Left: Halftone dots. Right: How the human eye would see this sort of arrangement from a sufficient distance.

Halftone is the reprographic technique that simulates continuous tone imagery through the use of equally spaced dots of varying size. Reprography is the reproduction of Graphics through mechanical or electrical means such as Photography or Xerography. A continuous tone image is one where each color at any point in the image is reproduced as a single tone and not as discrete Halftones such as one single color for Monochromatic ^[1] 'Halftone' can also be used to refer specifically to the image that is produced by this process. ^[1]

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 article is mainly concerned with Still photography film For Motion picture film please see Film stock. The binary numeral system, or base-2 number system, is a Numeral system that represents numeric values using two symbols usually 0 and 1. This binary reproduction relies on a basic optical illusion—that these tiny halftone dots are blended into smooth tones by the human eye. This article is about visual perception See Optical Illusion (Album for information about the Time Requiem album

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. "Color film" redirects here For the motion picture equivalent see Color motion picture film. An optical filter is a device which selectively transmits light having certain properties (often a particular range of Wavelengths that is range of Colours A subtractive color model explains the mixing of Paints Dyes Inks and natural colorants to create a range of Colors where each such color CMYK (short for '''c'''yan, '''m'''agenta, '''y'''ellow, and k ey ( Black) and often referred to as process color ' ^[2] The semi-opaque property of ink allows halftone dots of different colors to create another optical effect—full-color imagery. An ink is a Liquid containing various Pigments and/or Dyes used for coloring a surface to produce an Image, text, or ^[1]

History

The idea of halftone printing is due to William Fox Talbot. William Henry Fox Talbot (11 February 1800 – 17 September 1877 was the inventor of the negative / positive photographic process the precursor to most photographic processes of In the early 1850s, he suggested using "photographic screens or veils" in connection with a photographic intaglio process. Intaglio (pronounced in-TAL-yo ɪn'tælɪəʊ is a family of Printmaking techniques in which the image is incised into a surface known as the matrix or plate ^[3]

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. Stephen H(enry Horgan, b February 12, 1854, d August 30, 1941, was the inventor of the Half tone process of engraving while working They 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. Events 51 - Nero, later to become Roman Emperor, is given the title Princeps iuventutis (head of the youth Year 1880 ( MDCCCLXXX) was a Leap year starting on Thursday (link will display the full calendar of the Gregorian calendar (or a Leap year ^[4]

The first truly successful commercial method was patented by Frederic Ives of Philadelphia in 1881. Frederick Eugene Ives (1856–1937 was a US inventor born at Litchfield, Connecticut. Philadelphia (ˌfɪləˈdɛlfiə Year 1881 ( MDCCCLXXXI) was a Common year starting on Saturday (link will display the full calendar of the Gregorian calendar (or a Common ^[3][4] 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. Year 1882 ( MDCCCLXXXII) was a Common year starting on Sunday (link will display the full calendar of the Gregorian calendar (or a Common 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. A relief print is an image created by a Printmaking process such as Woodcut, where the areas of the matrix (plate or block that are to show printed black (typically ^[3]

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. ^[3]

The relief halftone process proved almost immediately to be a success. A relief print is an image created by a Printmaking process such as Woodcut, where the areas of the matrix (plate or block that are to show printed black (typically The use of halftone blocks in popular journals became regular during the early 1890s. ^[3]

Traditional screening

The most common method of creating screens—amplitude modulation—produces a regular grid of dots that vary in size. Amplitude modulation ( AM) is a technique used in electronic communication most commonly for transmitting information via a Radio Carrier wave

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

Resolution of halftone screens

The resolution of a halftone screen is measured in lines per inch (lpi). Lines per inch ( LPI) is a measurement of Printing resolution in systems that use a Halftone screen 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. Posterization (pronounced Poe-ster-ize-ation of an image occurs when a region of an image with a continuous gradation of tone is replaced with several regions of fewer tones resulting Therefore file resolution is matched to the output resolution.

Multiple screens and color halftoning

Three examples of color halftoning with CMYK separations. From left to right: The cyan separation, the magenta separation, the yellow separation, the black separation, the combined halftone pattern and finally how the human eye would observe the combined halftone pattern from a sufficient distance.

This close-up of a halftone print shows that magenta on top of yellow appears as orange/red, and cyan on top of yellow appears as green.

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. CMYK (short for '''c'''yan, '''m'''agenta, '''y'''ellow, and k ey ( Black) and often referred to as process color 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^[5]. The Mergenthaler Linotype Company was founded in the United States in 1886 to market the linecaster invented by Ottmar Mergenthaler. The Linotronic Imagesetters are a now common type of high-quality printer, capable of printing at resolutions of up to 2540 Dots per inch. PostScript ( PS) is a dynamically typed concatenative Programming language created by John Warnock and Charles Geschke in 1982 A raster image processor (RIP is a component used in a Printing system which produces a raster image also know as a Bitmap.

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. Dither is an intentionally applied form of Noise, used to randomize Quantization error, thereby preventing large-scale patterns such as contouring that are more objectionable

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. In Computer graphics, a raster graphics image or bitmap, is a Data structure representing a generally rectangular grid of Pixels In Digital imaging, a pixel ( pict ure el ement is the smallest piece of information in an image 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. Dither is an intentionally applied form of Noise, used to randomize Quantization error, thereby preventing large-scale patterns such as contouring that are more objectionable

References

1. ^ ^{a b c} Campbell, Alastair. The Designer's Lexicon. ©2000 Chronicle, San Francisco.
2. ^ McCue, Claudia. Real World Print Production. ©2007, Peachpit Berkeley.
3. ^ ^{a b c d e} Twyman, Michael. Printing 1770-1970: an illustrated history of its development and uses in England. Eyre & Spottiswoode, London 1970.
4. ^ ^{a b} Meggs, Philip B. A History of Graphic Design. John Wiley & Sons, Inc. 1998. p 141 ISBN 0-471-291-98-6
5. ^ Linotype History - 1973–1989

See also

• Raster image processor (RIP)
• Error diffusion
• Screentone
• Dot gain
• Benday dots

External links

• Halftone Screens and Dots
• Learn to create halftone shading with Adobe Photoshop
• Dr. Daniel Lau's Website about halftoning
• "The Rasterbator": Software that allows creation of large halftone type posters