What does the index of a lens really mean?
You might think that if you have a strong vision prescription, your glasses have to frame thick, heavy lenses. This may have been true in the past, but it’s no longer the case. High-index lenses enable people with strong prescriptions to see clearly with thinner, lighter and less noticeable lenses than standard glass or even plastic lenses.
High-index glasses lenses are made from plastics that are specially manufactured to bend light more effectively than regular lenses. These lenses use less material because they can be cut more thinly than regular lenses.
· CR39 (standard) plastic – 1.50
· Trivex – 1.53
· Mid-index plastic – 1.54 to 1.56
· Polycarbonate – 1.59
· 1.60 high-index plastic – 1.60
· 1.67 high index plastic – 1.67
· 1.74 high-index plastic – 1.74
The higher the refractive index, the thinner the lens. If you choose lenses with a refractive index of 1.67 or higher, they will likely be less than half as thick as regular plastic lenses.
If you want your lenses to be lighter as well as thinner, you’ll need to consider the specific gravity of the lens. This measures how dense a lens is. The thinnest and lightest lenses have lens material with a high index of refraction and a low specific gravity.
These lenses can be 25% to 30% lighter than regular plastic lenses and can weigh less than half as much as regular glass lenses.
What’s the Difference between 1.67 and 1.74 High-Index Lenses?
Both 1.74 high-index lenses and 1.67 high-index lenses have a high index of refraction and can accommodate strong prescriptions.
The lenses differ, however, in the following ways:
· 1.74 high-index lenses are for especially strong prescriptions of +/-8.00 or higher
· 1.74 high-index lenses cost more than 1.67 high index lenses
· When you wear lenses with a strong prescription, it can cause your eyes to look bigger or smaller than they are. Both of these lenses help correct this, but 1.74 high-index lenses do it better