Anti-Reflection coatings (AR)
Number of layers
A simple single layer (EAR) decreases the natural reflection of an uncoated surface, best if the refractive index of the layer is near to the square root of the index of the substrate. For Fused Silica (FS) no coating material can be found which fits perfectly. The best matching material is MgF2.
By using two layers of different coating materials the reflection can be minimized for one single wavelength (VAR).
Anti-reflection coatings even for broadband areas (BBAR) can be realized through an increased number of layers. The larger the bandwidth the higher the achievable reflection.