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Making the scanner behave consistently is the issue here. Densitometers are highly calibrated instruments that are designed to be very repeatable, scanners however are not.
Then there is the fact that digital imaging is a linear representation and sensitometry is exponential (it's actually more complicated than that thanks to gamma curves). Also by their very nature a digital scanner is designed to compress contrast (thanks to those gamma curves again). Scanners also usually have a real-world density range sensitivity which is lower than a densitomer as well.
This all together makes using the scanner as a reliable densitometer problem pretty tough.
david
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