Any image—such as a photograph—that exists as a series of small dots of varying size and color density that serve to simulate the appearance of continuous gradations of tone. Halftones are necessary in the reproduction of photographic images; most printing presses cannot print continuous tones, so photographic images must first be converted to a series of dots in order to be effectively printed.
Lightness and darkness of portions of an image are effected by varying the size and density of the dots; small dots spaced far apart produce light areas (highlights), while large dots clustered more closely together produce dark areas (shadows).
Halftones are produced either as film positives or negatives by photographing a continuous tone original through a halftone screen or fine grid. The screen pattern and frequency of the dots that are produced determine the ultimate quality of the reproduction. A 150-line screen, for example, will produce 150 rows and 150 columns of dots, or 22,500 dots per square inch.
Halftones can also be produced electronically, using digital data. (See Electronic Dot Generation.)