Hi-Index

High index lenses polymers typically refer to products with an index higher than 1.58. High index lenses require flatter curves than their lower index counterparts, resulting in thinner and lighter lenses. Furthermore, aspheric curves come standard in many high index products, particularly 1.66 and 1.70 products, and are available in 1.60. Asphericity reduces spatial distortion, reduces magnification or minification, and further helps maintain a thin and flat lens profile. High index material, however, tends to have a lower Abbe value which could potentially affect patients who are sensitive to chromatic aberration. Also higher index and flatter curves tend to result in more backside and inner-surface reflections. AR coatings are usually recommended for high index lenses to eliminate these reflections.

Index: 1.60, 1.66, 1.70

Pros
Thinner and lighter than glass and plastic
Better optical quality than polycarbonate

Cons
Susceptible to scratching (correctable by coating)
Susceptible to backside and inner-surface reflections (correctable with AR)