As a graduated eyeglass wearer, as I leave an enclosed environment for the sunny exterior, I have often thought that my life would be much easier and more comfortable if I had a pair of glasses that could correct my vision and at the same time use sunglasses to wear when necessary. If you're like me and you've already thought this through, then you'll like the photochromatic lenses. 

One of the trends that seems to be emerging this year is photochromatic lenses. But what are photochromatic lenses anyway? The lenses we usually see are completely transparent or, in the case of sunglasses, completely darkened.

What makes photo-sensitive lenses different is that they do not have a fixed "shade", they darken depending on the brightness. They react to UV rays, and darken when exposed to this type of radiation. When they are not exposed to UV rays they become transparent. 

Developed by William H. Armistead and Stanley Donald Stookey, these lenses have properties that were acquired through the incorporation of molecules, such as silver chloride, that darken when absorbing the sun's rays, thus eliminating the need to change glasses and making them an optimal complement for outdoor sports.

Because the photosensitive molecules revert to their natural state by a thermal process, the higher the temperature, the lower the darkening of the lenses. This "thermal dependence" prevents glasses from reaching the same "darkness" level as sunglasses when inserted into an environment with a high temperature. On the other hand, if the lenses are inserted in a cold climate, they get darker, which makes them great for winter sports. However, in cold environments, when returning to enclosed spaces the lenses take longer to recover translucency.

Photochromatic properties can be used in progressive or single focal length lenses, but these properties cannot be applied to too thin lenses and are available in a very limited range of colours. 

Despite the advantages of this type of lenses, there are also drawbacks in their use. Their main drawback is that they do not immediately adjust to light conditions. They always take a few minutes, depending on the brands that produce them. 

As a general rule, photochromatic lenses darken in less than one minute, and continue to darken slowly for the next 15 minutes. In the opposite case, the lenses begin to lighten in about 2 to 5 minutes, taking on average 15 minutes to become completely translucent.

Another disadvantage is that they may not darken when worn inside a car. This is because the material used in the windscreen often absorbs 100% of the UV rays. For this reason, it is not advisable to use it for driving.

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