Wiley-X uses the term Light Adjusting, while Oakley refers to the brand they share the technology with, Transitions. In the absence of ultraviolet light, non-tinted versions of these lenses in their natural state have only slight light reducing properties. With exposure to UV light, the lenses automatically darken for added protection against bright light.
How do they work?
When photochromic lenses first came into existence in the 60’s, only glass lenses were used. The embedding of molecules (usually Silver Chloride) was the most common means of achieving the desired effect. Exposed to UV rays, these molecules would undergo a chemical reaction, change shape, absorb some of the passing light and cause a darkening result.
Since polycarbonate and other “plastic” lenses have become the dominant material in the sunglass world, a slightly different process has taken over the darkening job. The size and shape of the carbon-based molecules (organic) now used are temporarily changed with UV exposure. These photochromic molecules can also be affected by significant variations in temperature, which can be both good and bad*.
The lightness or darkness of a lens is typically measured by its VLT. Visible Light Transmission is how much of the available white (visible) light passes through the lens. This is identified by the percentage.
If we see 100% of the visible light when not viewing through glasses, we may only see 80% of that same light when looking through a typical yellow lens. Through average Gray lenses, we may only see about 13% of the same light. It may seem lighter than that because our pupils have dilated behind the dark lens to allow more light in.
Of the photochromic styles found on SafetyGlassesUSA.com, the range of VLT for lenses in their light or natural state is 33 to 85 percent. That is, some lenses are nearly clear in their lightened state, while others only get as light as 33% VLT.
The VLT range for lenses in their fully darkened state is 9 to 35 percent.
The graphic below illustrates the progression of VLT from dark to light or vice versa, and also provides an approximation of your lens' darkness for any tint.
Any Photochromic Problems?
Despite all the benefits of photochromic lenses, there are some potential drawbacks to be aware of.
*A) As stated earlier, the organic molecules used in today’s lenses are also sensitive to temperature. But they’re affected in a way that’s probably opposite from what most people would guess – or want. Photochromic lenses will tend to become lighter when warmer and darker when colder. Thus, if you’re in Arizona in August and are desperately in need of dark sunglasses, you’ll need to consider that the 110 temperature that tends to lighten the lens will conflict with the intense UV rays that want to darken the lens. On the other hand, you might find that your glasses are much more effective than you expected on your Alaska vacation.
B) The effectiveness of photochromic lenses can be reduced or eliminated when used in some vehicles. Click here for more information on this issue.
Click here to browse all the styles of Photochromic eyewear that SafetyGlassesUSA.com carries.
Note: While most of our photochromic eyewear is tested to past ANSI standards, currently only those by AOSafety and Wiley-X brands are marked for safety standard compliance.