Our quest began in the summer of 2003. An important advertising client called to tell us their printer was having trouble matching our proofs.
As the technically oriented owner of Precision Color Inc, it was my job to investigate the problem. Our prepress company was supplying files and proofs to this printer, who had recently installed a CTP system. It was the time of color anarchy, as film and matchprint gave way to digital replacements, and each printer had its own ideas on color calibration.
This printer was cooperative and showed us the problem. Its presswork was sharper than our proofs. It seemed like an easy thing to fix, using tone reproduction curves. So:
- It printed our test form, which we measured, then built tone curves using a tool from Scitex
- As a check, we made a new proof, which should have matched the press sheet. To our dismay, it did not
- We double-checked our work and measured the new proof. The TVI values were very close to the press sheet, but the color didn’t match
I showed this to my friend, Chuck Spontelli, who was visiting from Bowling Green State University (BGSU), where he taught printing and color to students in the department of Visual Communications and Technologies (VCT). The prepress proof was a Kodak Approval with real halftone dots, transferred to the same paper as the press sheet—not a color-managed inkjet proof. Our prepress business was heavily invested in the Kodak equipment. We touted the advantages of real halftone dots and the option to use the actual printing paper. So, it was important to find out why our proofs didn’t match this presswork. Meanwhile, Chuck was looking for a research topic, and found this intriguing.
Calibration Tools
I recalled the Tone Reproduction and Neutral Determination (TRAND) technique we used to calibrate our drum scanners (see Figure 1). It was developed by Zenon Elyjiw and H. Brent Archer of Rochester Institute of Technology (RIT), who described it in a 1972 Technical Association of the Graphic Arts (TAGA) paper. It involves printing a test chart with many near neutral patches, identifying the gray ones visually, and using the density and CMY coordinates of those patches to build a scanner gradation. Using color management, I adapted this technique to make tone reproduction curves. I made a new approval proof using these curves, and it matched the press sheet very nicely.
All figures and images courtesy of William Birkett
The idea was simple. By building curves from a gray CMY ramp, we achieved a good reproduction of the gray axis. Anyone who’s made color separations with a camera or scanner (there’s still a few of us around) will tell you: That is the essence of good color. We had to make an International Color Consortium (ICC) profile of both proof and press sheet to build the curves. So, I refined the technique by using a tone ramp, where each sample contained equal amounts of CMY, rather than gray.
Curves built this way were nearly identical to those made with actual gray patches. Chuck and I tested this improved technique in December 2003, at Homewood Press, a small print shop in northern Ohio. The owner, Scott Dubuc, generously donated time and materials to help us (see Figure 2).
The results of this work were very encouraging and we wanted to make them public. So, we contacted TAGA and submitted an abstract for a paper. It was accepted and we got to work. We presented our paper at the 2004 annual meeting in San Antonio, TX. View that presentation.
This paper explained our technique from the standpoint of print standards, which were then based on density and dot gain. It was one thing to match a proof to printing, but that wouldn’t help someone set up a printing process from scratch. We felt improved standards were needed.
Standards & Datasets
My interest in standards grew from the realization that our prepress business depended on them. It was not feasible to have a unique color setup for each client. Matchprint proofs were our standard for many years, but CTP put an end to that. We were desperate to find a commercial print standard with authority.
When we talked to clients about print standards, the one they knew of was Specifications for Web Offset Publications (SWOP). In 1995, the Committee on Graphic Arts Technology Standards (CGATS) ran a press test to characterize SWOP printing. It published the measurements as the TR 001 dataset. If only we had an equivalent dataset for commercial printing.
Fortunately, this work was already underway. In March 2004, we purchased a reference press sheet from the Association of Graphic Solutions Providers (IPA), which was printed to General Requirements for Applications in Commercial Offset Lithography (GRACoL) specifications. CGATS published the measurements of this pressrun as DTR 004. We planned to use this as our commercial color standard. But the pooh-bahs of standards giveth and then taketh away. Because of technical errors, the DTR 004 data was flawed, and eventually withdrawn by CGATS.
The people doing this work regrouped and decided that future pressruns would have smooth tonal characteristics, based on linear CTP plates. Because of our TAGA paper, I was invited to comment on these matters. I attended the Leyda Brain Trust meeting in San Diego, CA in January 2005, where I shared ideas on how to create a set of press characterizations that would have a common appearance. These ideas, combined with our color matching technique, were adopted by Idealliance and eventually became the G7 method.
Meanwhile, Chuck and I wrote a follow-up TAGA paper, which was presented at the 2005 annual meeting in Toronto, Canada. To view it, click here.
This paper introduced two key elements of The Optimal Method. We fitted measurements of color tone ramps to cubic spline curves using a simple computer algorithm. We were unaware that cubic spline curves are Bernstein polynomials, a class of functions used in computer modeling, and that far better methods existed for fitting curves to data. Despite our lack of technical polish, we had some good ideas for improving print standards and laid groundwork for The Optimal Method.
We received inquiries about our work and began using our calibration technique with consulting clients. The results were generally good, but not always. Sometimes, the color match was off in the shadows, due to oddly shaped curves in that region. We came up with a quick remedy, which was to build two sets of curves: one using the near neutral technique and the other using TVI. These curves were combined, favoring the near neutral curves in the highlights and the TVI curves in the shadows.
CTV to SCTV
This blending technique needed an alternative to TVI, computed from colorimetry rather than density. That led us to develop a metric called CTV, or “colorimetric tone value.” We thought this would be generally useful, and proposed it as a standard at the CGATS meeting in July 2005. The briefing can be viewed here.
There were a few comments after this meeting, then the idea sat dormant for nine years. With the help of Steve Smiley, it was resurrected as SCTV, or “spot color tone value.” In 2015, ISO 20654 was approved, making SCTV a standard for spot colors only. Today, a revision of ISO 12647-2 is under development that includes CTV for process colors, as an alternative to TVI, which was our original concept. Now, back to the main story.
At the ISO TC-130 standards meeting in San Diego, in April 2006, the US delegation proposed standard datasets adjusted to a common appearance using the G7 method. This was not well-received by the German delegation, which had just introduced a certification program built around TVI calibration. Idealliance was planning a similar program using their G7 method. A serious rift opened in international print standards, which continues to this day.
In the aftermath of that fateful TC-130 meeting, CGATS wanted to prove the near neutral technique was superior. We volunteered to work on that. At the CGATS meeting in Mesa, AZ in November 2006, we presented our findings. The presentation can be accessed here.