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8 Anti-reflection

Coated Lens

The normal eye is easily adapted to different lighting conditions. Thus, the
loss of light due to reflection is usually not serious enough to reduce visibility
except where poor lighting condition already exist. It is the reflected light
which enters the pupil of the eye along the same path as the light used in
“seeing” an object that is of primary importance. Whenever light is incident
on the boundary between two medium, some light is reflected and some is
transmitted, and the transmitted light undergoes refraction into the second
medium. Anti-reflection coating are applied on the lens surface to efficiently
manipulate the light transmission and reflection through it. It has been
seen that the surface reflectance can be reduced by coating the lens with a
film of some materials having a lower refractive index than that of the
lens. Although, there are now two reflections – one at the exposed surface
of the film and another at the lens surface, their combined effect produces
less reflection than the uncoated surface. The result is, patient experiences
a marked sensation of improved vision. There is no doubt that anti-
reflection coating leads to greater patient comfort.

THE REFLECTION OF LIGHT

Dr Rayton presented some of the problems encountered due to reflections
of light in spectacle lenses. There are four definable sources of reflection
that can cause annoyance to the spectacle lens wearer:
1. Frontal Reflection: Frontal reflection occurs when some of the light
incident on the front surface is reflected back towards an observer as
shown in Figure 8.1B. This can be disconcerting for the observer, as the
wearer’s eyes are difficult to observe. Communications between human
relies just not on spoken or written words but also by facial expressions,
much of which are centered on the eye’s reactions. Inability to see the
eyes of a person you are talking to can be rather “off - putting” as one
cannot deduce whether what is being said is actually being understood.
These reflections are also annoying from purely a cosmetic point of view
as they detract from the overall appearance.