[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 Austin Franklin
>
>
> 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.

That is likely, but the table values would be offsets and not mathematical
non-linearities per se.  The light versus signal function remains a y = mx +
b linear equation, where your table values would be an array of "b" values
to get the dark signal to something like a uniform very dark gray.

>
> > 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.

These do not count photons except in the gross aggregate where the actual
count is unknowable.  Some scientific ccd-like detectors can count photons,
but these cannot.


> >  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.

That is not a mathematical non-linearity at all: it's only a linear offset.

>
> > 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

They were Karl's and my note cited him and his words.

The technical and mathematical misinfo needed clarification, IMHO.  :-)


Regards,

Chuck




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