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90 Ophthalmic Lenses

On crown glass with refractive index of 1.523, the first layer of coating
is applied with the material which has a refractive index of 1.70. Either
beryllium oxide or magnesium oxide is used for this layer. And the top
layer is coated with magnesium fluoride which has a refractive index of
1.38. Both beryllium oxide and magnesium oxide are soft materials and
will not produce very durable coating. Although, it allows some freedom
in the choice of coating materials and can give very low reflectance, the
two layer coating is very restrictive in its design. As a consequence multi-
layer antireflection coatings have been developed to allow the refractive
index of each layer to be chosen. The complex computer design techniques
were developed to make multilayer coating based on the simple principles
of interference and phase shifts. All methods consider the combined effect
of various film elements. Each layer is influenced by the optical properties
of the layer next to it. The properties of that layer are influenced by its
environment. Clearly this represents at least a complex series of matrix
multiplication where each matrix corresponds to a single layer. The
efficiency is not necessarily directly related to the number of layers used,
rather on correct selection of coating materials for refractive index, the
thickness with which the layers are applied, and the adherence to the
previous layer of the materials are all qualifications for an efficient multiple
coating.

ADVANTAGES OF ANTI-REFLECTION COATING
Primarily reasons for recommending anti-reflection coating to the eyewear
consumer are:
1. People see better with anti-reflection coated lenses as anti-reflection
coated lenses transmit more lights. Conventional glass or plastic gains
8% in light transmission. High index lenses gain even more 11% to 16%
depending on the materials. Increased light means wearer sees things
brighter and clearer, with crisper details.
2. While looking at a person wearing uncoated lenses, we see reflection of
light that is actually coming from the front surface of the lenses. This
can be disconcerting for the observer, as the wearer’s eyes are difficult
to observe, creating a barrier between the observer and the wearer. By
getting rid of the reflections, the lenses seem to “disappear” into the
frame.
3. Ghost images are a common experience with uncoated lenses,
particularly while driving at night. These visual annoyances appear as
dull images created by reflections coming from the internal lens surfaces.
With minus correction, ghost images are crisp and reflected on the side
of the light image towards the optical centre. With plus lenses, ghost
images are larger, less distinct and appear away from the optical centre.