Computer to Plate Proofing

Digital proofing considerations

There are some basic considerations to consider before deciding on which proofing device to buy as a part of your direct to plate workflow. The first question to ask is whether you really need to obtain the exact same halftone screen dot on the proof as in the final print. In the past when using analog systems, you automatically got the same screening because your film imaged both the proofing material and the plates. But with a digital proofer, it’s not so straightforward. The digital proofer may not work at the same resolution as the platesetter or be able to produce the same screen dot and ruling.

The reason for insisting that the proofing device has the same screen appearance as the platesetter is to reveal a potential moir’ in the images (miss-matched screen angles). If you are concerned with the risk of moir’, you should narrow your search and consider systems that offer a true screen proof dot, sometimes called a halftone dot proof. Agfa, Best, Compose, Fujifilm and KPG are among the vendors of such systems.

This moir’ issue will not be a problem for printers using a stochastic screening system, and in this case it will be more important to achieve an accurate color proof. For printers who have made the transition to stochastic screening, there are more output proofing options available than there are for true-screened proofs. (And the proofing systems will cost less.)

The quality of the proof from a particular proofing device will depend greatly on the RIP being used at the front end. The RIP needs to support both Postscript and ICC profiles for accurate color. Any proofer for a CTP workflow should support ICC profiles, which capture the device’s characteristics so that color conversions are accurate. One of the most useful features of a good RIP and proofer is the ability to simulate the color output from a printing press. Once we create an accurate press profile, we can work it back to the RIP and proofing device to simulate the press output.

Soft and remote proofing

Soft proofing is also very critical in the proofing cycle so that both the customer and operator can see color accurate proofs on their monitors, all of which have to be carefully calibrated. In an all-digital workflow, soft proofing can be very useful, saving time and courier costs.

However working in a digital environment creates new proofing challenges, particularly when working with customers at remote locations. A remote proofing solution is vital, particularly in applications where time is short. Ideally, the same kind of proofing device should reside with the designers and document creators, as with the printer. The system needs to ensure that the local proof looks exactly the same as the proof at the remote site. EFI/BEST has a great product called Remote Proof that acts as a quality control checker, both in-house and at remote sites. (www.bestcolor.com)

Some systems, like Best’s Remote Proof, use the Job Definition Format (JDF) for the transfer of color information, which helps to ensure that the output from two proofers in different locations presents the same result. All necessary reference data can be stored in a file and sent to the remote location along with the job. The client prints the proof and reads it with a spectrophotometer, which provides a pass/fail grade.

Choosing your proofing device

What makes a printer qualify as a high-end color proofer is whether it has the color accuracy, resolution and predictability required to consistently and reliably represent what will appear on the press. If a color printer shows unstable behavior in the inkset, it cannot be used for contract proofs.

Inkjet printers are proving popular for proofing applications when correctly color managed and I believe that the Micro Piezo electric print-head technology has the edge over thermal print-heads. The key to inkjet proofing is the short to mid-term stability of the inks as well as the obvious color control required for the system. The Micro Piezo technology is more consistent and is not as affected by environmental conditions as thermal technology. Also, the thermal process requires that the print-heads be replaced frequently which means the system has to be calibrated on a regular basis or color distortions change can result.

Over the last two years Epson has greatly improved proofing qualities with the development of the Ultrachrome inkset. Ultrachrome inks are halfway between a dye and pigmented water-based ink and are very stable; with tests showing a Delta E shift of less than 1.1 over a 12-week period. A feature of Ultrachrome ink droplets is that it is contained in a resin particle that limits its spread when it is put on the substrate, thus enhancing Epson’s maximum resolution of 2,880 x 1,440 dpi.

I believe that Piezo inkjet printers are better suited to color proofing and higher-end color applications. Also, the Epson range of inkjets do have the limitation of being no wider than a maximum of 44 inches wide.

The success of inkjet CTP proofing depends on who installs and calibrates the printer and RIP. Profiling the RIP to the customer’s chosen proofing standard or press means narrowing down the color gamut to make it easier for the press operator to match the resulting inkjet proof.

High-End Screen Dot Proofers

Most high-end screen dot proofers use a high-end version of dye sublimation technology. The image is exposed onto some type of substrate that is subsequently transferred to the paper stock. KPG’s Approval proofing system is one of the best-known dedicated proofing systems in this class. It supports most high quality proofing output, including special inks and metallics. Other less expensive halftone proofers use software RIPs, such as EFI/BEST Screenproof and are based on inkjet technology, to produce halftone dots. Often this type of proofing systems uses the same RIP data as is sent to the platesetter. (PDF, TIFF/IT, Delta lists, Scitex CT/LW, Postscript, etc.)

For CTP applications however, it is important to invest in a stable, consistent and reliable proofing engine. While color laser printers are also attractive as digital proofers, the main concern is color accuracy and repeatability. While color laser printers are often fast and easy to operate, not all of them stand up to the high demands of color accuracy.

Color management standards

It’s important to get a RIP and proofer that is SWOP certified. SWOP (Standard Web Offset Printing) is one of the most widely used ink standards and there are several proofers that are SWOP certified. (Best, Epson, Canon, Creo, Dupont, GMG, HP, KPG and Oris.) This is an indication of the device’s output quality.