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: Mon, 03 Nov 2008 21:08:54 -0800
- Thread-topic: G7 press calibration, best press conditions or average?
In a message dated 11/3/08 2:27 AM, eric@poem wrote:
> Hi all,
>
> Adjustment or calibration? Seems to me this amounts to the same thing.
You're correct, but with a note of caution.
On the one hand, *not all* adjustments necessarily help in the desired
calibration of a piece of equipment (in this case, the achievement of G7's
Neutral Print Density Curves, or NPDCs).
On the other hand, any type of calibration *necessarily* requires certain
adjustments that make the equipment conform to one or more desired aimpoints
(as, for example, is the case with monitor displays: a D50 white point, a
luminance of 120 cd/m2, etc.).
> Printing, especially offset printing is an extremely complex set of
> relationships, a real witche's brew, grease, water, solvent, extreme
> heat, extreme cold...let alone paper and ink manufacturing
> variables...shheesh how does it ever work!
> I always understood that successful process control was "the close
> control of numerous variables" now that it seems to me is the press
> crew's responsibility, their adjustment abilities, knowledge and
> experience are required to bring the press into a "calibrated" state.
And, in the case of G7 calibration, *all* of that has to be adjusted
(calibrated) to produce results that eventually conform to the ISO 12647-2
solid ink colorimetric values.
For the ISO ISO 12647-2 Solid Ink Values, see Appendix H of "Calibrating,
Printing and Proofing by the G7 Method - Version 6, August 2006":
<http://www.idealliance.org/g7global//howto/G7_how-to_v6[final].pdf>
Appendix H is on page 53 of the PDF.
In other words, the press/ink/paper conditions keep being adjusted until the
colorimetric measurements from the printed results correspond to the desired
aimpoints, plus or minus a given tolerance. The GRACoL PDF explains the
procedure in detail.
If all of that is not calibration, then what is?
Besides, once the desired conditions have been achieved, all the press, ink
and paper parameters must be rigorously maintained so that the press'
behavior remains *stable* (within tolerances).
To sum it up: first calibration, then stability.
*Stability* is the other important necessary condition that the G7 procedure
is after, once the output conforms to expectations. As I understand it, the
results are checked periodically to make sure that the press' overall
behavior does indeed remain reasonably stable over time (within tolerances).
Marco
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