How many of us really know what these terms are? Where did
they come from? Do they even matter? I am going to tell you
the tale of one of these often used terms, Abbe Number.
Abbe Number got its start from Ernst Abbe. Ernst was a
German physicist and a professor at the University of Jena.
He was a professor of mathematics and physics. In 1868 he
invented the apochromatic lens system for use in
microscopes. The apochromatic lens system eliminated primary
and secondary color distortions in microscopes. His work
gained the interest of Carl Zeiss, who in 1866 hired Ernst
to help with various optical problems and the manufacturing
process of optical instruments. In 1888, Ernst became the
sole owner of Carl Zeiss!
Abbe's work led him to conclude that there is a value, or
number, that is a measure of a material's light dispersion
in relation to the refractive index. Dispersion is the
scattering of light into its component colors by prisms or
lenses. This number, or value, is sometimes referred to as
refractive efficiency. This is the Abbe number. This value
is also called the V-number. When light passes through a
lens and gets dispersed, colors with shorter wavelengths
travel more slowly then colors with longer wavelengths. The
result of this is chromatic aberration.
Lenses that have a higher Abbe number will disperse light
less. This produces less chromatic aberration. Lenses that
have a lower Abbe number will have more chromatic
aberration, and will disperse light more. The higher the
refractive index of a lens, the lower the Abbe number will
be.
Chromatic aberration can be broken down further into
axial chromatic aberration and lateral chromatic aberration.
Axial chromatic aberration is the measure of the difference
in focus between the red and blue ends of the color
spectrum. Lateral chromatic aberration is the measure of the
prismatic deviation between the red and blue ends of the
color spectrum.
Axial chromatic aberration can create a blurring or
smearing effect on objects viewed. Patients experiencing
axial chromatic problems will often express that things
"seem to be out of focus, or slightly blurry.”
Lateral chromatic aberration creates "color fringing”
around viewed objects. The color fringing issue is increased
as the wearer views an object through the periphery of a
lens.
Abbe Number/Values for the most commonly used lens
materials
|
Crown Glass |
58 |
|
|
Cr-39 |
58 |
|
|
Spectralite |
47 |
|
|
Trivex/Phoenix |
43-45 |
|
|
1.60 |
36 |
|
|
1.70 |
36 |
|
|
1.74 |
33 |
|
|
1.66/1.67 |
32 |
|
|
Polycarbonate |
30 |
|
These Abbe numbers are important to our understanding of
our lens material options. Patients that complain of color
fringing, or that things just seem out of focus, may be
suffering the effects of chromatic aberration and a low Abbe
number. When a patient mentions these issues, you will be
prepared to handle the situation. There are a number of
actions that you can take to help minimize Abbe number
issues.
-
Analyze the patient's current eyewear for Rx, base
curve, thickness, material (if possible), and any
dissatisfaction with the eyewear.
-
Choose the best lens material to accomplish patient
satisfaction for clarity, thickness, cosmetics, optical
performance, and overall value.
-
Proper frame selection. It is important that you pick
a frame that not only is cosmetically appealing, but also
optically correct. Pick a well-centered frame. Adjust the
frame for proper pantoscopic tilt and vertex distance.
Below is a statement about the effects of a low Abbe
number and the issues it creates with chromatic aberration.
The author is a fellow Optician and ECP Magazine
contributor, Mark Morris. Mark is a high myope and has tried
just about every available lens material. He describes his
experiences and observations with Abbe number, lens
materials and the chromatic aberration issues. This was
posted on the website www.optiboard.com.
"I have a perfectly good, perfectly useless pair of
progressives in 1.67. When I put them on (never now since
the Rx has gone south) I felt like I needed a stiff neck
brace so I would try to turn my head instead of my eyes. The
Abbe problem became very noticeable just a few mm from
center. Color fringes of the order lesser myopes cannot
appreciate become intolerable very quickly to the high myope.
I liken it to when the newspaper gets their Registration off
when printing so the different colors used do not match up.
So you get a picture of someone with four eye and parallel
lines around their head of different colors, like looking at
a 3-D picture without 3-D glasses.
Try this: Get a really high minus lens in 1.67 or poly
(about the same Abbe). Since you will not be able to see
clearly out of it, acquire a comparably high plus lens and
stack them (or just get along with the blurriness, you can
still notice the Abbe effect), then look out of the edges of
the lens at a light source. A single light bulb outside at
night (like a used car lot) is excellent. When you move from
the center, you will see the light bulb actually separate
and form into two other separate light bulbs, one red, one
blue either above or beside the original white one depending
on which way you move the lens.
Another example is you can see the difference between a
crystal chandelier and a glass one. The crystal one has all
those glints and rainbows and the glass does not. Now which
one would you rather look through?"
Mark has explained perfectly the visual issues that
patients can experience with a low Abbe number. I think many
of us have had patients describe similar situations that we
may have disregarded for various reasons. I think that the
Abbe number of a lens is important, but not nearly as
important as our understanding of where the term came from
and the impact it has on our daily professional lives.
I want to thank Mark Morris for his help and input
concerning his own experiences with Abbe number, index of
refraction and its effect on the visual system.
I also want to thank Darryl Meister for his gracious
sharing of his vast optical knowledge for the betterment of
our profession. His continuing education course: Chromatic
Aberration was a highly valued reference piece for this
article. Darryl has provided the optical profession an
invaluable resource tool at www.opticampus.com.
