Re: Determining tonal percentage?
Re: Determining tonal percentage?
- Subject: Re: Determining tonal percentage?
- From: Graeme Gill <email@hidden>
- Date: Mon, 28 Apr 2003 14:31:21 +1000
Tyler Boley wrote:
>
I get something much closer when I enter a value of around 2.2
>
(coincidence?) for the mysterious N factor in Murray-Davies. What is the
>
N factor, and why is there a purely arbitrary factor in a calculation
>
used for such a specific and precise purpose?
As I understand it, it's all about a fudge factor to map between
theoretical dot area (or dot area measured on plate making film),
and the final visual density of ink on paper. If all your doing
is measuring actual paper density, then you should never have
to deal with it. Density is just another unit for measuring
reflectance or transmittance. To get an idea of the visual
effect, you would probably be better to convert these numbers
into an L* value.
Given reflectance from 0.0 .. 1.0:
Density = -log10(reflectance)
if (reflectance < 0.008856)
L* = 903.3 * reflectance
else
L* = (116 * (reflectance ^ 0.333333)) - 16
If you are mapping between measurement ahead of the final print
(ie. RIP input value, film density etc.), and the final print
itself (ie. you want to predict the final density given the
density or percentage coverage further up the chain), then
will have to worry about modelling dot gain and other
things that affect the overall transfer curve. Murray Davies
is just one arbitrary way of doing this. There are many others,
and they are all pretty much equivalent in the end.
Graeme Gill.
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