The value and convenience of Transition lenses are well known to consumers, but there is one area where they suffer from seemingly uneven performance: they do not fully darken when worn inside an automobile, regardless of the brightness of the day and the amount of light to which they’re exposed. Are the lenses to blame for this anomaly in performance?
Not at all. Transition (or photochromic) lenses actually work the same way in the car as they do anywhere else – they variably darken in reaction to the presence of ultraviolent (UV) light. The problem stems from the fact that most modern automobile windshields already filter out harmful UV rays.
“Your windshield is normally quite different from the rest of your glass, in that it’s two pieces of glass laminated with a layer of plastic – vinyl – in between,” Rob Vandal, Vice-President of Product Engineering and Development at Guardian Automotive (a supplier of original equipment windshields to major automakers), explained in a recent interview. “So that triple-layer system…contains UV inhibitors that protect the plastic and as a result also protect any transmission of UV through it. So a windshield, laminated glass, blocks 98 to 99 percent of all UV.”
Ultraviolet light operates below the visible spectrum and is invisible to the human eye. Its presence and effect has nothing to do with the brightness of sunlight; that is why you can still get a sunburn if you lay out on cloudy day. This sometimes leads to confusion over the effectiveness of Transition lenses, and questions like, “If bright sunlight is entering my car (and my eyes!), why aren’t my photochromic lenses darkening to compensate? Are they defective?”
No, your Transition lenses are working just fine. They simply can’t react to a light source that is not present, and in an automobile, most of that UV light has already been taken out of the equation. You may notice some darkening of your lenses while in your car; this is because in typical automotive applications, only the windshield blocks a high percentage of UV light. Side and rear glass is rarely laminated, so while it has some UV-blocking properties, it also lets a good deal of ultraviolet light spill through. And, of course, open windows allow in some ambient UV light, sometimes enough for your Transitions to react.
So how to combat this effect? If your prescription photochromic lenses do not sufficiently darken to your comfort level while in the car, you have options. You can purchase a separate pair of prescription sunglasses for driving. You can buy clip-on sunglasses to apply temporarily over your prescription eyeglasses. And Transitions Optical has recently developed and marketed a series of eyewear called DriveWear, which does darken behind the windshield of a car. We’ll discuss that product in another blog entry.
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