We report the observation of external switching of an intrinsic optically bistable device by incoherent illumination. The InSb etalons used in this study were switched by a photographic flashlamp both from off to on resonance and, under different conditions, from on to off resonsance. It has been demonstrated experimentally that switching can be initiated as a result of either an electronic or a thermal effect. The parameters that determine which mechanism dominates in a particular instance are discussed. The results should have general application to a variety of materials of device interest.