Researching HALFTONES for content source and inspiration for my own designs in 'What is design for print?' project (a print-based ISSUU published manual)- developments of which can be found on my Design Practice
blog in the upcoming weeks.

THE DEFINITION OF HALFTONE
http://www.google.co.uk/search?hl=en&client=firefox-a&hs=iAK&rls=org.mozilla:en-US:official&q=halftones&tbs=dfn:1&tbo=u&sa=X&ei=eZisTpfIDsi28QON8Z2vCw&ved=0CBwQkQ4

Web definitions

(halftone) a print obtained from photoengraving

(halftone) an engraving used to reproduce an illustration

wordnetweb.princeton.edu/perl/webwn

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. ...

en.wikipedia.org/wiki/Halftones

(halftone) Half the interval between two notes on a scale; A picture made by using the process of half-toning; To reproduce a photograph or other continuous tone image by the use of dots of various sizes

en.wiktionary.org/wiki/halftone

(Halftone) Converting a continuous tone to dots for printing

www.coburgprintworks.com.au/printing-terms.html

(Halftone) Tone graduated image composed of varying sized dots or lines, with equidistant centers.

www.movadcorp.com/resources/glossary.html

(halftone) A picture in which the gradations of light are obtained by the relative darkness and density of tiny dots produced by photographing the subject through a fine screen.

www.worldatlas.com/mapterms.htm

(halftone) The process of converting images into a regular array of dots of various sizes with equal spacing between centers. Also the process of reproducing an image as a series of dots of various sizes within a fixed grid.

kendalsigns.com/Terminology/Terminology.html

(halftone) Picture with varying shades of tone created by varying size dots.

impressprint.net/glossary/

(Halftone) A reproduction of a continuous-tone photograph by simulating gradiations of tone using dots of varying size, shape, or proximity.

www.baysideprinting.com/resources/terms.asp

(Halftone) in traditional publishing, a continuous-tone image photographed through a screen in order to create small dots of varying sizes that can be reproduced on a printing press. ...

www.accuprint.org/terms.htm

(Halftone) The process by which a continuous tone image such as a photograph is reproduced and simulated using a pattern of printed or silkscreened dots of varying size and equal spacing. At a normal viewing distance the reproduced image appears continuous in tone.

www.datalizer.com/resources/glossary.html

(Halftone) an illustration reproduced by breaking down the original tone into a pattern of dots of varying size. Light areas have small dots and darker areas or shadows have larger dots. Simulating a continuous tone photograph using dots.

printing.ou.edu/printing-services/glossary/

(Halftone) Printing process that expresses itself in the form of colored patterns of dots.

www.checkadvantage.com/CheckGlossary.html

(Halftone) The reproduction of a continuous tone original, such as a photograph, in which detail and tone value are represented by a series of evenly spaced dots of varying size and shape.

www.bookmasters.com/print/printglossary.htm

(Halftone) The method by which photographs and other images are printed by using cells of dots to simulate the tones between light and dark. A printing press is not able to change the tone of ink, therefore dots of colour are used to trick the eye into seeing a continuous tone image. ...

www.kainosprint.com.au/Glossary.shtml

(Halftone) Refers to a method of representing the colors of an image with dots of varying sizes. If the dots are small enough, the colors of the image appear continuous. Halftones are created to prepare photographic images for reproduction across various print media.

https://www.authorlearningcenter.com/articles/book-publishing-t…

(halftone) n. A printed reproduction of a photograph or other illustration, using evenly spaced spots of varying diameter to produce apparent shades of gray. The darker the shade at a particular point in the image, the larger the corresponding spot in the halftone. ...

www.softwaretipspalace.com/windows-xp-info/Windows_XP_G…

(Halftone) Black and white dots that vary in pattern to simulate shades of gray in an image.

www.absorbentprinting.com/help/glossary/

(Halftone) The result of a process by which a photograph is changed into a system of dots of varying size and shape allowing the accurate printing of various tone values in the photograph. Usually printed in one color.

www.hourglasscalendars.com/definitions.html

(Halftone) a tonal gradation made up of dots of varying sizes.

www.healeys-printers.co.uk/print-glossary/

(Halftone) A technique most often used in newspaper pictures -- a series of fine (black) dots, that when viewed from a distance, give the appearance of varying shades of grey. This process is relatively cheap-- you only have to run one color through your offset press.

www.acaeum.com/library/definitions.html

(Halftone) An irregular pattern of tiny dots that can be used to print a full range of tones. Halftone screens are used to print reproductions of photographs and artwork that are not line art. Multiple halftones are combined in process color to give the illusion of a full-color image.

www.wrightlabels.com/printing/standards.html

(Halftone) A photomechanical reproduction process of a photograph made on a printing press. An original photographic image is re-photographed through a screen that transforms the continuous tones of the image into a series of dots, relative to the amount of darkness in the original. ...

www.afterimagegallery.com/photoglossary.htm

(Halftone) Screened reproduction of an original made up of dots varying in size to create the illusion of a variation in tone. Can be done in various colors.

www.neacrylics.com/glossary.html

Halftone

Halftone is the reprographic technique that simulates continuous tone imagery through the use of dots, varying either in size, in shape or in spacing. "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 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

William Fox Talbot is credited with the idea of halftone printing. 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 Georg 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.
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. By the 1880s, Hoen was working on halftone methods that could be used in conjunction with either hand-worked or photolithographic stones.

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

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; 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

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 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), 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.

Dot shapes

Though round dots are the most common used, there are different dot types available, each of them having their own characteristics. They can be used simultaneously to avoid the moiré effect. Generally, the preferred dot shape is also dependent on the printing method or the printing plate.

Round dots: most common, suitable for light images, especially for skin tones. They meet at a tonal value of 70%.
Elliptical dots: appropriate for images with many objects. Elliptical dots meet at the tonal values 40% (pointed ends) and 60% (long side), so there is a risk of a pattern.
Square dots: best for detailed images, not recommended for skin tones. The corners meet at a tonal value of 50%. The transition between the square dots can sometimes be visible to the human eye.

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, 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 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.

IMAGES OF HALFTONES