Corrective Lens - Refractive Index

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 n_d. Lens materials are classified by their refractive index, as follows:

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

This is a general classification. Indexes of n_d 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.