| 摘要 |
PURPOSE:To reduce the comet tail after image by preliminarily preparing plural DC voltage having no correlations to a video signal and selecting one DC voltage corresponding to the quantity of incident light from these DC voltages as the voltage of a grating. CONSTITUTION:If the signal level exceeds a prescribed value, that is, if the beam quantity will be increased, the beam quantity is not caused to follow up the brightness, but the signal is substituted with a DC voltage having a certain value and this voltage is applied to the reference bias voltage of a first grating 3 to generate a maximum beam quantity which can be generated by an electron gun at a stroke. Consequently, the beam current is binarized with the prescribed value as the boundary, and a necessary condition in the beam quantity control system of the signal voltage feedback system is excluded because of no correlations between a beam current Ib and a signal current Is even if a device enters into the beam lacking state, and an oscillation phenomenon does not occur. Thus, the comet tail after image is suppressed and the oscillation phenomenon accompanied with the beam control is completely prevented.
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