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Custom KRS5

KRS5, or Thallium Bromo-Iodide, is a thallium halide material, manufactured for its ability to transmit into the deep IR. It is most commonly used as a spectroscopic infrared material due to its broad transmission band and low absorption. KRS5 is a deep red in colour.
Below is a brief summary of KRS5’s optical properties, a full data sheet is available here.

Transmission range

0.6 to 40µm

Refractive Index

2.371  @ 10µm


7.371 g/cm3

Melting Point



Knoop 40.2

Knight Optical can supply a variety of KRS5 optical components including ATR prisms , windows and ground blanks. We can also provide uncoated and coated optics, with broadband antireflective (BBAR) coatings. 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


< 40/20

Typical coatings

BBAR @ 2-40µm

DLC @ 2-40µm

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

  • KRS5 is non-hygroscopic with a high chemical resistance against strong acids.
  • It is manufactured by the sealed-ampoule Stockbarger technique, with a high purity starting material to remove possibility of absorption bands in the mid-IR region.
  • It also tends to cold flow, meaning it slowly deforms in shape under mechanical stress and pressure.


  • KRS5 is toxic and must be handled with extreme care. Gloves should be worn, and hands washed thoroughly after handling the optics.

Typical Applications

Fourier Transform Infrared (FTIR) Spectroscopy
  • KRS5 is widely used as a substrate for ATR prisms due to its wide transmission band, low absorption, and high refractive index. It is also stable under normal atmospheric conditions.
  • It is also used as transmission windows for liquid and gas samples.
Wire Grid IR Polarisers
  • Thallium Bromo-iodide is used as a substrate for wire grid IR polarisers. The grooves on the polarisers are manufactured using a holographic technique which can create submicron wire grid spacing. The grooves are filled with a metal deposit to create the wire grid.
  • Wire grid IR polarisers polarise light from an unpolarised IR laser source, and attenuate light from a polarised source.
  • These polarisers are used in a variety of applications including FTIR spectroscopy, medical imaging, and aerospace.
Athermalised imaging systems
  • KRS5 can be used in conjunction with germanium to correct for thermal defocussing within a system. It will often be used as a triplet along with another infrared material, such as zinc sulphide, to correct for the chromatic aberration that KRS5 can introduce.


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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