There are many types of lens and coatings available today for prescription eyewear. Isyte has tried to make some sense of the potential confusion...
Glass is probably now the least used but it was the world's first ophthalmic lens. It has the most durable surface of all lenses and so is less likely to scratch..
It is heavy and can break which makes it unsuitable for wearers where there is a danger of breakage during wear as glass could get into the eye. It is also much heavier.
It has the advantage that for high prescriptions it is available in higher indexes. The 1.9 index is the highest of any ophthalmic material available.
CR39 resin is the world's most used ophthalmic lens material. It is very light but can scratch but this can be avoided to a large extent by hard coats.
CR39 resin can be
2. Surfaced to special Rx's
3. Hard coated.ted
4. Special UV absorbers can be added
5. Available as photochromic
CR39 Resin is available with the greatest variety of any lens material. It can shatter and so is not suitable in sports glasses and is not the best option for rimless frames.
Polycarbonate is undoubtedly the world's strongest ophthalmic material. It may not stop a nail from piercing its surface but the lens would remain in tact.
Its quality has improved over the years and can now be regarded as an equal amongst other ophthalmic lens materials.
It has a few drawbacks it scratches the most easily of all the lens materials but hard coating can minimize this. It is more difficult to tint but is available in transition lenses.
It dislikes benzene chemicals so it's no good for people working in the petrochemical industry! Graduated tints are not available.
It is however the ultimate sporting lens.
These coatings will eliminate distracting reflections from both surfaces of the spectacle lens. They improve the efficiency of the lens and reduce glare especially when driving at night. They also improve the cosmetic appearance of the lens especially in higher prescriptions.
Anti-Scratch Coatings (Hard coated lenses)
This is a worthwhile addition to CR39 lenses and polycarbonate lenses. It takes the form of a quartz type coating usually on both sides of the lenses to reduce lens scratching. If a lens is scratched because there is no coating the scratch cannot be polished out.
These absorb all the harmful UV wavelengths of light between 300 and 400 nm. They have virtually no colour but will protect the eye against potentially harmful UV and hence reduce the risk of cataract and macular degeneration.
This was once only available in glass however they are now available in lightweight CR39 lenses. These lenses darken with the amount of UV light that hits the lens surface. They all have UV filters and modern photochromic lenses react quickly and have a wide range of transmission.
High Refractive Index Lenses
These lenses are available in both glass and CR39 and are very useful for higher prescriptions. The material of the lens refracts light more efficiently and hence a much lighter and thinner lens is produced compared to the equivalent power in a standard index CR39 or glass lens.
These lenses stop reflected glare by only allowing one particular plane of light to be transmitted through the lens. These are ideal for fishermen and all water sport activities.