COLOUR PRINTING
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Colour printing Colour printing is the reproduction of an image or text in colour (as opposed to simpler black and white or monochrome printing). Colour printing involves reproduction techniques suited for printing presses 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-colour images, such as colour photographs, is referred to as four-colour-process printing, because four inks (CMYK) are used: three primary colours plus black. The “subtractive” primary ink colours are: • cyan (a bright blue) • magenta (a vivid red-purple) • and yellow. Two graphic techniques are required to prepare images for four-colour printing. In the “pre-press” stage, original images are translated into forms that can be used on a printing press, through “colour separation” and “screening” or “halftoning.” These steps make possible the creation of printing plates that can transfer colour impressions to paper on printing presses based on the principles of lithography.
Screening Colour printing can also involve as few as one colour ink, or multiple colour inks which are not the primary colours. Using a limited number of colour inks, or specific colour inks in addition to the primary colours, is referred to as “spot colour” printing. Generally, spot-colour 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 colour inks, much like paint, is nearly unlimited, and much more varied than the colours that can be produced by fourcolour-process printing. Spot-colour inks range from subtle pastels to intense fluorescents to
reflective metallics. Colour printing involves a series of steps, or transformations, to generate a quality colour reproduction. The following sections focus on the steps used when reproducing a colour image in CMYK printing, along with some historical perspective.
Likewise, negatives are produced of the green and blue components to produce yellow and cyan 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.
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 colour removal, and under colour addition. This printing technique is referred to as CMYK (the “K” being short for “key.” In this case, the key colour is black). Today’s digital printing methods do not have the restriction of a single colour space that traditional CMYK processes do. Many presses can print from files that were ripped with images using either RGB or CMYK modes. The colour reproduction abilities of a particular colour space can vary; the process of obtaining accurate colours within a colour model is called colour matching.
Colour separation process The process of colour separation starts by separating the original artwork into red, green, and blue components (for example by a digital scanner). Before digital imaging was developed, the traditional method of doing this was to photograph the image three times, using a filter for each colour. 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 magenta component of the image.
Cyan, magenta, and yellow are the three main pigments used for colour reproduction. When these three colours are combined 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 ink pigments, the darker colours are dirty and muddied. To resolve this, a black separation is also created, which improves
Inks used in colour 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 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 colour to lighten it, except the white is the paper itself. In process colour printing, the screened image, or halftone for each ink colour 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 halftone screens that create printed images under magnification. Traditionally, halftone screens were generated by inked lines on two sheets of glass that were cemented together at right angles. Each of the colour 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 highcontrast 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 colour 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.
The Modern Lithographic Process In offset lithography, which depends on photographic processes, flexible aluminum, polyester, mylar or paper printing plates are used in place of stone tablets. Moder n printing plates have a brushed or roughened texture and are covered with a photosensitive emulsion. A photographic negative of the desired image is placed in contact with the emulsion and the plate is exposed to ultraviolet light. After development, the emulsion shows a reverse of the negative image, which is thus a duplicate of the original (positive) image. The image on the plate emulsion can also be created through direct laser imaging in a CTP (Computer-ToPlate) device called a platesetter. The positive image is the emulsion that remains after imaging. For many years, chemicals have been used to remove the nonimage emulsion, but now plates are available that do not require chemical processing. Lithography press for printing maps in Munich The plate is affixed to a cylinder on a printing press. Dampening rollers apply water, which covers the blank portions of the plate but is repelled by the emulsion of the image area. Ink, which is hydrophobic,
to clean impurities known as “hickies”.
is then applied by the inking rollers, which is repelled by the water and only adheres to the emulsion of the image area--such as the type and photographs on a newspaper page. If this image were directly transferred to paper, it would create a negative image and the paper would become too wet. Instead, the plate rolls against a cylinder covered with a rubber blanket, which squeezes away the water,picks up the ink and transfers it to the paper with accurate pressure. The paper rolls across the blanket drum and the image is transferred to the paper. Because the image is first transferred, or offset to the rubber drum, this reproduction method is known as offset lithography or offset printing. Many innovations and technical refinements have been made in printing processes and presses over the years, including the development of presses with multiple units (each containing one printing plate) that can
print multi-colour images in one pass on both sides of the sheet, and presses that accommodate continuous rolls (webs) of paper, known as web presses. Another innovation was the continuous dampening system first introduced by Dahlgren instead of the old method which is still used today on older presses (conventional dampening), which are rollers covered in molleton (cloth) which absorbs the water. This increased control over the water flow to the plate and allowed for better ink and water balance. Current dampening systems include a “delta effect or vario “ which slows the roller in contact with the plate, thus creating a sweeping movement over the ink image
The advent of desktop publishing made it possible for type and images to be manipulated easily on personal computers for eventual printing on desktop or commercial presses. The development of digital imagesetters enabled print shops to produce negatives for platemaking directly from digital input, skipping the intermediate step of photographing an actual page layout. The development of the digital platesetter in the late twentieth century eliminated film negatives altogether by exposing printing plates directly from digital input, a process known as computer to plate printing.
Offset printing Offset printing is a commonly used printing technique where the inked image is transferred (or “offset”) from a plate to a rubber blanket, then to the printing surface. When used in combination with the lithographic process, which is based on the repulsion of oil
and water, the offset technique employs a flat (planographic) image carrier on which the image to be printed obtains ink from ink rollers, while the nonprinting area attracts a waterbased film (called “fountain solution”), keeping the nonprinting areas ink-free.
Screen Angles
Present day
Why should screen angles differ
Offset printing is the most common form of high-volume commercial printing, due to advantages in quality and efficiency in high-volume jobs. While modern digital presses (Xerox iGen3 Digital Production Press or the family of HP Indigo solutions or Kodak Nexpress solutions, for example) are getting closer to the cost/benefit of offset for high-quality work, they have not yet been able to compete with the sheer volume of product that an offset press can produce. Furthermore, many modern offset presses are using computer to plate systems as opposed to the older computer to film workflows, which further increases their quality. In the last two decades, flexography has become the dominant form of printing in packaging due to lower quality expectations and the significantly lower costs in comparison to other forms of printing.
In offset printing, the screen angle is the angle at which the halftones of a separated colour is output to a lithographic film, hence, printed on final product media.
In offset printing the colours are output on separate lithographic plates. Failing to use the correct set of angles to output every colour may lead to some kind of optical noise called moiré patterns. These patterns look like some bands or waves that mostly lead to unpleasant look of the final print. There is another disadvantage associated with incorrect sets of angle values, i.e. the colours will look dimmer due to overlapping. The angles at which the halftone screens must be place in relation to each other in order to avoid a moiré pattern from forming. The common angles used are black at 45°, magenta at 75°, yellow at 90°, and cyan at 105°.
Pantone Matching System The accuracy of colour is critical in design. Because what you see on your monitor is never what will appear on a printed sheet, designers need a standardized colour key.
to be printed. If you want PANTONE Red 032 on shiny paper, then you would specify the colour in this manner: PANTONE 032 C, where C stands
for “coated”. You then have U, which stands for “uncoated”, It can be very frustrating to see the logo and then M, which stands for you worked hard to create look deep “matte”. You get: blue on the client’s letterhead, bluegreenish on his business card, and light blue on his very expensive envelopes. A way to prevent this is by using a standardized colour matching system, such as the PANTONE MATCHING SYSTEM. Though PANTONE is not the only colour standardization system, it is the most widely used and the one that most printers understand. Aside from being able to have consistency, PANTONE Colours allow you to use colours that cannot be mixed in CMYK. Pantone offers chip books that help designers see how colours look on coated, uncoated, and matte stock. PANTONE Colours are distinguished by numbers and a suffix. While the number indicates the PANTONE Colour itself, and is standard across all types of stock, the suffix indicates the media or stock, which affects how the ink is formulated to achieve the specific colour.
Same Colour, Different Looks The type of paper used, will affect the appearance of colours. In separate swatch or chip books, Pantone shows you how their colours look on coated, uncoated, and matte paper. Therefore you have the number of the colour (for example, PANTONE Red 032) followed by a suffix, which indicates on what stock your PANTONE Colour is meant
C = coated M = matte
U = uncoated
These three are the most important PANTONE Library abbreviations. You may, however, encounter the abbreviation CV followed by C, U or M. CV stands for Computer Video, which is the electronic representation of the PANTONE Colours. Now discontinued, but still seen in old versions of software, CV merely meant that the colour was an onscreen simulation. References: http://en.wikipedia.org/wiki/ Colour_printing