I also have sunglasses that say they block 99% of UV light. When I’m wearing both of them are they both working on their percentage, therefore on allowing. .(01x.04=.0004= .04% of UV in?
Or are the rays that get through different, and once they go through the glasses, are going to go right through the lenses as well?
Good question. You clearly have some grasp of the idea that a bunch of rays all of which are not the same will not be reduced by the product of the two filter’s blocking ratios.
UV is a broad term that includes everything between violet and X-rays. In practice in the outdoors, UV is a narrower range from UV to wavelengths that are short enough that air absorbs them, so it’s the part of sunlight shorter than violet that the atmospher lets through.
There is UVA and UVB and UVC in this range, and they have somewhat different effects on the eyes and other living things and surfaces.
In this particular case, either of your options may be true, or some intermediate case between them may be true.
To understand what the number is, you’d have to do a numerical addition of all the wavelengths present in the outdoors, while multiplying each one by the blocking ratio for that particular wavelength in the contacts, and by the same for the sunglasses. So you’d need the transmission spectra for both.
It is hard to get ultraviolet to go through things, for roughly the same reason that yellows and oranges and reds and browns are so common in nature compared to blues (except the sky) and greens (except plants) and purples. So without any better info, I’d guess your second scenario is more nearly the case.
It ultimately depends on how much is blocked at each frequency. From your link, your lenses block “up to 96% UV-A and 99% UV-B”. I’ll assume your glasses block 99% of both UVA and UVB. If we assume that the percentages are constant across the ranges, then the two together would block 99.96% of UVA and 99.99% of UVB.
It’s possible that the combination would block only as much as the contact lenses, but that would require that the contacts block 100% of UV over most of the ranges, and block nothing over 4% of UVA and 1% of UVB, and similarly for the glasses, and that the portions of total transmission overlap. That’s very unlikely.
The more likely scenario is that the UV transmission varies some, and that you get more like, maybe, 99.9% of UV blocked. You’d need more information than you have to say what the actual value is.
FWIW, they’re not actually blocking anywhere close to 90% of UV from reaching your eyes; the numbers only indicate how much UV the actual lenses block. Your contacts won’t do anything to protect the sclera (white bit of the eye), and your sunglasses won’t block any light coming from above, below, or the sides. As a rule of thumb, if you can see out (past the edges of the frame), plenty of UV is getting in; if you’re wearing, say, a small wire frame, your glasses are only blocking about 50% of your eyes’ UV exposure, regardless of how much the lenses block.
IOW, whether or not your contacts and glasses are blocking the same wavelengths or not, the question is basically academic.
>they’re not actually blocking anywhere close to 90% of UV from reaching your eyes; the numbers only indicate how much UV the actual lenses block.
I don’t think this is so big a problem. As I understand it, the reason we want UV blocking sunglasses is not just that we don’t like UV in general. It is because eyes evolved to regulate the light entering the pupil to a safe level, both in the visible and the UV, by taking advantage of the fact that UV and visible are in a stable ratio, and adjusting the pupil diameter to cut back on both when there’s too much visible light stimulus. The problem with sunglasses is that they might block most of the visible light and little of the UV, so our safety regulation system doesn’t work right, and we take in too much UV in our field of view through the pupil. Sunglasses provide relief from overbright visible light because they reduce the visible light entering the pupil in our field of view, and they’d have the same effect on UV. Unless we’re being thoughtless next to an arc welder, we’re not going to have UV problems with our sclera, are we?
In the short term, UVB causes keratitis, which is uncomfortable, but not necessarily dangerous, and contacts will protect you from that.
However, unfiltered UV does affect the sclera in the long term. Remember, you get much more of it in most places than you would “naturally” thanks to the thinning ozone layer.
Also, exposure to visible light - not UV - causes (or contributes to) macular degeneration.
IANAEyedoctor, but I’ve heard in the last years that the more you’re exposed to UV light in your life, the more you’ll get degenerative grey cataracts, which includes UV rays that go around the lenses.
Also, how much of a problem the UV-rays that do get around the lenses etc are depends on where you are. When I was in the youth Alpine Club, they stressed to make glacier or ski tours only with real glacier glasses that wraparound the whole face or have rubber seals to protect the whole eye, otherwise the reflection would cause snow blindness. (And on my first ski tour in March/April, one of the boys whose parents hadn’t wanted to buy a special glass only for this and given him normal sunglasses instead did get infected cornea after the first day and had to spend the next days with a wet cloth on his eyes and dark sweater wrapped around his head to block out all light, and was in some pain. I only got swollen lips and serious sunburn for not taking high enough sun cream and special lip balm with Sun Protection.)
I wonder what the deal with sailors and others on open water is - do they wear wraparound/special glasses today? I’ve seen ads that mostly stress the polarizing effect of canceling out the glare and irritation from the moving waves.