What Are High-Index Lenses for Eyeglasses?

High-index lenses are an alternative choice to standard, plastic eyeglass lenses. Although high-index lenses are more expensive than plastic lenses, they can reduce the thickness and weight of an eyeglass lens, particularly for individuals with a strong prescription. For most prescriptions, high-index lenses offer the same optical clarity as plastic lenses in a significantly smaller package. An eye care professional or optical laboratory is the best source for determining what lens index should be used for a given prescription and eyeglass frame.

Function
High-index lenses are able to bend light to a stronger degree than a lens with a lower index, therefore the higher the index of the lens the less material needed to achieve a given prescription.

Types
High index lenses are categorized according to how strongly they bend light. The higher the index, 1.53 to 1.74 for high-index lenses, the better the lens is at bending light.

Benefits
High index lenses are 20 percent to 65 percent thinner than standard plastic lenses. In general, the stronger the prescription, the greater the difference in thickness between standard plastic and high-index plastic.

Considerations
A reduction in lens thickness and weight will occur in high-index lenses for prescriptions stronger than +2.00 or -2.50. Weaker prescriptions are not thinner than standard lenses when made with high-index materials.

Polycarbonate versus High Index
Polycarbonate lenses offer the same aesthetic benefits as high-index lenses, but polycarbonate lenses scratch easier, are difficult to coat or tint and are more likely to create distortions in colors and vision.