filmscanners: LED Illumination for Film Scanners

[Raphael Bustin <rafeb@channel1.com>]

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 
technology.

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.


rafe b.