Anti-reflective coatings

An easy way to apply an anti-reflective coating to glass is the sputtering or thermal evaporation of what is called a λ/4 layer of, for example, magnesium fluoride (MgF₂). If the layer thickness d is determined according to formula d = λ/(​4 x n), where n is the refractive index of the layer material and λ the wavelength of the light, then the light waves reflected at the two boundary surfaces (air/coating and coating/glass) interfere destructively. The result is a reduction of the reflection.

Another possibility, for example, is the deposition of triple layers with n (air) < n1 < n2 < n3 < n (glass). Here, the anti-reflective effect is achieved by well-chosen refractive indices, which produce a gradual transition of the refractive index of air into that of glass (gradient layer).

Multi-layer stacks are required to produce complex optical coatings that minimize reflections and maximize transmission to the greatest possible extent. In general, these multi-layer coatings consist of alternating layers of low- and high-refractive index materials with a well-defined layer thickness each. The many boundary surfaces and their associated reflections in the layer system lead to interference, destructive as well as constructive, and determine the optical characteristic of the whole stack.  

Low-index dielectrics

• Magnesium fluorite (MgF₂) where n = 1.38
• Silicon oxide (SiO₂) where n = 1.46
• Aluminium oxide (Al₂O₃) where n = 1.67   

High-index dielectrics

• Titanium oxide (TiO₂) with n = 2.55
• Tantalum oxide (Ta₂O₅) with n = 2.20
• Zirconium oxide (ZrO₂) with n = 2.15
• Silicon nitride (Si₃N₄) with n = 2.05