  Designing a photometric system to best fulfil a set of scientific goals is a
complex task, demanding a compromise between conflicting requirements and
subject to various constraints. A specific example is the determination of
stellar astrophysical parameters (APs) - effective temperature, metallicity
etc. - across a wide range of stellar types. I present a novel approach to this
problem which makes minimal assumptions about the required filter system. By
considering a filter system as a set of free parameters it may be designed by
optimizing some figure-of-merit (FoM) with respect to these parameters. In the
example considered, the FoM is a measure of how well the filter system can
`separate' stars with different APs. This separation is vectorial in nature, in
the sense that the local directions of AP variance are preferably mutually
orthogonal to avoid AP degeneracy. The optimization is carried out with an
evolutionary algorithm, which uses principles of evolutionary biology to search
the parameter space. This model, HFD (Heuristic Filter Design), is applied to
the design of photometric systems for the Gaia space astrometry mission. The
optimized systems show a number of interesting features, not least the
persistence of broad, overlapping filters. These HFD systems perform as least
as well as other proposed systems for Gaia, although inadequacies remain in
all. The principles underlying HFD are quite generic and may be applied to
filter design for numerous other projects, such as the search for specific
types of objects or photometric redshift determination.
