Dear Shilesh:
Thank you very much for your reply. Very helpful, because it
clarifies the procedure. What you explain is what I was thinking
of, originally. But I was misguided by the descriptions in the QTR
User Guide and in the Calibration Guide (from the QTR website).
In the Calibration Guide it is stated on page 4: "But it's necessary to have all the relative
densities to a common value". So far so good. It is
logical and in line with what you explain. But on the same page it
is written: "Since the LLK will be
transitioning to the LK ink in the profiles the comparison is
most accurate by comparing the LLK ink to the LK ink not to
the K ink." Thereafter, a calculation is made in
which the relative density of LLK to LK is multiplied by the
relative density of LK to K, suggesting that the result of this
product would yield a more accurate relative density of LLK to K.?
- I am not convinced that the result is more accurate, because
for 3 gray inks the multiplication of fractions involves the
determination of 4 data points compared to the involvement of
only 2 data points for direct determination of the density ratio
of LLK to K. But it may depend on the method used to measure the
curves.
- More importantly, the multiplication of fractions only yields
the desired ratio of LLK to K if the functions L(K) for all inks
would be linear, i.e. if L(K) = a + b*K with some constants a
and b for all inks. But in our case the functions are
exponential of the form L(K) = a + b*exp(-c*K) with a>0,
b>0, c>0. For such functions the above multiplication of
fractions will NOT relate the gray densities to the common K
reference curve. Rather, in the example plot in my
original post for HFA-PR-308 and GCVT inks the multiplication of
fractions yields densities given by the abscissas of the
diamonds. The densities of the gray inks (all inks are gray
except for the toner) relative to the common reference ink K are
given by the abscissas of the squares (confirmed by you).
For an example, below I show part of my previous plot. C, LK and
K denote the cartridge slots. The carts are filled as follows.
C: 50%MK, LK: 100%PK, K: 100%MK. The LK cart with 100%PK prints
dark gray on matte paper. From the fits (thin curves) of the
data (circles) one obtains
- LK to K: fraction = 0.4821 (vertical red line)
- C to LK: fraction = 0.4786 (blue X)
- The product of these fractions is = 0.2307
(vertical blue dotted line)
- Compare this to the 'correct' value of the C to K density
(vertical blue line): 0.3307.
Concerning the global ink limit, you are correct. I was thinking
of using the Black Boost, because the slope of the L(K) curve for
the K channel is already very small at K=100% (with global ink
limit of 50%) and already L(K=100%) < 20 for the black ink.
You are correct regarding the toner. For the toner curve (open
blue squares on the dashed blue line in my original plot) I have
not calculated its relative density, because it requires a
separate curve in the Ink Setup to neutralize the print for all
gray values.
Thank you very much again!
Hendrik
Am 04.02.2025 um 04:35 schrieb shileshjani via groups.io:
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Hello Hendrick,
I hope this helps.
- First, I am not familar with dual K inkset described by Paul
Roark. But you mention that you printed at 50% limits on HPR
308, which is a coated MK paper, and as-such should yield Lmin
of approximately 17.7 to 16 (Dmax ~ 1.6 to 1.7) using single
MK. So you may want to look into, perhaps increase ink limit
to >50% or use "Black Boost" ~70%
- Please refer to picture below on how to set the density of
lighter shade of grey inks. Mind you this is only for 3 shades
of grey (K, LK, and LLK) as an example. Toner inks utilize the
density numbers also, but rely on "Limit" to determine how
much of the toner gets used. You that either visually, or
ideally with a spectro.
- Say at the calibration ink density, the solid blue line
represents density of K in 5% step file. Solid orange line
represnts LK ink, and solid green line is LLK ink. What you
want to do is to find Lmin of LK and LLK (dashed orange and
green lines respectively), and find the patch of K which
prints at the same density. In the case of LK (orange vertical
dash) gives you ~25%, and LLK (green vertical dash) gives you
~15%. These are the numbers you input into "Density." If I
undersand your graph, it seems you are looking at these
numbers from succesive lower density inks. That would not be
correct. All densities should be relative to K.
Hendrik Kuhlmann