There are two kinds of lenses inside the laser head — collimating lens and focusing lens, both of them are sensitive to any type of contamination. Especially the material splatter while the laser cutting the metal. Normally need to replace a piece a day depends on different metal cutting, Comparing the cost of collimating lens and focusing lens, the price of the protective lens is inexpensive.
That’s why we need the protective lens (protective windows) to protect the focusing lens and the collimating lens to ensure the optic system working stability during laser cutting. Because it protects the focusing lens and collimating from back spatter and debris.