The differences between lens materials
Back when glasses were first invented, all the lenses were made of glass. Fast forward to today, and you'll find lenses are offered in a variety of materials, with a variety of coatings available.
Knowing the basic differences can aid you when you're looking around to buy glasses.
With the advances that have been made in optical technology, few retailers offer glass as a standard any longer. While it served it's purpose in years past, most other materials tend to be more than ideal. Here are a few notes about glass lenses:
+ Scratch resistant
+ Resistant to discoloration
+ Resistant to chemicals
- Easy to chip or shatter
- Dangerous when shattered
The real downsides to glass are that it's heavy, and that it shatters easily and can be dangerous. Dropping a pair of glasses with glass lenses on a solid surface often results in it breaking. Particularly in the case of children playing, glass is the least preferable option - a baseball to the eye can result in a trip to the emergency room followed by surgery to remove shards of glass from the eye. It's also quite heavy - for those with high prescriptions, glass lenses tend to cause glasses to slide down the nose, or leave stronger impressions from the nosepads.
The only real advantage is it's scratch-resistance and durability. Anything that doesn't cause the glass to break usually leaves the glass itself relatively clear and unscathed. It's when something happens to break the glass that serious problems arise...
CR-39 is the most common material used in glasses today. It's lighter and less shatter-prone than glass. Here are a few notes about CR-39 (plastic) lenses:
+ Shatter resistant
- Scratches more easily
- Subject to discoloration (chemical or otherwise)
The real up-sides to CR-39 is that are that it's both light and shatter-resistant. This makes them both easier to wear, and much safer than glass. The largest down-side is that it's easier to wear - dropping a pair of glasses with CR-39 lenses on the ground probably won't break them, but it may leave them with a deep gouge or scratch. Anything that tends to cloud plastic can cloud CR-39 lenses over time as well. A well-kept pair of CR-39 glasses will usually last many years, but the lenses typically won't be in as good of shape as well-kept glass lenses after a long period of time. Since most people tend to replace their glasses every few years, this tends not to be a big deal.
Polycarbonate's a space-grade material, and has been used in space shuttles. The characteristics that made it great for space travel tend to make it great for eyewear:
+ Very Light
+ Virtually shatter-proof
+ More scratch-resistant than CR-39
+ Built-in UV protection
- Scratches more easily than glass
Polycarbonate lenses are the best choice for children's glasses. There have been videos across the internet showing a bullet effectively bouncing off polycarbonate - it's very very difficult to break and almost impossible to shatter. It has the added benefit of being the lightest material available. It's less susceptible to scratching than plastic/CR-39 lenses, but it's not quite as hard as glass and therefore can be scratched. It's used in most high-quality safety glasses in industrial workplaces. With many pros and very few cons, it's commonly known as the "best" lens material for any glasses.
Just about any optician would agree, you can't go wrong with polycarbonate. It's the safest lens to have near your eyes. In the case of children, most should consider it the only option. If you're looking to spend a little less, CR-39 is still a good option. Glass however isn't usually recommended anymore unless you have a specific reason for wanting it.
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