| 摘要 |
In the method, the halftone film to be corrected or its negative is exposed, first directly and then with the insertion of a transparent layer, on a hard film, in particular a lith film, to produce a screen copy. Depending on whether the positive or the negative of the halftone film to be corrected is used, an attenuation or an intensification of the tone value is achieved. The tone value correction can be controlled since, for a given copier, the exposure time necessary to achieve a certain change in tone value can be derived from a calibration curve. The transparent layer used may either be a blank sheet which may carry a coating layer for protecting the image areas not to be retouched against a change in screen dot size, or the transparent layer used may be a halftone negative of the halftone film to be corrected, thereby making it possible to modify the gradation of the screen values within the part of the image treated. Instead of a halftone negative, it is also possible to use a sheet having phototropic properties which spontaneously builds up a certain grey level on exposure to light and later becomes clear again so that it can be reused.
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