halftone

A halftone is a printed image built from a grid of small dots of varying size. Ink-jet printers, offset presses, and lithography machines cannot print true gray or intermediate colors directly. Instead, they use halftoning: a mechanical or digital technique that breaks a continuous photograph or artwork into thousands of dots. Where the original is light, dots are tiny or absent; where it is dark, dots are large and tightly packed. From normal viewing distance, the human eye blends these discrete marks into perceived shades of gray or color.

Halftone screens are defined by their frequency, measured in lines per inch (lpi), which specifies how many rows of dots appear along a one-inch span. Common newspaper halftones run 65 to 85 lpi; high-quality magazine work uses 150 lpi or finer; digital textile or ceramic printing may employ 200 lpi. The angle at which the screen is applied also matters. In four-color process printing (cyan, magenta, yellow, and black), each color channel is screened at a different angle, typically 0, 15, 45, and 75 degrees, to prevent visible moiré patterns where the grids would align and create unwanted interference patterns.

The actual mechanics vary by process. Traditional halftone screens were physical: fine glass plates with ruled lines that cast a grid shadow onto film during photographic reproduction. Modern digital halftoning uses software algorithms, most commonly error diffusion (which scatters rounding errors across neighboring pixels) or ordered dithering (which applies a fixed pattern). The choice affects quality: error diffusion typically produces smoother tones but can introduce artifacts; ordered patterns are more predictable but may show visible texture.

Halftone quality breaks down at very coarse settings or when the original image has strong repetitive patterns. Moiré fringes become visible when a halftone grid interferes with fabric weave, window blinds, or brick patterns. Dot gain (the unwanted enlargement of halftone dots during printing, caused by ink spread or press pressure) can crush shadow detail if not compensated during prepress. Conversely, undercompensation leaves highlights too thin and prone to plugging (ink filling the gaps entirely, leaving no white).

The term halftone originated in photographic lithography. A photographic negative of the original artwork was exposed through a fine glass screen held at a small distance from the lithographic stone or plate. The screen created dot-shaped exposures proportional to the density of the original image. This produced a printing plate that could transfer ink dots of varying size to paper. The method revolutionized newspaper and magazine reproduction in the late 1800s, replacing hand-drawn engraving and making rapid, economic reproduction of photographs possible for the first time.