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Lenses
LENS
MATERIALS:
Thirty years
ago this was an easy decision: Glass. Today, we have
glass, plastic, polycarbonate, photochromics, polarized,
and hi index materials. The patient needs to select an
optical expert that will guide you through the different
pros and cons for each individual prescription and the
different coatings and edge treatments available. Very
few patients select glass. Glass is heavier and has a
higher potential for bre akage. Its sole advantage is
that glass is very scratch resistant. However, there is
little advantage to purchasing something designed to
last for ten years when the average patient replaces
their eyewear every 2-3 years.
Plastic lenses are the
gross majority of lenses sold today. The thinness of the
plastic is determined by its index of refraction and by
the surfacing skills of the optician. Standard plastic
has an index of 1.48. Hi-index lenses are those with
indices of 1.56, 1.58, 1.60, and 1.66. Generally
speaking, patients with prescriptions over –3.00 should
consider getting a higher index lens. The higher index
or density of the material theoretically will produce a
thinner lens. However, life is never that simple and
another factor is the “center thickness of the lens”.
Some of the higher index lenses cannot be cut as thin
because the material will distort. Therefore, a –5.00
lens may actually be thinner overall if a 1.58 is
selected over a 1.66. Another variable affecting what
lens material to select is the abbe number or clarity
factor. Some materials are optically clearer. This is
especially important in higher corrections. Finally, the
lens base curve must be considered. This measurement is
used for your airplane windows and motorcycle fairings.
Its principal advantage is, it naturally absorbs UV
radiation and is extremely impact resistant. It can
actually repel a 22-caliber bullet without breaking! The
downside is its abbe number is lower and I don’t feel
it’s quite as scratch resistant as other plastics. For
children and sports eyewear it is the best choice.
Polarization, photochromics, transitions, and sunglass
clips, please refer to our sunglass page.
LENS
DESIGNS:
Lens design
can be divided into two groups: single vision and
multi-focal. A discussion of single vision lenses is
similar to our discussion of lens materials. However,
multi-focals is another story. Again, 30 years ago if
you needed bifocals you received lenses with a heavy
line called an executive lens. Then came a smaller
version known as a flat top or fused bifocal. This
second group of lenses was thinner, lighter, and had
less distortion. Next came round segment lenses and the
first “invisible” lens, which was known as a blended
lens. Other types of bifocals such as ribbon and
trifocal lenses soon followed. Finally, the progressive
lens was developed. This lens is marketed, as an
invisible bifocal but is in reality an invisible
trifocal. It is an aspheric lens, which gradually
increases magnification as the patient looks down. These
lenses are constantly being improved and newer designs
of progressives are being constantly being introduced.
Similar to the lens materials, progressive lens designs
need to be carefully selected. The lens design chosen
will be dependent upon the actual prescription and the
patient’s visual requirements. Hard, soft, multi design,
and specialty progressive designs should be considered
for each patient.
LENS COATINGS AND
TREATMENTS:
Once the proper material has
been decided different coatings and processing can be
done to customize the lenses even more to your needs.
Tinting is both therapeutic and fashionable. Some people
select tints solely on its appearance while other
patients find certain tints make their eyes more
comfortable.
Some common tints and their clinical
benefits are: light pink, which helps many people with
glare from fluorescent bulbs, light yellow, which
reduces glare from natural light sources, and many
people feel they see better in the distance. Pilots and
skiers especially like yellow tinting on overcast days.
A light grey helps people who are overly light sensitive
without making the lenses to dark to be unsafe for
daytime driving. Another popular coating is AR or
anti-reflective coating. These coatings were originally
designed to camouflage submarines from being observed
from planes in the air. These coatings eliminate the
reflections on the front and rear surfaces of the lens.
This enables the patient to see much clearer at night,
improves visual comfort on the computer and allows
people to see your eyes much clearer.
Everything that
deals with optics, even the inexpensive disposable
cameras have coated (anti-reflective) optics. I strongly
advise patients to consider having this applied to their
lenses. The negative side of these coatings is sometimes
they craze and have to be stripped and re-applied, and
they make “cleaning” the lenses more difficult. Edge
polishing makes the edge of the lens as clear as the
front surface. Cosmetically this makes your lenses look
thinner. All of our lenses in rimless and metal frames
are automatically polished.
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