EP0439884A1

EP0439884A1 - Shadow mask frame for prevention of halation

Info

Publication number: EP0439884A1
Application number: EP90301017A
Authority: EP
Inventors: Byeong-Gak Jeong; Keun-Bae Lee
Original assignee: Samsung Display Devices Co Ltd; Samsung Electron Devices Co Ltd
Current assignee: Samsung SDI Co Ltd
Priority date: 1990-01-31
Filing date: 1990-01-31
Publication date: 1991-08-07

Abstract

A shadow mask frame (4) for prevention of halation in a color picture tube is disclosed which is characterized in that a bent end portion (9) is provided around the periphery of an opening portion of the frame (4) in a predetermined inclination angle (α) relative to the plane of the frame, the bending being formed toward the panel of the picture tube. According to the device of the present invention, the ineffectively overscanned electron beams are positively reflected away toward the opposite side of the luminescent screen (2), with the result that not only the halation prevention effect becomes superior, but also there is no difficulty in the manufacturing and assembling.

Description

The present invention relates to a shadow mask frame for prevention of halation, and particularly to the structure of a novel shadow mask capable of preventing the halations due to the reflections of the overscanned or excessively deflected electron beams in a shadow mask type color picture tube.
As shown in Figure 1, the ordinary shadow mask type color picture tube is constituted such that a luminescent screen 12 is formed on the inner surface of the face plate of a panel 11, a shadow mask 13 supported by a frame 14 is installed by way of a mounting (not shown) so that it is spaced by a predetermined gap from the luminescent screen 12, and electron beams emitted from an electron gun (not shown) are made to selectively land on the luminescent screen 12 by means of the shadow mask 13 so that the intended image should be formed by visible light emitting of the luminescent screen 12.
The electron beams emitted by the electron gun are deflected by deflecting means such as deflecting coils and the like, so that the beams should land on the visible region of the luminescent screen 12. However, the so-called over-scanned beams which are produced by being departed from the effective region after excessively deflected are reflected from the side face of the panel or the side end of an opening portion, resulting in that a twice luminescence is caused, thereby degrading the image quality through the aggravations of the luminance and chromatility, and producing the so-called halation phenomenon, in other words "visible electron flooding".
As measure against this phenomenon, an electron shield 15 made of a thin metal sheet is installed in the space between the side face of the panel 11 and the outer face of the frame 14 as a blocking means against the reflections from the side face of the panel, while one of various kinds of means is provided against the reflections from the side end of the opening portion of the frame 14.
To described in a more detail the reflection mechanism of the beams from the side end of the opening portion of the frame 14, this frame 14 has usually a thickness of 1 to 2mm, and therefore, ineffectiveelectron beams of the outermost region of the effective region i,e., the ineffectively overscanned electron beams having a deflection angle a little larger than the maximum deflection angle will be incoming to the side end of the opening portion, while the reflected electron beams (shown by the dotted lines in Figure 1) are scanned on the luminescent screen 12, thereby producing the halations.
The extensively used conventional method for preventing such a phenomenon is constituted such that, instead of the frame, the electron shield 15 is extended up to the inside of the opening portion of the frame 14, thereby limiting the outer boundary of the effective region. In such a case, the leading end of the electron shield 15 having a thickness (usually 0.2mm or less) far thinner than that of the frame 14 has a small reflection area, and therefore, it can effectively prevent the halation. However, in order to limit the outer boundary of the effective region by means of the electron shield 15 having so thin a thickness, an extremely high degree of precision is required in its fabrication and assembling, as well as making the workability very fastidious, thereby making it difficult to expect a saving of the manufacturing cost.
Under this circumstance, various techniques have been proposed, attempting to limit the outer boundary of the effective region by means of a shadow mask frame, and to modify the shape of the leading end of the opening portion of the frame, in order to prevent the halations. However, almost all of them were of the technical constitution such that the reflection area for the ineffective electron beams at the side end of the opening portion of the frame is to be reduced.
For example, one of these techniques is disclosed in Japanese Patent Publication No. 58-9539, and is illustrated in Figure 2. This technique is constituted such that, in the ordinary shadow mask type color picture tube described above referring to Figure 1, the portion related to the opening portion of the frame 14′ of the shadow mask 13 is bent toward the electron gun which is the electron beam source, and the leading end of the bent portion is provided with a sharp burr 19 in order to minimize the reflection area for the ineffective electron beams.
However, the method of providing the burr 19 to the leading and of the opening portion of the frame 14′ is constituted such that the burrs produced between the punch and die when carrying out the blanking process is executed, and therefore, the shapes of the burrs are not uniform, with the result that not only is the halation prevention effect insufficient, but also there are many problems in handling the components because the burrs form a sharp edge.
The present invention is intended to overcome the disadvantages of the conventional techniques.
Therefore, it is the object of the present invention to provide a shadow mask frame for prevention of halations, in which the halation prevention effect is superior, and it is convenient to manufacture, assemble and handle.
In achieving the above object, the device of the present invention is constituted such that, instead of minimizing the reflection area of the ineffective electron beams as in the conventional techniques, the reflection direction of the ineffective electron beams are positively controlled by providing a reflecting face near the outer boundary of the effective region.
The reflection law that the incident and outgoing angles for the light ray are same each other is not exactly resorted to, but movements similar to it can be observable as for the electron beam. For example, if the reflecting face is disposed substantially perpendicularly to the direction of the incident ineffective electron beams, then the ineffective electron beams will be reflected in a direction almost same as the incident direction. Accordingly, the incident angle of the ineffective electron beams near the effective region, which causes halations due to the reflections from a plane crossing perpendicularly the axis of the picture tube, can be positively controlled.
That is, if the reflecting face is disposed such that it should have a slope same as the maximum deflection angle or substantially same as that, the incoming ineffective electron beams will be reflected along a path substantially same as the incoming path, and therefore, the problem that halation is produced upon mislanding of the beams on the luminescent screen is overcome. (Here, if observed from the axial direction, the electron beams are deflected toward the outer side of the picture tube through a deflecting means, and therefore, it is expected that the reflecting path will be formed slightly to the outer side relative to the incoming path.)
The shadow mask frame for prevention of halations according to the present invention based on the principle as described above is constituted as follows. The said shadow mask frame supporting a shadowmask to be maintained by a certain gap to the luminescent screen formed on the inner surface of a panel, and having an opening portion for limiting the outer boundary of the effective region for the electron beams,
characterized in that a bent end portion is provided arond the periphery of the said opening portion of the said frame in a predetermined inclination angle relative to the plane of the said frame, the bending being formed toward said panel.
The above object and other advantages of the present invention will become more apparent by describing the preferred embodiment of the present invention with reference to the attached drawings in which:

Figure 1 is a sectional view of the critical portion of the vicinity of the panel for the color picture tube of the usual conventional shadow mask type color picture tube;
Figure 2 is a sectional view of a conventional color picture tube in which a conventional type of shadow mask from for preventing the halation is provided;
Figure 3 is a schematical sectional view of the color picture tube according to the present invention;
Figure 4 is a partly cut-out perspective view of the color picture tube of Figure 3; and
Figure 5 is a sectional view of the critical portion of the vicinity of the color picture tube panel for showing the operation of the device of the present invention.

Figure 3 is a schematical sectional view of the shadow mask type color picture tube provided with a shadow mask frame according to the present invention. This color picture tube is constituted such that a luminescent screen 2 formed by depositing the R,G,B luminescent materials in the form of dots or stripes is provided on the inner surface of the face plate of a spherical glass panel; an electron gun 7 for emitting electron beams (indicated by the solid arrow mark in the drawing) is installed at the neck 6 of the picture tube; and a deflecting means 8 such as a deflecting coils for deflecting the electron beams so as for the electron beams to be guided to the effective region of the luminescent screen 2 is installed around the neck 6.
As a position separated from the luminescent screen 2 by a certain distance, a frame 4 supports a shadow mask 3 made of a thin metal sheet, which is provided with a plurality of apertures for selectively guiding the electron beams emitted from the electron gun 7 to the respective luminescent pes formed on the luminescent screen 2. An electron shield 5 is installed between the frame 4 and the panel 1 so as for the shield 5 to shield the space between the frame 4 and the panel 1.
According to the unique feature of the present invention, the frame 4 of the shadow mask 3 is bent in a predetermined inclination angle α toward the panel 1 at its related portion of the periphery of the opening portion, thereby forming a bent end portion 9. The side face of this bent end portion 9 located nearer to the electron guns serves as a reflecting face for the ineffectively overscanned electron beams.
The inclination angle α of the bent end portion 9 is determined as follows.
The electron beams emitted from the electron gun 7 are scanned to the effective region of the luminescent screen 2 after changing their advancing direction through the deflecting means 8. The intersecting point between the extended axis of the electron gun 7 and the advancing direction of the electron beam after being deflected is the deflection centre C which the angle formed by the above mentioned two extended lines at the deflection centre C is the deflection angle, the deflection angle formed by the electron beam passing through the outermost boundary of the effective region being called the maximum deflection angle ϑ.
In a color picture tube provided with an electron gun assembly consisting of three R,G,B electron guns, the deflection centres C and the maximum deflection angles are separately formed for the respective electron guns. However, in approximate terms, even if the deflection centre and the maximum deflection angle for the centrally located electron gun are used as the typical values, no faulty results will be given in practical terms, and therefore, the descriptions below will be made based on this assumption.
The incident angle of the ineffective electron beams which cause halations upon being reflected at the side end of the opening portion of the frame 4 can be thought to be slightly larger than the maximum deflection angle ϑ described above, and therefore, if the incident angle of the problematic ineffective electron beam is assumed to be equivalent to the maximum deflection angle ϑ in an approximate term, then the bent end portion 9 of the frame 4 perpendicularly crossing thereto and constituting a reflecting face can be seen to have an inclination angle α as large as the maximum deflection angle ϑ geometrically relative to the plane of the frame 4.
The bent end portion 9 is bent toward the panel 1, and its inclination angle α should be desirably either same as or slightly smaller than the above mentioned maximum deflection angle ϑ.
Particularly as shown in Figure 4, the maximum deflection angle ϑ₁ for the direction of the longer side of the panel 1 and the maximum deflection angle ϑ₂ for the direction of the shorter side of the panel 1 are different each other, and therefore, in the frame 4 also, the inclination angle α₁ of the bent end portion 9a extended in the direction of the longer side is close to the value of the maximum deflection angle ϑ₁ for the direction of the longer side, while the inclination angle α₂ of the bent end portion 9b extended in the direction of the shorter side is close to the value of the maximum deflection angle ϑ₂ for the direction of the shorter side. In exact terms, the deflection centres for the directions of the longer and shorter sides are different each other, but the drawing shows that they are same as the deflection centre C in an approximate term.
Alternatively depending the requirement, the average value of the maximum deflection angles ϑ₁, ϑ₂ for the directions of the longer and shorter sides can be taken as the overall inclination angle α of the whole bent end portion 9 around the periphery of the opening portion of the frame 4.
The shadow mask frame according to the present invention constituted as above will now be described as to its functions referring to Figure 5. The ineffective electron beams(indicated with the arrow mark of solid lines in the drawing) incoming with an incident angle approximately close to the maximum deflection angle ϑ are reflected from the bent end portion 9 of the frame 4, which perpendicularly crosses the incoming path of the beams, and the reflected beams advance along a reflection path (indicated with the arrow mark of dotted lines in the drawing) approximately same as the incoming path toward the opposite side of the panel 1. The ineffectively overscanned electron beams thus reflected are collided with the side wall of the funnel near the neck 6, and then, are dissipated by being grounded through an inner graphite layer coated on the inner face thereof.
Accordingly, the ineffective electron beams are prevented from advancing farther toward the panel 1, so that they can not produce undesirable visible light emitting on the luminescent screen 2, i.e., halations. Therefore, the luminance and the purity of the image are protected from being deteriorated, thereby producing clear images.
According to the present invention as described above, the ineffective electron beams are positively reflected away toward the opposite side of the luminescent screen, with the result that not only the halation prevention effect becomes superior, but also there is no difficulty in manufacturing and assembling. Therefore, the productivity is improved, and clear images can be economically obtained from the color picture tube of the present invention.
The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the scope of the invention.
There are described above novel features which the skilled man will appreciate give rise to advantages. These are each independent aspects of the invention to be covered by the present application, irrespective of whether or not they are included within the scope of the following claims.

Claims

1. A shadow mask frame for prevention of halation in shadow mask type color picture tube, the said shadow mask frame supporting a shadowmask to be maintained by a certain gap to the luminescent screen formed on the inner surface of a panel, and having an opening portion for limiting the outer boundary of the effective region for the electron beams,
characterized in that a bent end portion is provided around the periphery of the said opening portion of the said frame in a predetermined inclination angle relative to the plane of the said frame, the bending being formed toward said panel.

2. A shadow mask frame for prevention of halation as claimed in claim 1, wherein the inclination angle of said bent end portion, is approximately same as or slightly smaller than the maximum deflection angle of said electron beams.

3. A shadow mask frame for prevention of halation as claimed in claim 2, wherein the inclination angle of said bent end portion extended in the direction of the longer side and the inclination angle of said bent end portion extended in the direction of the shorter side are respectively determined to be approximately same as or slightly smaller than the maximum deflection angle for the direction of the longer side and the maximum deflection angle for the direction of the shorter side.

4. A shadow mask frame for prevention of halation as claimed in claim 1, wherein the inclination angle of said bent end portion is determined to be approximately the same as or slightly smaller than the average value of the maximum deflection angle of said electron beams for the direction of the longer side and the maximum deflection angle for the direction of the shorter side.

5. A shadow mask frame for prevention of halation, substantially as hereinbefore described with reference to Figures 3 to 5 of the accompanying drawings.

6. A shadow mask frame assembly for a color cathode ray tube and comprising a shadow mask (3) mounted at its periphery on a peripheral frame (4) which is to be mounted in the tube with the shadow mask spaced by a predetermined gap from the front screen (2) of the tube, said frame having a generally inwardly directed flange portion, the inner edge of which defines the boundary of the region in which electron beams are passed to the shadow mask and thence the screen, characterised in that at least the inner edge portion (9) of said flange is inclined toward the shadow mask so as to reflect electron beams lying just outside said region generally away from said shadow mask and front screen, thereby to inhibit halation in operation of the tube.