Abbe Number …

What is it?

The optical business is filled with technical terms that we hear all the time. We read articles, sit in education classes, listen to sales representatives, and talk with colleagues seeing, hearing and talking about these terms. 

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

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.

1. Analyze the patient’s current eyewear for Rx, base curve, thickness, material (if possible), and any dissatisfaction with the eyewear.
2. Choose the best lens material to accomplish patient satisfaction for clarity, thickness, cosmetics, optical performance, and overall value.
3. 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.

Bob Fesmire, ABOC