| 摘要 |
PURPOSE:To prevent the deterioration of the resolution by changing the spot shape according to the inputted signal level in a CRT display. CONSTITUTION:A detecting circuit 10 detects the average brightness level in one field by the inputted signal, and outputs the detected results to a ROM 11. The output value where the voltage value at a terminal 14 is the voltage value in the just-focus condition corresponding to the detected results is preliminarily stored in the ROM 11. This voltage is applied to a third grid 5, and the electron beam is converged by an electron lens to be constituted by this voltage, and a fluorescent surface is irradiated with the electron beam. This constitution converges the electron beam at the convergence voltage corresponding to the inputted signal even when the signal of any brightness level may be inputted, and leads to the constantly just-focus condition. |
