| 摘要 |
The disclosure relates to cathode-ray tubes that have to be used at a low level of luminance. The cut-off voltage of the tube is modified by using two servo-control loops, the first one as a function of the value of the cathode current and the second one as a function of the luminance value of a zone that is located outside the operational image of the screen and consists of a particular luminophore. Selector means are used to change over from one loop to the other and vice versa as a function of the values of the cathode current and of the luminance of the zone with respect to certain predetermined thresholds.
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