On the Nikon-Filmscanner we use the infrared channel
as forth channel but it is stored in the alpha channel
of the png format (or other formats) or it is transfered
to gimp as alpha-channel.
If we think of the dust as beeing a filter which occludes
some of the image, the interpretation as "apha" channel is
quite acceptable.
As there might be other methods to scan an "alpha" channel
than by scanning with infrared ligth I think the
renaming to RGBA is a good idea.
Andreas
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