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Specifying the halftone screen ruling


    The Halftone menu displays one or more sets of line screens (lines per inch, or lpi) and resolutions (dots per inch, or dpi) available on the printer or imagesetter that prints the color separations.

    A high line-screen ruling (for example, 150 lpi) spaces the dots used to create an image closely together to create a finely rendered image on the press; a low screen ruling (60 lpi to 85 lpi) spaces the dots farther apart to create a coarser image. The size of the dots is also determined by the line screen. A high line-screen ruling uses small dots; a low screen ruling uses large dots. The most important factor in choosing a line-screen ruling is the type of printing press your job uses. Ask your print shop how fine a line screen its press can hold, and make your choices accordingly.

    The PPD files for high-resolution imagesetters offer a wide range of possible line-screen rulings paired with various imagesetter resolutions. The PPD files for lower-resolution printers typically have only a few choices for line screens, and they are coarser screens of between 53 lpi and 85 lpi. The coarser screens, however, give optimum results on lower-resolution printers. Using a finer screen of 100 lpi, for example, actually decreases the quality of your image when a low-resolution printer is used for final output.

To specify a halftone screen ruling and resolution:

  1. In the Separation Setup dialog box, choose one of the following options:
    • To select one of the preset halftone screen ruling and printer resolution combinations, choose an option from the Halftone pop-up menu.
    • To specify a custom halftone screen ruling, select the plate to be customized and then enter the lpi value in the Frequency text box and a screen angle value in the Angle text box.

    Note: The default angles and frequencies are determined by the selected PPD file. Be sure to check with your print shop for the preferred frequency and angle before creating your own halftone screens.

  2. Specify another separation option, or click OK.