Setting ink limits

[Doombrain]

Does anyone have any preferred methods of determining ink limits?

Some RIPs allow per channel, others a overall. Is it just a case of looking for bleeding and grouping or is there more to it?

TIA

[Ernst Dinkla]

Does anyone have any preferred methods of determining ink limits?

Some RIPs allow per channel, others a overall. Is it just a case of looking for bleeding and grouping or is there more to it?

TIA

I'm no expert on this either but bleeding is just one thing and can be observed with the right targets where small white and less contrasty lines in full ink areas etc will not print sharp enough. See some QTR (B&W Rip) targets I modified for control like that. http://www.pigment-print.com/Quad%20QTR/Index.html

In far more cases you will check the black for the point it reaches its maximum density and throttle the black ink channel with an ink limit that sets that Dmax at the 100% patch in the target instead of for example the 85% patch. More ink doesn't have to mean more density, that is more obvious in pigment inks than in dye inks.

For the different hues CMY(XXX) you check the maximum chroma, it can be at a lower patch than the 100% patch too. In unbalanced inksets there can be a huge difference between the inklimits per hue. Some RIPs have a balance slider to adjust it in an analogue way to ink limitation, in other cases you will have to find the balance with the ink limits if possible. Ink chroma and density will in that case both play a role then I guess.

After that it still can occur that with CMYK or CcMmYK printers there's a too heavy load of ink mixes somewhere from 50% upwards in composite greys. Either the diluted c and m should have a transfer point lower than at 50% or the black generation should be extended to below 50%. Both will degrade the print quality in a way. A lower total ink limit can also be used. Compromises.

All diluted inks like c and m and the extra grey inks in some inksets like K3, Quad, essentialy have more ink medium and lower density so will challenge any paper coating that can't handle high ink loads. The partitioning of the grey inks, and of the c>C, m>M inks has to be controlled that they shift to the higher density inks before the ink load gets too heavy. In balanced inksets that will work without problems on good paper coatings.

In N channel printers like the Z3100 the ink load of color mixes should in theory be reduced compared to CMYK printers as the extra hues + grey inks replace the respective CcMmY mixes  to a degree without degrading the image quality. That's a complex balance between replacing three hue mixes with equivalent six hue mixes and replacing composite greys with grey + black inks. The gamut differences between the Z3100 and the  Epson K3 versions are related to that.
On the Z3100 there's also the gloss enhancer "varnish" that can come into this total ink load.


Ernst Dinkla

Try: http://groups.yahoo.com/group/Wide_Inkjet_Printers/

[Scott Martin]

Back in the "old days", the thinking was that more density equaled more saturation, so we tried to put down as much ink as we could while avoiding bleeding and excessive moisture. Over time it has become obvious that maximizing densities can lead to two problems: hue shifting and saturation reduction.

With most pigmented inksets the hues start to vier away from pure cyan, magenta and yellow when too much ink is used. Cyans actually lean towards blue, yellows towards orange and magentas toward red. This hue shifting is particularly dangerous for press proofing where colorimetric accuracy is important. For this reason some RIPs choose to limit the inks at the base of the hue "hooks."

When saturation levels are graphed out it is obvious that saturation (or more accurately, chroma) does increase with density up to a certain point, and then actually decreases. This "chroma" method of determining ink limits is best for fine art printing as is the method I choose to use for fine art and proofing applications. While some RIPs like ColorBurst have tools for chroma ink limiting, others don't and you have to do this separately from the RIP and this is one of many tricks that good color management consultants perform on jobs.

Excellent question. Some other good questions are:

What's the best method for composite (not per channel) linearization (chroma, density or Lab)?
What's the best method for determining total ink limit (as opposed to per channel ink limits)?
What's is the best method for balancing per channel and total ink limits for overall quality?
What separation parameters are optimal for different inksets?
How should one determine optimal usage of additive color (RGB) inks?

As a side note, Pigment and UV-curable inksets are really easy to work with. Solvent inks are another matter altogether. Solvent inks allow for fairly high per channel ink limiting but low total ink limits. Since total ink limits lead to poor shadow coloration, one must lower the per channel ink limits to keep the total ink limits high, and this balance is a tricky one to perform quickly.