Digital BW, The Print

Austin (or others)-

You seem to be in scanner mode so perhaps you could clarify something 
for me.  For as long as I can remember you have been saying things like 
this:

> The Leafscan 45 and 35 both have dedicated B&W scanning modes.  They 
> use a
> single channel with a neutral density filter.  IMO, they produce 
> superior
> scans than just scanning B&W film with a typical CCD scanner that 
> scans the
> B&W film in RGB.
>

but it has never been clear to me precisely why this is.

My scanner is a polaroid ss120.  It shines a white(ish?) light through 
my silver film (which I assume is fairly neutral but with varying 
density) and the intensity of resultant light is detected by a CCD 
after the light passes through a colour filter.

Why would the colour of the filter affect the relative intensities of 
the light hitting the CCD.  If my assumption that silver film is 
neutral is correct then aren't the intensities of the blue frequencies 
affected by passing through the film in a similar way to the red 
frequencies.  Why would it matter if the filter is neutral, red, green, 
blue, or whatever.

The only explanations I could come up with were:

1. film is not neutral (ok probably true but it seems to me the effect 
would be marginal),
2. the sensor's sensitivity varies with the wavelength of the light 
(ok, but since I am only interested in the intensities relative to each 
other why does that matter?)

Is one of these correct or, in your opinion, it is something else.

Thanks,

--
Kevin Gulstene