 4A and 4B, the step section has been
 formed continuously around all the circumference of the skirt section in
 such a manner that the step section encloses the effective surface. In a
 mask body 50 shown in FIGS. 5A and 5B, a plurality of discontinuous step
 sections 53 are formed in the direction of width of the skirt section 49
 at the boundary between the skirt section 49 and non-aperture section 48.
 Each step section 53 is extended in the direction perpendicular to the
 direction of the width. In a sectional view of the step section 53 taken
 in the direction perpendicular to the axis of the tube as shown in FIG.
 5A, portions where the step sections 53 have been formed alternate with
 portions where no step section 53 has been formed in such a manner that
 they enclose the effective surface 47.
 In the present embodiment, the width w (size) of the step section 53 in the
 direction (the directions of X and Y) perpendicular to the direction (the
 direction of Z) of the width of the skirt section 49 and the spacing
 between the step sections 53 on the long sides 57 differ from those on the
 short sides 56. The width of the step section 53 and the spacing between
 the step sections on the short sides 56 are smaller than those on the long
 sides 57.
 When the step section 53 are provided discontinuously this way, the
 strength of the skirt section 49 is made higher by a plastic deformation
 caused by forming the skirt section 49 by press molding than that of the
 mask body shown in FIGS. 4A and 4B. This improves the curved-surface
 retaining strength of the effective surface 47 more effectively, making
 the color cathode ray tube less liable to permit color purity to
 deteriorate.
 While in the above embodiments, the step section has been provided on both
 of the long sides and short sides in such a manner that the step sections
 enclose the effective surface of the mask body, the step section may be
 provided on only either the long sides or the short sides. This would
 improve the curved-surface retaining strength of the effective surface.
 Specifically, the curved-surface retaining strength of the effective
 surface can be improved in a well-balanced manner by just forming
 selectively step sections in portions where the curved-surface retaining
 strength of the effective surface is liable to decrease. This approach
 produces a similar effect to that of the above embodiments.
 Continuous step sections may be combined with discontinuous step sections
 in such a manner that, for example, continues step sections are provided
 on the short sides and discontinuous step sections are provided on the
 long sides.
 Furthermore, the width w of the step sections may be changed on a single
 side and the spacing between the step sections be variable there.
 In the above embodiments, the effective surface of the mask body and the
 non-aperture section enclosing the periphery of the effective surface are
 made up of a continuous plane. In the mask body 50 shown in FIG. 6, the
 boundary between the non-aperture section 48 and the skirt section 49 is
 formed by bending the non-aperture section 48 at a place close to the
 effective surface 47 in such a manner that the boundary between the
 non-aperture section 48 and skirt section 49 projects higher than the
 boundary between the effective surface 47 and the non-aperture section 48
 toward the fluorescent screen. The step sections 53 are provided at the
 boundary between the non-aperture section 48 and the skirt section 49.
 With the mask body 50 constructed as described above, the non-aperture
 section 48 functions as a bead that improves the curved-surface retaining
 strength of the effective surface 47. The non-aperture section 48,
 together with the step sections 53, further improves the curved-surface
 retaining strength of the effective surface 47, which enables the
 construction of a color cathode ray tube less liable to permit color
 purity to deteriorate.
 Furthermore, in the mask body 50 shown in FIGS. 7A and 7B, a non-aperture
 section 48 is provided on the periphery of the effective surface 47 in
 such a manner that the non-aperture section has a continuous surface with
 the effective surface 47. The bent skirt section 49 is formed on the
 periphery of the non-aperture section 48. In the mask body 50, the step
 sections 53 are provided particularly at the boundary between the skirt
 section 49 and the non-aperture section. Slits 55 are made in the skirt
 section 49.
 The step sections 53 are provided at the boundary between the skirt section
 49 and non-aperture section 48 on the horizontal axis (X-axis) and the
 vertical axis (Y-axis). In contrast, the slits 55 are made in the portions
 where the step sections 53 have and have not been formed on the open end
 side of the skirt section. The length of the slit 55 in the direction of
 width of the skirt section 49 is set to be not larger than one-half the
 width of the skirt section 49, usually to the length ranging from 5 to 20
 mm.
 For example, in a shadow mask incorporated in a color cathode ray tube with
 a 68-cm-long diagonal, the length d of the step section 53 in the
 direction perpendicular to the axis of the tube is set to 3 mm, its depth
 e in the direction of width of the skirt section 49 is 5 mm, and its width
 w in the direction perpendicular to the direction of width of the skirt
 section 49 is 130 mm as shown in FIG. 8. In a flat-plate mask of FIG. 9,
 slits 55 with a length of 12 mm and a width w of 1.6 mm are made in
 positions 70 mm and 230 mm from the vertical axis on the open edge side of
 the skirt section 49 in the portions where the step sections 53 have been
 formed on the long and short sides, and on the long sides. On the short
 sides, such slits are made in positions 80 mm and 150 mm from the
 horizontal axis.
 Such a mask body 50 is formed as follows. As shown in FIG. 9, when a
 flat-plate mask 56 is formed by photoetching, slits 55 are made at the
 same time when electron beam through holes are made. Thereafter, when step
 sections are molded at the same time the flat plate mask 56 is
 press-molded, the mask body 50 is thereby formed.
 When the step sections 53 and slits 55 are provided at the skirt section 49
 of the mask body 50 as described above, the curved-surface retaining
 strength of the effective surface 47 improves. Furthermore, the step
 sections 53 absorb deforming stress in manufacturing a shadow mask or
 thermal deforming stress in manufacturing a color cathode ray tube,
 preventing the effective surface 47 from being deformed. Moreover,
 resonance caused by sound from a speaker is less liable to take place,
 which makes the color cathode ray tube less liable to permit color purity
 to deteriorate. In addition, the configuration facilitates the assembly of
 the shadow mask.
 Specifically, when the step sections 53 are provided on the skirt section
 49 of the mask body 50, wrinkles or nodules occur near the step sections
 53 in press molding, which widens the open edge side of the skirt section
 49, making it difficult to insert the skirt section 49 inside the sidewall
 of the mask frame. As in the mask body 50, however, when slits 55 are made
 suitably in the portions where the step sections 53 have and have not been
 formed on the open edge side of the skirt section 49, the slits 55 absorb
 wrinkles or nodules caused by the formation of the step sections 53 in
 press molding. This prevents the skirt section 49 from spreading on the
 open edge side, making it easier to insert the skirt section 49 inside the
 sidewall of the mask frame.
 In the mask body shown in FIGS. 7A and 7B, the step sections have been
 provided only near the boundary between the skirt section and the
 non-aperture section in the vicinity of the horizontal axis and vertical
 axis, and slits have been made in the skirt section. Similarly, making
 suitable slits in the skirt section in the following cases would produce
 similar effects: a case where a step section is formed continuously around
 all the circumference of the skirt section so as to enclose the effective
 surface; a case where plural step sections are formed discontinuously in
 the direction perpendicular to the axis of the skirt section and the width
 of the step section in the direction perpendicular to the direction of
 width of the skirt section and the spacing of the step sections on the
 long sides differ from those on the short sides; or a case where a
 continuous step section is combined with discontinuous step sections.
 In FIGS. 7A and 7B, the mask body has a continuous surface constituting
 both the effective surface of the mask body and the non-aperture section
 enclosing the periphery of the effective surface. Alternatively, applying
 the invention to a mask body where the boundary between the non-aperture
 section and the skirt section is projected higher than the boundary
 between the effective surface and the non-aperture section toward the
 fluorescent screen by bending the non-aperture section at a place close to
 the effective surface, as shown in FIG. 6, would produce a similar effect.
 Furthermore, the step section and the sections associated with the step
 section may have a stricture as shown in FIGS. 10A and 10B. The shadow
 mask 36 is composed of an almost rectangular mask body 50 and an almost
 rectangular mask frame 52. The mask body 50 has an effective surface 47
 composed of a curved surface facing the fluorescent screen and a
 non-aperture section 48 provided so as to surround the periphery of the
 effective surface 47. In the effective surface 47, a large number of
 electron beam through holes 46 are made. On the periphery of the
 non-aperture section 48, a bent skirt section 49 is formed. The mask frame
 52 has a sidewall section 51 on the inside of which the skirt section 49
 of the mask body 50 is provided.
 In the embodiment, as shown in FIG. 10B, at the boundary between the skirt
 section 49 of the mask body 50 and the non-aperture section 48, there is
 provided a step section 53 stepped in the direction of width, i.e., height
 (the direction of Z) of the skirt section 49 and extended in the direction
 (the directions of X and Y) perpendicular to the direction of width (the
 direction of Z). The open end of the skirt section 49 where the step
 section 53 has been formed is welded to the mask frame 51. The step
 sections 53 are formed at several parts on the skirt section 49 so as to
 enclose the effective surface 47. The lateral length d of the step in the
 direction (the directions of X and Y) perpendicular to the tube axis
 (Z-axis) is set to 1 to 10 mm.
 The step section 53 of the mask body 50 can be formed easily at the same
 time when a flat-plate mask in which electron beam through holes have been
 made by photoetching is press-molded.
 When the step section 53 is provided at the boundary between the skirt
 section 49 and the non-aperture section 48 of the mask body 50 and near to
 a welding portion for fixing the mask frame 50 to the skirt section 49,
 the strength of the skirt section 49 is increased by the plastic
 deformation caused by forming the skirt section 49 by press molding. This
 improves the strength with which the curved surface of the effective
 surface is retained. The step section 53 can absorb deforming stress in
 manufacturing a shadow mask or thermal deforming stress in manufacturing a
 color cathode ray tube, which is applied through the welding section,
 thereby preventing the effective surface 47 from being deformed.
 Furthermore, when a color cathode ray tube is incorporated into a
 television set, the improvement of the curved-surface retaining strength
 makes sound from a speaker less liable to cause resonance. Therefore, use
 of the step section 53 makes the color cathode ray tube less liable to
 permit color purity to deteriorate.
 Furthermore, to improve the strength of the effective surface 47, it is
 preferable to provide step sections 53 at the boundary in the directions
 (X and Y directions) perpendicular to the tube axis (Z axis). It is more
 preferable that the step sections are provided near the welding portions
 between the skirt section and the mask body, and non-step sections are
 provided between the step sections and the step section and non-step
 sections are alternatively arranged around the periphery of the effective
 surface 47 instead of proving the continuous elongated step around the
 periphery of the effective region. The combination of the step sections
 formed near to the welding sections and non-step sections arranged between
 the step sections further improve the strength of the effective surface
 47.
 For instance, it has been verified that the movement of the beam landing on
 the three-color fluorescent layer caused by the resonance of sound from a
 speaker was reduced about 50%, as a result of providing the steps having
 same dimension with step length d=4 mm, the elongated length of 140 mm
 along the X and Y directions and the height of 5 mm along the Z direction
 for the skirt section in a shadow mask incorporated in a color cathode ray
 tube with a 76 cm-long diagonal.
 The reason why the length d of the step in the step section 53 in the
 direction perpendicular to the axis of the tube was set to 1 to 10 mm is
 that use of the length d less than 1 mm cannot improve the curved surface
 retaining strength for the effective surface 47 effectively. Another
 reason is that when the length d exceeds 10 mm, not only is a balance
 between the outside diameter of the shadow mask and the dimensions of the
 effective surface lost for practical use, but also the effect of improving
 the curved-surface retaining strength of the effective surface 47
 decreases. If the step length of the step section is set to be large, the
 improvement of the strength may be deteriorated. In consideration of the
 improvement of the strength, it was found that the step length d of the
 step section is set to be within about 160 mm.
 In the above embodiments, a color cathode ray tube with a panel having a
 flattened effective region has been used. Alternatively, applying the
 invention to a shadow mask in an ordinary color cathode ray tube with a
 panel the inside and outside faces of whose effective region are composed
 of curved surfaces with relatively large radii of curvature would produce
 a similar effect.
 As described above, providing the step sections on the skirt section of the
 mask body of the shadow mask increases the strength of the skirt section
 and thereby improves the curved-surface retaining strength of the
 effective surface. The step sections absorb deforming stress in
 manufacturing a shadow mask or thermal deforming stress in manufacturing a
 color cathode ray a tube, preventing the effective surface from being
 deformed. When the color cathode ray tube has been incorporated into a
 television set, resonance caused by sound from a speaker is less liable to
 occur. Therefore, by constructing a cathode ray tube as described above,
 the color cathode ray tube is less liable to permit color purity to
 deteriorate. Furthermore, the effective region of the panel is
 particularly flattened. Therefore, applying the invention to a color
 cathode ray rube in which the effective surface of the mask body of the
 shadow mask has been flattened would produce great improvement.
 Furthermore, making slits in the skirt section enables wrinkles or nodules
 aggravated by the formation of the step sections to be absorbed, which
 prevents the skirt section from spreading on the open edge side,
 facilitating the assembly of a shadow mask.
 Additional advantages and modifications will readily occur to those skilled
 in the art. Therefore, the invention in its broader aspects is not limited
 to the specific details and representative embodiments shown and described
 herein. Accordingly, various modifications may be made without departing
 from the spirit or scope of the general inventive concept as defined by
 the appended claims and their equivalents.