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Custom Silicon

Monocrystalline Silicon (Si) is one of the hardest optical materials available that transmits in the infrared region. It is also relatively lightweight, at half the density of germanium, so ideal for weight-sensitive optical systems.
Silicon has many benefits including:

  • Low dispersion
  • Low density
  • Excellent thermal stability
  • High thermal conductivity
    Below is a brief summary of silicon’s optical properties, a full data sheet is available here.

    Transmission range

    1.2 to 15µm

    Refractive Index

    3.4223 @ 5µm


    2.33 g/cm3

    Melting Point



    Knoop 1150

    Knight Optical supplies high quality silicon components, including windows, lenses and prisms, made to your custom specification. We can offer uncoated and coated optics, including broadband antireflective (BBAR) and diamond-like carbon (DLC) coatings operating at 3 to 5µm. Our typical manufacturing specifications are listed below however we are always expanding our capabilities so if your requirements are not mentioned below, please contact our technical sales team who will guide you through your enquiry.


    < 5mm to 300mm +

    Form error (@633nm)

    < 0.25 waves

    Centration /parallelism

    < 1 arc minute


    < 60/40

    Typical coatings

    BBAR @ 3-5µm

    DLC @ 3-5µm


    Uncoated, 1mm thick

    With our in-house state-of-the-art metrology laboratory you can be confident that your optical components will meet your requirements. You can find out more about our testing facilities here.

    Additional Information

    • Silicon can be produced via different methods—most notably the Czochralski (Cz) and Float-Zone (Fz) techniques. When grown by the Czochralski pulling technique, there is an absorption peak at 9µm due to the presence of oxygen, but this is vastly reduced when prepared by the Float-Zone technique.
    • Optical grade silicon is usually doped with either boron (p-type) or phosphorous (n-type) to increase its resistivity (5 to 40 ohm cm).
    • Like germanium, silicon is a semiconducting material with an indirect bandgap at 1.1µm at room temperature.
    • Silicon’s transmission is at around 50% between 1-6µm, and above 50µm, therefore an AR coating is recommended to achieve higher transmission, and a DLC coating can be applied to an exposed surface to increase its durability.

    Typical Applications

    Thermal imaging
    • Silicon windows and lenses are often used for NIR and MWIR thermal imaging applications. Though the image quality may not be as superior as germanium and zinc selenide, silicon does provide a more lightweight and cost effective solution.
    Laser Mirrors
    • Due to silicon’s high thermal capacity it is an ideal substrate for reflectors used in applications such as CO2 laser cutters.
    FTIR Spectroscopy
    • Silicon is often used as the substrate for infrared optical filters that are incorporated into FTIR spectrometers.
    IR Detectors
    • Silicon optics are often used in NIR and MWIR detectors due to its transmission from 1 to 6µm. Silicon based detectors are used in applications such as LiDAR and muzzle flash detection.

    Our 2022/2023 Catalogue

    Inside Knight Optical’s catalogue you’ll find details of the industries we serve as well as the many other services we can offer. It also showcases hundreds of our most popular products, supported by useful technical information, to help you choose the right part for your application.
    Click below to view our digital catalogue now.

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