Thallium Bromoiodide (KRS-5) discs are a medium refractive index material (2.37 @ 10 microns) with excellent long wavelength transmission in the infrared spectral region. It is non-hygroscopic with very good chemical resistance; however, KRS-5 will occasionally exhibit cold flow behavior. KRS-5 is extremely toxic and is absorbed through the skin and should only be handled with glove protection.