Shadow mask with specifically shaped apertures

Details Shadow mask with specifically shaped apertures Shadow mask with specifically shaped apertures

1999-07-16

2001-11-06

Patel, Vip (Department: 2879)

Electric lamp and discharge devices

Cathode ray tube

Shadow mask, support or shield

C313S403000

Reexamination Certificate

active

06313574

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a color cathode-ray tube, and particularly to an improvement of the shape of the slot apertures of a color cathode-ray tube that employs a shadow mask having slot apertures.
2. Description of the Related Art
A shadow mask-type color cathode-ray tube composed of a striped fluorescent screen and a slot-aperture shadow mask is a glass vacuum tube formed from panel and funnel
32
, as well as neck tube
33
, as shown in FIG.
1
. Electron gun
34
is located inside neck tube
33
, and three electron beams
5
corresponding to fluorescent stripes of the three colors red, green, and blue are emitted from this electron gun
34
. Electron beams
5
are electromagnetically deflected by deflection yoke
36
arranged outside funnel
32
and transmitted through slot apertures of color-selection shadow mask
4
. The beams strike the fluorescent stripes of three colors, red, green, and blue, formed on fluorescent screen
8
on the inner surface of panel
31
, causing the fluorescent material in the fluorescent stripes to emit light and thus generate an image.
Typically, a combination of a striped fluorescent surface and a slot-aperture shadow mask has been employed in color cathode-ray tubes for television of the prior art, while a combination of a dotted fluorescent surface and a round-aperture shadow mask has been employed in color cathode-ray tubes for high-resolution display. As a standard, the shadow mask of a color cathode-ray tube for television has a horizontal pitch of approximately 0.8 mm and a vertical pitch of approximately 0.8 mm, these values varying somewhat depending on the screen size. In contrast, the pitch of a shadow mask for a high-resolution display has a reference value of approximately 0.27 mm.
Slot-aperture shadow masks are used in color cathode-ray tubes for television chiefly because a brighter image can be obtained than with other types of shadow masks, and because the beam landing margin in the vertical direction is essentially infinite when a slot aperture shadow mask is used in combination with a striped fluorescent surface, thereby simplifying the landing design.
In contrast, round-aperture shadow masks are used in high-resolution display tubes because, in fabricating shadow masks that enable high-resolution display, round-aperture shadow masks are easier to fabricate than slot aperture shadow masks, and because round-aperture shadow masks have a uniform mechanical strength, thereby simplifying the press forming of shadow masks.
However, it has been recently suggested that a striped fluorescent surface is more suitable as the fluorescent screen for high-resolution display tubes than a dot fluorescent surface (for example, SID, EURO Display 1996, p. 138, 11. 1-18). Not only because a striped fluorescent surface is superior for high-resolution, but also due to the greater efforts now being expended toward the development of slot-type high-resolution tubes having striped fluorescent screen as well as due to improvements in shadow mask fabrication technology and shadow mask press formation technology, high-resolution color cathode-ray tubes using slot-type shadow masks are now being fabricated. A standard slot-type shadow mask has a horizontal pitch of approximately 0.25 mm and a vertical pitch of approximately 0.25 mm.
This reduction in pitch, however, has given rise to problems not encountered in color cathode-ray tubes for television. Among these problems, the projection onto fluorescent screen
8
of electron beams bends into electron beam projection
7
b
, which is in an inwardly curving banana-like shape as shown in FIG.
2
. This problem is encountered in the case that the electron beam is transmitted by the slot apertures of the shadow mask corresponding to the vicinities of the left and right ends of the display screen.
FIG. 2
is an enlarged view of fluorescent screen
8
in the vicinity of the right end of a display screen. The projections of electron beams are actually straight continuously from the upper end to the lower end of fluorescent screen
8
in the order of green fluorescent stripe
9
g
and blue fluorescent stripe
9
b
, but electron beam projection
7
b
corresponding to slot apertures in the vicinities of both the right and left ends of the display screen is bent approximately 10 &mgr;m in a banana-like shape.
If the electron beam projection is adjusted to achieve just landing of the central portion of the electron beam projection onto the fluorescent stripe, the upper and lower end areas of inwardly bending electron beam projection
7
b
will be shifted toward the center (inwardly) with respect to straight electron beam projection
7
a
as shown in FIG.
3
. In addition, inner portions
11
at the upper end and lower end of electron beam projection
7
b
approach the adjacent stripe on the inward side and thus tend to strike the stripe of another color, resulting in color interference. Moreover, outward portions
12
(in the direction opposite the center when viewed in the horizontal direction of the screen) at the upper and lower portions of electron beam projection
7
b
deviate from the fluorescent stripe and therefore tend to give rise to areas of diminished light emission. This phenomenon was not recognized as a problem in color cathode-ray tubes for television.
To examine this phenomenon using actual numerical values, in a 17-inch high-resolution tube in which the shadow mask has a horizontal pitch of 0.25 mm, the width of the fluorescent stripes is approximately 42 &mgr;m, the width of the graphite stripes is approximately 45 &mgr;m, and the width of an electron beam projection is approximately 75 &mgr;m. Even in cases in which the electron beam projection is straight and free of bending, a drop in luminance begins with a mislanding of 17 &mgr;m with respect to a stripe of 42 &mgr;m, and color interference (impingement on stripes of other colors) begins with a mislanding of 29 &mgr;m. Accordingly, the banana-like bending of 10 &mgr;m described above causes the landing margin to be reduce by 10 &mgr;m, thereby causing color interference to occur, i.e., another color to be hit, with a mislanding of only 19 &mgr;m.
A bend of approximately 10 &mgr;m in the projection of an electron beam is consequently a considerably large value for luminance and landing margin in the case of a high-resolution tube, and the correction of this bending is extremely important for the production of a slot-type high-resolution tube.
Japanese Patent Laid-open No. 320738/89 relates to a color cathode-ray tube for television. The document points out the problem that, when the plate thickness of the shadow mask is increased in order to meet such requirements as larger size and wider deflection angle, the projection onto the fluorescent screen of the electron beams blocked by the inner wall of the slot transmission apertures becomes seed-shaped as a persimmon seed.
One method that has been proposed to solve this problem involves partially widening the slot apertures by retreating only the outer side-surface of the aperture in the outward direction of the screen, as shown in
FIG. 4
, so as to prevent the electron beam projection from being deformed by collision with the outer side-surface.
Japanese Patent Laid-open No. 6741/93 describes the blocking of the electron beam by the corner areas of the siedewalls of slot apertures as one cause for the deformation of a beam projection. To prevent this deformation and improve the beam form, the corners of slot transmission aperture
1
and front-side large aperture
2
are extended outward in the horizontal direction as shown in FIG.
5
.
The problems disclosed in the above-described documents are similar to the previously described banana-shaped deformation in that the problems relate to the shape of the projection of an electron beam that is transmitted by a slot aperture of a shadow mask. However, the thickness of the shadow mask of the cited color cathode-ray tubes for television is relatively thick, from 0.15 to 0.18 mm or fro

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