> Then there are about a hundred other problems that you haven't even
> thought of yet. This is at the root of basic metrology, has been the
> basis for many peoples careers, and is the reason the national standards
> labs exist.
I appreciate this having studied a little in the area.
However if you do have access to a transmission densitometer you can
easily make your own that are sufficiently accurate for any alt-process
work following the method I suggested. It also means they are cheap enough
to throw them away whenever they get dirty or damaged at all.
I was giving a simple and practical solution rather than a theoretical
discussion - for which I'm sure there are plenty around better qualified
than me.
Peter Marshall
On Fixing Shadows and elsewhere:
http://www.people.virginia.edu/~ds8s
Family Pictures, German Indications, London demonstrations &
The Buildings of London etc: http://www.spelthorne.ac.uk/pm/