Lab-to-Spectral: Create a characterization from an MX4 proof profile

To move from old, lab-based technology (GMG ProfileEditor) to new technology (GMG OpenColor), you have the possibility to generate characterizations from existing MX4 profiles. During import, the Lab values from the MX4 profile are converted into spectral data, creating a spectral characterization within GMG OpenColor.

1. Select the printer gamut or proofing condition

If you can specify the proofer gamut that was linked to the MX4 proof profile, GMG OpenColor will incorporate any manual corrections or adjustments contained within that profile. If the corresponding gamut is not known or available, these corrections will be ignored during import.

The path of the gamut file can be seen in GMG ProfileEditor on the first tab of the MX4 profile. If the gamut file is in the specified location, GMG OpenColor will find it automatically during the import. If it is not there, you can search for the file name on your file system. The file name might also give you a hint on the proofing condition, so that you can select a proofing condition from the GMG OpenColor database instead of the gamut file.

2. Calculate the transformation

In most cases, you can proceed with the basic settings and just click Calculate.

If you want to adjust the profile gamut, load a high-resolution test chart or a reference characterization, you can also calculate a characterization from the advanced settings:

• Characterization fulcrums: As default, all existing fulcrums in the MX4 profile are used. Generally, the data resolution of the profile is sufficient to achieve a good data quality for the characterization. If you observe quality issues, you can repeat the Lab-to-Spectral process using a test chart with more patches to increase the data resolution. You can select a high-res test chart for this purpose. Choose the chart that matches the one in the MX4 profile.  If you do not know or if you need a very high data resolution, you can select "MX4-HighRes-Import_UndefinedTestChart". The generation of spectral data could take longer, and the resulting characterization data will require a large disk space.
  
  When is it needed to load a high-res test chart? 
  MX4 profiles generally use fewer fulcrums than MXN profiles. There are cases where the MX4 fulcrums are not sufficient to describe the tone value behaviour in the profile. Increasing the fulcrums can increase the quality, however this will also increase the profile calculation times and profile size.
• Spectral characterization: Here you can manually select a reference characterization from the database as a kind of template for generating the spectral values. The best reference characterization is one that refers to the same, or most similar, printing process.

3. Result and validation

• Green check: Generation of spectral data successful. You can expect the characterization to be close to the true characterization (from spectral measurement values).
• Orange icon: Generation of spectral data successful. When calculating a CMYK proof profile from a project using this characterization, you can expect a good quality. However, a recombination of the colors with other colors is recommended only if the colors are not overprinting.
• Red icon: Generation of spectral data failed. This is a rare result and strongly indicates that there is something wrong with the MX4 profile.

Note: The checksum of the imported MX4 can later be looked up by clicking Edit > Info.

Limitations and recommendations

• PhotoProof profiles are not supported.
• MX4 profiles where one of the channels have been swapped with a different color such as Violet or Green will most likely not give useful results. However, if the swapped color is very similar to the default color, for example, Dark Brown instead of Black, you might achieve satisfying results.
• It is always better to have "real" measured spectral data than to create spectral data from an MX4 profile. If you have characterization data available or if you can measure test charts from the print production, please do so instead of importing an MX4 profile.
• Creating a characterization by converting an ICC profile, works the same way as described above, except that some options are not offered (gamut, checksum).