Aluminum reacts easily with oxygen and water vapor in air to form aluminum oxide (Al₂O₃), resulting in a decrease in reflectivity (especially in the UV and visible wavelengths).
Solution: protective coating (transparent dielectric protective layer on the surface of aluminum).

Protective coating material	Applicable bands	Feature
MgF2	UV-VIS (optimal)	High hardness, good transmittance in the UV, but general resistance to moisture and heat (thickness about λ/4, λ=550nm).
SiO2	VIS-IR	Chemically stable and resistant to wiping, but with high UV absorption (thickness needs to be optimized).
Al2O3	Broadband	Prepared by atomic layer deposition (ALD), they are ultra-thin and dense, but costly.