Printer resolution is measured in the number of ink dots produced per inch (dpi). Most desktop laser printers have a resolution of 600 dpi, and imagesetters have a resolution of 1200 dpi or higher. Inkjet printers produce a microscopic spray of ink, not actual dots; however, most inkjet printers have an approximate resolution of 300 to 720 dpi.

When printing to a desktop laser printer, but especially to imagesetters, you must also consider screen frequency. Screen frequency is the number of halftone cells per inch used to print grayscale images or color separations. Also known as screen ruling or line screen, screen frequency is measured in lines per inch (lpi)—or lines of cells per inch in a halftone screen.

A high line-screen ruling (for example, 150 lpi) spaces the dots used to create an image close 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. That’s because increasing the lpi for a given resolution decreases the number of colors that can be reproduced.