The word "photochromic" comes from two Greek words "photos" meaning light and "Chroma" meaning colour—so photochromic simply means something that changes colour in response to light. In relation to sunglasses, photochromic lenses darken or lighten dependent on their exposure to ultraviolet (UV) radiation. For example, once the UV is removed by walking indoors or reduced by cycling through a forest, the lenses will gradually return to their clear state or a lighter tint respectively.
Photochromic lenses were originally developed in the 1960’s and made from glass. They worked a bit like pieces of old-fashioned, photographic film. Film darkens because it contains silver based chemicals that clump together when light falls on them. Early photochromic lenses contained similar silver chemicals and darkened the same way. Unlike photographic film, which darkens permanently, the photochromic lenses could change back again and clear when the light level fell back to normal.
Modern photochromic lenses are now made out of polycarbonate or plastic and instead of silver chemicals they contain organic (carbon-based) molecules that react to light in a slightly different way. Instead of clumping together, they change their colour and size when ultraviolet light strikes them. Although each molecule changes by only a fraction of a nanometre, when many molecules respond in the same way the effect can be dramatic as more and more light is progressively blocked out.
Today's photochromic lenses can offer different categories of sun protection to meet specific needs. For example, you can buy lenses that will start clear and change colour. These are ideal in glasses. There are others that will start as a tint and become progressively darker. These are ideal in sunglasses.
Typically, photochromic lenses darken substantially in response to UV light in less than one minute (see image on left where the right lens has been exposed to sun light for one minute), and then continue to darken very slightly over the next fifteen minutes. The lenses will begin to clear as soon as they are away from UV light, and will be noticeably lighter within two minutes and mostly clear within five minutes. However, it normally takes more than fifteen minutes for the lenses to completely fade to their non-exposed state. The best quality photochromic lenses can offer very quick transition times. Julbo’s Zebra lens for example boast high speed activation of between 22 to 28 seconds.
However, as photochromic compounds fade back to their clear state by a thermal process, the higher the temperature, the less dark photochromic lenses will be. This thermal effect is called "temperature dependency" and prevents these devices from achieving their full effect in very hot weather. Conversely, photochromic lenses will get very dark in cold weather conditions, which make them more suitable for snow skiers than beachgoers while outside. Once inside, away from the triggering UV light, the cold lenses take longer to regain their clear color than warm lenses.
There are both advantages and disadvantages to photochromic lenses. These are as follows:
Because Transitions lenses and other popular photochromic lenses rely on UV light to darken, general purpose photochromic lenses do not darken behind automobile and truck windshields that block the sun's UV rays. To overcome this problem, lens manufacturers have introduced photochromic lenses that are designed primarily for outdoor wear and for use when driving in sunlight.
The best lenses on the market to date are “Drivewear” lenses which are photochromic polarized lenses. By combining photochromic and polarization technologies, Drivewear lenses reduce glare and maximize visual acuity in bright light conditions. The lenses are capable of sensing and reacting to varying light conditions both outdoors and behind the windshield of a car or heavy goods vehicle. Drivewear lenses currently are available in plastic and polycarbonate lens materials and in single vision, bifocal and progressive lens designs.
Experts say the risk for cataracts and other age-related eye problems is associated with a person's lifetime exposure to the sun's UV rays, so protecting your child's eyes early on could pay dividends when he or she is a mature adult. As kids tend to spend more time outdoors than most adults, photochromic lenses are ideal as they block out all UV radiation. However, if you do not want to go to the expense of having photochromic lenses, at eyekit.co you can also buy glasses that have a UV coating on the lenses. They will not act as sunglasses but will block all UVA radiation.
The brand manufacturers have created many brand names for photochromic lenses. These include Transitions, Reactolite, Reactions and Graduations. However, they are all photochromic lenses.
The most common photochromic brands are
If you want a prescription that includes photochromic lenses, we will provide you with the latest ‘Transition VII’ lenses. At eyekit.co we believe these are the best lenses on the market and most suitable for outdoor activities. You can be reassured to know that they are also the same lenses used by many of the best named sporting brands such as Oakley and Julbo and are a higher level of transiton than many opticians on the highstreet.
We offer two types of transitions prescription lenses:
Transitions Signature VII
These lenses replace transitions VI lenses as the most balanced everyday lenses yet. They are more responsive in more situations. They provide the ideal combination of darkness, fast fade back speed and indoor clarity.
These offer increased darkness outdoors, even in hot temperatures. They are ideal for wearers who spend much of their day in bright, sunny conditions. Indoors, they have a comfortable hint of tint. Specifically designed to be extra dark outdoors, they also moderately activate inside the car or under an aeroplane canopy.