OPTICAL COATINGS

The reduction of reflection on glass surfaces can be achieved by applying a so-called λ/4 coating – such as magnesium fluoride (MgF2). If the coating thickness d is selected so that d = λ/(4 x n) - where n is the refractive index of the material – the reflected light waves of wavelength λ are canceled out at the two interfaces (air/coating and coating/glass) by superposition. The result is reduced reflection.

An alternative method uses triple coatings, whereby the refractive indices are selected so that they lie between air and glass. The anti-reflective effect is achieved here by carefully selected refractive indices that create a gradual transition from the refractive index of the air to that of the glass.

The use of multi-coating systems is recommended for demanding optical coatings that minimize reflections and increase light transmission. Such multiple coatings consist of material coatings with different refractive indices in precise thicknesses. The numerous interfaces and the associated reflections in the system lead to superimpositions – both destructive and constructive – and influence optical behavior.

Uniformly high reflectivity in the UV, VIS, and IR range

1. Magnesium fluorite (MgF2) with n = 1.38
2. Silicon oxide (SiO2) with n = 1.46
3. Aluminium oxide (Al2O3) with n = 1.67
    

Highest reflectivity in the VIS range

1. Titanium oxide (TiO2) with n = 2.55
2. Tantalum oxide (Ta2O5) with n 2.20
3. Zirconium oxide (ZrO2) with n = 2.15
4. Silicon nitride (Si3N4) with n = 2.05