Separated worlds

Prepress mechanics

Separated worlds

The PostScript-based imagesetter produced solid black type and vector graphics, plus subtle greyscale tints and continuous- tone black-and-white photographs, but it wasn’t a local desktop device, so you needed to send your artwork to an outputting bureau. PostScript’s device independence allowed you to just send them the file on disk, but this meant the output was absolutely fixed, with no scope for any alterations by the printer. To enable printers to fine-tune your output for their press, it was better to send the original DTP and component image files – and although this might sound straightforward, it wasn’t. Not only did you both need to use the same DTP application, but crucially you needed the same fonts – this created a minefield of compatibility problems, especially for pioneering PC-based designers working with Mac-based printers.

Thankfully, Adobe eventually developed a superior alternative by reworking PostScript into PDF (portable document format), which can embed layouts, fonts and graphics into a single, cross-platform digital master. It still isn’t seamless and transparent, as the need for preflighting and print-orientated standard specifications such as PDF/X demonstrate, but it can be done and it’s a significant step forward.

A bigger problem is that the commercial print process bears little resemblance to local laser printing: imagesetters produce their output photographically, and often the resulting film or bromide has then to be transferred to a plastic or aluminium printing plate, although the latest Computer to Plate (CTP) setters cut out this additional stage. Developing the plate is an analogue photochemical process, so densitometer bars need to be output alongside your design to let the printer check exposure and make sure your 50% halftone really is 50%.

Producing a printing plate is only the first step: the image is now transferred from plate to paper. Today, relief printing plates with raised image areas that stamp ink directly onto the paper are rare and lithography is the norm. This works by making image areas of the plate repel water and non-image areas attract it by treating them with gum arabic and acid, so that ink sticks only to the image. In offset lithography, the ink image is transferred onto an intermediate roller that finally transfers it to paper, reversing it in the process. Your DTP application needs to be able to produce negative and mirror-reversed output to accommodate these various process variants.

Before the presses can roll, you have to decide what paper to print on, and every different paper stock has subtly (or not so subtly) different printing properties. Coated papers have a layer of pressed kaolin clay that makes them slightly glossy, while uncoated papers are matt, and newsprint is usually recycled and very coarse. Ink absorbency rises on as you go from coated to uncoated to newsprint, causing the ink dots to spread out (dot gain) and making halftones look darker than intended, an effect that needs to be allowed for. On coated paper, solid black-and-white areas of a halftone may stand out undesirably, so print quality can be improved by reducing the overall tonal range by darkening 0% highlights and lightening 100% shadows.

Adding colour

Having covered these monochrome basics, we can finally start thinking about colour. We’ve seen that the paper isn’t just a neutral backdrop and there’s no single standard for paper white. If your paper is off-white, slightly cream or has a hint of green that will affect the look of your final print. What’s more, the same ink can look entirely different on coated, uncoated or newsprint stocks. Ideally, you’d see a trial print before the presses roll, but that isn’t generally practical. What you can do is consult a reference guide, typically based on the Pantone Matching System (PMS), that shows your chosen inks on typical coated and uncoated stocks.

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