In order to optimize function of these mirrors, different substrate materials are used - the most common ones are silicon(Si) and copper (Cu). Silicon mirrors have light weight and are therefore preferred in flying optics where high accelerations are needed. Copper has high thermal conductivity, and channels for cooling water can be included directly into the Co2 laser mirrors. Therefore, copper mirrors are preferred if best-possible cooling is important, for example in Co2 laser machines with very high laser power. - The optical properties of a mirror (reflectance, phase shift, etc) are determined by its coating. So in order to realize different mirror functions, different coatings are needed.