I looked at the image you provided, but the jpeg artifacting makes it
difficult to analyze what you are speaking of exactly.
However, there are some knowns regarding scanning.
1) Blooming: high contrast areas with high brightness push the CCD
sensor limits in terms of the amount of electrons they can handle. The
nature and design of Charged Coupled Devices is to try to spill excess
current into non-recording channels between the sensor rows, but in
extreme situations, even the channels cannot flow the excessive amount
away so it jumps to the next row or set of sensors.
This is a design issue and there isn't a heck of a lot to be done if
that is the cause. Different types of CCDs handle this issue with
differing designs.
2) Using a glass carrier doesn't help this situation, because glass
doesn't transmit light perfectly in straight columns. There is all sorts
of internal reflection (bouncing between the surfaces and due to the
density differences between air and glass surface, film and glass
surface, etc.)
If you are using a double sided glass carrier, you have the following
interfaces taking place:
Air/glass surface/glass interior/glass surface/air/film non-emulsion
surface/film plastic base/film emulsion/air/glass surface/glass
interior/glass surface/air, and that doesn't include any surface defects
like scratches, surface residue on the glass or film, etc.
That's quite a few places for diffraction, reflection, and general
bouncing around of photons.
If you can, I would try scanning without the glass carrier and see if
that helps (I realize it adds the problem of the surface sagging causing
focus issues), but it might tell you where the problem is coming from.
If you do determine that the glass is adding a lot of this smearing, you
might consider using a wet mount system, where a liquid is used (usually
an oily substance) that the film is floated in between the glass
surfaces. This liquid removes the diffraction differential between the
air surfaces and the glass and film surfaces, which reduces the total
number of surface for reflection and diffraction. It does demand careful
use, to prevent dripping, air bubbles, picking up dust and dirt, and
careful cleaning of the film and carrier afterward.
Art
Here's some explanation from the following website:
Separate Registers from Sensors
A third type of CCD places columns of shift registers between the
sensors. After acquiring an image, the array quickly transfers all of
the charges into the column shift registers. These registers then feed a
shift register that transfers the charges to an amplifier. As soon as
the camera transfers an image to these interline shift registers, it can
start to acquire another image.
By separating the shift registers from the sensors, the design
eliminates smearing. But the shift registers in these interline devices
take space. As a result, the sensors have spaces between them and they
convert only a fraction of the light that reaches the CCD into useful
information.
No matter which CCD comes in a camera, all CCDs are subject to an effect
called “blooming,’’ which occurs when a bright light shines on a sensor
or sensors. The bright light causes the sensor to quickly fill, or
saturate, its charge well. The excess electrons can flow into adjacent
wells and saturate them, too. When saturation occurs, the image obtained
from the CCD show a large white splotch at the place of bright
illumination. The size of the splotch determines how much blooming
occurred in the CCD.
CCD manufacturers can overcome blooming. One technique provides a CCD
with an electronic overflow that operates much like an overflow drain in
a bathroom sink. The electron well is set up so any excess electrons
combine with holes, thus moving the electrons into the CCD substrate. A
second approach lets users adjust the ratio of photon hits to electrons
produced. Producing fewer electrons per burst of photons will reduce
blooming.
Some CCDs and cameras let you control anti-blooming effects.
Anti-blooming lets the CCD withstand much more light than it could
otherwise before saturating charge wells. Of course, you could reduce
the aperture of a camera, or add a filter to reduce light levels, too.
lotusm50@sprynet.com wrote:
>Does anyone have any thoughts or suggestions on the following issue:
>
>I just did a scan of a Fuji Astia 6x7 slide It is about 2/3'rds dark,
>shadow and silhouette, one thirds correctly exposed, bright image
>through a window (of sorts). The scan was with the Nikon LS-8000 with
>the glass film holder, set to 8x multi-pass scan, with a single CCD (so
>it took forever, but should be good quality). What I got was a scan
>where some of the dark areas next to the bright areas got light
>spill-over making them lighter. It almost looks like a faint light leak
>into the dark areas -- a slight fogging of the some of the dark areas.
>It is especially evident above the window, with the seated figures, and
>on the middle of the right edge of the image. It is definitely NOT in
>the slide. You can see it here (depending on your monitor, some of it
>might be hard to see): boncratious.info/CherryBlossomDining.jpg Does
>anyone know what causing this and how I can avoid it or stop it from
>happening?
>
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