The refractive index of a lens (n) is measured for the yellow ray of helium (d) or the green ray of mercury (e), at the midpoint of the visible spectrum.

Lenses of index nd = 1.5, 1.6, 1.7, 1.8 and 1.9 are used for single-vision, solid bifocal, and progressive prescription requirements; another range of high-index barium glasses is used for near-vision correction in multifocal lens segments.

Index 1.5 is often called “low index”; indices up to 1.56  are generally described as “mid index” and those including 1.59 and above are called “high index”.

Plastic materials (such as nylon, polycarbonate,…) can be found within the 1.5 – 1.71 range of indices, whereas glass reaches 1.9.

In simple terms, the higher the refractive index of a lens, the more it will allow thickness reduction when designing its surface geometry, according to the type of refractive error to be corrected and the power.

High-index lenses are more complex than mid-index ones, and logically more expensive. This is why ophthalmic professionals will also consider the cost-benefit ratio when determining which index is most suitable for which correction.

Our two animations (based on Corning ophthalmic glass data) will show you in detail the benefits of using high-index lenses.

Among the advantages of glass as an ophthalmic material is its structure, which enables lenses to be produced in a range of refractive indices, without significantly altering its intrinsic optical, physical, or chemical properties.

Corning’s premium clear glass lenses are available in a wide range of refractive indices up to 1.9, to provide the ultimate in thin, optically superior prescription lenses.