Cadmium stannate has been found to be useful as a window for solar heat absorbing systems in which advantage is taken of the unique optical properties of cadmium stannate. When cadmium stannate is in an electrically conductive state, having a conductivity of greater than 10.sup.3 ohm.sup.-.sup.1 cm.sup.-.sup.1, it becomes transparent to solar radiation and reflective of thermal infrared radiation. This enables the cadmium stannate to function as a window which allows passage of solar radiation through it and incident onto a black body, and at the same time serve as a reflector of thermal infrared radiation emitted from said body. This results in more efficient solar heat transfer to the solar heat collector provided with such a window.