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halftone process

halftone process

In printing, a technique of breaking up an image into a series of dots to permit reproduction of the full tone range of a photograph or artwork. It is traditionally done by placing a glass screen printed with a tight grid of lines over the plate being exposed. The grid breaks up the image into hundreds of tiny dots, each of which is read by the camera as either black or white—or, in the case of colour art, as either a single printing colour or white. The resulting image, called a halftone, is then rephotographed for printing. Screens are made with a varying number of lines per inch, depending on the application; for newspapers the range is about 80–120, whereas glossy magazines usually require 133–175 lines per inch.

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Halftone is the reprographic technique that simulates continuous tone imagery through the use of equally spaced dots of varying size. 'Halftone' can also be used to refer specifically to the image that is produced by this process.

Where continuous tone imagery (film photography, for example) 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.

History

The idea of halftone printing is due to William 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.

The first truly successful commercial method was patented by Frederic Ives of Philadelphia 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 George Meisenbach patented a halftone process in England. His invention was based on the previous ideas of Berchtold and 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.

Shortly afterwards, Ives, this time in collaboration with Louis and Max Levy, improved the process further with the invention and commercial production of quality cross-lined screens.

The relief halftone process proved almost immediately to be a success. The use of halftone blocks in popular journals became regular during the early 1890s.

Traditional screening

The most common method of creating screens—amplitude modulation—produces a regular grid of dots that vary in size.

The other method of creating screens—frequency modulation—is used in a process named "stochastic screening."

Resolution of halftone screens

Typical Halftone Resolutions
Screen Printing 45-65 lpi
Laser Printer (300dpi) 65 lpi
Laser Printer (600dpi) 85-105 lpi
Offset Press (newsprint paper) 85 lpi
Offset Press (coated paper) 85-185 lpi
The resolution of a halftone screen is measured in lines per inch (lpi). This is the number of lines of dots in one inch, measured parallel with the screen's angle. Known as the screen ruling, the resolution of a screen is written either with the suffix lpi or a hash mark. E.g. 150lpi or 150#.

The higher the pixel resolution of a source file, the greater the detail that can be reproduced. However, such increase also requires a corresponding increase in screen ruling or the output will suffer from posterization. Therefore file resolution is matched to the output resolution.

Multiple screens and color halftoning

When different screens meet, a number of distracting visual effects can occur, including the edges being overly emphasized, as well as a moiré pattern. This problem can be reduced by rotating the screens in relation to each other. This screen angle is another common measurement used in printing, measured in degrees clockwise from a line running to the left (9 o'clock is zero degrees).

Halftoning is also commonly used for printing color pictures. The general idea is the same, by varying the density of the four primary printing colors, cyan, magenta, yellow and black (abbreviation CMYK), any particular shade can be reproduced. In this case there is an additional problem that can occur. In the simple case, one could create a halftone using the same techniques used for printing shades of grey, but in this case the different printing colors have to remain physically close to each other to fool the eye into thinking they are a single color. To do this the industry has standardized on a set of known angles, which result in the dots forming into small circles or rosettes.

The dots cannot easily be seen by the naked eye, but can be discerned through a microscope or a magnifying glass.

Digital halftoning

Digital halftoning has been replacing photographic halftoning since the 1970s when 'electronic dot generators' were developed for the film recorder units linked to color drum scanners made by companies such as Crosfield Electronics, Hell and Linotype-Paul.

In the 1980s halftoning became available in the new generation of 'imagesetter' film and paper recorders that had been developed from earlier 'laser typesetters'. Unlike pure scanners or pure typesetters, imagesetters could generate all the elements in a page including type, photographs and other graphic objects. Early examples were the widely used Linotype Linotronic 300 and 100 introduced in 1984, which were also the first to offer PostScript RIPs in 1985.

Early laser printers from the late 1970s onward could also generate halftones but their original 300 dpi resolution limited the screen ruling to about 65 lpi. This was improved as higher resolutions of 600 dpi and above, plus dithering techniques were introduced.

All halftoning uses a high frequency/low frequency dichotomy. In photographic halftoning, the low frequency attribute is a local area of the output image designated a halftone cell. Each equal-sized cell relates to a corresponding area (size and location) of the continuous-tone input image. Within each cell, the high frequency attribute is a centered variable-sized halftone dot composed of ink or toner. The ratio of the inked area to the non-inked area of the output cell corresponds to the luminance or graylevel of the input cell. From a suitable distance, the human eye averages both the high frequency apparent gray level approximated by the ratio within the cell and the low frequency apparent changes in gray level between adjacent equally-spaced cells and centered dots.

Digital halftoning uses a raster image or bitmap within which each monochrome picture element or pixel may be on or off, ink or no ink. Consequently, to emulate the photographic halftone cell, the digital halftone cell must contain groups of monochrome pixels within the same-sized cell area. The fixed location and size of these monochrome pixels compromises the high frequency/low frequency dichotomy of the photographic halftone method. Clustered multi-pixel dots cannot "grow" incrementally but in jumps of one whole pixel. In addition, the placement of that pixel is slightly off-center. To minimize this compromise, the digital halftone monochrome pixels must be quite small, numbering from 600 to 2,540, or more, pixels per inch. However, digital image processing has also enabled more sophisticated dithering algorithms to decide which pixels to turn black or white, some of which yield better results than digital halftoning.

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