[filmscanners] RE: scanner dmax discussion

["Austin Franklin" <austin@darkroom.com>]

Hi Chuck,

> > 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:

There are LUTs (Look-Up Tables) in the scanner that compensate for the
non-linearities that are determined during calibration.  These are simply
offset values that are added/subtracted from the actual data values.

> 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.

Basically, they simply count photons, and the output is directly
proportional to the number of photons counted.

>  Nearly all non-linearities and most noise effects are introduced
> after the detectors and are minimal effects.

Hum.  I disagree with that.  You aren't taking into account non-linearities
across sensor elements.

> 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.

Agreed.

> Dunno about his, but yours is flawed.

These comments you commented to weren't mine...but I also think some of your
interpretation of what was said may not be what was actually meant...and/or
required further info.

Regards,

Austin

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