Digital BW, The Print

--- In DigitalBlackandWhiteThePrint@yahoogroups.com, Steve Kale 
<stevekale@b...> wrote:
> QTR creates a greyscale from predetermined inks and then linearizes 
this
> greyscale.  The process, in simple terms, is determine ink limits 
for each
> component of the greyscale; print a calibration page which prints a
> greyscale for each ink given the determined ink limits; measure the 
peak
> densities of the lighter inks and determine where they sit on the K
> greyscale, eg peak LK is 45% K, LLK is 15% K. QTR constructs a 
greyscale
> using these inks, their limits and the partitioning points.  Print a
> greyscale with this "ink curve".  Measure the L* and feed this info 
back
> into the curve as a linearization function.  The maths behind the 
smoothing
> of rolling from one ink to the next in the greyscale is hidden and 
well
> beyond me.  So if one sets the ink limit of LK higher, all else 
being equal
> it will print darker at peak and hence be 'partitioned' at a higher
> percentage of K.
> 
> 


In that case, it doesn't make that much difference (within reason). 
As long as you set enough density and overlap to cover the paper with 
no visible dots you should be fine. The only thing you might want to 
think about is how much ink is going onto the paper at a given point. 
With some inks, it is easy to get more ink than the paper can hold. 
This normally comes up with colors where you may be mixing three inks 
together to form the target. QTR may take this into it's calculations 
and compensate to prevent excess ink on paper.

Example:

You have a target (input) color of 40 C, 40 M, and 40 K (120%). Your 
3 color limit is set at 150%, because that's all the paper can hold, 
should be safe right? Now if your printer needs to use 30% LC, and 
20% C to make that portion of the color, and 40% LM, and 30% M, and 
40% K (total 160%), you are in trouble because the limiting may only 
see the 120% (the input value) and bleeding or puddles may form. The 
darker the full shade and the lower (more dense) the cross over 
point, the more light shade you need to fill in the dots. And with 
primarily only 2 inks going down at any one time, you're chances of 
going above what a paper can hold are greatly reduced. his example 
can easily happen when you linearize each channel first, and then 
have to match the light ink into the full ink's linearity.

Now I have seen one RIP that suggests you should not use more than 
about 50% of the available density from the light inks, just to try 
and avoid the above problem. I don't think that rule can be applied 
to every ink set though. And that was after a global ink reduction 
step, my printer runs best at around 60% maximum, or 40% global 
reduction, with individual limits done after that.