Color picture tube with a tension mask

Details Color picture tube with a tension mask Color picture tube with a tension mask

2000-08-01

2002-12-03

Day, Michael H. (Department: 2879)

Electric lamp and discharge devices

Cathode ray tube

Shadow mask, support or shield

C313S043000

Reexamination Certificate

active

06489713

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention refers to a colour television set or a colour monitor and in particular a colour picture tube with a tension mask which is pretensioned in the vertical and in the horizontal direction.
2. Description of the Related Art
Colour television sets and (computer) monitors serve to convert electrical signals into colour pictures. Colour television sets and monitors have nowadays an interface for various video signal formats (such as e.g. composite colour picture signals, analog or digital component signals). These signals are converted into analog RGB signals for controlling a colour picture tube in a television set or in a monitor. The respective video signals fed to a television set or a monitor are converted in such way that each individual pixel of a reproduction screen can have associated therewith specific brightness and colour values. For reproducing a picture contained in a video signal, three electron beams are produced in a colour picture tube contained in a colour television set or a monitor. Each of these electron beams corresponds to one of the three primary colours of the additive colour mixture: red, green, blue. Depending on the reproduction position, the pixel information, i.e. the brightness and colour information, of the video signal is associated with a respective pixel on a luminescent screen of the colour picture tube.
By means of pixelwise superposition of three colour separation pictures, an additive colour mixture is obtained in the case of a colour picture tube. The luminescent screen of a colour picture tube contains approx. 400,000 colour triads, i.e. phosphor dots which are arranged in groups of three, each group comprising a red-light, a green-light and a blue-light phosphor dot. The diameter of such a phosphor dot is approx. 0.3 mm.
When a video signal is being reproduced, each of these dots is accessed by one of the three electron beams and caused to emit light. The electron beams are generated by an electron beam generation system in the neck of a colour picture tube. In
FIG. 4
, such a colour picture tube is shown in a cross-sectional view. A colour picture tube essentially consists of a glass element
15
. On the inner side of the front screen
16
of the colour picture tube, a phosphor layer
17
comprising the phosphor dots is arranged. The electron beams for accessing the phosphor dots are produced by an electron gun
18
in the neck of the colour picture tube. The electric signals for controlling the electron gun are supplied to said electron gun from outside via contact pins
20
. By means of a deflection unit which is not shown and which is arranged on the outer side of the colour picture tube, the electron beams are deflected in such a way that all the pixels of the luminescent screen are accessed successively. At a distance of approx. 15 mm from the luminescent screen
17
, a shadow mask with a mask frame
19
is provided in the interior of the colour picture tube. In said shadow mask a separate aperture is associated with each colour triad on the luminescent screen. These apertures or holes have a diameter of approx. 0.25 mm and are etched into the shadow mask/hole mask at regular intervals. The three electron beams meet in the respective aperture accessed by the joint beam deflection and fall on the phosphor dots of the luminescent screen
17
located behind said aperture. In the course of this process, a major part of the electrons generated by the electron beam generation system land on the shadow mask. This causes warming and a corresponding thermal expansion of the shadow mask. As a result, the apertures in the shadow mask will change their position relative to the phosphor dots associated therewith. The colour purity of the pixels reproduced deteriorates due to this change of position. Such a deterioration becomes apparent especially with regard to the apertures located at the periphery of the mask.
Shadow masks are implemented not only in the form of aperture masks, but they are also used in the form of strip masks. In the case of these strip masks, the luminescent screen
17
of a colour picture tube is not provided with individual phosphor dots but with phosphor strips extending in the direction of the strips of the shadow mask. Accordingly, the shadow mask is provided with strip-shaped apertures for the individual electron beams, the respective strip-shaped apertures being associated with the strips on the luminescent screen. Such a strip mask often consists of “wires” that extend in parallel.
As can be seen in
FIG. 4
, the shadow mask is held by a mask frame
19
so as to impart mechanical stability to the mask and so as to make it easily handleable. Also the mask frame of modern colour picture tubes consists of a thin metal sheet.
In view of the small thermal expansion, shadow masks are nowadays also produced from iron-nickel alloys having a very small coefficient of thermal expansion. Since such iron-nickel alloys are many times more expensive than iron, mask frames are, however, produced from iron. The connection of shadow masks and mask frames consisting of materials with different coefficients of thermal expansion is problematic. When such mask/frame combinations become warm, deformations of the shadow mask may occur. This has the effect that the positions of the holes in the shadow mask change relative to the positions of the associated phosphor dots or phosphor strips.
The most widely used shadow masks are shaped, self-supporting shadow masks. Such a shadow mask is shown in FIG.
1
. The shadow mask arranged behind the luminescent screen
1
comprises hole- or strip-shaped apertures
3
corresponding to the arrangement of the luminescent colours on the luminescent screen. The mask is secured to a frame
2
. The contour of such a mask can be varied in the vertical as well as in the horizontal direction. The holding frame for the mask need not absorb any major forces in this case. The material of the frame can therefore be chosen substantially from the economical point of view.
Another type of colour picture tubes is pretensioned in the vertical direction. The major use of such shadow masks, so-called tension masks, is essentially in Trinitron tubes. These shadow masks are fixedly connected to the mask frame on the upper and lower boundaries thereof. The pretension of the shadow mask is necessary so as to guarantee that the distance between the luminescent screen and the shadow mask remains constant in the direction of the longitudinal axis
21
of the colour picture tube. Due to the pretension, the vertically oriented “wires” are held under tension. This imparts mechanical strength to the strip mask. The pretension necessitates high tension forces which may assume values of several kN. Hence, the mask frame must be very solidly built. Upon selecting the material, it is especially necessary to take into account the high process temperatures used in the production process of a colour picture tube. Such masks have, however, the advantage that they are highly transparent and that the mask contour has a high thermal stability. They are, however, disadvantageous insofar as they always have only a cylindrical curvature of the mask. In addition, the tight mask wires tend to react to mechanical vibrations with strong oscillations.
Such a tension mask with tension in the vertical direction is shown in FIG.
2
. Also this mask is provided with a luminescent screen
1
and a mask frame
2
. The wires
5
of the mask are pretensioned in the vertical direction. This pretension is indicated by the arrows
7
,
8
in FIG.
2
. In order to avoid oscillations of the wires and in order to keep the distances between the wires constant, so-called damping wires
6
are placed on top of the tight mask wires
5
such that they extend transversely thereto. These damping wires are provided for suppressing mechanical oscillations of the mask wires
5
and for keeping the distances between the individual wires constant. A disadvantage of these damping wires
6
is that they

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