Photochromic lenses, often known as Transition Lenses, automatically darken when exposed to sunlight and clear again indoors. This adaptive technology combines prescription glasses and sunglasses into a single pair. However, many wearers notice that these lenses fail to darken inside a vehicle. The simple answer for most standard photochromic lenses is no, because of the specific light required for the darkening reaction to occur.
The Science of Light Activation
Standard photochromic lenses rely on a precise chemical process to initiate the tinting action. These lenses contain specialized organic molecules, such as naphthopyrans, embedded within the lens material. These molecules are highly sensitive to ultraviolet (UV) light, which is a high-energy component of sunlight.
When UV radiation strikes the lens, the energy triggers a reversible chemical reaction that causes the molecules to change their structure. This altered structure absorbs visible light, which is perceived as a darkening of the lens. The intensity of the UV light directly correlates with the degree of molecular change.
Once the source of UV light is removed, such as when moving indoors, the molecules revert to their original, clear state. This mechanism is powered almost exclusively by UV radiation, as visible light alone is insufficient to start the reaction.
Why Standard Lenses Do Not Darken Behind Glass
Standard photochromic lenses remain clear inside a car because of the vehicle’s construction, not a malfunction of the glasses. Modern car windshields use laminated glass, consisting of two layers of glass bonded with a plastic interlayer, typically polyvinyl butyral (PVB). This PVB layer is a highly effective barrier against UV radiation.
The laminated windshield is designed to block approximately 96% to 99% of both UVA and UVB rays. Since the vast majority of the UV energy needed to activate the lens molecules is filtered out, the lenses receive insufficient stimulus to darken. Consequently, the glasses stay clear, even on bright days.
Side and rear windows are often made of tempered glass, which provides less UV protection, typically blocking around 70% of UVA radiation. This lower blockage may allow a slight, barely noticeable tinting if the sun streams directly through a side window. However, this minimal activation rarely provides meaningful glare protection for the driver.
Specialized Lenses for Driving
The demand for light-adaptive lenses that work while driving has prompted the development of specialized photochromic technologies. These newer lenses are engineered to be activated by a broader spectrum of light, including visible light, which passes through the windshield.
Transitions XTRActive
This technology is sensitive to both UV and visible light. This dual-activation capability allows the lenses to darken moderately behind the windshield, providing comfortable tinting for driving conditions. These lenses also feature a slight tint indoors for those sensitive to harsh artificial lighting.
Transitions Drivewear
Drivewear is specifically designed for drivers. These lenses combine visible light photochromic technology with polarization, which eliminates glare from reflective surfaces like the road. Drivewear lenses change color, typically from a green or yellow-green hue in low light to a dark copper-red tint in bright sunlight. They are not intended for use as clear indoor lenses or for night driving. For those who prefer a non-photochromic solution, polarized sunglasses remain an excellent choice for superior glare control.