Expanded Gamut Calibration Workflow

Expanded Gamut systems (those using CMYK and additional inks, ie “nColor”) can be calibrated using Curve4/+ using a mixture of G7/+ calibration for the CMYK channels and SCTV calibration for the additional ink channels.

First, it’s important to note that Curve4/+ only accepts CMYK-specified data for target definition files (for scanning) and for measurement files. For SCTV calibration, it’s no problem having different colored inks, in any order, you just need to specify them (label them) as C, M, Y or K in the files. More about this below.

Second, Curve4/+ does not inherently “understand” nColor workflows or data. This is not a show-stopper limitation, it simply requires some specific handling of the measurements and the splitting of the calibration into the CMYK portion and the additional-ink portion.

Overall Calibration Workflow:

Determine the target images you want to use. The P2P51/75 and SCTV targets included with Curve4/+ will work fine (and the SCTVi target has the data properly labelled as CMYK).
Print them through your printing workflow, ensuring that the non-CMYK ink channels are assigned to the appropriate channels in the additional-ink target.
Measure the resulting target prints.
Create a G7/+ document for the CMYK data, import the CMYK measurements and generate the G7/+ curves as you normally would.
Create a separate document for the additional inks, setting its calibration method to SCTV. Import the additional-ink measurements, and generate the SCTV curves as normal.
Export the G7 and additional-ink curves from each document and import them into your printing system.

Using an nColor Target

If you want to use a true nColor target, for example the 7-Color ECG target from Idealliance, follow these steps.

For Step #2 above, ensure your printing system assigns the appropriate inks to each of the first four (CMYK) channels and the remaining additional-ink channels.

For Step #3, measure the resulting targets in software other than Curve4/+ as it does not handle non-CMYK target definition files or measurement files. ColorThink 4 (with PrePress Pack), Maxwell, or ColorLogic software can perform this task.

For Step #4 you’ll need to split the measurements into two files, both with their channels specified as CMYK. One file will contain the first four channels (the actual CMYK ink channels) and the other will contain the additional-ink channels, labelled as CMYK channels (inspect the SCTVi target file included with Curve4/+ for an example).

ColorThink 4 (with PrePress Pack) has a function to help with this separation. Here are the steps for splitting the file in ColorThink 4:

Split the CMYK data:

Open the file in ColorThink 4, it will open into the Worksheet.
From the popup menu immediately above the color list, select Change Color Space/Remap Channels
for New Space select CMYK and click OK
From the same popup menu, select Save List As..., ensure Source and Destination are both checked and, if it’s available, check Save spectral values. Click OK and save your CMYK data to a file.

Split the additional inks data:

Open the file again in ColorThink 4, it will open into the Worksheet.
From the popup menu immediately above the color list, select Change Color Space/Remap Channels
for New Space select CMYK
For each of the popup menus beside the CMYK channels, select an additional-ink channel, mapping the non-CMYK channels into the CMYK-named channels. If you have fewer than 4 additional inks, select <ignore> for the unneeded CMYK channels. Then click OK.
From the same popup menu, select Save List As..., ensure Source and Destination are both checked and, if it’s available, check Save spectral values. Click OK and save your additional-Ink data.

Now follow steps #4 through #6 above and calibrate the CMYK and additional-ink data separately.