Electric lamp and discharge devices – Cathode ray tube – Shadow mask – support or shield
Reexamination Certificate
1998-11-12
2002-05-07
Patel, Ashok (Department: 2879)
Electric lamp and discharge devices
Cathode ray tube
Shadow mask, support or shield
C313S407000
Reexamination Certificate
active
06384522
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a color cathode ray and particularly to a color cathode ray tube which restricts a landing displacement of electron beams on a phosphor layer caused by thermal expansion of a shadow mask.
BACKGROUND ART
In general, a color cathode ray tube comprises a vacuum envelope, which includes a face panel having a substantially rectangular effective portion in form of a curved surface, and a funnel connected with the face panel. A phosphor screen made of a three-color phosphor layer which radiates in blue, green, and red is formed on the effective portion of the face panel. A shadow mask is arranged inside the phosphor screen with a predetermined distance maintained from the face panel. The shadow mask comprises a substantially rectangular mask body and a substantially rectangular mask frame equipped at a peripheral portion of the mask body.
The mask body comprises a main surface portion having a number of electron beam apertures formed in a predetermined array and made of a curved surface opposed to the phosphor screen, a non-aperture portion surrounding the main surface portion, and a skirt portion provided around the main surface portion with the non-aperture portion interposed therebetween. The mask frame is formed to have a L-shaped cross-section and is welded to the skirt portion of the mask body.
Meanwhile, an electron gun which emits three electron beams is provided in the neck of the funnel. The three electron beams emitted from the electron gun are deflected by a magnetic field generated by a deflector equipped outside the funnel so as to scan horizontally and vertically the phosphor screen, thereby forming a color image.
In a color cathode ray tubes constructed in a structure as described above, and particularly, in an inline type color cathode ray tube having an electron gun which emits three electron beams arranged in line and running on one same horizontal plane, the three-color phosphor layers are formed of strip-like layers elongated in the vertical direction (or short axis direction or Y-axis direction) perpendicular to the tube axis (or Z-axis). On the other hand, electron beam apertures are arranged such that rows each consisting of a plurality of apertures aligned in the vertical direction and the rows are disposed in the horizontal direction (or long axis direction or X-axis direction).
The shadow mask is provided to select three electron beams, which pass through beam apertures at different angles respectively, so that the electron beams land on predetermined phosphor layers. Further, in order to obtain excellent color purity of an image displayed on the phosphor screen by scanning by respective electron beams, three electron beams passing through the electron beam apertures must correctly land on predetermined phosphor layers, respectively. The mask body therefore must be correctly positioned and aligned in a predetermined relationship to the phosphor screen, and the relationship must be maintained during operation of the color cathode ray tube. In particular, the distance (or q-value) between the inner surface of the effective portion of the face panel and the main surface portion of the mask body must be maintained within a predetermined tolerable range.
However, from operational principles of a color cathode ray tube, those electron beams that pass through electron beam apertures of the mask body and reach the phosphor screen are ⅓ in amount of the entire electron beams emitted from the electron gun, and most of the rest of the electron beams collide with the mask body and are converted into thermal energy, thereby heating the mask body to about 80° C. Therefore, the surface portion of the mask body locally expands toward the phosphor screen due to thermal expansion, i.e., so-called doming occurs, particularly in case of a shadow mask whose mask body is mode of a cold-rolled plate having a large thermal expansion coefficient (1.2×10
−6
/° C.) and thickness of 0.1 to 0.3 mm, and whose mask frame is made of a cold-rolled plate having a thickness of about 1 mm and having a greater mechanical strength than the mask body. Consequently, the distance between the inner surface of the effective portion and the main surface of the mask body exceeds a tolerable value, and landing of electron beams onto the three-color phosphor layers is displaced thereby deteriorating color purity.
There are two types of landing drift of electron beams on the three-color phosphor layers, one being landing drift which occurs due to thermal expansion of the entire mask body in the initial period when the color cathode ray tube is started operating, and the other being landing drift due to localized doming which occurs when a high-luminance image is displayed locally. The amount of landing drift differs depending on the luminance of an image pattern displayed on the screen, the duration thereof, and the like. For example, when a high-luminance image is displayed on the entire screen, deterioration of color purity occurs over a large area of the screen. When a high-luminance image is displayed locally, localized doming of the shadow mask occurs and landing positions are greatly drifted in a short time period, resulting in localized deterioration of color purity.
Landing drift due to localized doming is the greatest at an elliptic area in a middle portion of the phosphor screen in the horizontal direction when a high-luminance pattern is displayed at a position which is distant from the center of the screen by about ⅓ W where the length of the phosphor screen in the horizontal direction is expressed as W.
Conventionally, several measures have been developed to restrict landing drift caused by doming of the mask body. For example, the following (a) and (b) are known as techniques for restricting landing drift in the initial period of staring operation of a color cathode ray tube.
(a) According to the technique disclosed in U.S. Pat. No. 2,826,538, a graphite layer containing graphite as a main component is provided on the surface of a main surface a mask body and is used as a radiator for decreasing the temperature of the mask body, in order to promote thermal radiation of a mask body.
(b) Japanese Patent Application KOKAI Publication 60-54139 discloses a mask body in which a glass layer made of lead-borate glass or the like is formed on the surface of a main surface portion of the mask body facing an electron gun. If a lead-borate glass layer is thus provided, less calories are transmitted to the mask body since the thermal conductivity of the layer is smaller than that of the mask body, and therefore, an increase of the temperature of the mask body can be restricted. In addition, by providing a lead-borate glass layer, the mechanical strength of the mask body is improved. Further, if the lead-borate glass is welded to the mask body and crystallized, a compressive stress acts on the glass layer and a tensile stress acts on the mask body, so that the tensile strength of the mask body is improved.
It is also possible to restrict localized doming of the mask body by the techniques as described above.
In addition, the following method (c) is known as a conventional measure for restricting localized doming of the mask body.
(c) The method is to increase the curvature of the mask body. As is known, it is effective for this method to increase the curvature of the mask body in the short axis thereof.
However, in the technique (a) of providing a graphite layer on the surface of a main surface portion of the mask body, adherence of the graphite layer is deteriorated by a heat treatment repeated in steps of manufacturing a color cathode ray tube, so that the graphite layer easily peels off by a vibration applied to the color cathode ray tube. Small fragments of the layer which peeled off stick to the mask body, thereby clogging electron beam apertures, so that the quality of an image displayed on the phosphor screen is deteriorated. Small fragments of the layer also stick to an electron gun or the vicinity thereof, inducing a spa
Inoue Masatsugu
Murai Takashi
Saotome Ichiro
Tani Munechika
Kabushiki Kaisha Toshiba
Patel Ashok
Pillsbury & Winthrop LLP
Santiago Mariceli
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