Refractive index




In the UK and the US, the refractive index is generally specified with respect to the yellow He-d Fraunhofer line, commonly abbreviated as nd. Lens materials are classified by their refractive index, as follows:

  • Normal index: 1.48 ≤ nd < 1.54
  • Mid-index: 1.54 ≤ nd < 1.60
  • High-index: 1.60 ≤ nd < 1.74
  • Very high index: 1.76 ≤ nd

This is a general classification. Indexes of nd values that are ≥ 1.60 can be, often for marketing purposes, referred to as high-index. Likewise, Trivex and other borderline normal/mid-index materials, may be referred to as mid-index.

Advantages of higher indicesedit

  • Thinner, sometimes lighter lenses (See below).
  • Improved UV protection over CR-39 and glass lenses.

Disadvantages of increased indicesedit

  • Lower Abbe number, meaning, amongst other things, increased chromatic aberration.
  • Poorer light transmission and increased backside and inner-surface reflections (see Fresnel reflection equation), increasing the importance of anti-reflective coating.
  • Manufacturing defects have more impact on optical quality.citation needed
  • Theoretically, off-axis optical quality degrades (oblique astigmatic error). In practice this degradation should not be perceptible – current frame styles are much smaller than they would have to be for these aberrations to be noticeable to the patient, the aberration occurring some distance away from the optical centre of the lens (off-axis).

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