Crystalline silicon is opaque in the visible and becomes a practical IR window material beyond roughly 1.2 µm, with useful transmission through much of the SWIR and into the MWIR depending on thickness, resistivity grade and coating. For many OEM sensor windows, Si is the economical, scalable answer.
Strengths
Mature wafer infrastructure, large diameters available relative to some exotic IR crystals, good mechanical properties for windows, and a high refractive index that enables thinner components for a given optical power (with the usual AR burden). For 1.5 µm-class laser and SWIR imaging windows, Si is often first on the shortlist.
Limits
Do not expect Si to replace Ge for 8–12 µm thermal imagers — phonon absorption closes the LWIR window for typical thicknesses. Multi-spectral systems that need true LWIR plus NIR usually become multi-material (Si + Ge, or ZnS/ZnSe architectures), not a single Si optic.
Coatings and resistivity
Specify the band and whether the part sees laser fluence. Resistivity and doping can influence free-carrier absorption in IR; treat “optical grade Si” as a procurement callout, not a vague hope. AR stacks for 1.5–5 µm differ from telecom anti-reflectors on fibre collimators — match the stack to the sensor or laser line.
Explore cut-on behaviour in the Spectral Explorer (Si drops out by 10.6 µm), then formalise via Request a Quote.