Convergence (screen)
In the case of CRT monitors for color reproduction, the colors red, green and blue, which make up an image, have to be brought into convergence with one another. This is called a convergence setting . If the color separations are not congruent, the television picture has color fringes (the color edges are clearly next to each other).
In order to converge the colors red, green and blue, a test image with a so-called convergence grid is used. It consists of thin white horizontal and vertical lines on a black background. The lines consist of the colors red, green and blue and result in a grid of white lines if the three colors are correctly superimposed. Since the color green on the screen lies between red and blue in the arrangement of the colors ( color trio ), it is used as a reference, the position of the colors red and blue is set.
In the production of color picture tubes, the white screen is only used to control the convergence and to adjust the focus (line focusing). That is why it is also called a focus grid .
Some PC monitors also allow convergence to be set via the OSD menu.
technology
The convergence is achieved by magnetic fields that act on the electron beams in the vicinity of the kinescope neck in addition to the deflection coils and deflect them in the desired direction. A distinction is made between static convergence , which is usually realized by movable permanent magnets , and dynamic convergence . The latter depends on the intensity of the beam deflection and is implemented by means of electromagnets , the coil currents of which are influenced by the horizontal and vertical deflection voltage.
The magnets are all arranged outside the picture tube, which is why one setting for a certain beam system also affects the other two to a lesser extent.
In the early days of color television technology , Delta tubes were used in which the beam systems for red, green and blue were arranged in a triangle shape. This required a very complex convergence setting. Up to about 15 adjusting elements had to be operated, the settings of which influenced each other in some cases. The adjusters were usually combined in a separate circuit block (convergence unit). With the introduction of the inline tubes in the mid-1970s, the setting was made much easier, as the jet systems were now next to each other (see adjacent picture).