Photochromatic Sunglasses

The science behind protochromatic sunglasses (or "photo-chromatic", with a dash in between, as they used to be referred to) 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.