Re: Qualifying a CMYK Press Profile
Re: Qualifying a CMYK Press Profile
- Subject: Re: Qualifying a CMYK Press Profile
- From: Graeme Gill <email@hidden>
- Date: Tue, 26 Nov 2002 12:03:52 +1100
Darrian Young wrote:
>
Can you explain what you mean by 200% ink converage? Do you mean double the
>
density of of the 100% patch? For example, 100% has a density of 1.8, and
>
another patch has a density of 3.6? I have never heard of a 200% dot and
>
usually % refers to dot.
It's a matter of dot size and dot addressabiliy. At a "natural"
resolution, then they are about equal, so that turning on all the
pixels just covers the media with ink, leaving no gaps between
the pixels. The actual density is determined by the ink characteristics
and the exact quantity of ink in each drop, but let's just call
this 100% coverage (I'm assuming we're just talking about one
channel here.) Normally one would expect a useful media to handle
at least two channels running at 100%, ie. a TAC of 200%+.
Now most inkjet manufacturers are in a spec. war, so naturally
they'll try and make the specs look as good as they can. By slowing down
one of the stepper motors (either horizontal or vertical), they
can increase the addressability of the pixels by a factor of 2
in one direction. The inkjet technology doesn't let them decrease
the dot size to correspond (because they would have to squirt
elliptical dots, due to the asymmetric resolution), so now each dot
will overlap at least half of each of its two neighbours in the high res.
direction, meaning that if you fire all the pixels, you'll get
double the amount of ink, hence "200% coverage" (and by this
stage it's so thick that it's probably dribbling on your shoes !).
With some of the pigment inks, we've noticed "funnies" when
over 100% of some of the channels is laid down. This seems
to be an result of the way the pigment particles settle out
and dry when starting with such thick layers of wet ink.
>
>Since the purpose of calibration is to put a printer back in a known
>
>operating mode, it doesn't matter whether the density reading
>
>corresponds to visual results or not.
>
>
Maybe, but if the software is using the density readings to adjust your dot
>
gain curves, it will cause a problem. Your density is gong up, but if the
>
software thinks it is going down, it adjusts them in the wrong direction.
Not at all. For one thing, I've rarely seen such
behaviour of individual channels (the only time I can
remember it was on a device that had its own electronic
linearisation system, that was obviously throwing a kink
into things), and certainly haven't seen anything like this
on inkjets. In any case, the calibration system protects itself
against such reversals (non-monotonicity), and doesn't venture
into that region (a useful calibration system is a robust one.)
>
If then, visually it is evident that there is more density, but the density
>
meter says there is less, and the Spectroeye (which can do density readings
>
of spot colors also says it is going up), what is your explanation?
Without access to the printer, the RIP and the instruments, It is
difficult to venture an explanation. The detective work always
depends on pinning down every little detail.
If all your instruments are saying that the density is increasing,
I would be wondering if it is a color appearance trick (bronzing
for instance).
Graeme Gill.
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