[filmscanners] RE: rebuild your scanner and get better results

["Austin Franklin" <darkroom@ix.netcom.com>]

Hi Andras,

> > I disagree that banding (in general) can't be caused by differences not
> > accounted for in calibration between each of the LEDs, both of the same
> > color, and between the different colors.
>
> Just to make sure we are talking about the same thing: there is only
> ONE LED of each colour in the LS-8000.

Three different color LEDs.  One green, one red and one blue.  This is in
contrast to using filters over either the light source, or the sensor.

> > Typical scanner calibration does calibrate all three colors,
> and calibrates
> > each individual sensor for all three colors.  Every scanner
> I've designed
> > does that, and every one I've done work on does.  I KNOW the
> Leafscan does
> > it that way.  I can't vouch for any other desktop scanners
> though, but it
> > makes absolutely no sense that a competent programmer wouldn't
> do it that
> > way, because not doing it that way gives less accurate results.
>
> The Coolscans don't work this way. It's not necessary simply because
> the CCDs have identical colour response, given by the semiconductor
> material in use and independent of particularities of the microscopic
> structures of the semiconductor device.

That's not relevant.  The position of each line of the sensor is different,
as well as the position of each of the light sources (for three color LEDs).
That is what necessitates calibration for each of them.

> Calibrating for each colour is
> only necessary if your light source has different spatial profiles for
> the three colours, which in the Coolscans is not the case.

Well, I'm not sure what you mean by "spatial profiles", but spatially, the
light sources AND the sensors are in "spatially" different places.  Also,
changing the color of the light source means differing the exposure time,
hence calibration for that light source.  In my experimentation having
designed a few scanners, I found that the optimum exposure times for each
color can vary widely, and thus, calibration for each color is very
important.

> > Something and/or someone is confusing/ed here.  For example,
> you have an out
> > LED, which does leave a gap...that is in the X direction, but
> the banding is
> > in the Y direction, just like a clogged print nozzle.  Film
> staging problems
> > are what typically cause banding in the X direction on a film scanner.
>
> I've never seen x banding on a Coolscan, and it can't even exist in
> principle (that's what the CCD is calibrated for).

Yes, that is one of the reasons calibration is for, but it does not
eliminate other possible causes of banding, like I said, the LED pattern.

> > LEDs in the hardware.  There is a reason for doing it, as it
> can decrease
> > smear and bloom, which can lead to a better scan.  Now, they
> may have tuned
>
> How does the intensity of the LEDs affect anything but exposure and
> thus (indirectly) signal-to-noise ratio?

The sensor operates best within a range of intensity, and adjusting the
light level to maintain that range can help prevent, as I previously stated,
bloom and smear.  How do you feel exposure effects SNR?  I'm not saying it
doesn't, but I am curious what you believe.

Austin

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