Digital BW, The Print

Truman writes:

> The only way you could not reconstruct the
> intensity of white light would be if there
> was a frequency that fell outside the passband
> of all three of the RGB filters.

True, as long as the light is truly white, that is, as long as the intensity
of the light at each frequency in the spectrum is the same (the spectral
distribution is a flat line).  In this case, you can easily reproduce the
distribution in order to transform RGB into anything else.

If the light is not absolutely white, the rules change, and most
transformations are not possible.  If you can somehow reconstruct the
curve--e.g., if you know it has a certain shape--you can do the
transformation, but since there is typically no way to know this, there
isn't much you can do.

In real world scenes, of course, the light is never white.  The only time
all the light is white is if you take a picture of a white sheet of paper
illuminated by a perfect white light source, or the equivalent.