Image generation in a color picture tube

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The path of the electron beams through a section of a slit mask in a color picture tube.
Schematic path of the electron beams through a slot in the mask

The image generation in color picture tubes always works in the same way with slit , stripe and shadow masks and was patented by Werner Flechsig in 1938 . The picture tube has an electron gun to excite the dye phosphors of each of the three primary colors . The electron beam sources lie next to one another or in a triangular arrangement in the neck of the picture tube. The electron beams they emit are magnetically deflected with the help of deflection coils so that they scan the entire screen line by line in 0.04 seconds ( PAL , SECAM ), or 1/30 s ( NTSC ). To ensure that they only stimulate the phosphors of the color for which they are responsible, a mask is located approx. 20 mm in front of the luminescent screen that shades or captures those electrons that would hit the wrong colors.
The three electron beams intersect in the plane of the shadow mask - they have to meet there ( convergence ) in order to avoid color fringes. The mask consists of a thin sheet of metal that contains a regular pattern of punctiform or slot-shaped holes or - in the case of the Trinitron picture tube - consists of vertical metal wires.

The three electron beams themselves have such a large diameter that they usually sweep over several of the holes distributed in the uniform pattern on the shadow mask. As a result, each electron beam excites several phosphors corresponding to its color on the screen. However, the shadow mask prevents an electron beam from exciting a color for which it is not responsible.

The grid in the shadow mask therefore has nothing to do with the number of lines or the horizontal resolution of the television standard used. It is therefore possible for the picture tube of a television to display images of different television standards with different numbers of lines and horizontal resolutions. Examples: NTSC M with 525 lines (480 visible), PAL B / G with 625 lines (576 visible) or HDTV with 1250 lines (1152 visible) and double the horizontal resolution of PAL B / G. The display of digital HDTV image data, e.g. B. 1280 × 720 or 1920 × 1080 pixels is possible.

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