Electric lamp and discharge devices – Cathode ray tube – Shadow mask – support or shield
Patent
1998-08-18
2000-10-17
Day, Michael H.
Electric lamp and discharge devices
Cathode ray tube
Shadow mask, support or shield
H01J 2907
Patent
active
061336823
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention relates to a color cathode ray tube and particularly to a color cathode ray tube comprising a shadow mask having a number of apertures.
BACKGROUND ART
In general, a color cathode ray tube comprises a vacuum envelope having a face panel, a phosphor screen formed on an inner surface of the face panel and including three color phosphor layers capable of radiating in blue, green, and red, a shadow mask opposed to the phosphor screen, and an electron gun provided in a neck of the vacuum envelope. The shadow mask includes a mask body having a number of apertures for passing electron beams, and a mask frame supporting the peripheral edge portion of the mask body. In this color cathode ray tube, three electron beams emitted from the electron gun scan the phosphor screen through the shadow mask, thereby displaying a color image.
The shadow mask is provided to select the three electron beams to be respectively landed on predetermined positions on the three color phosphor layers, and this selection must be correctly carried out such that three electron beams are respectively landed correctly on predetermined positions of the three color phosphor layers, in order that a color image displayed on the phosphor screen obtains an excellent color purity. Therefore, the shadow mask must be arranged so that a predetermined positional relationship is always maintained with respect to the phosphor screen during operation of the color cathode ray tube, i.e., the distance (q value) between the shadow mask and the phosphor screen must always fall within a predetermined tolerance range.
However, in a color cathode ray tube of a shadow mask type, only 1/3 or less of the entire electron beams emitted from the electron gun reach the phosphor screen, and the other remaining beams collide onto the shadow mask. Further, the shadow mask is heated by those colliding electron beams and expands towards the phosphor screen, i.e., so-called doming occurs. The doming can be divided into two types.
One type that occurs is at the beginning of starting operation of a color cathode ray tube. Specifically, at the starting operation, the mask body of the shadow mask is mainly heated and a temperature difference occurs between the mask body and the mask frame which is provided on the peripheral edge portion of the mask body. Due to the temperature difference, doming occurs.
The other type that occurs is locally in a relatively short time when an image having a high luminance is locally displayed and the mask body is thereby locally heated and expanded.
Once doming of a shadow mask occurred, the position of the shadow mask relative to the phosphor screen changes and the q value derives from the tolerance range. Landing positions of electron beams with respect to the phosphor layers are then dislocated from predetermined positions, and as a result, the color purity of an image displayed is degraded. Landing dislocations thus caused by doming vary depending on the position of an image pattern to be displayed, the luminance thereof, and the continuation time of a high-luminance image pattern.
In addition, a landing dislocation of an electron beam caused by local doming when an image having a high luminance is displayed locally tends to easily occur at an intermediate region between the center of the shadow mask and an end of the horizontal axis thereof. This can be associated with doming of the shadow mask and the deflection angle of an electron beam. For example, even when doming occurs in the vicinity of the vertical axis of a shadow mask, the deflection angle of electron beams is small within this portion, so that the electron beam is not much affected by doming and a landing dislocation caused therefrom is small. Meanwhile, the peripheral portion of the mask body is supported on the mask frame which has a large heat capacitance by a non-aperture portion, so that heat in the mask body diffuses into the mask frame even when the peripheral portion of the mask body is locally heated. Therefore, doming which occurs
REFERENCES:
patent: 3652895 (1972-03-01), Tsuneta et al.
patent: 4159177 (1979-06-01), Elshof et al.
patent: 5055736 (1991-10-01), Yun et al.
patent: 5243253 (1993-09-01), Marks et al.
Patent Abstracts of Japan vol. 006, No. 173 (E-129), Sep. 7, 1982.
Murai Takashi
Saotome Ichiro
Tani Munechika
Day Michael H.
Kabushiki Kaisha Toshiba
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