If the instrument supports an ambient
measurement capability, then it will be used. If the insrument does not
have an ambient mode, then an emissive measurement mode can be used,
although typically many illuminants are too bright to directly point
the instrument at. A work-around is to reflect the illuminant from a
spectrally flat white surface. A good candidate for this is a piece of
white, fine textured polystyrene foam. [The suitability of a reflector
can be checked using
spotread -S
to check that the reflection characteristic is close to flat.]

This is done by placing the paper such
that it is uniformly illuminated with reasonable brightness, and then
placing the instrument so that it receives the reflected light from the
paper. This is typically achieved by placing the instrument close to
the paper at about 45º, so that it's aperture has a clear view of
the illuminated paper, but avoiding shadowing the region that is in
view.
