Transition glasses, formally known as photochromic lenses, function as both clear glasses indoors and sunglasses outdoors. These specialized lenses automatically darken when exposed to sunlight, eliminating the need to switch between two pairs of eyewear. Photochromic lenses are engineered to provide full protection from ultraviolet light while adapting to varying light conditions.
Constant UV Protection Regardless of Tint
Photochromic lenses offer a constant shield against ultraviolet radiation, regardless of the tint they display. This protection is an inherent property of the lens material, not a surface coating that can wear away. Lenses that darken in sunlight typically block 100% of both UVA and UVB rays, the primary forms of invisible UV radiation that pose a risk to eye health.
It is important to distinguish between the two types of light that interact with the lens. Ultraviolet light triggers the darkening reaction and is blocked entirely, while visible light is what the eye perceives. The lens material provides complete UV protection even when the lens is perfectly clear indoors. The darkening feature is simply a mechanism for comfort, reducing glare and eye strain.
The Science Behind Photochromic Activation
The ability of photochromic lenses to change from clear to dark is based on a reversible chemical process involving specialized molecules embedded within the lens material. For plastic lenses, these molecules are typically organic compounds, such as naphthopyrans or spirooxazines. When exposed to ultraviolet light, they undergo a rapid and reversible change in their molecular structure. The altered shape causes them to absorb a greater amount of visible light, resulting in the lens darkening. When removed from UV exposure, the molecules revert to their original, transparent state.
Practical Limitations and Performance Factors
A common observation is that photochromic lenses often fail to darken inside a vehicle. This is a direct consequence of how modern automobiles are constructed. Most car windshields contain a layer of lamination that is highly effective at blocking ultraviolet light, the very trigger that activates the photochromic molecules. Since the activating UV rays are filtered out before reaching the lens, the glasses remain mostly clear during driving.
The performance of these lenses is also affected by temperature, a factor that influences the speed of the molecular reaction. In cold weather, the photochromic molecules can achieve a deeper, darker tint than in warmer conditions. However, the colder temperature also slows the rate at which the molecules can revert, meaning the lenses will take longer to clear once you return indoors.
Conversely, in hot environments, the lenses darken less intensely because the heat encourages the molecules to favor their clear state. The increased temperature also causes the lenses to fade back to clear faster when stepping out of the sun. While most of the darkening occurs quickly, reaching maximum tint can take up to fifteen minutes, and returning to a fully clear state indoors also involves a time lag.