Digital BW, The Print

Hi Kevin,

> 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.

Passes through THREE different color filters...  One, red, one green and one
blue.  Resulting in three channels of color information.

> Why would the colour of the filter affect the relative intensities of
> the light hitting the CCD.

Right or wrong, that doesn't have a thing to do with the reason.  It's the
property of the CCD response/artifacting to different color lights.  Each of
the three lights gives different responses to the CCD and has different
artifacting.  Red, because it has the highest energy of the three, tends to
be the fuzziest, simply because of what are called bloom and smear.  Bloom
is basically saturation of the sensing element, smear is basically crosstalk
between adjacent sensing elements, because of the intensity (this is the
artifact that PMT scanners do NOT suffer from, as they scan one "spot" at a
time).  Blue is the next worst, then green is the best...but not always.

> Why would it matter if the filter is neutral, red, green,
> blue, or whatever.

CCD response is the primary reason.  Neutral density has the lowest
artifacting.

> 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),

That is true, but as you suggest may 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?)

Yes, and there is more to that, as I said, the artifacting varies as well.

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

Yes, and yes ;-)

Regards,

Austin