Transition lenses are photochromic optical lenses that darken when exposed to ultraviolet light. Originally it was only available in a heavy glass lens, but in 1990, lens manufacturer Transition Optical began making the lenses out of a thinner, lighter plastic material.
Transition lenses are produced in a sterile, dust-free manufacturing environment so airborne particles and dust do not lodge in the lens material. A liquid containing millions of photochromic molecules is applied to the front surface of the lenses after they are heated gently so that the lenses become porous.
The lens acts like a sponge, absorbing the photochromic material into the top layers. Once the liquid has dried, the lenses are UV sensitive and will change from clear to dark.
When exposed to UV light, the molecules become energised and activated and cause the lens to darken.
After the lenses are coated with the photochromic liquid, they can scratch very easily in their raw state.
A second process is then applied. Transition lenses receive a scratch resistant coating that was originally developed by NASA engineers for protecting space helmets.
The lenses are manufactured into large, thick chunks called blanks, uncut lenses that are sent to optical labs to be ground down into prescription lenses and sized to fit into frames.