What are Hi Index lenses and why would I want them?
High-index eyeglass lenses are the right choice if you want thinner, lighter lenses and glasses that are as attractive and comfortable as possible.
Types of Plastic:
Standard (CR39) Plastic
The plastic used for most lenses, CR39 as it is also known, is approximately half the weight of glass and offers wearers UV protection and shatter resistance.
Polycarbonate lenses are thinner and lighter than standard plastic lenses and offer far greater impact resistance compared to standard plastic. UV protection is also inherent in this material. This makes polycarbonate an ideal choice for sports, children's and safety eyewear.
High-Index is used to describe a category of plastics that are lighter and thinner than standard (CR39) plastic. High-Index includes 1.54, 1.67 and 1.74 (known as Ultra High-Index). As the index increases, the thinner and lighter the lens. A 1.67 index material is half the thickness of standard plastic. UV protection is also inherent in High-Index plastic lenses.
Importantly, High-Index plastic material enables those with stronger prescriptions to achieve a more aesthetically appealing look (no more coke bottle glasses).
Most eyeglass wearers are nearsighted, which requires corrective lenses that are thin in the center but thicker at the edge of the lens. The stronger the prescription, the thicker the edges.
Refractive errors and lens powers required to correct them are measured in units called diopters (D). If you are mildly nearsighted, your lens prescription might say -2.00 D. If you are highly myopic, it might say -8.00 D.
If you are farsighted, you need "plus" (+) lenses, which are thicker in the center and thinner at the edge.
Lefthand image indicates: High-index lenses can be much thinner and lighter, even in a strong prescription.
Righthand image indicates: The three basic types of optical lenses: