Halftone is the reprographic technique that simulates continuous tone imagery through the use of dots, varying either in size, in shape or in spacing. "Halftone" can also be used to refer specifically to the image that is produced by this process.
Where continuous tone imagery contains an infinite range of colors or greys, the halftone process reduces visual reproductions to a binary image that is printed with only one color of ink. This binary reproduction relies on a basic optical illusion—that these tiny halftone dots are blended into smooth tones by the human eye. At a microscopic level, developed black and white photographic film also consists of only two colors, and not an infinite range of continuous tones. For details, see film grain.
Just as color photography evolved with the addition of filters and film layers, color printing is made possible by repeating the halftone process for each subtractive color—most commonly using what is called the "CMYK color model". The semi-opaque property of ink allows halftone dots of different colors to create another optical effect—full-color imagery.
The idea of halftone printing is due to William Henry Fox Talbot. In the early 1850s, he suggested using "photographic screens or veils" in connection with a photographic intaglio process.
Several different kinds of screens were proposed during the following decades. One of the well known attempts was by Stephen H. Horgan while working for the New York Daily Graphic. The first printed photograph was an image of Steinway Hall in Manhattan published on December 2, 1873. The Graphic then published "the first reproduction of a photograph with a full tonal range in a newspaper" on March 4, 1880 (entitled "A Scene in Shantytown") with a crude halftone screen.