A sharp spectral rise in reflectance or transmittance of a suitable thin film filter can be used to construct a thermo-optical controllable mirror or phase retarder. A Fabry-Perot filter or a (lambda) /4-multilayer stack are such devices. Because of its comparatively high temperature dependence of refractive index Germanium is used as the active substance. The design wavelength is 10.6 micrometers . Reflectance of the filter can be tuned either in a quasistatic way by temperature variation of the whole device or in a dynamic regime with light pulses of visible spectral range which are strongly absorbed in Germanium. This increases temperature and causes a transmission change by displacing the resonance peak of the Fabry-Perot. Thermal relaxation time is of about 10 microsecond(s) .