The advantages of having photochromic lens on your sunglasses are not only convenient (ability to go from outdoors to indoors with changing eyewear) but they enhance vision of patients with various ocular pathologies (diseases) and those who have light to moderate sensivity to light.

Photochromic refers to a type of lens that darkens when exposed to UV light and clears up when not exposed. These lenses are commonly used in eyeglasses and sunglasses to provide protection from sunlight.

The technical term for a lens that has a variable tint is aphotochromic lens (also called photochromatic if you are in America). One of the most well known and commonly sold brands of photochromic lenses are "TRANSITIONS". Photochromic lenses get darker when they are exposed to ultra-violet light, not when you are indoors in a brightly lit room with artificial lights. Photochromic lenses are best suited to slightly cooler climates as they react much quicker and will go darker when they're cold compared to using them in a very hot climate.

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You can purchase photochromic (color changing) paints at most automotive paint stores.