Anti-reflective coatings

Anti-reflective dielectric coatings

Enable application of anti-reflective coatings on surfaces

An easy way to apply an anti-reflective coating to glass surfaces consists in the sputtering or vapour deposition of what is called a λ/4 layer of, for example, magnesium fluorite (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, then the light waves of wavelength λ reflected at the two boundary surfaces (air/coating and coating/glass) cancel each other out by destructive interference. 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).

Systems with multiple layers will need to be used to produce complex optical coatings that minimize reflections to the greatest possible extent and at the same time maximize the transmission of light waves. In general, these multi-layer coatings consist of alternating layers of low- and high-refractive materials with a precise layer thickness. The many boundary surfaces and their associated reflections in the layer system lead to interference, destructive as well as constructive, and determine the optical effect.   

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

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