Cathode ray tube

Details Cathode ray tube Cathode ray tube

2002-12-10

2004-10-19

Patel, Ashok (Department: 2879)

Electric lamp and discharge devices

Cathode ray tube

Shadow mask, support or shield

C313S479000

Reexamination Certificate

active

06806633

ABSTRACT:

TECHNICAL FIELD
The present invention relates to a cathode ray tube, and in particular to the shapes and form of a mask frame and an inner magnetic shield being joined together, designed to reduce negative effects caused by terrestrial magnetism.
BACKGROUND ART
FIG. 7
shows a conventional cathode ray tube (hereinafter referred to as a CRT) used in televisions and personal computers and the like. With a CRT such as the one indicated in this drawing, an image on the screen is produced when an electron beam
60
emitted from an electron gun is horizontally and vertically deflected by deflection coils
62
and then is scanned onto the entire display screen. During this process, if an external magnetic field, such as a geomagnetic field, is applied from a direction that is orthogonal to the direction in which the electron beam
60
is traveling, then the beam will become distorted in a manner such as that indicated by
61
(somewhat exaggerated in the drawing). This will result in mislandings of the beam, as it will fail to light the phosphor
64
on the panel
63
in the proper location. As a countermeasure to this, it is common practice to install into the interior of the CRT (in this case, the interior of the funnel section) an inner magnetic shield
65
that surrounds the scanning path of the electron beam.
However, it is impossible to entirely shield out an external magnetic field. Therefore, the actual role played by the inner magnetic shield
65
is that of (a) blocking out a certain amount of the external magnetic field, (b) changing the direction of the magnetic flux so that the electron beam is not affected, and (c) correcting the electron beam when it is affected by the magnetic line of force.
With a few exceptions, terrestrial magnetism is almost always the source of an external magnetic field. This terrestrial magnetism is broken down into a horizontal component (a vector component that is horizontal with the screen) and a vertical component (a vector component that is vertical to the screen). Of these two, it is well known that the vertical component does not pose a problem to the functioning of the CRT. This is due to the fact that the vertical component affects electron beam landing in a uniform way across almost the entire screen, and this phenomenon can easily be counteracted by using a correcting lens to ensure the proper formation of the phosphor surface so that it will compensate for the presence of the vertical component.
On the other hand, as shown in
FIG. 8
, the horizontal component
70
of terrestrial magnetism is more complicated in that its direction can change depending on its directional position relative to the CRT. Generally, this horizontal component
70
can be broken down into two directions: the CRT tube axial direction
71
and the CRT lateral direction
72
.
Accordingly, when one considers a shield against terrestrial magnetism, one must ultimately consider the characteristics of a lateral magnetic field and a tube axial magnetic field, both of these forces being subcomponents of the horizontal component of terrestrial magnetism.
By measuring the amount of change that occurs in the phosphor surface beam landing when an external magnetic field equal to or more powerful than terrestrial magnetism is applied, it is possible to assess the various properties of the CRT. Points to be measured can be, among those shown in
FIG. 9
, the four corner sections of the screen and the upper and lower central sections of the length of the screen (hereinafter referred to as the NS sections), for example. It is particularly important that the following properties be noted: (1) The properties of the corner sections when a lateral magnetic field is applied (hereinafter referred to as “lateral corner” properties). (2) The properties of the NS sections when a tube axial magnetic field is applied (hereinafter referred to as the “tube axial NS” properties).
FIG. 10
shows the shape of the inner magnetic shield
65
. As shown in
FIG. 10
, the inner magnetic shield
65
is a pyramid including two long sides
71
opposite to each other and two short sides
72
opposite to each other, where an opening
73
is formed at the top.
Each bottom of these sides is outwardly bent to form a bending edge, and in recent shadow masks to which tension is applied, the inner magnetic shield
65
is attached to the mask frame by fixing the bending edges to the mask frame.
As indicated in
FIG. 11
, the mask frame is composed of a pair of spanning members
81
(L-shaped in a sectional view) and a pair of U-shaped holding members
82
. The pair of U-shaped holding members
82
are arranged opposite to each other. The pair of U-shaped holding members
82
are welded and fixed at two pairs of opposite ends of the pair of spanning members
81
. A plurality of tensed wires are spanned between the pair of spanning members
81
, the plurality of wires forming a shadow mask Ma. The plurality of wires are spanned at certain positions of the holding members
82
which are determined to hold the tension of the shadow mask Ma and to increase the strength of the frame in the direction of tension.
In recent years CRTs with larger screens and flat face plates have become the norm. In the case of CRTs with flat face plates in particular, the above-mentioned method of applying tension to a shadow mask is commonly used.
Conventional inner magnetic shields of CRTs that incorporate this method have tended to produce a dramatically higher amount or mislandings due to terrestrial magnetism. It is believed that this is because the magnetic properties of the shadow mask extremely change when it is placed under tension (Murai et al., SID2000DIGEST, pp582-585). For example, a conventional 25-inch CRT that has a lateral corner property of approximately 10 &mgr;m and a tube axial NS property of approximately 10 &mgr;m would degrade to have a lateral corner property of 15 &mgr;m and a tube axial NS property of 30 &mgr;m when tension is applied to the shadow mask.
Efforts have been made to improve the capabilities of inner magnetic shields constructed as in FIG.
10
. For instance, the overall design includes a “V”-shaped section
74
that is cut out of the aforementioned short sides
72
. The depth and width of the cuts have been optimized, but the amount of change in beam landing due to external magnetic fields equivalent to terrestrial magnetism have only been improved to:
(lateral corner property, tube axial NS property) (21 &mgr;m, 23 &mgr;m).
Additionally, the rates of change for the lateral corner properties and the tube axial NS properties ate roughly the same, with their measurements being in an inverse trade-off relationship, making it impossible to improve both properties at the same time.
DISCLOSURE OF THE INVENTION
An object of the present invention is to provide an inner magnetic shield that will solve the above problems and decrease the occurrence of drifted or uneven color throughout an entire display screen by decreasing the amount of mislandings caused by distorted electron beams occurring as a result of external magnetic fields such as terrestrial magnetism.
The present invention is a CRT designed to solve the above problems by being provided with an inner magnetic shield, a mask frame, a shadow mask that is affixed to the mask frame, and a face plate in which the mask frame and shadow mask are installed. The mask frame is made up of a pair of attachment members to which the shadow mask is attached, and a pair of positioning members that are joined with the attachment members to keep them in a predetermined position. The inner magnetic shield is of an outwardly square tube shape with a plurality or side surfaces to which at least one skirt is provided that extends, at a location where the inner magnetic shield is not in contact with either of the positioning members, to the vicinity of an attachment member. It is at this location that the skirt is magnetically coupled with the attachment member.
The present invention can also be a CRT designed to solve the above problems by being provided with

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