Photo-chromatic Sunglasses

The science behind photo-chromatic sunglasses can be a
little difficult to understand, but the first step is to
know what photo-chromatic sunglasses are.

Photo-chromatic sunglasses are prescription sunglasses that
change from clear to shaded sunglasses instantly when
exposed to the sun then back again when inside.

It is a reversible application commonly known as
photochromism.

Photochromism is the reversible transformation of a
chemical species between two forms by the absorption of
radiation where the two forms have different absorption
spectra.

As previously stated, it is quite difficult to understand
the science behind it. Though they are highly popular, the
science is still slightly unstable and the color changing
lenses found in eyeglasses cant withstand thousands of
hours of outdoor exposure.

The changing speed from clear to shaded lenses is highly
sensitive to the rigidity of the environment around the
dye.

Photo-chromatic lenses can be made of either glass or
plastic. The glass version was developed around 1960 by the
Corning Company. The plastic version is used more as it is
more affordable.

Transitions Optical introduced the first successful plastic
photo-chromatic sunglass lens in 1991. What makes them
special is they do not change indoors under bright lights.

It takes the UV that the sun provides to activate the
chemicals that darken the lenses to shade and protect the
eyes.

Because photo-chromatic compounds fade back to a clear
state by a thermal process, the higher the temperature, the
less dark the lenses will become. It prevents the glasses
from becoming true sunglasses in very hot temperatures.