Colour management removes human error and matches on-press to proof
Tuesday, June 12, 2007
Print quality is not the differentiator it once was (although finishing quality probably still is, but thats another story). Image quality disqualifies, rather than qualifies, printers for certain jobs. Todays offset presses, allied to CTP, allow excellent reproduction to be achieved pretty straightforwardly.
Increasingly, printers are taking control of their colour with new technology, but the majority still don’t, relying on skilled press minders to achieve the result. The problem with this approach is that as press minders are not all the same in terms of colour vision and decision-making, results will vary.
The tools required to manage colour are relatively straightforward. All you need are a consistent proofer, press and proof colour measurement equipment – a densitometer and spectrophotometer – with a colour computer. This may sound a bit daunting, but it’s impressive to see new presses, linked to platesetters and proofing with automatic closed-loop control systems, setting up in just a few minutes. The technology can be applied to older machines as well, and can bring a great boost to productivity.
Proofs must simulate the visual characteristics of the production print, but off-press proofing uses different inks and substrates. Solid colours and dotgains vary because of the gloss, light scatter (within the print substrate or the colorant), metamerism and transparency of the proof. Inkjet proofs use an opaque substrate compared to a double-sided print on a lightweight paper. The result is the two methods have different colour profiles and colour management is necessary to make both the same.
Now, I have been bored senseless by the theory of colour management over the years, but it is an important topic, so here is my understanding. You determine the colour profile of a device by measuring a large number of colour patches with known amounts of CMYK across the whole colour gamut, and do even more at visually critical regions, such as fleshtones. Calibrate the proofer and on the press, make sure the plates are linear and run the same inks and conditions as normal production. (A common error is to make the press run too well; it will not then work in operation.) This gives a set of look-up colour tables for reference patches; this is the colour profile. Colour management tries to match these references between devices. A suitable computer can then adjust a file to give the same visual result.
Working back from the press means a printer can control all inputs to let the press run at its most effective, saving valuable makeready time and waste to match the proof. It also allows printers with many presses to run them consistently, matching UV and conventional print if required.
For print buyers, this is a very useful tool. Independent bodies push alternative standards that define printing conditions, including Gracol/Swop, Sicograf, Fogra, European Colour Initiative, and the ISO with its 12647 series. ISO 12647 specifies the technical properties of halftone printing – covering screening method, tone value increase (or dotgain), the colours of solids, overprints and the paper. Conforming to the specified values for the proof and press result in a good visual match, while measuring and recording prove consistency through a run. The result is proofs that match press results.
All pass sheets will be measured against a ‘traffic light’ system to the ISO 12467 standard; the sheet either passes or fails. This process improves customer satisfaction and reassurance, removing subjectivity in operation. A spectrophotometer means there is no dependence on individuals as quality arbiters. Smart printers have taken quality to the next stage, as a key part of productivity improvement to make their whole operation more effective.
Sean Smyth is an independent ‘techie’ providing support for organisations looking to apply technology effectively. Smyth can be contacted on firstname.lastname@example.org