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Printing in Spot and Process Colours

Producing your work can be a challenge if you wish to use spot colours.

Common software like Microsoft Word and Powerpoint do not support spot colour printing but work in a RGB (a specification for full colour) model designed to be additive, like a computer screen or television, rather than a sheet of paper.

Even single professional software like Corel  Draw, Adobe Photoshop or Adobe InDesign can produce strange effects particularly where colours have been given different names (for example Pantone 300CV or Pantone 300CVU will print to different printing plates). From our point of view the biggest problem is that these programmes can describe the same colour differently (Corel Draw was particularly troublesome at one time, we could have 4 different versions of the same colour).

Offset Litho is printed from a set of printing plates. A set of plates must be produced before anything is printed. A plate is usually produced in a platemaking machine these days. Essentially a powerful laser burns the image onto a  photosensitive printing plate. The plate may have an aluminium or polyester base. The plate ends up with ink receptive and repellent areas (typically black and white respectively)

Every plate is monochrome (black or white); the printed colour depend on the ink used. A spot colour will usually involve mixing the colour from a series of base ink colours.

Printing in full or process colour usually uses four monochrome plates on a press which is set up with cyan, magenta, yellow and black Inks. Sometimes print can be in hexachrome where six plates are used (adding green and orange to the cyan, magenta, yellow and black to get a wider colour range or colour gamut. Colour plates produce the end result by effectively mixing the ink colours on the page from the base colours. Sometimes the job may involve a spot colour as well as the process set, where for a metallic colour like gold or silver or a corporate colour is used (like the NHS blue for instance) when the correct result cannot be produced from process colours.

Pictures are made up from a series of dots of various sizes of the base colours cyan, magenta, yellow and black. The size of the dots varies and they are normally printed next to each other. For printing we refer to the number of dots per inch. Normally a picture needs to be 300 dots per inch at its final or intended size so that the print process (offset or digital) retains enough quality to be acceptable to the eye. This is particularly important with larger images. It is easy to stretch an image on a computer but it is the original image that is important.

If you see a reference to stochastic screen that refers to the shape of the dots in the image. You can fit more dots in a given area so the image tends to appear sharper and brighter.