Si(Silicon)

Silicon crystals are ideal for infrared applications, with good transmittance from 1.2 to 7.0 μm. With a density of 2.329 g/cm³, roughly half that of germanium, silicon is preferred for weight‑sensitive applications, especially in the 3–5 μm band. It also features a Knoop hardness of 1150, making it harder and less brittle than germanium. Additionally, silicon offers significant cost advantages over germanium, so it is widely adopted by military brands for lenses and windows.