Electric lamp and discharge devices – Cathode ray tube – Shadow mask – support or shield
Reexamination Certificate
2000-09-25
2004-10-12
Patel, Nimeshkumar D. (Department: 2879)
Electric lamp and discharge devices
Cathode ray tube
Shadow mask, support or shield
C313S403000
Reexamination Certificate
active
06803710
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a shadow mask for a Braun tube formed with substantially a rectangular slot and a curved slot for forming a beam, having substantially a rectangular shape, uniformly on a fluorescent surface of a color Braun tube.
BACKGROUND ART
FIG. 6
is a general view of a shadow mask for a color Braun tube having a plurality of slots each in substantially a rectangular shape. A conventional shadow mask
61
is composed of a slot forming portion
62
and a skirt forming portion
63
. An electron beam passing a slot enters directly linearly into the slot at a central portion S of the shadow mask, but it enters obliquely to the slot as directed to an outer peripheral portion thereof. For this reason, a front side opening and a back side opening of the slot of the conventional shadow mask are adjusted in the opening forming positions thereof.
FIG. 7
is a schematic view showing a positional relationship between the front side opening and the back side openings of the respective slots of the conventional shadow mask. As shown in FIG.
7
(i), the central slot of the shadow mask is composed of a front side opening
72
subjected to an etching process with a large area so as not to obstruct the passing of the electron beam and a back side opening
71
, which is disposed substantially the central portion of the front side opening
72
, as electron beam
73
entering side. On the other hand, the slots on the outer peripheral side of the shadow mask, for example, a slot formed at the point P on the Y-axis of coordinate in
FIG. 6
, a slot formed at the point R on the X-axis thereof and a slot formed at the point Q on the diagonal coordinate in
FIG. 6
, are shown in FIGS.
7
(ii), (iii) and (iv), respectively, and the front side openings
72
of these slots are arranged so as to be shifted towards the outer peripheral side of the shadow mask
61
with respect to the back side openings
71
so as not to obstruct the passing of the electron beams
73
obliquely entering with respect to the slots.
In the shadow mask of such structure, in order to prevent a thermal deformation (referred to as doming) caused through a collision of the electron beam with the shadow mask, a thin metal plate formed of a material having a small thermal expansion coefficient such as nickel-iron alloy is used as a thin metal plate for the shadow mask. However, because the shadow mask manufactured with such thin metal plate is expensive, a shadow mask formed by making thick a cheap soft steel plate has been used to thereby suppress the thermal expansion of the shadow mask applied to the Braun tube and, hence, to prevent the occurrence of the doming.
The use of the thickened plate for the shadow mask makes large a height of a sectional area of the slot formed through the etching working. For this reason, as the slots formed on the side of the outer periphery as shown in FIGS.
7
(ii), (iii) and (iv), with the slots whose front side openings
72
are merely shifted, the obliquely entering electron beam
73
is shut off by the thickened portion of the slot. As a result, the electron beam
73
cannot landed with a predetermined shape on a fluorescent surface of the Braun tube, thus providing a problem.
FIGS. 8
to
10
are schematic views for explaining such problem. FIG.
8
(i) is a view showing the shape of the slot formed at the R point on the X-axis of coordinate shown in
FIG. 6
, in which the front side opening
72
is formed through the etching working with the back side opening
71
being shifted from the front side opening
72
. The electron beam
73
passing through the central portion A of the slot can pass, with a desired width W as shown if FIG.
8
(ii), a portion between side wall sections
83
and
84
to which thin steps
81
and
82
are formed through the sufficient etching process. On the other hand, the electron beam
73
passing through the upper end portion B in the longitudinal direction of the slot is shut off by a step
86
, having a large thickness, formed to a side wall section
88
, which is not subjected to the sufficient etching process, as shown by the sectional view of FIG.
8
(iii), and hence, this electron beam
73
cannot pass with the desired width W. As mentioned above, the difference of the shapes of the side wall sections, particularly, the thicknesses of the steps are the ventral portion A and the longitudinal upper end portion B resides in the difference in the etching progressing g speeds caused by the positional relationship between the front side opening
72
and the back side opening
71
. That is, at the central portion A of the slot, the etching progressing speed is large (high) and this portion is etched with a sufficient speed to thereby form the thin steps
81
and
82
. On the other hand, at the upper end portion B, the etching progressing speed is small (low) and this portion is not sufficiently etched, so that the etching progresses from the back side opening
71
having a small opening width and, hence, the steps
85
and
88
having large thickness are formed. As a result, a spot of the electron beam passing the slot and landing on the fluorescent surface provides a curved shape in which upper and lower end portions of a boundary line of the outer peripheral side of the Braun tube because the incident electron beam
73
is shut off by the thickened step
86
formed to the side wall section
88
on the outer peripheral side which is not subjected to the sufficient etching working.
Furthermore, though mentioned hereinlater with reference to
FIG. 3
, a boundary line
39
(central side of the shadow mask) of an electron beam
31
varies in the electron beam passing position by a back side opening
11
having an enlarged opening area. For this reason, in the case of the slot having substantially a rectangular shape, the electron beam
31
passing the central portion of the slot cannot pass the same position as the boundary line
39
mentioned above, and in the spot landing on the fluorescent surface, both the ends of the longitudinal direction of the boundary line
39
may provide a curved shape towards the central side of the shadow mask.
Accordingly, when the conventional shadow mask
61
is used, both the longitudinal ends of the spot
91
of the electron beam passing the slot and landing on the fluorescent surface provides a shape, as shown in
FIG. 9
, curved so as to approach the axis of ordinate passing the central portion of the fluorescent surface of the Braun tube. Such deformation of the spot
91
becomes large as the incident angle of the electron beam
73
becomes large, that is, as the electron beam
73
becomes apart from the axis of ordinate mentioned above and is directed in the bilateral direction.
FIG. 10
is a schematic view showing the deformed spot
91
landing on the fluorescent surface of the Braun tube. Such deformation of the electron beam spot
91
may provide a problem of not obtaining sufficient luminance which is obtained in essential by landing the spot on the fluorescent surface with the rectangular shape. Moreover, the shapes of the spots differ from each other at various portions of the fluorescent surface of the Braun tube, so that there is a fear of causing difference in the luminance at different portions and generating an illumination variety of R, G, B colors, thus also providing a problem.
DISCLOSURE OF THE INVENTION
In order to solve the above problems, an object of the present invention is to provide a shadow mask so that a spot of an electron beam landing on a fluorescent surface of a Braun tube takes a desired rectangular shape.
The present invention provides a shadow mask for a Braun tube having a plurality of slots forming beam spots, each having substantially a rectangular shape, uniformly on a fluorescent surface of a color Braun tube, the shadow mask being characterized in that the slots include a slot having substantially a rectangular shape and formed at a portion near an axis of ordinate passing a central portion of the shadow mask and a curved slot formed on an outer periphe
Hatori Toshihiro
Ikegami Takeshi
Colón German
Dai Nippon Printing Co. Ltd.
Keefer Timothy J.
Patel Nimeshkumar D.
Seyfarth Shaw LLP
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