Re: Dot Gain
Re: Dot Gain
- Subject: Re: Dot Gain
- From: Roger Breton <email@hidden>
- Date: Wed, 30 Apr 2003 11:29:18 -0400
>
Roger wrote:
>
> A typical "dot area" reading on a densitometer does not separate
>
> "mechanical" from "optical" gain : they're both taken into account
>
> by the measurements. In fact, this difficult separation of the two
>
> can be accomplished by the addition of an "N-factor" or by the use
>
> of a device called a ScanDot. (please correct me if I am wrong or
>
> you want to include more qualification of this process or the gist
>
> of the underlying Yule-Neilson equation)
>
>
I thought that the "optical" dot gain is something "only" perceived by the
>
visual system. Look at the so called Herman grid
>
(http://www.michaelbach.de/ot/lum_scgrid/index.html). The white area between
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the black dots is perceived as gray which would explain that the perceived
>
dot gain is larger than the measured dot gain. So I would say that a
>
densitometer just measures the amount of light which is reflected or
>
transmitted by the paper or film, in other words the "mechanical" dot gain
>
due to the actual distribution of ink on the paper or silver in the film.
>
The "N-factor" would then serve as a correction factor for the "optical" dot
>
gain and the logarithmic behaviour of the visual system when lightness is
>
involved.
>
>
Am I missing something?
>
>
Regards,
>
Peter Baumbach
I never heard that there was such thing as a "perceived" dot gain, in the
context of measuring printing dot gain on a printing press for the purpose
of linearizing or compensating for dot growth.
I am unable to visit your link but I know what the Herman grid illusion is
all about. It's a perceptual phenomenon where one sees gray squares that
don't exist between solid black rectangles separated by white gaps on a
grid. I don't think that phonemenon is at play here, in the context of dot
gain, because we're talking aabout ratios of density measurements, and, as
far as the instrument is concerned, there is no "perception" involved on the
part of the densitometer.
To me, mechanical dot gain means a growth in the original size of a dot
(we're talking fractions of an inch) as a function of ever slight blanket
pressure applied to a substrate on a printing press, what some people call
"squeeze", in order to image a paper. Whereas optical dot gain refers to a
scattering of light around that very same dot image, it is an interaction of
light and the dot color with the substrate. Light is reflected in all
directions around the dot shape. How far around into the substrate? That's
one type of problems that has kept scientists busy for a number of years and
that is well understood scientifically nowadays, I suppose. I think this
dates as far as the 1930's with the Kubelka-Munk theory. But was later
revisited and expanded by a number of printing scholars lile Clapton and
Yule -- yeah, the same guy in the Yule-Neilson equation.
Total dot gain includes both mechanical and optical and is returned by a
single densitometer measurement. Thank you X-Rite and Tobias and Beta and
GretagMacbeth for making all these wonderfull instruments. And thank you Mr.
Murray and Davies for coming up with a historic formula to relate, on a
logarithmic scale, the amount of light going on the substrate to the amount
of light being reflected by the substrate.
I'll rest my case.
Roger Breton
Laval, Canada
email@hidden
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