So many factors are considered in determining which lens material is best for each situation. Some criteria are: How strong is the prescription? What is the type of frame (metal/plastic/rimless)? What are the sizes of the lenses (larger=thicker)? And, what is your tolerance for edge thickness? These are some of Visual Effects high-tech, thin lens options.
High-index = Lenses that are thinner & lighter in weight because the material is denser.
Standard plastic (CR-39) is the material of choice with low to moderately strong prescriptions. It is relatively inexpensive, lighter than glass and scratch resistant. From moderately strong to severe prescriptions different “high-index” materials are called for. The term “high-index” refers to any lens material with an index of refraction greater than the 1.49 of CR-39.
All lens materials being denser than air have higher refractive indices ranging from the 1.49 of CR-39 through 1.60 through 1.67 to finally 1.74, corresponding to progressively denser and thinner high-index lenses. 1.60 lenses are about 30-35% thinner than CR-39, 1.67 lenses are about 40% thinner, and 1.74 are about 50% thinner than CR-39.
Originally designed for farsighted (plus) lenses to reduce the thick bulge, this special design now is available for nearsighted (minus) lenses as well. This type of lens uses a computer design that flattens the curves of both surfaces to reduce thickness, decrease the magnification effect and improve optics. Couple this design with high-index materials to get great looking strong prescription lenses. The latest addition to this category is “atoric” surfaces, which help improve optics by reducing distortion in astigmatic lenses.
Newer designs have decreased the distortion that was a problem with this material in the past.
This was the first lens material introduced that was thinner and lighter than CR-39. It’s most significant benefit is its impact resistance – 10x stronger than plastic. Because of this feature we use it for sport and safety frames, kids glasses, and drill mounts.