Mirrors or total reflectors are used in laser cavities as rear reflectors and fold mirrors, and externally as beam benders in beam delivery systems. Silicon is the most commonly used mirror substrate; its advantages are low cost, good durability, and thermal stability. Copper is typically used in high-power applications for its high-thermal conductivity. Molybdenum’s extremely tough surface makes it ideal for the most demanding physical environments. Molybdenum is normally offered uncoated.