Electric lamp and discharge devices – Cathode ray tube – Shadow mask – support or shield
Reexamination Certificate
2000-03-03
2002-06-18
Patel, Nimeshkumar D. (Department: 2879)
Electric lamp and discharge devices
Cathode ray tube
Shadow mask, support or shield
C313S402000
Reexamination Certificate
active
06407490
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to color cathode ray tubes (CRTs), and more particularly, to a tension mask with a color selection function and a tension mask and frame assembly for a color CRT.
2. Description of the Related Art
In color CRTs, three electron beams emitted from an electron gun land on a phosphor screen through apertures in a shadow mask with a color selection function to excite red, green and blue phosphor lines in the phosphor screen formed on the inner surface of a panel. The panel of a conventional color CRT, which forms an image as mentioned above, is designed with a predetermined inner curvature taking into account a deflection trajectory of electron beams, which have been deflected by a deflection yoke after being emitted from the electron gun. The shadow mask thereof is also designed with a curvature corresponding to that of the panel.
In the operation of a CRT, the mask manufactured to have a curvature in the same order as that of the inner surface of the faceplate is heated as kinetic energy of electron beams emitted from the electron gun is converted to thermal energy, to thereby distort the mask into the shape of a dome resulting from thermal expansion. which is referred to as a “doming phenomenon”. The doming phenomenon displaces positions of the apertures formed on the mask, so that the landing positions of the electron beams are shifted. As a result, undesired phosphor lines are excited, so that the color purity of a display image is deteriorated.
The mask described above is made of steel foil having a thickness of 0.1-0.25 mm. A plurality of apertures are formed in the steel foil via etching, and then the steel foil is molded to have a predetermined curvature. If the curvature of the mask is less than a predetermined level, the mask is readily subjected to a permanent thermal distortion during manufacture of CRT. Accordingly, the mask cannot perform a normal color selection function due to its structural weakness.
Also, the mask having the above-mentioned configuration has limitations in the manufacture of flat CRTs, while the need for flat CRTs is increasing.
In order to meet the need for the flat CRTs as well as to prevent the doming of the mask, U.S. Pat. No. 3,638,063 has suggested an aperture grill type mask. As shown in
FIG. 1
, a mask
10
including a plurality of strips
12
, grid elements, which are disposed at predetermined intervals, is supported by a frame
11
under one directional tension. The mask
10
having the grid structure is designed such that thermal strain which occurs during operation of a CRT, can be opposed by the tension applied by the mask-and-frame assembly, thereby preventing doming of the mask. However, the mask
10
with the strips
12
is made of steel foil that is 0.1 mm thick and is only supported by the frame
11
at two edges thereof, without any interconnection between adjacent strips, and thus the individual strips
12
easily vibrate when subjected to a small impact, thereby causing a howling phenomenon to arise in a display image. The tension applied to the strips
12
is proportional to the thickness of a single strip. Thus, in order to make the structural intensity withstand the thermal expansion during CRT operation, the weight of the frame
11
is inevitably increased.
To account for this problem, U.S. Pat. No. 4,942,332 discloses a mask illustrated in
FIGS. 2 and 3
. As shown in
FIGS. 2 and 3
, a mask
20
includes a series of parallel strips
22
disposed at predetermined intervals to define slits
21
, and a plurality of tie bars interconnecting adjacent strips
22
. Also, the longer sides of the mask are fixed to a support member (not shown).
In the mask
20
, the tie bars
23
interconnecting the adjacent strips
22
can reduce howling of a display image, resulting from vibrations of the mask by an external impact, but not contributing to a reduction in the Poisson contraction. In particular, when a tension is applied in the vertical direction of the mask within the elastic limit of the material of the mask, the mask
20
is stretched in the longitudinal direction, but contracted in the transverse direction. Due to the transverse contraction, the outermost slits of the mask
20
are displaced. Furthermore, as thermal expansion raises the mask
20
during operation of a CRT, the shorter sides of the mask
20
expand outward.
The disclosure also defines a vertical-to-horizontal pitch ratio of the slits (PV/PH) to be greater than 16 such that the transverse displacement of the slits at the side of the usable picture region of the mask.
The instant inventors has carried out the following numerical analysis during research into the amount of displacement of slits at the edge of a mask with respect to vertical-to-horizontal pitch ratio (PV/PH) of slits. In particular, the specification of the mask and experimental conditions therefor were: the width of the mask (W) was 298.4 mm; the height of the mask (H) was 312 mm; the horizontal pitch (PH) of slits was 0.8 mm; the width of a single strip (W
1
) was 0.6 mm; the thickness of the mask (t) was 0.1 mm; Young's Modulus (E) was 2.1×10
−6
kgf/mm
2
; the thermal expansion coefficient (&agr;) was 13×10
6
/° C.; Poisson's ratio was 0.27; and the material used was aluminum killed (AK) steel.
In the mask having the above specifications. it was assumed that the area of the usable picture region was 387.4×288 mm, and that the total number of strips was 484. To keep the tension constant when the temperature is raised to 100° C., the amount of initial tension applied to the strips of the mask, expressed by &egr;=&agr;&Dgr;T should be equal to 0.0013 (=13×10
−6
/100), wherein the tension applied to a single strip, expressed by ƒ
strip
=&Egr;·&egr;· (area of a single strip), is equal to 1.638 kgf. Also, the amount of transverse contraction of a single strip, expressed by &Dgr;W
1
=&ngr;·&egr;·W
1
, is equal to 0.211 &mgr;m. Here, since the total number of strips is 484, after application of the tension, a maximum transverse contraction of the useful picture region (&Dgr;W) becomes 102 &mgr;m. As a result the edge of the useful picture region is shifted outward by 52 &mgr;m.
FIG. 4
comparatively illustrates the amount of transverse displacement of slits in the disclosure of U.S. Pat. No. 4,942,332 and the numerical analysis by the instant inventors, with respect to the vertical-to-horizontal pitch ratio (PV/PH) of the slits. As shown in
FIG. 4
, according to the disclosure, the transverse displacement of the slits was 64 &mgr;m for a PV/PH of 5; 8 &mgr;m for a PV/PH of 15; and 15 &mgr;m for a PV/PH of 30. Meanwhile, the numerical analysis performed under the same conditions as those of the disclosure showed a transverse displacement of 50 &mgr;m for a PV/PH of 5 and 32.2 &mgr;m for a PV/PH of 30.
As can be noted from the above result, a decreasing tendency of the displacement with a PV/PH increase appears both in the disclosure and the numerical analysis result. However, a decrease in the Poisson contraction at the edge of the useful picture region of the mask appears to be exaggerated in the disclosure, in contrast to the numerical analysis.
In detail, according to the numerical analysis by the instant inventors, at a PV/PH of 30, the outermost slits of the mask contract by 33.7 &mgr;m from the initial positions upon the application of tension, while the outermost slits expand by 28.1 &mgr;m from the initial positions when the temperature of the mask increases to 80° C. during operation of a CRT. Accordingly, the total amount of the transverse displacement of the outmost slits, arising from the Poisson contraction and the thermal expansion, becomes 61.8 &mgr;m.
As described above, the amount of Poisson Contraction can be somewhat reduced by increasing the PV/PH ratio of slits as indicated in the disclosure. However, unlike the prediction in the prior art, it is likely that increasing the PV/PH ratio is insufficient for reducing the
Kim Chan-yong
Pyun Do-hun
Shin Soon-cheol
Clove Thelma Sheree
Lowe Hauptman & Gilman & Berner LLP
Patel Nimeshkumar D.
Samsung SDI & Co., Ltd.
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