1. A cathode ray tube wherein an electron beam is made to scan on the outside of an effective screen, comprising: A beam shielding plate disposed between a frame of a color selecting electrode system and a deflection center of the electron beam so as to prevent the electron beam from striking the frame when the electron beam is made to scan on the outside on the effective screen, said beam shielding plate formed so that the front end surface of the said shielding plate at the side inside of the screen becomes approximately parallel with respect to the path of the electron beam or is swept back from the path of the electron beam.

2. A cathode ray tube as set forth in claim 1, wherein said beam shielding plate has a front end portion which is bent to the color selecting electrode system side and with a front end surface which is substantially parallel to the path of the electron beam.

3. A cathode ray tube as set forth in claim 1, wherein said beam shielding plate is formed With the front end surface shaped in a taper.

4. A cathode ray tube as. set forth in claim 1, wherein said beam shielding plate is formed with a front end portion bent to the color selecting electrode system side and with a front end surface formed in a taper.

5. A cathode ray tube substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a cathode ray tube which prevents halation when it is used with over-scanning.
2. Description of the Related Art
In a color cathode ray tube, as shown in Fig. 1, a phosphor screen 4 is formed on an inner surface of a faceplate 3 of a tube envelope 2. A color
selecting electrode system 10 called an aperture grill or a shadow mask is disposed facing the phosphor screen 4. An electron beam E emitted from an electron gun 5 disposed in a neck 2a of the tube envelope 2 is deflected by a deflection yoke 6 to scan the phosphor screen 4.
In a cathode ray tube used for a TV monitor in broadcasting station etc., as shown in Fig. 2, which is a schematic view showing a color selecting mechanism in the cathode ray tube from above, sometimes the cathode ray tube is used with over-scanning where the electron beam E is made to scan the outside of a normal effective region defined by the endmost slit of the
color selecting electrode system 10 in the figure. When the cathode ray tube is used with such over-scanning, the over-scanned electron beam is reflected irregularly at a frame 11 of a color selecting electrode system. When the irregularly reflected electrons reach the phosphor screen 4, frame halation is caused. For this reason, in the TV monitors for broadcasting stations etc., a long and thin beam shielding plate 12 made of metal is disposed between the frame 11 and a deflection center DC of the electron beam attached to an internal beam shielding plate 13 so as not to allow the electron beam to strike the frame 11. This beam shielding plate 12 is made of iron or stainless steel usually having a thickness of about 0.2 mm, absorbs electrons when a high voltage is applied, or reflects the electrons to the electron gun side and therefore shields the frame 11 from the electron beam and prevents halation due to the irregular reflection.
Depending, however, on the application of the TV monitor for a broadcasting station, there are cases where it will be used in a dark room in a low current state. When it is used by such a method of use, slight generation of halation is observed. Suppression of this halation has been desired.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a cathode ray tube which is used with over-scanning and in addition with which the generation of halation can be prevented even when it is used in a dark room or the like.
The present invention provides the following cathode ray tubes.
(1) A cathode ray tube wherein an electron beam is made to scan on the outside of an effective screen as well, said cathode ray tube having a beam shielding plate disposed between a frame of a color selecting electrode system and a deflection center of the electron beam so as to prevent the electron beam from striking the frame when the electron beam is made to scan on the outside of the effective screen, characterized in that it has a beam shielding plate formed so that the front end surface of the beam shielding plate at the side inside of the screen becomes schematically parallel with respect to the path of the electron beam or is swept back from the path of the electron beam.
(2) A cathode ray tube as set forth in the above (1) which has a beam shielding plate with a front end portion which is bent to the color selecting electrode
system side and with a front end surface which is substantially parallel to the path of the electron beam.
(3) A cathode ray tube as set forth in the above (1) which has a beam shielding plate formed with the front end surface shaped in a taper.
(4) A cathode ray tube as set forth in the above (1) which has a beam shielding plate formed with a front end portion bent to the color selecting electrode system side and, at the same time, with a front end surface formed in a taper.
The present invention was made based on the discovery that the halation at the time of over-scanning is caused by the fact that the beam shielding plate has a certain thickness and therefore the front end surface thereof faces the direction of emission of the electron beam at a slant, the electron beam is Irregularly reflected at this front end surface, and the reflected electrons strike the phosphor screen side.
Namely, the cathode ray tube of the present invention has the beam shielding plate ingeniously shaped so that the front end surface of the beam shielding plate becomes substantially parallel to the path of the electron beam or is swept back from the path of the electron beam whereby the electron beam is prevented from being emitted onto the front en the front end surface la will be oriented in the direction of emission of the electron beam, the electron beam will strike the front end surface la, and electrons will be reflected to the phosphor screen 4 and cause halation. On the other hand, if the bending angle is larger than this, there will be a possibility that the electrons reflected at the bending surface lb will reach the phosphor screen 4. Further, the bending position is preferably made a portion near the edge as much as possible from the viewpoint of the ease of positioning of the front end surface.
A cathode ray tube of the present invention having a beam shielding plate given a shape as shown in
Fig. 5 and the cathode ray tube having the beam shielding plate 12 as shown in Fig. 3 before the improvement were used for performing an experiment comparing the margin of safety until halation appears on the screen 4 in the case of different amounts of over-scanning (amount of deflection from the edge of the effective screen of the panel to the non-effective screen side).
In the conventional cathode ray tube shown in Fig. 2, the halation was caused at about 3 to 4 mm (a state where the reflection occurs at the front end surface of the beam shielding plate), but in the cathode ray tube of the embodiment of the present invention, the halation was caused first at about 15 mm (a state where the electron beam strikes the internal magnetic shield). A monitor for a broadcasting station is usually used with 105 to 107 percent over-scanning. In for example a 20 inch model, there is about 10 mm over-scanning on one side, but in the cathode ray tube of the embodiment of the present invention, it was confirmed that a sufficient allowance could be secured.
Further, a beam shielding plate 1A shown in Fig. 6 is a flat plate similar to the conventional beam shielding plate, but is configured so that the front end surface is swept back from the electron beam by
shaping the front end surface as a taper so that the electron beam will not strike the surface. This taper angle can be appropriately selected by considering the orientation of the beam reflection plate etc. As to the method of forming the front end surface into a taper, it is possible to form it by the well known etching process.
Further, in a beam reflection plate IB shown in Fig. 7, since the front end portion is bent and, a taper is provided at the front end surface, even in a case where the bending angle is slightly deviated from a shown in Fig. 5 or mounting error occurs in the beam shielding plate, since the tolerance is increased by the taper of the front end surface, the electron beam can be reliably prevented from striking the front end surface.
In the cathode ray tube of the present invention, by ingeniously shaping the beam shielding plate, the generation of halation at the time of over-scanning can be reliably prevented.

We Claim:-
1. A cathode ray tube wherein an electron beam
is made to scan on the outside of an effective screen,
comprising:
A beam shielding plate disposed between a frame of a color selecting electrode system and a deflection center of the electron beam so as to prevent the electron beam from striking the frame when the electron beam is made to scan on the outside on the effective screen,
said beam shielding plate formed so that the front end surface of the said shielding plate at the side inside of the screen becomes approximately parallel with respect to the path of the electron beam or is swept back from the path of the electron beam.
2. A cathode ray tube as set forth in claim 1, wherein said beam shielding plate has a front end portion which is bent to the color selecting electrode system side and with a front end surface which is substantially parallel to the path of the electron beam.
3. A cathode ray tube as set forth in claim 1, wherein said beam shielding plate is formed With the front end surface shaped in a taper.
4. A cathode ray tube as. set forth in claim 1,

wherein said beam shielding plate is formed with a front end portion bent to the color selecting electrode system side and with a front end surface formed in a taper.
5. A cathode ray tube substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.