  Previously, a method has been developed to find the best colour filter for a
given camera which results in the new effective camera sensitivities that best
meet the Luther condition. That is, the new sensitivities are approximately
linearly related to the XYZ colour matching functions. However, with no
constraint, the filter derived from this Luther-condition based optimisation
can be rather non-smooth and transmit very little light which are impractical
for fabrication.
  In this paper, we extend the Luther-condition filter optimisation method to
allow us to incorporate both the smoothness and transmittance bounds of the
recovered filter which are key practical concerns. Experiments demonstrate that
we can find physically realisable filters which are smooth and reasonably
transmissive with which the effective "camera+filter" becomes significantly
more colorimetric.
