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filmscanners: LED Illumination for Film Scanners
I did a bit of google-searching on this topic and
came up with some interesting hits.
Unfortunately I can't cut/paste URLs into this
email program, but the search phrase was
"scanner LED illumination."
Some interesting points...
1. A white paper from Kodak describes a scheme with
LEDs of several different colors -- the advantage
being that the spectral content (of the illuminant)
can be fine-tuned. The Kodak paper cites specific
benefits of this scheme for scanning color negatives.
2. No need to warm up the light source. LED
can be turned on/off or varied in real-time.
3. No aging effects.
4. Ability to control intensity of each color
illuminant separately -- eg., the "Analog Gain"
control in NikonScan.
5. Nikon is not the sole practitioner of this
6. Benefits of true "solid state" technology. <g>
What I don't yet understand is how the illuminant
is evenly distributed over the film width, or how
the uniformity of a LED illuminant compares with
cold-cathode or fluorescent light sources.
However, similar issues exist for cold cathode
and fluorescent lamps. In all cases, there has
to be white-point compensation for each pixel
location in the CCD array. It's not just the
illuminant that needs compensation, but the
specific pixel locations in the CCD sensor array
vary dramatically in their sensitivity. This is
true for all CCD arrays.
I certainly have seen banding in both of my previous
film scanners, on occasion, arising from problems
with faulty lamps and/or lamp power supplies.
(And you have to admit, a LED power supply is a
lot simpler than the high-voltage, high-frequency
supply needed for fluorescent bulbs.)
A further advantage to the LED illumination
scheme is that it works with a monochrome CCD sensor.
The monochrome sensor has one less source of
non-uniformity, since it doesn't have any
color filtering over the sensor itself. (I
imagine these filters themselves must introduce
some degree of non-uniformity.)
The Leaf also "works around" this issue by
doing three scans -- presumably using a different
filter with each pass. But the Leaf can't
control illuminant intensity or spectral content
the way that the Nikon scanners do.