How fast do lenses that darken by themselves change?
Most take about half a minute to a minute to go dark and a few minutes to clear. Heat makes them less dark, and many do not darken fully in a car.
How they work.
These lenses are called photochromic. In plastic ones, molecules change shape in ultraviolet light and then absorb much of the visible light, so the lens darkens. Out of that light they change back, and the lens clears.
Most react to the UV in sunlight, not to the light of indoor lamps.
How fast they change.
The American Academy of Ophthalmology says most of the darkening takes about half a minute, and clearing takes about five minutes.
An encyclopedia gives close figures: much darker within about a minute, and mostly clear within five minutes of leaving the sun. A public science site adds that full clearing can take up to an hour.
Heat makes them less dark.
They clear by a process driven by heat, so the warmer the lens, the less dark it gets. In the cold they get darker and take longer to clear.
In a university study of 12 lenses, the time to clear to a very light tint was more than six times as long at about 6 degrees Celsius as at about 21 degrees.
In a car.
Car windscreens filter out most UV. Behind one, many of these lenses darken slowly and only part of the way, so they can end up too dark or too light for driving.
The light sets the level.
The wearer cannot choose how dark they get. The light and the heat decide that.
Sources:
- American Academy of Ophthalmology: Recommended Types of Sunglasses
- Wikipedia: Photochromic lens
- Wikipedia: Sunglasses
- Explain that Stuff: Photochromic lenses
- Department of Optometry, Kangwon National University, in PLoS One: Differences in the optical properties of photochromic lenses between cold and warm temperatures
- Georgia Institute of Technology: News release on electrochromic polymers for sunglasses and windows
Other guides
Where Photenic fits.
Photenic is being made for this gap: a tint you set, on the glasses you already own. It is in development.