Color printing
Encyclopedia
Color printing or Colour printing is the reproduction of an image or text in color (as opposed to simpler black and white or monochrome
printing). Any natural scene or color photograph can be optically and physiologically dissected into three primary color
s, red, green and blue, roughly equal amounts of which give rise to the perception of white, and different proportions of which give rise to the visual sensations of all other colors. The additive combination of any two primary colors in roughly equal proportion gives rise to the perception of a secondary color
. For example, red and green yields yellow, red and blue yields magenta (a purple hue), and green and blue yield cyan (a turquoise hue). Only yellow is counter-intuitive.
Yellow, cyan and magenta are merely the "basic" secondary colors: unequal mixtures of the primaries give rise to perception of many other colors all of which may be considered "tertiary
."
es capable of thousands or millions of impressions for publishing newspapers and magazines, brochures, cards, posters and similar mass-market items. In this type of industrial or commercial printing
, the technique used to print full-color images, such as color photographs, is referred to as four-color-process or merely process printing. Four inks are used: three secondary colors plus black. These ink colors are cyan
, magenta
and yellow
; abbreviated as CMYK
. Cyan can be thought of as minus-red, magenta as minus-green, and yellow as minus-blue. These inks are semi-transparent or translucent. Where two such inks overlap on the paper due to sequential printing impressions, a primary color is perceived. For example, yellow (minus-blue) overprinted by magenta (minus green) yields red. Where all three inks may overlap, almost all incident light is absorbed or subtracted, yielding near black. It is because of this poor "subtractive" black that a separate black ink is used. The secondary or subtractive color
s cyan, magenta and yellow may be considered "primary" by printers and watercolorists (whose basic inks and paints are transparent).
Two graphic techniques are required to prepare images for four-color printing. In the "pre-press" stage, original images are translated into forms that can be used on a printing press, through "color separation," and "screening" or "halftoning
." These steps make possible the creation of printing plates that can transfer color impressions to paper on printing presses based on the principles of lithography
.
An emerging method of full-color printing is six-color process printing (for example, Pantone
's Hexachrome
system) which adds orange and green to the traditional CMYK inks for a larger and more vibrant gamut
, or color range. However, such alternate color systems still rely on color separation, halftoning and lithography to produce printed images.
Color printing can also involve as few as one color ink, or multiple color inks which are not the primary colors. Using a limited number of color inks, or specific color inks in addition to the primary colors, is referred to as "spot color" printing. Generally, spot-color inks are specific formulations that are designed to print alone, rather than to blend with other inks on the paper to produce various hues and shades. The range of available spot color inks, much like paint, is nearly unlimited, and much more varied than the colors that can be produced by four-color-process printing. Spot-color inks range from subtle pastels to intense fluorescents to reflective metallics.
Color printing involves a series of steps, or transformations, to generate a quality color reproduction. The following sections focus on the steps used when reproducing a color image in CMYK printing, along with some historical perspective.
preceded printing on paper in both Asia and Europe, and the use of different blocks to produce patterns in color was common. The earliest way of adding color to items printed on paper was by hand-coloring , and this was widely used for printed images in both Europe and Asia. Chinese woodcut
s have this from at least the 13th century, and European ones from very shortly after their introduction in the 15th century, where it continued to be practiced, sometimes at a very skilled level, until the 19th century—elements of the official British Ordnance Survey
maps were hand-colored by boys until 1875. Early European printed books often left spaces for initial
s, rubric
s and other elements to be added by hand, just as they had been in manuscript
s, and a few early printed books had elaborate borders and miniatures
added. However this became much rarer after about 1500.
. The chiaroscuro woodcut was a European method developed in the early 16th century, where to a normal woodcut block with a linear image (the "line block"), one or more colored "tone blocks" printed in different colors would be added. This was the method developed in Germany; in Italy only tone blocks were often used, to create an effect more like a wash drawing. Jacob Christoph Le Blon
developed a method using three intaglio plates, usually in [mezzotint]; these were overprinted to achieve a wide range of colors.
In Europe and Japan, color woodcuts were normally only used for prints rather than book illustrations. In Chinese woodblock printing
, where the individual print did not develop until the nineteenth century, the reverse is true, and early color woodcuts mostly occur in luxury books about art, especially the more prestigious medium of painting. The first known example is a book on ink-cakes printed in 1606, and color technique reached its height in books on painting published in the seventeenth century. Notable examples are the Treatise on the Paintings and Writings of the Ten Bamboo Studio of 1633, and the Mustard Seed Garden Painting Manual published in 1679 and 1701.
In Japan color technique, called nishiki-e
in its fully developed form, spread more widely, and was used for prints, from the 1760s on. Text was nearly always monochrome, as were images in books, but the growth of the popularity of ukiyo-e
brought with it demand for ever increasing numbers of colors and complexity of techniques. By the nineteenth century most artists worked in color. The stages of this development were:
In the 19th century a number of different methods of color printing, using woodcut (technically Chromoxylography
) and other methods, were developed in Europe, which for the first time achieved widespread commercial success, so that by the later decades the average home might contain many examples, both hanging as prints and as book illustrations. George Baxter patented in 1835 a method using an intaglio
line plate (or occasionally a lithograph), printed in black or a dark color, and then overprinted with up to twenty different colors from woodblocks. Edmund Evans
used relief and wood throughout, with up to eleven different colors, and latterly specialized in illustrations for children's books, using fewer blocks but overprinting non-solid areas of color to achieve blended colors. Artists such as Randolph Caldecott
, Walter Crane
and Kate Greenaway
were able to draw influence from the Japanese prints now available and fashionable
in Europe to create a suitable style, with flat areas of color.
Chromolithography
was another process, which by the end of the 19th century had become dominant, although this still used multiple prints with a stone for each color. Mechanical color separation, initially using photographs of the image taken with three different color filters, reduced the number of prints needed to three. Zincography
, with zinc
plates, later replaced lithographic stones, and remained the commonest method of color printing until the 1930s.
was developed, the traditional method of doing this was to photograph
the image three times, using a filter for each color. However this is achieved, the desired result is three grayscale
images, which represent the red, green, and blue (RGB) components of the original image:
The next step is to invert each of these separations. When a negative image of the red component is produced, the resulting image represents the cyan component of the image. Likewise, negatives are produced of the green and blue components to produce magenta and yellow separations, respectively. This is done because cyan, magenta, and yellow are subtractive
primaries which each represent two of the three additive
primaries (RGB) after one additive primary has been subtracted from white light.
Cyan, magenta, and yellow are the three basic colors used for color reproduction. When these three colors are variously used in printing the result should be a reasonable reproduction of the original, but in practice this is not the case. Due to limitations in the inks, the darker colors are dirty and muddied. To resolve this, a black separation is also created, which improves the shadow and contrast of the image. Numerous techniques exist to derive this black separation from the original image; these include grey component replacement
, under color removal
, and under color addition
. This printing technique is referred to as CMYK (the "K" is the color abbreviation for black as "B" is already designated for blue).
Today's digital printing methods do not have the restriction of a single color space
that traditional CMYK processes do. Many presses can print from files that were ripped with images using either RGB
or CMYK modes. The color reproduction abilities of a particular color space can vary; the process of obtaining accurate colors within a color model is called color matching.
for each ink color is printed in succession. The screen grids are set at different angles, and the dots therefore create tiny rosettes, which, through a kind of optical illusion
, appear to form a continuous-tone image. You can view the halftoning, which enables printed images, by examining a printed picture under magnification.
Traditionally, halftone screens were generated by inked lines on two sheets of glass that were cemented together at right angle
s. Each of the color separation films were then exposed through these screens. The resulting high-contrast image, once processed, had dots of varying diameter depending on the amount of exposure that area received, which was modulated by the grayscale separation film image.
The glass screens were made obsolete by high-contrast films where the halftone dots were exposed with the separation film. This in turn was replaced by a process where the halftones are electronically generated directly on the film with a laser. Most recently, computer to plate
(CTP) technology has allowed printers to bypass the film portion of the process entirely. CTP images the dots directly on the printing plate with a laser, saving money, increasing quality (by reducing the repeated generations), reducing lead-times, and saving the environment from toxic film-processing chemicals.
Screens with a "frequency" of 60 to 120 lines per inch (lpi) reproduce color photographs in newspapers. The coarser the screen (lower frequency), the lower the quality of the printed image. Highly absorbent newsprint
requires a lower screen frequency than less-absorbent coated paper
stock used in magazines and books, where screen frequencies of 133 to 200 lpi and higher are used.
The measure of how much an ink dot spreads and becomes larger on paper is called dot gain
. This phenomenon must be accounted for in photographic or digital preparation of screened images. Dot gain is higher on more absorbent, uncoated paper stock such as newsprint.
Monochrome
Monochrome describes paintings, drawings, design, or photographs in one color or shades of one color. A monochromatic object or image has colors in shades of limited colors or hues. Images using only shades of grey are called grayscale or black-and-white...
printing). Any natural scene or color photograph can be optically and physiologically dissected into three primary color
Primary color
Primary colors are sets of colors that can be combined to make a useful range of colors. For human applications, three primary colors are usually used, since human color vision is trichromatic....
s, red, green and blue, roughly equal amounts of which give rise to the perception of white, and different proportions of which give rise to the visual sensations of all other colors. The additive combination of any two primary colors in roughly equal proportion gives rise to the perception of a secondary color
Secondary color
A secondary color is a color made by mixing two primary colors in a given color space. Examples include the following:-Light : red + green = yellowgreen + blue = cyan blue +...
. For example, red and green yields yellow, red and blue yields magenta (a purple hue), and green and blue yield cyan (a turquoise hue). Only yellow is counter-intuitive.
Yellow, cyan and magenta are merely the "basic" secondary colors: unequal mixtures of the primaries give rise to perception of many other colors all of which may be considered "tertiary
Tertiary color
A tertiary color is a color made by mixing one primary color with one secondary color, in a given color space such as RGB or RYB.Unlike primary and secondary colors, these are not represented by one firmly established name each, but the following examples include some typical names.Brown and grey...
."
Today
While there are many techniques for reproducing images in color, specific graphic processes and industrial equipment are used for mass reproduction of color images on paper. In this sense, "color printing" involves reproduction techniques suited for printing pressPrinting press
A printing press is a device for applying pressure to an inked surface resting upon a print medium , thereby transferring the ink...
es capable of thousands or millions of impressions for publishing newspapers and magazines, brochures, cards, posters and similar mass-market items. In this type of industrial or commercial printing
Printing
Printing is a process for reproducing text and image, typically with ink on paper using a printing press. It is often carried out as a large-scale industrial process, and is an essential part of publishing and transaction printing....
, the technique used to print full-color images, such as color photographs, is referred to as four-color-process or merely process printing. Four inks are used: three secondary colors plus black. These ink colors are cyan
Cyan
Cyan from , transliterated: kýanos, meaning "dark blue substance") may be used as the name of any of a number of colors in the blue/green range of the spectrum. In reference to the visible spectrum cyan is used to refer to the color obtained by mixing equal amounts of green and blue light or the...
, magenta
Magenta
Magenta is a color evoked by light stronger in blue and red wavelengths than in yellowish-green wavelengths . In light experiments, magenta can be produced by removing the lime-green wavelengths from white light...
and yellow
Yellow
Yellow is the color evoked by light that stimulates both the L and M cone cells of the retina about equally, with no significant stimulation of the S cone cells. Light with a wavelength of 570–590 nm is yellow, as is light with a suitable mixture of red and green...
; abbreviated as CMYK
CMYK color model
The CMYK color model is a subtractive color model, used in color printing, and is also used to describe the printing process itself. CMYK refers to the four inks used in some color printing: cyan, magenta, yellow, and key...
. Cyan can be thought of as minus-red, magenta as minus-green, and yellow as minus-blue. These inks are semi-transparent or translucent. Where two such inks overlap on the paper due to sequential printing impressions, a primary color is perceived. For example, yellow (minus-blue) overprinted by magenta (minus green) yields red. Where all three inks may overlap, almost all incident light is absorbed or subtracted, yielding near black. It is because of this poor "subtractive" black that a separate black ink is used. The secondary or subtractive color
Subtractive color
A subtractive color model explains the mixing of paints, dyes, inks, and natural colorants to create a full range of colors, each caused by subtracting some wavelengths of light and reflecting the others...
s cyan, magenta and yellow may be considered "primary" by printers and watercolorists (whose basic inks and paints are transparent).
Two graphic techniques are required to prepare images for four-color printing. In the "pre-press" stage, original images are translated into forms that can be used on a printing press, through "color separation," and "screening" or "halftoning
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...
." These steps make possible the creation of printing plates that can transfer color impressions to paper on printing presses based on the principles of lithography
Lithography
Lithography is a method for printing using a stone or a metal plate with a completely smooth surface...
.
An emerging method of full-color printing is six-color process printing (for example, Pantone
Pantone
Pantone Inc. is a corporation headquartered in Carlstadt, New Jersey, USA. The company is best known for its Pantone Matching System , a proprietary color space...
's Hexachrome
Hexachrome
Hexachrome was a six-color printing process designed by Pantone Inc. In addition to custom CMYK inks, Hexachrome added orange and green inks to expand the color gamut, for better color reproduction. It was therefore also known as a CMYKOG process....
system) which adds orange and green to the traditional CMYK inks for a larger and more vibrant gamut
Gamut
In color reproduction, including computer graphics and photography, the gamut, or color gamut , is a certain complete subset of colors. The most common usage refers to the subset of colors which can be accurately represented in a given circumstance, such as within a given color space or by a...
, or color range. However, such alternate color systems still rely on color separation, halftoning and lithography to produce printed images.
Color printing can also involve as few as one color ink, or multiple color inks which are not the primary colors. Using a limited number of color inks, or specific color inks in addition to the primary colors, is referred to as "spot color" printing. Generally, spot-color inks are specific formulations that are designed to print alone, rather than to blend with other inks on the paper to produce various hues and shades. The range of available spot color inks, much like paint, is nearly unlimited, and much more varied than the colors that can be produced by four-color-process printing. Spot-color inks range from subtle pastels to intense fluorescents to reflective metallics.
Color printing involves a series of steps, or transformations, to generate a quality color reproduction. The following sections focus on the steps used when reproducing a color image in CMYK printing, along with some historical perspective.
History of color printing
Woodblock printing on textilesWoodblock printing on textiles
Woodblock printing on textiles is the process of printing patterns on textiles, usually of linen, cotton or silk, by means of incised wooden blocks. It is the earliest, simplest and slowest of all methods of textile printing. Block printing by hand is a slow process...
preceded printing on paper in both Asia and Europe, and the use of different blocks to produce patterns in color was common. The earliest way of adding color to items printed on paper was by hand-coloring , and this was widely used for printed images in both Europe and Asia. Chinese woodcut
Woodcut
Woodcut—occasionally known as xylography—is a relief printing artistic technique in printmaking in which an image is carved into the surface of a block of wood, with the printing parts remaining level with the surface while the non-printing parts are removed, typically with gouges...
s have this from at least the 13th century, and European ones from very shortly after their introduction in the 15th century, where it continued to be practiced, sometimes at a very skilled level, until the 19th century—elements of the official British Ordnance Survey
Ordnance Survey
Ordnance Survey , an executive agency and non-ministerial government department of the Government of the United Kingdom, is the national mapping agency for Great Britain, producing maps of Great Britain , and one of the world's largest producers of maps.The name reflects its creation together with...
maps were hand-colored by boys until 1875. Early European printed books often left spaces for initial
Initial
In a written or published work, an initial is a letter at the beginning of a work, a chapter, or a paragraph that is larger than the rest of the text. The word is derived from the Latin initialis, which means standing at the beginning...
s, rubric
Rubric
A rubric is a word or section of text which is traditionally written or printed in red ink to highlight it. The word derives from the , meaning red ochre or red chalk, and originates in Medieval illuminated manuscripts from the 13th century or earlier...
s and other elements to be added by hand, just as they had been in manuscript
Manuscript
A manuscript or handwrite is written information that has been manually created by someone or some people, such as a hand-written letter, as opposed to being printed or reproduced some other way...
s, and a few early printed books had elaborate borders and miniatures
Miniature (illuminated manuscript)
The word miniature, derived from the Latin minium, red lead, is a picture in an ancient or medieval illuminated manuscript; the simple decoration of the early codices having been miniated or delineated with that pigment...
added. However this became much rarer after about 1500.
Europe
Most early methods of color printing involved several prints, one for each color, although there were various ways of printing two colors together if they were separate. Liturgical and many other kinds of books required rubrics, normally printed in red; these were long done by a separate print run with a red forme for each page. Other methods were used for single leaf printsOld master print
An old master print is a work of art produced by a printing process within the Western tradition . A date of about 1830 is usually taken as marking the end of the period whose prints are covered by this term. The main techniques concerned are woodcut, engraving and etching, although there are...
. The chiaroscuro woodcut was a European method developed in the early 16th century, where to a normal woodcut block with a linear image (the "line block"), one or more colored "tone blocks" printed in different colors would be added. This was the method developed in Germany; in Italy only tone blocks were often used, to create an effect more like a wash drawing. Jacob Christoph Le Blon
Jacob Christoph Le Blon
Jacob Christoph Le Blon, or Jakob Christoffel Le Blon, was a German painter and engraver who invented the system of three- and four-colour printing, using an RYBK color model similar to the modern CMYK system.[2] He used the mezzotint method to engrave three or four metal plates to make prints...
developed a method using three intaglio plates, usually in [mezzotint]; these were overprinted to achieve a wide range of colors.
Asia
Woodblock printing
Woodblock printing is a technique for printing text, images or patterns used widely throughout East Asia and originating in China in antiquity as a method of printing on textiles and later paper....
, where the individual print did not develop until the nineteenth century, the reverse is true, and early color woodcuts mostly occur in luxury books about art, especially the more prestigious medium of painting. The first known example is a book on ink-cakes printed in 1606, and color technique reached its height in books on painting published in the seventeenth century. Notable examples are the Treatise on the Paintings and Writings of the Ten Bamboo Studio of 1633, and the Mustard Seed Garden Painting Manual published in 1679 and 1701.
In Japan color technique, called nishiki-e
Nishiki-e
refers to Japanese multi-colored woodblock printing; this technique is used primarily in ukiyo-e. It was invented in the 1760s, and perfected and popularized by the printmaker Suzuki Harunobu, who produced a great many nishiki-e prints between 1765 and his death five years later.Previously, most...
in its fully developed form, spread more widely, and was used for prints, from the 1760s on. Text was nearly always monochrome, as were images in books, but the growth of the popularity of ukiyo-e
Ukiyo-e
' is a genre of Japanese woodblock prints and paintings produced between the 17th and the 20th centuries, featuring motifs of landscapes, tales from history, the theatre, and pleasure quarters...
brought with it demand for ever increasing numbers of colors and complexity of techniques. By the nineteenth century most artists worked in color. The stages of this development were:
- Sumizuri-e (墨摺り絵, "ink printed pictures") - monochrome printing using only black ink
- Benizuri-eBenizuri-e' are a type of “primitive” ukiyo-e style Japanese woodblock prints. They were usually printed in pink and green, occasionally with the addition of another color, either printed or added by hand. The production of benizuri-e reached its peak in the early 1740s...
(紅摺り絵, "crimson printed pictures") - red ink details or highlights added by hand after the printing process；green was sometimes used as well - Tan-e (丹絵) - orange highlights using a red pigment called tan
- Aizuri-eAizuri-eAizuri-e literally means “blue printed picture”. The term usually refers to Japanese woodblock prints that are printed entirely or predominantly in blue. When a second color is used, it is usually red...
(藍摺り絵, "indigo printed pictures"), Murasaki-e (紫絵, "purple pictures"), and other styles in which a single color would be used in addition to, or instead of, black ink - Urushi-eUrushi-eUrushi-e , literally meaning "lacquer picture," refers to two types of Japanese artworks: paintings painted with actual lacquer, and particular woodblock printing styles which use regular ink but are said to resemble the darkness and thickness of black lacquer.-Prints:Urushi-e woodblock prints were...
(漆絵) - a method in which glue was used to thicken the ink, emboldening the image; gold, mica and other substances were often used to enhance the image further. Urushi-e can also refer to paintings using lacquerLacquerIn a general sense, lacquer is a somewhat imprecise term for a clear or coloured varnish that dries by solvent evaporation and often a curing process as well that produces a hard, durable finish, in any sheen level from ultra matte to high gloss and that can be further polished as required...
instead of paint; lacquer was very rarely if ever used on prints. - Nishiki-e (錦絵, "brocade pictures") - a method in which multiple blocks were used for separate portions of the image, allowing a number of colors to be utilized to achieve incredibly complex and detailed images; a separate block would be carved to apply only to the portion of the image designated for a single color. Registration marks called kentō (見当) were used to ensure correspondence between the application of each block.
19th century
| The modern revival of colour-printing from wood-blocks, inaugurated by Whittingham, Leighton, and others, owes its full success to the energy, enterprise, and artistic skill of Edmund Evans. It is this printer that we have to thank for the delightful coloured plates by Caldecott, Greenaway, and Crane, that during the last thirty years have won the affection of old and young. |
| —English coloured books (1906) |
In the 19th century a number of different methods of color printing, using woodcut (technically Chromoxylography
Chromoxylography
Chromoxylography was a colour woodblock printing process popular from the mid 19th to the early 20th century, commonly used to produce illustrations in children's books, serial pulp magazine such as mysteries and romances, and cover art for yellow-backs and penny dreadfuls.In the 19th century...
) and other methods, were developed in Europe, which for the first time achieved widespread commercial success, so that by the later decades the average home might contain many examples, both hanging as prints and as book illustrations. George Baxter patented in 1835 a method using an intaglio
Intaglio (printmaking)
Intaglio is a family of printmaking techniques in which the image is incised into a surface, known as the matrix or plate, and the incised line or area holds the ink. Normally, copper or zinc plates are used as a surface, and the incisions are created by etching, engraving, drypoint, aquatint or...
line plate (or occasionally a lithograph), printed in black or a dark color, and then overprinted with up to twenty different colors from woodblocks. Edmund Evans
Edmund Evans
Edmund Evans was a prominent English wood engraver and colour printer during the Victorian era. Evans specialized in full-colour printing, which became popular in the mid-19th century...
used relief and wood throughout, with up to eleven different colors, and latterly specialized in illustrations for children's books, using fewer blocks but overprinting non-solid areas of color to achieve blended colors. Artists such as Randolph Caldecott
Randolph Caldecott
Randolph Caldecott was a British artist and illustrator, born in Chester. The Caldecott Medal was named in his honor. He exercised his art chiefly in book illustrations. His abilities as an artist were promptly and generously recognized by the Royal Academy. Caldecott greatly influenced...
, Walter Crane
Walter Crane
Walter Crane was an English artist and book illustrator. He is considered to be the most prolific and influential children’s book creator of his generation and, along with Randolph Caldecott and Kate Greenaway, one of the strongest contributors to the child's nursery motif that the genre of...
and Kate Greenaway
Kate Greenaway
Catherine Greenaway , known as Kate Greenaway, was an English children's book illustrator and writer, who spent much of her childhood at Rolleston, Nottinghamshire. She studied at what is now the Royal College of Art in London, which at that time had a separate section for women, and was headed by...
were able to draw influence from the Japanese prints now available and fashionable
Japonism
Japonism, or Japonisme, the original French term, was first used in 1872 by Jules Claretie in his book L'Art Francais en 1872 and by Philippe Burty in Japanisme III. La Renaissance Literaire et Artistique in the same year...
in Europe to create a suitable style, with flat areas of color.
Chromolithography
Chromolithography
Chromolithography is a method for making multi-color prints. This type of color printing stemmed from the process of lithography, and it includes all types of lithography that are printed in color. When chromolithography is used to reproduce photographs, the term photochrom is frequently used...
was another process, which by the end of the 19th century had become dominant, although this still used multiple prints with a stone for each color. Mechanical color separation, initially using photographs of the image taken with three different color filters, reduced the number of prints needed to three. Zincography
Zincography
Zincography was a planographic printing process that used zinc plates. Alois Senefelder first mentioned zinc's lithographic use as a substitute for Bavarian limestone in his 1801 English patent specifications . In 1834, the Frenchman Breugnot patented a zincographic printing process, producing...
, with zinc
Zinc
Zinc , or spelter , is a metallic chemical element; it has the symbol Zn and atomic number 30. It is the first element in group 12 of the periodic table. Zinc is, in some respects, chemically similar to magnesium, because its ion is of similar size and its only common oxidation state is +2...
plates, later replaced lithographic stones, and remained the commonest method of color printing until the 1930s.
Color separation process
The process of color separation starts by separating the original artwork into red, green, and blue components (for example by a digital scanner). Before digital imagingDigital imaging
Digital imaging or digital image acquisition is the creation of digital images, typically from a physical scene. The term is often assumed to imply or include the processing, compression, storage, printing, and display of such images...
was developed, the traditional method of doing this was to photograph
Photography
Photography is the art, science and practice of creating durable images by recording light or other electromagnetic radiation, either electronically by means of an image sensor or chemically by means of a light-sensitive material such as photographic film...
the image three times, using a filter for each color. However this is achieved, the desired result is three grayscale
Grayscale
In photography and computing, a grayscale or greyscale digital image is an image in which the value of each pixel is a single sample, that is, it carries only intensity information...
images, which represent the red, green, and blue (RGB) components of the original image:
The next step is to invert each of these separations. When a negative image of the red component is produced, the resulting image represents the cyan component of the image. Likewise, negatives are produced of the green and blue components to produce magenta and yellow separations, respectively. This is done because cyan, magenta, and yellow are subtractive
Subtractive color
A subtractive color model explains the mixing of paints, dyes, inks, and natural colorants to create a full range of colors, each caused by subtracting some wavelengths of light and reflecting the others...
primaries which each represent two of the three additive
Additive color
An additive color model involves light emitted directly from a source or illuminant of some sort. The additive reproduction process usually uses red, green and blue light to produce the other colors. Combining one of these additive primary colors with another in equal amounts produces the...
primaries (RGB) after one additive primary has been subtracted from white light.
Cyan, magenta, and yellow are the three basic colors used for color reproduction. When these three colors are variously used in printing the result should be a reasonable reproduction of the original, but in practice this is not the case. Due to limitations in the inks, the darker colors are dirty and muddied. To resolve this, a black separation is also created, which improves the shadow and contrast of the image. Numerous techniques exist to derive this black separation from the original image; these include grey component replacement
Grey component replacement
Within the CMY color space, any hue angle can be achieved by combining two of the three primaries. The intention of the third color is to move that hue towards grey , and is known as the greying agent. However, as that greying agent has an inherent hue of its own, it also shifts the hue as it...
, under color removal
Under color removal
Ideally, CMY without black should complete the printing process; however, sufficiently pure CMY ink has not been developed, and for this reason, black ink is used to add details and darkness in shadowed areas...
, and under color addition
Under color addition
In four-color printing , under color addition is a technique for darkening areas of the printed image by adding colored inks...
. This printing technique is referred to as CMYK (the "K" is the color abbreviation for black as "B" is already designated for blue).
Today's digital printing methods do not have the restriction of a single color space
Color space
A color model is an abstract mathematical model describing the way colors can be represented as tuples of numbers, typically as three or four values or color components...
that traditional CMYK processes do. Many presses can print from files that were ripped with images using either RGB
RGB color model
The RGB color model is an additive color model in which red, green, and blue light is added together in various ways to reproduce a broad array of colors...
or CMYK modes. The color reproduction abilities of a particular color space can vary; the process of obtaining accurate colors within a color model is called color matching.
Screening
Inks used in color printing presses are semi-transparent and can be printed on top of each other to produce different hues. For example, green results from printing yellow and cyan inks on top of each other. However, a printing press cannot vary the amount of ink applied to particular picture areas except through "screening," a process that represents lighter shades as tiny dots, rather than solid areas, of ink. This is analogous to mixing white paint into a color to lighten it, except the white is the paper itself. In process color printing, the screened image, or halftoneHalftone
Halftone is the reprographic technique that simulates continuous tone imagery through the use of dots, varying either in size, in shape or in spacing...
for each ink color is printed in succession. The screen grids are set at different angles, and the dots therefore create tiny rosettes, which, through a kind of optical illusion
Optical illusion
An optical illusion is characterized by visually perceived images that differ from objective reality. The information gathered by the eye is processed in the brain to give a perception that does not tally with a physical measurement of the stimulus source...
, appear to form a continuous-tone image. You can view the halftoning, which enables printed images, by examining a printed picture under magnification.
Traditionally, halftone screens were generated by inked lines on two sheets of glass that were cemented together at right angle
Right angle
In geometry and trigonometry, a right angle is an angle that bisects the angle formed by two halves of a straight line. More precisely, if a ray is placed so that its endpoint is on a line and the adjacent angles are equal, then they are right angles...
s. Each of the color separation films were then exposed through these screens. The resulting high-contrast image, once processed, had dots of varying diameter depending on the amount of exposure that area received, which was modulated by the grayscale separation film image.
The glass screens were made obsolete by high-contrast films where the halftone dots were exposed with the separation film. This in turn was replaced by a process where the halftones are electronically generated directly on the film with a laser. Most recently, computer to plate
Computer to plate
Computer to plate is an imaging technology used in modern printing processes. In this technology, an image created in a Desktop Publishing application is output directly to a printing plate....
(CTP) technology has allowed printers to bypass the film portion of the process entirely. CTP images the dots directly on the printing plate with a laser, saving money, increasing quality (by reducing the repeated generations), reducing lead-times, and saving the environment from toxic film-processing chemicals.
Screens with a "frequency" of 60 to 120 lines per inch (lpi) reproduce color photographs in newspapers. The coarser the screen (lower frequency), the lower the quality of the printed image. Highly absorbent newsprint
Newsprint
Newsprint is a low-cost, non-archival paper most commonly used to print newspapers, and other publications and advertising material. It usually has an off-white cast and distinctive feel. It is designed for use in printing presses that employ a long web of paper rather than individual sheets of...
requires a lower screen frequency than less-absorbent coated paper
Coated paper
Coated paper is paper which has been coated by a compound to impart certain qualities to the paper, including weight, surface gloss, smoothness or reduced ink absorbency. Kaolinite or calcium carbonate are used to coat paper for high quality printing used in packaging industry and in magazines...
stock used in magazines and books, where screen frequencies of 133 to 200 lpi and higher are used.
The measure of how much an ink dot spreads and becomes larger on paper is called dot gain
Dot gain
Dot gain is a phenomenon in offset lithography and some other forms of printing which causes printed material to look darker than intended. It is caused by halftone dots growing in area between the original printing film and the final printed result...
. This phenomenon must be accounted for in photographic or digital preparation of screened images. Dot gain is higher on more absorbent, uncoated paper stock such as newsprint.
See also
- Block printing
- FlexographyFlexographyFlexography is a form of printing process which utilizes a flexible relief plate. It is basically an updated version of letterpress that can be used for printing on almost any type of substrate including plastic, metallic films, cellophane, and paper...
- Letterpress printingLetterpress printingLetterpress printing is relief printing of text and image using a press with a "type-high bed" printing press and movable type, in which a reversed, raised surface is inked and then pressed into a sheet of paper to obtain a positive right-reading image...
- Offset printingOffset printingOffset printing is a commonly used printing technique in which the inked image is transferred from a plate to a rubber blanket, then to the printing surface...
- RotogravureRotogravureRotogravure is a type of intaglio printing process; that is, it involves engraving the image onto an image carrier...
- TypographyTypographyTypography is the art and technique of arranging type in order to make language visible. The arrangement of type involves the selection of typefaces, point size, line length, leading , adjusting the spaces between groups of letters and adjusting the space between pairs of letters...
Further reading
- Bruno, Michael H. (Ed.) (October 2000). Pocket Pal: A Graphic Arts Production Handbook Graphic Arts Technical Fndtn; 18th edition, ISBN 0883623382