A tunable IR filter capable of scanning from 8.2 to 12.8 µm has been designed, constructed, and tested. It is a first-order Fabry- Pe´ rot interferometer with piezoelectrically driven cavity spacing. Multilayer dielectric coatings for the partially transmitting mirrors were designed to minimize the wavelength-dependent phase change produced by reflection. The transmission bandwidth ranged from 2.8 to 4.0% across the tuning range. Continuous scanning at 20 Hz rates was demonstrated.