[filmscanners] RE: scanner dmax discussion

["Chuck Cole" <cncole@earthlink.net>]

> -----Original Message-----
> From: filmscanners_owner@halftone.co.uk
> [mailto:filmscanners_owner@halftone.co.uk]On Behalf Of Karl
> Schulmeisters
> Sent: Wednesday, July 09, 2003 12:44 AM
> To: cncole@earthlink.net
> Subject: [filmscanners] Re: scanner dmax discussion
>
>
  <snip>

> So in part, the 'wizardry' of the electronics inside the scanner A/D
> converters is to try and compensate for the non-linearities of
> the detectors
> themselves.

This is just not an accurate statement: silicon (and similar)
photoconductive photosensors are about the most linear of all detectors, and
little else is used these days.  One elegant variety of ccd-like sensor is
strictly linear to within better than 0.1% over about 9 decades of light
level.  Nearly all non-linearities and most noise effects are introduced
after the detectors and are minimal effects.

> Note also, most of the detectors have photo response curves not that
> dissimilar from film, so by changing their detection threshold, you can
> drive them into different parts of the response curve.  Not all scanners
> allow you to do this, but in theory it is possible.

The non-linear aspects of this are quite wrong also, but thresholding after
the detectors can affect placement of the "active range" of signal handling.
Nothing affects the detection threshold except the as-built physics of the
semiconductor devices.


> George DeWolf is wrong in his analysis.

Dunno about his, but yours is flawed.  Some of your points may have validity
if described in other terms, but your electrtonics and physics is way off.


Chuck




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