  While tunable filters are a recent development in night time astronomy, they
have long been used in other physical sciences, e.g. solar physics, remote
sensing and underwater communications. With their ability to tune precisely to
a given wavelength using a bandpass optimized for the experiment, tunable
filters are already producing some of the deepest narrowband images to date of
astrophysical sources. Furthermore, some classes of tunable filters can be used
in fast telescope beams and therefore allow for narrowband imaging over angular
fields of more than a degree over the sky.
