Is the spectral response of any digital sensor identical with that of any
monochrome film?  

In broad terms a red filter will darken blues and greens, a green filter
will lighten greens and darken blues, and so on, but the nuances are likely
to be subtly different between different films and between different
digital sensors, let alone between film and digital.  

Through-the-lens exposure meters are probably similar in their response
whether used with film or digital - and their spectral response is not
identical with either film or digital sensors.  

The human eye responds differently from any of them.

Experiment with 'bracketing' your exposures to see how that changes the
relative lightening and darkening effects of your filters - worth trying
with either film or digital media  :))

Not that I've ever tried it, but there is a possible difference between
using film and a digital sensor when it comes to filtering.

When you buy a 6MPixel digital camera, it is actually 2MPixels, because what
manufacturers usually mean is 6MSensors. A pixel on a computer screen can
be any colour, but on a camera sensor it usually refers to just one colour.

I can envisage that spatial information from all three colours is used in
the imaging software, esp working with raw images, to contribute to the
luma information (i.e. picture brightness). This would result in a high
luma information content and a lower chroma (colour) content, which is just
like the human eye, so would be a Good Thing (TM) when it comes to
perceived image quality. If this happens, then placing a red filter in
front of the sensor would be worse than taking the output of all of the
pixels into account before applying filtering in The Gimp or some other
image processing program.

Of course, I don't know if this is what happens, because manufacturers are
usually so embarrassed about their software that they never release the
source code, and I've not tested it because I'd only ever consider wet
photography for my beloved mono shots, but if you are a digital user it
might be worth experimenting with both approaches.

Perhaps there's a knowlegable employee of some high-end digital camera
manufacturer lurking on the list who can tell us the whole truth...

Nick/.