TLDR Summary: UV420 lenses block UV as well as a little of the visible violet light, which gives extra protection against high-energy visible light - good for your outdoor activities!
[Note: These lenses are often marketed similarly with blue-light lenses, sometimes known as PC lenses or computer lenses. We recommend reading that article as well.]
UV LIGHT AND THE EYE
We first need to understand UV light, and how it affects the eye.
Ultraviolet has a wavelength around 10nm to 400nm. Between 400nm and 700nm is visible light - with 400nm being violet light and 700nm being red light. UV is absorbed by both the cornea (that's the front, transparent part of the eye) and the crystalline lens (that's the flexible lens behind the pupil that helps us to focus at objects, and also the part that gets the infamous "cataracts"). UV can cause photo-keratitis as well as cataracts, which of course is why we are concerned and try to block as much of it as possible.
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UV420 LENS - WHAT DOES IT DO?
Not to be confused with blue-light blocking lenses; these lenses are not quite the same! These are like extended range UV blockers - blocking off transmission up to 420nm, which is UV and some of the visible violet light as well.
Note: Technically UV's wavelength is from 10nm to 400nm, but these lenses may be called UV420 anyway, for simplicity's sake, while some manufacturers brand these as "blue-light blocking lenses" instead.
We sometimes see this being demonstrated in optical shops using a laser (of 405nm wavelength), which would be blocked by the UV420 lenses but not the usual UV380 or UV400 lenses. Clearly this looks impressive and fear spreads that your mobile phone is actually laser-ing your eye but, well, good to remember that:
1) Lasers are focused, intense beams of light, not like the sun's rays or your mobile phone screen
2) The UV or blue-light coming from your mobile phone or computer screen is unlikely to pose a hazard (as we have explored in the blue-light blocking lenses post) - unless you're using your phone while driving then that's a real hazard...
3) If there is that much UV coming from your screen, it would probably turn your Transitions lenses tinted. Or you could see the fluorescence ink on currency notes. Or help set your gel manicure.
I'm sure some of you will realise that the light from the laser pointer looks more blue than violet; actually it is more 'purple' than 'blue' - however they can appear quite blue when pointed at a white wall or white surface - this is due to the use of optical brighteners - chemicals added to paper, paint, and other products, that fluoresce under UV light. This causes it to look more blue than it actually is.
BUT WHY 420NM?
The intention of blocking up to 420nm is meant to give an extra protection against high-energy visible light (HEV Light) - basically the violet part of the visible light spectrum, rather than stopping at the traditional "UV380" or "UV400".
As we know, the shorter the wavelength, the more energy there is. Which is why ultraviolet (UV) does so much harm to our skin and eyes, and for that same reason, why not extend and block part of the violet light as well.
Hence, these lenses block both the well-known UV rays as well as the violet portion of the visible light which has higher energy than the other colours of visible light.
WHAT ABOUT THE USUAL MULTI-COATED LENSES?
The good news is, multi-coated lenses also have UV protection - they just stop at 380nm (so it's called UV380) or 400nm (so it's called UV400).
Side note: Have you ever seen stickers on sunglasses stating "UV380" or "UV400"? Yup - that number is basically the wavelength.
So yes, chances are that your glasses, if they are multi-coated lenses, are already giving you UV protection.
DO I NEED IT?
There isn't a set standard on how much UV protection we should be getting (or in this case, high energy visible light protection as well), but perhaps more is better than less.
While is can sometimes be marketed as a "protection from LED screens" product, the real use for such lenses may be found in the opposite direction - outdoors! Since our dear sun is an excellent source of both UV and violet light, that would be where these lenses come into play. So if you spend lots of time in the sun, this is something you should consider having.
If you're aphakic (do not have the crystalline lens in the eye), or if your IOL implant (cataracts surgery) does not absorb UV, then this lens would surely be preferred - since the natural crystalline lens that we have in our eyes help absorb UV and the absence of such protection means the retina (the back layer of the eye) gets more UV than it usually would in an average eye. There's even a paper on violet and blue light blocking IOLs.
That being said...
UV protection, of course, doesn't stop at a pair of spectacles. Using a wide-brimmed hat or using an umbrella also reduces UV exposure, and so does avoiding the midday sun or using a pair of good sunglasses.
We carry both the blue-light lenses (PC lenses) as well as these UV420 lenses. Still unsure? Drop by our store and talk to us - 194 Kim Keat Ave, Toa Payoh.