Photochromic Lenses

SAHAYAN ZIASARFARZ AHMEDTEHSEEN JAVAID

•  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.

• 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).

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.

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.

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.

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.

REFERANCE

• AMERICAN ACEDEMY• MA JALIO• FRIENDS