| 摘要 |
PURPOSE:To arrange so that an indicated brightness on a cathode ray tube may always be kept below a constant tolerable value by repeating sampling of brightness data and correction action of a brightness command signal until a deflection amount reaches less than a tolerable value. CONSTITUTION:A brightness detector 6 is provided in a certain sampling area 5 set on a cathode ray tube 1, and a sampling action is carried out in a single cycle according to a start correction command 12 from a brightness control part 11 and in this way, an actual brightness of the CRT1 surface is detected. Next, the detected brightness data and standard data for appropriate brightness preset by a setting circuit 14 are compared by a comparison circuit 13. And then the sampling action is carried out while a brightness command signal 16 from a brightness command signal generation circuit 15 is corrected until a deflection amount obtained through the comparison circuit reaches lower than the tolerable value. Thus it is possible to prevent a change in the density or linear width of printed characters projected on a photosensitive material, etc. 4 through an optical system 3 and obtain a printing quality of predetermined standard.
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