Iterative proofing: Color management is changing — for the better

By Ron Ellis

 

The state of proofing has changed

There have been significant changes in the inkjet proofing world in the past year. In the past getting an accurate match depended on technique and a good eye. Tuning a RIP and inkjet proofer required a bit of black magic. New RIP technologies are taking the guesswork out, and making the process of matching a press or proof more automated. The result is that these new technologies are making it easier to hit a proofing target with certainty. While not any faster, the results of the process are based less on guesswork and visual comparison than on mathematical matching. The results of this were dramatically apparent in the International Prepress AssociationŐs Proofing Round up conducted in June. In the tests, two systems using iterative non-ICC (International Color Consortium)  based methods leaped ahead of the competition. The results and feedback from those who install proofing systems confirms that these new methods are a dramatic improvement over previous proofing systems. Both CGS and GMG proofing products appear to have jumped ahead of the competition. (More information on the IPA Proofing RoundUP can be found at http://www.ipa.org/webinars/04roundup.php3)

 

How proofing systems are tuned

Both now and in the recent past, most inkjet proofing systems are based on ICC profiles. These traditional ICC-based RIPs all use a combination of profiles to achieve the desired result, with some being more accurate than others.

Here are the typical steps involved:

1.      To create a profile of the proofing paper;

2.      To create a profile of the system to be matched (such as Matchprint, Approval, etc. or press sheet;

3.      Use of the RIP and the two profiles to make the conversions for the proof;

4.      Output the proof and evaluate against the system to be matched;

5.      Make visual corrections until the proof achieves an agreed upon visual match.

The last step is based upon subjective examination of the proof and correction. If two people see color differently, or if the person approving the color has color deficits, then the color will never be accurate. The profiling is often done with an IT8 or other target that contain a number of patches.

If the profiling steps are not done correctly (and every color consultant has their own magic recipe for ink limiting, total ink limits, black generation) then the profiles may be so far out that a good visual match is not even possible, even after extensive corrections.

Very often during this correction process the color can go out even further as false missteps are taken. After all this, the proof can be mathematically compared with the target. Using this method, a measurement of both systems is taken and a deltaE is calculated. DeltaE is the numerical value specifying the distance between two colors in a color space. The value is an indication of the difference in colors, with the color space and the math used to calculate the color with the DeltaE playing a major role. In the past a deltaE of three or less was considered an acceptable match. Using the new iterative techniques most installs of proofing systems are now showing a deltaE of less than one. Less than one is so close that there is often no visual difference. (With a deltaE of three you can normally see a visual difference.)

 

A new way of tuning proofers

               During the IPA Proofing Round up, two systems leaped ahead of all the other proofing systems. These were GMG Color Proof and CGS Oris Color Tuner. Both had a deltaE of under one. (Many of the proofing systems that hold market share showed poor results). Both GMG Colorproof and CGS Oris Color Tuner work with an iterative process that autocorrects the color differences until the deltaE is below one.

With these systems the paper and match are calculated by the software rather than using an ICC profile. Rather than using third party software to create the profile, the color management is handled directly within the RIP software. A good example is CGS Oris Color Tuner, which calls the process Automatic Printer Calibration and Automatic Color Calibration.

When using Oris, a color target such as the ECI target containing a large number of patches is recorded in order to create the paper match (rendering color as accurately as possible on the paper). The software then compares the readings to the reference patch data and produces a deltaE. The software then calculates corrections for all the patches that are not a match, and another corrected patch set is output and re-recorded.

The process is repeated until the deltaE for the paper is less than one. Once the paper profile is accurate, the match (Matchprint, Approval, presssheet, etc.) target is recorded and the process is again repeated until the match target is again under a deltaE of one. At this point you have a very accurate match, and the visual comparison is printed and compared to the Matchprint, Approval, presssheet, etc. If there are any visual differences a slight correction can be made, however, unlike the old way, instead of making five or six major edits, this usually results on one or two very minor edits, if any.

This method is not necessarily faster than the old way, but the results you get are much closer and are more predictable. With the resulting low deltaE there are less colors out of whack. One thing that makes this auto correction process possible are faster spectrophotometers like the GretagMacbeth ICCcolor, and the soon to be released X-Rite DTP 70. The process usually requires reading a large patch set such as the ECI chart (1500 patches) over and over up to six times or more. Feeding by hand with an older spectrophotometer strip by strip could be very time-consuming and frustrating. These new devices allow you to read these charts very quickly, allowing you to get the chart data into the software quickly so you can rerun and reread. The GretagMacbeth ICCcolor can read the 1,500 patches of the ECI chart in about 10 minutes. Combined with the autocorrect functions of the software, both GMG Color Proof and CGS Oris Color Tuner can make color matches that are nearly impossible with the current versions of other software.

               This is not to say that other software will not be able to do this as well in the near future. EFI Colorproof software (formerly Best) can be purchased with a premium option that performs the same iterative process using ICC profiles and the ECI chart. The EFI premium option is the only ICC-based iterative process on the market, and shows promise. Seeing the lead taken by GMG Colorproof and CGS Oris Color Tuner, many other companies are looking at these methods.

               Inkjet proofing is getting closer and closer, and in large part this is due to the new methods that take away some of the subjective aspects of the process and rely more on math to get to the point where the visual edits are undertaken. While the visual edit may always be needed, just because everyone sees color differently, the days of doing hours of edits to obtain a match may be over, and thatŐs good news for everyone.

 

About the author: Ron Ellis is a prepress consultant specializing in workflow training and integration. He worked in the commercial printing industry for 18 years and brings a strong background to all aspects of prepress. He has consulted on numerous CTP installations and he provides color management, integration, training, workflow development, and troubleshooting solutions to the graphic arts community. He can be contacted at 603-498-4553 or through his web site at www.ronellisconsulting.com.