About Lens Materials
Introduction
There are two features that all BrightEyes' Sunglasses share in common. All the sunglasses we carry have optically correct lenses and provide 100% UV protection.
An optically correct lens prevents headaches after wearing for an extended period of time. When the eye has to look through a lens that is not optically correct, it has to continually refocus because of distortion. This places a large amount of strain on the eye and can eventually lead to headaches, nausea and dizziness.
Sunglass lenses are made of three different materials:
Plastic (CR39) Lens Technology
CR39 stands for Columbia Resin batch 39 manufactured by the Pittsburgh Plate Glass company, which had laboratories in Columbia, this is made up of hard cast resin (CR).
Advantages
- Half the weight of glass lenses.
- Much more shatter-resistant (but not shatter-proof).
- Consistent colour no matter the thickness (preferable for prescription lenses).
- Can be ground and polished like glass.
Disadvantages
- Must be dyed to achieve colour; dyes are not as permanent.
- Tint consistency is difficult to achieve.
- Scratch easier than glass, even with scratch-resistant coating.
- Susceptible to colour and coating fading (thermal bleaching) when exposed to heat.
- Can warp when exposed to excessive heat (>120ºC)
Polycarbonate Plastic
Polycarbonate materials are very similar to CR39 materials, although the molecular structure of the lens does differ and contributes to a much more impact resistant material. It is a remarkably strong plastic; in fact, aircraft windshields motorcycle windshields and sport safety glasses are made of polycarbonate.
A good choice for people who engage in outdoor activities and fast-action sports.
Advantages
- The most shatter-resistant lenses available.
- Very lightweight. (approx 27% lighter than CR39)
- Preferred for oversized lenses to wrap around the face.
- Highest absorption of UV
Disadvantages
- Naturally very soft material, it must be coated for scratch resistance.
- Least scratch-resistant material (even with coating).
- Can not be tinted.
Glass Lens Technology
The clearest of the three lenses ground and polished like a fine camera lens, so the surface is free of imperfections. These days’ glass lenses make up a very small percentage of the sunglass market.
Advantages
- Provides the best scratch resistance.
- Permanent tint and UV protection can be added during manufacture.
- Can have an anti-reflection coating to help with glare reduction.
- Can be vacuum coated with tint (a more permanent method).
Disadvantages
- Least shatter-resistant material.
- Tints can be darker in thicker areas or lighter in thinner areas (as in prescription lenses) due to tint being a coating on surface.
- Shapes are limited due to weight and size.
- Twice as heavy as plastic.
- You can not coat a finished glass lens.
- Fogs up more easily.
- Tints are limited in colour and depth.