From: Richard Urmonas (rurmonas@tpg.com.au)
Date: 10/31/03-11:11:13 AM Z
Quoting Sandy King <sanking@clemson.edu>:
> Richard,
>
> What is the convention for the conversion factors of -1.42 and -1.36
> you use in the formulas below.
>
> Also, if you would be so kind, please show me how your derived the
> 13% change in density difference between log 1.42 and log 1.36 in a
> practical walk through of the conversion.
Sandy,
I was thinking about how to explain this, when I realised I have got it wrong.
So I must appologise for misleading both you and any other readers
of my reply. There are two issues. The first, my mistake is that when
I converted to a linear scale I referenced it to zero (i.e. no light reflected
at all) rather than to the paper (i.e. no image metal).
The second issue is more complex. This is that we cannot get to zero
reflectivity. No matter how much metal we may deposit on the paper,
there will always be some reflection. Hence we cannot relate the
density measurement to the amount of metal present.
Let me illustrate this with an (extreme) example. Let us say we have
managed to form a 1mm thick layer of image on our paper. This will have some
reflection density. Now if we scrape away 0.5mm of this image layer, it is
still sufficiently thick to stop any light reaching the paper surface, so we will
measure the same reflection density. Thus we have removed half of the
image metal with no measurable change in density. Obviously in a real
case we have very much thinner layers, but hopefully you can see my
point, in that as the density approaches some asymptote,
we can no longer relate the density measurement to the amount of image metal.
Anyway thank you for your investigation. It was something I had
wondered myself, before you raised this topic on the list.
Richard
--- Richard Urmonas
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