Before revision: 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.
After revision: Lens materials are classified by their refractive iex, as follows: Normal iex: 1.48 ≤ < 1.54 Mid-iex: 1.54 ≤ nd < 1.60 High-index: 1.60 ≤ nd < 1.74 Very high index: 1.76 ≤ nd This is a general classification.