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BREXIT UPDATE: Knight Optical will continue to provide quality assured service for all your optical requirements both custom & stock. We will handle all requirements caused by the new regulations.


Looking For a Custom Optical Solution? Enquire

Custom Caesium Iodide

Caesium Iodide (CsI) is a water soluble material that transmits from UV all the way to the Far IR, with its useful transmission extending to 55µm it has the deepest known IR transmission of the common infrared materials. It is very resistant to mechanical and thermal shock but can degrade under certain atmospheric conditions, especially when in contact with moisture.
Below is a brief summary of caesium iodide’s optical properties, a full data sheet is available here.

Transmission range

0.25 to 55µm

Refractive Index

1.73916 @ 10µm


4.51 g/cm3

Melting Point



Knoop 20 with 200g indenter

Knight Optical can supply high quality caesium iodide components, including windows, lenses and prisms, made to your custom specification. 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 50mm +

Form error (@633nm)

< 0.25 waves

Centration /parallelism

< 1 arc minute


< 20/10

Typical coatings

BBAR @ 1.2-3µm

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.


  • Caesium iodide is a very soft material with a hardness of HK20 and therefore can be difficult to polish.

Typical Applications

  • Caesium Iodide can be doped with Thallium to create CsI(Tl) which is creates one of the brightest scintillators, emitting at wavelengths that work well with Silicon photodiodes.
Security Imaging Systems
  • Arrays of CsI (often doped with Thallium) can be used in security imaging systems including baggage scanners.
Infrared Spectroscopy/FTIR spectrometers
  • CsI is commonly used in infrared spectroscopic windows due to its exceedingly large transmission band.
  • Due to the materials degradation when in contact with moisture, it is often coated and stored within a desiccator to minimise any interactions with the atmosphere.


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.
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