Photochromic lenses change color in response to UV light. They darken when exposed to UV rays and lighten when UV exposure is removed. Various chemical compounds like silver chloride or naphthopyrans are used in glass or plastic lenses to enable this reversible color change. Photochromic lenses provide visual comfort both indoors and outdoors by automatically adjusting tint as needed.

1. PHOTOCHROMIC
LENSES
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Made By: Zahida Zia
BS Chemistry
8th Semester (E)
GCWUF

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 Photo means light and Chromic means color.
 Photochromism is the reversible transformation of a chemical
species between two forms by the absorption of electromagnetic
radiation, where the two forms have different absorption spectra
 Photochromism is a phenomenon that was discovered in the late
1880s by Markwald. Markwald called the process phototropy
until the 1950s, when Yehuda Hirshberg proposed the use of the
term photochromism.
 Photochromic lenses were invented by William H. Armistead and
Stanley Donald Stookey of Corning Glass Works in the early

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 A photochromic effect is a reversible change in chemical species
caused by the absorption of electromagnetic radiations.
 That is, when photochromic materials are exposed to ultraviolet
radiation, they change their structure, hence causing a change in
color (from colorless to dark). In the absence of UV radiation, they
retain their initial color.
 A lens is an optical device that either disperses or focuses a beam of
light.
 Photochromic lens is basically an optical device with photochromic
substrates.
 They turn black on exposure to UV radiations and retain their
original state (clear) in the absence of UV radiations.

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TIME FOR WORKING
 It takes about 30 seconds to a minute to darken and will continue to
darken for a further 10 minutes to full sunglasses, returning to an
almost clear state within 2 minutes and fully cleared within 5
minutes.
MATERIAL USED
 The most common photochromic lens materials are glass and
polycarbonate sheet (plastic).

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Also Known As
 Day and light lenses
 Transition
PROPERTIES
 Visual comfort both indoor and outdoor.
 Available in single vision and bifocal forms.
 Responds to temperature variations.
 Blocks 100% of the sun’s eye damaging UV rays reducing eye
fatigue.

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TYPES OF PHOTOCHROMIC LENSES
GLASS PLASTIC

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GLASS PHOTOCHROMIC LENSES
 The first photochromic lenses were made from glass.
 the main photochromic compounds were either silver chloride or silver
halide.
 The compounds were embedded within the glass substrate.
 To make a glass photochromic lens, you need to dope silver chloride with
copper.
 when you subject glass photochromic lens to UV light, it will begin to dark
gradually.
 This implies that, the silver chloride will begin to break into silver and
chlorine.
 This is what leads to the darkening process.
 Again, when you remove the UV light, silver will begin to combine with
chlorine. It is at this stage that copper plays an integral role.
 That is, the presence of copper (I) chloride will help to reverse the process.

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Darkening Process
Lightening Process

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PLASTIC PHOTOCHROMIC LENSES
 Plastic photochromic lenses are made of polycarbonate materials
that are carbon-based molecules. Depending on the type of molecules or
compounds, they react differently by changing their molecular structure on
exposure to ultraviolet radiation.
Naphthopyrans
 When a naphthopyran is exposed to UV light , a weak bond (red)
breaks and the molecule rearrange to a species that absorbs light at
longer wavelengths.

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Spiropyrans and spirooxazines
Diarylethenes

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AVAILABLE IN DIFFERENT COLORS
Mostly 3 colors are used, brown, graphite green and grey.

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Photochromic Lens Temperature Dependency
 Photochromic lenses change state in a chemical and thermal
process which can be affected/limited by a process known as
'temperature dependency'.
 In extreme heat a photochromic lens is unlikely to achieve full
sunglasses shade and may only achieve a 95% state.
 On the other hand, photochromic lenses will become very dark
in a cold environment/weather conditions.
 Cold weather also increases the time taken to return to a clear
shade

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ADVANTAGES
 They protect your eyes against the harmful UV rays of the sun.
 They provide the convenience of having clear lenses indoors
while having a tinted lens outdoor without the need of having
multiple pairs.
 They can be placed in almost any
frame on the market.
 Provide an advanced eye safety.

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DISADVANTAGES
 Do not get dark quickly enough in cold weather.
 They take time to lighten and darken. Darkening takes
about a minute while lightening can take 15 minutes or
more.
 Mostly don't work effectively in a car.
 They can hold a slight tint, so the lenses may never be
completely clear.
 Price of Photochromic lenses.
 Photo chromatic lenses are not as effective in hot climate.
When temperature increase , the reaction slows.

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REPUTABLE PHOTOCHROMIC LENSES BRANDS
 Transitions
 WeeTect
 Sensity
 Essilor Sun Solution
 Zeiss
 Rodenstock
 Signet Armorlite

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CONCLUSION
 As you can see, the working principle of photochromic lenses is
based on invisible chemical reactions or change in molecular
structure.
 Their effects manifest in the form of color change. Again, to
benefit from all these, you need to buy high quality
photochromic lenses from reputable and trusted manufacturer.

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