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Photochromic Lenses Explained

What are Photochromic Lenses?

Photochromic lenses, often called transitions or reactolights, darken to a sunglasses tint when exposed to sunlight, or U/V ultraviolet, and return to a clear state when indoors, away from U/V light.

Photochromic lenses are made of many lens materials including plastic, glass or polycarbonate. They are typically used as sunglasses that conveniently switch from a clear lens indoors, to a sunglasses depth tint when outdoors, and vice versa.

They typically take around 30 seconds to fully darken and approx 2 - 5 minutes to return to a clear state.

Photochromic lenses were created in the 1960's at the Corning Glass Works Inc, and became the first variable tint/photochromic lenses brought to the marketplace.

How Do Photochromic Lenses Work?

Tiny molecules of silver halide and chloride are embedded within a photochromic lens which is invisible and clear until exposed to sunlight/UV rays. A chemical process takes place when exposed to sunlight/UV and the molecules effectively move, change shape and absorb the light. This process changes the molecules and they darken to a sunglasses shade/tint.

The photochromic lens will remain in this 'darkened' sunglasses state whilst exposed to sunlight/UV and the molecules will reverse the process once the lenses are removed from the sunlight/UV rays returning to the original 'transparent/clear' lens.

Photochromic Lenses Reaction

How Long Do Photochromic Lenses Take To Change Colour or React?

Photochromic lenses, when exposed to sunlight/UV rays, will on average take approx 30 seconds to a minute to darken and will continue to darken for a further 10 minutes to full sunglasses tint/shade, returning to an almost clear state within 2 minutes and fully cleared within 5 minutes although excess temperature will affect this.

What are The Advantages and Disadvantages of Photochromic Lenses?

Photochromic lenses are an extremely convenient, cost effective and beneficial purchase for a number of reasons, although, there are some weaknesses.

There are a few pro's and cons of photochromic lenses and sunglasses. Here is a list of the common ones.

Advantages & Benefits of Photochromic Lenses:

  • Reduced costs – buying a single pair of dual purpose photochromic glasses that can provide vision for both indoors when clear and providing sunglasses protection outside is more cost effective than buying two separate pairs.
  • UV protection - Photochromic lenses provide full protection against the harmful UV rays of the sun and direct exposure to sunlight can cause serious eye problems and in some cases blindness.
  • Convenience - You only need to own 1 pair of glasses for both indoors and outdoors in the sun as opposed to needing to switch between 2 pairs of regular glasses to sunglasses. You only need to remember to take one dual purpose pair of glasses with you as no doubt at some point we have all forgotten to take our sunglasses out with us only to be caught out by the glare of the sun or even left them behind never to be seen again.
  • Eye Health – Reducing your exposure to the suns harmful UV rays can lower the risk of cataracts or other eye/age related problems

Disadvantages of Photochromic Lenses:

  • Reaction Times – Although photo lenses reaction from clear to sunglasses is quite quick, typically within 30 – 60 secs, the reversal takes a little longer as much as 5 minutes depending upon the temperature of the lens which could be dangerous if entering a dark room or certainly inconvenient at best.
  • Driving – Standard photochromic lenses do not work inside a car due to the windscreen blocking out the UV rays which are needed to active the chemical process which causes the lens to darken. (Although there are special photochromic lenses that can react inside a car or behind glass which are specialist lenses known as 'transition drivewear lenses' which combine both photochromic and polarisation technology.
  • Not Dark Enough – Photochromic lenses as convenient as they are at blocking out UV rays and protecting you from the suns glare will likely not be sufficient for anybody who suffers from extreme high light conditions. Fortunately, the lenses are suitable for the majority of wearers but advice must be taken if you are affected by any light conditions.

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

Plastic Photochromic Lenses Vs Glass Photochromic Lenses:

Plastic photochromic lens molecules of silver and halide are usually layered or embedded evenly onto the surface of the lens providing an even shade of tint when darkening.

Glass photochromic lenses have the molecules contained within the glass and this can sometimes cause an uneven tinting effect on a lens with higher prescription due to the different thickness of the lens from one edge to the other

Can I use Photochromic sunglasses for driving?

Standard photochromic lenses do not work inside a car due to the windscreen blocking out the UV rays which are needed to active the chemical process which causes the lens to darken.

A new lens has been brought to the market that can react inside a car or behind the windscreen without UV rays and uses daylight alone to trigger the reaction. These lenses are known as 'transition drivewear lenses' which combine both photochromic and polarisation technology.

As the name suggests, they were invented for use in the car whilst driving and offer full protection from the suns glare during the day which makes them perfect for wearing whilst driving.

This image shows how transitions Drivewear photochromic lenses react in different conditions:

DriveWear Photochromic Lenses

Conclusion:

Photochromic lenses are a great way to protect your eyes, potentially reduce costs and add some convenience to your routine but to some they will be inconvenient or inadequate and not worth purchasing. It is up to you to decide the benefits but please feel free to contact us and speak to one of our opticians should you require any further information or advice.

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