Real World Computing

But what inks should you choose? Again, there's no such thing as a single standard black. Inks are physically mixed by the printer using a preset formula, and Pantone offers several blacks, most with a subtle but significant admixture of an undercolour. But you don't have to print only black: printing plates can deliver any colour ink you choose, and nothing makes a job stand out more than judiciously applied colour. Pantone offers more than 1,100 colour formulations. If you're using only a single colour, you'll want it to be a dark one for contrast and legibility, but that still leaves hundreds to choose from. Most design software now lets you choose a Pantone colour onscreen, but this is only the crudest of guides to the finished result. This is partly due to paper effects, as just discussed, but mostly because the colour space of your computer monitor depends on illuminated red, green and blue phosphor pixels, and it can't reproduce many of the bright Pantone ink colours at all.

If your budget stretches that far, you can add great impact to your design by including splashes of a second, or spot, colour, which requires two passes through the press for each printed page and two separate master printing plates. In the past, these plates were often produced by covering your artwork with transparent overlays and using white Tippex to hide one ink on each. Using a computer, your design software can nowadays automatically output such ink-based colour separations. Being able to automatically produce separations also lets you combine tints of each colour to generate more tints and shades. This might sound simple, but it's possible only thanks to an optical illusion, where the eye can't resolve the tiny separate spots of coloured ink and so averages them to generate a third in the eye.

Maintaining this illusion requires each tint to be represented by a greyscale halftone on its own printing plate, and the screens need to be angled such that during each pass, when ink is slapped onto the paper, their respective halftone spots (each less than 0.01in wide at 100lpi) butt up to each other without completely overlapping.

Once you've mastered mixing two inks, it's an obvious next step to mix three, and the number of possible colour combinations expands exponentially. In fact, by choosing your inks carefully, you can produce a full rainbow spectrum from red through to violet. The ink colours you need are the subtractive equivalents of the additive light-based red, green and blue primary colours; namely, cyan, magenta and yellow (CMY). Cyan and magenta together produce the illusion of blue; magenta and yellow produce red; and cyan and yellow produce green. Mixing different percentages of these CMY inks can reproduce 1,000,000 different colours (100 x 100 x 100), and by using a continuous halftone for each of the C, M and Y plates you can reproduce a full-colour photograph. With just three colour separations and three passes of the printing press, we've leapt from greyscale to full colour!

While three-colour CMY print is an extraordinary illusion, it's by no means perfect. With the effectively fixed grids of the computer screen and a single-pass inkjet printer, it's relatively straightforward to control the process of mixing colour levels so that the eye is tricked into seeing the desired end colour. By contrast, with CMY print, you have to overlay three separate press passes, and the most obvious danger is misregistration, where one pass doesn't quite fit on top of the others, which can lead to colour fringes and undesirable slivers of the underlying paper showing through where colours should be abutting. The solution to this is to slightly expand the area that one of the inks will print, usually the lightest, and set it to overprint the colour below it. With a program such as QuarkXPress, you can set the width of this trapping on both an ink-to-ink and object basis.

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