| 摘要 |
In a colour display tube (1), the shadow mask (13) serves as the colour selective element. The major part of the electrons emitted by the electron gun (10) are intercepted by the shadow mask (13). These electrons can be absorbed by the shadow mask (13) or they can be reflected. In the first situation, where the electrons are absorbed, the shadow mask (13) is heated as a result of which it will be deformed, leading to misregistration of the electron beams (7, 8, 9) when they impinge upon the phosphor screen (6), causing colour impurities on the colour display tube (1). This phenomenon is commonly referred to as doming. In the second situation, where the electrons are reflected, degradation of the contrast performance takes place because the reflected electrons hit the phosphor screen (6) at totally different positions, leading to the generation of stray light. Both doming and contrast performance are important for the quality of a colour display tube (1). The problem however is that in order to optimize the doming performance, all electrons should be reflected and to optimize the contrast performance all electrons should be absorbed by the shadow mask (13). This invention provides a solution for this problem in that the shadow mask (13) is coated only at the surface at the gun side (20) with a heavy metal or an oxide thereof, having a high backscatter coefficient, which has a favorable effect on doming. Preferably, the screen side (21) of the shadow mask (13) and the walls (24, 25) of the apertures (22) are covered with a coating with a low backscatter coefficient, thereby improving the contrast in the vicinity of the position of the electron beam (7, 8, 9). In terms of visual perception a very good compromise between contrast and doming performance-i.e. colour purity-of a colour display tube (1) is achieved.
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