Story of Polycarbonate Lenses !

"Polycarbonate lens offered 100% protection from the harmful UV rays and were up to 10 times more impact resistant glass or plastic lenses that made polycarbonate lenses an instant hit. They were first developed in the 1970's for use in space related programs like visors on space suites and shuttle wind shields. In 1983, polycarbonate lenses were formally introduced to the marketplace by Gentex Corporation in response to the demand for light weight, durable and impact resistant lens."

|| Polycarbonates got their name because they are polymers containing carbonate groups (-O-(C=O)-O-) ||

 Polycarbonate is a type of tough and versatile plastic that is used mainly for the manufacture for various things from bulletproof windows to compact discs (CDs). These days, polycarbonate is also used for making lenses that are light weight and almost damage proof. Though there are different eyeglass lenses types, when it comes to eye safety, polycarbonate lenses are considered the best option. These lenses can be used for eyeglasses, sports eye wear and sunglasses etc. Clear polycarbonate is used in the manufacture of eyeglasses, since it is transparent, durable and has a high infraction index. Polycarbonate lenses are the first plastic high index lens having one of the highest index ratings (1.586). They are thinner and more durable than other conventional and glass lenses and are used as prescription glasses for vision correction of even very high numbers.

 Polycarbonate lenses are also used for making sunglasses that make the use of filters to block the harmful UV rays of the sun. These lenses are also polarized to block glare and are also perfect for sports wear since they are high impact resistant. Polycarbonate lenses are an excellent choice for making children's glasses and safety glasses since they are resistant to scratches as compared to the standard plastic lenses.

Advantages of polycarbonate lenses

• Greater protection: Polycarbonate lenses in your glasses protect your vision by holding up to rough play or sports practice. With high impact resistance, polycarbonate lenses provide greater protection against eye damage and vision loss from broken or shattered lenses.
  
• Lighter weight: Polycarbonate material is lighter than standard plastic or glass, which is highly suitable for people with strong prescriptions. As well as being lighter, polycarbonate lenses are thinner than standard plastic or glass, which contributes to the lighter weight.
  
• Scratch-resistant: Although no lens is scratch proof, polycarbonate lenses come with a scratch-resistant coating to keep them clear as long as possible, even when worn by children. Experts believe that a polycarbonate lens is significantly more shatter-resistant than glass or regular plastic lenses.
  
• UV protection: Polycarbonate lenses also offer inherent ultraviolet protection. About 99 percent of potentially damaging UV rays are filtered out by these lenses, whether the rays come from sunlight, fluorescent lights or a computer screen.
  
• Safety: Currently, the lenses and frames made from polycarbonate materials provide the highest level of impact protection. When glass or plastic lenses break, they do not break into harmless granules, but can break into sharp shards that can enter your eye and destroy your vision. That is why polycarbonate is far and away the safest of all the lenses made.
  

Drawbacks of polycarbonate lenses

• One of the very few weaknesses or drawbacks of polycarbonate lenses is that they are not as good optically as high index or plastic lenses.
  
• People in prescriptions with higher powers sometimes have trouble seeing out the edges of the lenses -- your clear field of vision is not as wide as with glass or plastic lenses. Similarly, you should also remember that polycarbonate lenses bend light differently. As your vision is corrected by light passing through a prescription lens and focusing an image on your retina, polycarbonate bends light to a greater degree than glass or plastic lenses of equal thickness.
  Therefore, if you previously wore a glass or plastic lens, there may be an adjustment period needed to adapt to polycarbonate lenses.
  
• Certain lens coatings may reduce the impact effectiveness of polycarbonate and some lens tints may be difficult or impossible to apply.
  
• Polycarbonate lenses will not shatter, they are more prone to scratching than other materials if not protected with scratch-resistant coating.
  
• Polycarbonate lenses are harder to make, they require more time to manufacture.

Physical Properties - (from Wikipedia)

Density (ρ)	1.20–1.22 g/cm3
Abbe number (V)	34.0
Refractive index (n)	1.584–1.586
Flammability	V0-V2
Limiting oxygen index	25–27%
Water absorption –Equilibrium(ASTM)	0.16–0.35%
Water absorption – over 24 hours	0.1%
Radiation resistance	Fair
Ultraviolet (1-380nm) resistance	Fair

External Link for polycarbonate lens

What is UV400, UVA, UVB and UVC?

Ultraviolet radiation (UV) comes naturally from the sun. There are also some manmade lamps and tools (welding tools, for instance) that can produce UV radiation. For most of us, however, the sun is the primary source of UV. UV is divided into at least three different categories based on wavelength:

Does the amount of UV protection in sunglasses matter?

• UVA wavelengths(320-400 nm) are only slightly affected by ozone levels. Most UVA radiation is able to reach the earth's surface and can contribute to tanning, skin aging, eye damage, and immune suppresion. UV 400 Protection lenses are engineered to block 100% of all harmful blue light up to 400 nanometers
  
  
• UVB wavelengths(280-320 nm) are strongly affected by ozone levels. Decreases in stratospheric ozone mean that more UVB radiation can reach the earth's surface, causing sunburns, snow blindness, immune suppression, and a variety of skin problems including skin cancer and premature aging.

• UVC wavelengths (100-280 nm) are very strongly affected by ozone levels, so that the levels of UVC radiation reaching the earth's surface are relatively small.

The effects of UV radiation on earth's ecosystems are not completely understood. Even isolating the effects of UVA versus UVB is somewhat arbitrary. All UV radiation can be damaging. This knowledge has prompted many manufacturers of sun screen and sunglasses to offer products that protect against both UVA and UVB wavelengths.

While humans can choose various courses of protection, for instance avoiding noon-time sun, plants and animals are not so fortunate. Studies have shown that increased UV radiation can cause significant damage, particularly to small animals and plants. Phytoplankton, fish eggs, and young plants with developing leaves are particularly suspectible to damage from overexposure to UV.

Solar UV radiation levels are highest during the middle of the day. In total, almost half the daytime total UV radiation is received during the few hours around noontime. Clouds, as well as ozone, have a tremendous affect on UV radiation levels. However, cloudy skies generally do not offer significant protection from UV. Thin or scattered clouds can have minor impacts on UV and even, for a short time, increase UV above what it would be on a blue sky day by further scattering the radiation and increasing the levels that reach the surface.