|Fig. I: Dispersion|
|Notice how the blue wavelengths refract shorter than the red wavelengths. The spread between them illustrates how much a lens (or transparent material) disperses light.|
EACH ISSUE, TECH TIPS WILL EXPLORE SOME INTERESTING ASPECT OF OPTICAL TECHNOLOGY. THIS MONTH WE LOOK AT UNDERSTANDING ABBE VALUE.
HOW DO WE DETERMINE A LENS MATERIAL’S ABBE NUMBER?
The method eyecare professionals (ECPs) use to assess a lens material’s usefulness for a patient’s needs is by evaluating its optical performance data. One of these performance characteristics is its Abbe number.
Born to impoverished parents in Germany in 1840, Ernst Karl Abbe became a German physicist and optometrist. He taught physics at the University of Jena and later formed a partnership with Carl Zeiss in 1866, in which he developed the scientific optical principals used by Zeiss to produce high-quality optical devices like microscopes and telescopes. One of his contributions was what is commonly known today as Abbe value (also known as Abbe number in the world of physics).
The wavelengths of light that the eye can detect range from about 380nm to 760nm. When the eye experiences all of these wavelengths simultaneously, it interprets them as white. White light can be dispersed (spread apart) and this dispersion can create detectable color aberration (known as chromatic aberration) for the lens wearer.
|Fig.2: Abbe Number for Selected Materials
A lens material’s Abbe number is determined by measuring how much wavelengths of light are separated as they pass through the material. To do this, the following formula is used:
V=v=(nd – 1)/(nF – nc)
Note: n(d), n(F) and n(c) are the refractive indices for the Fraunhofer lines 587.6nm, 486.1nm, and 656.3nm.
This formula provides a number—a V value that indicates the dispersive nature of the lens material (see Fig. 1). If the Abbe number is high, the lens material controls dispersion well. If the number is low, the material has the potential to cause color aberration for someone wearing a lens made from it. Fig. 2 illustrates the Abbe number of a few commonly utilized lens materials.
Allvar Gullstrand, a Swedish ophthalmologist, did research into the optics of the human eye and how it refracted light that passed through it. One of his many revelations was the concept that the human eye was an optical system just like any other optical system, and because of this, it had an Abbe number of its own. According to Gullstrand, that value falls between 43 and 45. This means that if a lens material’s Abbe number falls in the 43 to 45 range, the person wearing lenses made from that material will likely not experience troubling chromatic aberration, especially with lower powered lenses. Lenses made from materials with much lower Abbe numbers have the potential for the wearer to experience color aberration. This aberration is usually characterized by patients as seeing red and blue ghost images around objects they view through the lenses, especially out in the periphery.
Because of Gullstrand’s data, ECPs generally look for lens materials with Abbe numbers at or near 43 to 45 when the other optical performance factors of the lens material they are considering meet the needs of the patient. Remember, Abbe number is only one of a number of very important optical performance characteristics. In the end, the best lens material for any patient is the one that offers the best performance with the least compromise.
Ed De Gennaro is Director, Professional Content, First Vision Media Group.