A transmissive optical device comprising: a layer (10) of light absorber material in the solid state, preferably made of a phase-change material with switchable refractive index such as GeSbTe; a partially-reflective layer (12), and a spacer layer (14) between the layer (10) of light absorber material and the partially-reflective layer (12). The spacer layer (14) and an optional coverlayer (16) may be transparent conductive ITO layers which may serve to electrically switch the phase of the phase-change material layer (10), thereby switching the transmission/reflection properties of the transmissive optical device.