Electric lamp and discharge devices: systems – Cathode ray tube circuits – Cathode-ray deflections circuits
Reexamination Certificate
2002-04-30
2003-07-08
Wong, Don (Department: 2821)
Electric lamp and discharge devices: systems
Cathode ray tube circuits
Cathode-ray deflections circuits
C315S370000, C313S447000
Reexamination Certificate
active
06590352
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to cathode ray tubes (CRTs) and is particularly directed to an arrangement for preventing the buildup of electrostatic charge on the display screen of a CRT by directing the charge to neutral ground.
BACKGROUND OF THE INVENTION
CRTs operate at high voltages causing the glass display screen, or faceplate, surface of the CRT to become electrically charged by static induction. The static electricity attracts dust and other contaminates in the air causing them to collect on the display screen's outer surface which degrades the video image presented on the display screen. When a person touches the CRT's display screen, he or she may receive a slight shock by discharge of the static electricity, particularly under relatively low humidity conditions. In addition to being unpleasant to the touch, this static discharge may disrupt the operation of other electronic equipment located nearby such as a computer when the CRT is employed in a computer terminal. Other types of self-emitting video displays such as liquid crystal displays (LCDs), plasma discharge displays (PDPs), vacuum flourescent screens, and gas discharge screens also suffer from the aforementioned problems. The buildup of static charge on the faceplate of a video display arises because of the dielectric nature of glass. In addressing problems arising from static electricity buildup on the display screen, an electrically conductive antistatic coating is typically applied to the outer surface of the display screen and is coupled to neutral ground for dissipating static charge on the display screen's outer surface.
The outer surface of the CRT's display screen reflects approximately 4-8% of the light incident on the display screen. This reflected light not only degrades video image resolution and contrast, but also causes viewer eye fatigue. To reduce this light reflection and improve video image viewing, an outer antireflective layer, or layers, is deposited on the display screen's inner antistatic layer. In some cases, the antireflective and antistatic layers are mixed together to form a single solution which is applied to the display screen's outer surface as a single layer coating. Even when the antistatic and antireflective layers are sequentially deposited on the display screen, there is to some extent fusion between the two layers, particularly when the layers are applied by a wet coating method such as by spray or spin coating.
To avoid the problems of static charge buildup on the display screen, the display screen's conductive antistatic layer is typically connected to neutral ground for dissipating this charge. One approach to grounding the display screen's outer surface coating using the CRT's implosion protection, or tension band is disclosed in U.S. Pat. No. 5,025,490 and involves the application of an electrical conductive tape to the outer layer of the 2-layer coating and attaching the conductive tape to the grounded implosion protection band. The electrically conductive tape used in this application is rather expensive and cannot be used where the antireflective layer is separate from and covers the antistatic layer such as when the coating is applied by sputtering.
Referring to
FIG. 1
, there is shown a typical prior art installation for electrically grounding the glass display screen
14
of a CRT
10
. In the following discussion, common elements in the various arrangements are described with the same terminology and have assigned the same element identifying number. In addition, the terms “display screen,” “display panel” and “faceplate” are used interchangeably as are the terms “layer” and “coating”. CRT
10
includes a sealed glass envelope
12
having a rear neck portion
18
, an intermediate funnel portion
16
, and the aforementioned glass display screen
14
on a forward portion thereof. Disposed within the CRT
10
are one or more electron guns (not shown for simplicity), each of which directs a respective electron bean on the inner surface of the glass display screen
14
. Disposed on the inner surface of the glass display screen
14
is a phosphor screen (also not shown) which includes plural discreet phosphor deposits, or elements, which emit light when an electron beam is incident thereon to produce a video image on the display screen. Disposed about the outer periphery of the sealed glass envelope
12
is an implosion protection, or tension, band
70
typically comprised of a high strength, electrically conductive material such as steel. Attached to respective corners of the outer periphery of the implosion protection band
70
are first, second, third, and fourth band ears
28
a
-
28
d
. Each of the band ears
28
a
-
28
d
is securely attached to the outer surface of the implosion protection band
70
by conventional means such as weldments. The band ears
28
a
-
28
d
are used for mounting the CRT
10
in a chassis which is not shown in the figure. Disposed on the outer surface of the glass display screen
14
is a composite antistatic/antireflective coating
52
. The composite antistatic/antireflective coating
52
may be in the form of a single layer or in the form of an inner antistatic layer applied directly to the glass display screen
14
and an outer antireflective layer applied over the inner antistatic layer. The composite antistatic/antireflective coating
52
is electrically conductive, as is the antistatic layer in the case of the two-layer coating.
FIG. 2
is a simplified sectional view of a corner of the CRT's sealed glass envelope
12
showing a single composite antistatic/antireflective coating
52
such as produced by a wet coating application, i.e., a spray or spin coating, on the outer surface of the CRT's glass display screen
14
. In the wet coating process, there is typically diffusion between the antistatic and antireflective layers to produce a composite coating comprised of molecules
51
arranged in a staggered array. In the two-layer arrangement as shown in
FIG. 3
, an inner antistatic layer
46
is maintained separate from an outer antireflective layer
48
on the outer surface of the CRT's glass display screen
14
. The separate inner antistatic layer
46
and outer antireflective layer
48
are typically applied using a dry coating method such as by sputtering. In both cases, the prior approach involves grounding the antistatic/antireflective coating by electrically coupling it to the CRT's implosion protection band
70
which is connected to neutral ground by means of a grounded connection
44
(shown in dotted line form). In the case of the composite antistatic/antireflective coating
52
shown in
FIG. 2
, an edge of the outer surface of the composite antistatic/antireflective coating is electrically coupled to the implosion protection band
70
by means of a strip of conductive aluminum tape
50
. Applied over the conductive aluminum tape
50
is a plastic film of conductive aluminum foil
60
having an adhesive backing
60
b
for adhesion to the outer surface of the conductive aluminum tape. This arrangement is also shown in the perspective view of FIG.
1
. In the case of the two-layer arrangement comprising the inner antistatic layer
46
and the outer antireflective layer
48
, the conductive aluminum tape
50
is applied to an outer edge of the outer antireflective layer as well as the CRT's implosion protection band
70
. A plastic film of conductive aluminum foil
60
is placed over the conductive aluminum tape
50
as in the previously described arrangement. Because the outer antireflective layer
48
is not as good a conductor as the inner antistatic layer
46
, the grounding arrangement shown in
FIG. 3
is not as effective in grounding an electrostatic charge which may be present on the display screen as the arrangement shown in
FIG. 2
for the composite antistatic/antireflective coating
52
. In order to improve the grounding capability of the arrangement shown in
FIG. 3
, a conductive element
55
is sometimes formed o
Chunghwa Picture Tubes Ltd.
Emrich & -Dithmar
Vu Jimmy T.
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