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.