Photochromic Lenses:

Photochromic lenses darken to a sunglasses tint when exposed to sunlight, or UV ultraviolet, and return to a clear state when indoors, away from UV light.

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 between two to five minutes to return to a clear state.

Advantages:

Reduced cost (single pair of glasses work indoors, outdoors, and during the night as well)

UV protection (Absorbs UV light and changes its color, thus protecting eyes from harmful UV rays)

Eye health (Reducing your exposure to the sun harmful UV rays can lower the risk of cataracts or other eyes/age-related problems)

Disadvantages:

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

Little expensive compared to normal lenses.

In extreme heat, a photochromic lens is unlikely to achieve full sunglasses shade and may only achieve a 95% state of tint.