Expanded Gamut Study Evaluates Spot Color Reproduction in Multicolor Printing

Expanded gamut (EG) printing requires a new approach in terms of color management software and, of course, in the number of colors on press. There are many solutions in this new technology area, but are they ready for prime time, and what kind of results can you expect?

(Spoiler alert—They are ready! And our data shows that you can expect to accurately reproduce 90 percent of a spot color library.)

EG printing expands the color gamut of conventional CMYK printing processes via the use of additional colorants, such as orange, green and violet (OGV) inks. While EG printing can be used for images and commercial printing, the biggest and most immediate ROI is in spot colors for labels and package printing.

Spot colors are generally used to reach colors that are outside the gamut of conventional CMYK printing, however, printing with them requires estimating ink quantities, followed by formulation and delivery of the ink and use of a separate printing unit with requirements for cleanup and changeover for the next print job. Excess spot color ink must be stored, creating inventory, or discarded in an environmentally sound manner. The whole process of spot color printing is inefficient—It is expensive and arduous and represents custom manufacturing, which is difficult to justify in a challenged economy.

The benefits of EG printing include the ability to reproduce a large number of spot colors without spot color inks, better accuracy on press, ability to gang jobs, fewer ink changes and washups. Barbara Braun-Metz, CEO, ColorLogic GmbH, puts it succinctly: “In essence, an expanded gamut system can avoid the overhead of printing with spot color inks, and replace them with a fixed set of expanded gamut inks while still achieving the customer’s requested color.”

Expanded Gamut Study

In this study, we evaluated the following software solutions:

  • Alwan — Alwan ToolBox 1.0.3, Alwan ColorHub 6.5
  • CGS ORIS — X GAMUT 2.0
  • ColorLogic GmbH — ColorAnt 5.1.1, CoPrA 5, ZePrA 7.04
  • GMG Color — OpenColor 2.2.1, ColorServer 5.0
  • Heidelberg — Prinect Color Toolbox 2019, Prinect PDF Toolbox 2019
  • Kodak — Kodak Spotless Software 2.1, Prinergy 8.3

As part of the broader investigation, testing also including PACKZ from HYBRID Software, i1Profiler from X-Rite and a RIP and DFE, EFI Fiery XF and HP SmartStream Production Pro, respectively. Every major supplier was contacted and invited to participate (We reached out repeatedly to Esko but were unable to secure their participation).

The study was essentially a “proof of concept” for EG printing, done using two digital printing systems. Tests were conducted on an Epson SureColor P9000 inkjet printer/proofer at Ryerson University in Toronto and an HP Indigo 7900 at HP’s Graphics Experience Center in Alpharetta, GA, shown in Figure 1. Both printing systems are available in CMYKOGV modes as confirmed under a microscope (see Figure 2).

(Due to the many different small jobs, we used an Indigo cut-sheet system, the Indigo 7900, however the same HP Indigo 7-color technology is used in a number of digital web presses for labels and packaging, such as the Indigo 6900 and Indigo 30000.)

Figure 1: The cut-away for the HP Indigo 7900 shows seven available ink stations. In our testing we used IndiChrome C-M-Y-K-O-G-V electroinks and UltraDigital 100# Gloss Cover media.
All images courtesy of Dr. Abhay Sharma

Pantone Library

In an ingenious experiment, the whole PANTONE+ Solid Coated spot color library, consisting of 1,846 spot colors, was placed as named spot color channels within a three-page PDF file and analyzed using PACKZ (see Figure 3). The test form was printed on the printers described earlier, using CMYKOGV colorants. The color accuracy of the reproduction for each software system was evaluated in terms of CIE Delta E 2000, between target L*a*b* values of the Pantone digital library vs printed values from each device.

Implications of the research are profound—If a hardware-software solution is able to reproduce 90 percent of the spot color library to < 2 CIE Delta E 2000, then it is possible to dispense with the traditional approach to spot colors and adopt an EG solution instead.

Color management is based on a characterization process involving a characterization chart, so in the first part of the test, each vendor’s characterization chart was printed on the Epson P9000 and Indigo 7900. The number of color patches in EG characterization test charts is still an area of proprietary, non-standardized working practice—There is no standardized IT8.7/4, as used in CMYK printing.

In narrow web flexography there are real estate limitations, and too many patches are a printing and measuring nightmare. The study found vendors use proprietary test charts in which the number of patches ranged from 875, the lowest number (for Alwan) to 3,528 (ColorLogic) and 3,536 (Kodak). A number of standards committees and organizations are involved in development of a standard test chart for CMYKOGV printing. The findings from this study can contribute to the ongoing work in that area.

Figure 2: Output for the Epson SureColor P9000 and the HP Indigo 7900 was examined under a USB microscope. Here we show the EG O-G-V colorants in each printing system. Note the clearly defined larger pattern of the Indigo screening.

Hitting 90 Percent

A practical tolerance in package printing is perhaps Delta E < 2, so the percentage of spot colors that are within this limit were recorded (see Figure 4). The gamut of the Epson P9000 CMYKOGV printer allowed for a large enough gamut and the software vendors were able to properly model its response and determine appropriate inking to create the required L*a*b* printed color.

The tests prove vendors are able to reproduce 89 percent to 94 percent of the PANTONE+ Solid Coated library on the Epson P9000 inkjet printer to a Delta E < 2. The colors that are not reproduced with an acceptable tolerance are likely to be out of gamut and therefore not physically realizable.

The results for the HP Indigo 7900 suggest that the vendors are able to reproduce 77 percent to 87 percent of the PANTONE+ Solid Coated library to a Delta E < 2. The smaller number compared to the Epson P9000 is due to gamut limitations of the inks and paper used in the Indigo 7900 production press.

Implications of these results are profound—As the systems were able to reproduce most of the spot color library, it is possible today to dispense with the traditional approach to spot colors and adopt an EG solution instead. In all of the previous discussion, the reader is reminded that small differences are not usually significant, especially given variations in printing and measuring.