Re: Reading Dots & Density with a Spectrophotometer
Re: Reading Dots & Density with a Spectrophotometer
- Subject: Re: Reading Dots & Density with a Spectrophotometer
- From: Graeme Gill <email@hidden>
- Date: Mon, 06 Jan 2003 11:54:44 +1100
"Bruce J. Lindbloom" wrote:
>
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
>
> the Status "X" spectral weightings you want to compute and do the
>
> multiplication there. You'll have four multiplications to do, one for the
>
> red filter (cyan), one for green filter (magenta), one for the blue filter
>
> (yellow) and one for Visual (black).
>
>
>
> Here's an example for the blue filter (ISO 5/3 page 15):
>
>
>
> wavelength 350 360 370 380 390 400 410 420 430 440 ...
>
> 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
>
> reflectance factor that you got from the EyeOne (less than 1.0) by the
>
> corresponding wavelength weighting and sum all the products at every 10 nm,
>
> from 350 to 760. The total should be the Status density.
>
>
>
> 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
>
further processed to obtain density. The additional steps are:
>
>
a) Divide the value by the sum of all of the spectral products for the given
>
density response (e.g. divide by [1.000 + 1.301 + 2.000 + 2.477 + ...]).
>
>
b) Take the base 10 logarithm of this value.
>
>
c) Multiply by -1.
Note the weighting factors are not being used correctly in the above discussion.
The ISO factors are log10 weighting factors, and have to be converted to linear
weighting factors before multiplying by the spectral reflectance values, normalized,
etc.
Graeme Gill.
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