The index of a plastic lens material is an expression of its thinness and a higher number means the lens is thin and a smaller index number means the lens is thicker than one with a higher index number. Generally, the higher the index number of a lens material also means it is lighter but that is not the case with Trivex materials. Trivex lens material has an average index value of 1.53 and it is the lightest lens material available today.
In high-power prescription situations, thinner and lighter lens materials are suggested to help make the lens be more aesthetically pleasing. Nearsighted prescriptions, for example, require a lens to be very thin at the center and much wider at the edge. Using thinner materials can minimize the edge thickness and improve the overall appearance of the glasses. If you are farsighted, your lenses might require a much thicker center point to make the needed correction. Using a higher index material will help reduce the distortion of your appearance through your glasses when viewed by others. The higher the index value, the higher the cost. High index lenses are often good choices for rimless drill-mount glasses because the higher index plastic accepts the drill hole well and the edge profile of the lens is greatly reduced the higher the index value of the lens, generally speaking.
Lens materials vary in their index. Common index values are shown below for the common lens materials in use today.
Material Index Value Referred To As
CR39 1.49 Standard Plastic, CR39
Trivex 1.54 Trivex
Polycarbonate 1.58 Poly, Polycarbonate
Plastic Mid Index 1.56-1.60 Mid Index or (index value)
Plastic High Index 1.61 High Index or 1.61 High Index
Plastic High Index 1.67 High Index of 1.67 High Index
Plastic High Index 1.74 Super High Index or 1.74 High Index