Standard photochromic lenses are activated by UV light (mainly UV-A). Car windshields block over 99% of UV-A, so the lenses remain clear. Only specialty lenses (e.g., "transitions Xtractive") respond to visible light and work behind windshields.

Initial tint appears within 30–60 seconds in direct sunlight. Full dark (maximum density) takes 2–3 minutes. Newer generations can reach 70% dark in under 60 seconds. Speed also depends on temperature (faster when cold).

Typical lifespan is 2–3 years of regular use before noticeable performance decline (slower darkening, lighter maximum tint). After 3–4 years, many users replace them.