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Printing In order to be printed, colour photographs need to be separated into primary colours. The primary ink colours are called "Four Colour Process", the colours being cyan (bluish), magenta (pink/reddish), yellow and black. Similar inks are used in colour inkjet printers.
Printing presses can only print solid colour in the image area, while no ink prints in the non-image areas. To achieve the required result the photograph is converted into a pattern of very small and clearly defined dots of varying size. This process is called screening. The printed result is an optical illusion relying on the eye to mix the dots of the four process colours. The pictures displayed right show the final result (top) with the four colour process separations underneath. Amounts of the four process colours differ across the image combining to give the final mix of colours. Other Ink Colours
Special colours are those where four colour process is not used. Examples being metallic, fluorescent and Pantone (PMS) inks. Four colour process cannot adequately simulate all special colours. At times results can be quite similar, but in many cases it is not accurate enough and additional special colour inks are used to achieve the required result. Short for Cyan-Magenta-Yellow-Black, and pronounced as separate letters. CMYK is a colour model in which all colours are described as a mixture of these four process colours. CMYK is the standard colour model used in offset printing for full-colour documents. Because such printing uses inks of these four basic colours, it is often called four-colour printing. In contrast, display devices generally use a different colour model called RGB, which stands for Red-Green-Blue. One of the most difficult aspects of desktop publishing in colour is colour matching — properly converting the RGB colours into CMYK colours so that what gets printed looks the same as what appears on the monitor. Computer monitors emit colour as RGB (red, green, blue) light. While all colours of the visible spectrum can be produced by merging red, green and blue light, monitors are capable of displaying only a limited gamut (i.e., range) of the visible spectrum. Whereas monitors emit light, inked paper absorbs or reflects specific wavelengths. Cyan, magenta and yellow pigments serve as filters, subtracting varying degrees of red, green and blue from white light to produce a selective gamut of spectral colours. Like monitors, printing inks also produce a colour gamut that is only a subset of the visible spectrum, although the range is not the same for both. Consequently, the same art displayed on a computer monitor may not match to that printed in a publication. Also, because printing processes such as offset lithography use CMYK (cyan, magenta, yellow, black) inks, digital art must be created as CMYK colour or must be converted from RGB colour to enable use. Digital art that is comprised of spot colours (e.g., special colours: any colours that are not CMYK process colours), generally require conversion to the CMYK colour space to enable file use. Because colour gamut's for spot colour libraries, such as those associated with the PANTONE MATCHING SYSTEM, usually extend beyond the ranges of the CMYK colour gamut, some spot colours may not be represented effectively using CMYK process inks. CMYK is a subtractive colour model used in colour printing. This colour model is based on mixing pigments of the following colours in order to make other colours:
• C=Cyan
• M=Magenta
• Y=Yellow
• K=Key (black) The mixture of ideal CMY colours is subtractive (Cyan, Magenta and Yellow printed together on white produces black). CMYK works through light absorption. The colours that are seen are from the part of light that is not absorbed. In CMYK magenta plus yellow produces red, magenta plus cyan makes blue, cyan plus yellow generates green and the combination of cyan, magenta and yellow form black. Because the 'black' generated by mixing the subtractive primaries is not as dense as that of a genuine black ink (one that absorbs throughout the visible spectrum), four-colour printing uses a fourth, black, ink in addition to the subtractive primaries yellow, magenta and cyan. Use of four-colour printing generates a superior final printed result with greater contrast. However, the colour a person sees on a computer screen is often slightly different from the colour of the same object on a printout since CMYK and the RGB colour model used in computer monitors are different. RGB colour is made by the reflectance of light whereas CMYK works by the absorption of it. |
