Electric lamp and discharge devices: systems – Cathode ray tube circuits – Plural concentrating – accelerating – and/or de-accelerating...
Reexamination Certificate
2002-08-22
2003-09-16
Wong, Don (Department: 2821)
Electric lamp and discharge devices: systems
Cathode ray tube circuits
Plural concentrating, accelerating, and/or de-accelerating...
C313S414000, C315S382000
Reexamination Certificate
active
06621221
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to a cathode ray tube comprising
a display screen for converting an electron-optical image into a light image, and
an electron-optical system comprising
electron sources, juxtaposed in a plane, for emitting electrons;
a beam-shaping section for forming a first outer electron beam, a middle electron beam and a second outer electron beam from the electrons emitted by the respective electron sources;
a main lens for focusing the electron beams on the display screen;
deflection means for deflecting the electron beams across the display screen, and
a DAF section for dynamically adapting the focusing and astigmatism of the electron beams in dependence upon a landing spot of the electron beams on the display screen.
The invention also relates to a picture display device comprising such a cathode ray tube.
An embodiment of such a cathode ray tube is known from U.S. Pat. No. 4,814,670.
In a picture display device comprising the cathode ray tube, three electron beams are generated by the electron gun, which beams are imaged on the display screen. The display screen has lines or dots of phosphors which luminesce when they are impinged upon by one of the electron beams.
For displaying color images, use is made of an electron gun in which three electron beams are generated which are juxtaposed in what is called an “in-line” plane. The three electron beams are focused on the display screen by the main lens. The display screen is provided with red, green and blue phosphors. Furthermore, the cathode ray tube is provided with means which ensure that each electron beam lands on its own phosphor, which means comprise, for example, a shadow mask. Each electron beam thus corresponds to one of the colors red, green and blue.
In a frequently used configuration of the electron beams, the first outer electron beam particularly corresponds to the color red, the middle electron beam corresponds to the color green and the second outer electron beam corresponds to the color blue.
The cathode ray tube has deflection means for deflecting the electron beams. Generally, the cathode ray tube has a neck around which magnetic deflection means are arranged. For deflecting the electron beams, the deflection means receive, in operation, a deflection current which is synchronized with a picture signal received by the picture display device.
As the electron beams are deflected by the deflection means, the electrons cover a longer path between the electron source and the landing spot on the display screen. More particularly, the electrons cover a longer path between the main lens and the display screen, in dependence upon the extent of deflection. As a result, the electron beam is out of focus on at least a part of the display screen and is imaged as a relatively hazy picture.
Furthermore, when deflecting the electrons, the deflection means act as an electron-optical quadrupolar lens which will hereinafter also be referred to as deflection lens. Due to this quadrupolar lens, astigmatism occurs and the shape of the electron beam changes in dependence upon the deflection. The strength of the quadrupolar lens increases with an increasing extent of deflection of the electron beam.
The resolution of a cathode ray tube is dependent on the size and shape of the image of the electron beam, referred to as the spot. The change of the extent of focusing and of the astigmatism of the electron beam due to the deflection reduces the quality of a spot. Consequently, the resolution of the cathode ray tube decreases, particularly in the corners of the display screen.
To reduce this effect, the electron gun is provided with a DAF section as is known from the above-mentioned U.S. Pat. No. 4,814,670. Particularly, the DAF section comprises an intermediate electrode which is provided with horizontal, elongated apertures on the side facing the focusing electrode. The focusing electrode is provided with vertical elongated apertures. “Horizontal” is herein understood to mean the direction parallel to the “in-line” plane and perpendicular to the direction of propagation of the electrons. “Vertical” is herein understood to mean the direction perpendicular to the “in-line” plane.
In operation, a dynamic focusing voltage is applied to the intermediate electrode so that an electron-optical quadrupolar lens is formed between the focusing electrode and the intermediate electrode. The strength of the main lens can also be adapted by means of the dynamic focusing voltage.
Generally, the deflection means are self-convergent in the horizontal direction. This means that the electron beams in the horizontal direction are substantially in focus throughout the display screen, which is at the expense of an increased overfocusing in the vertical direction.
The known color electron gun has a substantially equal design for the three electron beams. Consequently, the DAF section for the three electron beams has the same effect, i.e. the electron-optical quadrupolar lens formed, in operation, between the focusing electrode and the intermediate electrode has an equal strength for all of the three electron beams.
However, since the three electron beams are juxtaposed in the in-line plane and are situated at a given mutual distance from each other, for example, at a distance of 6 mm at the location of the deflection means, they travel along different paths through the magnetic field of the deflection means deflecting the three electron beams. Consequently, the deflection lens has mutually different strengths for the three electron beams. This effect is referred to as “color-dependent defocusing”.
It has been found that color-dependent defocusing affects the resolution of the cathode ray tube to a considerable extent, notably in a cathode ray tube for a computer monitor, a cathode ray tube having a relatively large deflection angle of the electron beams and a cathode ray tube without a shadow mask, referred to as Flat Intelligent Tracking (FIT) cathode ray tube.
Generally, the DAF section is adjusted in such a way that, in operation, the middle electron beam is substantially in focus throughout the display screen, while the outer electron beams are then no longer in focus at the edges and particularly in the corners of the display screen.
The color-dependent defocusing is then notably visible because the first outer electron beam is overfocused in the vertical direction on the east side of the display screen, which, viewed from the exterior, is the right-hand side. Generally, the first outer electron beam corresponds to the color red, and in this case the red spot is hazy. On a relatively large color monitor with a relatively high resolution, red characters may be out of focus on the east side of the screen.
Moreover, the second outer electron beam is overfocused in the vertical direction on the west side of the screen, which, viewed from the exterior, is the left-hand side. Generally, the second outer electron beam corresponds to the color blue and in this case the blue spot is hazy. On a relatively large color monitor with a relatively high resolution, blue characters may be out of focus on the west side of the screen.
The known cathode ray tube has the drawback that color-dependent defocusing occurs.
SUMMARY OF THE INVENTION
It is an object of the invention to provide a cathode ray tube having an improved focusing of the outer and inner electron beams at the edges, and particularly in the corners, of the display screen.
It is a further object of the invention to provide a picture display device comprising such a cathode ray tube.
In the cathode ray tube according to the invention, the first object is achieved in that the DAF section comprises a first electron lens, which has mutually different strengths for the electron beams, and a second electron lens, which has mutually different strengths for the electron beams, the strengths of the second electron lens being changeable independently of the strengths of the first electron lens.
In operation, the DAF section in the cathode ray tube according to the invention c
Ijzerman Willem Lubertus
Krijn Marcellinus Petrus Carolus Michael
Dinh Trinh Vo
Koninklijke Philips Electronics , N.V.
Wong Don
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