An Optical Solar Reflector (OSR) consists of a top layer made out of quartz

Quartz

Quartz is the second-most-abundant mineral in the Earth's continental crust, after feldspar. It is made up of a continuous framework of SiO4 silicon–oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall formula SiO2. There are many different varieties of quartz,...

, over a reflecting layer made of metal

Metal

A metal , is an element, compound, or alloy that is a good conductor of both electricity and heat. Metals are usually malleable and shiny, that is they reflect most of incident light...

.

OSR is used for radiators on space craft. The quartz outer layer lets the solar

Sun

The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...

 light through which reflects on the metal layer. This results in a low absorption coefficient. The quartz layer is a good IR

Infrared

Infrared light is electromagnetic radiation with a wavelength longer than that of visible light, measured from the nominal edge of visible red light at 0.74 micrometres , and extending conventionally to 300 µm...

 emitter. The result of these properties is a good emitting, low absorbing material. Thus making it a "cold material".

Optical Solar Reflectors are a type of Second Surface Mirror (SSM)