Profiling for CMYKRGB media presents additional considerations especially because you're dealing with 700% ink load. Pay particularly close attention when setting ink restrictions and ink limits. Also, it is vital to choose an ICC workflow for this type of printing. Anything else will yield poor quality output.
Ink Restrictions
On the Advanced Ink Restrictions tab, use the drop-down menu to select a CMYKRGB swatch, either: "darkinkrestrictions CMYKRGB" or "Ink Resrtiction Swatch CMYKRGB". Turn off the light inks (lc,lm, lk and llk) by setting the value to 0 and print the swatch.
There are two valid methods for determining where to set the Ink Restrictions.
The first way to set the ink restrictions would be to read the swatches and use the "Chroma" method. When you see a change in the chroma value, or when a "hook" appears in the plot, set the ink restriction value to the percentage just before the chroma shift begins. This method is really good for the RGB channels because there are not industry standards for density values using these colors.
Another method is to evaluate the swatch by using a densitometer to find where the highest density begins to drop. By measuring successive patches from the 100% level and moving down you will find a point where the density (volume) begins to change. For example: If you read 100%, 98%, 96%, 94%, 92%, & 90% and find that the density for 96% is the same as for 100% but the reading for 94% is lower, 92% lower still, etc. then you would set the ink restriction for that color at 96%. Once you have determined your dark ink levels, set the light inks to approx 30-50% of the darkest level.
Print the "Ink Restriction Verification Swatch CMYKRGB" swatch to verify the settings for any light inks. For more information about setting light ink restrictions click here. Pay careful attention to steps 6 & 7 in the How To section.
Calibration
When linearizing use the Basic Density Curves option to create target densities.
Ink Limits
For setting ink limits you must use the Advanced option.
See Ink Limits for additional details.
When you look at row 1 you need to also look at the rows "min 1,2,3" and basically choose the "Lowest Common Value" where the patch is acceptable in both places. Repeat these steps for rows 2 and 3 which also reviewing the rows "min1,2,3". When you decide a column value for the row , it must also meet the minimum requirements for bleeding, artifacting, etc...within the rows labeled "min".
For section 1, 2, & 3 set the ink limit for the group to equal to the highest low ink limit for that section. That may sound confusing until you consider that you want to set the ink limit as "low as you must but as high as you can..." The ink limit for sections 1, 2, & 3 is linked to the ink limits for Red, Green, & Blue.
For sections 1 set the ink limit to lowest that you would set for Green or Blue; for section 2 set the ink limit to the lowest that you would set for Red or Blue, etc.
When you evaluate sections 4, 5, 6 - the same rules apply. You want to review and set the limit for that section to the lowest acceptable value for the entire section and it must also be a valid number when reviewing the rows labeled min(4,5,6).
Set the ink limit for section 7 to the limit needed to minimize bleeding or puddling while still realizing that some bleeding or puddling may occur. This will still be acceptable.
ICC
When printing the ICC swatches it is very important that you use the CMYKRGB swatches. Click > Setup > Options and choose the swatch from the list. You can also check Scrambled Swatch. Build the ICC profile with the default options.
Improvements in the latest version
We have made big advances in our color engine for CMYKRGB and CMYKOG since version 7 was first released. Here are a few printer specific observations.
- The color is more consistent between printers, media, and ink sets. Comparing identical prints from CMYK, CMYKOG, and CMYKRGB printers, the overall color balance and accuracy are very consistent, with the added advantage of the extra gamut of the n-color printers.
- There aren't really any extra special skills needed to profile with the extra inks. Just be aware that there will be extra ink load, and restrict and limit appropriately. Using the Measure Tool in Media Manager to determine maximum chroma and density helps to find a good spot to set ink restrictions, but it is always a good idea to trust your eyes as well. There are separate ink restriction swatch for OG and RGB.
- For solvent printers like the Epson GS6000, it is best to use the basic curve media model. You may need to set n-factors on the OG inks around 1.5 to help the ink limit.
For aqueous printers like the HP Z3100 or Canon iPF8100 with CMYKRGB inks, you may be able to use the advanced grayscale. - When ink limiting, the biggest difference is the ink combinations that will appear on the ink restriction swatch. There are still only seven zones to set in the advanced ink limit, but the lowest setting for some zones will determine the ink limit for the extra OG/RGB combinations. For instance, the lowest value set for zones 1-3 will be the ink limit value used for the single O and G channels.
- In general, it is best to use the ICC profile swatch designed for the extra ink channels. It should appear at the top of the list for ICC swatches to print. You may notice that some of the patches bleed, even when you set your ink limits so that there was no bleeding. Max has said to just let those patches dry and read them in anyway.
- For ICC build settings, it is best not to use interpolation, since RGB/OG profiles are calculated based more on a color model and less on direct calculation of points (the extra dimensions would make it prohibitively long to build a profile with direct calculations). Interpolation MAY help you achieve better shadow definition, but it may affect brighter colors on the edge of the gamut. Note also the building with interpolation may double the amount of time it takes to build the ICC. (See Build Times below)
- For standard OG/RGB ink sets, it is not recommended to use the new "Dynamically Generate Ink Separations" options, as the static separations build into the software are already based on these ink types. If it is used, it may cause loss of saturation in areas related to the extra inks in the printer (i.e. reds, greens, and blues at the edge of the gamut may become less saturated).
Build Times
Some approximate ICC build times:
(based on 7.2 alpha III on a Dell Optiplex 745 dual core with 3GB ram on XP Pro)
CMYKRGB ICC without interpolation: 25 minutes
CMYKRGB ICC with interpolation: 1 hour
CMYKOG ICC without interpolation: 10 minutes
CMYKOG ICC with interpolation: 20 minutes
On some slower machines, CMYKRGB profiles may take several hours to build, even without interpolation.
FAQ
The best way to maximize the use of RGB inks is to determine the ink restrictions based on the chroma method. This will ensure that the maximum chroma for RGB inks is considered when creating secondary colors in the ICC.
During the Ink Restrictions step, remember to print the swatch labeled "CMYKRGB" and choose an ink percentage for your primary colors where the chroma value increases until the value begins to decrease or "hook".
Q: Do the RGB inks replace full strength CMY at some stage in the profile for printing secondary colors, or are the CMY inks only used for printing C, M, Y and composite black?
A: RGB inks are used for printing secondary colors when the chroma value is higher than what the two color combination can reproduce using CMY inks alone. RGB inks do not replace the secondary two color combination entirely, but is used to enhance the color chroma and produce the maximum possible gamut. This process uses a fixed separation algorithm for all CMYKRGB printers. CMY inks are only used for printing composite black.
Q: Is it the case that only dilute CMY inks used to form low density secondary colors, or are RGB involved here as well?
A: Low density secondary colors are created with either a CMY combination or RGB + CMY inks. The inks that are used to create the color are defined by the greatest chroma value. For example if a green color value that has a greater chroma using C + Y + G ink than a C+Y ink combination, C + Y + G ink is used to reproduce that color.
Q: Do we have any control over these processes in the profile build?
Currently the customer cannot modify the fixed separation set-up for RGB inks anywhere within the profiling process. It is important to note that you print the ICC swatches labeled "CMYKRGB" in order to create a multi-color profile. These swatches have been optimized for the multi-color separation processing. If you use any of the other set of ICC swatches, the processing uses CMYKRGB, but you are only creating a CMYK profile.
