Photographs are continuous-tone images because you cannot distinguish divisions, or gradations, between colors. Printed images are not continuous-tone images. Instead, printers create the illusion of many colors using halftone dots. Most printed images are made up of rows of dots of varying sizes, called halftone screens, which produce the appearance of multiple tones when printed.

The quality of a printed image is greatly influenced by the refinement of its halftone screen. The finer the screen, the less apparent the dot pattern, because there are more dots per area and they are closer together. A high-resolution imagesetter, which can produce a high number of dots per inch, will accommodate a fine halftone screen.

Definitions
Dots per inch (dpi)
Dots per inch is the value that measures the resolution of an output device. Most desktop printers have a resolution of 300 to 600 dots per inch. Imagesetters used for offset printing can print 1,200 or more dots per inch. Dots per inch, or output device resolution, is often confused with image resolution (see Pixels per inch, below).

Lines per inch (lpi)
Lines per inch is the value that measures halftone resolution. The appropriate halftone resolution is dependent on output device resolution, printing paper, and, for offset printing, printing factors such as the type of printing plate and the quality of the press and press person.

Pixels per inch (ppi)
Pixels per inch measures the resolution of the image. Image resolution measures all the sample points that represent color and value in your image. The more pixels your image is divided into, the greater resolution your image has.

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