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Surface Treatments 189

lenses are thermosetting materials. Higher index materials are usually softer
and must be hard coated. Thermoplastic materials can be reheated and
reformed. They consist of long chains of molecules that do not cross link
with each other. Most common thermoplastic material is polycarbonate.
All known thermoplastic materials are very soft and they must be hard
coated.
Hard coating is a procedure by which a thin layer of material usually
0.5 micron to 10 micron thick is applied to the lens surface. The most
common material used is lacquer which is applied either by dipping or by
spinning procedure. Vacuum deposited hard coats are less common. It may
be tintable or non tintable. Non-tintable hard coating offers more scratch
resistance than the tintable.
It is not possible to see the layer of hard coating except of the edges of
bifocal segments and there is no optical effect of hard coating if the refractive
index of the lacquer used is same as that of the base lens material. If the
refractive index differs, there is a change in surface reflection.

HYDROPHOBIC COATING
Anti-reflection coating needs to be kept clean. The reason is that a very
small amount of oil or grease on top of coating can destroy its effect. The
result is loss of the purpose for which anti reflection is being applied.
Since the effect of this does not occur evenly across the lens surface , this
could be very annoying to the patient. In fact, they are not so dirtier, the
wearer is just more aware of it. This problem can be eased if a hydrophobic
coating is applied to the lens surface. The basic principle is to create a
high surface wetting angle, which allows the water or oil to run off rather

Fig. 13.2: Top layer coating is hydrophobic coating