Potassium Chloride (KCl) is commonly used for optical components in wide spectrum band from the ultraviolet to the infrared, since it is transparent over the entire range of wavelengths (about 0.3 µm to 20 µm). Its transmission extends beyond that of Sodium Chloride.
Potassium Chloride has a low refractive index (1.454 at 10.6 microns), its damage threshold is high. That's why optical windows from Potassium Chloride are particularly useful for spatter barrier windows in CO2 lasers.
Potassium Chloride can be used up to 400 °C. The material is hygroscopic and highly soluble in water, that's why it must be kept in a dry environment or protected against moisture degradation of polished surfaces.
Potassium Chloride is commonly used for optical windows, lenses, prisms and other optical items in the range from the ultraviolet to the infrared. It is one of the most useful materials for IR spectroscopic components and optical items for Fourier transform infrared (FTIR) spectroscopy. Due to its low cost it is mainly used for CO2 laser protection windows.