Photo-chromatic lenses are lenses that adjust to the amount of Ultra Violet (UV) light directly exposed to the lens. These lenses are based on inorganic photochromes (AgX), and have been available since the 1960's. The more UV light the lens is exposed to the darker the lens becomes. Degree of darkness also varies by manufacture, but most reach a relatively dark shade. Many consider photo-chromatic lenses as comfort lenses, which adjust to the amount of sunlight that the wearer is exposed to, hence creating a comfortable view for most eyewear users. Photochromatic lenses also provide 100% protection from harmful UVA and UVB rays. Photo-chromatic lenses are primarily available in brown and gray in most materials and types of lenses. Some exclusions and exceptions may apply with certain lens manufactures.
One thing to consider about driving with photochromatic lenses is that all vehicle wind shields come with a degree of UV filter to help protect the diver and passenger(s) from harmful UV rays. Since photochromatic lenses are activated by their exposure to UV light, photochromatic lenses will not reach their mature level of darkness.
Most photochromatic lenses experience a fatigue process, where after repeated exposure to UV light the lenses eventually maintain a slight tint and sometimes do not get as dark as when they were brand new. The fatigue factor may vary in respect to the amount of UV exposure the lenses experience on a consistent basis and manufacture, but is generally noticeable after about one year. The down fall of fatigued photochromatic lenses is that if only one lens is damaged and/or has to be replaced, the fatigued lens will generally be noticeable next to a brand new lens, so it is usually recommendable to replace both lenses to avoid an awkward looking pair of glasses.
Photo-chromatic lenses usually change from light to dark in 10 to 15 seconds and change back from dark to light in 10 to 15 minutes, but may vary from one manufacture to another. There are many different manufactures that produce photo-chromatic lenses. Below is a list of manufactures with a brief description and links to their respective websites.
One of the most notable photo-chromatic lens manufactures on the market is Transitions. Transitions have a patented photochromatic dye within the lens that produces an ultra clear and ultra dark effect with a quick transition time. Transitions are available in Gray and Brown. Transitions used to make a lens called the Splitz that changed from red to purple, green to blue, orange to yellow and a couple of other variations. These lenses were discontinued around 2003-2004. However, Transitions have teamed up with Nupolar to create the first polarized photocromatic lens called Drivewear. The Drivewear lens changes from a yellow (for low light environment) to a brown (for bright environment) accommodating the lighting condition. Both colors of the Drivewear are attributed to creating contrast for almost any lighting condition.
Corning is attributed to inventing the photochromatic lenses in the 196's. Corning is also accredited for in-mass photochromic technology. This technology has photochromatic molecules that are dispersed evenly throughout the lens material. Corning claims that this self replaced photochromatic molecules technology prolongs the life of the lenses changing effect. The corning Sunsensor is also a mid-index lens that is lighter and slightly thinner that conventional plastic CR-39 lenses. The Sunsensor lenses are available in brown and gray. lenses. The Sunsensor lenses are available in brown and gray.
Rodenstock created the Colormatic lens, which also has the photochromatic technology built in through out the lens medium. Rodenstock claims to have a four year lifespan non-fatigue lens anatomy, which will also darken up to 50% inside a vehicle. The non-fatigue factor is relevant in the event that only one lens is damaged and needs to be replaced. With most photochromatic lenses both lenses have to be replaced because after photochromatic lenses are exposed to UV light some lenses experience fatigue, which leads to the lenses carrying a shade of tint and sometimes do not get as dark as when they were new. So when replacing one lens a noticeable difference may be noted when a new photochromatic lens is placed next to a fatigued photochromatic lens. Rodenstock also claims that there Colormatic technology allows the Colormatic lenses to tint and lighten in record time. The Colormatic lens is offered in three colors: Brown, Gray and Green
Hoya has its own in-mass process photochromatic lens offered in a 1.55 index plastic lens. They call this lens the Hoya SunGray IV and has been available in single vision, bifocal, and its wide designed progressive lenses. However Hoya also uses the Transitions technology on some of its other lens designs including its Phoenix photochromatic lenses available in Trivex.
The LifeRx photochromatic lens is offered by Vision-Ease as a polycarbonate lens. Polycarbonate lenses are thinner and lighter than conventional plastic lenses. LifeRx uses a molded laminate film technique that places a photochromatic dye layer .04mm below the front surface of the lens. Vision-Ease Claims that the Life RX deactivates 30-180% more quickly than the latest generation of its respective competitors with a longer life performance. However, because of the laminated technique, these lenses might not be the best option for frames that require a grooved lens. Like polarized lenses laminated lenses run a risk of splitting depending on how close the groove is to the laminate seem.
These lenses are the original photochromatic lens, and are only available in glass. Being that Photogray/brown lenses are only available in glass they are the most scratch resistant photochromatic lens. However, these lenses do not get as dark or as clear and are considerably heavier than other materials photochromatic lenses.