Optical grade Silicon is normally specified with a resistivity of 10 to 40 Ohm-cm which is higher than most semiconductor applications. The usual Silicon material, unless otherwise specified is Czochralski (Cz) Grade which has an absorption band at 9μm and so not important if the windows are used in the 3 to 5μm thermal band. Where required Float Zone Garde Silicon (FZ) material can be supplied which does not have this absorption. Very high resistivity (>1000 Ohm*cm) material can be supplied on special applications, particularly for THz applications.

Silicon (Si) Windows manufactured from optical grade silicon are popular for the 1.2 - 7µm spectral region due to their low cost (1/10 of Germanium and ZnSe price) and low density. Due to its low density (half of Germanium and Zinc Selenide), Silicon windows is ideal for weight sensitive applications, especially those in the 3 - 5µm region. Density is 2.329 g/cm3 and Knoop Hardness is 1150, making it harder and less brittle than Germanium.