Measuring Samples from the Forum 2018 Print Project at Different Locations
In the last article, Richard Black described how measurement devices and software tools are used to guide the press characterization as closely as possible to a standard, then how new aims are established for ongoing production work. The same tools and methods were then used to guide the production job back to the press characterization run.
This article will assess color accuracy from random print samples of the production run measured at a different location (my dining room table). Such measurements simulate the way a critical print buyer might assess results.
The Press Characterization: The Logical Aim
The goal of the production run is to hit the aims established in the press characterization run. This characterization run was extremely complex and consisted of nine colors plus white. The nine colors can be divided into seven process colors (CMYKOGV) and two spot colors (FTA Red and FTA Blue). Control elements can be broadly divided into two categories: solids and tints. For solids, the production run must match the press characterization run as closely as possible. For tints, the production run must match the specification to which the press characterization is “synchronized” as closely as possible (see “Synchronized Profile” sidebar). In assessing the production run, it’s logical to compare the solids first. The software used for comparison is shown in Figure 1. This is different software than what was used in the production run, but the measurement devices and the aims were the same.
Results are shown in Figure 2. Six of the seven solids on the production run were less than a 1.2 Delta E 2000 compared to the press characterization run.
The results of the two spot colors are shown in Figure 3. At a Delta E 2000 of 0.70, the match to FTA Blue is excellent. At a Delta E 2000 of 2.1, the match to FTA Red is in the “in acceptance” range for most print buyers. Interestingly, as a side note, the match to FTA Red using expanded gamut (magenta + orange) was closer than using spot colors. Apparently, the standardization of CMYKOGV inks that is part of the expanded gamut (EG) setup process led to greater color consistency than the less standardized custom blending of bases in the inkroom.
Tint Matching: CMYK to G7
There were multiple control patches placed on the production job to enable the assessment of tint values. For this project, it was agreed to use G7 as the aim for CMYK tints and to use Linear Spot Color Tone Value (SCTV) as the aim for the “non-CMYK” tints (the spot color tints and the EG tints). With eight locations to measure on a single repeat, Figures 4a, 4b and 4c represent the “worst case” of the eight readings. These graphs show how closely the print sample measurements (points on graph) match the G7 aim (solid lines in graph).
As can be seen, the black printed heavy compared to the aim. If we were sure this was a systematic characteristic of black and not a one-time event, we would make a curve adjustment to the black to reduce the dots’ sizes for future pressruns. These points can actually be measured directly into the curve software, so that such an adjustment can be calculated from the measured data. The CMY tone values printed closer to the aim and would warrant no further manipulation. The CMY gray balance “suffered” from a CMY solid that was extremely far off neutral. As can be seen from Figure 4c, the A of the L*a*b* value of CMY solid was close to -14 (very green) while the B of the L*a*b* was close to +11 (very yellow).
While a true G7 advocate would establish a plan to get such values closer to zero, it’s interesting to note that this large imbalance had no negative effect on the production run. As can be seen, the curve adjustment established accurate neutrality up to the midtone (solid lines). Because 100 percent Gray Component Replacement (GCR) was used for both the 4-color and 7-color images, the large CMY imbalance from the midtone through the solid had no effect. This is because there are no large CMY overprint areas in a high GCR color separation.
Tint Matching: Non-CMYK to Linear SCTV
With the introduction of SCTV, the assessment of tint match accuracy for non-CMYK colors has become much more intuitive. This formula is now built into most of the common measurement devices and software. The beauty of SCTV is that the aim is linear. Unlike Murray-Davies Tone Value—in which a 50 percent dot must print to read about 67 percent in order to appear “halfway to solid” to the eye—with SCTV, a 50 percent dot must print to 50 percent in order to appear “halfway to solid” to the eye.
Figure 5 shows the measurement of the 50 percent tint patch for the spot color “FTA Blue.” The measured value is 51.8 percent. Comparing this to the desired value of 50 percent, we note this tint is slightly too dark.
Table 1 shows a comparison of the 50 percent tint patch aim to the 50 percent tint patch actually measured on the print for the five “non-CMYK” colors in the job. As can be seen, they all printed close to the aim, but they all printed a little darker than the aim. Combining this knowledge with the measurement from the CMYK tints, we note that all nine color tints printed darker than the aim. And since the profiles were synchronized to the aim, it can also be said that the tints of all nine colors printed darker on the production run than on the press characterization run. A curve adjustment should be made based on this data to “pull” future production run tints a little closer to the aim.
Assessing Color Accuracy of EG Tint Builds
In assessing the color accuracy of a printed job, color elements can be divided into two categories: process control elements and direct color elements. Process control elements are placed in control bars and serve the purpose of assuring that your press is consistently printing to the specification at which you are aiming. Such elements likely do not directly match any of the color in the job. The G7 “50/40/40” patch is an example of a process control element. Comprised of 50 percent cyan, 40 percent magenta and 40 percent yellow, it is designed to quantify all three colors in a single measurement. But rarely is such a tint build found anywhere in a production job.
Direct color elements represent colors in the actual job. In some cases, these colors are built into color control bars, in other cases the measurements are taken directly from the graphics on the job. It is the direct color elements that are the ultimate concern of the brand owner.