Shadow mask in color CRT

Details Shadow mask in color CRT Shadow mask in color CRT

2001-08-28

2004-04-06

Patel, Ashok (Department: 2879)

Electric lamp and discharge devices

Cathode ray tube

Shadow mask, support or shield

C313S403000

Reexamination Certificate

active

06717342

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a shadow mask, and more particularly, to a shadow mask in a color CRT (Cathode Ray Tube) having an electron reflective film on a surface thereof opposite to an electron gun for prevention of doming.
2. Background of the Related Art
Referring to
FIG. 1
, a related art color CRT is provided with a panel
1
having a coat of fluorescent film
3
applied to an inside surface thereof, a funnel
2
welded to the panel
1
with fusion glass having a coat of conductive graphite applied to an inside surface thereof, and an electron gun
6
in a neck portion
4
of the funnel
2
for emitting three electron beams
5
corresponding to the
3
colors. There is a shadow mask
7
, a color selecting electrode, having a plurality of slots or dotted holes opposite to, and in the vicinity of the fluorescent film
3
supported by a frame
8
, and there is a deflection yoke
9
on an outer circumference of the funnel for deflection of electron beams in left and right directions. There is an inner shield
10
fitted to rear side of the frame, when seen from a panel side, for prevention of deflection of the electron beam by an influence of geomagnetism when the electron beams pass through the hole in the shadow mask and reach to the fluorescent film.
In general, as shown in
FIG. 2
, a hole of the shadow mask has a section enlarged both in an electron gun direction and a fluorescent film direction.
Upon reception of a video signal at the electron gun
6
, in the foregoing CRT, thermal electrons are emitted from cathodes of the electron gun, and travel toward the panel as the electrons pass through acceleration and focusing processes by voltages applied to electrodes in the electron gun. A path of the electron beam
5
is controlled by a magnetic field of a magnet fitted to a neck portion
4
of the funnel
2
, and directed onto the inside surface of the panel by the deflection yoke
9
, of which approx. 20% pass through the hole in the shadow mask and is involved in color selection, by hitting onto the fluorescent film
3
on the inside surface of the panel, to make the fluorescent film luminant, that reproduces the video signal.
In the meantime, the rest 80% of the electron beam hits on a surface of the shadow mask, to raise a temperature of the shadow mask accompanied by a thermal expansion of the shadow mask, to cause doming in which a landing position of the electron beams are varied with an extent of bulge of the shadow mask, that makes a color purity of a picture poor.
In order to solve this problem, a length and welding points and the like of the shadow mask are varied in the related art to shift position of the shadow mask near to, or away from, the fluorescent film, to adjust a distance between the fluorescent film and the shadow mask, for compensation of doming.
However, since the compensation of doming in the shadow mask is made in a state the doming is steady when the shadow mask completes thermal expansion, an initial deterioration of resolution caused by initial doming can not be prevented.
In order to prevent occurrence of the doming, U.S. Pat. Nos. 674,934, 4,528,246, and JP laid open patent S59-15861 suggest to use invar steel as a material for the shadow mask. Though the shadow mask of invar steel can reduce a thermal expansion of the shadow mask, the shadow mask has problems in that cost is high with poor economy, and machinability is very poor to require annealing at an elevated temperature higher than 900°, and heating a mold therefor in forming the shadow mask.
EP 0139379 suggests a method for reducing a thermal expansion of the shadow mask, in which a coat of a material having a low thermal expansion, such as lead or borate, is applied to a surface of the shadow mask, and, as another method for preventing doming, a method for coating a thermal insulating material which has an excellent thermal insulating property is known. This method prevents transmission of a heat caused by the electron beam to the shadow mask. In this instance, a material of ceramic is mostly used as the insulating material.
As another method, a method is suggested, in which a coat of a material having an excellent thermal emissivity is applied to a surface of the shadow mask, or a surface of the shadow mask is blackened, for enhancing a thermal emission rate, and a Korea Patent No. 85-1589 to Philips suggests application of a coat of aqueous suspension containing a electron reflective material on a surface of an electron gun.
As another method, a method for coating a thermal insulating material, a thermal emissive material, or an electron reflective material on the surface of the shadow mask or the electron gun by spray or sputtering has been used. However, even with a precise control of the spray, a part of the shadow mask holes are plugged frequently, and the surface coated by this method is not even. The shadow mask having the sputtering method applied thereto has a thin coat, and a deposition equipment therefore is expensive and has a poor productivity.
Korea laid open patent Nos. 97-63326, 98-66424, 98-66425, 98-71999, and EP WO97-29504 disclose development of a screen printing composition having 30-78% of WO
3
or Bi
2
O
3
as an electron reflective material, and carbon, a manganese oxide, or an aluminum oxide as the thermal emissive material, for screen printing on the shadow mask to reduce doming and improve the hole plugging by forming an even film. It is understood that there is much room for achieving the improvement of the doming by development of new electron reflective materials, rather than using the related art methods and different from the foregoing patents.
However, referring to
FIG. 2
, the related art electron reflective film
11
mostly on a surface
13
of the shadow mask outside of the hole
12
facing the electron gun shows doming when the electron beams hit onto the inside surfaces
14
of the holes as shown in FIG.
3
.
SUMMARY OF THE INVENTION
Accordingly, the present invention is directed to a shadow mask in a color CRT that substantially obviates one or more of the problems due to limitations and disadvantages of the related art.
An object of the present invention is to provide a shadow mask in a color CRT, in which electron reflective films are formed, not only on an outer surface of the shadow mask, but also on an inside surfaces of holes in the shadow mask, to reduce thermal deformation of the shadow mask caused by electron beam hitting on the surface of the shadow mask, with a consequential suppression of the doming, thereby preventing mislanding of the electron beams and color dispersion of the color CRT to improve color purity.
Additional features and advantages of the invention will be set forth in the description which follows, and in part will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings.
To achieve these and other advantages and in accordance with the purpose of the present invention, as embodied and broadly described, the shadow mask in a color CRT includes an electron reflective film formed, starting from a surface of the shadow mask, down to an inside surface of each of holes.
In another aspect of the present invention, there is provided a shadow mask in a color CRT including an electron reflective film formed on a surface of the shadow mask facing an electron gun, or on the surface of the shadow mask and an inside tapered surfaces of holes of the shadow mask, and another electron reflective film formed on a surface of the electron reflective film formed on the shadow mask facing an electron gun.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed.


REFERENCES:
patent: 3760214 (197

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