KRS-5, also known as thallium halogenide, is a crystalline material used in a variety of optical applications. It is a heavy metal halide with a chemical formula of TlI and is known for its high refractive index and low absorption in the visible and near-infrared spectrum. This makes it useful as a window material for spectrometers, as well as in other optical systems that require high transmission in the visible and near-infrared regions. Additionally, KRS-5 has a high thermal stability, making it suitable for use in high-temperature environments, and it is easily polished, allowing for precise optical surfaces.