A clear lens on its edge, light passing through its thick edge splitting into a faint rainbow fringe.

Lens materials

Dispersion

Optical glossaryLens materials

The splitting of white light into colors as it passes through a material. The optical price of high index.

  • 1.50: Abbe number 58
  • 1.60: Abbe number 41–42
  • 1.67: Abbe number 31–32
  • 1.74: Abbe number 32–33
Abbe number by index — the higher it is, the less the lens splits white light into colored edges. It falls as the index climbs: the catch that comes with a thinner lens.

From CLIFF's index data · Abbe by lens material

In more depth

A material's refractive index is not quite the same for every wavelength: blue light is slowed and bent a little more than red. That spread is dispersion. In a prism it makes a rainbow; in a spectacle lens it is normally too small to notice.

Materials that bend light strongly tend to disperse it strongly too, which is why dispersion rises with index. The Abbe number is the figure that measures it — the higher the Abbe number, the lower the dispersion.

Related terms

Abbe numberA measure of how much a material disperses light into colors. It falls as index rises; lower Abbe can mean faint color fringing at the edges of strong lenses.Refractive indexHow strongly a material bends light. Higher index means less material for the same power — a thinner lens.High indexIn the CLIFF range, materials of 1.67 and above, used to keep strong prescriptions thin. Buys thinness, not clarity.

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