QTR-Quadtone RIP

Hi Mel

**
>
> ...
> Let's say I'm going to make a neutral curve with QTR for Epson Premium Semi
> Gloss, with UT2 inks for the Epson 1290 printer and like the "supplied"
> curve for Epson Premium Lustre I don't use Black ink. I will use the
> existing profile for the Lustre paper as a template.
>
OK, here's the quick and dirty approach first.  Open the
UT2-EPremLuster-neutral-3 curve with the Curve Creator.  (By the way, I'm
Windows and don't know if Macs have the same type of interface.)   I'd
assume Epson made their papers to have about the same ink limits and
characteristics.  So, I'd simply go to the Linearization tab of the Curve
Creator, clear it, save the profile under a new name, and print a 21-step
test file with the new profile.  Then read that profile with the spectro and
put those Lab numbers into the Linearization boxes.

In short, start by just re-linearizing an existing profile.  That may be all
you need to do.

So, after printing out the test page "With a loupe or a densitometer I look
> for a patch where it appears that all the paper is covered with ink".
>

With the black ink, you want the maximum black.  With the lighter inks, you
don't need or necessarily want to be at their maximum.  You want to look for
where they are still efficiently contributing to the image and smooth.  Then
find the relative density for purposes of the cross-over.

So, with respect to setting the lighter ink limit, where you set it is
rather flexible.  I often just use the black ink limit for all.  Denser inks
virtually always hit their maximums before the lighter inks.

If you graph the results of any single ink separation print (or other
21-step test print) you'll see that the slope is continuously decreasing.
 That is, you get less density per unit of ink as you add more ink.
 Eventually, the paper becomes flooded and you'll see mottling/roughness in
the print.  You want to stay well below that point.

Whatever point you choose for the ink limit, the QTR "density" is found by
comparing the lighter ink at its limit to the black ink that is printed at
its limit.  So, if the highest density of the light ink is equal to the
black ink density at the 35% patch of the black ink, then 35 is the relative
density.  You don't need to consider or use any other intermediate inks to
find this.  It's simply the light ink relative to the black ink.


> Presumably in this case I am only interested in M, LM, C and LC.
>

Yes, but look carefully at the UT2-EPremLuster-neutral profile.  The C and
LC are "gray" inks and the M and LM are "toner" inks.  These are 2 different
cross-over progressions.  Look at the curves that profile has. The M and C
are, in effect, dual black inks -- one for each progression.

Note that I suggested making 2 totally separate profiles.  This allows you
to use the sliders to mix different tones -- even different for different
parts of the image.  For these two profiles, only 2 inks would be used in
each.  You'd have one "carbon" (see the existing PremLuster carbon profile)
and one "cold" (similar to the existing "cool" curve, but without any toners
added).


>
> "Now we must measure the density of each ink in terms of the next darker
> ink".
>

You only have 2 inks per (what I call) progression.  That is, the LC-C and
LM-M are in separate channels and do not cross over with each other.  I
always simply compare the lighter inks directly to the "black" (whatever is
the darkest in the channel -- gray or toner).


> "You are also deciding the order of the grays to be used during this
> process".
>
Again, it's simply LM to M, and then LC to C.  There is only one "black" and
one lighter ink per channel.


> I interpret this as M in terms of C
> LC in terms of M
> LM in terms of LC
>

No, there are 2 separate channels -- gray and toner.  Better yet, have them
be in totally independent profiles and mix them with the sliders.



>
>
> Am I correct in using the X-Rite 810 Visual channel setting for evaluation
> of these "coloured" inks ?
>

I use the Datacolor Spyder 3 print, so I'm not familiar with that X-Rite.
 If it'll give you a LAB output, the Lab L is what most use.

 Paul
www.PaulRoark.com


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