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ABBE Number and Chromatic Abberation

 

In optics, the Abbe number, is a measure of the material’s light dispersion (Dispersion is the scattering of white, visible light into its component - colors by lenses “rainbow effect”).) in relation to the refractive index. It is named for Ernst Abbe (1840–1905), the German physicist who defined it.

 

Light dispersion leads to undesired chromatic aberration in a lens which expresses in blurring or smearing effect on the objects viewed or “color fringing” around objects viewed. Lenses that have a higher Abbe number will disperse light less. This produces less chromatic aberration.

 

 

Abbe numbers are used to classify optically transparent materials. For example values of Abbe range from around 20 for very dense flint glass, around 30 for polycarbonate plastics, and up to 58 for crown glass.

 

Abbe Number/Values for the most commonly used lens materials

Chromatic Abberation Chart

 

These Abbe numbers are important for understanding of our lens material options. Patients that complain of color fringing, or that things just seem out of focus, despite of rite prescription, may be suffering from 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. Opt for the higher Abbe number if possible.
  • 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.



Specific Gravity of Different Optical Lens Materials

 

Specific gravity (SG) is defined as the ratio of the density of a given material to the density of water at a specific temperature and pressure, typically at 4 °C (39 °F) and 1 atm (760.00 mmHg).

 

The density of a material is defined as its mass per unit volume. The symbol of density is ρ (the Greek letter rho).

 

Mathematically:

 

p=m/v

 

Where: ρ is the density,

 

m is the mass,

 

V is the volume.

 

We use specific gravity to compare between different lens materials in the same prescription and similar optical index in order to provide patient with as light as possible lenses.

 

Abbe Number/Values for the most commonly used lens materials

 

Specific Gravity of Different Materials