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Learn more about "Halftone"

Halftone

Halftone is the reprographic technique that simulates continuous tone imagery through the use of dots, varying either in size or in spacing.Campbell, Alastair. The Designer's Lexicon. ©2000 Chronicle, San Francisco. "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 numeral system|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 Filter (optics)|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". McCue, Claudia. Real World Print Production. ©2007, Peachpit Berkeley. 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 (printmaking)|intaglio process.Twyman, Michael. Printing 1770–1970: an illustrated history of its development and uses in England. Eyre & Spottiswoode, London 1970. 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.''LIFE''. "100 Photographs That Changed the World". Time, Inc. August 25, 2003, p 18. 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.Meggs, Philip B. A History of Graphic Design. John Wiley & Sons, Inc. 1998. p 141. ISBN 0-471-291-98-6. The first truly successful commercial method was patented by Frederic Eugene Ives|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 print|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 print|relief halftone process proved almost immediately to be a success. The use of halftone blocks in popular journals became regular during the early 1890s. The development of halftone printing methods for lithography appears to have followed a largely independent path. In the 1860s, A. Hoen & Co. focused on methods allowing artists to manipulate the tones of hand-worked printing stones.August Hoen, ''Composition for etching stone'', U.S. Patent 27,981, Apr 24, 1860. By the 1880s Hoen was working on halftone methods that could be used in conjunction with either hand-worked or photolithographic stones.August Hoen, ''Lithographic Process'', U.S. Patent 227,730, May 15, 1883.August Hoen, ''Lithographic Process'', U.S. Patent 227,782, May 18, 1880.

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 also known as stochastic screening. Both modulation methods are named by analogy with the use of the terms in telecommunications.

Resolution of halftone screens

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; for example, "150 lpi" 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 are combined, 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 color model|CMYK), any particular shade can be reproduced.Halftone Line Screens in Printing "Use of halftone line screens for printing digital images on press". (last checked on 2009-04-20) 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 Mergenthaler Linotype Company|Linotype Linotronic 300 and 100 introduced in 1984, which were also the first to offer PostScript Raster image processor|RIPs in 1985.Linotype History - 1973–1989 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, and 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 graphics|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 Dither#Dithering algorithms|dithering algorithms to decide which pixels to turn black or white, some of which yield better results than digital halftoning.