Silicon (Si) is a naturally occurring chemical element that is used in various areas. It is widely used as a semiconductor in electrical engineering and is found in most photovoltaic systems.

In optics, silicon is used to manufacture lenses, scanner windows and prisms for the IR range. It has a high transmission in the range between 3 µm and 5 µm. Silicon with a coating is also used as a mirror material for CO2 lasers. Due to the low density of the material, optical components made of silicon are particularly suitable for applications where weight has to be saved.