Re: G7 press calibration, best press conditions or average?
Re: G7 press calibration, best press conditions or average?
- Subject: Re: G7 press calibration, best press conditions or average?
- From: Marco Ugolini <email@hidden>
- Date: Fri, 07 Nov 2008 01:00:14 -0800
- Thread-topic: G7 press calibration, best press conditions or average?
In a message dated 11/6/08 2:50 PM, Henry wrote:
> Marco replied:
>>
>> The one that also (a) achieves stability and (b) maintains it with
>> the least amount of effort.
>>
>> My bet is that the colorimetric route (G7, as opposed to
>> densitometric-based methods) is the better one.
>
> _______________________
>
> Marco, I just don't follow what your reply to my question. I have
> read it several times and it sounds as if you didn't understand what
> I wrote. The fact is that densitometry does achieve stability, with
> arguably the same, and possibly less effort.
Yes, I see how my reply may have been confusing.
I am not a G7 certified expert. I know about print technologies enough to
grasp the outlines of what G7 is about, after reading version 6 of the
"how-to" PDF for the G7 method:
<http://www.idealliance.org/g7global//howto>
See page 6 for notes on TVI (densitometric) vs colorimetric measurements
(section 1.6 in particular: "Solving the TVI problem").
That's what I was referring to when I made the statement that you quote
above.
> The bet that G7 is *better* than densitometric-based method is, um,
> somewhat deceptively attractive. If we were to use real money, what
> would the ground rules be?
I can only say that the theory certainly sounds convincing to me based on
what I know about colorimetry, but if you wish to place a bet, perhaps you
want to do that with a certified G7 expert, who would be far more grounded
in actual G7 practice than myself.
> Since I don't believe that we are ever going to agree on the rules
> anyhow, I'll propose a simple one, such as:
>
> 1. The one and only rule: The press is to be in a repeatable state.
Repeatable, yes. To that, G7 adds: and in a state of neutral output verified
colorimetrically (via NPDCs, either by means of press controls and/or by
means of platemaking curves).
> How is it that one could prove that G7 is better at bringing a press
> to this state? This would only be a matter of quibbling over the
> accuracy of Lab vs. density values. They are both accurate
> measurements, are they not?
For the sake of fairness, I cannot say that one is better than the other in
all cases, though I will admit that I tend to think that colorimetry has the
edge. After all, spectral measurements can also be used to determine ink
densities -- whereas strict densitometry simply measures light absorption,
and cannot produce precise colorimetric readings.
> And, since when does a press not move
> about during a run? (I ask this in advance of the inevitable
> disagreement over the acceptable range of the measurements).
Of course, presses have to be kept in check for the duration of a run.
G7 proposes both colorimetric targets for the SIDs (Appendix H, on page 53
of the PDF) and suggested tolerances (Appendix G, on page 51).
> Don't you suspect that a press operator can use a densitometer to
> bring a press to its target in an acceptable manner?
It can be done, sure, but colorimetry does not make assumptions about
neutrality: it actually *measures* neutrality on the a* and b* axes -- it
does not base the achievement of neutrality on assumptions of density
numbers for inks and paper color that may not exactly correspond to the
actual inks and paper on hand.
Anyway, the above-mentioned G7 PDF is far more instructive and detailed than
I can be on all this. It's not an easy read, and I don't pretend to
understand its suggested procedures completely quite yet. But it presents a
challenge to established ways that is tested and true, with its adoption
rapidly expanding in the print industry. (In my print production work, I'm
pleased to say that I now run into print providers who eagerly promote their
adoption of G7 procedures as a quality-enhancing and value-added feature.)
It would be a mistake either to dismiss or underestimate G7 because we think
that we don't need it since the established ways work just fine: it
certainly deserves a full, open-minded and careful hearing.
Best.
Marco Ugolini
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