Sunglasses are a form of protective eyewear designed primarily to
prevent bright sunlight from damaging or discomforting the eyes.
Healthcare professionals recommend eye protection whenever the sun
comes out to protect the eyes against damaging UV radiation. Experts
recommend sunglasses that reflect or filter out 99% of UVA and UVB
light with wavelengths of up to 400 nm. Sunglasses which meet this
requirement are often labeled as "UV400" The U.S. standard is called
ANSI Z80.3-2001, which includes three transmittance categories.
According to the ANSI standard, the lens should have a UVB (280 to 315
nm) transmittance of no more than one per cent and a UVA (315 to 380
nm) transmittance of no more than 0.3 times the visual light
transmittance. There is also an ANSI standard Z87.1-2003 which
includes requirements for basic impact and high impact protection. In
the basic impact test, a 1 inch steel ball is dropped on the lens from
a height of 50 inches. In the high velocity test, a 1/4 inch steel
ball is shot at the lens at 150 feet/second. To pass both tests, no
part of the lens may touch the eye.
ThinkFreebies uses only the best materials in the manufacture of their
sunglasses lenses. All ThinkFreebies meet the above requirements
for UV400 lenses. ThinkFreebies Sunglasses feature two types of lenses;
polycarbonate (high impact lens) and polarized (basic impact lens).
Polycarbonate material was originally designed for canopies covering
cockpits in fighter planes. Polycarbonate offered an impact-resistant,
optically clear window. Polycarbonate lenses for sunglasses are up to
10 times more impact-resistant than plastic or glass lenses and they
provide 100% protection from the sun's harmful rays. They are also
lightweight, adding to the comfort of your eyeglasses and sports
eyewear. Polycarbonate lenses are the most impact-resistant lenses on
the market because of its manufacturing process. The majority of other
plastic lenses utilize a cast molding process. This is when a liquid
plastic material is baked for long periods, in lens form, until the
plastic solidifies into a lens. Polycarbonate, on the other hand, is a
thermoplastic material produced from small pellets. Therefore, the
earliest phase of polycarbonate lenses begin from a solid source,
eliminating the cast molding process and instead use a process called
inject molding in which the pellets are heated to a melting point.
Once in a liquid form, the polycarbonate is rapidly injected into lens
molds. At this phase in the process, the lens molds are compressed
under high pressure and cooled to form a final lens product in a
matter of minutes.
Polarized sunglasses first became available in 1936, when Edwin Land
began experimenting with making lenses with his patented Polaroid
filter. Polarized lenses improve comfort and visibility by reducing
glare reflected at angles off shiny non-metallic surfaces such as
water. Light reflected from surfaces such as a flat road or smooth
water generally is horizontally polarized. This means that, instead of
light being scattered in all directions in a more usual way, reflected
light generally travels in a more horizontally oriented direction.
This creates an annoying and sometimes dangerous intensity of light
that we experience as glare. Polarized lenses contain a special filter
that blocks this type of intense reflected light, reducing glare.
Polarized sunglasses also can be worn indoors by light-sensitive
people such as post-cataract surgery patients. Polarized material is
more expensive than glass, plastic, or polycarbonate.