Halftone

Halftone is the reprographic technique that simulates continuous-tone imagery through the use of dots, varying either in size or in spacing, thus generating a gradient-like effect. "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 an image that is printed with only one color of ink, in dots of differing size (pulse-width modulation) or spacing (frequency modulation) or both. This reproduction relies on a basic optical illusion: when the halftone dots are small, the human eye interprets the patterned areas as if they were smooth tones. 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. Since the location of the individual dots cannot be determined exactly, the dots partially overlap leading to a combination of additive and subtractive color mixing called autotypical color mixing.

While early mechanical printing processes (most notably the Woodburytype, patented in 1864) could imitate the tone and subtle details of a photograph, expense and practicality prohibited their use in mass commercial printing that used relief printing.

Before the development of halftone printing, most pictures in newspapers used woodcut or wood-engraving techniques utilising hand-carved blocks of wood that, while they were often copied from photographs, resembled hand-drawn sketches. Commercial printers wanted a practical way to realistically reproduce photographs onto the printed page, but most common mechanical printing processes can only print areas of ink or leave blank areas on the paper and not a photographic range of tones; only black (or coloured) ink, or nothing. The half-tone process overcame these limitations and became the staple of the book, newspaper and other periodical industry.

William Fox Talbot (1800-1877) is credited with the idea of halftone printing. In an 1852 patent 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. William Leggo produced an early version with his leggotype while working for the Canadian Illustrated News. The first printed halftone photograph was an image of Prince Arthur published on October 30, 1869. The New York Daily Graphic would later publish "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.

Frederic Ives of Philadelphia patented the first truly successful commercial method in 1881. Although he found a way of breaking up the image into dots of varying sizes, he did not make use of a screen. In 1882, the German Georg Meisenbach [de] patented the "autotype" halftone process in Germany which he named Autotypie [de]. His invention was based on the previous ideas of A. J. Berchtold and J. W. Swan. He used single-lined screens which were turned during exposure to produce cross-lined effects. He was the first to achieve any commercial success with relief halftones.