When a customer enters an electronics store, they are not
offered an analog, black and white television and a VCR.
Instead, they are offered the latest in technology. The same
should happen when a patient enters an optical dispensary.
He or she should not be offered uncoated, CR-39 lenses.
Instead, the patient should be offered the latest in the
technology that meets his or her needs. In many instances,
this would mean offering the patient high index lenses.
What is a high index lens?
Although the technical definition for high-index lenses
is any lens with a refractive index higher than 1.52, the
high index lenses that will be discussed in this article are
those with an index of 1.64 or higher.
First things first, what is an index? In simple terms,
index is a measurement of how much light is bent by the lens
material. The higher the number, the more the light is bent
by the lens. What this means to the eye care professional
and the patient, the higher the index, the less material
that is needed to bend the light to fill the doctor’s
prescription. For example, a -7.00D lens ground on a 1.70
index material will be approximately 50 percent thinner than
a -7.00D lens ground on a 1.50 index material if all
parameters are equal.
High index lens designs
As technology has improved, so has the availability of
high index lens designs. High index is most commonly
available in multiple progressive and single vision designs.
However, lined multifocals can sometimes be found in the
material. Aspheric and atoric designs are also becoming more
readily available to the eye care provider in high index
materials. In aspheric and atoric designs, the lenses are
gradually steepened along the periphery in minus designs and
the periphery is gradually flattened in plus designs. The
benefit is greater peripheral vision as well as less
magnification or minification of the eye. High index is also
available with polarization or photochromic options. Glass
is another high index lens option; however, it is not
usually the best option due to weight and safety
When to recommend a high index lens
A high index lens is thinner and lighter (except in the
case of glass lenses) than other conventional materials.
Therefore, it is an ideal lens material for individuals with
a higher prescription. A general guideline is to recommend
high index materials for any patient with a prescription of
+/-3.00 D or more. Anything less than this prescription and
the weight and thinness benefits are usually minimal.
High index, especially the index of 1.67 and 1.70, is
also an excellent choice for individuals who want the
minimalist look of drilled rimless eyeglasses. Due to the
thinness of the material’s edge, the lens does not look
out of place or heavy when placed in a drilled rimless. In
addition, high index materials do not crack as easily when
drilled and the hole maintains its size and shape more
readily than lower index materials such as CR-39.
High index is also essential for the newer, larger frame
designs. With the increasing popularity of larger frames,
edge thickness problems due to decentration are going to
become more prevalent. High index can eliminate some of this
but remember that it is very important to consider the lens
prescription when fitting a larger frame.
Lifestyle can also be an important factor when
recommending high index lenses. Certain professions such as politicians, lawyers, and
those who engage in public speaking prefer the cosmetic
benefits of high index lenses. Individuals who are fashion
conscious are also good candidates for the thinner profiles
that are provided by high index.
There are some exceptions to fitting a high index lens,
however. One instance is children or individuals who have
severely decreased vision in one eye. In these cases, the
safety and impact resistance of polycarbonate or Trivex® is
necessary to protect the patient’s vision. Also, the
flatter base curves of high index lenses prevent the lenses
from fitting into the wrap designs that are popular in some
frame designs and are therefore not recommended.
The Importance of Anti-Reflective Coating
Do not sell a high index lens without anti-reflective
(AR) coating. This may seem extreme, but an uncoated CR-39
lens reflects 8% of light, whereas an uncoated high index
lens will reflect up to 50% more than CR-39. A rule of thumb
is the higher the index, the greater the amount of light
reflected. This can lead to increased difficulty with night
driving as well as eye fatigue due to decreased light
transmittance. With an AR coated lens however, the light
transmittance can increase to 99.5%. As a result,
reflections and chromatic aberrations are reduced, the
patient has a clearer view of his or her surroundings, and
the glasses have a better cosmetic appearance. By applying
AR, the rate of non-adapt to the visual differences in high
index material over lower index materials will decrease.
Fortunately, AR coatings have really improved over the last
few years and are an essential enhancement to high index
with the new scratch, dirt, and oil resistant formulas
How to fit High Index Lenses
Although there are several factors that help to ensure
patient satisfaction when fitting a high index lens, the
main factor is taking accurate measurements. It is important
that the eye care professional take monocular PDs and
determine the optical centers. When taking measurements it
is essential that the frame fits well, there is minimal
decentration, and that the pantoscopic tilt is between 10
and 15 degrees. Once these measurements are taken, it is
best to determine the lens thickness to ascertain if it
meets the needs of the patient. This can be done by
utilizing the sag approximation formula and adding the
result to the predetermined center thickness or by applying
optical calculators such as those found on OptiCampus.com.
For reference, the sag approximation formula is:
Sag = ((d/2)2 X D) / 2000(n-1)
where d is diameter in mm
D is power
n is index
Everybody wants the look and feel that high index lenses
can provide. Low index lenses are thicker, thinner lenses
look better. Lower index lenses are heavier, lighter lenses
are more comfortable. Therefore, it is no surprise that high
index lenses are the wave of the future in the optical
industry. High index lenses provide the best solution for
most patients with higher prescriptions. With the proper
understanding of the different designs of high index
materials and fitting techniques, an eye care provider can
give the best possible vision solution to their patients.