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88 Ophthalmic Lenses

Fig. 8.5: Principle of anti-reflection coating
However, in practice it is impossible to attain total extinction of reflection
just by depositing a single layer of coating on the lens. In fact, the reflection
could be almost eliminated by meeting two conditions:
1. The refractive index of the coating is the square root of that of the lens
material.

n film = n Substrate
2. The thickness of the coating, multiplied by its refractive index, is exactly
one – quarter of a wavelength of light or any odd number of quarter
wavelengths.
Unfortunately, this is not always obtainable. Crown glass with a refractive
index of 1.523 would need a material with an index of 1.234, whereas the
most suitable material available for glass is Magnesium Fluoride which
has a refractive index of around 1.38. So it is not very well suited for crown
glasses but its performance is better on higher index materials. Also the
two necessary conditions can be met for only one wavelength. This is the
reason why single layer anti-reflection coating cannot reduce the reflectance
in the entire range of visible spectrum. Single layer coatings are designed
to give their maximum effect in the middle and brightest region of the
visible spectrum. The reflectance gradually increases towards both violet
and red ends of the spectrum. As a result, the coated surface presents the
characteristics of purplish appearance which is described as bloom.

MULTILAYER ANTI-REFLECTION COATING
The limitations of the single layer antireflection coating can be overcome
by multilayer coating which are now capable of almost extinguishing
surface reflections from one end of the visible spectrum to the other. The