What if you could choose lens materials for each patient by having the
lens materials audition for their part in the eyewear? They’d have to really sing and dance to get the part. In each case, you’d be able to have them
describe their talent, the reason they are best for this part and how they
could make their wearer smile and enjoy their performance.
If choosing lenses were a movie, no doubt the material would be the
star, but we all know stars who have made bad movies because they had
a poor script or the wrong supporting cast. Ever put the wrong material in a frame and the lenses were thicker than you expected? The
patient wasn’t too pleased either.
So to make thinner and lighter lens materials center stage and be able to
choose the best cast, let’s meet those auditioning. Let’s let them tell us how
they would form the best foundation for a particular patient’s needs and
the lens prescriptions they
wear. Remember, they provide the building blocks
that house the elements of
design, protection and treatment. How do we really
describe what the patient is
to expect from this cast that
we put together for them?
In order to design a lens
that is thinner and lighter,
the material can be the
headliner, but the rest of the
cast—frame size, shape,
style, decentration, asphericity and specified center or
edge—must be carefully
blended in order to turn in
an Oscar winning performance. As the director and producer, the dispenser is responsible for blending the elements in
order to produce the best in “thin and light” chemistry.
Patients who have been promised thinner lenses ask this question
thousands of times a day, “How thin will my new lenses be?” The
fact is there is no one “super ultra premium lens,” but rather a great
number of material and design options. They rarely receive a straight
answer from their eyecare professional.
Answers are usually veiled with vague references to a percentage
since the material properties do not address all of the true elements of
the finished product.
Note: The patient is not asking a general question in the academic
sense of one scientist to another on the relative difference in thickness
between two separate indices of refraction all other variables being
equal. The patient is asking very specifically, when spending for this
great new product and hearing promises of thinness, “Exactly how
thick are my new lenses going to be?” That question cannot be
answered with a phrase like “They will be 25 percent thinner.” That
answer lacks accuracy because the next question will be, “25 percent
thinner than what?” The old cast and the new cast are completely
different. The old lenses are often of a lesser lens power, but the new
lenses will be stronger. The old lenses were for a frame that is most
often different in size and shape than the new frame. The decentration of the old job will probably vary from the new job. What about
the specified center or edge thickness of the old job versus the new
job? Certainly all of these elements are of utmost importance to the
finished product and yet the use of a percentage regarding these
design elements is at best a guess.
Another explanation to answer this seemingly easy question comes
when the optician abandons a stated percentage in favor of a high-index demonstrator. The optician upon hearing the dreaded “How
thick will my new lenses be,” reaches for a standard predetermined
demo and can say:
Optician: “Look at this demonstrator, it shows this -5.00 regular lens as
thicker than the new ‘super ultra lens.’ Unfortunately, since your prescription is different, a -8.00 and the frame you chose is also a different
size and shape this doesn’t exactly tell the thinness. It will look just like
the demo but different. Does that answer your question?”
Patient: “Not really—what I want to know is how thick my new lenses will be?”
Optician: Frustrated at this point they return to the inaccurate answer
from the first explanation of thinness, the percent;“They will be 25 percent thinner.” (Thinking all the while that if the lab doesn’t do something to make this job look good the patient will be dissatisfied. They
cross their fingers, call in the order and hope the editing room can turn
a stinker into an all time classic.)
The unfortunate reality is, if the optician has not gotten a handle on
all of the elements that truly affect finished lens thickness the laboratory can do very little to improve on a faulty series of choices and
designs. What is an optician to do?
To effectively predict the finished thickness, the optician must
realize how all of the design elements affect the final outcome.
There is no substitute for knowledge and tools to better know the
final result when it is critical to the patient. Before one can predict
an end point, the path used in arriving at “thin and light” must be
clearly understood. Finished thickness can be predicted, but only
when all of the design elements that affect the outcome have been
identified and controlled.
ELEMENTS OF THINNESS
The all-important supporting cast, in their general order of importance,
directly affects the thinness, lightness and comfort of a pair of glasses.
Frame size is the most significant element of thin lens design. Bigger results in thicker lenses and smaller results in thinner lenses. To
the dispenser’s advantage this element is controllable.
Lens shape is a significant factor in reducing lens thickness. Simply
stated, rounder shapes result in thinner lenses.
The shape factor can be quantified in terms of Effective Diameter
(ED). The ED represents the smallest theoretical blank that can be
used to achieve accurate centration and cutout on a given shape. This
value is equal to two times the longest radius of that shape. The prediction of finished thickness demands precision. The ED should be
looked up in an industry source such as “Frame Facts” or call the
manufacturer. It is not the longest diameter of a frame, however a
rule of thumb that can be used when the real number is unavailable
is add 2mm to the longest diameter. Your lab, when tracing the
frame, will accurately determine the ED.
Decentration is a hidden element that is often overlooked. Many
dispensers do not compute the decentration at the time of dispensing. If more decentration means more thickness then how can a dispenser who has not calculated this key element promise thinner lenses? They cannot. So always determine decentration.
Index of Refraction determines thinness; the higher the index, the
thinner the lens, all other things being equal. The “all things being
equal” refers to frame size, shape and decentration.
Aspheric design offers plus lens patients a dramatic advantage
over steeper and thicker spherical lenses that they may currently
be wearing. Aspheric lenses provide flatter base curves and lens
peripheries that further flatten for plus prescriptions and base
curve peripheries that steepen slightly in minus prescriptions. This
delivers great cosmetics while still preserving good vision. All
aspheric designs are not alike and dispensers should consider this
when choosing them.
Specified center thickness can help to control the final result. Since
most labs produce CR lenses at 2mm centers and polycarbonate or
high index around a 1.5mm centers the absolute best difference in
finished edge thickness is the result of 0.5mm reduction at the center
plus the index and design effects. This interaction is sometimes complex, especially in plus Rxs so when the patient really wants to know,
a software program that can be downloaded from the web or a call to
the lab is in order. For programs that can be run on any PC, visit
www.optiboard.com and see the download section.
The type of frame affects thickness. For example, some high-plus
lens prescriptions may not lend themselves to rimless mountings,
drilled or grooved unless thinner and lighter materials are used. Plus
lens center thickness is a function of the minimum edge thickness
that is needed at the farthest edge. It is also affected by the most plus
power and the meridian in which it is located. Therefore, thin edges
are critical for great looking eyewear. If grooved and CR, then thin
edges may flake, so to reduce thickness and the effect of flaking or
chipping use Trivex or polycarbonate.
Also understand the minimum edge thickness your lab will provide
when jobs are drilled or grooved. Most labs add thickness to accommodate room for a groove or for stability around drill holes. In the
case of minus lenses, since thickness is inherent at the edge it would
be safe to say that the mounting type (rimless or full rim) will be of no
consequence to the finished thickness.
If the dispenser chooses the right star and surrounds them with the
proper supporting cast the eyewear can have a thrilling conclusion.
If the dispenser wants a really powerful ending, he/she can achieve
that by using the thickness calculation to accurately answer the
patient’s question and bring down the house.
EASY FINISHED LENS THICKNESS COMPUTATIONS
For the professional dispenser who pays close attention to the elements of design, the question, “How thick will my new lenses be”
need not be a problem. In fact, with a calculator and a minute of
time, dazzle patients with your expertise. In addition to giving the patient a straight and accurate answer the advantage of pre-calculating thickness is that if for any reason the patient was expecting a thinner lens the design elements can be re-addressed to arrive at the
desired outcome before time, effort and money are wasted. Calculate
finished edge thickness using the Sag Formula (sagittal value). Using
a calculator, finished thickness can be answered with precision. The
sag is a millimeter value for the distance between parallel tangents to
the front and back surface of a lens (thickness). Sag Value is a mathematical measurement of the center thickness of a hypothetical plus
lens with a zero edge or of the edge of a hypothetical minus lens with
a zero center thickness.
To make the formula work as an answer to “How thick are my
lenses going to be,” the dispenser works the formula with all of the
known variables to get the sag, then to that value they must add the
desired center for a minus lens or the desired edge thickness for plus
lenses. With the resulting value the dispenser may draw the proper
thickness in millimeters on a piece of paper—look the patient directly in the eye and respond, “Your new lenses will be this thick.”
THE PROCEDURE/FORMULA
Using the patient’s PD, frame selected, decentration computed, lens
material chosen and patient assured they made a great choice, follow
these steps.
Step #1 — Double the decentration and add that value to the effective diameter of the frame chosen. This number represents the minimum blank size (MBS) or diameter of the smallest theoretical blank
from which that job will cut out. Halve that value to arrive at the
radius the sag formula requires.
Should this be two times Dec Dec + ED = MBS
MBS/2 = Radius
Step #2 — Use the following formula to compute the sag value:
Radius2 X Lens Power (D) = Thickness
2000 X (n - 1)
Where:
2000 = Constant (Never changes)
1 = Constant (index of refraction of air)
n = the index of refraction of the material being used
Radius = Computed in step #1
Power = For spherical powers use the sphere Rx power. In cylinder
lenses use the strongest meridian power (Unless the strongest power is
located at the ED meridian, there will be slight error on the safe side.
The lens will be thinner than predicted.)
Step #3 — Add the center (shows the edge thickness in minus lenses) or
edge thickness (shows the center thickness in plus lenses) to this value,
2mm for CR and 1.5mm for all other materials. For Trivex and poly-carbonate, where a 1mm center or edge will be ordered, add 1mm.
Step #4 — Using a millimeter ruler, draw the answer in millimeters as
a line on a piece of paper. In plus powers the answer will represent
the center thickness of a lens with a 1.5mm edge and in the case of
minus powers the answer will represent the edge thickness of a lens
with a 1.5mm center.
EXAMPLE
Rx: OU -6.25 Sphere A Box: 53mm
Material: 1.59 Polycarbonate DBL: 15mm
Patient PD: Monocular 32/32mm ED: 57mm
Step #1 — Compute Decentration:
- A Box plus DBL ÷ 2 = Monocular “Frame PD”
53 + 15 ÷ 2 = 34
- Monocular Frame PD - Patients Monocular Pd = Decentration
34 - 32 = 2mm Decentration (same for R & L)
- Double Decentration + E.D. = Smallest Blank Diameter
2 x 2 = 4, 4 + 57 = 61
Radius = 1/2 of 61 or 30.5mm
Step #2 - Sag Value
(30.5 x 30.5) x 6.25 = 930.25 x 6.25 = 5814 = 4.8mm
2000 x (1.59 – 1) 2000 x .59 1200
Step #3 - Add 1.5mm Center to Minus Lens Edge:
1.5mm + 4.8mm = 6.3mm finished thickness
Step #4 — Draw the answer to present it to the patient
Upon presenting the finding, call the patients attention to their old
lenses for the purpose of a favorable comparison. This comparison
shows the improvement in the new lenses and answers the question
directly on a positive high note. The sag formula is only accurate in
the calculation of spherically designed lenses; however, if the variables
were calculated for spherically designed lens and an aspheric design
was substituted the result will be a further reduction in thickness.
CONCLUSION
A professional optician who understands lens materials properties and
other elements that contribute to and control thickness i.e., the sag
formula, a calculator, a minute of time, and the practiced ability for
calculation, will not fear the question of thickness. Not only can they
answer the question with accuracy they can also look the patient in
the eye and say “How about a large bucket of buttered popcorn and
a soft drink with your new lenses.” That’s a wrap. |