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Process and spot colours

This colour system is often wrongly understood, which is why we explain here first of all the difference between process and spot colours.

Process colours in Calamus should be understood as the four preset colours available for the whole Calamus Colour management system: C (Cyan), M (Magenta), Y (Yellow) and K (blacK or contrast). This does not mean arbitrary shades or tints of these colours, but strictly defined colour values that are used in the so-called Euroscale, which is uniformly used in Europe for four-colour printing.

Please note that at present it is not possible to simulate other process colour systems in Calamus, such as SWAP or similar. The colour separation in Calamus is only realized as Euroscale four-colour separations.

During colour separation, a continuous-tone coloured document is split up into four separate parts, one for each ink colour. In professional use these are each output on an imagesetter to a separate film, which is used to produce a printing plate for this colour. Different intensities of each colour are produced via the rasterizing process, which breaks up tones into a series of closely-spaced dots whose spacing (or size with some printing processes) determines the intensity of the printed colour – the further apart (or smaller) they are, the lighter the colour will be at that position.

A spot colour is a freely chosen colour that is added in a separate printing step (or even in a different printing works) to the four process colours C-M-Y-K. In general it costs less to print than separated process colours and may be used instead of them for documents that only use one (or a few) solid colours apart from black. But it can be used equally well together with process colours to produce large solid areas of colour.

Each spot colour requires its own film to produce a suitable printing plate and uses an ink that is pre-mixed to give just one desired shade or tint which is fed from its own ink reservoir on the printing machine. Unlike process colours it is printed solid, without a screen or rasterization. Note that it may be a metallic colour such as gold, silver or bronze, for instance; it may equally well be a firm's customized house colour. BT blue, Royal Mail red or German Post Office yellow are typical examples of house colours as spot colours.

A spot colour can also be a print lacquer, a clear coating that is used in some types of printing to add brilliance to printed colours. As this print lacquer often covers the 4-colour printing (though with printed packaging, for instance, some areas that will later carry adhesive address-labels may have to be kept uncoated), this printing process will require its own print-film.

In Calamus you can not alter the four process colours C-M-Y-K. The only thing you can change are the names that represent these colours. This may be useful when preparing separation films for foreign print works; for German printers you may prefer to use Zyan in place of Cyan and Gelb instead of Yellow etc. These colour names will be output in the margins alongside the picture during printing or imagesetting.

To create spot colours, first make sure an entry is selected in the colour list on the left (if necessary click on New colour to place one there), select Process /Spot colours in the Col.system popup and then click on the New button under the colour patch on the right. A copy of the currently selected process or spot colour will be created and added to the (central) list. You can now change both the name and colouring of this newly produced spot colour. Note that the colouring is only required originally for the screen representation, since each spot colour will be imageset as a separate film. So if you want to have a Gold spot colour, it is sensible to alter the colour name produced after clicking on New from, say Cyan #2 to Gold. After this switch from the Intensity to the Colour radio button to let you assign a CMYK colour mixture to this new spot colour. With gold it is recommended that you mix Y with M until you like the resulting colour in the colour patch.

But as soon as the radio button Colour is selected you can also take directly the already preset CMYK colour values of a palette colour in order to assign them to a spot colour. It may be quite sensible to create an HKS-colour, for instance, not as a palette colour (see below) but as a spot colour. This was you can choose subsequently during imagesetting whether the colour (say HKS 25) should be output to its own print film, or whether the preset CMYK colour values that form the basis of this palette colour as well as the spot colour should be output as colour separations instead. In that case no separate film would be output for HKS 25, but its colour portions would be distributed over the four C, M, Y and K films.

If you click on the Assign name button once you have achieved the desired colour mixture, the selected spot colour name will be included in the document colour list (at the left of the dialog); this saves you having to type in the spot colour name again in the document colour list.

If you want to use spot colours at a reduced the intensity (say 50 % BT blue or 25 % gold), proceed as follows:

First create a new document colour in the list on the left with New colour.

Next create a spot colour of the desired hue as described above and leave its intensity set to 100 %. Assign the name of this spot colour to the document colour. This includes the 100 % colour in the list.

Now copy the document list spot colour (click once more on the New colour button). The new colour automatically adopts all the values of the 100 % spot colour. In the Process / Spot colours part of the dialog you can alter the Intensity slider for this new colour to, say, 25 %. Now alter the name of the copied spot colour in the document list so that you can recognize that it has reduced intensity (for instance Gold 25 %).

It is not sensible to create the spot colour in the colour system itself (the list on the right) again and keep it there with an intensity of 25 %. When you later output that spot colour during imagesetting, the result would be not just one film for this spot colour (with different spot-colour intensities), but that several films would be produced. This is normally not what you want.

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Last updated on June 24, 2015

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