Learn About: Inks & Separations
Traditional printing methods require that artwork is translated to printing plates, one plate for each ink that will be used to produce the final images/type. While separations have long been the responsibility of the print shop, modern graphics software has given the designer the ability to greatly control separations.
While design work may not be directly performed by many of our clients, a basic understanding of inks and separations can help you with some of your print planning, and maybe even spark some ideas.
Overview
Traditional presses and, in most cases, modern digital print machines both rely on a fundamental principle of applying single inks in succession to a sheet of paper, such that the inks may at times combine visually to create particular effects. The variation in whether the ink is applied as a solid (100%) shape, or in tiny dots of varying size, is what allows us to print the countless elements that could make up a printed page: text, artwork, images, etc.
Standard Inks
The most commonly used library of inks is the Pantone Colour Matching System (PMS), which is a sort of recipe book for printers to create specific ink colours, and strives to ensure that a customer can get the same spot colour at different print shops.
Looking at the application of a single ink to the page (in this case, a dark blue) we find a couple standard variations in style:
Even though they appear different, each style demonstrates the use of a single colour of ink (a "spot" ink). The difference is in how the overall circular shape is formed: firstly, as a solid shape; secondly, as a circular region of dots (gives the impression of a lighter blue, even though it is printed with the same dark blue ink); and thirdly, as a gradient from blue to white.
Next, we can add a second ink (a medium red) and vary the ways these two interact:
In the first three, the shapes may touch but are still discreet. But the fourth style, duotone, has the inks interacting and appearing to make a third colour. This is the principle behind process colour (below).
Process Colour
Recreating full colour images in print is most often accomplished by printing "process." This is a 4-ink method sometimes noted as CMYK (
) each letter representing an ink: Cyan, Magenta, Yellow, and Key (black). As with duotones, above, process colour is a method that combines and varies the dots of these four inks, which have been found to effectively reproduce many colours typically seen in nature. This provides the solution for the vast majority of full colour printing.
There are also "Hexachrome" presses which add to CMYK the inks Orange and Green (yes, making 
); this broadens the gamutGamut: (in offset printing) the subset of colors which can be accurately represented in a given circumstance, such as within a given color space or by a certain output device. of colours that can be effectively reproduced, but requires a specialty press. Another method that aims to improve upon process printing results adds two lighter cyan and magenta inks (ergo 
); this can particularly be found in some retail printers designed for high quality photographic output.
The compromise with process printing is that there are limitations on what colours can be reproduced when combining cyan, magenta, yellow, and black. The possible colours are suitable for reproducing most photographs, for example, but may not be able to effect a particular Pantone colour (such as an official corporate colour). In this situation, it may be valuable to print process colour, as well as one or two spot inks.
Example
Here is our original image of an attractive bird:
If our print job is one colour (let's assume black ink only), we can render the image fairly well by using halftones (ink dots of varying size). Shown here is a grayscale image, plus a zoomed-in look at how the ink dots might appear close up.
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Alternatively, we might print in full colour (process), and be able to render the image quite true to the original. Notice how the cyan, magenta, yellow, and black dots are arranged to sometimes combine, other times not, but that the four inks give the appearance of many colours.
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But what if we're only printing with two inks, such as black and a corporate blue? Well, we can make a duotone image that uses the blue ink to accentuate the bird photo.
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This is a brief demonstration of how print technology is quite flexible, and that in planning a print job there are options to consider in choosing what inks to use.
Separations
The method of creating printing plates for each ink that will be used is aptly called separation. Using our process colour printed bird photo from above, the CMYK separations would look something like this:
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Each printing plate (four in total) would have the necessary dots so that these inks, when printed over top each other, will produce an image that appears like the original.
The grayscale (black only) image would just have one plate, while the duotone image would have two. There are also tritone images, quadtone images, and so forth.
RGB Colours
There is a fundamental physical difference between printing inks, as described above, and the colours you see on your computer screen (which are made of light). What you are looking at is something like the halftone dots of process colour, but instead the graphics are made of red, green, and blue points of light (pixels), or RGB (
). These three colours of light, much like the four basic colours of ink, have been found to be most versatile in combining to create the appearance of millions of other colors.
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With ink, you typically start with a white page and add inks to make colours, through the spectrum to black; with light, you start with an absence of light (black) and add light through the spectrum to white. So, for example, an image that is RGB may appear the same as one that is CMYK, but before it can be separated for ink plates it must be converted to CMYK.
This creates challenges for the modern, digital design process. There has to be some compromise between the degree to which ink printing is able to render colour, and how much 'predictability' there is for the designer using a computer to "get what she sees"—for the final printed piece to resemble how it looked on screen. It is a challenge both ways, though, since a computer screen cannot display such print effects as metallic inks, foiling, embossing, etc.
Needless to say, this is a complicated arrangement that provides no end of study. For the purpose of this article, it is simplest to advise that not all screen colours can be printed exactly, and that the planning of inks and separations on a computer requires some conceptualization of what will happen at press time.
Digital Printing
We look on another page at the differences between digital and offset printing, but some clarification is important when talking about inks and separations.
A digital printer will still typically use either one ink/toner (such as with a black laser printer) or four (CMYK, in fact, for most colour printers). But the technology behind digital printing in many ways removes the complications of dealing with separations, because it accepts any combination of grayscale, duotone, process, even RGB artwork, and does its best to print them together. There is still a separation stage, but it is largely automated.
Summary
The computer has been smartly integrated with print technology but will always provide certain challenges due to some of the fundamental differences between the two. Planning a print job on a computer, so that the eventual separation of colours into inks gives the desired results, is made easier as the software evolves—but this is still greatly helped when the user understands and can visualize how her design must translate to print. A good starting point is to consider what the finished print piece should look like, and what inks would be necessary to achieve that. The play between these limitations and capabilities is a constant consideration during the time leading right up to when the first copy comes off the press.
