This tutorial contains:
There are two basic types of images:
Line art images contain only black and white (no shades of gray).
Continuous tone images contain shades of gray, or a range of colors and shades of color.
Line art and continuous tone images can be in different forms:
In order to print a continuous tone image on a conventional printing press, it must be converted to a halftone: a screen pattern of variable size dots which create an impression of tone values with one ink color.
Halftones are generated by the output device when a file is printed.
Note: Some imagesetters and 600-dpi laser printers use screening technologies other than halftoning. If you are printing an image on a nonhalftone printer, consult your service provider or your printer documentation for the recommended image resolutions.
Halftone screens are measured in lpi (lines per inch). This refers to how many rows, or lines, of dots fit in a linear inch.
The number of lpi in a halftone screen is called the screen frequency. It is also referred to as screen ruling or line screen.
The appropriate halftone frequency depends on the paper stock and type of press used for printing. Newspapers commonly use an 85-line screen. Magazines use higher resolution screens, such as 133 lpi and 150 lpi.
Note: Always check with your print shop for correct screen frequencies.
Image Resolution for Halftones
The relationship between image resolution and screen frequency determines the quality of detail in the printed image. To produce a halftone image of the highest quality, you generally use an image resolution that is from 1.5 to at most 2 times the screen frequency. But with some images and output devices, a lower resolution can produce good results.
To determine your printer's screen frequency, check your printer documentation or consult your service provider.