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92 Ophthalmic Lenses

strip anti-reflection coating from the old lens and recoat both lenses together
for an exact colour match. Never use a stock anti-reflection coated lens for
one eye and order custom anti-reflection coating for the other. Inevitably,
there will be a slight colour difference in the anti-reflection coating. The
difference in colour has no effect on vision but is always noticed by the
patient.
Be sure to explain that anti-reflection lenses must never be cleaned dry,
as this can scratch the coating. The best cleaner is one that is made especially
for anti-reflection coated lenses. Patient should not use acetone, windex,
caustic solution or soaps to clean the anti-reflection coated lenses. Make
sure that every eyewear patient going through the office gets an explanation
and demonstration of anti-reflection coatings benefits. This ensures that
every patient understands you are trained and skilled in dispensing what
has become accepted as the most advanced modern eye wear available.

TECHNOLOGY OF ANTI-REFLECTION COATING
Anti-reflection coating requires great technical expertise and very high
quality pure coating material. The technology consists of stacking thin layers
with specific characteristics to achieve overall result and is done inside a
vacuum chamber. The lens to be coated must be absolutely free of surface
defects, perfectly cleaned and rid of dust or any other impurities before
they are placed in the vacuum chamber. Extremely low pressure is created
inside the chamber by removing the air from it and a very high temperature
has to be ensured which is fine for mineral glass lens but not for resin lens.
Material used for coating must be heated to 1000 deg C/1800 deg F to 2200
deg C/4000 deg F to bring it to gaseous state. The several layers of
transparent materials are evaporated by a very high energy evaporator.
The lens surfaces are charged electrically to create an attraction between
the lens surface and the material. Coupled with the use of sophisticated
microprocessor controls of pressure, temperature and evaporation rate and
other technological advances with regard to adhesion of coating to lens
surface has allowed the application of very durable coating to most lens
types. The end result is lenses that provide enhanced cosmetic appeal, fewer
ghost images, improved contrast and therefore, improved optical
performances.
However, glass lenses can be heated up to 300 deg C/570 deg F, but
plastics heated above 100 deg C/210 deg F turn yellow and then decompose
very fast. Magnesium fluoride used to coat glass lens has no adhesion, if
applied below 200 deg C/400 deg F. Hence, plastic lens coating requires
more complex stacking procedure with low index layers made of silica.