Re: Reading Dots & Density with a Spectrophotometer
Re: Reading Dots & Density with a Spectrophotometer
- Subject: Re: Reading Dots & Density with a Spectrophotometer
- From: Ibis Itiberê S Luzia <email@hidden>
- Date: Sat, 04 Jan 2003 14:10:16 -0200
Date: Thu, 02 Jan 2003 10:11:00 -0600
Subject: Re: Reading Dots & Density with a Spectrophotometer
From: "Bruce J. Lindbloom" <email@hidden>
To: ColorSync User List <email@hidden>
CC: <email@hidden>
On 24 December 2002, Roger Breton wrote:
>
Well, I believe a more direct route is to save the spectral reflectance
>
factor out of Measure Tool and go straight to Excel. Then setup a sheet with
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the Status "X" spectral weightings you want to compute and do the
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multiplication there. You'll have four multiplications to do, one for the
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red filter (cyan), one for green filter (magenta), one for the blue filter
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(yellow) and one for Visual (black).
>
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Here's an example for the blue filter (ISO 5/3 page 15):
>
>
wavelength 350 360 370 380 390 400 410 420 430 440 ...
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product 1.000 1.301 2.000 2.477 3.176 3.778 4.290 4.602 4.778 4.914
>
>
So, you see, to compute optical densities you need to multiply the
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reflectance factor that you got from the EyeOne (less than 1.0) by the
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corresponding wavelength weighting and sum all the products at every 10 nm,
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from 350 to 760. The total should be the Status density.
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>
Please someone correct me if I've got this wrong. Always willing to learn!
>
This is not correct. The sum of products that you calculate above must be
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further processed to obtain density. The additional steps are:
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a) Divide the value by the sum of all of the spectral products for the given
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density response (e.g. divide by [1.000 + 1.301 + 2.000 + 2.477 + ...]).
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b) Take the base 10 logarithm of this value.
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c) Multiply by -1.
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You now have density.
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--
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Bruce J. Lindbloom
>
email@hidden
>
http://www.brucelindbloom.com
--__--__--
Dear ColorSync mates:
I was reading this message and perhaps I can help you with some knowledge.
Several years ago (1993) I was studing the so called Murray-Davies and
Yule-Nielsen equations which are responsible to correlate dot areas with
density readings. The answer from the coleague is correct (Mr Bruce) for
Desity computation. There are more usefull information on the link below
where you can also download a very comprehensive spreadsheet that
illustrates this proccess with examples and can also be used for your
day-to-day work as it has all the responses for ISO Visual, ISO Status T and
E:
http://www.digitalcolour.org/understanding/Density.htm
But for dot area you have to deal with Murray-Davies and Yule-Nielsen which
are a chapter apart. Lets go deep with them:
a- Murray-Davies: is a simple equation that correlates Dy (density of the
half tone area) and Ds (Density of the "solid" area or simply 100% dot area)
with A (dot area). It is writen:
A = (1-10^(-Dy)) / (1-10^(-Ds)) * 100
This equation, although very simple, functional and yet the 'standard" in
"modern" densitometry for dot area computation, leaks of some imprecision
due to the "Yule" effect, also known as "optical dot gain". This is because
some of the light that penetrates the paper is absorbed internaly or in the
base of the dots leading to the visual sensation that the dot area is
greater than it realy is or visualy "darker". So, for this reason, we have
to put a factor for the so called "n" effect in the Murray-Davies equation.
b- Yule-Nielsen: adding the "n" factor for optical dot gain in the
Murray-Davies equation (above) we have the Yule-Nielsen equation, which is
writen:
A = (1-10^(-Dy/n) / (1-10^(-Ds/n)) * 100
The advantage of Yule-Nielsen over Murray-Davies is the fact the once
knowed, the "n" factor is the "key" to know the real fisical dot area on the
paper/substrate. But, at this point you may ask me: "Well, so how do I
calculate or evaluete this "n" factor, so to speak?" Well, my friend, this
is realy not an easy quest. And thats because for two primary reasons:
b.1- Transcendental equations: if you try to isolate the "n" in the
Youle-Nielsen equation you will fall in a math problem also knowed as
"transcedental equations". Those equations don't have a direct solution
because you can't isolate the variable in the equation directly. In order to
handle this kind of problem you have to make a computer program that works
by consecutive logical approaches till it reaches a point where the "n" has
a satisfatory value for the computation;
b.2- Ironicly, even if you have such a program in hands, you have
firstly to know the real fisical area of at least one halftone area measured
(and then measure Dy for this area). This task can only be accomplished by
very precise and sofisticated devices that instead of measuring dot areas
with density readings they measure by real analisys of amplified images
taken thorugh state-of-the-art CCD cameras like Centurfax's CCDot. Although
spensive these devices don't suffer from fisical light limitations like the
ordinary optical densitometers does.
___________________________________________________________________________
Practical applications for Murray-Davies and Youle-Nielsen:
1- Murray-Davies: digital proofs, contract proofs, press proofs and any kind
of systems where the optical dot gain isn't a matter. For these kind of
purposes, at my company we use optical devices like the XRite 500 series and
CPC2-S Closedloop Autoscan Desitometer from Heidelberg (production
environment), for exemple;
2- Yule-Davies: CTP and some Rips like Best Inkjet Rips (for linearization
purposes). CTP is the most known issue because there are some optical
densitometers designed specially for CTP that needs the "n" factor to
calculate precise dot areas over Aluminium or Polyester plates. These kind
of medium/systems have a great "n" effect over optical measurements and
therefore you have always to concern if the "n" factor is well calculated
because otherwise the precision of your measurements will decrease
accordingly. At my company we do not use optical devices for CTP purposes
and so we acquired one CCDot from Centurfax for this reason.
Best Regards and Happy New Year for everybody
Ibis Itiberj Salgado Luzia
IT Analyst - Litokromia - Brazil
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