These fibers are made of two different core materials. Chalcogenide (CIR) fibers exhibit high transmittance from 2 to 6 µm, and are characterized by low optical loss and high flexibility. Polycrystalline (PIR) fibers exhibit high transmittance from 4 to 18 µm. The internal core and cladding are designed to prevent light form leaking through the fiber, and has a high enough damage tolerance to withstand damage from sources as intense as a continuous-wave CO₂ laser. Chalcogenide IR fibers have a max operating temperature of 96˚C and Polycrystalline IR fibers have a max operating temperature of 120˚C.