[filmscanners] RE: Terms used in Dynamic Range calculations...

["Paul D. DeRocco" <pderocco@ix.netcom.com>]

> From: Austin Franklin
>
> The dynamic range equation is, using these terms, and the
> provided diagram, can be either of the following:
>
> DR (dB) = 10log10 (largest/smallest)
>
> DR (dB) = 10log10 ((maximum signal level - minimum signal level)
> / smallest)

For large ranges, these are essentially the same, but the first one is
correct, and the second one doesn't make sense. If the maximum signal level
was only twice the minimum, then the second formula would call that 0db
dynamic range.

> The ISO spec states:
>
> "DR = Dmax - Dmin"
>
> Since density values are in log form, this is the correct equation.
>
> The ISO spec defines:
>
> "Dmax = Density where the Signal to noise ratio is 1"
>
> Which is correct, as it is the lowest signal level that will come from the
> scanner electronics when measuring the highest density, and where
> the signal
> and noise will be equal.  This value obviously takes into account noise.
> They make the assumption that the noise is the same as the "minimum signal
> level", which is fine for the case they are using it for (more below).

Also, keep in mind that noise spectrum is very important. Electronic noise
is typically white plus some low frequency 1/f noise. However, the RMS noise
of an ink jet print is huge, but it's all pushed up into spatial frequencies
that you can't see without a magnifier. But try to scan an ink jet print on
a modern 2400dpi flatbad, and quite a bit of it will come barreling through.

--

Ciao,               Paul D. DeRocco
Paul                mailto:pderocco@ix.netcom.com

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