Photochromic lenses are lenses that darken on exposure to specific types of light, most
commonly ultraviolet radiation. Once the light source is removed, the lenses will gradually
return to their clear state. Photochromic lenses may be made of glass, polycarbonate,
or another plastic. Photochromatic lenses contain millions of
molecules of silver chloride or silver halide embedded within them. These molecules are
transparent to visible light in the absence of ultraviolet rays, which is normal for artificial
lighting. When exposed to ultraviolet rays, as in direct sunlight, the molecules undergo
a chemical process that causes them to change shape and absorb portions of the visible light,
causing the lenses to darken. This process is reversible; once the lens is removed from
strong sources of UV rays the silver compounds return to a state which allows all light through.
Invention Photochromic lenses were developed by Roger
Araujo at the Corning Glass Works Inc. in the 1960s, and the process was used in the
first mass-produced variable tint lenses. Technical details
The glass version of these lenses achieve their photochromic properties through the
embedding of microcrystalline silver halides, or molecules in a glass substrate. Plastic
photochromic lenses rely on organic photochromic molecules to achieve the reversible darkening
effect. The reason these lenses darken in sunlight but not indoors under artificial
light, is that room light does not contain the UV found in sunlight. Automobile windows
also block UV so these lenses would darken less in a car. Lenses that darken in response
to visible light would avoid these issues, but they are not feasible for most applications.
In order to respond to light, it is necessary to absorb it, thus the glass could not be
made to be clear in its low-light state. This correctly implies photochromic lenses are
not entirely transparent: they filter out UV light. This does not represent a problem,
because the human eye does not see in the UV spectrum.
With the photochromic material dispersed in the glass substrate, the degree of darkening
depends on the thickness of glass, which poses problems with variable-thickness lenses in
prescription glasses. With plastic lenses, the material is typically embedded into the
surface layer of the plastic in a uniform thickness of up to 150 µm.
Typically, photochromic lenses darken substantially in response to UV light in less than one minute,
and then continue to darken very slightly over the next fifteen minutes. The lenses
fade back to clear along a similar pattern. The lenses will begin to clear as soon as
they are away from UV light, and will be noticeably lighter within two minutes and mostly clear
within five minutes. However, it normally takes more than fifteen minutes for the lenses
to completely fade to their non-exposed state. A report by the Institute of Ophthalmology
at the University College London has suggested that even in dark conditions photochromic
lenses can absorb up to 20% of ambient light. Because photochromic compounds fade back to
their clear state by a thermal process, the higher the temperature, the less dark photochromic
lenses will be. This thermal effect is called "temperature dependency" and prevents these
devices from achieving true sunglass darkness in very hot weather. Conversely, photochromic
lenses will get very dark in cold weather conditions, which makes them more suitable
for snow skiers than beachgoers while outside. Once inside, away from the triggering UV light,
the cold lenses take longer to regain their transparency than warm lenses.
A number of sunglass manufacturers/retailers offer products that use photochromism to make
lenses that go from a dark to a darker state. Because these products are tinted in the bleached
state, they are typically used only outdoors and are not considered general-purpose lenses.
References
External links "Photochromic lenses", How stuff works .
"Optometry Glossary", Perfect glasses, USA .