Adobe InDesign CS5 - Manual (Page 651)

645 USING INDESIGN
Color separations
Last updated 11/16/2011
About halftone screen frequency
In commercial printing, continuous tone is simulated by dots (called halftone dots) printed in rows (called lines or line
screens). Lines are printed at different angles to make the rows less noticeable. The Screening menu in the Output
section of the Print dialog box displays the recommended sets of line screens in lines per inch (lpi), and resolution in
dots per inch (dpi), based on the currently selected PPD. As you select inks in the ink list, the values in the Frequency
and Angle boxes change, showing you the halftone screen frequency and angle for that ink.
A high line-screen ruling (for example, 150 lpi) spaces the dots closely together to create a finely rendered image on
the press; a low line-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 line-screen ruling uses
large dots. The most important factor in choosing a line-screen ruling is the type of printing press your job will use.
Ask your service provider how fine a line screen its press can hold, and make your choices accordingly.
Line screens
A. 65 lpi: Coarse screen for printing newsletters and grocery coupons B. 85 lpi: Average screen for printing newspapers C. 133 lpi: High-quality
screen for printing four-color magazines
D. 177 lpi: Very fine screen for printing annual reports and images in art books
The PPD files for high-resolution imagesetters offer a wide range of possible screen frequencies, paired with various
imagesetter resolutions. The PPD files for low-resolution printers typically have only a few choices for line screens,
usually coarser screens of between 53 lpi and 85 lpi. The coarser screens, however, give optimum results on
low-resolution printers. Using a finer screen of 100 lpi, for example, actually decreases the quality of your image when
you use a low-resolution printer for final output.
Specify a halftone screen frequency and resolution
❖ In the Output section of the Print dialog box, choose one of the following options:
• To select one of the preset screen frequencies and printer resolution combinations, choose an option in the
Screening menu.
• To specify a custom halftone screen frequency, 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: Before creating your own halftone screens, check with your service provider for the preferred frequencies and angles.
Also, be aware that some output devices override the default frequencies and angles.
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