Photochromic lenses are lenses that darken when exposed to UV light such as sunlight and lighten when removed from the UV light source. They work by using small amounts of silver halide compounds like silver chloride, which darken when exposed to UV light. This darkening process occurs rapidly within a minute of UV exposure and lightening takes slightly longer at around 5-15 minutes without UV light. While Transitions lenses are a popular brand, many companies produce photochromic lenses with different technologies for achieving the lightening and darkening effect.

1. Photochromic
Lenses
SAHAYAN ZIA
SARFARZ AHMED
TEHSEEN JAVAID

2. • The word "photochromic" comes from two Greek
words "photos" meaning light and "Chroma"
meaning colour—so photochromic simply means
something that changes colour in response to light.
• Photochromic lenses are lenses that darken on
exposure to specific types of light, most commonly
ultraviolet (UV) radiation.

3. • Traditional photochromic eyeglasses are generally alkali
boroaluminosilicates with 0.01 to 0.1 percent silver halide
and a small amount of copper. Upon absorption of light,
the silver ion reduces to metallic silver, which nucleates to
form colloids about 120 angstroms in size. This is small
enough to keep the glass transparent, but the colloids are
dense enough to make the glass look gray or brown. In
photochromic eyeglasses, darkening is reversed either by
the removal of light (optical bleaching) or by raising the
temperature (thermal bleaching).

5. Darkening & Lightening timings
• 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.

7. Photochromics and transitions
• Because the most popular photochromic lenses sold in the
United States are made by Transitions Optical, many
people — including some eye care practitioners—
mistakenly call all photochromic lenses "transitions
lenses" or "transition lenses." But there are many brands
of photochromic lenses offered by different lens
manufacturers.
• The glass version of these lenses achieve their
photochromic properties through the embedding of
microcrystalline silver halides (usually silver chloride), To
achive the reversible darkening effect.

8. Advantages
• They provide the correct amount of protection for
the varying light conditions.
• They absorb 100% UV radiation that can otherwise
damage eyes.
• These lenses sometimes are used to enhance the
vision of patients who have various eye pathologies,
including macular degenerations.

9. Disadvantages
• They do not adjust immediately. It takes a few minutes for
the lenses to change from light to dark or vice versa.
• For lenses that start clear, they are likely to darken more
or less every time you go outside whether it's sunny or not
as it reacts to the ultraviolet light which is still around on
cloudy days. However, they will quickly go clear again
when indoors.
• They don’t always work effectively in cars because
windscreens naturally screen out most of the ultraviolet
light.

10. REFERANCE
• AMERICAN ACEDEMY
• MA JALIO
• FRIENDS