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High-Index Lenses: Thin is Most
Definitely In
High Index lenses are made from material that is of much
greater density with a higher index of refraction than standard
plastic lenses. Because of these characteristics, less material
is required to provide the prescribed amount of vision correction
and results in thinner, lighter lenses. These lenses are
especially beneficial for patients with moderate to strong
prescriptions who wish to avoid thick, heavy, and unsightly
lenses.
Lens materials with a higher index of refraction (IOR)
allow light to pass more quickly through the lens, so less
of the actual lens material needs to be used. That's why
lenses with a higher index of refraction, or high index
lenses, can be thinner than traditional lenses.
High index lenses are available in either glass or plastic
(CR-39, for example). Different manufacturers make different
high index lenses, and what sets each lens apart from the
others is its index of refraction. The higher the IOR, the
denser the material. All things being equal, a 1.66 IOR
material will result in a thinner and lighter-weight lens
than a 1.57 IOR material will.
Best Candidates
High index lenses are traditionally used for myopes. High
index materials don't provide as much benefit to patients
wearing plus corrections as minus. Plus lenses made in high
index will be thinner, but the thinning takes place in the
center of the lens where it is not visible and does little
to improve the cosmetic appearance of the eyewear. For plus
patients, aspheric lenses with their flatter, slimmer profile
offer the greatest cosmetic benefits for plus lenses. Most
aspheric lenses are made of higher index materials, and
they provide improved cosmetic benefits and better vision
for plus patients.
Pros and Cons
The advantages of high index lenses are:
They are thinner, flatter and lighter than conventional
plastic or glass
They can fit into neat and fashionable frames
They have a higher scratch resistance than normal plastic
and higher impact resistance than glass.
Built-in UV protection |
The disadvantages are:
They can be expensive
They are not as scratch resistant as glass.
Ultra-High Index |
Without question, "ultra-high index" lenses are
designed for people who want the latest in thin and light
lens technology. Depending on the design (e.g., aspheric)
and the final center thickness, lenses with refractive indices
of 1.67 and above can be as much as twice as thin as polycarbonate
lenses after processing. For higher prescriptions?say, +/-10.00D
and above?this can be quite significant.
Generally, the higher the IOR, the higher the cost of the
lenses will be. The thinnest, most lightweight lenses are
typically the most expensive.
Ultra-high index lenses provide their best benefits when
they are matched with aspheric curves. These lenses have
flatter front curves than their lower index counterparts,
resulting in thinner and lighter lenses?clearly a plus for
patients. Aspheric curves are especially helpful. The asphericity
reduces spatial distortion, reduces the eye's magnification
or minification, and maintains a thin and flat lens profile.
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