BY PALMER R. COOK, OD
Our patients expect to see well, look good and
be comfortable in their new eyewear. They also
expect, quite reasonably, to perceive a value
equal to or greater than the cost of the eyewear.
If you are a full-service provider, you have a
responsibility to meet these four expectations.
But sometimes things don’t go as planned and
the disappointed patient may turn up on your
doorstep. If you think about it, that’s probably
the best of a bad situation. It gives you a chance
to resolve the problem and set things right.
Unfortunately, many patients try to tolerate
marginally satisfactory aspects of their eyewear
experience. These patients are prone to
tell friends, neighbors, co-workers and family
members of their less than ideal experience.
They are also likely to take their prescription
“elsewhere to have it filled” the next time.
I regularly field questions from doctors and
dispensers about patients who are distressed
in one way or another about their new eyewear,
and I have come to think of this as “eyewear
distress syndrome.” This is not the same
as “eyewear buyer’s remorse,” but sometimes
the two are often almost inextricably intertwined.
A major cause for this distress lies in
the difficulty of selecting the lens material best
suited for the prescription and the patient. Of
all the eyewear design decisions you make one
of the most confounding and frustrating is
selecting the best lens material.
When we only needed to select either glass
(clear, scratch-resistant, but twice-the-weight)
or standard 1.498-index plastic (half-the-
weight, more shatter resistant, but prone to
scratch easily), the presentation to the
patient was relatively easy and straightforward.
Today there is a growing array of lens
materials available and deciding which will
best meet the needs of your patient can be a
daunting and confusing task. After reading
Professor Mo Jalie’s discussion of curve variation
factors in Ophthalmic Lenses &
Dispensing, I realized that this concept could
be converted from a standard of glass to one
based on plastic. Doing this conversion to
standard index plastic (n = 1.498) makes
sense because it’s the material with which
U.S. dispensers have greatest familiarity.
This conversion yields an easy-to-use
“decoder,” truly a kind of Rosetta Stone that
allows you to better apply your optical
knowledge. The examples and “how to” discussion
that follows will enable you to begin
to make the best lens material decisions. As
with any tool, this Rosetta Stone for Eyecare
is not a stand-alone, single answer for every
lens material decision. It provides a benchmark
that can be used with your experience
and professional judgment to achieve the
best eyewear for each of your patients.
Although there are many root causes for
“eyewear distress syndrome,” a preponderance
of these calls involve an inappropriate
choice of lens material. Practitioners who
contact me are generally experienced, skilled
and knowledgeable. They usually identify
and deal with problems such as “unhappy
with the frame I picked,” “issues of cost” and
predictable “disappointment in best correctable
vision” (e.g. macular degeneration or
advancing cataracts) before they call. This
makes my perspective on “eyewear distress
syndrome” somewhat skewed, but be assured
that a reconsideration of lens materials should
be part of resolving eyewear distress problems.
Playing the part of an optical Sherlock
Holmes, a role every practitioner must
assume from time to time, is always made easier
by carefully considering the patient’s primary
complaint about the eyewear. Some
common complaints and how they relate to
the issues of lens material include:
“I don’t like how I look in my new eyewear.”
If the material is
chosen with a refractive
index that is too low,
there is usually either too
much lens thickness or
the eyewear has been
made inordinately small
to minimize the low
index/thickness and curvature
issues. Of course,
using a smaller eye size
reduces the edge thickness
of minus lenses and
allows plus lenses to be
ground thinner, but this
reduction in size has its practical limits.
If the refractive index is too high, the cosmetic
issue is primarily reflectance. As index
increases, lenses reflect more light. Light
reflecting from both the front and back surfaces
toward anyone looking at the patient creates
a veiling glare that obscures the most
expressive part of the face—the eyes. For standard
plastic (1.498), 3.97 percent of light
falling on the front of the lens will be reflected.
If the index is increased to 1.59 (poly), the
reflectance is 5.15 percent, an increase of 29.7
percent. Using anti-reflective lenses is the
obvious and easy answer for this problem.
Regardless of the material chosen, reflectance
and related veiling glare will be greatly
reduced if lenses with high-quality, anti-reflective
(AR) properties are used.
“I don’t see well in my new eyewear.” Not
seeing well with new eyewear can be the result
of power or axis errors, inappropriate base
curves, incorrect placement of the MRPs (i.e.
major reference point—the point that gives
the correct prescription), internal or surface
imperfections or even an incorrect prescription.
When the cause for not seeing well is an
inappropriate choice of lens material, the difficulties
are usually related to increased
reflectance, chromatic aberration or the
combination of the two. The increase in
reflectance that causes glare from the front
surface of the patient’s lenses also causes ghost
images when night driving,
annoying reflections
from the backs of the
lenses and less clarity due
to veiling glare from light
reflected within the lenses.
The night driving difficulties
result from light
that either passes through,
or is reflected from, four
interfaces of differing
index. Clinicians should
realize that the ghost
image from a non-antireflective
1.58 lens is ultimately
approximately 65 percent brighter
than the ghost image from a non-AR, standard
index lens. Is it any wonder that patients complain?
Again, this difficulty is best dealt with
by using high-quality AR lenses.
When lens power varies depending on the
color (wavelength) of the light passing through
it, chromatic aberration results. As patients
look through the optical centers of their
lenses, chromatic aberration is usually unnoticed.
When the lines of sight pass through any
point in the lens other than the optical center,
the induced prism displaces the images
formed by the red, orange, yellow, green and
blue portions of the spectrum to a greater or
lesser extent depending on the wavelength
you consider and significant degradation of
the retinal image can occur.
When looking one centimeter away
from the center of a -5.00 lens, the patient
experiences 5 prism diopters of image displacement
if there is no chromatic aberration
in the lens (i.e. all wavelengths are bent equally
by the lens material). Chromatic aberration
displaces images when the line of sight passes
through the periphery of the lens according to
Prentice’s Law. (Prentice’s Law states that the
prismatic effect in
prism diopters at any
point in a lens is equal
to the lens power times
the distance in centimeters
from the optical
center (e.g. 1 centimeter
from the center of
a 1 diopter lens will
cause a prismatic deviation
of 1 prism
diopter). If the power of the lens is greater for
blue light and lesser for red light, then the prismatic
effect for these wavelengths varies and
the image is degraded by an “image spread” or
“rainbow fringe” that can be seen best along
image contours which are more or less perpendicular
to the base-apex line of the
induced prism. The width of the “rainbow
fringe” naturally becomes greater as the line
of sight strays further from the optical center
of the lens.
Patient tolerance for this aberration varies.
An Abbe value is assigned to lens materials to
indicate how much chromatic aberration they
will yield. (The Abbe value is equal to: (nD –1)
/ (nF – nC) where nD is the index of the material
for YellowGreen, nF is the index for Blue
and nC is the index for red.) A lens with an
Abbe around 30 will give about twice as much
image spread as one with an Abbe of about 60,
and a lens with an Abbe of about 45 will give
about 25 percent more image spread than one
with an Abbe of about 60.
Patient complaints related to chromatic aberration
can include blurring, color fringes or
may simply be of a vague nature. Patients with
prism corrections and those who tend to turn
their eyes away from the optical center for
viewing are more prone to be troubled by
materials with low Abbe values. Be wary of
using frames with large lens sizes, especially
ones that require significant decentration of
the Rx, because this gives your patient more
opportunity to be troubled by chromatic aberration.
Using AR lenses will not reduce chromatic
aberration, but patients with AR lenses
tend to be more tolerant of these problems.
“I’m not comfortable in my new eyewear.” If
issues of visual discomfort are ignored, discomfort
complaints are mostly related to
incorrect frame alignment or result from the
lenses being too heavy. Using lenses with a
higher index (more light bending muscle)
decreases lens volume because these lenses
don’t have to be curved as much to bend light.
If the volume of one lens is less than another,
the weight of the first lens will naturally be
lighter, provided the materials used for both
lenses have the same specific gravity.
Both specific gravity and index vary from one
material to another, so knowing which material
will give the least lens weight is not easily
determined, especially
in a clinical setting.
Selecting a lens material
of a higher index
may yield a finished
lens of less volume, but
the differences may not
be as great as you (and
your patient) expect. A
high-index plastic lens
(1.66) in a -2.00 power
will have roughly the same volume as a -1.50
standard plastic lens. Therefore if your patient
needs a -2.00, and if you switch him to a 1.66
material, he will end up with a lens that is
about the same volume and curvature as if he
were given a -1.50 standard plastic lens but
with a power of -2.00. Most patients would
have difficulty detecting
the thickness and
weight difference
between a -2.00 and a
-1.50 standard plastic
lens, but they will not
have any problem
recognizing that the
cost of the higher
index lens cost is
greater. Problems of
perceived value and
buyer’s remorse can arise from situations of
this sort. Not only that, decreased optical performance
of the higher-index material can add
fuel to the flames of perceived value and
buyer’s remorse problems.
“My new eyewear just isn’t right.” Non-specific,
vague and many-faceted complaints (e.g.
these hurt my ears, I can’t see with them, they
don’t look good on me, etc.) can indicate
buyer’s remorse rooted in low perceived value.
These patients may feel they did not receive
benefits in fair proportion to the eyewear cost.
Using more expensive lens materials is best
accepted if the results are positive, “patient
perceptible” and with few or no induced optical
problems.
Fortunately there is a simple way to predict
what lens volume and thickness
will result when a different
index is selected. Not only
can volume and thickness be
easily predicted, the relative
weights of the lens for
various materials can be
estimated with good clinical
accuracy for lens powers
between + 10 diopters. To do
this, a two-digit multiplier is
used to give a comparable standard plastic lens
equivalent. For example: all experienced clinicians
in the U.S. have a good idea of how thick
a -8.00 lens will be in standard plastic if they
know the size of the lens. The real question is,
what material should be selected if you want
your patient to have a lens with the same volume and curvatures as a lower
power standard plastic lens in
the same size? And, compared
to a lower power lens in standard
plastic, what will the new
lens weigh?
ROSETTA STONE
FOR EYECARE
The Rosetta Stone, an antiquity
discovered in Rosetta (now
el-Rashid), Egypt in 1799,
allowed us to decode then
unreadable hieroglyphics by
comparing a hieroglyphic text
with a known language. There
is a multiplier known as the
curve variation factor (CVF),
which reveals the power of a
lens in one material that would
have the same volume and curvature
as a lens of another
power in another material.
Clinicians can easily use this multiplier as a
Rosetta Stone for selecting the best lens material.
For example, if your patient requires
-10.00 lenses, you know that although a standard
plastic lens would cost less, it would also
be unnecessarily thick. You can certainly
reassure your patient that, through the use of
some other material, you can reduce both the
thickness and possibly the weight of their
lenses. You can also easily quote the added
cost for this improvement, but it has been difficult
to visualize and communicate how
much improvement will result. The CVF
enables you to tell your patient that going to
polycarbonate, for example, will yield a lens
with the needed -10.00 power but with
approximately the curvature and thickness of
a -8.50 standard plastic lens and with a weight
that is at least 9 percent lighter than the -8.50
in standard plastic. Alternatively, if you consider
using a 1.66 material, the CVF allows
you to predict that the finished -10.00 lens
will have the curvatures and volume approximately
the same as a -7.60 standard plastic
lens, and it will have a weight 2 percent heavier
than the -7.60 standard plastic lens. By
using the CVF, you can balance the increased
cost, reflectance and Abbe (chromatic aberration)
issues of higher-index material against
the lower cost, thickness and optical advantages
offered by standard plastic lenses. This
CVF multiplier has great potential for helping
you give the very best lens design advice
to every one of your patients.
HOW IT WORKS FOR
CURVES AND THICKNESS
Because of the widespread use of standard
plastic lenses, nearly every clinician is conceptually
grounded in their use. In other words,
you have a pretty good idea of how a lens of
about any power is going to look in standard
plastic, so the approximations presented are
relative to standard plastic. Also, the CVFs are
rounded for simplicity of use and practicality.
The formula is CVF = (1 – nstandard)/(1 –
nnew). In this formula, nstandard represents
whatever material you wish to use as your standard
(in the case of this article
1.49 was used), and nnew
is the index of whatever
material you are considering
using. (For a more thorough
discussion of Curve
Variation Factors refer to
Professor Mo Jalie’s excellent
book, Ophthalmic
Lenses & Dispensing.) The
formula is clinically accurate
to up to about + 10.00
diopters. Any clinician can
use this CVF formula to
develop his or her own CVF
chart and carry it to as many
decimal places as they wish.
It is also possible to use the
formula to determine the
CVF for new materials of
indexes that are not currently
available.
HOW IT WORKS FOR WEIGHT
To predict the weight of the finished, higherindex
lens, you must first determine the
equivalent standard plastic lens. The density
of the higher-index material can be used to
predict the final relative weight the lens will
have. For example, if a 1.66 material (CVF
=.76) with a specific gravity of 1.35 is being
considered, then the finished -10.00 lens will
have the curves and volume equal to a standard
plastic lens with a -7.60 power. The
weight of the finished 1.60 lens will be about
2 percent heavier than the -7.60 standard
plastic lens because the ratio of the density of
the 1.66 (i.e. 1.35) to the density of standard
plastic (i.e. 1.32) is 1.02.
USING CVF IN YOUR OFFICE
Patients want perceptible differences when
they purchase new eyewear and they want any
trade-offs in decreased optical performance to
be compensated with significantly thinner and
lighter lenses. It is your professional judgments0
that must lead to the best recommendations. A
-4.00 Rx that is fabricated in 1.60 material will
have about the same thickness and flatter
curves as a -3.25 standard plastic lens—a difference
that may only be discernable by an
expert. The light reflected from the front surface
of the 1.60 material will be 34 percent
brighter than the light reflected from the
standard plastic. Ghost images during night
driving will be 175 percent brighter with the
1.60 material and there will more chromatic
aberration because of the decrease in Abbe
from 58 to 43.
An easy way to demonstrate how changing
index will affect finished lens thickness and
curvature is to have your lab make up a set of
plus and minus standard plastic lenses in some
suitable diameter. For a patient with a -10.00
Rx, you can show them a -10.00 standard plastic
lens (at perhaps 55mm diameter) and a -8.50 standard plastic lens of the same diameter
if you want to show how going to polycarbonate
will affect the curves and thickness of
their lenses. If they have a -5.00 Rx and if you
are considering poly, you can show them a -
5.00 and a -4.25 in standard plastic to demonstrate
the same relative comparison. Because a
minus lens made of polycarbonate or PPG’s
Trivex material is usually surfaced .8mm thinner
than other materials, the weight of a disc
50mm in diameter and .8mm in thickness can
be deducted from the weight of a 50mm
round lens in a minus power if you want to
compensate for the thinner centers. This
reduces the weight by 1.88 grams for poly or
1.74 grams lighter for
Trivex. Plus lens are usually
surfaced .2 to .3mm
thinner for these materials,
the weight savings
due to thinning is somewhat
less in plus powers.
Since the surface areas of
most lenses today are significantly
less than that of
a 50mm round lens this
thickness and volume difference
has been ignored
for the purpose of simplifying
this presentation.
A common error is to try
to make the thinnest, flattest
lenses possible for
every patient regardless of
their prescription. This often leads to patient
frustration, adaptation problems and disappointment,
especially in the mid-range and
lower powers. This is because ghost images are
especially troublesome in these powers. Also,
chromatic aberration that was negligible with a
high Abbe value may cause image degradation
when areas away from the center of the lens
are used. It is important to recognize that
patients with high prescriptions tend to
view through the centers of their lenses, but
in mid-range and lower powers, off-center
viewing is more common.
THE GOOD NEWS
The use of today’s top-quality AR products
greatly reduces the optical issues of
the increased reflectance of all higherindex
materials. In addition, although
chromatic aberration is an attribute of
each lens material and cannot be
reduced, patients are better able to tolerate
its effects when the reflectance problems
are reduced. It is important to keep
in mind that AR lenses improve the performance
of all lens materials, but the need
for them becomes imperative when higherindex
materials are used.
A WORD ABOUT
ASPHERICS
The use of aspheric base
curves has long been
known to improve the
performance of lenses in
the higher plus powers.
However, the use of an
aspheric base curve will
neither significantly
reduce nor eliminate
chromatic aberration,
which tends to increase
as index increases.
Because reflectance
increases in direct proportion
as index increases,
the use of an aspheric
base curve will not improve most reflectance
related problems. Using an aspheric lens
design can be a good alternative to increasing
index, especially in the plus powers, if
you want thinner and flatter lenses.
FRAME SELECTION CAN BE CRITICAL
Using frames with wider bridges and smaller
eyesizes to keep the frame PD as close as possible
to the anatomical PD is another way of
allowing lower index materials to be used.
Frames with wider eyewires can also decrease
lens size and may help hide the edge thickness
of minus lenses. Turnback temples can help
give the eyewear the appearance of needed
overall width while keeping the A measurement
of the eyewire and the frame PD smaller.
The bottom line is to use all the frame design
techniques you can first and only then increase
the index if you must. LT
Palmer R. Cook, OD is director of professional education for Diversified Ophthalmics in Cincinnati.
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