lenses are photochromic lenses that change color depending on light intensity.
When we go abroad, ultraviolet light reaches the lens, change the chemistry of it, causing it to darken. On the contrary, to enter the house or at night, loses its UV light and retrieves the initial membership is again transparent. Also, on cloudy days where the sun intensity is not as high the pitch of the lens is in an intermediate grade.
The intensity of the color change and the speed of change are dependent on temperature, 23 ยบ up to get darker 83%, equivalent to a sun lens filter category 3 (which is what most often wear sunglasses), and in just over 30 seconds and reached 70% of color.
Clarifying, if the lens is anti-glare, a 95% transmission, so they are as transparent as a conventional lens.
The advantages of these lenses, and protect from UV light and not having to change between sunglasses and tinted glasses, is protected from glare by adjusting speed of light conditions external, while protecting against eye strain that produces the continuous variations light throughout the day.
You can see more information and examples of its performance:
