Photochromic lenses are lenses that darken automatically when exposed to sunlight, then fade back when you return indoors. 
In most cases, photochromic lenses are clear (or nearly clear) indoors and darken to a medium sun tint outdoors. But there are exceptions.Terms sometimes used for photochromic lenses include “light-adaptive lenses” and “variable tint lenses.”
Photochromic lenses are available in nearly all lens materials and designs, including high-index lenses, bifocals and varifocal lenses An added benefit is that they shield your eyes from 100 percent of the sun’s harmful UVA and UVB rays.
Because a person’s lifetime exposure to sunlight and UV radiation has been associated with cataracts later in life, it’s a good idea to consider photochromic lenses for children’s eyewear as well as for eyeglasses for adults.
Though photochromic lenses cost more than clear lenses, they offer the convenience of reducing the need to carry a pair of prescription sunglasses with you everywhere you go.
Although there are many types of photochromic lenses we are particular fans of Hoya Sensity lenses.
Traditionally, the performance of light reactive lenses has been strongly influenced by the weather and temperature conditions.
For example in a skiing environment with high levels of mountain UV and low air temperatures, or a dry, dusty desert with high temperatures and very little UV. In the past, photochromic lenses had a hard time dealing with such circumstances. This meant that on the ski slopes lenses were too dark – and too slow in returning to clarity. In hot sands, colour change was too little, too late.
Sensity lenses swiftly darken to a sun lens outdoors, and revert faster than before to full clarity indoors. They are tested across different climates, regions, altitudes and temperatures and provide consistent light reactive performance. Meanwhile, UV protection is maintained at a consistently high level.
Regular photochromic lenses work by the lens molecules being activated by the sun’sultraviolet radiation. Because UV rays penetrate clouds, photochromic lenses will darken on overcast days as well as sunny days. But because the windscreen and window glass of cars and lorries blocks most UV rays, photochromic lenses typically won’t get as dark inside a vehicle as they do outdoors. There is however an exception to this – the so called Transitions XTRActive lens
.XTRActive lenses are the darkest Transitions lenses available: adjusting up to 90% tint at 23°C. They get darker faster, even in hot temperatures with 80% tint at 35°C.XTRActive lenses also darken behind the windscreen of a car. By adjusting to both UV and visible light, the lenses activate to a comfortable level of tint for driving: up to 50% at 27°C*. Transitions XTRActive lenses ensure glare protection and optimal comfort in all outdoor situations and temperatures. Wearers will appreciate the versatility compared to ordinary clear lenses. Transitions XTRActive provides 100% blockage of harmful UV-A and UV-B rays.
Our hectic modern lifestyle means that fast reactions have become second nature to us. Nowhere is this more apparent that when driving a car, which often requires us to react swiftly to unexpected situations. Vision plays a crucial if not defining role in this process and is the key to driver safety
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Drive Wear lenses maximise light transmission in cloudy and overcast conditions, providing the sharpest possible vision. The polariser protects against the glare that would normally hinder vision in low light. The lenses have a high-contrast yellow-green colour in these conditions.Our hectic modern lifestyle means that fast reactions have become second nature to us. Nowhere is this more apparent that when driving a car, which often requires us to react swiftly to unexpected situations. Vision plays a crucial if not defining role in this process and is the key to driver safety.