Re: Convert UV-excluded to UV-included
Re: Convert UV-excluded to UV-included
- Subject: Re: Convert UV-excluded to UV-included
- From: Andreas Kraushaar <email@hidden>
- Date: Wed, 23 Dec 2009 12:32:34 +0100
On 23/12/2009, at 10:21 AM, Mike Strickler wrote:
Hi Andy,
Thanks so much for writing. I am slightly handicapped at the moment
due to the late hour as well as brain fatigue: I am currently trying
to untangle the dense prose of one of your countrymen (his English,
not his German) as I edit his book--on design of superconducting
magnets, of all things. But I'll try to tackle this; comments below:
On Dec 22, 2009, at 11:21 PM, Andreas Kraushaar wrote:
Hi all,
thats why defined viewing conditions are extremely important (the
ISO 3664 in the latest 2009 revision takes care of that with a more
stringent UV metamerism index - so a controlled portion of UV light).
However, it is difficult enough to make two papers (with different
amount of OBA) matching there :-)
Since we will start working on that topic soon I would be
interested in your opinion how good the existing "problem
categorization" has been? In other words, do you think the main
problems are addressed in a nice (mutually exclusive, collectively
exhaustive) way?
1) printing RGB files on substrates with much OBA (and you don't
want the neutrals (R=G=B) being rendered yellow
I see your point, but it's equally true with CMYK files, when the
target a* and b* are both 0, for example a 50 40 40 0 gray in most
ISO print spaces. And this assumes (with some justification) that
the assumed RGB space is "gray-balanced" such that when R=G=B, a*
and b* are both 0. (If the user forgets to convert from his scanner
profile to such a working space--which can happen--this relation of
course doesn't hold. So can we just say that (whether RGB or CMYK),
when the assumed/assigned a* and b* values are 0 the appearance is
indeed neutral?
if you will. The point is that you have only one substrate with x
amount of OBA (to be compared against a mental references or of your
memories of a softproof etc).
2) you have a prima A, e.g. a proof (no or less OBA since you want
a light fast and permanent proof) and a print B, e.g. paper stock
with more/much OBA.
- How do you find out if a match is possible at all?
Good question! I am an empirical practitioner, Andy, so I would just
say that for me, in practice, I can usually predict an acceptable
visual match in paper white and lighter colors (dark ones generally
being no problem) when the proofing paper has sufficient reflectance
(high L*) and "blueness" (-b*). I'm speaking here of inkjet proofs,
where we can add a bit of cyan and/or magenta to match a bluer press
substrate. I know, for example, that proof made on a paper that
measures 95 0 0, or even 95 0 -2 (Fogra 39/Graco "white") I will
never be able to get an acceptable match to a print job run on McCoy
Gloss (95 0 -6?) since by correcting the hue mismatch (abs. col
rendering) I must darken the "paper" color and all the lighter
colors as I add cyan and magenta. But with a proof paper that
measures 96 0 -3 I can do it. (This presents a complication, as
these higher L* values generally imply higher levels of OBAs.)m The
other way around--matching more yellowish papers (often but not
always with low levels of OBAs) is little problem, as even
substantial amounts of yellow added to the proof "white" hardly
changes the L*. But as I say, this is a "seat of the pants" approach
only. It does seem to hold under a 3664-conforming booth, in other
words, I can generally match the appearance of a press sheet loaded
with OBAs, though in each case my proofing papers had modest levels
of OBAs. Obviously, if the proof papers themselves had high OBA
levels this could add some additional unpredictability to the process.
That is already part of the answer :-) I was looking for agreement in
the questions :-)
Since you have a combination of a self luminous and a reflective
part (for a paper with OBA) you have to find out if the "blue
hat" (the luminous part in the spectrum) results in lightness level
that can't be matched by the proof (with no "mac light":-)
- What is the best visual technique to modify the proof in order to
get a visual match (under controlled viewing conditions first -
such as a 3664 conforming light booth with adjustable UV).
See above answer for inkjet proofs. For laminate proofs one may of
course use the job stock itself.
- What is the best metrology to
a) measure the bispectral reflection (and if that is to much the
spectral radiance factor for the intended viewing condition - what
a telespectroradiometer would measure hitting on it)
I'll defer to the experts on this.
b) measure (paper specific) aim values (of whatever kind) to have a
visual match when those values (objectively) match
Huge problem. This is not my area, but I've noticed a couple of
things in practical work. One is that UV cutoff filters almost
always produce a greater disparity with visual impression than
software "filtering" of any kind. I include in this the "filter by
absence" approach taken by Gretag/X-Rite in the iSis LED source,
though as I said in my post this is not so bad as the cutoff filter.
Another thing I've experienced occasionally is a paper that appears
neutral but measures as quite blue (high -b*), undoubtedly owing to
OBAs. I imagine that there is some oversight in the way the spectral
measurements were converted to XYZ--perhaps including too much of
the deep violet where the eye's sensitivity is in fact not very high?
So it seems it's not just "how much" a paper fluoresces but "where"
and "how" that determine it's perceived "blueness" or "brightness."
3) what is a practical method for nowadays graphic arts use cases
to help the printer in adjusting the solid coloration on the press
aiming for an (ISO 12647-2 conforming) print accompanied by a
(FOGRA39 or JanColor or Gracol Proof)
I'm not sure I understand this question--are we now moving away from
the UV/non-UV question and into another area? If you're speaking of
running to SIDs, this is a fraught issue owing to printers'
discomfort with L*a*b* numbers. A good approach can be having the
consultant run to correct colorimetric values and then measuring the
solids with the "house" densitometer for the pressmen's target
Status E or T densities. Gray balance is another question, and the 3-
color grays must be assessed either by L*a*b* measurement or by eye.
One nice tool is an accurate proof of a press control bar to be
placed against the one on the press sheet, possibly with a little
chart that indicates which way they must adjust each color. My
friend Glenn Andrews advocates replacing the 50K with a 53K, as this
should match the 50 40 40 0, and any error can be clearly seen. I
should say that pressmen here often make the mistake of trying to
adjust tints and gray balance by raising or lowering solid densities
from their proper values rather than adjust their plate curves or
look into press problems such as ink-water balance and packing. We
want to encourage running presses "to the numbers" and keeping them
there rather than engaging in "custom" adjustments on a job-by-job
basis. In America we still very far from standardizing printing as a
manufacturing process, and whatever you can do to help will be
wonderful.
what problem scenarios did I miss?
Not sure.
if I touched on all your issues, I am fine :-)
sincerely
Andy
sincerely
Andy
Best regards,
Mike
On 22/12/2009, at 8:40 PM, Mike Strickler wrote:
Any formula you used would be specific to the illuminant and (if
applicable) the reflecting medium. Change illuminants (for
example, going from a spectro with a halogen bulb to one with an
UV-included LED source) and/or papers and your numbers won't be
valid. You'll also find that UV-filtered readings won't match
readings taken with a spectro, like the iSis, that uses for its
"filtered" LEDs that have no (or nearly no) UV component to begin
with. The conventional cutoff filters take out a bit of the visual
spectrum as well, so they result in a more "yellowish" reading in
comparison with the unfiltered reading--how different will depend
on how much UV fluorescence there is in the paper being measured.
------------------------------
Message: 2
Date: Mon, 21 Dec 2009 16:03:18 -0500
From: "Chris McFarling" <email@hidden>
Subject: Convert UV-excluded to UV-included
To: <email@hidden>
Message-ID: <4A752153D4F94EB5BE5AEDAB9A57D263@helo>
Content-Type: text/plain; format=flowed; charset="iso-8859-1";
reply-type=response
I'm wondering if there is formula that exists to convert
measurements from a
UV filtered spectro into data that would simulate that of a non-
filtered
device?
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