These lenses darken on exposure to ultraviolet (UV) radiation. Once the UV is removed (for example by walking indoors), they gradually return to their clear state. Photochromic lenses rely on organic photochromic molecules to achieve the reversible darkening effect. These lenses darken in sunlight but not indoors under artificial light, as typical room lights do not contain the UV (short wavelength light) found in sunlight. These are perfect all-day lenses for sunrise-to-sundown epic rides, treks or road trips.