Silicon is another material that is frequently utilized in infrared systems – however due to absorption it is not suitable for CO2 transmitting optics. Silicon can be used from 1.2 to 7μm but only has about 50% transmission in this range. Because of this, we have added an AR coating to enhance the performance from 3-5μm. With a density of 2.329 g/cm3, Si is ideal for use in systems where weight is a concern.



Transmission Range 1.2µm to 7.0µm & from 25µm out to beyond 300µm
Density 2.329 g/cm3
Thermal Expansion 2.55 x 10-6 /°C @ 25°C
Surface Quality Typical specifications for surface quality in the infrared are a 40-20 scratch-dig in the 1.2 to 3µm spectral region & 60-40 scratch-dig for the 3-7µm area. Diamond Turned surface finishes of 120 Å rms or better are typical.
Surface Accuracy In the infrared, typical required surface figure ranges from 1/2 wave to 2 waves @ 0.6328µm & are usually specified depending on the system performance requirements.
AR Coating Options The most common anti-reflectance coating for Silicon is BBAR for 3 to 5µm. Many other specialized wavelength bands are possible within the 1.2 to 7.0µm range.
Typical Applications Thermal imaging, FLIR.
Wavelength (µm) Index of Refraction (n)
1.5 3.484
2 3.456
3 3.436
4 3.429
5 3.426
6 3.424
7 3.423


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