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A halftone is a reproduction of a photograph or other continuous tone picture, in which only various-sized dots of black ink or ink: of a single shade are used to create the effect of intermediate or middle tones of gray. The illusion of different tones is due to the observer's vision, in which the dots are blended. The halftone technique is used to reproduce pictures in newspapers, magazines, leaflets, and other matter printed by letterpress, offset, rotogravure, or silk screen processes.
Making a Halftone
In making a halftone, the original continuous tone picture, such as a photograph, painting, or pencil or pastel drawing. is photographed through a screen, which is usually scribed into glass or plastic and placed between the camera lens and the film. Each tiny square of the screen transmits light from a very small area of the picture. Areas that are light in the original appear in the negative as large, often overlapping opaque dots.
Transparent, pinpoint dots (highlight dots) appear, even in the darkest areas on the negative; these correspond to the intersections of the screen lines. Dark areas in the original appear on the negative as large transparent, dots with only tiny black centers (shadow dots), which correspond to the centers of the screen's square holes.
The screened negative is used to make a halftone plate, or cut, by any of several methods, such as conventional photoengraving, planographic, or gravure techniques. The halftone plate and the halftones printed with it are positives.
In the printed halftone, light areas of the original are reproduced as areas of tiny dark dots (highlight dots). Dark areas in the Original are reproduced as masses of large dark dots with tiny central light dots (shadow dots). Intermediate tones in the original are reproduced as areas with dots of various sizes, depending on the tone.
There are several theories explaining the halftone screen's action. In one theory the dot pattern is thought to be due to the diffraction of light as it passes through the screen. In another theory, the pattern is assumed to be formed by the shadow cast by the screen.
Special Techniques and Screens
Sometimes the highlight dots are removed from a halftone plate photographically, chemically, or by routing, to produce highlight, or dropout, halftones. Halftones may also be silhouetted by removing background dots, or vignetted to produce an image that appears to fade gradually around the edges.
Portions of the halftone plate may be removed and replaced with type or a line cut. Also, portions of the screened negative can be removed and replaced with line negatives. Plates made with line and halftone negatives are known as combination plates.
In addition to conventional, rectangularly scribed screens, there are screens scribed with only parallel lines, concentric circles, or other patterns, some creating effects of mezzotints or crayon drawings.
To reproduce full-color pictures using the halftone technique, four halftone plates are made. One each is made to print a dot pattern in yellow, cyan, magenta, or black ink. The dots in the reproduction made with all four inks are mixed in the viewer's vision so that the halftone appears to contain all the variations of colors of the Original.
The fineness or coarseness of a screen is indicated by its size-that is, the number of lines it has in a linear inch. Generally the finer the screen used, the greater the detail.
However, fineness is limited by such factors as the smoothness and porosity of the paper and by the printing process. On coarse paper, such as newspaper, 55-line screens are used for letterpress printing; finer screens may be used in offset and gravure printing, even on coarse papers. Screens finer than 200-line have been used, but this is the finest commercially practical. The dots formed by using a 140-line screen are invisible to the naked eye.
Henry Fox Talbot, a British pioneer in photography, invented the halftone process around 1852; the first printed halftone appeared in the Montreal Canadian Illustrated News in 1869, apparently the work of William A. Leggo.
The first commercially manufactured screens were produced in Philadelphia in 1891 by the Levy brothers, Louis and Max, who made the process practical for all printing.