An optical solar reflector (OSR) consists of a top layer made out of quartz, over a reflecting layer made of metal.
OSRs are used for radiators on spacecraft. The quartz outer layer lets the solar light through which reflects on the metal layer. This results in a low absorption coefficient. The quartz layer is a good IR emitter. The result of these properties is a good emitting, low absorbing material, thus making it a cold material.