Thalium Bromoiodide (KRS) is a deep IR material with a high refractive index. KRS-5 crystalises by the sealed-ampoule Stockbarger technique. Starting materials of the highest purity are selected to ensure that there are no anionic absorption bands between 2 and 16 microns and all crystals are checked for quality by using a path length of 120mm.

KRS-5 is only slightly soluble in water but can be dissolved in alcohol and nitric acid. This material is considered toxic and should be handled with care.

The refractive index of KRS-5 can vary due to small compositional changes and can sometimes be seen as a gradient across the material. KRS-5 is used extensively in spectroscopy for ATR (Attenuated Total Reflection) prisms, windows and lenses. The low solubility, high refractive index and low absorption make KRS-5 ideal for this application. In conjunction with Germanium, KRS5 can also be used in athermalised IR imaging systems.