Photochromic lenses contain molecules that change shape under ultraviolet light, darkening the lens outdoors and clearing again indoors. One pair does the job of clear glasses and sunglasses.
Most lenses darken noticeably within a minute outdoors and take a few minutes to clear fully indoors. They darken more in cold weather and less in heat.
Windscreens block most UV, so standard photochromic lenses only darken slightly while you drive. If driving in bright sun is the main use, a dedicated pair of sunglasses is still the better option.
Photochromic readers suit people who read outdoors: on holiday, in the garden, at the café. Distance photochromics suit people who walk, cycle or are in and out of buildings all day.
Yes. The lenses block UV whether they are clear or dark.
Only a little, because most glass blocks the UV that triggers the change.
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