When purchasing prescription lenses it is important to know which material of lens is best for vision. There are a few factors to consider and know about the 4 main materials of lenses that determine which gives the clearest vision and why
The most common materials of prescription lenses are CROWN GLASS, CR39 HARD RESIN, TRIVEX and lastly POLYCARBONATE
Index of Refraction
Lenses have an index of refraction, This is a measure of how well the material refracts light. Depending on the strength of the prescription, the higher the Index, the “thinner” the lenses will be, or less material is required to refract light. Some common index of lenses are 1.49, 1.60 1.67. 1.74
Abbe Value is a measure or value of how a lens disperses light as it passes through it. It is the clarity of transparent materials. This dispersion causes something called chromatic aberration. Chromatic aberration is a perceived fringing of light or fuzziness or blurr noticeable around the peripheral of a prescription lens. The higher the abbe value the better the clarity.
100 would be an ideal perfectly clear value. The human eye has an abbe value of 45-50 because light needs to pass through the crystalline lens.
Crown Glass has the highest abbe value of optical materials it s about 59 therefore it has the least aberration. It has the best optics, however it is heavy, it can break easily and is no longer used much at all for prescription glasses
CR39 , has a value of 58, the closest to glass, and THE BEST FOR VISUAL CLARITY. It is light weight, it is a hard material therefore does not scratch easily, it works well with Anti-Reflective coating and Absorbs Tint well, it is also available as a safety lens when made with a centre thickness to be impact resistant.
Trivex, has a value of 45, this lens is a newer material, it is known to be impact resistant light weight, vision clarity is mid range, an is easy to drill for specialty frames
Polycarbonate, this lens has a value of 30, it has the most aberration, originally it was used for safety glasses it is known to be light and impact resistant. This material scratches very easily, it does not work to well with Anti Reflective coating and the least clear.
In an ideal world the ABBE VALUE should not be lower than that of the eye. The BEST Material for optical lenses is CR39.
Michael Edward McGrath
Licensed Optician #1336